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Course Title: ENERGY SOURCES AND UTILIZATION (2.0)

Course Content:

1.

Fossil fuel: their processing and utilization

2. Renewable source of energy

3. Solar energy utilization; flat-plate collector design; economics of solar energy

equipment and their operation

4. Wind energy; wind mill design

5.

Geothermal energy: its recovery and utilization; environmental consequences of

geothermal energy exploitation

6. Biomass

7. Tidal waves

8. Principles of operation of nuclear reactors; safety problems in nuclear reactors



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1.0 FOSSIL FUEL: PROCESSING AND UTILIZATION

1.1 Introduction

Fossil fuels are fuels obtained from fossil sources. Basically, the substances called fossil fuels

derived their name from the archeological term fossil which means ‘dug up’. Fossil fuels are

carbon containing sources of energy which originates from the decomposed remains of

prehistoric plants and animals. They are formed as a result of the geological processes which

have taken place on dead and decomposed organisms, be it of plants or animals. They have

rich energy resource base and naturally exist deep down under the earth and can only be

brought to the surface of the earth by exploration. Chemically, fossil fuels consist largely of

hydrocarbons which are compounds composed of hydrogen and carbon. Moreover, some

fossil fuels also contain additional compounds of substances such as nitrogen and sulphur.

The most commonly used fossil fuels are those of petroleum products, coal and natural gas.

1.2 Formation of fossil fuels

Just as aforementioned, fossil fuels are fuels formed as a result of natural processes of

anaerobic decomposition of dead and buried organic matter. They are the products of the

fossilized remains of dead plants and animals which have been exposed, over several millions

of years, to heat and pressure within the earth’s crust .

The type of fossil fuel obtained depends typically on the parent materials and the

materials’ environmental influence. For instance, oil and natural gas are formed from remains

of marine organisms (e.g. planktons, plants) which have been trapped within the sea floor

over millions of years. As the sediments pile up under anaerobic condition, in very deep

situation, the organics become subjected to heat and pressure which leads to the formation of



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oil and then gas. Coal on the other hand, is formed in a non-marine environment, basically

from the remains of vegetables. It could be formed from vegetables on lowlands, swampy

(i.e. stagnant water swamps) or mire environments. The steps of conversion from plant to

coal are illustrated as:

Fig. 1.1: schematic steps of conversion from plants sources to coal

In a more generalized form, the complete steps of coal formation from plants are represented

as in Fig. 1.2.

Fig. 1.2: Generalized (or complete) steps of formation of coal from plant (Steve Mohr, 2010. Projection of world fossil fuel production with supply and demand interaction, Ph. D dissertation, University of Newcastle, Australia, p. 1)

1.3 Classification of Fossil fuels

Fossil fuels are broadly classified into three groups: hydrocarbon oils, Natural gas and Coals.

1.3.1 Hydrocarbon oils

Heat + Pressure

Coalification

PLANT PEAT COAL



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These are mixtures of hydrocarbons in liquid state. The common example of these is the

crude oil. Petroleum products are produced from crude oil via the processes of distillation and

catalytic cracking of complex hydrocarbons to form the simpler hydrocarbon units/distillates.

These distillates include gasoline, diesel, kerosene, fuel oils, petrochemicals, liquefied

petroleum gas, asphalt and tar. Of the different examples and components of fossil fuels,

petroleum products are the most consumed. The majority of transportation systems run on

petroleum products. Also, most of industrial and domestic processes are powered by energy

derived from petroleum products. More so, global economies have been designed and built

around consumption of petroleum products to the extent that changes in its availability or

price determines fluctuation in economic variables.

13.2 Natural gas

Natural gases are gaseous mixtures of saturated hydrocarbons, notably methane. They

contain, at a minimum level, about seventy five percent methane gas, with other

hydrocarbons of the alkane group which include ethane, propane, butane and pentane in very

small (about 20%) proportions. They can be used domestically for heating and cooking and

industrially for different processes ranging from electricity production to powering various

industrial processes. It has also found applications in various other fields which include it

serving as fuels in baking of bricks and ceramic tiles, in the production of cements, in boilers

to generate steam and in glass making and food processing industries. Natural gases also play

a major role in petrochemical industries, where they are used in the making of fertilizers,

detergents and plastics. Importantly, natural gases can be used to power gas turbines for the

generation of electricity. Many countries like Nigeria depend so much on power generated

from gas-fired turbines. The combustion of natural gases produces huge energy and can be



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useful to power automobiles, airplanes, trains, ships, industries and homes. It is abundant and

cleaner than the other types/categories of fossil fuels.

Natural gases are classified as either conventional or unconventional natural gas.

Those mentioned above are the conventional sources while shale gas, methane hydrates,

aquifer gas, tight gas, coal bed methane etc. are unconventional natural gases.

- Shale gas: This is a natural gas trapped within shale (a sedimentary rock of fine grains

composed of layers of compressed clay, mud or silt ) formations. Shale is organically

rich with very low permeability.

-

Methane hydrates: These are methane trapped in ice crystals particularly in sea floors.

More specifically, methane hydrates are solid clathrate compounds (a chemical

surbstance consisting of lattice of one type of molecule trapping and containing

another type of molecule). They are compounds in which large amount of methane are

trapped within a crystal structure of water, forming a ice like solid.

-

Aquifer gas: This is a natural gas found trapped in water aquifers

- Tight gas: This is a gas with permeability of as low as 10-9

D (darchy). It is always

difficult to extract from its source because of the nature of the rock and surrounding

sand.

-

Coalbed methane gas: This is a methane gas produced by coal and entrapped in the

coal by the process of adsorption. The methane is in a near liquid state and linning the

solid matrix of the coal. It contains very little heavier natural gas (e.g. propane or

butane) and it is free of natural gas condensate. It is usually called sweet gas as a

result of the absence of hydrogen sulphide (H2S) in the gas mixture.

Note: Unconventional sources of natural gas that have very low permeability require

fracturing from the process of drilling to extract the gas from them. Permeability can be

estimated using the relation:



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P v

t

where:

v = superficial (or bulk) fluid flow rate through a medium

k = permeability of the medium

∆ P = pressure difference

μ = dynamic viscosity

t = thickness of the medium

1.3.3 Coals

Coal is a black or brownish-black combustible solid formed by the decomposition of

traditional biomass of plants origin in the absence of oxygen. It is a combustible rock of more

than 50 wt% carbonaceous material. It has the same chemical constituent of carbon and

hydrogen as other fossil fuels but also contain oxygen and small amounts of sulphur and

nitrogen. It can be used directly as fuel, giving enormous amount of energy and heat when

burnt. Also, it is a chemical reactant and a source of organic chemicals, and can be converted

to gaseous and liquid fuels. Its use as a means of producing electricity has come a long way.

Today some countries (especially South African countries) depend on coal-fired turbines for

electricity generation. It also has wide applications in the making of fertilizers, pesticides and

solvents, and could be heated in the presence of steam and oxygen to produce synthesis gas.

Synthesis gas is useful either directly as fuel or refined to become burning gas. Coal is the

most abundant of the three types/categories of fossil fuels.

Coal can be classified into four broad categories. These are: Anthracite, Bituminous,

Subbituminous, and Lignite. The differentiation of the categories of clay lies in their moisture

content and the capacity of the available energy. The capacity of available energy in a

particular type of coal can be directly linked to the size or number of its carbon content.



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- Anthracite: This is coal with the highest carbon content. The carbon content is

between 86 and 98%. Its heat value is also nearly 15,000 BTU per pound (BTU/lb). It

is the hardest form of coal and scarcely available all over the world.

-

Bituminous: This has carbon content of between 45 and 86%. Its heat value is

between 10, 500 and 15, 500 BTU/lb. It is sometimes called soft coal.

- Subbituminous: This has carbon content of between 35 and 45%. Its heat value is

between 8, 300 and 13, 000 BTU/lb. It is a dullish black in appearance.

- Lignite: This has carbon content of between 25 and 35%. Its heat value is between 4,

000 and 8, 300 BTU/lb. It is soft and brownish-black in colour. It is commonest form

of coal.

NB: BTU is the acronym for British Thermal Unit. 1 BTU is the amount of heat required to

raise the temperature of a pound of water by 1 degree Fahrenheit.

1.4 Processing of fossil fuels

Fossil fuels in their raw form, such as crude oil, are usually not useful (except the light sweet

oil). It required processing for it to become usable and applicable with modern technologies.

The processing of fossil fuels refers to the activities carried out in order to convert the raw

fossil fuels to the usable and applicable form of oils, natural gases, and coals. Generally,

processing fossil fuels can be divided into two major parts. These are Refining and Mining.

The refining processes are used for producing the various types of oils and natural gases

while mining is used for coals.

1.4.1 Refining of crude oil

Oil refining is an industrial process that involves the breakdown of the complex hydrocarbon

chains of crude oil into more useful, but simpler hydrocarbons, products (i.e. petroleum



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products). These products include diesel fuel, gasoline, kerosene, liquefied petroleum gas,

asphalt, tar, fuel oils, Naphtha, Lubricants, bitumen, etc.

The refining process involves the separation of crude oil into its fractions by

fractional distillation in a distillation chamber. The distillation begins when crude oil, which

is a mixture of different hydrocarbons, is heated under very high pressure to a temperature of

600oC. The different mixtures of oil become vapour as the temperature goes beyond their

boiling points and rises in the distillation column. It cools as it rises and condenses in the

trays along the column. The fractions are separated based on their difference of boiling points

(bp.). Those with lower bp. occupy the topmost part of the fractionating column in succession

while those with higher bp. are at the lower parts, also in succession according to the value of

their bp. The heavy bottom distillates are often cracked by catalytic action into lighter

products.



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Fig. 1.3: Flow diagram of a typical oil refinery (Source: www.globalpetroleum.com/oil%20Refinery.pdf)



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1.4.2 Mining of coal

Mining is simply the process of excavating materials from the ground. Thus, coal mining is

the process of removing coal from its natural habitat – the ground. There are basically two

methods of coal mining. These are the surface (or opencut/opencast) method and the

underground method. The difference in the method is based on the location of the coal

beneath the top soil. Coals located less than 38 m below the top soil are mined using surface

mining method while those deeper than 38 m below the top soil are mined using the

underground method. The underground mining method is also of two types: the room-and-

pillar mining and the longwall mining. Whereas, the room-and-pillar mining leaves pillars (or

blocks) of coal in the mine to support the roof of the mine, the longwall allows the roof to

collapse in controlled sequence after the mining operation is completed. The advantage of the

room-and-pillar mining over the longwall is that it aids quick start-up of coal production and

it is cheaper. However, the longwall mining method leads to the removal of more coal.

1.5 Utilization of fossil fuels

The use of fossil fuels has various advantages and disadvantages. Some of the advantages

include the fact that fossil fuels are a very dependable source of commercially generated

electricity. More than 70% of the world’s energy is generated from fossil fuels annually.

Also, fossil fuels is one of the cheapest sources of energy, easy to harness and can be stored

over a long period of time. The majority of today’s industrial, commercial and domestic

systems and processes run on the various grades of fossil fuels. The transportation sector, be

it road, rail or air transport systems depends basically on fossil fuel for operation. Ethylene, a

product of petrochemical constituent of crude oil refining can be used to make anaesthetics,

antifreeze and detergents. Also, paraffin wax can be employed as insulators in drywall for



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insulating buildings. It can also be used to make candles. Lubricants are mostly employed to

reduce friction and wear between contacting surfaces moving relative to each other.

Liquefied petroleum gas can be used for different heating purposes. Other fossil fuels can be

used domestically for cooking, lighting via the use of generators and for many other

purposes.

Utilization of fossil fuels for gainful purposes has had tremendous effects on the

economics of nations, promoting industrialization, rural-urban integration, supporting

education and also the development of information technology. It use however poses some

major challenges to the environment, humans, and even national economies.

Environmentally, burning fossil fuels for energy releases a lot of carbon monoxide

(CO), carbon dioxide (CO2), water vapour (H2O(g)) and other deleterious compounds of

nitrogen and sulphur. Some of these compounds could be methane, nitrous oxides (NOx) and

sulphurou oxides (SOx). These products of fossil fuels combustion are active gases that

terribly affect the environment. It causes global warming and may leads to climate change.

Combustion of coal have been known to lead to production of fly ash and smog which are

harmful to health. Acid rain is another effect of fossil fuels burning. Sulphur (IV) oxide (SO2)

released to the atmosphere can undergo serial combination with water vapour to form

sulphuric acid. This is acid rain. It has corrosive tendency on the buildings, roofing materials

and paints. It is also reported that, acid rains have damaging effects on crops, forest, streams,

lakes and rivers. When humans are exposed to large volumes of some of these gaseous

products of methane, NOx, and SOx it could lead to respiratory infections. Nationally,

depending on fossil fuels for energy can lead to foreign exchange depletion as part of it will

be used to fund the purchase of petroleum products for national consumption.



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More so, the exploration of crude oil has over the years led to various oil spills. Oil

spills on rivers and oceans have been known to affect aquatic life and also fishing activities.

Additional Reading

Go through all the attached documents for more understanding



VII-Energy-A-Refining Crude Oil-1

REFINING CRUDE OIL

New Zealand buys crude oil from overseas, as well as drilling for some oil locally. Thisoil is a mixture of many hydrocarbons that has to be refined before it can be used for fuel.All crude oil in New Zealand is refined by The New Zealand Refining Company at theirMarsden Point refinery where it is converted to petrol, diesel, kerosene, aviation fuel,

bitumen, refinery gas (which fuels the refinery) and sulfur.

The refining process depends on the chemical processes of distillation (separating liquids by their different boiling points) and catalysis (which speeds up reaction rates), and usesthe principles of chemical equilibria. Chemical equilibrium exists when the reactants in areaction are producing products, but those products are being recombined again intoreactants. By altering the reaction conditions the amount of either products or reactantscan be increased.

Refining is carried out in three main steps.

Step 1 - Separation The oil is separated into its constituents by distillation, and some of these components(such as the refinery gas) are further separated with chemical reactions and by usingsolvents which dissolve one component of a mixture significantly better than another.

Step 2 - Conversion The various hydrocarbons produced are then chemically altered to make them moresuitable for their intended purpose. For example, naphthas are "reformed" from paraffinsand naphthenes into aromatics. These reactions often use catalysis, and so sulfur isremoved from the hydrocarbons before they are reacted, as it would 'poison' the catalysts

used. The chemical equilibria are also manipulated to ensure a maximum yield of thedesired product.

Step3 - Purification The hydrogen sulfide gas which was extracted from the refinery gas in Step 1 is convertedto sulfur, which is sold in liquid form to fertiliser manufacturers.

The plant at Marsden Point also manufactures its own hydrogen and purifies its owneffluent water. This water purification, along with gas 'scrubbing' to remove undesirablecompounds from the gases to be discharged into the atmosphere, ensures that the refineryhas minimal environmental impact.

INTRODUCTION

Our modern technological society relies very heavily on fossil fuels as an important source ofenergy. Crude oil as it comes from the ground is of little use and must undergo a series ofrefining processes which converts it into a variety of products - petrol for cars, fuel oil forheating, diesel fuels for heavy transport, bitumen for roads.

The NZ Refining Company, situated 40 km from Whangarei at Marsden Point, processesabout 4 500 000 tonnes of oil per year. About 90% of the feedstock comes from overseas,




and about 10% is the local crude obtained as a byproduct from the production of natural gas

at Kapuni. The sources of oil used are given in Table 1.

In this chapter we will describe the physical and chemical processes by which NewZealand's refinery converts crude oil into the variety of useful products required to meet New

Zealand's needs.

Table 1 - Feedstocks used at the Marsden Point refinery (1992 figures)

Origin Quantity / kT % Wt.

Indigenous feedstock 888 19.8

Foreign crudes 2464 55.2

Synthetic gasoline 419 9.4

Other 700 15.6

Total 4471 100

Uses of refined oil The refinery produces a range of petroleum products. These are listed on the simplified flow

scheme of the refinery in Figure 1, and the relative quantities made are given in Table 2.

Table 2 - Products (1994 figures)

Type Quantity / kT % Wt

Super or Premium petrol 732 15.0

Regular petrol 640 14.6

Jet/DPK 669 15.2

Diesel oil 1482 31.0

Fuel oils 478 10.0

Bitumen 144 3.0

Sulfur 23 1.0

Exports 143 3.0

Refinery fuel and loss 317 7.0

Total 4716 100

Petrol Petrol (motor gasoline) is made of cyclic compounds known as naphthas. It is made in twogrades: Regular (91 octane) and Super or Premium (96 octane), both for spark ignitionengines. These are later blended with other additives by the respective petrol companies.



Figure 1 - Schematic representation of the Marsden Point Refinery

AGO = Automotive Gas Oil H.C.U. = Hydrocracking unit H.V.U.2 = HB.D.U. = Butane deasphalting unit H.D.S. = K.H.T. =C.D.1 & 2 = Crude distillation units H.D.T. = S.R.U. = SuD.A.O. = Deasphalted oil H.M.U. = Hydrogen manufacturing unit

Asphalt

Waxy dust and DAO

Utilities

H.D.T.

K.H.T.

H.D.S.(as required)

Bitumen

S.R.U.

Platformer

H.M.U.

Crude from tankage

C. D. 1 & 2

H.V.U.2& B.D.U.

Hydrocracker

All Services to Process Units

H 2S from

Process Units

Sour Naptha

Sour Kero

Occasionally Sour G. Oil

Long residue

Sweet G. Oil

Sweet Kero

Sweet Naphtha

Tops

Gas Oil

H2

Platformate

Vacuum G. Oil

H.C.U

Kero Naphtha

Cracker tops




Jet fuel/Dual purpose kerosene The bulk of the refinery produced kerosene is high quality aviation turbine fuel (Avtur) used by the jet engines of the domestic and international airlines. Some kerosene is used forheating and cooking.

Diesel Oil This is less volatile than gasoline and is used mainly in compression ignition engines, in roadvehicles, agricultural tractors, locomotives, small boats and stationary engines. Some dieseloil (also known as gas oil) is used for domestic heating.

Fuel Oils A number of grades of fuel oil are produced from blending. Lighter grades are used for thelarger, lower speed compression engines (marine types) and heavier grades are for boilersand as power station fuel.

Bitumen This is best known as a covering on roads and airfield runways, but is also used in industry asa waterproofing material.

Sulfur Sulfur is removed from the crude during processing and used in liquid form in themanufacture of fertilisers (see article).

THE REFINING PROCESS

The Marsden Point refinery takes crude oil and refines it by first separating it into its various

constituents, then converting the undesirable elements to desirable ones before purifyingthem to make the final products. These processes are based on some chemical principleswhich are outlined below.

Chemical principles The processes used to refine oil are based on the chemical principles which governdistillation, and require an understanding of chemical equilibria and the effect on reactionrates of catalysts. The theory behind these is outlined below.

Fractional distillation When a mixture of two liquids of different boiling points is heated to its boiling point, the

vapour contains a higher mole fraction of the liquid with the lower boiling point than theoriginal liquid; i.e. the vapour is enriched in the more volatile component. If this vapour isnow condensed, the resultant liquid has also been enriched in the more volatile component.For example, if a 1:1 molar mixture of benzene (b.p. 80.1oC) and toluene (b.p. 110.6oC) isheated it boils at a temperature of 92.2oC and the composition of the vapour (and hence itscondensate) is 71.3 : 28.7 benzene:toluene. If this vapour is condensed and then brought toits boiling point its vapour has a mole ration of 86.4 : 13.6. A third cycle produces acondensate of 94.4 : 5.6 benzene:toluene. This is the principle of batch fractional distillation,and in a distillation column many, many such cycles are performed continuously, allowingalmost complete separation of liquid components. In this process the more volatilecomponents are drawn from the top of the column and the the least volatile ones concentratein the lower part. In practice, the distillation is carried out continuously. A generalised

distillation column is shown in Figures 2 and 3.




HEAT EXCHANGER -cooling, condensation

SEPARATOR

Uncondensed gases

FURNACE - partialvapourisation

Tray 1

Tray 2

Tray 3

Tray 4

Tray 5

FEED

BOTTOM PRODUCT

HEAT EXCHANGER - partial vapourisation

Reboil

Reflux

Figure 2 - Fractional distillation

LIQUIDS

DOWN-COMER

VAPOURS

PRODUCTSIDE - DRAW

Vapours

Liquids

Figure 3 - Schematic representation of the inside of the distillation column

When two species A and B (A more volatile than B) are fed into the distillation column andheated partial vapourisation occurs. The vapour, richer in A, rises from the feed tray (tray 3)

through the bubble caps, bubbling through the liquid on tray 4, setting up a vapour/liquidequilibrium. Tray 4 is cooler than tray 3, so some of component B will condense, leaving the




vapour rising from tray 4 richer in A. The same process is repeated on tray 5, and we are leftwith a vapour leaving tray 5 very much richer in component A than was the original feed. Tokeep a temperature differential between the trays, the top vapour leaving tray 5 is condensed by cooling it, and some is fed back to the top of the column, the remainder leaving as the top product.

