modern ways for concentrating solar power

7/27/2019 Modern Ways For Concentrating Solar Power

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Modern ways forconcentrating solarpower(CSP)

ABSTRACTThis essay is a summary of modern ways for concentrating

solar power, within this essay all types of CSP will bediscussed. Things like their main components, costs andperformance will be discussed. Finally, a little forecast andconclusions about it will be implied.

Advanced andalternativeenergysystems(302.064)

Eric Martnez Lara

1228149


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Eric Martinez Lara 2

302.064 - Advanced and alternativeenergy systems

Modern ways for concentrating solar

energy

Contents

Introduction .................................................................................................................................. 3

Different types of CSP (concentrating solar power) ..................................................................... 5

parabolic trough collector technology ...................................................................................... 5

linear fresnel plants ................................................................................................................. 10

solar dish systems ................................................................................................................... 13

power tower solar plants ........................................................................................................ 18

Thermal Storage Systems for Concentrating Solar Power ...................................................... 22

forecast and conclusions ............................................................................................................. 26

Figure list ..................................................................................................................................... 31

References list ............................................................................................................................. 32


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Introduction

The origin of this kind of power production comes from a legend, this legend explainthat Archimedes using a "burning glass" concentrated sunlight on the invading Romanfleet and repelled them from Syracuse. Then, in 1973 a Greek scientist, Dr. IoannisSakkas, which was curious about whether Archimedes could have destroyed theRoman fleet in 212 BC, put near of 60 Greek sailors holding oblong mirrors tipped tocatch the sun's rays and direct them at a target plywood silhouette 160 feet away. Thisship caught fire after few minutes.

Figure 1: Ioannis Sakkas experiment

On the other hand, in 1866, Auguste Mouchout used a parabolic trough to producesteam for the first solar steam engine. Then, the first patent for solar collector wasobtained by the Italian Alessandro Battaglia, in the same year. Over the followingyears, inventors such as John Ericsson and Frank Shuman developed concentratingsolar-powered devices for irrigation, refrigeration and locomotion.

Then, in 1968, Professor Giovanni Francia designed and built the first concentrated-solar plant. This plant had the architecture of today's concentrated-solar plants with asolar receiver in the center of a field of solar collectors. This plant was able to produce1 MW with a superheated steam of 100 bar and 500 degrees Celsius. The nextdevelopment came in 1981, in Southern California, it was a Solar One power tower of 10MW. But the parabolic-trough technology of the nearby Solar Energy GeneratingSystems, begun in 1984, which was more workable.

Concentrated solar power (also called concentrating solar power, concentrated solarthermal, and CSP) are systems which use mirrors or lenses to concentrate a large areaonto a small area. This concentrated light is converted to heat, which drives a heat


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engine (usually a steam turbine) connected to an electrical power generator, in orderto produce electrical power.

Figure 2: CSP example

The innovative aspect of CSP is that it captures and concentrates the suns energy toprovide the heat required to generate electricity, rather than using fossil fuels ornuclear reactions. Another attribute of CSP plants is that they can be equipped with aheat storage system in order to generate electricity even when the sky is cloudy or

after sunset. This significantly increases the CSP capacity factor compared with solarphotovoltaics and, more importantly, enables the production of dispatchableelectricity, which can facilitate both grid integration and economic competitiveness.CSP technologies therefore benefit from advances in solar concentrator and thermalstorage technologies, while other components of the CSP plants are based on rathermature technologies and cannot expect to see rapid cost reductions.


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Different types of CSP (concentrating solar power)

CSP plants can be divided into two different groups, based on whether the solarcollectors concentrate the sun rays along a focal line or on a single focal point (withmuch higher concentration factors). Line-focusing systems include parabolic trough and linear Fresnel plants and have single-axis tracking systems. Point-focusing systemsinclude solar dish systems and solar tower plants and include two-axis trackingsystems to concentrate the power of the sun.

The next step will be to intensely present, one by one, all systems that I havementioned above.

Line-focusing systems

parabolic trough collector technology

The parabolic trough collectors (PTC) consist of solar collectors (mirrors), heatreceivers and support structures. The parabolic-shaped mirrors are constructed byforming a sheet of reflective material into a parabolic shape that concentratesincoming sunlight onto a central receiver tube at the focal line of the collector. Thearrays of mirrors can be 100 metres (m) long or more, with the curved aperture of 5 mto 6 m. A single-axis tracking mechanism is used to orient both solar collectors andheat receivers toward the sun (A.T. Kearney and ESTELA, 2010). PTC are usually alignedNorth-South and track the sun as it moves from East to West to maximize the

collection of energy. The receiver comprises the absorber tube (usually metal) insidean evacuated glass envelope. The absorber tube is generally a coated stainless steeltube, with a spectrally selective coating that absorbs the solar (short wave) irradiationwell, but emits very little infrared (long wave) radiation. This helps to reduce heat loss.Evacuated glass tubes are used because they help to reduce heat losses.


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Figure 3: Solar/Rankine parabolic trough system schematic

Components of a Parabolic trough solar collector

The basic component of a parabolic trough solar field is the solar collector assembly orSCA. A solar field consists of hundreds or potentially thousands of solar collectorassemblies. Each solar collector assembly is an independently tracking, parabolictrough solar collector composed of the following subsystems:

Concentrator Structure

Mirrors or reflectors

Linear receiver or heat collection element

Collector balance of system

Also, each parabolic trough solar collector assembly consists of multiple, torque-tubeor truss assemblies (often referred to as solar collector elements or modules).


