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USE OF THERMAL SOLAR ENERGY IN DISTRIBUTED SMALL-SCALE

PLANTS FOR WATER DESALINATION

Joan Carles Bruno, Alberto Coronas

Universitat Rovira i Virgili, Dept. Enginyeria Mecànica,

Avda. Països Catalans, 26, 43007 Tarragona, Spain

Fax: +34 977559691, E-mail: juancarlos.bruno@urv.cat

ABSTRACT

During recent years there has been a renewed interest for the development of efficient renewable energy driven desalination systems for distributed small-scale production that would supply remote and small communities. In this paper is presented a short review of these technologies driven by thermal solar energy including fundamentals, technical performance data and economics. Also in the second part of the paper are described the recent activities in this field of the Applied Thermal Engineering research group (CREVER) at the Universitat Rovira i Virgili (Tarragona, Spain).

INTRODUCTION AND OBJECTIVES

Seawater represents 97.5% of all water’s planet and

is an attractive resource to obtain the desired

freshwater. Sea water consists of around 70 elements,

where six of them are almost 99% of the total contribution. Salinity compositions of seas and oceans

and gulfs range between 28000 and 36000 ppm

although it is higher for almost closed seas

or gulfs (El-Dessouky and Ettouney, 2002).

Increment of desalination plants installed

in a short time, indicates that the need to access

to water is growing and their desalination cost

is reduced. The world desalination installed capacity

has increased from a bit more than

10 million cubic meter per day in 1986 to more than 42

million in 2006. Only in Spain the capacity

of newly commissioned plants in 2006 was higher than 400 000 m3/day (IDA Desalination, 2007) and the total

desalination capacity is higher than 2·106 m3/day.

Reverse Osmosis (RO) process is mostly used.

Nowadays, the specific main energy consumption of

RO with an efficient energy recovery is about 3

kWh/m3. It results in an annual electricity consumption

of 2190 GWh, mainly generated by fossil fuels with the

consequent environmental damage. Therefore, the

introduction of renewable energy-driven desalination

would be an environmental-friendly solution for the

problem of fresh water shortage. In addition, renewable energy-powered desalination is frequently the only

possibility of a secure fresh water supply in developing

countries, in rural communities where no electricity

grid is available.

Adoption of solar desalination can be made for

either small-scale distributed units, which are suitable

for remote and small communities, or for larger size

plants, which can be integrated in the national energy

supply system. On the other hand, new small-scale

desalination units are being made available in a limited

number or are under development and are more

expensive than conventional large scale desalination

units (Ettouney and Rizzuti, 2007). The objective of this paper is to present a short

review of desalination technologies suitable

to be integrated into thermal solar energy systems to

produce fresh water and also report on the research

activities in this field of the research group on Applied

Thermal Engineering (CREVER) at the University

Rovira i Virgili. Only solar thermal energy system will

be covered although other renewable energy

technologies exist such as the solar PhotoVoltaic (PV)-

driven RO desalination systems which normally use

batteries as energy storage. The main drawbacks of

such systems are low performance and very high costs of PV cells and problems of batteries – i.e. high

maintenance and replacement as well as toxic wastes –

or also wind energy driven systems.

FUNDAMENTALS OF WATER DESALINATION

The term desalination refers to the process

of withdrawing the solvent water from saline water

obtaining almost pure water with a very low content of

dissolved salts and brine with a high concentration of solutes as is illustrated Figure 1.

This separation process produces two streams, fresh

water and a saline solution (brine) with some required

energy input. Saline water is usually classified as

brackish water when the salt concentration, mostly

sodium chloride, is less than 10000 ppm and sea water

when the salinity exceeds about 30000 ppm.

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Fig. 1. Schematic diagram of a desalination process

Seawater desalination can be achieved using

different technologies and also the required energy can

come from fossil conventional, waste heat or renewable energy sources. Using desalination

processes freshwater can be obtained from oceans with

the possibility to apply renewable energy resources in

them (Kalogirou, 2005, Mathioulakis, 2007).