The liquid portion of the feed entering the column falls from tray 3 to tray 2, which is hotterthan the feed tray 3. On tray 2, because of the increase in temperature, the vapour is richer inA than the liquid, so the composition of the liquid on tray 2 is richer in component B than theoriginal feed. This liquid falls to tray 1, where the process is repeated. To ensure that thetemperature falls from tray to tray going up the column, some of the bottom product (rich inB) is heated and fed back to the bottom tray. The remainder leaves as bottom product rich inB.

Thus, by the use of cooled top product (reflux) and heated bottom product (reboil) thetemperature difference between trays is maintained, and fractional distillation occurs. It canalso be seen that liquid drawn from any of the trays will contain varying concentrations of Aand B, changing from a mixture rich in B at the bottom tray, to a mixture rich in A at the toptray.

In distilling petroleum we are considering not just 2 components, but many components.However, the same principles apply and by feeding heated oil to a fractional distillationcolumn, we can, by withdrawing liquid from various trays, separate the oil into varyingfractions. However, when withdrawing liquid from intermediate trays, some of the light product that bubbles through the liquid on each tray is present. This can be removed by passing steam through the withdrawn fraction in a small distillation column (called a

stripper). The mixture of steam and light material obtained as a top product from the stripperis returned to the main distillation column.

Figure 2 shows only 5 trays with 1 bubble cap per tray. In practice, several tens of trays areused, with of the order of 100 caps per tray.

Chemical equilibrium Many chemical reactions are reversible, i.e. the products react to reform the reactants. Whenthe reactants are mixed products start to form. As the reactant concentrations decrease andthe product concentrations increase, the rate of the forward reaction decreases and the rate ofthe back reaction increases, reforming the reactants. In a closed system where neither

products nor reactants can escape, a state of dynamic chemical equilibrium is eventuallyreached where the rates of the forward and back reactions are equal (thus meaning that thequantities of reactants and products remain the same) and no apparent change is occurring.For the reaction:

aA + bB ! cC + dD

where A and B are reactants, C and D are products and a, b, c, d are the stoichiometriccoefficients (i.e. the numbers in the chemical equation), the concentrations of reactants and products existing at equilibrium are related by the equation:




where K is called the equilibrium constant. The value of K is extremely important to the

chemist since it shows the extent to which the reactants can be converted into products undera given set of conditions.

Since chemical reactions are employed generally to convert reactants into desirable products,chemists try to convert as much of the reactants as possible into products. Alteration of theconditions of a system is one method whereby the yield of desirable products may beincreased. The effect of changes to the system can be predicted by Le Chatelier's principlewhich states: "If a change is made to a system in equilibrium, the system will adjust itself soas to overcome the effects of the change."

In practice this means that if, for example, reaction products are removed from the system,

the reaction can proceed to completion, i.e. all of A and B can be reacted to produce C and D.Also alterating the pressure of a gaseous reaction mixture, where the total number of moles ofreactants differ from the total number of moles of products, causes the concentrations atequilibrium to change. If the total number of moles of products is less than the total numberof moles of reactants, an increase in pressure will cause the reaction to move to the right.Temperature also affects equilibrium concentrations. The value of the equilibrium constant K changes with temperature. Using Le Chatelier's principle we can see that increase intemperature will cause the reaction to shift to the right if the forward reaction is endothermic(i.e. heat is 'used up' in the reaction).

Reaction Rates and Catalysts Knowledge of the value of the equilibrium constant K at different temperatures is importantto the chemist as it enables prediction of equilibrium concentrations to be made. However,such knowledge does not enable predictions to be made as to the rate at which equilibrium isattained. In industrial processes it is not unusual for equilibrium never to be attained, as thereaction rates are low.

We may consider a reaction taking place via the formation of an activated complex whichdecomposes into the reaction products. Formation of the activated complex requiresrearrangement of the molecular structure of the reactants, which requires additional energy,called the activation energy of the reaction. The further rearrangement of the molecular

structure of the activated complex to form products also has an associated energy change.

Assuming that the decomposition of the activated complex is fast, the rate at which theactivated complex is formed will govern the overall reaction rate. Thus, if a differentactivated complex can be formed, a different reaction rate will result. A catalyst is asubstance that enables a different, easier-to-form activated complex to be formed, and thusincreases the overall reaction rate. Put another way a new faster chemical pathway (ormechanism) from reactants to products is possible. The activated complex can be thought ofas a combination of A, B and catalyst.

][B ][A

][D ][C = K

ba

d c




Reaction coordinate

Energy A + B

C + D

Energy difference between reactantsand products

Activated complexes

a E 2

a E 1

activation energy with catalyst

a E 2

activation energy without catalyst

a E 1

Figure 4 - Reaction pathways with and without a catalyst

The rate of formation of the activated complex with catalyst is faster than the rate offormation of the complex without catalyst. Thus by finding a suitable catalyst, the overallreaction rate can be speeded up. The search for suitable catalysts for various reactions is an

important part of industrial research. (Figure 4 just shows the reaction as one step forsimplicity. In fact both catalysed and uncatalysed reactions probably occur in a series ofsteps, with the activation energy being the difference between the energy of the reactants andthe highest barrier.)

In general all reactions go faster at higher temperature, and the rate of increase withtemperature is greater the higher the activation energy. Thus many reactions which are too

slow at room temperature go at an appreciable rate at elevated temperatures.

Step 1 - Separation The oil is separated by boiling points into six main grades of hydrocarbons: refinery gas(used for refinery fuel), gasoline (naphthas), kerosene, light and heavy gas oils and longresidue. This initial separation is done by distillation. The long residue is further separatedin the butane desaphalting unit, and the refinery gas is separated into hydrogen sulfide in theShell ADIP process.

Distillation The first step in the refining of crude oil, whether in a simple or a complex refinery, is the

separation of the crude oil into fractions (fractionation or distillation). These fractions aremixtures containing hydrocarbon compounds whose boiling points lie within a specifiedrange. A continuous flow of crude oil passes from the storage tanks through a heating coilinside a furnace, where it is heated to a predetermined temperature. The heated oil then

enters the fractionating column (Figures 2 and 3), which is a tall cylindrical towercontaining trays suitably spaced and fitted with vapour inlets and liquid outlets.

Upon entering the column, the liquid/vapour mixture separates - the vapour passing upwardsthrough the column, the liquid portion flowing to the bottom from where it is drawn off as "longresidue". The vapours rise through the tray inlets, become cooler as they rise, and partially

condense to a liquid which collect on each tray. Excess liquid overflows and passes through theliquid outlets onto the next lower tray. The bottom of the column is kept very hot but




temperatures gradually reduce towards the top so that each tray is a little cooler than the one below it.

Ascending hot vapours and descending cooler liquids mix on each tray and establish atemperature gradient throughout the length of the column. When a fraction reaches a tray wherethe temperature corresponds to its own particular boiling range, it condesnses and changes intoliquid. In this way the different fractions are separated from each other on the trays of thefractionating column and are drawn off for further treatment and blending.

The fractions that rise highest in the column before condensing are called light fractions, andthose that condense on the lowest trays are called heavy fractions. The very lightest fraction isrefinery gas, which is used as a fuel in the refinery furnaces. Next in order of volatility comesgasoline (used for making petrol), kerosene, light and heavy gasoils and finally long residue.

High vacuum distillation unit This works on the principle that lowring the pressure lowers the boiling point of the compoundsconcerned. This is used to separate gasoline into gasoline (which boils under these conditions)

and short residue (a mixture of asphaltic compounds and oil which does not).

The butane deasphalting unit (BDU) The BDU uses a solvent extraction process to separate the gasoline and asphaltic compounds inthe short residue. The short residue from the high vacuum distillation unit is mixed with liquid butane (a 2:1 mixture of n-butane and iso-butane) at a specified temperature and pressure close tothe critical point of butane. Two liquid phases are formed, one rich in butane containing theextracted oil (Deasphalted Oil - DAO), the other of asphaltic compounds with low butane and oilcontent (ASPHALT).

The extraction column operates isothermally (without change in temperature) and has a

continuous butane rich phase with droplets of the asphaltic phase mixed into it. The densitydifference between phases drives the dispersed phase downwards in countercurrent flow to thecontinuous phase which is being forced upwards. This makes an interface level form in the bottom where the two phases are forced past each other. The extractor has perforated trays toenhance the contacting of the two liquid phases, and the butane solvent is recovered from the two phases (DAO and ASPHALT) by flashing and stripping to be recycled for reuse. A diagram of

the BDU is given in Figure 5.

Butane has been selected as the optimum solvent because it provides the highest deasphalted oilyield with good selectabilily at low solvent-to-feed ratios. The butane easily dissolves the lower

boiling hydrocarbons, but its solvent power is limited with respect to the higher boilinghydrocarbons, especially aromatic and asphaltic compounds.




Figure 5 - The Butane Deasphalting unit

x x x x

EXTRACTION

FLASHINGFLASHING

STRIPPING

STRIPPING

FLASHING

Short residue feed

Butane

Recycling

Exchanger

Steam

Butane

Butane

Compressor

Mixer

Steam

Key

Heating

Pump




Shell ADIP process The refinery fuel gas contains H2S, and this must be removed from the gas stream before the gasis burnt as fuel to prevent excessive SO2 emissions. This is done by reacting the H 2S with asolution of diisopropanol amine (DIPA, a base) known as ADIP. This is called the Shell ADIP process, and it is a regenerable absorption process, meaning that the ADIP is regenerated for

further use. The overall reaction can be represented as follows:

HS-

N

CH3CHCH2 H

H CH2CHCH3OH

OH

+ +

N

CH3CHCH2

H

CH2CHCH3

OH OH

H2S +

The solution is used to absorb H2S at higher pressures and lower temperatures, during whichtime the equilibrium lies to the right (Le Chatelier's principle). The solution is then regenerated,

releasing H2S, by using higher temperatures and lower pressures. Figure 6 outlines the ADIP process of absorption and regeneration. The DIPA is dissolved in water at a concentration of 4mol L-1, and the H2S concentration of the lean ADIP (the ADIP which is fed into the absorber) istypically 80 ppm wt.

Sour gas

Treated gas

Heatexchanger

LeanADIP Reboil

heater

Steam

Cooling

Water to sour water strippe

H2S to sulphur

recovery

Figure 6 - ADIP process

Step 2 - Conversion Of the oils separated out from the original crude (refinery gas, gasoline, kerosene, light andheavy gasoils and asphalt), only refinery gas can be used as is, and even this is usually ADIP

treated. All the others require some further treatment before they can be made into the final




product. This firstly involves the removal of sulfur (as it interferes with the success of some laterlater processes) and then the chemical conversion of the oils into more desirable compounds.

Desulfurisation The oil products all naturally contain some sulfur compounds. These must be removed fromgasoline, kerosene and diesel oils before catalytic reforming (the next conversion process) asotherwise the sulfur poisons the catalyst used. The sulfur is removed by reacting the sulfurcompounds with hydrogen, forming hydrogen sulfide, which can be removed as a gas from thecooled liquid oil. The process is carried out over a catalyst at a pressure of about 20 atmospheresand a temperature of about 350 C. Under these conditions the oils are gaseous.

The reactions occurring can be simplified to:

2 H2 C SR

H

R CH3 H2S++

(where R is an alkene), and thiol degradation:

H3C CH2 CH2 CH2 SH + H2 CH3 CH2 CH2 CH3 + H2S

Unfortunately, at the temperatures required to cause the reaction to go to the right at a fast rate(see previous section on equilibrium) the hydrocarbons decompose to lighter hydrocarbons andfinally to carbon. However, from Le Chatelier's principle we can see that there is another way toforce the reaction to the right. The left side of the reaction contains three gaseous moles (thehydrocarbons are gaseous) whereas the right hand side only contains two. By increasing the pressure, the reaction is forced to the right. In addition, if the hydrogen is kept in excess, thereaction is also forced to the right. Thus having an excess of hydrogen and keeping the mixtureat high pressure prevents the need to use a high temperature. The rate of conversion of sulfur

compounds to H2S is improved, and the formation of carbon is inhibited. After a period of somemonths the catalyst does become deactivated by laydown of carbon, and regeneration is carriedout by passing a mixture of air and steam at high temperature through the catalyst, thereby burning off the carbon. A mixture of air and steam is necessary, as use of pure air would resultin uncontrolled combustion, and the resulting high temperature would adversely affect thecatalyst.

Catalytic reforming This is used to increase the octane ratings of the oil concerned. Octane number is a measure ofthe 'knock' or 'pinking' that occurs in an internal combustion engine when an unsuitable (too lowoctane) fuel is used. The octane number is defined by an arbitrary scale which allocates zero

octane number to n-heptane and 100 to2,2,4-trimethyl pentane (iso-octane). The octane numberof a fuel is defined as the percentage by volume of iso-octane in a mixture with n-heptane thatgives the same degree of knocking as the fuel in a special test engine.

In proper operation of an internal combustion engine, combustion is initiated by the spark produced by the spark plug at the correct time in the cycle. A flame front then passes throughthe cylinder at the relatively slow rate of about 100 km/hr, the increase in pressure providing themotive power. Considering, for example, combustion of pentane, we have:

CH3CH2CH2CH2CH3 + 8O 2 ! 5CO 2 + 6H 2O9 volumes 11 volumes




As the reaction produces 11 volumes of product from 9 volumes of reactants but the volume iskept constant, the pressure increases, and the rising temperature also caused by the combustioncauses an even greater temperature increase. This increase in pressure provides the motive power. As the pressure increases on ignition, the pressure in the fuel/air mixture in the cylindernot as yet traversed by the flame front rises, with a consequent increase in temperature (PV = n

RT). If the fuel has too low an octane number, this increase in temperature causes the remainingfuel/air mixture to ignite before the flame front passes through. This ignition is explosive, andcauses great impact force to be transmitted by the piston to the bearings. Continual operation ofan engine in which such knocking is occurring causes rapid destruction of bearings. Octanenumbers of hydrocarbons vary with their oxidation stability. Straight chain paraffins have thelowest octane numbers (e.g. n-butane has an octane number of 25) with branched chain paraffins being higher (2 methyl pentane has an octane number of 70) followed by naphthenes (C6H12 =83) and then aromatics (benzene = 100).

The desired reactions (ring forming and aromatising) are endothermic so, to ensure that thereactions take place at an acceptable rate, a temperature of about 500oC is used. At thistemperature all the hydrocarbons considered are gaseous. A pressure of about 25 atmospheres isalso used and the process is carried out using a platinum catalyst (platinum dispersed through1mm spheres of alumina). The process is also sometimes called platforming, because of the useof a platinum catalyst.

Because the reactions are endothermic, 3 furnaces and reactors in series are used. Figure 7 outlines the process.

As the desirable reactions are endothermic, high temperature favours the forward reaction (LeChatalier's principle), which leads to the formation of the desired aromatics. The high

temperature also ensures that the reactions occur at a reasonably fast rate, but in addition favoursthe undesirable cracking reaction (see desulfurisation above), which gives lay-down of carbon onthe catalyst. To suppress coking a high hydrogen pressure is used by recycling some of the lightgases (about 70% vol. of hydrogen) from the separator even though this inhibits the formation ofthe desirable aromatic products to some extent.

As in many industrial processes, this is a compromise. At the previously stated conditions of500 C and 25 atmospheres, conversion of alkylhexanes into alkylbenzenes is fast. Paraffins(long-chain alkanes of more than twenty carbon atoms) are thus driven to be converted toalkylhexanes as the reaction product is removed almost as soon as it is formed. Also,isomerisation reactions that give molecules capable of forming alkyl hexanes are helped, as again

the reaction product is removed.

After a period of several months, deactivation of the catalyst by coke lay-down becomes so pronounced that removal of the coke is necessary. This is achieved by passing a mixture of airand nitrogen through the catalyst, thus burning off the coke. The nitrogen is necessary to controlthe rate of burn off, as if pure air was used the efficiency of the catalyst would be destroyed bythe resulting high temperature. (Steam is not used as in the regeneration of the desulfurisingcatalyst, as water adversely affects the platinum catalyst.) It is interesting to note that during thelifetime of the catalyst each platinum atom leads to the reaction of about 20 000 000 moleculesof gasoline, a truly catalytic act.




isomerisation, all of which are exothermic and all of which, except for isomerisation, consumehydrogen. The heat released is absorbed by injecting cold hydrogen quench gas between thecatalyst beds. Without the quench the heat released would generate high temperatures and rapidreactions leading to greater heat release and an eventual runaway. All of the reactions, except fordenitrogenation, desulfurisation and deoxygenation which only occur in the first stage, happen in

both stages.

DenitrogenationCarbon-nitrogen bonds are ruptured with the formation of ammonia.

N

+ 5 H2 H3C CH2 CH2 CH2 CH3 + NH3

Pyridine n-pentane ammonia

DesulfurisationCarbon-sulfur bonds are ruptured with the formation of hydrogen sulfide.

H3C CH2 CH2 CH2 SH + H2 CH3 CH2 CH2 CH3 + H2S

Butyl mercaptan n-butane hydrogen sulfide

DeoxygenationCarbon-oxygen bonds are ruptured with the formation of water.

OH

+ H2 + H2O

phenol benzene

HydrogenationThe saturation of carbon-carbon double bonds of the olefins or aromatics.

H3C CH2 C C

HH

H

+ H2 H3C CH2 CH2 CH3

butylene n-butane




+ 2H2

naphthalene tetralin

decalin

CH2CH2CH2CH3H2

H23

tetralin butyl benzene

HydrocrackingThe splitting or breaking of straight or cyclic hydrocarbons and hydrogenation of theruptured bonds.

CH2CH2CH2CH3

H2 CH3CH2CH2CH3+ +

butyl benzene benzene n-butane

IsomerisationThe change of one compound into another by rearrangement of its atoms.

CH3CH2CH2CH2 CH3CHCH3

CH3

n-butane iso-butane

Condensation

All the previous reactions yield more or less useful products. A less desirable but importantreaction is the condensation of aromatics to produce large unsaturated polyaromatics whichdeposit on the catalyst to form coke. Polyaromatics are much more susceptible to thisreaction than monoaromatics. Hence rapid catalyst deactivation due to coke laydown willoccur if the feedstock contains a high amount of polyaromatic asphaltenes.

2 H2+ 2

anthracene hexacene




Waxydistillate from HVU-2

DAOfromBDU

FIRST STAGE REACTORS

SECOND STAGE

REACTORS

H2

H2

Residue recycle (to extinction)

H2 quench

H2 quench

Cooling

Hydrogenrecycle gas

Compressor

2 stageflashing

Reboilheater

Cooling

Distillation

Figure 8 - The two-stage hydrocracking process



Figure 9 - The sulfur recovery unit

H2S

fuel gas

Air Air

fuel gas

1st stage(THERMAL)

2nd stage(CATALYTIC)

3rd stage(CATALYTIC)

Condensor

Air

fuel gas

Condensor

fu

Sulphur pit Pump

Sulphur loading tank




ANCILLARY PROCESSES

Hydrogen manufacture The large consumption of hydrogen, particularly in the hydrocracker, has meant that the Marsden

Point refinery has its own hydrogen manufacturing unit (Figure 10). The hydrogen is produced

by converting hydrocarbons and steam into hydrogen, and produces CO and CO2 as byproducts.The hydrocarbons (preferably light hydrocarbons and butane) are desulfurised and then undergothe steam reforming reaction over a nickel catalyst. The reactions which occur during reformingare complex but can be simplified to the following equations:

CnHm + nH 2O ! nCO + (( 2n + m )/2)H 2

CO + H2O ! CO 2 + H2

The second reaction is commonly known as the water gas shift reaction.

The process of reforming can be split into three phases of preheating, reaction and superheating.

The overall reaction is strongly endothermic and the design of the HMU reformer is a carefuloptimisation between catalyst volume, furnace heat transfer surface and pressure drop.

In the preheating zone the steam/gas mixture is heated to the reaction temperature. It is at theend of this zone that the highest temperatures are encountered. The reforming reaction then

starts at a temperature of about 700"C and, being endothermic, cools the process. The final phase of the process, superheating and equilibrium adjustment, takes place in the region wherethe tube wall temperature rises again.