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Concentrator Structure

The structural skeleton of the parabolic trough solar collector is the concentratorstructure. The concentrator structure:

Supports the mirrors and receivers, maintaining them in optical alignment

Withstands external forces, such as wind

Allows the collector to rotate, so the mirrors and receiver can track the sun.

Mirrors or Reflectors

The most obvious object within the parabolic trough solar collector are its parabolic-shaped mirrors or reflectors. Those mirrors are curved in the shape of a parabola,which allows them to concentrate the sun's direct beam radiation on the linearreceiver.

All current parabolic trough power plants use glass mirror panels manufactured by

Flabeg. The mirrors are second-surface silvered glass mirrors (which means that thereflective silver layer is on the backside of the glass). The glass is a 4-milimeter-thick,special low iron or white glass with a high transmittance. The mirrors have a solar-weighted specular reflectivity of about 93.5%. A special multilayer paint coatingprotects the silver on the back of the mirror. And each mirror panel is approximately 2square meters in area.

The glass mirror panels have performed very well during the operation of the SEGS(solar electric generating system) power plants. They've maintained high reflectivity

and suffer low annual breakage rates. However, mirror breakage does occur andreplacements have been relatively expensive. A number of alternative mirror conceptshave been under development to reduce cost, improve reliability, or increaseperformance.

Linear Receiver or Heat Collection Element

The parabolic trough linear receiver, also called a heat collection element (HCE), is oneof the primary reasons for the high efficiency of the original Luz parabolic troughcollector design.


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The receiver is a 4-meter-long, 70-mm diameter stainless steel tube with a specialsolar-selective absorber surface, surrounded by an anti-reflective evacuated 115-mmdiameter glass tube. Located at the mirror focal line of the parabola, the receiver heatsa special heat transfer fluid as it circulates through the receiver tube.

The receiver has glass-to-metal seals and metal bellows to accommodate for differingthermal expansions between the steel tubing and the glass envelop. They also helpachieve the necessary vacuum-tight enclosure.

The vacuum-tight enclosure primarily serves to significantly reduce heat losses at high-operating temperatures. It also protects the solar-selective absorber surface from

oxidation.

The selective coating on the steel tube has good solar absorptance and a low thermalemittance for reducing thermal radiation losses. The glass cylinder features an anti-reflective coating to maximize the solar transmittance. Gettersmetallic compoundsdesigned to absorb gas moleculesare installed in the vacuum space to absorbhydrogen and other gases that permeate into the vacuum annulus over time.

The original Luz receiver design suffered from poor reliability of the glass-to-metal

seal. Solel Solar Systems and Schott Glass have developed newer designs that havesubstantially improved:

Receiver reliability

Optical and thermal performance

The lifetime of receivers.

Collector Balance of System

The localize controller for a LS-2 parabolic trough solar collector assemblycommunicates with a computer in a central control building.

A number of other key components make up the balance of system in the parabolictrough solar field, including pylons and foundations, drive, controls and collectorinterconnect


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Performance and cost discussion

Plant Performance

Increasing the performance of the solar collectors and power plant are one of theprimary opportunities for reducing the cost of trough technology. Collectorperformance improvements can come from developing new more efficient collectortechnologies and components but often also by improving the reliability and lifetime of existing components.

The year 2000 technology shows a 20% improvement in net solar to electric efficiency

over the 1997 baseline system performance. This is achieved by using currenttechnologies and designs, by reducing HCE heat losses and electric parasitic. New HCEshave an improved selective surface with a higher absorptance and a 50% loweremittance. This helps reduce trough receiver heat losses by one third.

The 2005 technology shows a 7% increase in efficiency primarily as a result of addingthermal storage. Thermal storage eliminates dumping of solar energy during powerplant start-up and during peak solar conditions when solar field thermal delivery isgreater than power plant capacity. Thermal storage also allows the power plant to

operate independently of the solar field. This allows the power plant to operate nearfull load efficiency more often, improving the annual average power block efficiency.The thermal storage system is assumed to have an 85% round-trip efficiency. Minorperformance improvements also result from scaling the plant up to 160 MW from 80MW. Annual net solar-to-electric efficiency increases to 13.8% [1].

The 2010 technology shows a 6% increase in net solar-to-electric efficiency primarilydue to the use of the tilted collector. Power plant efficiency improves slightly due tolarger size of the 320 MW power plant. Thermal storage has been increased to 10hours and the solar field size increased to allow the plant to operate up to a 50%annual capacity factor. As a result, more solar energy must be stored before it can beused to generate electricity, thus the 85% round-trip efficiency of the thermal storagesystem tends to have a larger impact on annual plant performance. The resultingannual net solar-to-electric efficiency increases to 14.6%.The 2020 and 2030 technologies show 5% and 10% improvements in performance overthe 2010 trough technology.

Cost Reductions


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The technology shows a 30% cost reduction on a /kW basis and a 55% reduction on a/m2 basis. These cost reductions are due to: larger plants being built, increasedcollector production volumes, building projects in solar power park developments, andsavings through competitive bidding. In general, the per kW capital cost of powerplants decreases as the size of the plant increases. For trough plants, a 49% reductionin the power block equipment cost results by increasing the power plant size from 30to 320 MW. The increased production volume of trough solar collectors, as a result of larger solar fields and multiple plants being built in the same year, reduces troughcollector costs by 44%. Power parks allow for efficiencies in construction and costreduction through competitive bidding of multiple projects. A 10% cost reductionis assumed for competitive bidding in later projects.