There are to date no generally agreed-on norms

or standards to assess the performance of solar

desalination systems. The parameters usually used to

the rating of thermal desalination processes

performance are the gained output ratio (GOR) and the

performance ratio (PR). In general, if thermal energy

drives the desalination process, two parameters are normally used in order to measure the process

efficiency:

· GOR (Gain Output Ratio), defined as the rate

between kg of external steam supply and kg of

product obtained.

· PR (Performance Ratio), defined as kg of product

obtained per 2300 kJ, that is, approximately, the

latent heat at temperatures normally used in

distillation processes.

In spite of this Burgess and Lovegrove (2005) and

Banat et al, (2007) have surveyed and described the performance metrics and units found in the desalination

literature for the report of desalination performance and

pointed out the that a lack

of uniformity in units and terminology that

is apparent in the desalination literature.

There are many classifications for desalination

methods according to the type of energy consumed:

electrical, mechanical or thermal or the kind

of separation process applied to separate salts from

water. Desalination technologies can be classified

according to several criteria:

Type of energy source used:

· Thermally driven technologies: Multi-Effect

Distillation, Multi-Stage Flash, …

· Electricity driven technologies: Reverse Osmosis,

Electrodialysis, …

Size of the application:

· Conventional large scale units: Multi-Effect

Distillation, Multi-Stage Flash, …

· Medium-Small scale units: Membrane

Distillation, Humidification Dehumidification, …

· Ship on-board units: single-effect evaporation

systems For a distributed renewable energy driven

desalination plant a classification according

to its size could be: small plants (1-50 m3/day),

medium (50 – 250 m3/day) and large plants (> 250

m3/day). Size of conventional plants is in the order of

several thousands of m3/day.

It should be mentioned that thermal processes are

not sensitive to the type of feed water with regard

to salinity and contamination, because the product

water is the condensed water vapour, leaving all the

impurities in the liquid phase. This is not the case of membrane desalination processes which

do not involve phase change.

DESALINATION TECHNOLOGIES DRIVEN BY

THERMAL ENERGY

In this section are briefly described all the

desalination technologies that could be driven

by thermal energy both conventional and small-scale

distributed technologies and also those technologies such as reverse osmosis that are driven by mechanical

energy which could be generated

by conversion from thermal energy and are suitable for

small-scale applications.

Depending on the type of desalination technology the

right type of thermal solar collector has

to be selected. High temperature concentrating

collectors, such as linear Fresnel collectors, producing

both electricity, and eventually heat through a

cogeneration configuration, could feed all types of

processes and hybrid systems also.

Multi-Stage Flash (MSF)

MSF process is based on the generation of vapour

from seawater or brine due to a sudden pressure

reduction when hot seawater enters an evacuated

chamber. The process is repeated stage by stage

at successively decreasing pressure. The number

of stages varies from 15-25 with a maximum of 40.

This process requires an external steam supply,

normally at temperature around 100ºC.

Brine

Energy

Fresh water Sea water Desalination

process

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The performance ratio is about 7-8 with

a maximum of 12. The maximum temperature

is limited to avoid scaling. The required thermal and electrical consumption is about 50-52 kWh/m3 and 4-6

kWh/m3, respectively.

Multi-Effect Distillation (MED) In the MED process, the steam generated in one

effect is transferred to the next effect to heat the salt

solution because the next effect is at lower temperature

and pressure. The performance of the process is

proportional to the number of effects which is usually

between 8 and 16. MED plants are commonly driven

by saturated steam at 70ºC. This technology is provided by several manufacturers: IDE Technologies

Ltd, Entropie, Alfa Laval, etc. A modified MED

process includes a Mechanical or Thermal Vapour

Compression (MVC, TVC) of the initial vapour

generated from the saline solution to generate

additional production. The consumption of thermal and

electric energy is around 60 kWh/m3 and 2 kWh/m3,

respectively. The integration of an absorption heat

pump has been proposed to increase the system

efficiency (Alarcón-Padilla et al, 2007)). The typical

PR is about 9-10 and can increase to 25 using a heat recovery system such as a heat pump. In another

of the proposed arrangements to drive a MED plant

with solar energy the required low pressure steam

could be obtained by the steam at the outlet

of a steam turbine driven by the steam directly

generated in concentration solar collectors

or by means of thermal oil as intermediate fluid.

