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Manuel RomeroCentro de Investigaciones Energeticas,

Medioambientales y Tecnologicas,Avenida Complutense, 22,

E 28040 Madrid, Spaine-mail: romero@ciemat.es

Reiner BuckDeutsches Zentrum fur Luft- und Raumfahrt,

Institut fuer Technische Thermodynamik,Pfaffenwaldring 38-40,

D-70569 Stuttgart, Germanye-mail: reiner.buck@dlr.de

James E. PachecoSandia National Laboratories,

Solar Thermal Technology Department,PO Box 5800, M/S 0703,

Albuquerque, NM 87185-0703e-mail: jepache@sandia.gov

An Update on Solar CentralReceiver Systems, Projects,and TechnologiesCentral Receiver Systems that use large heliostat fields and solar receivers locatedof a tower are now in the position to deploy the first generation of grid-connected cmercial plants. The technical feasibility of the CRS power plants technology can be vas sufficiently mature after the pioneering experience at the early 1980s of severaplants in the 0.5–10 MW power range and the subsequent improvement of key conents like heliostats and solar receiver in many projects merging international collaration during the past 15 years. Solar-only plants like Solar Tres and PS10 or hyschemes like SOLGAS, CONSOLAR, or SOLGATE are being developed and suportfolio of alternatives leading to the first scaling-up plants during the period 200–2010. Those projects with still non-optimized small sizes of 10–15 MW are already re-vealing a dramatic reduction of costs versus previous feasibility studies and give thefor the formulation of a realistic milestone of achieving a LEC of $0.08/kWh by the2010 and penetrating initial competitive markets by 2015 with LECs betw$0.04/kWh–$0.06/kWh.@DOI: 10.1115/1.1467921#

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1 IntroductionSolar Thermal Power Plants~STPP! with optical concentration

technologies are important candidates for providing a major shof the clean and renewable energy needed in the future, ethough they still suffer from lack of dissemination and confidenamong citizens, scientists and decision makers@1#. Although thesolar radiation is a high quality energy source because of thetemperature and exergy at its source, its power density atearth’s surface makes it difficult to extract work and achieve rsonable temperatures in common working fluid@2#. Therefore,using STPP unequivocally means optical concentration. Thethe case of power towers, incident sunrays are tracked by lmirrored collectors~heliostats! which concentrate the energy flutowards radiative/convective heat exchangers, called solar reers, where energy is transferred to a working thermal fluid. Aenergy collection by the solar subsystem, optical concentrator,solar receiver, the conversion of thermal energy to electricitymany similarities with the one from fossil-fueled thermal powplants.

Usually reflective solar concentrators are used to attain the tperatures required for the operation of the thermodynamic cy@3#. In power towers or central receiver systems~CRS!, the solarreceiver is mounted on top of a tower and sunlight is concentraby means of a large paraboloid that is discretized into a fieldheliostats. Central Receiver Systems have a large potentiamid-term cost reduction of electricity produced since they allmany intermediate steps between the integration in a conventiRankine cycle up to the higher exergy cycles using gas turbinetemperatures above 1300°C, and this subsequently leads to hefficiencies and larger throughputs.

The typical optical concentration factor ranges from 2001000 and plant sizes of 10 to 200 MW are chosen becauseconomy-of-scale constraints, even though advanced integraschemes are claiming economic sense for smaller units as@4#. The high solar fluxes impinging on the receiver~average val-ues between 300–1000 kW.m22! allow working at relatively hightemperatures up to 1000°C and to integrate thermal energy

Contributed by the Solar Energy Division of THE AMERICAN SOCIETY OF ME-CHANICAL ENGINEERSfor publication in the ASME JOURNAL OF SOLAR ENERGYENGINEERING. Manuscript received by the ASME Solar Energy Division, Sep. 20final revision, Jan. 2002. Associate Editor: R. Pitz-Paal.

98 Õ Vol. 124, MAY 2002 Copyright © 20

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more efficient cycles in a step-by-step approach. CRS can eaintegrate in fossil plants for hybrid operation in a wide varietyoptions or have the potential to generate electricity with highnual capacity factors by using thermal storage. With storage, Cplants have the capability to operate more than 4500 hoursyear at nominal power@5#. Main characteristics of CRS plants arsummarized in Table 1.