O&M costs show a reduction of almost 80%. This large cost reduction is achievedthrough increasing size of the power plant, increasing the annual solar capacity factor,operating plants in a solar power park environment, and continued improvements inO&M efficiencies. Larger plants reduce operator labor costs because approximatelythe same number of people are required to operate a 320 MW plant as are requiredfor a 30 MW plant. The solar power park assumes that five plants are co-located andoperated by the same company resulting in a 25% O&M savings through reduced

overhead and improved labor and material efficiencies. In addition, about one third of the cost reduction is assumed to occur because of improved O&M efficiency resultingfrom improved plant design and O&M practices based on the results of the KJCO&M Cost Reduction Study [4].

linear fresnel plants

Linear Fresnel collectors (LFCs) are similar to parabolic trough collectors, but use aseries of long flat, or slightly curved, mirrors placed at different angles to concentratethe sunlight on either side of a fixed receiver (located several meters above theprimary mirror field). Each line of mirrors is equipped with a single-axis tracking systemand is optimized individually to ensure that sunlight is always concentrated on thefixed receiver. The receiver consists of a long, selectively-coated absorber tube.

Unlike parabolic trough collectors, the focal line of Fresnel collectors is distorted byastigmatism. This requires a mirror above the tube (a secondary reflector) to refocusthe rays missing the tube, or several parallel tubes forming a multi-tube receiver that iswide enough to capture most of the focused sunlight without a secondary reflector.

The main advantages of linear Fresnel CSP systems compared to parabolic troughsystems are that:


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LFCs can use cheaper flat glass mirrors, which are a standard mass-producedcommodity;

LFCs require less steel and concrete, as the metal support structure is lighter.This also makes the assembly process easier;

The wind loads on LFCs are smaller, resulting in better structural stability,reduced optical losses and less mirror-glass breakage; and.

The mirror surface per receiver is higher in LFCs than in PTCs, which isimportant, given that the receiver is the most expensive component in bothPTC and in LFCs.

These advantages need to be balanced against the fact that the optical efficiency of LFC solar fields (referring to direct solar irradiation on the cumulated mirror aperture)

is lower than that of PTC solar fields due to the geometric properties of LFCs. Theproblem is that the receiver is fixed and in the morning and afternoon cosine losses

are high compared to PTC. Despite these drawbacks, the relative simplicity of the LFCsystem means that it may be cheaper to manufacture and install than PTC CSP plants.

However, it remains to be seen if costs per kWh are lower. Additionally, given thatLFCs are generally proposed to use direct steam generation, adding thermal energystorage is likely to be more expensive.

Components of Linear Fresnel reflector plant

Reflectors

The reflectors are located at the base of the system and converge the suns rays intothe absorber. A key component that makes all LFRs more advantageous thantraditional parabolic trough mirror systems is the use of "Fresnel reflectors". Thesereflectors make use of the Fresnel lens effect, which allows for a concentrating mirrorwith a large aperture and short focal length while simultaneously reducing the volumeof material required for the reflector. This greatly reduces the systems cost sincesagged-glass parabolic reflectors are typically very expensive. [2] However, in recentyears thin-film nanotechnology has significantly reduced the cost of parabolicmirrors. [5]


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A major challenge that must be addressed in any solar concentrating technology is thechanging intensity of the incident rays (the rays of sunlight striking the mirrors) as thesun progresses throughout the day. The reflectors of a CLFR are typically aligned in anorth-south orientation and turn about a single axis using a computer controlled solartracker system. This allows the system to maintain the proper angle of incidencebetween the suns rays and the mirrors, thereby optimizing energy transfer.

Abs or be rs

The absorber is located at the focal point of the mirrors. It runs parallel to and abovethe reflector segments to transport radiation into some working thermal fluid. The

basic design of the absorber for the CLFR system is an inverted air cavity with a glasscover enclosing insulated steam tubes, shown in Figure 4. This design has beendemonstrated to be simple and cost effective with good optical and thermalperformance.

Figure 4: Incident solar rays are concentrated on insulated steam tubes to heat working thermal fluid

Figure 5: LFR solar systems alternate the inclination of their mirrors to focus solar energy on multiple absorbers,improving system efficiency and reducing overall cost.


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For optimum performance of the LFR, several design factors of the absorber must beoptimized.

First, heat transfer between the absorber and the thermal fluid must bemaximized. [1] This relies on the surface of the steam tubes being selective. Aselective surface optimizes the ratio of energy absorbed to energy emitted.Acceptable surfaces generally absorb 96% of incident radiation while emittingonly 7% through infra-red radiation. [7] Electro-chemically deposited blackchrome is generally used for its ample performance and ability to withstandhigh temperatures. [1]

Second, the absorber must be designed so that the temperature distributionacross the selective surface is uniform. Non-uniform temperature distributionleads to accelerated degradation of the surface. Typically, a uniformtemperature of 300 C (573 K; 572 F) is desired. Uniform distributions areobtained by changing absorber parameters such as the thickness of insulationabove the plate, the size of the aperture of the absorber and the shape anddepth of the air cavity.

Point-focusing systems

solar dish systems

The dish/engine system is a concentrating solar power (CSP) technology that producesrelatively small amounts of electricity compared to other CSP technologiestypicallyin the range of 3 to 25 kilowatts. Dish/engine systems use a parabolic dish of mirrors todirect and concentrate sunlight onto a central engine that produces electricity. Thetwo major parts of the system are the solar concentrator and the power conversionunit.

Figure 6: Solar dish System


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Components of solar dish systems

Solar Concentrator

The solar concentrator, or dish, gathers the solar energy coming directly from the sun.The resulting beam of concentrated sunlight is reflected onto a thermal receiver thatcollects the solar heat. The dish is mounted on a structure that tracks the suncontinuously throughout the day to reflect the highest percentage of sunlight possibleonto the thermal receiver.