Reverse Osmosis (RO) RO requires electricity or shaft power to drive

a pump that increases the pressure of the saline solution

to that required which depend on the salt concentration. It is normally around 70 bar for seawater desalination.

Electric energy consumption ranges from 3 kWh/m3 for

the highest capacity and efficiency units to 5 kWh/m3

for the smallest and less efficient units, when energy

recovery is used. Without energy recovery the

electrical consumption is about 10 kWh/m3 and may

exceed 15 kWh/m3 for small units. The ratio between the flow rate of production and intake raw water

(recovery ratio) for sea water is about 30-60%. With

regard to selection, RO requires skilled workers and

availability of chemical and membranes supplies. Also

fluctuations of the available energy would damage the

system so intermediate energy storage is required.

Electrodialysis (ED) ED is suitable for brackish water. The electrical

consumption can vary from 0.5 to 10 kWh/m3

depending on water salinity. In remote areas, ED is most suitable than RO for brackish water

because it is more robust and its operation and

maintenance are simpler and able to adapt

to changes of available energy input.

Solar Stills A conventional solar still has simple geometry. The

still is formed of a square or rectangular box, which is

equipped with a sloped glass cover.

The top cover of the box must be made

of transparent glass to allow for the passage of solar energy. The desalination mechanism is similar

to that of nature. A shallow pool of brackish

or seawater absorbs solar energy and as a result

a vapour of fresh water is formed in the space above

the water. The vapour condenses on the inside of the

glass cover and is then collected

in a side through. Figure 2 shows a schematic of the

most basic form of solar stills. A simple solar still has a

low production rate of 1 l/m2/d. However, some

modifications in the design can increase the production

rate significantly (fourfold) for example using vertical

solar stills, use of multi-effect stills or combining it with thermal solar collectors. However,

the application of this technology

is mainly limited to domestic uses.

Brine

Water Vapor

Solar Radiation

Distillate

Collection

Condensation

Distillate

Collection

Fig. 2. Schematic of a simple solar still

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Humidification Dehumidification (HDH) Conventional HDH process is formed of three main

components (Fig. 3). These are the humidifier, where the intake air humidity is increased

to saturation conditions, the condenser, where the

humidified air is cooled to condense the product water,

and the feed seawater heater. The intake seawater is

preheated in the dehumidification unit and further

heated in the feed heater to achieve the desired design

conditions with the aid of an external low grade energy

source such as solar collectors.

Multi-effect humidification-dehumidification (MEH)

systems are based on the same HDH principles but

offer higher efficiency. The energy demand is in the range of 120 kWh/m3. According to Müller-Holst

(2007) the MEH is competitive

in comparison with RO-PV for small scale applications

if electricity cost is at or higher than 0.15 €/kWh. This type of units has entered the commercialisation stage

(Tinox-Mage and Rewater system).

Fig. 3. Humidification dehumidification desalination process (Ettouney and Rizzuti, 2007)

Membrane distillation

Contrary to membranes for RO, which have

a pore diameter in the range of 0.1 to 3.5 nm, membranes for membrane distillation have a pore

diameter of about 0.2 mm. The separation effect

of these polymer membranes is based on their

hydrophobic nature. This means that up to a certain

limiting pressure, liquid water can not enter the pores

(Fig. 4). Molecular water in the form of steam can pass

through the membrane.

On one site of the membrane there is salt water, for

example at 80ºC. if there is a lower temperature on the other side of the membrane, created for example by a

condenser coil to 75ºC, then there exists a partial pressure difference for water vapour across the membrane. The water vapour condenses on the low-

temperature side and distillate is formed.

Membrane distillation technology for desalination

is under development by several European research

centres and companies (Fraunhofer Institute,

TNO - Memstill and Scarab).

RESEARCH ON COMBINED THERMAL

SOLAR ENERGY AND DESALINATION

In this section are described two research projects

related with desalination technologies using solar

energy and waste heat at the research group Crever in

the Univ. Rovira I Virgili.