The CO2 in the hydrogen produced by reforming is removed by absorption (see purification below), but trace quantities of both CO and CO2 do remain. These are converted to methane

(CH4) by passing the hydrogen stream through a methanator. The reactions are highlyexothermic and take place as follows:

CO + 3H2 ! CH 4 + H 2O

CO2 + 4H 2 ! CH 4 + 2H 2O

Finally, all produced hydrogen is cooled and sent to the Hydrocracker.

In contrast to the desulfurisation process described earlier, the HMU is an example of anon-reversible absorption process applied to protect a valuable catalyst. In this case,desulfurisation is by absorption of H2S in zinc oxide, according to the equation:

ZnO + H2S ! ZnS + H 2O

The reaction is non-reversible and consequently the saturated absorbent must be discharged andreplaced.



High temperatureshift converter Low temp.

shiftconverter CO 2

removal sulfinol

absorption

process

Cooling Cooling Heating Cooling

Methanator

Cooling

Mixer

Reformer furnaceVapourised feed Desulphurisation

A B

Steam

Figure 10 - The hydrogen manufacturing unit

Purification The hydrogen produced must be purified (to remove the carbon oxides) before it can be usd inthe hydrocracker. A regenerable absorption process is applied by circulating a "Sulfinol"solution. This is a mixed solvent, consisting of 50% DIPA, 25% water and 25%tetraydrothiophenedi-oxide (sulfolane).

SOO

sulfolane

CO2 is removed by absorption in the sulfinol at low temperatures and high pressure.Subsequently, the "fat" sulfinol solution is regenerated by removal of CO2 at high temperatureand low pressure. The CO2 is further purified by chilling to give food grade product. Thechemistry of the process is as follows:

CO2 + 2

N

CH3CHCH2

H

CH2CHCH3

OH OH N

CH3CHCH2 HOH

CH3CHCH2 H

OH

++

N

CH3CHCH2

C

CH2CHCH3

OH OH

O

O-

DIPA




CO2 + H2O +

N

CH3CHCH2

H

CH2CHCH3

OH OH

HCO3-

+

N

CH3CHCH2 H

H CH2CHCH3

OH

OH

+

N

CH3CHCH2

C

CH2CHCH3

OH OH

O

O-

+ N

CH3CHCH2 H

H CH2CHCH3OH

OH

+

CH2

CCH3

H O

C

O

N CH2 CH

OH

CH3 H2O

N

CH3CHCH2

H

CH2CHCH3

OH OH

+ +

oxazolidone

The water treatment plant A major ancillary facility of the expanded refinery is the effluent water treatment plant.Extensive facilities, costing $50 million (1985), ensure the continued protection of the marineenvironment of Whangarei Harbour. The effluent water discharge is continuously monitoredfor oil content and other contaminants, according to the comprehensive requirements of the

discharge permit issued by the Northland Regional Council. Figure 11 is a simplified blockflowscheme of the effluent water treating facilities.

There are two types of water to be dealt with:

# Water which has been, or is likely to have been, in contact with oil. This water must pass through gravity separators to remove the oil and may require other treatment.Such water comes from the refining processes or ballast discharged by coastaltankers.

# Water which has only accidentally been in contact with oil. This water passesthrough oil traps before being routed to the holding basin and final buffer basin. Suchwater comes from the boiler plant and collected rainwater.

The treatment of effluent water is as follows. Process water is deodorised in sour-waterstrippers where the gas (H2S and NH3) is stripped off. The stripped water has oil removed inthe gravity separators and then, together with some rainwater, is homogenised in a buffertank. From this tank, the effluent water is piped to a flocculation/flotation unit where air and polyelectrolytes are injected in small concentrations to make the suspended oil and solidsseparate from the water. The latter are skimmed off and piped to a separate sludgehandling/disposal unit. The remaining watery effluent from the flotation unit is passed toadjoining biotreater where the last of the dissolved organic impurities are removed by theaction of micro-organisms in the presence of oxygen (biodegradation). On a continual basis,sludge containg micro-organisms is removed to the sludge handling/disposal unit, while thetreated effluent water is held first in the retention basin and finally discharged into Whangarei

Harbour.



Process units Drainage

Continuous

ex utility plant

Continuous Intermittent

BALLAST WATER

SEWAGE WATER ACCIDENTALLY

CONTAMINATED BY OIL

Sour water stripper

Gravity oil separator

Oil trap

Bio treater

Processing areas

Metering/sampling point

WATER EXPECTED TO BE

CONTAMINATED BY OIL

OIL FREE WATER

Receiving tanks


Comminutors


Homogenisation tank

Flocculationflotation unit

Aeration basin

Holding basin

Buffer basin

DISCHARGE

OF TREATED EFFLUENT

TO MARINE OUTFALL

Figure 11 - The water treatment plant

The discharge is through a continuous flow meter and sampling system and terminates attwin diffusers located in deep water where tidal currents ensure the rapid and extensivedilution of the treated effluent water.

A comprehensive system of waste management has also been established at the refinery.This is based on internationally accepted concepts of waste minimisaiton, such as reduction,reuse and recycling. The disposal of any remaining residual waste follows strictly controlled procedures.

ENVIRONMENTAL IMPLICATIONS

The refinery operates under an air discharge permit, monitored by the Northland RegionalCouncil (NRC), since the enactment of the Resource Management Act (RMA). The current




permit requires monthly reports to be prepared for the NRC, relating to sulfur dioxide (SO2)emissions, ground level measurement of SO2, smoke from chimneys, from flaring and fromfire fighting training.

The permit also required that the quantity of SO2 emitted should be significantly reduced in

three steps between 1992 and 1996. Capital costs to achieve these reductions were in theregion of $30 million. In addition, operating costs of several million per annum wereincurred. The SO2 limit is now amongst the lowest of any similar refinery in the world.

In addition, the water treatment plant described above ensures that minimal quantites of oiland other effluent are discharged with the water into the Whangarei Harbour.

Article written by Heather Wansbrough, combining the articles from volumes one and two ofedition one and information brochures supplied by Tony Mullinger (The New ZealandRefining Company Ltd). Edited by John Packer and John Robertson.



Safety inspectors carefullymonitor the drilling and blastingprocess.

The truck will take the topsoil to aspecial place where it will be saveduntil mining is finished in this area.Then the topsoil will be replaced so

plants can grow again.

How is coal mined?

Mining is the process of removing coal from the ground.There are two types of mining: underground mining and surface mining. When the coal seam is fewer than125 feet under the surface, it is mined by surface mining.Coal that is deeper than 125 feet is removed from theground by underground mining.

Surface Mining

Surface mining is used when acoal seam is located close to thesurface. Heavy equipment isused to clear the land of trees,shrubs and topsoil.

Holes are drilled into the rockand explosives are placed inthese holes. The explosionbreaks up the dirt and rock

called overburden.



Large earth-moving machinesmove the overburden to exposethe coal seam.

When the coal is uncovered,bulldozers and shovels scoopup the coal and load it intolarge trucks. All of the coal ismined.

In 2000, there were six surface mines in Illinois. The surface minesproduced 3,800,000 tons of coal and employed 330 miners.

After mining the topsoil is replaced for plants and wildlife to growagain.

When the trucks are loaded, they will haul the coalto the preparation plant.

Land that has been mined can be used inmany ways.



This is a diagram of an undergroundroom and pillar mine.

Underground Mining

Underground mining is used when the coal seam lies deep in theearth. In an underground mine onlysome of the coal is removed. Thecoal that remains helps support themine roof.

Underground mines look like asystem of tunnels. The tunnels areused for traveling throughout themine, moving coal from place toplace and allowing air to circulate in

the mine.

The coal that is mined isput on conveyor belts. Theconveyor belts take the coalto the surface.

There are three types of underground mines: slope, drift, and shaft.When the coal seam is close to the surface but too deep to usesurface mining, a slope mine can be built. In a slope mine a tunnelslants down from the surface to the coal seam.

It is very dark underground.

A conveyor belt takes coal out of themine. The pillars are covered with awhite powdered limestone to preventspontaneous combustion.



The shaft can be 30 feet indiameter.

Men and materials ride anelevator down to the coalseam at a shaft mine.

A drift mine is built when the coal seam lies in the side of a hill ormountain. Drift mines may also be built in a surface mine that hasbecome too deep. There are many drift mines in the eastern UnitedStates.

The most common type of

mine in Illinois is the shaftmine. These mines may be125 to 1,000 feet deep. Alarge hole, or shaft, is drilleddown into the ground until itreaches the coal seam.

In a slope mine, the minersand materials ride in special

cars to get to the coal seam.



Corn and soybeans grow abovethis underground coal mine.

The longwall panel shows howmuch coal the longwall miningmachine cuts.

The shields are shown in yellow inthe pictures.

The shearer is shown in orange. It shears thecoal away. The conveyor belt is shown in gray.

Longwall mining removes more coal than room and pillar mining.Large panels of coal are extracted. The panels are 750 to 1,000 feetwide. The continuous miner cuts tunnels 18 to 20 feet wide.

The longwall machine has large shields thatsupport the roof andprotect the miners duringmining.

A rotatingdrum, called ashearer, cuts the coal. The coal dropsonto a conveyor belt. As more of thecoal is cut, the machine moves forward.The roof behind the machine falls in aplanned order.

In 2000, there were 12underground mines in Illinois.The 3,131 employed minersproduced 29,700,000 tons ofcoal.



A Math Code Word Puzzle

2+9

4-3

10-5

1+3

3-1

2+6

4+6

5-4

4+5

5+1

8+4

5-3

2+4

5-1

12-7

5-2

7+7

2+5

5+3

6+1

7-2

5+8

1+7

Letter Code10

2I

3R

4L

5A

6N

7E

8S

9U

10F

11C

12D

13M

14G

- - - - - - - - - - - - - - - - - - - - - - -

DIRECTIONS: Add or subtract the problems and then find the answersin the letter code. Put the letters on the dotted lines. Write the decodedsentence on the line below.

________________________________________________________.



The Coal Resource: A Comprehensive Overview of Coal 7

The choice of mining method is largely

determined by the geology of the coal deposit.

Underground mining currently accounts for

about 60% of world coal production, although

in several important coal producing countries

surface mining is more common. Surfacemining accounts for around 80% of production

in Australia, while in the USA it is used for

about 67% of production.

Underground MiningThere are two main methods of underground

mining: room-and-pillar and longwall mining.

In room-and-pillar mining, coal deposits are

mined by cutting a network of ‘rooms’ into the

coal seam and leaving behind ‘pillars’ of coal to

support the roof of the mine. These pillars canbe up to 40% of the total coal in the seam –

although this coal can sometimes be recovered

at a later stage. This can be achieved in what is

known as ‘retreat mining’, where coal is mined

from the pillars as workers retreat. The roof is

then allowed to collapse and the mine is

abandoned.

Longwall mining involves the full extraction of

coal from a section of the seam or ‘face’ using

mechanical shearers. A longwall face requires

careful planning to ensure favourable geology

exists throughout the section before

development work begins. The coal ‘face’ can

vary in length from 100-350m. Self-

advancing, hydraulically-powered supports

temporarily hold up the roof while coal isextracted. When coal has been extracted from

the area, the roof is allowed to collapse. Over

75% of the coal in the deposit can be

extracted from panels of coal that can extend

3km through the coal seam.

The main advantage of room–and-pillar

mining over longwall mining is that it allows

coal production to start much more quickly,

using mobile machinery that costs under $5

million (longwall mining machinery can cost

$50 million).

The choice of mining technique is site specific

but always based on economic considerations;

differences even within a single mine can lead

to both methods being used.

Surface MiningSurface mining – also known as opencast or

opencut mining – is only economic when the

coal seam is near the surface. This method

recovers a higher proportion of the coal

SECTION TWO

COAL MINING

>> Coal is mined by two methods – surface or ‘opencast’mining and underground or ‘deep’ mining. >>



8 World Coal Institute

deposit than underground mining as all coal

seams are exploited – 90% or more of the coal

can be recovered. Large opencast mines can

cover an area of many square kilometres and

use very large pieces of equipment, including:

draglines, which remove the overburden; power

shovels; large trucks, which transport

overburden and coal; bucket wheel excavators;and conveyors.

The overburden of soil and rock is first

broken up by explosives; it is then removed

by draglines or by shovel and truck. Once the

coal seam is exposed, it is drilled, fractured

and systematically mined in strips. The

coal is then loaded on to large trucks or

conveyors for transport to either the

coal preparation plant or direct to where

it will be used.

Coal PreparationCoal straight from the ground, known as run-

of-mine (ROM) coal, often contains unwanted

impurities such as rock and dirt and comes in a

mixture of different-sized fragments.

However, coal users need coal of a consistent

quality. Coal preparation – also known as coal

beneficiation or coal washing – refers to the

treatment of ROM coal to ensure a consistent

quality and to enhance its suitability for

particular end-uses.

The treatment depends on the properties of

the coal and its intended use. It may require

only simple crushing or it may need to go

through a complex treatment process to

reduce impurities.

To remove impurities, the raw run-of-mine coal

is crushed and then separated into various size

fractions. Larger material is usually treated

using ‘dense medium separation’. In this

process, the coal is separated from other

impurities by being floated in a tank containing

a liquid of specific gravity, usually a

suspension of finely ground magnetite. As the

coal is lighter, it floats and can be separated

off, while heavier rock and other impurities

sink and are removed as waste.

The smaller size fractions are treated in a

number of ways, usually based on differences

in mass, such as in centrifuges. A centrifuge is

a machine which turns a container around very

quickly, causing solids and liquids inside it to

separate. Alternative methods use the

different surface properties of coal and waste.

In ‘froth flotation’, coal particles are removed in

a froth produced by blowing air into a water

bath containing chemical reagents. The

bubbles attract the coal but not the wasteand are skimmed off to recover the coal

fines. Recent technological developments

have helped increase the recovery of ultra

fine coal material.

Definition

Overburden is the layer of

soil and rocks (strata)

between the coal seams and

the surface.

Longwall mining involves the

full extraction of coal from a

section of the seam using

mechanical shearers.

Photograph courtesy ofJoy Mining Machinery.

Definition

DWT – Deadweight Tonnes

which refers to the

deadweight capacity of a

ship, including its cargo,

bunker fuel, fresh water,

stores etc.



Continuous MinersDeveloping Roadways

Next Longwall Panelto be Mined

Direction of Mining Mined Area

Coal Conveyor

Coal Pillar

Coal Shearerand Roof Supports

Coal Pillars Retainedfor Roof Support

Coal Conveyorto Surface

Mine SurfaceFacilities

Previously MinedLongwall PanelMined Area

Coal Shearerand Roof Supports

Coal Seam


Coal TransportationThe way that coal is transported to where it

will be used depends on the distance to be

covered. Coal is generally transported by

conveyor or truck over short distances. Trains

and barges are used for longer distances

within domestic markets, or alternatively coal

can be mixed with water to form a coal slurry

and transported through a pipeline.

Ships are commonly used for international

transportation, in sizes ranging from

Handymax (40-60,000 DWT), Panamax (about

60-80,000 DWT) to large Capesize vessels

(about 80,000+ DWT). Around 700 million

tonnes (Mt) of coal was traded internationally

in 2003 and around 90% of this was seaborne

trade. Coal transportation can be very

expensive – in some instances it accounts for

up to 70% of the delivered cost of coal.

Underground Mining Operations

Diagram courtesy of BHP Billiton Illawara Coal



10 World Coal Institute

Measures are taken at every stage of coal

transportation and storage to minimiseenvironmental impacts (see Section 5 for more

information on coal and the environment).

Safety at Coal MinesThe coal industry takes the issue of safety very

seriously. Coal mining deep underground

involves a higher safety risk than coal mined in

opencast pits. However, modern coal mines

have rigorous safety procedures, health and

safety standards and worker education and

training, which have led to significant

improvements in safety levels in bothunderground and opencast mining (see graph on

page 11 for a comparison of safety levels in US

coal mining compared to other industry sectors).

There are still problems within the industry.

The majority of coal mine accidents and

fatalities occur in China. Most accidents are in

small scale town and village mines, often

illegally operated, where mining techniques are

labour intensive and use very basic equipment.

The Chinese government is taking steps toimprove safety levels, including the forced

Graded embankmentto act as baffle against

noise and dust

Topsoil and subsoilstripped by motor scrapers

and carefully stored

Overburden from benchesdug by shovels and hauled

by dump trucks

Overburden being excavatedby dragline

Coal seams Overburden Draglineexcavation

Froth flotation cells at

Goedehoop Colliery are usedfor fine coal beneficiation.

Photograph courtesy of

Anglo Coal.

Surface Coal Mining Operations and Mine Rehabilitation




closure of small-scale mines and those that fail

to meet safety standards.

Coal Mining & the Wider CommunityCoal mining generally takes place in rural areas

where mining and the associated industries are

usually one of, if not, the largest employers in

the area. It is estimated that coal employs over

7 million people worldwide, 90% of whom are in

developing countries.

Not only does coal mining directly employ

millions worldwide, it generates income and

employment in other regional industries thatare dependent on coal mining. These industries

provide goods and services into coal mining,

such as fuel, electricity, and equipment, or are

dependent on expenditure from employees of

coal mines.

Large-scale coal mines provide a significant

source of local income in the form of wages,

community programmes and inputs into

production in the local economy.

However, mining and energy extraction can

sometimes lead to land use conflicts and

difficulties in relationships with neighbours and

local communities. Many conflicts over land use

can be resolved by highlighting that mining is

only a temporary land use. Mine rehabilitation

means that the land can be used once again for

other purposes after mine closure.

Spoil pileDragline bucketunloads burden

After the soils are replaced in their proper sequence,they are ripped to relieve compaction and

then cultivated, limed and fertilised

Draglinebackfill

levelled bybulldozers

Tippingoverburden

from benchesto backfill

Subsoil andtopsoil being

replacedand shaped

Grass andtrees

Service Providing

Leisure & Hospitality

Trade, Transportation& UtilitiesEducation &Health Services

Coal Mining

Agriculture, Forestry,Fishing & Hunting

Manufacturing

Construction

0 1 2 3 4 5 6 7 8

Injury Rates in Selected US Industries, 2003

(per 100 full-time employees)Source: Bureau of Labor Statistics, US Department of Labor

SECTION TWO END



13

2.0 RENEWABLE SOURCES OF ENERGY

2.1 Introduction

Energy has been defined as the ability to do work. It is a force multiplier that enhances man’s

ability to convert raw materials into useful products, providing varieties of useful services. Its

broad division includes the various forms which includes but not limited to mechanical,

chemical, electrical, sound, light, nuclear etc. However, its consumption and utilization has to

a great extent determine the factor of increase or decrease in the population of a community

or nation, with the high rate of energy use proportionate to population stagnation and high

national product; the converse is linked to population explosion. Countries with low energy

availability and high energy demand have been found to have correspondingly high

proportion of poverty, illiteracy and migration. This human factor in energy analysis cannot

be overemphasized; the socio-economic importance of energy must take the center stage in

addressing the world population growth. Women right programmes, literacy programmes and

birth control policies will not do well if the current trend of energy shortages experienced by

developing nations is not addressed globally.

The world’s system of energy is of different kinds and forms with broad division under the

renewable and non-renewable energy sources. The most widely used has been the non-

renewable, which also have been described as not too friendly to the environment due to its

harmful emissions and byproducts. Such examples include the use of coal, fossil fuels,

nuclear reactors, etc. to mention a few. These sources are depleting, and do not produce

adequate and consistent power for national consumption, even the cost of maintenance has

never been of help.

More so, the energy needs of developing countries like those of Africa are

increasingly growing, more than one out of three people is reported to still depend on



14

traditional biomass for cooking and heating. An international projections reported that the

energy dependence of the world is expected to rise by over 34% between 2002 and 2025

when that by developing nations only, is expected to double the present demand. Thus,

bringing about growing concerns in creating alternative energy sources which will be capable

of meeting the energy needs of the world’s population, as those of the present sources and

usage is unsustainable and unsuitable. The way out of this, lie in the use of renewable sources

of energy for power generation, as they contain enormous, largely untapped and sustained

opportunity for meeting the world’s energy needs; for example, the sun radiates enormou s

amount of energy in one year than people have used since the world began, they are

environmentally friendly as they do not contribute harmful and toxic emission to it. In

addition, they do not create the concern of eventual depletion as those of non-renewable

sources. Another example of renewable sources of energy is the wind energy; it is one of the

cleanest and most environmentally friendly sources of energy, capable of generating a high

amount of electricity.