2 Experiences in Central Receiver SystemsAlthough the number of projects of STPP with towers has be

large, only few have culminated in the construction of entexperimental systems. In Table 2, systems that have been teall over the world along with new plants that are likely to be buare listed. In general terms, as it can be observed they are chterized as being small demonstration systems, between 0.510 MW, and most of them had their period of operation in t80s @3,6–9#. The thermal fluids used in the receiver have beliquid sodium, saturated or superheated steam, nitrate-basedten salts, and air. All of them can easily be represented by flcharts, where the main variables are determined by working fluand the corresponding interface between power block and sportion.

The set of referred experiences has served to demonstrattechnical feasibility of the CRS power plants, whose technologsufficiently mature. The most extended experience has beenlected by several European projects located in Spain at themises of the Plataforma Solar de Almerı´a @8# and in the USA the10-MW Solar One and Solar Two plants@10,11#, that can be seenin Fig. 1. Annual efficiencies of 7% have been demonstrated,the predictions are to reach efficiencies of 23% at design point20% annual by 2030, with investment costs of $0.9/Wp @6#, butthe first generation of commercial demonstration plants have tobuilt to validate the technology under real operating and marconditions. This first generation of commercial plants was studextensively in the early 90s where a joint USA/German stuidentified the potential for molten salt and air cooled plants@12#.Several penetration strategies have been proposed since thenmany more may be developed in the future since solar towhave the advantage of permitting a very open integration dedepending on dispatching scenarios, annual capacity factors,hybridization schemes. Three of most promising power towtechnologies that are expected to lead to commercial plants

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described in this paper: 1! molten salt technology, 2! open orclosed loop volumetric air technologies, and 3! saturated steamtechnology. A longer range concept involves upgrading methto hydrogen and carbon monoxide as means to store solar enchemically. The technology favored by industry in the UnitStates of America is based on solar-only power plants that utlarge thermal storage capacity with molten nitrate salts as workfluid as targeted in projects Solar Two and Solar Tres. The usvolumetric receivers either with closed air loops for efficienttegration into gas turbine cycles or open air for intermediate sage and/or hybridization solutions have been promoted in Eurand Israel with projects like SOLGATE, PS10, and Consolar.nally, a more conservative approach where solar saturated sreceivers are used for cogeneration purposes is also undeSOLGAS initiative.

Table 1 Characteristics of solar thermal power central re-ceiver systems, adapted from †6‡

Typical Size 10–200 MW*Operating Temperature

- Rankine 565°C- Brayton 800°C

Annual Capacity Factor 20–77%*Peak Efficiency 16–23%*Annual Net Efficiency 12–20%*Commercial Status Scale-Up Demonstration~10–30 MW!Technology Development Risk MediumStorage available Nitrate salt for molten salt receiver

Ceramic bed for air receiversHybrid designs YesInvestment cost$.W21 4.4–2.5*$.Wp

21** 2.4–0.9*

*Values indicate changes over the 1997–2030 time frame.** $.Wp

21 removes the effect of energy storage or solar multiple, asually made in PV.~In the present study a conversion rate of 1 Euro50.91 $ as of Sept-ember 2001 has been used!

Journal of Solar Energy Engineering

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2.1 The Solgas Concept. Production of superheated steaat the solar receiver has been demonstrated in several plantsSolar One, Eurelios or CESA-1, but the operational experierevealed critical problems related to the control of zones wdissimilar heat transfer coefficients like boilers and superhea@10#. Better results regarding absorber panels lifetime and contlability have been reported for saturated steam receivers. Inticular, the STEOR pilot plant~Solar Thermal Enhanced Oil Recovery! for oil extraction using direct injection of steam wasuccessfully operated in Kern County, CA, during 345 days1983 with a high reliability@13#. The good performance of saturated steam receivers was also qualified at the 2 MW Weizmreceiver tested in 1989 that produced steam at 15 bar forhours @14#. Even though utilizing saturated steam receiversduces technical risks, the outlet temperatures are significalower than those of superheated steam making it necessary toapplications where this technology can be integrated into pcesses where fossil-fuel provides superheating.