Solar ORC systems for reverse osmosis

Description

For small capacity desalination systems RO

is so far one of the most efficient desalination

alternatives taking care of all the mentioned

limitations. Their combination with photovoltaic solar

collectors is limited primarily by their cost and the

need to use batteries for energy storage.

A possible solution could be to take advantage

of the combination of thermal solar energy and Organic

Rankine Cycles (ORC) to drive the RO compressor as

proposed and studied by some authors (Bruno et al, 2008 and Manolakos et al, 2007).

An Organic Rankine Cycle (ORC) is a

thermodynamic Rankine or vapour power cycle that

uses an organic working fluid instead of water and is

known to yield excellent performance at temperatures

up to about 350ºC and especially at low power capacities. The research and development of the ORC

technology has been focused in the production of

electricity, mainly related to recovery of low

temperature waste heat, geothermal heat, biomass, or

solar energy. Many references to these applications are available in the literature. However, the development

of solar ORC for desalination is very limited.

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Fig. 4. Principle of membrane Distillation (Banata et al, 2007)

In the proposed system (Fig. 5) the working fluid is

heated to boiling, and the expanding vapour

is used to drive a turbine and more generally any

expander. This expander provides all the energy

required to drive the high-pressure RO pump and the high pressure ORC and solar plant circulation pumps.

The energy could be transferred directly

as shaft power in a direct coupling

or by intermediate conversion to AC electricity.

Power cycle alternatives

Different technologies can be used to expand the

fluid to produce mechanical energy: positive

displacement systems (reciprocating and rotary),

turbomachinery (turbines) and even specific designs. Different selection criteria can be applied. According to

the size of the ORC system, for large capacity systems

the choice should be a turbine. For low capacity

systems the high rotating speed

Fig. 5. Diagram of the proposed solar desalination ORC system (Bruno et al, 2008)

of turbines make them not useful for direct drive of

machinery requiring some kind of special gearbox and

also a special high speed electric generator and power

converter is required for electric generation similar to

the ones used for micro gas turbines. For medium

capacity systems the most suitable technology is a

screw expander and for smaller systems, just a few kW,

the preferred option could be the scroll expander. One

of the most extended ideas in the field of expander

development is to adapt mass-produced (cheap)

displacement compressors for use as reasonably

efficient expanders. Scroll expanders are operated at

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lower speed than the other technologies

so it is suitable for a direct mechanical coupling

between the expander and the high pressure pump of the RO subsystem. The drawback is that although the

operation in reverse of a scroll compressor is feasible it

will not work at its optimal conditions working as an

expander (Huff and Radermacher, 2003). Screw-type

expanders can

be more easily adapted from compressors.

Modelling and results

In Bruno et al 2008 the ORC system was modelled

using the Aspen Plus process simulator, with the

required inputs from other programmes to model Reverse Osmosis (ROSA, 2006) and thermal

solar plants. The first part of this paper analyses a

comprehensive list of candidate working fluids for the

ORC desalination application, and

a selection is made of the most interesting fluids

according to the type of solar collectors used

in each case. For example, in the case of compact

Parabolic Trough collectors the isopentane working

fluid was selected for the ORC cycle. The optimal

operating temperature was calculated for the solar ORC

integrated with the RO plant that optimises the global ORC-thermal solar plant efficiency.

In the second part of the paper were examined two case

studies one from Almería and another one

in Barcelona that can be considered representative of two different levels of solar radiation characteristic

of the Mediterranean area of Spain.

In these case studies the area of the solar field

collectors was calculated, considering both brackish

and sea water desalting applications for a handling

capacity of 15 m3/day modelling the solar and

desalination system using the TRNSYS software.

An economic estimation was also reported

comparing the presented results with those

of an equivalent Photovoltaic-RO plant. The technical-

economic results obtained for the two locations suggested the adequate thermal solar technology to be

that represented by the compact Parabolic Trough

Collector (PTC solitem) collector system (Table 1)

instead of the other solar collector options (FPC: Flat

Plate Collectors and ETC: Evacuated Tubes Collectors

or PTC Eurotrough: conventional Parabolic Trough

collectors).