2.2 Sources and Types of Renewable energy

Renewable energy has been defined as the energy obtained from natural sources. The sources

are those available either directly around us or indirectly in the earth’s crust. Depending on

the renewable energy sources, there are several types of renewable energy. These sources

include: solar, wind, biomass, geothermal, water (or hydropower), ocean thermal, tidal, wave

and hydrogen fuel/energy.

Solar energy: This is energy from the sun. The sun is a potent source of usable energy. Its

potential is estimated to be greater than the world’s total energy demand. Every year the earth

receives an estimated 8.33 x 1016 MW of solar energy. This amount of energy is many



15

thousand times sufficiently larger than the total global energy demand. It can be utilized

either as solar thermal or as photovoltaics. The areas of application of solar energy at present

include: for heating and cooling of residential buildings, water heating, drying of agricultural

and animal products, electricity generation, lighting and other industrial and agricultural

processes. Apart from the sun being a source of solar energy, it also the active agent in wind

flow. As a result of the heating from the sun, hot air tends to rise and cooler air fill the space

created by the rising hot air. This air heating and movement of air gives momentum to air and

the speed of its motion (wind speed) can be employed as wind energy. Also, the presence of

the sun and its activities aid in photosynthesis and plant growth and development, it is also

the agent behind transpiration and evaporation. Secondarily, precipitation is a result of the

activities of the sun’s heat on the earth.

Wind energy: Wind is created when hot air rises and cooler air moves in to replace the rising

hot air. The momentum in the wind as a result of its movement can be employed for gainful

purposes. Therefore, the means of capturing the mechanical energy in a moving wind and

converting it to produce electricity or do meaningful work is termed wind energy or power.

Historically, wind energy has been employed for water pumping, grinding and to power

ships. However, as a result of technological improvements, wind energy is being employed to

generate MW to GW of electricity around the world. Wind speeds from 3.0 m/s and above

can be employed with wind turbine for electricity purposes. However, speed greater than 4.0

m/s are economically viable for community electricity generation.

Biomass energy: Biomass is simply described as organic materials made from living and

recently living plants and animals. It is the organic matter in trees, living plant materials and

agricultural crops. In the context of nature, biomass means natural materials of plants and



16

animals. It contains stored energy from the sun. Biomasses are carbon based compounds

made up of carbon, oxygen, hydrogen (chemical constituents of carbohydrates) and probably

also containing Nitrogen and other substances in small quantities. These substances may

include those of heavy metals, alkali earth metals and alkali metals found in functional

molecules like porphyrins, a group of chemical compounds most of which occur in nature

such as in chlorophyll found in green leaves of plants and also in red blood cells.

Biomass energy refers to energy derived from natural material of mostly plants and

animals. The energy can be employed for electricity generation and also for heating purposes.

When biomass materials are burnt, they produce heat which can be used for different

purposes. First, the heat can be used directly for heating. It can also be used to raise steam

that can be employed with steam turbines to generate electricity. Another way of generating

electricity from biomass is via the process of biological degradation. Animal wastes or food

wastes can be treated or controlled in such a way that it is anaerobically decomposed to

produce methane (biogas). This biogas can be employed with turbines to generate electricity.

Potential of biomass energy from agricultural residue alone is estimated to be about

480 mt (about 480,000 kg). Small scale power generation from biomass as reached about 1

MW. Large scale generation is still being developed.

Geothermal energy: This is the natural heat from the centre of the earth. The earth’s core has

a temperature of over 70,000oC. Some rocks closer to the ground surface (about 1 km below

the surface) also have temperature in the neighborhood of 500oC. Moreover, areas where

there are volcanic eruptions, hot springs and geysers (streams gushing hot water and steam)

typically have very hot rocks underlying the surface. Areas with geothermal energy potential

usually have a reservoir of hot water below the surface. The heat from this source can be



17

employed for heating buildings. It can also be sued for cooling using vapour absorption

system and also for electricity. It is reported that potential of up to 3400 MW exist in New

Zealand, United States of America, japan and Iceland. China and Philippines have been able

to generate hundreds of MW of electricity from geothermal sources. Geothermal reservoirs

far from the earth’s surface can be accessed by drilling.

Food for thought : Can Nigeria employ geothermal energy sources for electricity

generation?

Water (or hydropower) energy: Water is a renewable energy resource because of the

continuous activities of evaporation and precipitation. When water flow under gravity, it

poses immense kinetic energy. This kinetic energy can be employed to drive turbines for

production of electricity. Also flowing water can be channeled in a way that it can be

employed by turbines to generate electricity. Hydropower can be either small scale or large

scale. It is the most widely used source of electricity generation, capable of generating huge

amount of electrical energy. Globally, hydropower contributes about 19% of the world

electrical energy either from small or large scale generation. Historically, the development of

hydro-electricity has come a long way. It dates back to 1882, when moving water was used to

produce electricity via a water wheel on the Fox River in Wisconsin. Table 1.1 shows the

classification of hydropower generation.

Table 1.1: Classification of hydropower generation capacities

S/N Scale Capacity Usability1 Pico Up to 5 kW Domestic and small commercial systems2 Micro Between 6 and 100 kW Small community based power system

3 Mini Above 100 kW but below 1 MW Micro power grid4 Small Above 1 MW and up to 25 MW Grid connection5 Large Greater than 100 MW Grid connection



18

Ocean Thermal : Along the regions and layers of sea water from the surface to the deep sea,

there exist gradients of temperatures due to the heating effect of the sun. The ocean is

regarded as the world’s largest solar energy collector and storage systems because of its

vastness in coverage. As long as there is a gradient in temperature between the warm surface

water and the cold deep water by at least 20oC, an ocean thermal energy conversion system

(heat engines) can be used. It is estimated that the ocean has the potential to produce 10 TW

of electrical energy.

NB: Read the attached document for more on Ocean Thermal Energy Conversion (OTEC)

system

Tidal power/energy: Tides are the cyclic rising and falling of ocean surface caused by the

tidal forces of the moon and sun acting on the ocean. They are caused by the gravitational

attraction of the moon and the sun acting upon the ocean waters of the rotating earth. Tides

cause changes in the depth of the marine and estuarine water bodies and produce oscillating

currents known as tidal streams. Moreover, tidal power can be explained to mean the

generation of electricity using the mechanical force created by the rise and fall of ocean

surge. It is a renewable, largely abundant non-depleting and clean source of energy. Tides are

very powerful with the sea moving very quickly producing immense force of moving water

which can be adequately harnessed for gainful work. Tidal energy is produced by using

special energy generators purposely designed to convert the mechanical power of ocean

currents to electricity. These generators are generally called tidal ‘stream’ generators. They

contain turbines mounted on gear box shafts and operate in similar manner to wind turbines,

only that they produce more electrical energies at low tidal velocities compared with same



19

amount of wind speeds for wind turbines. The generators are placed under water in places or

areas where high tides are common and recurrent. They are designed to capture the kinetic

energy of moving water and subsequently convert it to electrical energy. A key feature of

most tidal stream turbines is that the turbines use technology already developed from the

wind industry. The only difference is the support structure.

Wave energy: Waves are produced as a result of the action of winds on the sea. It contains a

great deal of energy which can be harnessed to drive turbines for electricity generation.

Energy produced from sea or ocean waves are termed wave energy.

Hydrogen fuel : Hydrogen is a natural gas that exists in combination with other elements or

material. Hydrogen fuel or energy is an alternative energy resource. It has no environmental

implications and possesses huge energy content, the highest per unit of mass of any chemical

fuel. Hydrogen can be employed in fuel cells to produce electricity and also provide heat.

Although it is not primary an energy source like oils and coals or natural gas, it is however a

huge energy carrier. When electricity is generated from the different sources, it can be

converted and stored in hydrogen. To put energy into hydrogen requires that it must be

compressed or liquefied. A major challenge facing hydrogen energy utilization on a large

scale today is that the technology of its storage and commercial production is still under

development.

2.3 Prospects of Renewable energy Sources

Globally energy systems are of different kinds and forms with broad division under the

conventional (non-renewable) and unconventional (renewable) energy sources. The most

widely used however are the conventional energy sources of fossil fuels (oils, natural gas and



20

coal) and nuclear reactors. The environmental and health challenges posed by these sources

have been the major concern across the world. This is because of the emission and

byproducts from systems utilizing these conventional sources of energy. Another issue of

note about the conventional sources of energy is their finite nature. It is thought that because

it takes several millions of years for the organic matter to become fossil fuels, the rate of its

use today is so fast that more would have been extracted before its replacement. Also, the

cost of maintaining the energy conversion and generation systems for conventional sources is

very high. Although the technology of energy conversion is readily available, even with

major improvements, the environmental and health challenges associated with employing the

conversion systems are still present.

Further to this, global annual demand for energy is always on the rise. An

international projections reported that the energy dependence of the world is expected to rise

by over 34% between 2002 and 2025 when that by developing nations only, is expected to

double the present demand. Based on the need for energy, people in rural areas of developing

countries constantly depend on traditional biomass for cooking and heating. This invariable

leads to deforestation in affected regions. Another factor is rural electrification. In most

developing countries (e.g. Nigeria and other West African nations), communities in the rural

areas are hardly connected to the national electricity grid. The costs of connecting these rural

suburbs to the grid and maintaining the facilities are also not very feasible for poor nations.

One major way that have been recognized to posses the potential for resolving all the

highlighted challenges of rural electrification and associated environmental and health

challenges is the adoption of the unconventional (renewable) energy resources. These sources

contain enormous, largely untapped and sustained opportunity for meeting the world’s energy

needs. For instance, the sun radiates enormous amount of energy in one year than people

have used since the world began, they are environmentally friendly as they do not contribute



21

harmful and toxic emissions. In addition, they do not create the concern of eventual depletion

as those of non-renewable sources. They have no associated health challenges and can be

deployed as standalone facilities for rural electrification in places that are not connected to

the national electricity grid. The major challenges of utilizing renewable energy resources for

power generation however lie in the intermittence of the sources and the initial cost of

installation. For instance, employing wind for wind-to-electricity generation will require that

there is sufficient amount of wind speed to drive the turbines. Adequate and required wind

speeds are not however always available. There are times when the wind speeds come below

the required magnitudes. At this period, the turbine installation will be unavailable for

generation. Also, employing the sun for power generation would also require the availability

of good solar radiation over a place. This may not be the case for most places across the

world. Geothermal resources are also localized. They are only available in places with good

underground heat. Ocean thermal energy can only be employed in the tropical regions.

Based on all of these and other shortcomings, there are ongoing researches across the

globe to develop improvements in order to bring down the cost of generation modules. For

instance, there are ongoing researches to employ wood for wind turbine blades and also

works are under way to find a means by which better solar collector designs (and material

improvements) can be done to capture more radiant energy. Another area of research is the

development of appropriate storage technology for renewable energies for periods when the

installations are unavailable for generation.



22

References

El Bassam, N. and P. Maegaard, 2004. Integrated renewable energy for rural communities:

Planning guidelines, technological application, Elsevier LTD, Amsterdam.

Hermann, S, 2001. A solar manifesto, James and James LTD, London, 1-22.

Rai, G.D, 2004. Non-conventional energy sources, Khanna Publishers, New Delhi

Rathore, N.S and Panwar, N.L, 2007. Renewable energy sources for sustainable

development, New India Publishing agency, New Delhi, 1-3

Ridao, A.R., E.H. Garcia, B.M. Escobar and M.Z. Toro, 2007. Solar enery in Andalusia(Spain): present state and prospects for the future, Renewable and Sustainable Energy

Reviews, 11 (1), 148-161

Sorensen, H.A, 1983. Energy conversion systems, Wiley, USA, 485-491

Tippens, P.E, 2001. Physics, 6th, edn., McGraw-Hill, New York, 176-187.

U.S department of Energy. 2005. Energy information administration, international energy

outlook 2005. Available on the web [http://www.eia.doe.gov/oiaf/pdf/0484 (2005).pdf]

United Nations. 2005. The energy challenge for achieving the millennium development goals.

Available on the web [http://www.undp.org/energy/docs2/UN-ENRG%20paper.pdf]

Utlu, Z. and A. Hepbasli, 2007. A review on analyzing and evaluating the energy utilizationEfficiency of countries, Renewable and Sustainable Energy Reviews 11 (1), 1-29

Williams, R.H. and J.W. Carl, 1990. Energy from the sun, American Science Journal , 43, 41

Xiaowu, W. and H. Ben, 2005. Energy analysis of domestic-scale solar water heater,

Renewable and Sustainable Energy Reviews, 9, 638-645



Marshall DP (1997) Subduction of water masses in aneddying ocean. Journal of Marine Research 55:201}222.

Marshall JC, Nurser AJG and Williams RG (1993).Inferring the subduction rate and period over theNorth Atlantic. Journal of Physical Oceanography23: 1315}1329.

McDowell S, Rhines PB and Keffer T (1982) North Atlan-tic potential vorticity and its relation to the generalcirculation. Journal of Physical Oceanography 12:1417}1436.

Pedlosky J (1996) Ocean Circulation Theory. New York:Springer.

Pollard RT and Regier LA (1992) Vorticity and verticalcirculation at an ocean front. Journal of Physical Oceanography 22: 609}625.

Price JF (2001) Subduction. In: Ocean Circulation and Climate: Observing and Modelling the Global Ocean,G. Siedler, J. Church and J. Gould (eds), AcademicPress, pp. 357d 371.

Rhines PB and Schopp R (1991) The wind-driven circula-tion: quasi-geostrophic simulations and theory for non-symmetric winds. Journal of Physical Oceanography21: 1438}1469.

Samelson RM and Vallis GK (1997) Large-scale circula-tion with small diapycnal diffusion: the two-thermo-cline limit. Journal of Marine Research 55: 223}275.

Stommel H (1979) Determination of watermass propertiesof water pumped down from the Ekman layer to thegeostrophic Sow below. Proceedings of the National Academy of Sciences of the USA 76: 3051}3055.

Williams RG (1991) The role of the mixed layer in settingthe potential vorticity of the main thermocline. Journal of Physical Oceanography 21: 1803}1814.

Williams RG, Spall MA and Marshall JC (1995) DoesStommel’s mixed-layer ‘Demon’ work? Journal of Physical Oceanography 25: 3089}3102.

Woods JD and Barkmann W (1986) A Lagrangian mixedlayer model of Atlantic 183C water formation. Nature319: 574}576.

OCEAN THERMAL ENERGY CONVERSION(OTEC)

S. M. Masutani and P. K. Takahashi,

University of Hawaii at Manoa, Honolulu, HI, USA

Copyright ^ 2001 Academic Press

doi:10.1006/rwos.2001.0031

Ocean thermal energy conversion (OTEC) generateselectricity indirectly from solar energy by harnessingthe temperature difference between the sun-warmedsurface of tropical oceans and the colder deepwaters. A signiRcant fraction of solar radiation inci-dent on the ocean is retained by seawater in tropicalregions, resulting in average year-round surface tem-peratures of about 283C. Deep, cold water, mean-while, forms at higher latitudes and descends toSow along the seaSoor toward the equator. Thewarm surface layer, which extends to depths of

about 100}

200m, is separated from the deep coldwater by a thermocline. The temperature difference,T , between the surface and thousand-meter depthranges from 10 to 253C, with larger differencesoccurring in equatorial and tropical waters, as de-picted in Figure 1. T establishes the limits of theperformance of OTEC power cycles; the rule-of-thumb is that a differential of about 203C is neces-sary to sustain viable operation of an OTEC facility.

Since OTEC exploits renewable solar energy,recurring costs to generate electrical power areminimal. However, the Rxed or capital costs of

OTEC systems per kilowatt of generating capacity

are very high because large pipelines and heat ex-changers are needed to produce relatively modestamounts of electricity. These high Rxed costs dom-inate the economics of OTEC to the extent that itcurrently cannot compete with conventional power

systems, except in limited niche markets. Consider-able effort has been expended over the past twodecades to develop OTEC by-products, such as freshwater, air conditioning, and mariculture, that couldoffset the cost penalty of electricity generation.

State of the Technology

OTEC power systems operate as cyclic heat engines.They receive thermal energy through heat transferfrom surface sea water warmed by the sun, andtransform a portion of this energy to electrical

power. The Second Law of Thermodynamics pre-cludes the complete conversion of thermal energy into electricity. A portion of the heat extracted fromthe warm sea water must be rejected to a colderthermal sink. The thermal sink employed by OTECsystems is sea water drawn from the ocean depthsby means of a submerged pipeline. A steady-statecontrol volume energy analysis yields the result thatnet electrical power produced by the engine mustequal the difference between the rates of heat trans-fer from the warm surface water and to the colddeep water. The limiting (i.e., maximum) theoretical

Carnot energy conversion ef Rciency of a cyclic heat

OCEAN THERMAL ENERGY CONVERSION (OTEC) 1993



Less than 18°C

Depth less than 1000mMore than 24°C

40°S

30°S

20°S

10°S

Equator

10°N

20°N

40°N

30°N

40°E 80°E 120°E 160°E 160°W 120°W 80°W 40°W 0°W

L a

t i t u d e

18°_20°C20°_22°C

22°_24°C

Longitude

Figure 1 Temperature difference between surface and deep sea water in regions of the world. The darkest areas have the

greatest temperature difference and are the best locations for OTEC systems.

engine scales with the difference between the tem-peratures at which these heat transfers occur. ForOTEC, this difference is determined by T and isvery small; hence, OTEC ef Rciency is low. Althoughviable OTEC systems are characterized by Carnotef Rciencies in the range of 6}8%, state-of-the-artcombustion steam power cycles, which tap muchhigher temperature energy sources, are theoreticallycapable of converting more than 60% of theextracted thermal energy into electricity.

The low energy conversion ef Rciency of OTECmeans that more than 90% of the thermal energyextracted from the ocean’s surface is ‘wasted’ andmust be rejected to the cold, deep sea water. Thisnecessitates large heat exchangers and seawaterSow rates to produce relatively small amounts of electricity.

In spite of its inherent inef Rciency, OTEC, unlikeconventional fossil energy systems, utilizes a renew-able resource and poses minimal threat to theenvironment. In fact, it has been suggested thatwidespread adoption of OTEC could yield tangibleenvironmental beneRts through avenues such as re-duction of greenhouse gas CO2 emissions; enhanced

uptake of atmospheric CO2 by marine organismpopulations sustained by the nutrient-rich, deepOTEC sea water; and preservation of corals andhurricane amelioration by limiting temperature risein the surface ocean through energy extraction andartiRcial upwelling of deep water.

Carnot ef Rciency applies only to an ideal heatengine. In real power generation systems, irrevers-ibilities will further degrade performance. Given itslow theoretical ef Rciency, successful implementationof OTEC power generation demands careful engin-eering to minimize irreversibilities. Although OTEC

consumes what is essentially a free resource, poor

thermodynamic performance will reduce thequantity of electricity available for sale and, hence,negatively affect the economic feasibility of anOTEC facility.

An OTEC heat engine may be conRgured follow-ing designs by J.A. D’Arsonval, the French engineerwho Rrst proposed the OTEC concept in 1881, orG. Claude, D’Arsonval’s former student. Their de-signs are known, respectively, as closed cycle andopen cycle OTEC.

Closed Cycle OTEC

D’Arsonval’s original concept employed a pureworking Suid that would evaporate at the temper-ature of warm sea water. The vapor would sub-sequently expand and do work before beingcondensed by the cold sea water. This series of stepswould be repeated continuously with the sameworking Suid, whose Sow path and thermodynamicprocess representation constituted closed loops} hence, the name ‘closed cycle.’ The speciRc pro-cess adopted for closed cycle OTEC is the Rankine,or vapor power, cycle. Figure 2 is a simpliRed sche-

matic diagram of a closed cycle OTEC system. Theprincipal components are the heat exchangers,turbogenerator, and seawater supply system, which,although not shown, accounts for most of the para-sitic power consumption and a signiRcant fractionof the capital expense. Also not included are ancil-lary devices such as separators to remove residualliquid downstream of the evaporator and subsys-tems to hold and supply working Suid lost throughleaks or contamination.

In this system, heat transfer from warm surfacesea water occurs in the evaporator, producing

a saturated vapor from the working Suid. Electricity

1994 OCEAN THERMAL ENERGY CONVERSION (OTEC)



Working fluidcondensate

Warmseawaterdischarge

Evaporator

Workingfluid

vapor

Warm seawater in

Turbogenerator

Working fluidpressurizer

(boiler feed pump)

Condenser

Cold seawater in

Coldseawaterdischarge

Figure 2 Schematic diagram of a closed-cycle OTEC system. The working fluid is vaporized by heat transfer from the warm sea

water in the evaporator. The vapor expands through the turbogenerator and is condensed by heat transfer to cold sea water in the

condenser. Closed-cycle OTEC power systems, which operate at elevated pressures, require smaller turbines than open-cyclesystems.

is generated when this gas expands to lower pres-sure through the turbine. Latent heat is transferredfrom the vapor to the cold sea water in the conden-ser and the resulting liquid is pressurized witha pump to repeat the cycle.