The hybrid systems offer at present estimated costs of elecity production from the solar portion of $0.08–0.15 per kWwhereas solar-only plants can expect costs in the range of $00.20 per kWh. The implementation of hybridized systems is oof the paths to break the non-technological financial barriersdeploy solar electric technologies which enables the reductiothe initial investment@5#. The use of hybrid plants with alow technological risk by using a central receiver system wsaturated steam as working fluid serves as the starting pTwo projects subsidized by the European Commission, the proSOLGAS promoted by SODEAN and the project COLOSOLAR promoted by SEVILLANA@15#, have established thestrategy of penetration on the basis of the integration of the srated steam receivers into cogeneration systems and the repoing of combined cycles. The size of the cavity receiver was omized to supply 21.8 MWt to the fluid at 135 bar and 332.8°Coutlet temperature. The collector subsystem consisted of 489liostats~each with a 70 m2 reflective surface! and a 109-m tower.Integrating power towers into existing combined cycle plants c

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Table 2 Experimental power towers in the world

Project CountryPower(MW e) Heat Transfer Fluid Storage media

Beginningoperation

SSPS Spain 0.5 Liquid Sodium Sodium 1981

EURELIOS Italy 1 Steam Nitrate Salt/Water 1981

SUNSHINE Japan 1 Steam Nitrate Salt/Water 1981

Solar One U.S.A. 10 Steam Oil/Rock 1982

CESA-1 Spain 1 Steam Nitrate Salt 1982

MSEE/Cat B U.S.A. 1 Nitrate Salt Nitrate Salt 1983

THEMIS France 2.5 Hitec Salt Hitech Salt 1984

SPP-5 Russia 5 Steam Water/Steam 1986

TSA Spain 1 Air Ceramic 1993

Solar Two U.S.A. 10 Nitrate Salt Nitrate Salt 1996

Consolar Israel 0.5** Pressurized Air Fossil Hybrid 2001

Solgate* Spain 0.3 Pressurized air Fossil Hybrid 2002

PS10* Spain 10 Air Ceramic 2004

Solar Tres* Spain 15 Nitrate Salt Nitrate Salt 2004

*Projects under development.** Thermal

MAY 2002, Vol. 124 Õ 99

Fig. 1 Aerial views of the plants Solar Two of 10 MW in California „superior left … and CESA-1 of 1.2 MW in Almeria, Spain „right …

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create issues with respect to heliostat field layout, since the sfield is forced to make use of sites nearby gas pipelines anddustrial areas. Land becomes a non-negligible portion of the pcost and site constraints lead to some particular problems forout optimization and subsequent optical performance. Thisthe case for COLON heliostat field and represented a real clenge during the design phase, because of the important restions stemming from the available site@16#. The feasibility projectof the SOLGAS plant was completed at the beginning of 1996had its continuation in the project COLON SOLAR whose dtailed design was finalized in April 1998@17#. The SOLGASscheme~Fig. 2! predicts solar production costs below $0.11/kWand annual solar shares in the range of 8–15%. With the rechanges in the Spanish electrical sector the legal status of hyplants is being assessed, and once clarified, will allow to retthe SOLGAS concept and to promote a plant of commerdemonstration.

2.2 Solar One, Solar Two, and Solar Tres Plants. The So-lar One Pilot Plant successfully demonstrated operation outility-scale power tower plant. The Solar One receiver heasub-cooled water to superheated steam, which drove a turbThe superheated steam was also used to charge an oil-rockmocline storage system. Solar One operated for six years betw1982 and 1988, the last three of which were dedicated to a poproduction phase@10#. Although Solar One successfully demostrated the feasibility of the power tower concept, the thermstorage system was inadequate for operating the turbine atefficiency because the storage system operated only betw220–305°C, whereas the receiver outlet~and design turbine inlet!temperature was 510°C. The primary mode of operation wadirectly couple the receiver outlet with the turbine input, bypaing the thermal storage system. Storage provided auxiliary stduring offline periods.

To provide high annual capacity factors with solar-only powplants, a cost-effective thermal storage system must be integrinto the plants. One such thermal storage system employs monitrate salt as the receiver heat transfer fluid and thermal stomedia. The usable operating range of molten nitrate salt, a mixof 60% sodium nitrate and 40% potassium nitrate, matchesoperating temperatures of modern Rankine cycle turbines.