The use of an equivalent photovoltaic system

to generate electricity to drive the RO desalination

system had a higher cost than the optimised solar ORC-

RO system specially when using the best solar thermal technology (Table 1).

Table 1. Specific annual cost in €/m3 of the Solar ORC desalination plant for both geographic locations (Bruno et

al, 2008)

Location Raw water ORC-RO-FPC ORC-RO-ETC ORC-RO-PTC

Compact

ORC-RO-PTC

EuroTrough RO-PV

Almería Brackish 2.86 2.36 2.03 2.30 3.80

Seawater 7.99 6.28 4.32 4.90 12.83

Barcelona Brackish 3.31 3.29 2.36 2.33 4.31

Seawater 9.54 7.87 5.50 4.66 14.85

Single-effect evaporation systems

Single-effect desalination systems are also an interesting option for solar desalination that could

be an alternative to solar Reverse Osmosis systems

driven by PV, wind energy or ORC plants. Although

single-effect systems are not very efficient systems, its

roughness, reliability and low cost make them worthy

to be studied and compared with these desalination

systems. Currently these units are used on-board ships

to supply potable water using waste heat. Their

capacity ranges from only 0.5 m3/day up to about 300

m3/day with

a required thermal energy supply of about 700 kWh/m3. At the Crever – URV was characterised

a single-effect evaporation desalination system

(Fontemar TCS-1) with a maximum capacity of less

than 2 m3/day (Fig. 6). A simplified diagram

is shown in Figure 2.

The desalination unit consists basically of an ejector and two chambers interconnected with a

demister (Fig. 7). In this unit, the evaporator tubes

containing the driving hot water are submerged

in a liquid pool. The liquid surrounding the submerged

tubes reaches saturation temperature for the specific

conditions of salinity and pressure and evaporation

proceeds. The formed vapour flows through a demister

that removes the entrained liquid droplets. The vapour

flows to the condenser, where it condenses on the

outside surface of the condenser tubes. As

condensation takes place, the latent heat of condensation preheats the feed liquid in the condenser

before entering the evaporator unit. The ejector reduces

absolute pressure of equipment and removes strong

salinity seawater (brine).

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Fig. 6. Single-effect evaporation system connected to the test bench

Fig. 7. Schematic diagram of the single-effect seawater desalination unit

As an example of the results obtained in figure 8 is

shown the production capacity of the desalination unit as a function of the input temperature to the system that could be represented by the outlet temperature from the solar collector field and also as a function of the sea water input temperature. For these results the hot water input flow rate was 1.26 kg/s.

CONCLUSIONS

Combination of thermal solar energy and desalination systems would generate a sustainable source of fresh water reducing air pollutant emissions and green house gases. Conventional thermal driven desalination technologies (MED and MSF plants) are only suitable for large size plants and consume steam at 70-100ºC that is not usually available in solar collectors working at these temperatures. However, small-scale desalination units for remote or small communities are already available in a limited number or under

development but are more expensive than conventional large scale desalination units. According to recent studies (Fiorenza et al, 2007) the water cost for a conventional desalination unit is in the order of 0.5-1 $/m3 while for a thermal solar driven desalination unit of 5000 m3/day is the double and it could be around 3 $/m3 for smaller capacities (500 m3/day).

Electricity driven desalination systems suitable for small-scale applications such as RO and ED could be driven by thermal solar energy when some kind of power generation conversion system is used. Other technologies such as Humidification and Dehumification or membrane distillation can use directly the thermal energy generated by the solar collectors for small-scale applications.

Up to now all the renewable energy driven desalination systems are experimental or demonstrations plants but a renewed interest is expected as long as the cost of solar energy becomes more competitive against fossil fuels costs.

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Fig. 8. Production of water as a function of the input hot water and sea water temperatures for the tested single-

effect evaporation unit

ACKNOWLEDGEMENTS

The authors would like to acknowledge the partial

funding provided for this research to Ministry of

Education and Science of Spain under the projects:

ENE2005-08381-C03-03 and PSE-370300-2006-5.

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