Concentrators use a reflective surface of aluminum or silver, deposited on glass orplastic. The most durable reflective surfaces have been silver/glass mirrors, similar todecorative mirrors used in the home. Attempts to develop low-cost reflective polymerfilms have had limited success. Because dish concentrators have short focal lengths,relatively thinglass mirrors (thickness of approximately 1 mm) are required toaccommodate the required curvatures. In addition, glass with a low-iron content isdesirable to improve reflectance. Depending on the thickness and iron content,silvered solar mirrors have solar reflectance values in the range of 90 to 94%.

The ideal concentrator shape is a paraboloid of revolution. Some solar concentratorsapproximate this shape with multiple, spherically-shaped mirrors supported with atruss structure . An innovation in solar concentrator design is the use of stretched-membranes in which a thin reflective membrane is stretched across a rim or hoop. Asecond membrane is used to close off the space behind. A partial vacuum is drawn inthis space, bringing the reflective membrane into an approximately spherical shape.Figure 2 is a schematic of a dish/Stirling system that utilizes this concept.


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Figure 7: Schematic of a dish/engine system with stretched-membrane mirrors

Power Conversion Unit

The power conversion unit includes the thermal receiver and the engine/generator.The thermal receiver is the interface between the dish and the engine/generator. Itabsorbs the concentrated beams of solar energy, converts them to heat, and transfersthe heat to the engine/generator. A thermal receiver can be a bank of tubes with acooling fluidusually hydrogen or heliumthat typically is the heat-transfer mediumand also the working fluid for an engine. Alternate thermal receivers are heat pipes,where the boiling and condensing of an intermediate fluid transfers the heat to theengine.

Figure 8: Schematic which shows the operation of a heat-pipe solar receiver.


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The engine/generator system is the subsystem that takes the heat from the thermalreceiver and uses it to produce electricity. The most common type of heat engine usedin dish/engine systems is the Stirling engine. A Stirling engine uses the heated fluid tomove pistons and create mechanical power. The mechanical work, in the form of therotation of the engine's crankshaft, drives a generator and produces electrical power.

Figure 9: Schematic showing the principle of operation of a Stirling engine


The base-year technology (1997) is represented by the 25 kW dish-Stirling systemdeveloped by McDonnell Douglas in the mid 1980s. Southern California EdisonCompany operated a MDA system on a daily basis from 1986 through 1988. During itslast year of operation, it achieved an annual efficiency of 12% despite significant

unavailability caused by spare part delivery delays. This annual efficiency is better thanwhat has been achieved by all other solar electric systems, including photovoltaic's,


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solar thermal troughs, and power towers, operating anywhere in the world [11,12].The base-year peak and daily performance of near-term technology are assumed to bethat of the MDA systems. System costs assume construction of eight units. Operationand maintenance (O&M) costs are of the prototype demonstration and accordinglyreflect the problems experienced.

Performance for 2005 is largely based on one of the solarizable engines beingcommercialized for a non-solar application. Use of a production level engine will havea significant impact on engine cost as well as overall system cost. This milestone willhelp trigger a fledgling dish/engine industry. A production rate of 2,000 modules peryear is assumed. Achieving a high production rate is key to reducing component costs,especially for the solar concentrator.

Performance for years 2010 and beyond is based on the introduction of the heat-pipesolar receiver. Heat-pipe solar receiver development is currently being supported bySunLab in collaboration with industrial partners. The use of a heat-pipe receiver hasalready demonstrated performance improvements of well over 10% for the STM 4-120compared to a direct-illumination receiver [10]. While additional improvements inmirror, receiver, and/or engine technology are not unreasonable expectations, they

have not been included. This is, therefore, a conservative scenario. A production rateof 30,000 modules per year is assumed. By 2010 dish/engine technology is assumed tobe approaching maturity. A typical plant may include several hundred to over athousand systems. It is envisioned that a city located in the U.S. Southwest would haveseveral 1 to 50 MWe installations located primarily in its suburbs. A central distributionand support facility could service many installations. In the table, a typical plant isassumed to be 30 MW e.

Production levels for 2020 and 2030 are 50,000 and 60,000 modules per year,

respectively. No major advances beyond the introduction of heat pipes in the 2010time frame are assumed for 2020-2030. However, evolutionary improvements inmirror, receiver, and/or engine designs have been assumed. This is a reasonableassumption for a $2 billion/year, dish/engine industry, especially one leveraged by alarger automotive industry. The system costs are therefore 20 to 25% less thanprojected by MDA and SAIC at the assumed production levels. The MDA and SAICestimates are for their current designs and do not include the benefits of a heat-pipereceiver. In addition, the MDA engine costs are for an engine that is beingmanufactured primarily for solar applications. Advanced concepts (e.g., volumetric

Stirling receivers) and/or materials, which could improve annual efficiency by an


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additional 10%, have not been included in the cost projections. With theseimprovements installed costs of less than $1,000/kW e are not unrealistic.

power tower solar plants

In power tower concentrating solar power systems, numerous large, flat, sun-trackingmirrors, known as heliostats , focus sunlight onto a receiver at the top of a tall tower. Aheat-transfer fluid heated in the receiver is used to generate steam, which, in turn, isused in a conventional turbine generator to produce electricity. Some power towersuse water/steam as the heat-transfer fluid. Other advanced designs are experimenting

with molten nitrate salt because of its superior heat-transfer and energy-storagecapabilities. Individual commercial plants can be sized to produce up to 200 megawattsof electricity.