The success of the Rankine cycle is a consequenceof more energy being recovered when the vaporexpands through the turbine than is consumed inre-pressurizing the liquid. In conventional (e.g.,combustion) Rankine systems, this yields net electri-cal power. For OTEC, however, the remaining bal-ance may be reduced substantially by an amountneeded to pump large volumes of sea water throughthe heat exchangers. (One misconception aboutOTEC is that tremendous energy must be expendedto bring cold sea water up from depths approaching1000 meters. In reality, the natural hydrostatic pres-sure gradient provides for most of the increase inthe gravitational potential energy of a Suid particlemoving with the gradient from the ocean depths to

the surface.)Irreversibilities in the turbomachinery and heatexchangers reduce cycle ef Rciency below the Carnotvalue. Irreversibilities in the heat exchangers occurwhen energy is transferred over a large temperaturedifference. It is important, therefore, to selecta working Suid that will undergo the desired phasechanges at temperatures established by the surfaceand deep sea water. Insofar as a large number of substances can meet this requirement (because pres-sures and the pressure ratio across the turbine andpump are design parameters), other factors must be

considered in the selection of a working Suid includ-

ing: cost and availability, compatibility with sys-tem materials, toxicity, and environmental hazard.Leading candidate working Suids for closed cycleOTEC applications are ammonia and variousSuorocarbon refrigerants. Their primary disadvan-tage is the environmental hazard posed by leakage;ammonia is toxic in moderate concentrationsand certain Suorocarbons have been banned bythe Montreal Protocol because they depletestratospheric ozone.

The Kalina, or adjustable proportion Suid mix-ture (APFM), cycle is a variant of the OTEC closedcycle. Whereas simple closed cycle OTEC systemsuse a pure working Suid, the Kalina cycle proposesto employ a mixture of ammonia and water withvarying proportions at different points in the sys-tem. The advantage of a binary mixture is that, ata given pressure, evaporation or condensation oc-curs over a range of temperatures; a pure Suid, onthe other hand, changes phase at constant temper-

ature. This additional degree of freedom allows heattransfer-related irreversibilities in the evaporatorand condenser to be reduced.

Although it improves ef Rciency, the Kalina cycleneeds additional capital equipment and may imposesevere demands on the evaporator and condenser.The ef Rciency improvement will require some com-bination of higher heat transfer coef Rcients, moreheat transfer surface area, and increased seawaterSow rates. Each has an associated cost or powerpenalty. Additional analysis and testing are requiredto conRrm whether the Kalina cycle and assorted

variations are viable alternatives.




Warm seawater in

De-aeration(Optional)

Vacuumchamber flash

evaporatorTurbogenerator

Desalinatedwater vapor

Cold seawaterdischarge

Condenser

Noncondensablegases

Vent compressor

Desalinatedwater

(Optional)

Coldsea water in

Noncondensablegases

Warm seawaterdischarge

Figure 3 Schematic diagram of an open-cycle OTEC system. In open-cycle OTEC, warm sea water is used directly as the

working fluid. Warm sea water is flash evaporated in a partial vacuum in the evaporator. The vapor expands through the turbine and

is condensed with cold sea water. The principal disadvantage of open-cycle OTEC is the low system operating pressures, which

necessitate large components to accommodate the high volumetric flow rates of steam.

Open Cycle OTEC

Claude’s concern about the cost and potential bio-fouling of closed cycle heat exchangers led him topropose using steam generated directly from thewarm sea water as the OTEC working Suid. Thesteps of the Claude, or open, cycle are: (1) Sash

evaporation of warm sea water in a partial vacuum;(2) expansion of the steam through a turbine togenerate power; (3) condensation of the vapor bydirect contact heat transfer to cold sea water; and(4) compression and discharge of the condensateand any residual noncondensable gases. Unless freshwater is a desired by-product, open cycle OTECeliminates the need for surface heat exchangers. Thename ‘open cycle’ comes from the fact that theworking Suid (steam) is discharged after a singlepass and has different initial and Rnal thermo-dynamic states; hence, the Sow path and processare ‘open.’

The essential features of an open cycle OTECsystem are presented in Figure 3. The entire system,from evaporator to condenser, operates at partialvacuum, typically at pressures of 1}3% of atmo-spheric. Initial evacuation of the system and re-moval of noncondensable gases during operation areperformed by the vacuum compressor, which, alongwith the sea water and discharge pumps, accountsfor the bulk of the open cycle OTEC parasiticpower consumption.

The low system pressures of open cycle OTEC arenecessary to induce boiling of the warm sea water.Flash evaporation is accomplished by exposing thesea water to pressures below the saturation pressurecorresponding to its temperature. This is usuallyaccomplished by pumping it into an evacuatedchamber through spouts designed to maximize heatand mass transfer surface area. Removal of gasesdissolved in the sea water, which will come out of

solution in the low-pressure evaporator and com-promise operation, may be performed at an inter-mediate pressure prior to evaporation.

Vapor produced in the Sash evaporator is rela-tively pure steam. The heat of vaporization isextracted from the liquid phase, lowering itstemperature and preventing any further boiling.

Flash evaporation may be perceived, then, asa transfer of thermal energy from the bulk of thewarm sea water of the small fraction of mass that isvaporized. Less than 0.5% of the mass of warm seawater entering the evaporator is converted intosteam.

The pressure drop across the turbine is establishedby the cold seawater temperature. At 43C, steamcondenses at 813 Pa. The turbine (or turbine dif-fuser) exit pressure cannot fall below this value.Hence, the maximum turbine pressure drop is onlyabout 3000Pa, corresponding to about a 3:1 pres-sure ratio. This will be further reduced to accountfor other pressure drops along the steam path anddifferences in the temperatures of the steam andseawater streams needed to facilitate heat transfer inthe evaporator and condenser.

Condensation of the low-pressure steam leavingthe turbine may employ a direct contact condenser(DCC), in which cold sea water is sprayed over thevapor, or a conventional surface condenser thatphysically separates the coolant and the condensate.DCCs are inexpensive and have good heat transfercharacteristics because they lack a solid thermalboundary between the warm and cool Suids. Surfacecondensers are expensive and more dif Rcult to main-tain than DCCs; however, they produce a market-able freshwater by-product.

Ef Suent from the condenser must be discharged tothe environment. Liquids are pressurized to ambientlevels at the point of release by means of a pump,or, if the elevation of the condenser is suitably high,




can be compressed hydrostatically. As noted pre-viously, noncondensable gases, which include anyresidual water vapor, dissolved gases that havecome out of solution, and air that may have leakedinto the system, are removed by the vacuum com-pressor.

Open cycle OTEC eliminates expensive heat ex-

changers at the cost of low system pressures. Partialvacuum operation has the disadvantage of makingthe system vulnerable to air in-leakage and pro-motes the evolution of noncondensable gases dis-solved in sea water. Power must ultimately beexpended to pressurize and remove these gases. Fur-thermore, as a consequence of the low steam den-sity, volumetric Sow rates are very high per unit of electricity generated. Large components are neededto accommodate these Sow rates. In particular, onlythe largest conventional steam turbine stages havethe potential for integration into open cycle OTECsystems of a few megawatts gross generating capa-city. It is generally acknowledged that higher capa-city plants will require a major turbine developmenteffort.

The mist lift and foam lift OTEC systems arevariants of the OTEC open cycle. Both employ thesea water directly to produce power. UnlikeClaude’s open cycle, lift cycles generate electricitywith a hydraulic turbine. The energy expended bythe liquid to drive the turbine is recovered from thewarm sea water. In the lift process, warm seawateris Sash evaporated to produce a two-phase,liquid}vapor mixture } either a mist consisting of liquid droplets suspended in a vapor, or a foam,where vapor bubbles are contained in a continuousliquid phase. The mixture rises, doing work againstgravity. Here, the thermal energy of the vapor isexpended to increase the potential energy of theSuid. The vapor is then condensed with cold seawater and discharged back into the ocean. Flow of the liquid through the hydraulic turbine may occurbefore or after the lift process. Advocates of the mistand foam lift cycles contend that they are cheaper to

implement than closed cycle OTEC because theyrequire no expensive heat exchangers, and aresuperior to the Claude cycle because they utilize ahydraulic turbine rather than a low pressure steamturbine. These claims await veriRcation.

Hybrid Cycle OTEC

Some marketing studies have suggested that OTECsystems that can provide both electricity and watermay be able to penetrate the marketplace morereadily than plants dedicated solely to power gen-

eration. Hybrid cycle OTEC was conceived as a

response to these studies. Hybrid cycles combine thepotable water production capabilities of open cycleOTEC with the potential for large electricity genera-tion capacities offered by the closed cycle.

Several hybrid cycle variants have been proposed.Typically, as in the Claude cycle, warm surfaceseawater is Sash evaporated in a partial vacuum.

This low pressure steam Sows into a heat exchangerwhere it is employed to vaporize a pressurized,low-boiling-point Suid such as ammonia. Duringthis process, most of the steam condenses, yieldingdesalinated potable water. The ammonia vaporSows through a simple closed-cycle power loop andis condensed using cold sea water. The uncondensedsteam and other gases exiting the ammonia evapor-ator may be further cooled by heat transfer to eitherthe liquid ammonia leaving the ammonia condenseror cold sea water. The noncondensables are thencompressed and discharged to the atmosphere.

Steam is used as an intermediary heat transfermedium between the warm sea water and the am-monia; consequently, the potential for biofouling inthe ammonia evaporator is reduced signiRcantly.Another advantage of the hybrid cycle related tofreshwater production is that condensation occurs atsigniRcantly higher pressures than in an open cycleOTEC condenser, due to the elimination of theturbine from the steam Sow path. This may, in turn,yield some savings in the amount of power con-sumed to compress and discharge the noncondens-able gases from the system. These savings (relativeto a simple Claude cycle producing electricity andwater), however, are offset by the additional back-work of the closed-cycle ammonia pump.

One drawback of the hybrid cycle is that waterproduction and power generation are closelycoupled. Changes or problems in either the water orpower subsystem will compromise performance of the other. Furthermore, there is a risk that the pot-able water may be contaminated by an ammonialeak. In response to these concerns, an alternativehybrid cycle has been proposed, comprising de-

coupled power and water production components.The basis for this concept lies in the fact that warmsea water leaving a closed cycle evaporator is stillsuf Rciently warm, and cold seawater exiting thecondenser is suf Rciently cold, to sustain an indepen-dent freshwater production process.

The alternative hybrid cycle consists of a conven-tional closed-cycle OTEC system that produces elec-tricity and a downstream Sash-evaporation-baseddesalination system. Water production and electric-ity generation can be adjusted independently, andeither can operate should a subsystem fail or require

servicing. The primary drawbacks are that the




ammonia evaporator uses warm seawater directlyand is subject to biofouling; and additional equip-ment, such as the potable water surface condenser,is required, thus increasing capital expenses.

Environmental Considerations

OTEC systems are, for the most part, environ-mentally benign. Although accidental leakage of closed cycle working Suids can pose a hazard, undernormal conditions, the only ef Suents are the mixedseawater discharges and dissolved gases that comeout of solution when sea water is depressurized.Although the quantities of outgassed species may besigniRcant for large OTEC systems, with the excep-tion of carbon dioxide, these species are benign.Carbon dioxide is a greenhouse gas and can impactglobal climate; however, OTEC systems release oneor two orders of magnitude less carbon dioxide thancomparable fossil fuel power plants and those emis-sions may be sequestered easily in the ocean or usedto stimulate marine biomass production.

OTEC mixed seawater discharges will be at lowertemperatures than sea water at the ocean surface.The discharges will also contain high concentrationsof nutrients brought up with the deep sea water andmay have a different salinity. It is important, there-fore, that release back into the ocean is conducted ina manner that minimizes unintended changes to theocean mixed layer biota and avoids inducing long-term surface temperature anomalies. Analyses of OTEC ef Suent plumes suggest that discharge atdepths of 50}100m should be suf Rcient to ensureminimal impact on the ocean environment. Con-versely, the nutrient-rich OTEC discharges could beexploited to sustain open-ocean mariculture.

Economics of OTEC

Studies conducted to date on the economic feasibil-ity of OTEC systems suffer from the lack of reliablecost data. Commercialization of the technology is

unlikely until a full-scale plant is constructed andoperated continuously over an extended period toprovide these data on capital and personnel andmaintenance expenses.

Uncertainties in Rnancial analyses notwithstand-ing, projections suggest very high Rrst costs forOTEC power system components. Small land-basedor near-shore Soating plants in the 1}10 MW range,which would probably be constructed in ruralisland communities, may require expendituresof $10000}$20000 (in 1995 US dollars) per kWof installed generating capacity. Although there

appears to be favorable economies of scale, larger

Soating (closed cycle) plants in the 50}100MWrange are still anticipated to cost about$5000kW

1. This is well in excess of the$1000}$2000kW

1 of fossil fuel power stations.To enhance the economics of OTEC power sta-

tions, various initiatives have been proposed basedon marketable OTEC by- or co-products. OTEC

proponents believe that the Rrst commercial OTECplants will be shore-based systems designed for usein developing PaciRc island nations, where potablewater is in short supply. Many of these sites wouldbe receptive to opportunities for economic growthprovided by OTEC-related industries.

Fresh Water

The condensate of the open and hybrid cycle OTECsystems is desalinated water, suitable for humanconsumption and agricultural uses. Analyses have

suggested that Rrst-generation OTEC plants, in the1}10MW range, would serve the utility powerneeds of rural PaciRc island communities, with thedesalinated water by-product helping to offset thehigh cost of electricity produced by the system.

Refrigeration and Air Conditioning

The cold, deep sea water can be used to maintaincold storage spaces, and to provide air conditioning.The Natural Energy Laboratory of Hawaii Author-ity (NELHA), which manages the site of Hawaii’sOTEC experiments, has air-conditioned its buildings

by passing the cold sea water through heat ex-changers. A new deep seawater utilization test facil-ity in Okinawa also employs cold seawater airconditioning. Similar small-scale operations wouldbe viable in other locales. Economic studies havebeen performed for larger metropolitan and resortapplications. These studies indicate that air condi-tioning new developments, such as resort com-plexes, with cold seawater may be economicallyattractive even if utility-grid electricity is available.

Mariculture

The cold deep ocean waters are rich in nutrients andlow in pathogens, and therefore provide an excellentmedium for the cultivation of marine organisms.The 322-acre NELHA facility has been the base forsuccessful mariculture research and developmententerprises. The site has an array of cold waterpipes, originally installed for the early OTEC re-search, but since used for mariculture. The coldwater is applied to cultivate Sounder, opihi (limpet;a shellRsh delicacy), oysters, lobsters, sea urchins,abalone, kelp, nori (a popular edible seaweed used

in sushi), and macro- and microalgae. Although




many of these ongoing endeavors are proRtable,high-value products such as biopharmaceuticals, bi-opigments, and pearls will need to be advanced torealize the full potential of the deep water.

The cold sea water may have applications foropen-ocean mariculture. ArtiRcial upwelling of deepwater has been suggested as a method of creating

new Rsheries and marine biomass plantations.Should development proceed, open-ocean cages canbe eliminated and natural feeding would replaceexpensive feed, with temperature and nutrientdifferentials being used to keep the Rsh stock inthe kept environment.

Agriculture

An idea initially proposed by University of Hawaiiresearchers involves the use of cold sea water foragriculture. This involves burying an array of cold

water pipes in the ground near to the surface tocreate cool weather growing conditions not found intropical environments. In addition to cooling thesoil, the system also drip irrigates the crop via con-densation of moisture in the air on the cold waterpipes. Demonstrations have determined that straw-berries and other spring crops and Sowers can begrown throughout the year in the tropics using thismethod.

Energy Carriers

Although the most common scenario is for OTEC

energy to be converted into electricity and delivereddirectly to consumers, energy storage has been con-sidered as an alternative, particularly in applicationsinvolving Soating plants moored far offshore. Stor-age would also allow the export of OTEC energy toindustrialized regions outside of the tropics. Long-term proposals have included the production of hydrogen gas via electrolysis, ammonia synthesis,and the development of shore-based mariculture

systems or Soating OTEC plant-ships as ocean-going farms. Such farms would cultivate marinebiomass, for example, in the form of fast-growingkelp which could be converted thermochemicallyinto fuel and chemical co-products.

See alsoCarbon Dioxide (CO

2 ) Cycle. Geophysical Heat

Flow. Heat and Momentum Fluxes at the Sea

Surface. Heat Transport and Climate.

Further Reading

Avery WH and Wu C (1994) Renewable Energy from theOcean: A Guide to OTEC. New York: Oxford Univer-sity Press.

Nihous GC, Syed MA and Vega LA (1989) Conceptualdesign of an open-cycle OTEC plant for the production

of electricity and fresh water in a PaciRc island. Pro-ceedings International Conference on Ocean EnergyRecovery.

Penney TR and Bharathan D (1987) Power from the sea.ScientiTc American 256(1): 86}92.

Sverdrup HV, Johnson MW and Fleming PH (1942)The Oceans: Their Physics, Chemistry, and General Biology. New York: Prentice-Hall.

Takahashi PK and Trenka A (1996) Ocean Thermal Energy Conversion; UNESCO Energy Engineering Series. Chichester: John Wiley.

Takahashi PK, McKinley K, Phillips VD, Magaard L andKoske P (1993) Marine macrobiotechnology systems.

Journal of Marine Biotechnology 1(1): 9}

15.Takahashi PK (1996) Project blue revolution. Journal of Energy Engineering 122(3): 114}124.

Vega LA and Nihous GC (1994) Design of a 5 MWOTEC pre-commercial plant. Proceedings Oceanology94: 5.

Vega LA (1992) Economics of ocean thermal energy con-version. In: Seymour RJ (ed.) Ocean Energy Recovery:The State of the Art. New York: American Society of Civil Engineers.

OIL POLLUTION

J. M. Baker, Clock Cottage, Shrewsbury, UK

Copyright ^ 2001 Academic Press

doi:10.1006/rwos.2001.0055

Introduction

This article describes the sources of oil pollution,composition of oil, fate when spilt, and environ-

mental effects. The initial impact of a spill can vary

from minimal to the death of nearly everything ina particular biological community, and recoverytimes can vary from less than one year to more than30 years. Information is provided on the range of effects together with the factors which help to deter-mine the course of events. These include oil typeand volume, local geography, climate and season,species and biological communities, local economicand amenity considerations, and clean-up methods.

With respect to clean-up, decisions sometimes have

OIL POLLUTION 1999



23

3.0 SOLAR ENERGY UTILIZATION

3.1 Introduction

Solar energy is simply described as the energy from the sun. The sun is known to be the

major source of renewable energies. Its radiations have been either used directly or indirectly

in various ways for gainful purposes since the world began. The direct effects of the sun on

the earth and also its application have led to other renewable forms of energy which include

solar, hydropower, wind and biomass (through the processes of photosynthesis).

Gravitational forces of the sun and moon creates the tides from which tidal energy is

obtained. Solar electricity can be obtained by direct conversion of solar energy in

photovoltaic cells and modules, by producing steam in parabolic thermal collectors in a

normal steam turbine power plant, or by producing steam in reflector solar power tower again

for a steam turbine power plant. It is reported that the sun radiates enormous amount of

energy in one year than people have used since the world began (Williams and Carl, 1990).

Basically, energy from the sun is environmentally friendly. They do not contribute harmful

and toxic emissions to the environment and cannot be depleted by reason of use. The

instruments that are commonly used to measure solar radiation of a place are the pyranometer

and the pyrheliometer .

3.1.1 The sun and its energy

The sun is almost a perfectly spherical mass of about 1.4 x 106 km in diameter with a weight

of about 2.0 x 1030 kg (almost 330,000 times that of the earth). 75% of the sun’s mass is

hydrogen while about 23.32% is helium. The remaining mass is made up of minor quantities

(about 1.68%) of elements which include oxygen, carbon, iron, neon and other heavier

elements.