Figure 10: power tower solar plant

Two large-scale power tower demonstration projects have been deployed in theUnited States. During its operation from 1982 to 1988, the 10-megawatt Solar Oneplant near Barstow, California, demonstrated the viability of power towers byproducing more than 38 million kilowatt-hours of electricity.

The Solar Two plant was a retrofit of Solar One to demonstrate the advantages of molten salt for heat transfer and thermal storage. Using its highly efficient molten-saltenergy storage system, Solar Two successfully demonstrated efficient collection of solar energy and dispatch of electricity. It also demonstrated the ability to routinelyproduce electricity during cloudy weather and at night. In one demonstration, Solar


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Two delivered power to the grid 24 hours a day for almost 7 consecutive days beforecloudy weather interrupted operation.

Spain has several power tower systems. Planta Solar 10 and Planta Solar 20 arewater/steam systems with capacities of 11 and 20 megawatts, respectively. Solar Treswill produce some 15 megawatts of electricity and have the capacity for molten-saltthermal storage.

The dispatchability of electricity from a molten-salt power tower is illustrated in Figure2, which shows the load dispatching capability for a typical day in Southern California.The figure shows solar intensity, energy stored in the hot tank, and electric power

output as functions of time of day. In this example, the solar plant begins collectingthermal energy soon after sunrise and stores it in the hot tank, accumulating energy inthe tank throughout the day. In response to a peak-load demand on the grid, theturbine is brought on line at 1:00 PM and continues to generate power until 11 PM.Because of the storage, power output from the turbine generator remains constantthrough fluctuations in solar intensity and until all of the energy stored in the hot tankis depleted. Energy storage and dispatchability are very important for the success of solar power tower technology, and molten salt is believed to be the key to costeffective energy storage.

Figure 11: Dispatchability of molten-salt power towers.

Hybrid alternative

To reduce the financial risk associated with the deployment of a new power planttechnology and to lower the cost of delivering solar power, initial commercial-scale

(>30 MW ) power towers will likely be hybridized with conventional e fossil-fired


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plants. Many hybridization options are possible with natural gas combined-cycle andcoal-fired or oil-fired Rankine plants.In a hybrid plant, the solar energy can be used to reduce fossil fuel usage and/or boostthe power output to the steam turbine.In the power boost hybrid plant, additional electricity is produced by over sizing thesteam turbine, contained within a coal-fired Rankine plant or the bottoming portion of a combined-cycle plant (Figure 12), so that it can operate on both full fossil and solarenergy when solar is available. Studies of this concept have typically oversized thesteam turbine from 25% to 50% beyond what the turbine can produce in the fossil-onlymode. Oversizing beyond this range is not recommended because the thermal-to-electric conversion efficiency will degrade at the part loads associated with operatingin the fuel-only mode.

Figure 12: Power tower hybridized with combined cycle plant. Power is produced in the gas turbine (fossil only)and from the steam turbine (fossil and solar). Steam from the solar steam generator is blended with fossil

steam from the heat recovery steam generator (HRSG) before entering a steam turbine.

Environmental impact

No hazardous gaseous or liquid emissions are released during operation of the solarpower tower plant. If a salt spill occurs, the salt will freeze before significantcontamination of the soil occurs. Salt is picked up with a shovel and can be recycled if necessary. If the power tower is hybridized with a conventional fossil plant, emissionswill be released from the non-solar portion of the plant.


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At 2000, the following successful operation of Solar Two, the first commercial scalepower tower is assumed to be built in the Southwestern U.S. or within a developingnation. At the present time, the Solar Two business consortium is comfortable withscaling up the Solar Two receiver to 145 MW (3.3 times larger than Solar Two [14]).This larger receiver will be combined with a state-of-the-art glass heliostat field (> 95m2 each) [15], a next-generation molten-salt steam generator design (based onlessons learned at Solar Two), a high-efficiency steam turbine cycle, and will employmodern balance of plant equipment that will improve plant availability. As pointed outin the previous paragraph, these improvements are expected to increase annual

efficiency from 8.5 to 15%.

To reduce the financial risk associated with the deployment of this first commercial-scale plant and to lower the cost of delivering solar power, the plant will likely behybridized with a base-loaded fossil-fired plant. If the solar plant is interfaced with acombined cycle plant, the system layout could be similar to that depicted in Figure 10.Hybridization significantly reduces the cost of producing solar power relative to a solar-only design for the following reasons.

Capital costs for the solar turbine are reduced because only an increment tothe base-load fossil turbine must be purchased.

O&M costs are reduced because only an increment beyond the base-load O&Mstaff and materials must be used to maintain the solar-specific part of theplant.

The solar plant produces more electricity because the turbine is hot all the timeand daily startup losses incurred in a solar-only plant are avoided.

Power plant size is assumed to remain at 200 MW e . Power towers built between theyears 2010 and 2020 should have a receiver that has a significantly higher efficiencythan is currently possible with todays technology.Receivers within current power towers are coated with a highly absorptive black paint.However, the emissivity of the paint is also high which leads to a relatively largeradiation loss. Future power tower receivers will be coated with a selective surfacewith a very low emissivity that will significantly reduce radiation losses. Selectivesurfaces similar to what is needed are currently used in solar parabolic troughreceivers. Additional research is needed to produce a surface that wont degrade atthe higher operating temperature of the tower.

Given this improvement, scoping calculations at Sandia indicate that annual receiverefficiency should be improved to about 90%.


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By 2020, further improvements in heliostat manufacturing techniques, along withsignificant increases in annual production, are expected to lower heliostat costs totheir final mature value (~$70/m2, see Figure 13). The reflectance of the mirrors is alsoexpected to be improved from the current value of 94% to a value of at least 97%.Advanced reflective materials are currently being investigated in the laboratory. As thetechnology reaches maturity, plant parasitics will be fully optimized and plantavailability will also improve. Combining all the effects described above, annual plantefficiency is expected to be raised to 20% and annual capacity factor should be raisedabove 75%.

Figure 13: Heliostat price as a function of annual production volume.

Thermal Storage Systems for Concentrating Solar Power

One challenge facing the widespread use of solar energy is reduced or curtailed energy

production when the sun sets or is blocked by clouds. Thermal energy storage providesa workable solution to this challenge.

In a concentrating solar power (CSP) system, the sun's rays are reflected onto areceiver, which creates heat that is used to generate electricity. If the receiver containsoil or molten salt as the heat-transfer medium, then the thermal energy can be storedfor later use. This enables CSP systems to be cost-competitive options for providingclean, renewable energy.


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Several thermal energy storage technologies have been tested and implemented since1985. These include the two-tank direct system, two-tank indirect system, and single-tank thermocline system.

Two-Tank Direct System

Figure 14: Two-tank direct molten-salt thermal energy storage system at the Solar Two power plant

Solar thermal energy in this system is stored in the same fluid used to collect it. Thefluid is stored in two tanksone at high temperature and the other at lowtemperature. Fluid from the low-temperature tank flows through the solar collector orreceiver, where solar energy heats it to a high temperature, and it then flows to thehigh-temperature tank for storage. Fluid from the high-temperature tank flowsthrough a heat exchanger, where it generates steam for electricity production. Thefluid exits the heat exchanger at a low temperature and returns to the low-temperature tank.

Two-tank direct storage was used in early parabolic trough power plants (such as SolarElectric Generating Station I) and at the Solar Two power tower in California. Thetrough plants used mineral oil as the heat-transfer and storage fluid; Solar Two usedmolten salt.

Two-Tank Indirect System

Two-tank indirect systems function in the same way as two-tank direct systems, exceptdifferent fluids are used as the heat-transfer and storage fluids. This system is used inplants in which the heat-transfer fluid is too expensive or not suited for use as thestorage fluid.


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Figure 15: Two-tank indirect thermal energy storage system

The storage fluid from the low-temperature tank flows through an extra heatexchanger, where it is heated by the high-temperature heat-transfer fluid. The high-temperature storage fluid then flows back to the high-temperature storage tank. Thefluid exits this heat exchanger at a low temperature and returns to the solar collectoror receiver, where it is heated back to a high temperature. Storage fluid from the high-temperature tank is used to generate steam in the same manner as the two-tankdirect system. The indirect system requires an extra heat exchanger, which adds costto the system.

This system will be used in many of the parabolic power plants in Spain and has alsobeen proposed for several U.S. parabolic plants. The plants will use organic oil as theheat-transfer fluid and molten salt as the storage fluid.

Single-Tank Thermocline System

Figure 16: Single-tank thermocline thermal energy storage system


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Single-tank thermocline systems store thermal energy in a solid mediummostcommonly, silica sandlocated in a single tank. At any time during operation, aportion of the medium is at high temperature, and a portion is at low temperature.The hot- and cold-temperature regions are separated by a temperature gradient orthermocline . High-temperature heat-transfer fluid flows into the top of thethermocline and exits the bottom at low temperature. This process moves thethermocline downward and adds thermal energy to the system for storage. Reversingthe flow moves the thermocline upward and removes thermal energy from the systemto generate steam and electricity. Buoyancy effects create thermal stratification of thefluid within the tank, which helps to stabilize and maintain the thermocline.

Using a solid storage medium and only needing one tank reduces the cost of thissystem relative to two-tank systems. This system was demonstrated at the Solar Onepower tower, where steam was used as the heat-transfer fluid and mineral oil wasused as the storage fluid.


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forecast and conclusions

The most optimistic CSP industry development scenarios in public circulation forecastthat 7 percent of the power supply in 2030 may be generated with CSP technology,growing further to a possible share of 25 percent until 2050. More moderateassumptions of SolarPaces, the European Solar Thermal Electricity Association (ESTELA)and Greenpeace International assess the combined solar power output to contributebetween 3 - 3.6 percent in 2030 and 8 - 11.8 percent in 2050 to the worldwide powersupply. This would imply a capacity of over 830 GW in 2050 and deployments of 41 GWper annum. All in all, the CSP industry could be looking ahead to accumulated annualgrowth rates of 17 percent to 27 percent in the medium short term over the next fiveto ten years. MAN Ferrostaal, German Industrial Service Provider and ConcentratingSolar Power Industry Player, offers an assessment of worldwide CSP trends andtendencies and the solar market in the Middle East and North Africa in the "SolarReport" October 2009 on the international portal site solarserver.com.

Figure 17: Concentrating Solar Power plant example

Based on the Reference Scenario of the International Energy Agency (IEA), the by farmost conservative market prognosis, considerably lower growth rates may have to beexpected. On a strict "business-as-usual" basis, with legislative frameworks no morefavourable than existing policies, no binding commitments made to enactenvironmental standard reforms and steady low investor confidence, renewableenergies would never contribute significantly to global power generation. It is fact,however, that in 2008 CSP installations accounted for about 430 MW of generatedelectricity worldwide. Because of several projects in Spain, an addition of about 1 GW


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will foreseeably come online before the end of 2011. In the midterm, a capacity of some 20 GW by 2020 and an accumulated investment volume of about $160 billionseem realistic. "We all know the figures," says Tom Koopmann, Senior Vice Presidentof Solar Energy at Ferrostaal and chief strategist for the MENA region, "and we knowthat the numbers vary. To predict the market of 2050 with confidence today is to tell afortune based on assumptions."