24

The sun’s surface temperature is estimated to be about 5778 degree Kelvin while its internal

temperature is about 5 million degree Kelvin. It emits radiant energy in the form of

electromagnetic waves. The radiated waves have wavelengths (each corresponding to a

frequency and energy) that range from 0.2 to 4.0 μm. The size of the wavelength is indirectly

related to the frequency and the energy. A shorter wavelength corresponds to a higher

frequency and also, higher energy content (expressed in electron volts (eV)). The sun’s

energy is created by repeated fusion reaction at its core. This reaction involves the

exothermic combination of two hydrogen atoms to form a helium atom. It fuses about 6.2 x

1011 kg of hydrogen atoms in one second. The reaction is made possible because of the

interaction between the sun’s high temperature and pressure (which is about 70 billion times

higher than the earth’s atmospheric pressure)

The mean distance between the sun and earth is approximately 1.50 x 10 11 m. The

sun-earth distance varies based on the movement of the sun from in January to in July. Light

from the sun travels this distance on daily basis in about 8.32 minutes with a speed of light

(3.0 x 108 m/s). The amount of solar energy incident on a horizontal surface on the earth at a

specific time is referred to as insolation . Moreover, insolation received at a specific location

is dependent on certain factors which include: the sky’s clearness or cloudiness, the sun’s

angle (altitude) in the sky relative to the surface at the location, the atmospheric medium,

water vapour, location latitude etc. For instance, insolation from directly overhead sun in a

clear sky on a horizontal surface is about 1kW/m2 (this is the highest possible on the earth’s

surface, when no reflector is present (e.g. Mirrors and lenses)). This value decreases when the

sun is not directly overhead or the receiving surface is not oriented perpendicularly to the

sun’s rays. Also, the value of the insolation is affected when there are more atmospheric

medium between the sun and the receiving surface, the medium can absorb or reflect part of

the energy. The magnitude of insolation is also affected by humidity, equinox and solstice.



25

Fig. 3.1 shows the variation of the insolation with the months and also the equinoxes (when

the earth’s tilt is neither away from nor towards the sun) and solstices (when the sun reaches

its highest position in the sky as seen from the north or south poles). In the design of solar

energy based systems, the changes in the magnitude of insolations per season and also due to

the earth’s rotation are taken into cognizance.

Fig. 3.1: Variation of the magnitudes of insolution per month (Ghosh, T.K and Pretas, M.K, 2011. Energyresources and systems, vol. 2: renewable resources, Springer, New York, 82)

Light from the sun pass through the atmosphere to the earth. As it does, part of the

light is absorbed or reflected back by the various atmospheric contents which include clouds,

water vapour, oxygen, ozone, dust and carbon dioxide. The molecules of oxygen and ozone

absorb almost all the ultraviolet radiation while the carbon dioxide and water vapour absorbs

some of the infra red energy. In addition, the clouds and dust particles in the atmosphere

scatters part of the solar radiation. This phenomenon affects the amount of solar spectrum

that leaves the sun and reaches the earth. The portion of radiation that successfully reaches



26

the earth’s surface (without scattering or absorption) is called direct radiation . The other part

that is scattered and reflected (leading to reversal or shift in original direction) but reaches the

earth’s surface is referred to as diffuse radiation . Therefore, the total solar radiation

received from the sun on a surface is the summation of the direct and diffuse radiations.

Another name for this total solar radiation is called the solar insolation mentioned earlier.

The intensity of diffuse radiation varies among other factors with location latitude, time of

the day and time or month of the year. Therefore, because of the phenomenon of scattering

and absorption of radiant energies, only wavelengths between 0.29 and 2.5 μm are considered

for terrestrial application of solar energy. Fig. 3.2 shows the solar spectrum outside in space

and on the ground.

Fig. 3.2: Solar spectrum outside in space and on the ground (Ghosh, T.K and Pretas, M.K, 2011. Energyresources and systems, vol. 2: renewable resources, Springer, New York, 82)

Under favourable atmospheric conditions, maximum observed intensity at noon onan

oriented surface at sea level is 1 kW/m2. At higher altitude of 1 km, the value is about 1.5

kW/m2. From Fig. 3.2, it is noted that each portion of the solar spectrum is associated with



27

different energy level. For instance, in the region of visible spectrum, the red light is at the

low energy end and the violet light is at the high energy end. Photons in the ultraviolet region

therefore have more energy than those in the visible region while those in the infrared region

gave lesser energy than those in visible region.

3.2 Kinematics of solar radiation

3.2.1 Solar constant (I sc )

Definition: This is the magnitude of energy received in unit time per unit area perpendicular

to the sun’s direction at the mean sun-earth’s distance. It is the rate at which solar energy

arrives at the top of the atmosphere. The standard value of the solar constant is given as:

Isc = 1353 W/m2.

3.2.2 Solar intensity

The solar radiation intensity experienced outside the earth’s atmosphere (extraterrestrial

radiation intensity) varies due to the little variation in the sun-earth’s distance through the

year. Based on this, the intensity of the solar radiation (I) that reaches the earth is derived

mathematically from:

360 21 0.33cos

365 sc

n I I

3601 0.33cos

365 sc

n I I

3.1

where:

n = day number of the year

Isc = solar constant



28

3.2.3 Air mass

This is the ratio of the path of the sun’s ray through the atmosphere to the length of the path

when the sun is at zenith (directly overhead). Apart of very low solar altitude angles, the air

mass is given as:

cosm ec 3.2

where:

α = altitude angle

However, different values are associated with the air mass for different location of the sun at

sea level:

m = 1, when the sun is directly overhead (i.e. at zenith)

m =2, when zenith angle (θz) is 60o. Zenith angle is the angle subtended by the zenith and the

line of sight of the sun.

m = sec θz, when m > 3

m = 0 just above the earth’s atmosphere.

3.2.4 Declination (δ)

This is the angular distance of the sun’s ray north or south of the equator . It is zero on the two

equinox days (22 March and 20 September) and varies between 23.5 o on 22 June and - 23.5o

on 20 December. Mathematically, the sun’s declination is estimated from:

360 284

365

23.45sin n

3.3

3.2.5 The hour angle (ω)

This is the angle which the earth must turn to bring the meridian of a point in line with the

sun’s ray. The hour angle is equivalent to 15o per hour. It is measured from noon based on the

local solar time. It converts the local solar time into number of degrees whuch the sun moves

across the sky. It is 15o at solar noon. In the morning the hour angle may be taken as positive

and negative in the afternoon. The hour angle (ω) can be estimated from:



29

15 12o LST 3.4

where:

LST = local solar time

However, LST is given as:

60

TC LST LT

3.5

where:

LT = local time and TC = time correction factor

4TC LSTM longitude EoT 3.6

where:

LSTM = local standard time meridian (It is a reference meridian used for a particular time

zone and is similar to the prime meridian used for Greenwich Mean Time (GMT))

EoT = equation of time

15o

GMT

LSTM T

3.7

where:

GMT T LT GMT in hours 3.8

9.87sin 2 7.53cos 1.5sin EoT 3.9

where:

360 81365

n 3.10

where:

n = day number in a year

The equation of time correction (EoT) is given in Fig. 3.3. Fig. 3.3 can be used directly in leu

of the equation above.



30

Fig. 3.3: Equation of time correction (source: [http://pvcdrom.pveducation.org/SUNLIGHT/SOLART.HTM, May 15, 2012])

Class Exercise

Evaluate the local solar time at Lagos, Nigeria (Latitude 6.45o N, Longitude 3.38o E) at 9.00

am (GMT +1) on June 20, 1996.

Solution

Using the equations above solve for LST.

β = 88.77o

EoT = - 1.2416

∆TGMT = 1 hr.

LSTM = 15o

TC = 45.2384

LST = 9.754 am

3.2.6 Zenith angle (θ z )

This is the complementary angle of the sun’s altitude angle (α).

The altitude angle is the vertical angle between the projection of the sun’s ray on the

horizontal plane and the direction of the sun’s ray that passes through the point.

http://pvcdrom.pveducation.org/SUNLIGHT/SOLART.HTM





31

2 z

3.11

3.2.7 Solar azimuth angle (γ s )

This is the solar angle (in degree) along the horizontal east or west of north or it is a

horizontal angle measured from the north to the horizontal projection of the sun’s rays. It is

taken as positive when measure westward.

Moreover, α, θz, and γs can be expressed in terms of the latitude angle (φ) as:

cos cos cos cos sin sin z 3.12

cos sec cos sin cos sin cos s

3.13

sin sec cos sin s 3.14

3.2.8 The slope (s), surface azimuth (γ ), and incident angle (θ)

Slope: This is the angle made by the plane surface with the horizontal. It is positive when

surface is sloping towards the south.

Surface azimuth: This is the angle of deviation of the normal to the surface from the local

meridian. The zero point is south, east is positive and west is negative.

Incident angle: This is the angle being measured between the beam of rays and the normal to

the plane.

The relation between incident angle (θ) and other angles is given for spherical geometry as:

cos sin sin cos cos cos cos sin cos cos cos cos sin cos sin

cos sin sin sin

s s s s

s

...3.15

Equation 3.15 can be simplified by the following modifications:

a) When the surfaces are vertical, s = 90o. Thus, equation 3.15 becomes:

cos sin cos cos cos cos sin cos cos sin sin 3.16

b) When the surfaces are horizontal, s = 0o, θ = θz, i.e. cos θ = cos θz = sin α. Thus,

sin sin cos cos coscos z 3.17



32

c) When the surface is facing the south, γ = 0, θ = θ T (denoting that the surface is titled).

Thus:

sin sin - cos cos cos -

sin sin cos cos cos sin

=cos cos cos cos -sin sin

cos T

s s

s s

s s

3.18

d) When the vertical surface is facing southward, s = 90o, γ = 0. Then:

sin cos cos -cos sincos z 3.19

3.2.9 Day length (t d )

Thus is the time between sunrise and sunset.

Since each hour correspond to 15o, the day length (in hours) is given by:

2

15 s

d t 3.20

However, at the time of sunrise or sunset, θ z = 90o and from equation 3.17, the sunrise hour

angle ωs can be estimated after substituting θz = 90o as:

1 tan tancos s 3.21

Therefore,

1 tan tan2 cos

15d t 3.22

Class Examples

1. A flat collector was used to capture solar radiation on November 5, 2008 at 10 am solar

time for a location 29.5o N, 30o E. If the collector is facing southward and tilted at an angle of

33o with the horizontal, estimate the angle the solar radiation made with the normal to the flat

collector.

2. If the orientation of the collector (in 1 above) is changed in such a way that it is held at 90o

to the horizontal and facing due east with surface azimuth angle being 15o, what will be the

angle the solar radiation makes with the flat collector?



33

3. What will the angle be if the vertical collector (in 2 above) is facing due south? Also

determine the altitude angle α.

4. Determine the sunrise day length for a location with latitude 23.4o and declination 20o

Solutions

1. Use equations 3.3 and 3.18 to estimate θT

2. Use equations 3.3 and 3.16 to estimate θ

3. Use equations 3.3, 3.11 and 3.19 to estimate θz and α

4. Use equation 3.20 and 3.21 to estimate td

3.2.10 Irradiance

This is defined as the measure of the incoming solar radiation incident on a surface per unit

area of that surface. The unit is given as W/m2.

3.2.11 Radiant exposure

This is defined as the amount of solar energy incident on a surface per unit area of the

surface. It can be estimated by integrating the irradiance over a specified time, mostly 24

hours (a day). The unit is given as W/m2.

3.2.12 Radiosity

This is the rate at which radiant energy is simultaneously reflected, emitted and transmitted

away from a unit area of a surface. The unit is given as W/m2.

3.2.13 Emissive power

This is the rate at which solar radiation leaves a unit area of a surface by emission. The unit is

given as W/m2.

3.2.14 Peak sun hour

This is defined as the equivalent number of hours per day when Solar Irradiance averages

1KW/m2

.



34

3.2.15 Global solar radiation

This is the amount of solar radiation from the sun incident on a surface usually measured on a

daily basis. It could then be averaged monthly in order to deduce its monthly values. It could

be extraterrestrial (i.e., the amount of solar radiation incident on the earth surface before

diffusion by cloud cover), direct (beam radiation) or diffuse (scattered) components. It is

measured in W/m2.

3.2.16 Clearness index

This is defined as the ratio of radiation received on a horizontal surface in a period (usually

one day) to the radiation that would have been received on a parallel extraterrestrial surface

in the same period. Mathematically, it is represented as:

T

o

H

H K 3.23

3.2.17 Variation in day length

This is the number of hours between sunrise and sunset measured in hours. It can otherwise

be referred to as maximum hours of optimum sunshine. It is usually denoted as N and it is the

same as td. This is given mathematically as:

2

15 s

d N t 3.24

1 tan tan2 cos

15

d t 3.25

3.2.18 Extraterrestrial solar radiation (H o )

This is the amount of solar radiation experienced outside the earth’s atmosphere. It is

expressed mathematically as:

0

224 360(1 0.033cos )(cos cos sin sin sin )

365 360

sSC s

n H I

3.26

3.2.19 Relative sunshine



35

This is the ratio of the average daily hours of bright sunshine over a period to the maximum

daily hours of bright sunshine for the same period. It is expressed as:

T

d

n n

N t R 3.27

3.3 Estimation of average global solar radiation

Different models exist that correlates different meteorological parameters to evaluate the

global solar radiation (H) of a place. Some of these include those that evaluate H from the

correlation of parameters such as maximum and minimum temperatures, amount of cloud

cover, extraterrestrial radiation, relative humidity, latitude, elevation, soil temperature,

precipitation, evaporation and number of rainy days, etc.. Another model also exists that

correlates the global solar radiation to the relative sunshine hours. For the purpose of this

course the Angstrom-Prescott model type of equation 3.28 will be used. This is given as:

T o

H n K a b

H N 3.28

where:

K T = clearness index, Ho = extraterrestrial solar radiation, n = the monthly average sunshine

hours, N = maximum sunshine duration or day length, a and b are correlation coefficients (or

constants). The ration

N is called the relative sunshine.

Based on the Angstrom-Prescott model, page developed a model represented by equation

3.29. The model was reported to be applicable anywhere in the world. Thus for this course,

equation 3.29 will be very useful for determining the global solar radiation of a place.

0.23 0.48T T o

K R H H

3.29



36

3.4 Utilization of solar energy

Of the sources of renewable energy, solar energy has enjoyed wide utilization in various

applications whether of industrial, domestic or agricultural uses. However, worthy of note is

the fact that, the technologies that employ solar energy can be classified as either passive

solar or active solar depending on the way the solar energy is captured, converted and

distributed.

- Passive solar technologies use the indirect method of harnessing and discharging

solar energy in the form of thermal energy through the creation of channels and

spaces for material circulation. The material could be liquid (e.g. water) or gases (e.g.

air). The principle creates a system containing fluid or materials that receives heat by

direct solar heating and distribute the heat via spaces or channel located within it.

Applications of this include the design of passive solar buildings in which, the

windows, walls and floors are designed and made to receive and distribute heat.

Another is the thermosiphon solar water heater.

- Active solar technologies use the direct method of harnessing the solar energy either

with solar thermal collectors or photovoltaic panels. With these technologies, solar

energy can be converted to either heat or electricity via electrical or mechanical

conversion devices.

Adopting solar technology depends on certain factors which are capable or aiding the growth

of solar technology utilization across the globe, especially developing countries. These

factors are highlighted below.

Factors that may likely aid the adoption and promotion of solar technology utilization

1. Availability of efficient conversion system

2. Economics of solar energy equipments and their applications



37

3. Cost of operation and maintenance: this must be low

4. Ease of mobility: the device should be easy to transport

3.4.1 Modes of utilization of solar technology

There are basically two broad modes of utilizing solar energy technologies. These are:

1. Utilization as solar thermal energy

2. Utilization as photovoltaic cell generated electricity/solar lighting

3.4.1.1 Utilization as solar thermal energy

This is a simple way of utilizing solar energy directly. The solar thermal conversion system

receives the sun’s energy in the form of heat. This heat may therefore be employed for

different purposes ranging from general heating purposes, space heating and cooling of

residential buildings, solar cooking etc. Also, solar thermal energy system can be used to

generate electricity. In this type of system, various solar tracking and concentrating devices

are used to focus sunlight and achieve high temperature to generate steam for driving turbines

in solar thermal electric power plants. Sometimes, instead of the fluid being water, other

working fluids may be used. For instance, different but suitable gases and other volatile

liquids may be used. Sometimes, the solar tracking and concentrating devices are used to

focus sunlight to a receiver. At the receiver, the temperature increases to a very high value

and it is used to melt salt. The hot molten salt is then driven to generate steam in a cyclic

manner. The steam is used to generate electricity with a steam turbine. The maximum

theoretical thermodynamic conversion efficiency of using the hot molten salt to generate

steam that produces the electricity is given by the Carnot cycle efficiency as equation 3.30.

hot cold Carnot

hot

t t

t 3.30



38

A schematic diagram of the operating processes of a solar thermal power plant is displayed in

Fig. 3.4.

Fig. 3.4: Schematic diagram showing the cycle processes of solar thermal power plant (Source: Patel, M.R,1999. Wind and solar power system, CRC press, Boca Raton, 159)

There are basically three alternative configurations of solar thermal collectors. These are the

Dish, Central receiver and the Parabolic trough.

In employing solar thermal energy for space cooling, solar heated water is used to vaporize

ammonia or propane at a moderate pressure. The vapour is then channelled in such a way that

it drives a turbine which in turn drives a vapour compression cooling unit. The disadvantages

of this vapour compression system lie with the working fluid and the efficiency. While

propane is highly flammable, ammonia is toxic. The system’s efficiency is very low and

required improved design. Another alternative means of providing solar cooling is by the

principle of absorption cooling or absorption air conditioning. The schematic flow diagram of

absorption cooling is given in Fig. 3.5.



39

Fig. 3.5: Schematic flow diagram of an absorption cooling system. (Source: [http://www.google.com.ng/

imgres?imgurl=http://sustainablesources.com/images/heatcoolsolar4.gif&imgrefurl=http://solarheatcool.sustaina blesources.com/&h=449&w=350&sz=5&tbnid=0A7FAFQmkjHSGM:&tbnh=90&tbnw=70&zoom=1&docid=nfsFUKdnT_pxNM&hl=en&sa=X&ei=gzm1T7HSF-KM0AX6s2E&ved=0CKgBEPUBMAU&dur=5374, May17, 2012])

3.4.1.2 Utilization as photovoltaic cell generated electricity/solar lighting

One of the common ways of utilizing solar energy is for generating electricity. Solar energy

can be converted directly to electricity by the use of photovoltaic (PV) cells/panels. A PV cell

(sometimes called solar cell) is composed of crystalline semiconductor cells or materials. The

semiconductor cells (wafers) are made of ultra-thin layer phosphorous-doped silicon and

thick layer of boron-doped silicon. The phosphorous-doped silicon is the n-type

semiconductor while the boron-doped silicon is the p-type semiconductor. At the top surface

of the material, where the p-n junction is located, an electrical field is created. The electrical

http://www.google.com.ng/%20imgres?imgurl=http://sustainablesources.com/images/heatcoolsolar4.gif&imgrefurl=http://solarheatcool.sustainablesources.com/&h=449&w=350&sz=5&tbnid=0A7FAFQmkjHSGM:&tbnh=90&tbnw=70&zoom=1&docid=nfsFUKdnT_pxNM&hl=en&sa=X&ei=gzm1T7HSF-KM0AX6s2E&ved=0CKgBEPUBMAU&dur=5374











40

field, at the strike of sunlight, leads to the transfer of electrons from the semiconductor

terminal to the electrical appliance, thereby producing electricity.

A typical silicon photovoltaic cell produces between 0.5 and 0.6 volts direct current

under no load condition. However, the output from a PV system depends on: eff iciency, size

of the PV system and intensity of sunl ight . A PV system has proven to be very reliable in

space applications. Other areas of application include its use as standalone electricity

generation for remote areas, grid connected electricity generation, hybrid power systems, etc.

3.4.1.2.1 Advantages of the PV systems

1. It can be employed anywhere in the world, as long as the sun shines there.

2. It is quiet and has no issue of noise pollution

3. It has no sunshine magnitude constraint, unlike wind turbine that cannot produce electricity

below certain wind speed value.

4. It is easy to install

5. Needs no tall tower or stand unlike wind turbines

6. It does not produce vibrations and it’s not toxic

7. The application is simple to adapt.

8. It has no distribution and transmission losses

9. Its production is free of environmental pollution

3.4.1.2.2 Disadvantages of PV systems

The major disadvantages are in the high production and capital costs, and also its

intermittence. The high cost of production is due to the cost of material technology which

commonly is the crystalline semiconductor. However, the capital cost has been reducing from

the 1980s. Its cost reduced an average of US$ 20/watt to US$ 5/watt between the 1980 and



41

1990s. Also, the cost of PV electricity has also decline within the stated period from US$

0.20/kWh to US$ 0.15/kWh. Presently, the levelized cost is around US$ 20 to 40/kWh. This

is still high compared to electricity generated from fossil sources. Its intermittence nature

means it can only be used when there is sunshine. Other disadvantages include the lack of

competent energy storage system and the very low conversion efficiency which is below

30%.