"That we wont see any dynamic growth in CSP, we believe, is quite unlikely. There areseveral hundred MW in operation and almost 1 GW in construction. The cumulativecapacities announced to be in development amount to some 7 GW, but some cautionmust be exercised at this point. `Under development can be interpreted in manyways. It might mean almost anything from a feasibility study that has indicated apotential positive scenario up to a construction in process. At Ferrostaal we pursue asignificant amount of projects in early development stages in parallel, of which thensome result in an actual power plant in operation. During the pre-developmentprocess many factors might impact the final decision to execute a project." Projectionsand analyses that seemed reasonably optimistic two years ago, whether commissionedcorporate studies or publicly available outlooks, it appears, have been underestimatingthe market, he emphasizes. In Spain, for instance, renewable energy legislation hasbeen revised only a short time ago because too many CSP projects were proposedwhich could have created potentially too high subsidy spending.

Why CSP is Becoming Ever More Attractive?

Figure 18: Another CSP plant example

CSP plants have very low operating costs because of their fuel independence. About 80percent of the investment costs are spent on construction and debt pay-off. The


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required investment for a given project, of course, depends on its scale but also onlocal infrastructure, grid connection and project development expenses. Finally, thesolar irradiation is of great importance as it determines to a large extent the efficiencyof the plant.

In order for CSP to be fully competitive, the initial investment costs have to decreaseand components have to become more efficient. The same tendencies, which havebeen observed with other technologies in the past, can now be observed on the CSPmarket. Scaled up plant sizes, technological advancements and improved operationmodes (such as implementation of thermal storage) increase plant efficiency.Important external factors such as market regulations and policy initiatives designed topromote renewable energies and CSP investments provide incentivising frameworksfor the industry. Currently, these include long-term feed-in tariffs, government-issuedinvestment subsidies, tax incentives and regenerative energy quotas. Put in a nutshell:CSP projects parabolic trough plants in particular have become bankable.

There are intrinsic costs: investments in components, construction and operation, forexample. These costs must be lowered from within the industry to make CSP moreattractive. But whether the price per kWh of CSP, now or in the future, is competitivewith conventional generation depends not only on CSP technology.

Taking their Measure: Who Does What?

Throughout the entire region, interest in the sustainable use of regenerative energieshas grown. Due to the prevailing climate, solar power obviously has the appeal of anatural choice. Several countries have either repeatedly stated serious interest in CSPprojects or already have moved on to execute plant constructions.

The United Arab Emirates, Abu Dhabi especially, have started initiatives to use

renewable energy. The most notable outcome of this is Masdar City. Next to the usageof other energy sources, the main power supply for the City will be delivered through a100 MW CSP plant which is in the final phase of a tender process. Further projects arefirmly planned and will support to cater for the ever growing UAE power demand,which has doubled between 1993 and 2003 and already reached a consumption of 12,000 kWh per capita and year.

While the emirate of Abu Dhabi is about to execute the first large-scale CSP powerplant in the GCC region, many of her neighbours have their own projects in concrete

stages of planning and development. Workgroups have been established to determinehow solar power can best be integrated into grid expansion plans. Various feasibility


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studies have detailed the economic viability of constructing CSP capacities. As a result,projects are expected in Dubai, Bahrain, Oman, Saudi Arabia and other countrieswithin the next twelve months.

In Algeria, a national goal has been set to provide for 10 percent of the energy demandwith renewable energy by 2025. Almost five years ago, in 2004, the AlgerianGovernment introduced the first regenerative power feed-in-law of any OECD country

guaranteeing the power purchase from integrated solar combined cycle plants (ISCC)with over 20 percent solar generation for up to two times the regular tariff. At themoment, one solar thermal plant is under construction, and two more ISCC plants,each with an output of 400 MW and 70 MW CSP, will be developed between 2010 and2015.

Morocco has contracted a 470 MW station in the northeast of the country, due tocommence operation in 2009. In 2007, a Combined Cycle Power Island was contractedin Egypt, which is currently under construction and expected to start operation in theyear 2010. A first 140 MW ISCC plant with a 20 MW parabolic trough solar field, inwhich Ferrostaal was involved, has been built in Egypt already.

On the other hand, the market in each country requires individual assessment.

Countries rich in fossil resources with flourishing petrochemical industries, like SaudiArabia, Kuwait, the United Arab Emirates, or Qatar, generate huge revenues which canbe reinvested. These countries have the means to diversify with CSP and are interestedin acquiring the technology in order to stay a global player in the energy sector, evenwhen fossil fuel resources are depleted. The challenge is not only to invest intechnology, but also to use it, a step which needs to be managed politically, as localpower prices presently are extremely low and there is only a limited willingness toaccept price increases.

Other countries like Jordan, Bahrain, Syria or Lebanon, which have less or no availablefossil resources of their own, could use CSP in order to become less dependent onimported energy. These countries are relying on fuel imports or are consuming most of their own production, a production that then cannot be sold for profit on the globalmarket. Often in these countries the financing is more challenging to structure, but thehigher CSP kWh price is closer to what is being paid for fossil energy in any event. Eachcountry is different and has individual potentials for specific CSP applications.

The United Arab Emirates demonstrate exemplarily just how immediate air-

conditioning affects the overall energy demand. During the hot summer months, twicethe amount of electricity is consumed than during the winter. These seasonal peaks


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are typical for many countries and urban centres in the whole region. Equallycharacteristic is the comparatively low energy efficiency. According to the GermanEnergy Agency (dena), the impact of climatisation and cooling on electricityconsumption is particularly great because it is caused by the largest consumer group:private households, small and mid-sized businesses, office buildings and publicinstitutions. Hardly more than 10 percent of the demand originates with the industry.