3.5 Flat-Plate Collector Design

The flat-plate collector is the commonest type of solar collector for water heating systems and

space heating. It is mainly used for residential water heating and space heating installations.

This type of collector transfers the heat of the sun to water either directly or through the use

of another fluid and heat exchanger. They are mostly employed for low temperature (usually

below 90o C). A Flat-plate collector design makes use of a rectangular panel (less than 3.0

m2 in area). It has an insulated bottom surface in a metal casing with a dark-coloured

absorber plate on the top that absorbs most of the solar energy. The insulation prevents the

heat from escaping through the back of the collector via conduction and radiation. A glass or

plastic cover called the glazing surface is placed over the absorber plate. The cover allows

solar energy to pass through but reduces heat losses by radiation from the absorber. Also, the

glass cover act as a convection shield to reduce losses from the absorber plate beneath. The

glazing surface traps the heat within the collector and the absorber plate transfers energy from

solar radiation to a fluid that circulates within it. The fluid can be air, antifreeze or water. The

absorber plates are often coated to maximize the solar radiation collection. The absorber is

made of thermally stable polymers, steel, copper or aluminium sheet of 1 to 2 mm diameter.

Tubes of diameter 1 to 1.5 cm are placed inside the metal casing to heat liquid or air. They

are soldered, brazed or clamped to the absorber plate bottom. Worthy of note is that flat-plate



42

collectors can trap and absorb both direct and diffuse solar radiation. When sunlight passes

through the glazing, it strikes and heats up the absorber plates which in turn heat the fluid in

the tubing. Thermal insulation of thickness 5 to 10 cm is usually placed behind the absorber

plate to prevent heat losses. A picture of a flat-plate collector is shown in Fig. 3.6, while a

detail labelled diagram is shown in Fig. 3.7.

As a result of the fact that the cover is mostly made of glass, it permits radiation of

wavelength below 2 μm but is opaque to the longer wavelength of the infr a-red region. Based

on this, heat is trapped within the space between the absorber and the cover material.

However, because the enclosed air is warmer that the ambient air, there is always minimal

heat loss to the surrounding from the top of the cover by conduction, convection and

radiation. The rate of heat lost is however proportional to the difference in the temperature of

the enclosure and the surrounding. As the temperature of the air space increases more than

the surrounding, the rate of heat loss also increases. This phenomenon affects the collector

efficiency.

In addition, a certain percentage of the incident solar insolation is lost by absorption

in the glass cover plates. This can be reduced by the use of a clear glass (or white glass).

Losses also occur as a result of reflection from the glass cover. This reflection can be

invariably reduced by coating the glass cover with anti-reflecting substances such as thin

films of magnesium fluoride for example.



43

Fig. 3.6: A photograph of a flat-plate collector (Source: http://en.wikipedia.org/wiki /File:Solar_panels,_Santorini.jpg, accessed May 18, 2012]

http://en.wikipedia/

http://en.wikipedia/



44

Fig. 3.7: detailed component parts of a flat-plate collector created by sectioning through the module (source:Ghosh, T.K and Pretas, M.K, 2011. Energy resources and systems, vol. 2: renewable resources, Springer, NewYork, 82)

3.5.1 Thermal analysis and efficiency of a flat-plate collector

For a flat-plate collector of surface area A and incident solar radiation intensity I, the amount

of solar radiation received (as heat) from the sun can be represented as Q i:

iQ I A 3.31

However, a part of this radiation is reflected back to the surrounding/sky, while another

component is absorbed by the cover (glazing) material. The third component transfers to the

absorber plate through the cover. Thus the percentage of the solar radiation that passes

through the cover (transmissivity, τ) and that is absorbed (absorptivity, α) by the absorber

panel is the most important and should be accounted for. This percentage is given as the

product of τ and α. Hence, the energy input into the system is given now as:



45

.iQ I A 3.32

Based on the temperature difference between the system and the surrounding, heat is lost to

the surrounding. Therefore, the rate of heat loss (QL) depends on the collector temperature

and the collector’s overall heat transfer (or loss) coefficient (UL). The heat loss is therefore is

therefore given as:

L L c aU t t Q A 3.33

The useful energy (QU) is the difference between the heat input (Qi) and the heat lost (QL)

given as:

.U L c a

I A U A t t Q 3.34

Note that (τ.α) in equation 3.34 is referred to as the effective product transmittance-

absorptance (also called transmissivity-absorptivity product). It is defined as the ratio of the

radiation absorbed in the absorber plate to the radiation incident on the cover system. It is

given as:

1 1.

d

3.35

where:

ρd = diffuse reflectance which is approximately 0.16, 0.24 or 0.29 for one, two or three glass

covers respectively when the incident angle is 60o.

In addition to this, the heat that reaches the absorber is trapped away by the fluid. This can be

evaluated as:

U p f imc t t Q 3.36

where:

c p = specific heat capacity of the fluid

tf = temperature supplied by the fluid

ti = inlet temperature of the fluid = temperature of the upper surface of the absorber plate



46

m = mass of the fluid

Because it is difficult to estimate the collector temperature, t c, it is convenient to relate the

actual useful energy (heat) gain of a collector to the useful energy (heat) gain if the whole

collector were at the fluid inlet temperature (t i). The quantity that relates this is called the

collector heat removal factor (F) and given as:

.

p f i

L i a

mc t t F

I A U A t t 3.37

Equation 3.37 is obtained by dividing equation 3.33 by equation 3.34 after the assumption.

The maximum possible useful energy gain in a solar collector is known to occur when the

collector is at the inlet fluid temperature. Therefore, the actual useful energy gain is therefore

found by multiplying the maximum possible useful energy gain (equation 3.34) by the

collector heat removal factor (equation 3.37). It is given as:

. .U a ai i L L F AF Q I A U A t t I U t t 3.38

Equation 3.37 is the widely used equation for measuring collector energy (or heat) gain and is

generally referred to as Hottel-whitter-Bliss equation.

Efficiency of a flat- plate collector (η)

The efficiency of a flat-plate collector is a measure of the performance. It is the ratio of the

useful energy gained to the solar energy incident on the collector over a particular period.

This is:

U Q dt

A Idt 3.39

At a particular instant of time, it is:

. L c aU

AF I U t t Q

AI AI 3.40

. L c a

F I U t t

I 3.41



47

Note: (τ.α) is given by equation 3.35 in all the equations.

The transmittance can be determined from:

cosnkL

e 3.42

where:

k = constant of proportionality. The value of k for glass range from about 0.01/cm for

absolute clear (white) glass to 0.32/cm for poor quality glass with a greenish cast.

Class Exercise

1. A flat-plate solar collector was employed for water heating at Ibadan (7.39 o N, 3.9o E),

Oyo state, Nigeria on January 31, 2011 at 11 am LST. The data relating to the solar collector

are stated below. Determine the (a) energy input into the collector system, (b) useful energy

from the collector and (c) the collector efficiency.

Glass transmittance and absorptance are 0.8 and 0.85; number of glass cover = 2; solar

collector area = 2.5 m2; collector fluid temperature = 60o C; overall collector heat loss

coefficient = 8.0 W/m

2

o

C; ambient temperature = 28

o

C; ρd = 0.2; heat removal factor = 0.82;

Isc = 1353 W.

Solution

The steps required to solve the questions are:

- Determine (τ.α) using equation 3.35

- Determine I using equation 3.1

- Solve Qi using equation 3.32

-

Solve Qu using equation 3.38

- Solve η using equation 3.40

2. If for question 1 above, the glass transmissibility is required. You are therefore required to

estimate it. Given that the angle of refraction is 25o, distance travelled by the radiation in the

system is 0.2 cm, k = 0.2/cm.



48

Solution: Use equation 3.42

References/Bibliography

Aswathanarayana, U., 2010. Solar energy, In Aswathanarayana, U., Harikrishnan, T andThayyib Sahini, K.M. (eds), Green energy technology, economics and politics, CRC press,Boca Raton, 21

Bakirci K., 2009. Models of Solar Radiation with hours of bright sunshine: A review.

Renewable and sustainable Energy Reviews, 13, 2580-2588

El-Metwally, M., 2005. Sunshine and global solar radiation estimation at different sites inEgypt, Journal of Atmospheric and Solar-Terrestrial Physics, 67: 1331-1342

Ghosh, T.K and Pretas, M.K, 2011. Energy resources and systems, vol. 2: renewableresources, Springer, New York, 82

Hutchinson, M.F., Booth, T.H., McMahon, J.P., Nix, H.A., 1984. Estimation of monthlymean values of daily total solar radiation for Australia, Solar Energy, 32: 277-290

Janjai, S., Pankaew, P., Laksanaboonsong, J., 2009. A model for calculating hourly solarradiation from satellite data in the tropics, Applied Energy, 86: 1450-1457

Jin, Z., Yezheng, Wu, Gang, Y., 2005. General formula for estimation of monthly average

daily global solar radiation in China, Energy Conversion and Management , 46: 257-268

Lewis, G., 1983. Estimates of irradiance over Zimbabwe, Solar Energy, 31: 609-612

Odod, J.C., Sulaiman, A.T., Aidan, J., Yguda, M.M., Ogbu, F.A, 1995. The importance ofmaximum air temperature in the parameterisation of solar radiation in Nigeria, Renewable

Energy, 6: 751-763

Oyedepo, S.O, 2010. Renewable energy lecture note, Covenant University, Ota(Unpublished)


Rathore, N.S and Panwar, N.L, 2007. Renewable energy sources for sustainabledevelopment, New India Publishing agency, New Delhi, 1-3

Robaa, S.M., 2003. On the estimation of global and diffuse solar radiation over Egypt, Mausam, 54: 17-35

Robaa, S.M., 2009. Validation of the existing models for estimating global solar radiationover Egypt, Energy Conversion and Management, 50: 184-193

Struckmann Fabio, 2008. Analysis of a flat-plate solar collector, Project report 2008MVK160 Heat and Mass Transport, May 8, 2008, Lund Sweden.



49

Swartman, R.K., Ogunalade, O., 1967. Solar radiation estimates from common parameters,Solar Energy, 11: 170-172

Trabea, A.A., Shaltout, M.A.M, 2000. Correlation of global solar radiation withmeteorological parameters over Egypt, Renewable Energy, 21: 297-308

Twidell J. and Weir T., 2006. Renewable Energy Resources, Taylor & Francis, London and New York

Webb J. 1995. By the light of the Sun, New sci. J ., 121, 40



50

4.0 WIND ENERGY

4.1 Introduction

Wind is created by the movement of rushing air. When land air that has been sun heated rises

it leaves a space to be filled by cooler surrounding air. The filling takes place immediately the

space is created in a way that could be described and felt as a rush. This fast movement of air

is called wind. It is reported that one primary forcing function causing surface winds from the

poles towards the equator is convective circulation. In this, solar radiation heats the air near

the equator and the low density heated air rises to the surface where it is displaced by cooler,

denser and higher pressure air. At the upper atmosphere near the equator, the air tends to flow

back towards the pole. This movement of air from the pole to the equator and back to the pole

in a circular global convective travel of air gives rise to wind and its effects round the globe.

Another phenomenon causing wind is the temperature differential between land and

sea or water bodies. The absorptivities and thermal time constant of land and water are

different. During sunny periods or in the day the rate at which the temperature of the land

builds up is higher than that of the water bodies or sea. Therefore, air closer to the ground

tends to be heated up by conduction, radiation and convection. The heated air then rises and

travels to cooler areas of water bodies and sea. Cooler surface air from water and sea

environment also travels to fill the space left by the hot air. This circle of air movement

reverses during the night times. At night, the land losses its heat faster than water or sea, thus

hot air rises from the water sea side to the cooler land area.

Apart from the mentioned causes of wind flow, another phenomenon of wind

movement is caused by the boundary layer frictional effects between the wind (or moving air)

and the roughness of the earth’s surface. The presence of buildings, trees, mountains, and

other obstructions hinder the free movement of air stream. This creates mechanical

turbulence around the obstruction and the wind speed in a horizontal direction increases with



51

height near the surface. Also, the higher the altitude the lower the drag effect on moving

winds. Thus wind speeds at higher heights are always more than those at lower heights.

4.2 Wind power

Wind power (or energy) can be described as the means by which the mechanical energy in

the air movement is employed to produce electricity or do meaningful work. Wind power as

an energy source is a green energy source. It is a non-depleting, naturally available, non-toxic

and environmentally friendly source of valuable and usable energy. Moreover, wind energy

has historically been used directly to power ships, for pumping water or grinding grain, but

the principal application of wind power today is the generation of electricity. Large scale

wind farms are typically connected to the local electric power transmission network, with

smaller turbines being used to provide electricity to isolated locations. Transforming wind

power to electricity is very common around the world. Within the year 2006 alone, wind

turbines were used to generate up to a total of 26.6 billion kWh per year of electricity in the

United States of America, representing about 0.4 percent of the nation’s total e lectricity

generation and also corresponding to about 65.3 percent of the sum total of Nigeria’s

generation in year 2003 and 2004 put together.

The technology of wind power to electricity conversion works with a windmill (adevice used to harness wind power) or wind turbines (or wind generator). With the windmill,

the blades pick up the mechanical energy from the moving wind, turns a drive shaft

connected to an electric generator which in turn produce electricity. The wind turbines (Figs.

4.1 and 4.2) on the other hand, directly convert the kinetic energy of the wind into electrical

energy. The production of electricity from wind on a large scale involves creating wind

farms. Wind farms are areas of land, usually large flat area of land, containing clusters of



52

dozens of wind machines used for the production of electricity. The location of these farms

depends on factors ranging from availability of wind, wind speed, to absence or presence of

windbreaks. Moreover, the availability and speeds of wind varies throughout a country and

from season to season. Areas located in between deserts and large water bodies, like sea or

oceans, enjoy rich supply of wind during summer. This is because the heating of the deserts

by the sun makes hot air rise and cooler air from the surroundings of the large water bodies

moves to fill the space created by the rising hot air. The movement of these land and

sea/ocean breeze generates enormous wind power and can be used to do useful work. The

magnitude of wind power available in a place can be determined from the knowledge of the

mean wind speed of the place. Consequently, when a particular site appears promising for

wind farm development, detailed site specific measurements are carried out through erecting

meteorological masts, up to about 30 to 50 m in height depending on the terrain, for

measuring wind speed and wind direction at different heights. Typical hub heights for wind

turbines are 60 m and it is projected to reach about 100 m by year 2030 (FMPS, 2006).

Moreover, actual measurements are needed because the power output of a wind farm is

sensitive to wind speeds. Therefore wind speed can determine the viability or otherwise of a

wind farm project, while detailed and reliable information about variation in wind speeds and

direction over the year is vital for any prospective wind power development. Apart from the

wind speed, the wind speed frequency distribution, commonly described by a Weibull

distribution is also important (FMPS, 2006).

In addition, wind power is proportional to the cube of the mean wind speed, meaning

that doubling the average wind speed leads to increase in wind power by a factor of eight. A

mathematical relationship that can be used for the computation is given as (Stiebler, 2008):



53

3 31

2 p P AV C KV (4.1)

Where:

P = Wind Power,

ρ = air density which is assumed for practical calculation to be approximately 1.2

kg/m3,

A = swept area of the turbine rotor = 2

rotor diameter 4

V = mean wind speed,

C p = Coefficient of power (which in reality, maximum values range between 0.4 – 0.5

due to losses that can be profile loss, tip loss or loss due to wake rotation).

Power of wind rotor

Power available in the wind pC

K = constant of proportionality and equal to

1

2 pC

Knowing the average wind power for a place can also lead to an understanding of the average

wind energy flux density over an entire area. The relationship between wind energy flux

density and average wind power of a place over a certain period (which most times is taken

per year) is given mathematically as:

21

4

max

v

v E P T e

(4.2)

Where:



54

E Wind energy flux density = wind energy per unit area

η = turbine efficiency (a term that is used to take into account the various losses in

bearings, couplings and gear boxes),

P = wind power flux,

Tmax = maximum time period (which for a year (365 days) in hours is 8760 hrs.),

V = mean wind speed over the place.

Wind resources are usually expressed in wind speeds (measured in meters per second (m/s))

from which energy units can be obtained.

Fig. 4.1: Diagrammatic view of wind turbine major components (Sterzinger and Svrcek, 2004).



55

4.3 Criteria for establishing wind farm

Before embarking on wind farm development, it is worthwhile to fist know where a certain

wind speed is possible and what would be the corresponding energy output from such speed.

A complete assessment study begins with site selection and preparation, followed by

installation of wind speed measuring equipment. In order to have adequate measurement and

statistically significant analysis, wind speed measurements covering some years (or at least a

year) are always required. This is done in order to capture the associated fluctuations of wind

speeds across a location. Fig. 4.3 presents a flowchart showing the steps to complete resource

assessment and decision making. The analysis and modelling stage is critical as it exposes the

site’s potential and determines the degree of viability of wind-to-power project at the site.

The analysis of the wind speed over the area is of utmost importance because the

performance of a wind energy conversion system depends of the magnitude of the average

wind speed. Although daily wind speed values are highly variable over a period because of

the inherent fluctuations associated with it, wind speeds are however homogenous over a long

period of months or years. Therefore monthly average wind speeds are very useful in

analysing the suitability of a site for utilization of a wind energy conversion system (WECS).

Fig. 4.2: Diagrammatic representation of the internal component of a nacelle

(Sterzinger and Svrcek, 2004).



56

Three wind speeds are important to the design of WECS. These are: cut-in wind

speed, cut-out wind speed and the rated wind speed.

Cut-in wind speed : This is the wind speed below which the WECS will not operate.

Cut-out wind speed : This is the wind speed above which the WECS may suffer damage if

operated. It also called furling wind speed.

Rated wind speed : This is the speed for which the WECS is designed to produce maximum

power.

Based on the aforementioned, wind speeds closer to the rated wind speed are always

desirable at a site. However, the range of wind speeds for active WECS (e.g. wind turbine)

are those between cut-in and cut-out speeds. Just as important is the determination of the

wind profile characteristic prevalent in an area or site is, so also is the knowledge of using the

site’s wind speed range for appropriate turbine selection. In mak ing a decision over a site’s

suitability for wind farm development, the site’s wind data and analysis are considered to

determine the economic viability or otherwise of the site’s potential. If the report is

favourable, then a decision can be reached and the site can be employed for a wind farm

development. Wind speeds from 3.5 m/s and above are usually considered suitable for turbine

applications while those above 4 to 5 m/s are very good for electricity production.



57

Fig. 4.3: Step by step procedure for carrying out complete resource assessment study

4.4 Design parameters for wind energy conversion systems

A wind turbine is a rotating machine which converts the kinetic energy in the wind into

mechanical energy. If however, the mechanical energy is converted to electricity, the

machine is called a Wind Generator, or Wind Energy Converter (WEC), commonly referred

to as Wind Energy Conversion Systems (WECS). There are various design parameters which

are very necessary for proper consideration. These parameters are evaluated using different

models as stated below.

Tip-Speed Ratio (TSR) : This is an important parameter of WECS. It is the ratio of the

circumferential velocity of the blade tips to the wind speed, given by (Stiebler, 2008):

(4.3 (a))

Where:

Site selection

Site preparation

Installation of wind speedmeasuring instrument

Data collection and storage

Analysis and modelling

Results evaluation

Simulation with practical wind turbine

Decision Making



58

λ = TSR

u = blade speed (m/s),

ν = wind speed (m/s).

The circumferential speed of the blade tip is expressed in terms of angular speed (ω) (or

number of revolution per minute (N) of the rotating turbine) and related to u as (Çetin, 2005):

(4.3 (b))

Making Eq. ii into Eq. i give:

(4.3)

It has been shown empirically that the optimum TSR for maximum power output, depending

on the number of blades, occurs at (Ragheb, 2009):

(4.4)

Where:

r = Radius of the rotating turbine

D = Diameter of rotating turbine

n = number of blades.

Wind Turbine Torque Coeff icient (C T ) : This is derived from the power coefficient (C p) as;

(4.5)

The relationship between CP and λ, and also CT and λ are given by Figs 4.3 and 4.4 below.