Figure 19: Example of private household CSP

A CSP solution can address the demand, generating cold from heat. A solar thermal

collector can generate sufficient process steam to power an absorption chiller,providing an ecological alternative to conventional cooling systems. The advantage of using the sun itself for cooling is, of course, obvious. At present Ferrostaal markets acommercially feasible technology in this area. The system has been scaled for largebuildings hotels, shopping malls, airports and can provide air conditioning in thesummer months, heating and warm water in winter, or process steam for industrialapplications. While several plants are presently planned in Turkey, the UAE and LatinAmerica, the Iberotel Sarigerme Park hotel at the Turkish Aegean has been using thesystem since 2004.


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Figure list

Figure 1: Ioannis Sakkas experiment ................................................................................ 3Figure 2: CSP example ...................................................................................................... 4Figure 3: Solar/Rankine parabolic trough system schematic ........................................... 6Figure 4: Incident solar rays are concentrated on insulated steam tubes to heatworking thermal fluid ..................................................................................................... 12Figure 5: LFR solar systems alternate the inclination of their mirrors to focus solarenergy on multiple absorbers, improving system efficiency and reducing overall cost.12Figure 6: Solar dish System ............................................................................................. 13Figure 7: Schematic of a dish/engine system with stretched-membrane mirrors ........ 15Figure 8: Schematic which shows the operation of a heat-pipe solar receiver. ............ 15Figure 9: Schematic showing the principle of operation of a Stirling engine ................ 16Figure 10: power tower solar plant ................................................................................ 18Figure 11: Dispatchability of molten-salt power towers. ............................................... 19Figure 12: Power tower hybridized with combined cycle plant. Power is produced inthe gas turbine (fossil only) and from the steam turbine (fossil and solar). Steam fromthe solar steam generator is blended with fossil steam from the heat recovery steamgenerator (HRSG) before entering a steam turbine. ...................................................... 20Figure 13: Heliostat price as a function of annual production volume. ......................... 22Figure 14: Two-tank direct molten-salt thermal energy storage system at the Solar Twopower plant .................................................................................................................... 23Figure 15: Two-tank indirect thermal energy storage system ....................................... 24Figure 16: Single-tank thermocline thermal energy storage system ............................. 24Figure 17: Concentrating Solar Power plant example .................................................... 26Figure 18: Another CSP plant example ........................................................................... 27


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References list

[1]. Status Report on Solar Thermal Power Plants, Pilkington Solar International: 1996.Report ISBN 3-9804901-0-6.

[2]. Assessment of Solar Thermal Trough Power Plant Technology and ItsTransferability to the Mediterranean Region - Final Report, Flachglas SolartechnikGMBH, for European Commission Directorate General I External Economic Relations,and Centre de Developpement des Energies Renouvelables and Grupo Endesa,Cologne, Germany: June 1994.

[3]. O&M Cost Reduction in Solar Thermal Electric Power Plants - Interim Report onProject Status, KJC Operating Company, for Sandia National Laboratories: September 1,1994.

[4]. O&M Cost Reduction in Solar Thermal Electric Power Plants - 2nd Interim Reporton Project Status, KJC Operating Company, for Sandia National Laboratories: July 1,1996.

[5]. Texas Renewable Energy Resource Assessment: Survey, Overview &Recommendations, Virtus Energy Research Associates, for the Texas Sustainable

Energy Development Council, July, 1995, ISBN 0-9645526-0-4.[6]. http://en.wikipedia.org/wiki/Parabolic_trough

[7]. http://en.wikipedia.org/wiki/Linear_fresnel_reflector

[8]. http://www.nrel.gov/docs/fy12osti/54758.pdf

[9]. Montes Pita, M.J. Analisis y Propuestas de Sistemas Solares de Alta Exergia QueEmplean Agua como Fluido Calorifero. Universidad Politcnica de Madrid (ES) : Masterthesis, 2008.

[10]. Washom, B., Parabolic Dish Stirling Module Development and Test Results,Paper No. 849516, Proceedings of the IECEC, San Francisco, CA (1984).

[11]. Lopez, C.W., and K.W. Stone, Performance of the Southern California EdisonCompany Stirling Dish , Sandia National Laboratories, Albuquerque, NM: 1993. ReportSAND93-7098.

[12]. Kolb, G.J., Evaluation of Power Production from the Solar Electric GeneratingSystems at Kramer Junction: 1988 to 1993, Solar Engineering 1995, Proceedings of theASME Solar Energy Conference, Maui, HI (1995).

[13]. http://en.wikipedia.org/wiki/Dish_Stirling#Dish_designs


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[14]. Central Receiver Commercialization Plan, Bechtel National Inc., for the CaliforniaEnergy Commission: June 1995. Report 01-0444-35-3027-2777.

[15]. Strachan, J.W., and R.M. Houser, Testing and Evaluation of Large-Area Heliostatsfor Solar Thermal Applications, Sandia National Laboratories, Albuquerque, NM:February 1993. Report SAND92-1381.

[16]. Kolb, G.J., Economic Evaluation of Solar-Only and Hybrid Power Towers UsingMolten Salt Technology, Proceedings of the 8th International Symposium on SolarThermal Concentrating Technologies, Cologne, Germany (October 6-11, 1996).Accepted for publication in the journal Solar Energy.

[17]. http://en.wikipedia.org/wiki/Solar_power_tower

modern ways for concentrating solar power

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