59

Fig. 4.3: Curve showing the relationship between CP and λ for a 3 blade rotor (Stiebler, 2008)

Fig. 4.4: Curve showing the relationship between CT and λ for a 3 blade rotor (Stiebler, 2008)

However, the torque of a turbine is expressed in terms of its torque coefficient as:

(4.6)

Eq.2.6 shows that turbine torque varies with the square of wind speed while the power from it

varies with the cube of the wind speed as shown earlier (Eq. (4.1)). Based on this equation

also, the magnitude of the torque in a turbine depends on the shape of the rotor blades. This

shape determines the swept area of the turbine rotors and would lead to the maximum



60

developable torque. Thus a relationship exists between the turbine torque, swept area, wind

speed and the power developed.

Tur bine Rotor Forces : According to Stiebel (2008), the main rotor properties follow from

the lifting force and drag force of a blade as described by aerofoil theory. With an aerofoil

element of width and depth given by w and d respectively, subjected to a wind speed v, and

depending on the angle of attack, α, between wind direction and blade profile cord, the lift

and drag forces (F1 and F2) are given as:

(Force is normal to oncoming wind flow) (4.7)

(Force is parallel to oncoming wind flow) (4.8)

C1 and C2 are characteristic for a given blade profile. Moreover, C1 and C2 are both functions

of α. The ratio is called the glide ratio or the lift to drag ratio. Observing

Eq. (2.8) reveals that the turbine force is proportional to the square of wind speed (just like

the torque) and directly to the swept area. This can generally be interpreted to be:

(From P F

v ) (4.9)

Relating Eq. (2.6) to Eq. (2.9) gives the relationship between the torque on a wind turbine and

the force developed as a result of the turbine rotation as:

(4.10)

Also the power (P) and drag force (S) can be related to turbine force (F) as:

(4.11)

(4.12)



61

Where:

Cs = the drag coefficient.

The relationship between the drag coefficient and the TSR is given by Fig. 2.5 below.

Fig. 2.5: Curve showing relationship between Cs and TSR (λ) (Stiebler, 2008)

The concept of the Betz Law

Consider Fig. 2.6 below

Fig. 2.6: Idealized fluid model for a wind rotor (Stiebler, 2008)



62

According to Stiebler (2008), when a homogenous flow of wind, with initial value before the

turbine blade plane v1, moves through an ideal turbine rotor, it suffers a retardation, due to

the conversion process of kinetic energy to power, to another speed v 3 (after the turbine).

According to Newton’s second law:

2

2 2 1 3. . P F v m v v Av v v

However, according to the principle of conservation of Energy, Power is given as:

2 2 2 2

1 3 2 1 3

1 1

2 2

E P v v Av v v

t m

(4.13 (a))

Equating the two:

1 3

22

v vv

(4.13 (b))

Thus the wind speed in the plane of the turbine rotor is given by the average of the wind

speed before and after the turbine

Therefore, substituting (b) into (a) gives:

2 2

1 31 31

2 2 P A v vv v

3 2

3 3 3

1 1 1

3

1

1

41

v v v P A

v v vv

(4.13 (c))

The maximum or minimum power extractable from the wind can then be determined by

differentiating P with respect to 3

1

v

v:

2

3 3

1 13

1

3

1

1

41 3 2

v vdP A

v vvd

v

v

Based on this, the maximum power can be said to occur at:

3

1

1

3

v

v (4.13 (d))



63

Based on this, Betz asserted that the maximum useful power occurs when the relation above

is satisfied.

Therefore, according to Betz, the maximum useful power is given from (c) and (d) as:

max

3

1

16 1

27 2 P Av

(4.13)

where:16

0.59327

Therefore going by relation of Eq. 2.13, Betz law states that the maximum extractable power

from the wind by an ideal turbine is not more than 59.3% of the power available in the wind.

This Betz limit is therefore referred to as the turbine’s maximum coefficient of performance

or maximum power coefficient.

Capacity Factor

The capacity Factor (CF) of a WECS is a measure of the electrical energy extracted from the

system as a fraction of the theoretical maximum available to it. It can be mathematically

expressed as the ratio of electrical energy generated from the WECS during a given period to

that which it would have produced had it been running continually at maximum output, i.e.:

EP

EI h

P CF

P N

where: PEP = Electricity generated during the period (kWh); PEI = WECS’s installed capacity

(kW) and Nh = Number of hours in the period.

Moreover, the variation in CF is basically due to differences in local wind speeds and

WECS’s design parameters, particularly the ratio of the rotor swept area to the capacity of the

WECS (DTI, 2001).



64

4.5 Classif ication of wind energy conversion systems

There are two broad classifications of wind energy conversion systems. The first is according

to the electrical output while the second is according to the machines’ axis of rotation relative

to the wind direction.

4.5.1 Classification according to Machine output

This classification is as presented in Table 4.1.

Table 4.1: Classification of WECS according to output

S/N Scale Capacity Utilization

1 Small Up to 2 kWUtilized for places needing relatively

low power amd for remote applications

2 Medium Above 2 but not more than 100 kW

Can be employed as stand alone powergenerating units, water pumping andother agricultural purposes requiring

3 Large Above 100 kW

For community wide electricity and grid

connected electricity

4.5.2 Classification according to Machine s’ axis of rotation

This can be either Horizontal axis machines or Vertical axis machines

4.5.2.1 Horizontal axis machines

In this type of machine, the axis of rotation is horizontal and the aeroturbine plane is

vertically facing the wind.

4.5.2.2 Vertical axis machines

These machines have their axis of rotation vertical

4.5.2.3 Differences between the Horizontal and Vertical axes machines



65

Of these two types of turbine machines mentioned above, the horizontal axis machines have

been found to be more efficient and capable of producing higher values of electrical energy.

It is the most employed of the two. However, the disadvantages of horizontal axis wind

turbine machines include their weight, required height and the inability to produce in

turbulent winds. Horizontal axis turbine is always heavier that the vertical type machine.

Vertical axis on the hand can produce from winds coming from all directions and also is

suitable in areas with inconsistent wind supply. In addition, vertical axis wind turbines can be

employed at low heights because of its ability to withstand turbulence.

2.3.3 Advantages of Using Wind Energy Conversion Systems

Wind energy is one of the lowest-priced renewable energy technologies available today,

costing between 4-6 cents per kW-hr. depending on the wind resource base and financing of

the particular project (ECN-UNDP, 2005). The construction time of wind energy technology

is also less than other energy technologies, it uses cost-free fuel, the operation and

maintenance cost is very low, and capacity addition can be in modular form, making it

adaptable to increasing demand. In addition, it has ability for generating high amount of

electricity to meet world consumption. It is a clean, easily accessible, naturally applicably,

enormously available, environmentally friendly, non-depleting and non-toxic source of

valuable and usable energy. Its installations are also very durable.

2.3.4 Dis advantages of Using Wind Energy Conversion Systems

(i) Noise emission: Like many other mechanical systems, wind energy systems are sources of

noise pollution. When in operation, aerodynamic noise of the blades prevails over other

components and such could be disturbing and frustrating. Residents around wind turbines



66

installations have complained in time past of the noise effects of the energy systems. Typical

causes of turbine noise could be due to the high tip speed ratio (Ragheb, 2009).

(ii) Oscillating shadow: The rotating turbine blades cast shadows on the surroundings,

thereby causing optical disturbance for residents around the installations. This has created

agitations that wind turbines impinge on clarity of vision for vehicle drivers on roads close to

wind installations.

(iii) Dangers to flying animals: It has been reported that several birds have been killed by the

rotating blades of wind turbines.

(iv) Land vibrations: During the operation of wind turbines and their installations, the

mechanical parts cause some vibratory movements to diffuse into the ground, most especially

when the turbine operates at high tip speed ratio (Ragheb, 2009). These vibratory motions are

felt differently depending on closeness to the system.

(v) Dangers to sea animals: Offshore wind energy systems have their blades located below

the surface of water where fishes find their easy life. Unknowingly these fishes stray into the

path of the blades and are killed.

Understanding the implications and agitations surrounding these disadvantages, various

countries have taken mitigating steps at reducing or eliminating the effects of operating wind

energy systems on the surrounding residents and animals. These steps have been enshrined

into legislations and standards to guide the practices of establishing wind farms.



67


Çetin, N.S., Yurdusev, M.A., Ata, R., and Özdemir, A, 2005. Assessment of optimum tipspeed ratio of wind turbines, Mathematical and Computational Applications, 10 (1), 147-154

Federal Ministry of Power and Steel (FMPS), 2006. Renewable Electricity ActionProgramme, International Center for Energy, Environment and Development (ICEED),Abuja, available online [http://w ww.iceednigeria.org/REAP-postconference.pdf, accessedJanuary 19, 2010]

Ragheb, M. 2009. Optimal rotor tip speed ratio, available on the web [https://netfiles.uiuc.edu/mragheb/www/NPRE%20475%20Wind%20Power%20Systems/Optimal%20Rotor%20Tip%20Speed%20Ratio.pdf, accessed November 15, 2010]

Sterzinger, G and Svrcek, M. 2004. Wind turbine development: Location of manufacturingactivity, Renewable energy policy project, Technical report, 13-14, available on web

[http://www.repp.org/articles/static/1/binaries/WindLocator.pdf, accessed 25 May 2009]

Stiebler M. Wind energy systems for electric power generation, Green Energy andTechnology, Berlin: Springer; 2008




68

5.0 GEOTHERMAL ENERGY

5.1 Introduction

Geothermal energy can be explained to be the thermal energy stored in the sub-surface of the

earth (Imperial College, 2003). It is a non-conventional source of energy, naturally occurring

and capable of direct exploitation and application for gainful purposes. In actual sense, the

word ‘geothermal’ is coined from the Greek words geo meaning earth and thermal meaning

heat (Ghosh and Prelas, 2011). Thus geothermal energy can be directly described as the

energy derived from the earth’s heat content. All of the heat stored in the earth’s crust above

15oC to a depth of 10 km is termed geothermal (Rai, 2004). It has various applications which

include for space heating, building heating, water heating, industrial and agricultural

processes, heat pumps and electricity generation.

According to Ghost and Pretas (2003), the earth crust is extremely hot with a temperature of

about 4200

o

C. This heat transports through cracks or faults in the crust to the surface (Rai,

2004). In some cases, hot molten magmas which are commonly present at depths greater than

24 to 40 km travels towards the surface due to some geologic conditions which may lead to

volcanic eruptions. The rate at which heat flows to the surface from the earth is estimated to

be about 0.063 W/m2 with the total outflow amounting to about 32 x 1022 W (Imperial

College, 2003).

5.2 Geothermal energy recovery and utilization

The earth is described according to Imperial College (2003) as a giant furnace. Within the

earth crust both radioactive materials and radioactive decay processes of potassium, uranium

and thorium generate heat. Although there is no accurate estimate of the amount of



69

geothermal energy resources available around the world, its amount within the earth is

enormous. Despite this availability, the resources have been widely untapped. Few nations

have been reported to assess and utilize geothermal energy. With the presence of several hot

spots scattered around the world, the potential of geothermal energy as a viable source of

useful energy cannot be overemphasized. For instance, in Nigeria the hot springs of Erin

Ijesha in Osun State may be a pointer to the fact that there may be available potential for

geothermal energy in some places in the country. The mountainous terrains of areas closer to

the volcanic belts of Cameroon may also be significant for geothermal energy in Nigeria.

Sources of geothermal resources include dry and wet steams, hot brine, hot rock and molten

magma. These are explained below (Imperial College, 2003).

Dry steam : Dry steams are mostly obtained from vapour-dominated geothermal systems or

fields (Rai, 2004). The steams are dry saturated and superheated and produced from dry

geothermal fields at very high pressures. The dry steam occurs as a result of the heating effect

of hot rocks on ground water. This water gets heated at very high temperature to steam. The

steam therefore moves upward and reaches the surface at about 200oC and 8 Bar. Dry steams

are located at very deep drilling ranges and are very scarce. They are however the easiest and

most economical to tap because they have limited corrosion problems. The corrosiveness is

due to the presence of corrosive gases and erosive material (Imperial College, 2003; Rai,

2004).

Wet steam : Wet steam unlike the dry steam is mostly obtained from liquid-dominated

geothermal systems or fields (Rai, 2004). In this, the geothermal fields are wet and produce

hot pressured water at temperatures which range within 175 to 315oC (Rai, 2004). The hot

water often contains impurities such as sulphur compounds or gases. Moreover, when the

fluid is tapped, it flows to the surface, and by so doing there is a drop in pressure. The



70

reduction in pressure however causes about 10 to 20 % of it to flash into steam. Wet steam

geothermal sources are commoner than the vapour-dominated geothermal fields (Imperial

College, 2003; Rai, 2004).

Hot brine : Hot brines are very hot salt solutions. They occur in large deep sedimentary basins

containing moderately high temperature solution (about 160oC) under very high pressure

(about 1000 Bar). Fields containing hot brines are referred to as geopressured systems (or

fields). Geopressured brines are known to also contain, in dissolved form, large amounts of

methane (a natural gas) at temperature below 180oC. The gas is released when the pressure is

reduced. Geopressured brines are tapped from very deep underground acquifers at depth

ranging from about 2400 to 9000 m (Imperial College, 2003; Rai, 2004).

Hot rocks : These are also referred to as petrothermal systems. These are heated rocks at very

high temperatures. They occur at moderate depths below the ground to which water does not

have access, either because of the absence of ground water or due to the low or lack of

permeability of the rock. Based on this, the heat from the rock can be indirectly tapped by

drilling the rock and introducing cold water into it. The direct heat transfer between the rock

and water, heats up the water. The hot water (or steam) is then recovered to the surface and is

used. The temperature of the hot rocks varies from 150 - 290oC (Rai, 2004). Majority of the

potential geothermal explorable heat is stored in dry rocks. Globally, the normal temperature

range is between 20 and 30oC per kilometre. This temperature gradient is sufficient to

produce temperature of 80oC for space heating at depths of 2.2 km and temperature of 180

oC

at the accessible depth of 3.5 km (Imperial College, 2003).

Molten M agma : It is sometimes called molten lava. It is the extreme case of hot rock and has

temperatures higher than 650oC. It is found either at moderate depths, especially in recently



71

active volcanoes or volcanic regions or at depths below the volcanoes. Their very high

temperature is an impinging factor against its extraction.

5.2.1 Geothermal energy recovery

Worthy of note first and foremost is that, the total stored geothermal energy exceeds those of

nuclear resources and total fossil fuels. There are basic steps to follow in order to explore,

exploit and recover geothermal energy from its source. To know a potential geothermal field

may be through the simple process of observation. Fields where hot water or steam comes out

of the ground are potential geothermal fields. However, because of the high cost of

geothermal drilling, it is required to ascertain the energy potential of a reservoir before

drilling is undertaken (Rai, 2004).

According to Ghosh and Prelas (2011), the identification and quatification of geothermal

resources require geologic, hydrologic, geochemical and geophysical techniques that allow

gathering of information regarding the potential use of specific sites. This information is

required in order to first determine the suitability of the sites for geothermal energy recovery.

A preliminary surface indication of a geothermal resources in a site is given by fumaroles (or

steam vent), volcanoes and solfataras (vent in volcanoes) and geysers (or springs of hot water

or steam). Sometimes, the presence of a preliminary indicator is not enough to ascertain the

reservoir capacity. Important questions to be answerd for a potential geothermal field to be

ascertained as suitable include (Lumb, 1981; Ghosh and Prelas 2011):

1. Can geothermal phenomena be identified at the field/site?

2. Is the field useful for geothermal energy production?

3. Can production well be drilled at the site with the highest possibility of tapping into

the resources?

4.

What is the shape, size and depth of the field/site?



72

5. What is the classification of geothermal field?

6. What are the production zones?

7.

What is the heat content of fluids that will be discharged by the geothermal wells?

8.

Are there characteristics that might cause problems during field development?

9. What is the source of the recharge water?

10. What is the homogeneity of the water supply?

11. What is the characteristic of the geothermal system (i.e. is it water or vapour-

dominated)?

The methods of providing information regarding the geothermal reservoir characteristics

include heat flow rate determination, chemical compositional analysis of the surface or

ground water, electrical resistivity measurement and seismic measurement (Rai, 2004).

Heat fl ow rate in the ground : This is measured in bore holes to a depth of about 100 m or

more. It is commonly expressed in Heat Flow Units (HFU). 1 HFU is equivalent to 0.0418

W/m2. Values greater than 1 HFU, indicates potential geothermal field/site.

Chemical compositional analysis of the surface or ground water : The chemical

compositional analysis to determine the mineral salts present in the ground water may lead to

information relating to identifying the type of geothermal reservoirs. Also the analysis of hot

well or spring water can indicate the possible water origin and the reservoir temperature.

Electri cal resistivi ty measurement of the ground : The electrical resistivity usually depends

on the salinity and temperature of the ground water and the porosity of the rocks. A low value

may indicate the presence of hot and/or saline water. Hot water containing substantial

amounts of dissolved mineral salts is often present in wet geothermal reservoirs. The

measurements are made at a surface depth at many points and also at varying depths in a

particular location on site to be selected for geothermal energy recovery.



73

Seismic measurement : This could be passive or active. Passive seismic measurement

technique involves those associated with minor earthquakes or microseismic activities. It can

be detected by a seismometer. Active seismic measurements are similar to those done for oil

exploration where a shock wave is generated in the ground by means of explosives or impact

with heavy mass. The corresponding signals are then detected at a distance to the impact

location. This provides information regarding the underground structure.

Once a geothermal well/reservoir is established, the heat recovery from the well/reservoir is

recovered by drilling. The commonest drilling method is rotary drilling via rotary bits and

similar to that used for oil and gas. The difference is in the temperature. Geothermal wells

have higher temperatures (up to 350oC) than oil and gas wells.

5.2.2 Geothermal energy utilization

The application of geothermal energy varies but it is strongly dependent on the temperature

of the resources. For instance, for electricity generation, the temperature of the resources

must be greater than 150oC. Below 150oC, the resources can be employed for different

applications which include drying of agriculture products, space heating in buildings,

refrigeration (low temperature limit), soil warming, fish farming (e.g. hatching) etc. Apart

from its use for electricity at temperature above 150

o

C, it can also be employed for

evaporation of highly concentrated solutions, in the industry for some chemical processing,

timber drying etc. Some of the various uses of geothermal energy at particular temperature is

illustrated with Fig. 5.1.



74

Fig. 5.1: Some uses of geothermal energy at different temperature levels (Ghosh and Prelas 2011)

5.3 Environmental consequences of geothermal energy exploitation

Unlike other non-conventional sources of energy like wind and solar, the processes leading to

recovery of geothermal energy resources are not entirely clean. They release toxic gases to

the environment and cause environmental pollution. For instance:

1. Steam and hot water from hydrothermal systems contain dissolved solids in the water,

trapped solid particles as well as noncondensable gases. The gases are mostly carbon dioxide



75

(CO2) with varying amount of methane (NH4), hydrogen, ammonia (NH3) , hydrogen

sulphide (H2S) and nitrogen. CO2 and NH4 are greenhouse gases that have direct effect on

the atmosphere. H2S forms corrosive acid (or acid rain) in the atmosphere with the right

conditions. Acid rain has corrosive tendencies on buildings, roofing materials and paints and

also damages crops, forest, streams, lakes and rivers. Burning H2S can lead to the formation

of SO2. Humans exposed to large volumes of methane, NO x, and SOx could develop

respiratory infections. In addition to this, excess H2S in geothermal fields cause harmful

effects on the bearings of drilling machines and attacks the electrical equipment because of

the corrosive tendencies of its moist state.

2. Land erosion is another associated environmental effect of geothermal energy processing.

As a result of the large amount of water involved in exploitation of geothermal resources,

land erosion can result.

3. Noise pollution is another associated environmental problem. The noise emanates from

exhausts, blow-downs, and processes of centrifugal separation.

4. Contaminated water: Sometimes, the water from wet geothermal fields contains toxic

substances such as mercury, arsenic, ammonia etc. If this water is discharge into land water

or sea, it could be injurious both to human and aquatic organisms.

5. Seismicity or earthquakes: Fears have been expressed that continuous exploitation of

geothermal fields could lead to imbalance of the earth crust and could cause earthquakes in

the future. This is especially true if the practice takes place in zones of high shear stress

where fairly large temperature differentials occur.

5.4 Advantages and Disadvantages of geothermal energy



5.4.1 Some advantages

1.

It is cheaper than fossil based fuels

2.

It delivers more energy than other energy modules

3. It has vast applications

4.

It is the least polluting compared to other conventional sources

5. It is consistent and available throughout the year

5.4.2 Some Disadvantages

1.

Overall production efficiency is low

2. The steam and hot water coming out from the ground may contain impurities and

some gases that pollute the atmosphere

3. Noise from drilling


compiled note

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