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molten-salt power tower plant, cold salt at 290°C is pumped fra tank at ground level to the receiver mounted atop a tower whit is heated by concentrated sunlight to 565°C. The salt flows bto ground level into another tank. To make electricity, hot saltpumped from the hot tank through a steam generator to msuperheated steam. The superheated steam powers a Rancycle turbine. A schematic of a molten-salt power tower is shoin Fig. 3. The collector field can be sized to collect more powthan is demanded by the steam generator system, the excesaccumulates in the hot storage tank. With this type of storsystem, solar power tower plants can be built with annual capafactors up to 70%.

Several molten salt development and demonstration expments have been conducted over the past two and half decadthe USA and Europe to test entire systems and develop comnents. The largest demonstration of a molten salt power towerthe Solar Two project—a 10 MW power tower located near Bstow, CA. A picture of Solar Two is shown in Fig. 1.

The purpose of the Solar Two project was to validate the tenical characteristics of the molten salt receiver, thermal storaand steam generator technologies, improve the accuracy ofnomic projections for commercial projects by increasing the dabase of capital, operating, and maintenance costs, and to distrinformation to utilities and the solar industry to foster wider iterest in the first commercial plants. The Solar Two plant was bat the same site as the Solar One pilot plant and reused mucthe hardware including the heliostat collector field, tower struture, 10 MW turbine, and balance of plant. A new, 110 MWttwo-tank molten-salt thermal storage system was installed asas a new 42 MWt receiver, a 35 MWt steam generator system~535°C, 100 bar!, and master control system. An additional 10heliostats each 95 m2 were refurbished from a defunct photovotaic facility to supplement the original 1818 heliostats~each 39m2! @18#.

The plant began operating in June 1996. The project succfully demonstrated the potential of nitrate salt technology. Soof the key results were: the receiver efficiency was measured t88%, the thermal storage system had a measured round-tripciency of greater than 97%, the gross Rankine-turbine cycle eciency was 34%, all of which matched performance projectio

Transactions of the ASME

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Fig. 2 Diagram of SOLGAS scheme „Source: Final Project Report to the EC …

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The collector field under-performed relative to predicted valuprimarily due to low availability of heliostats~85–95% versus98% expected!, degradation of the mirrored surfaces, and pocanting of the heliostats. Most of the heliostat problems wattributed to the fact that the heliostat field sat idle and umaintained for six years between Solar One shut down and STwo start-up. The overall peak-conversion efficiency of the plawas measured to 13.5%. The plant successfully demonstrate

Fig. 3 Schematic of a Molten Salt Power Tower

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ability to dispatch electricity independent of collection. On ooccasion, the plant operated around-the-clock for 154 hostraight @19#. The plant met daily performance projections whthe actual heliostat availability was accounted for. Althoughplant had some start up issues and did not run long enougestablish annual performance or refine operating and maintenprocedures, the project identified several areas to simplifytechnology and to improve its reliability. On April 8, 1999, thdemonstration project completed its test and evaluations andshut down.

Since Solar Two was a demonstration project and quite smby conventional power plants standards, it could not compete enomically with fossil fired power plants without special subsidieSolar-only commercial power tower plants must be larger to tadvantage of economy of scales, to have more efficient desiand to distribute the costs of the maintenance crew over greenergy production.

To reduce risks associated with scaling-up hardware, thecommercial molten salt power tower will be approximately thrtimes the size of Solar Two. This project is called Solar Tres. Ibeing pursued by the U.S. company, Nexant~a subsidiary ofBechtel, Inc.! and the Spanish company, Ghersa@20#. Spain cur-rently offers favorable market opportunities for solar power plawith laws that specify a premium be paid for solar genera

MAY 2002, Vol. 124 Õ 101

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Table 3 Characteristics of Four Sizes on Molten Salt Power Towers

ParameterSolar Two„Mature… Solar Tres

Solar 50ÕSolar Cuatro Solar 100

Plant Rating 10 15 50 100

Location Barstow,CA, USA

Cordoba,Spain

SouthernSpain

SouthwesternUSA

Annual Solar Insolation, kWh/m2 2700 2067 2067 2700

Capacity Factor, % 20 65 69 70

Field Area, m2 81400 263000 971000 1466000

Receiver Thermal Rating, MW 42 120 466 796

Thermal Storage Size, MWh 110 610 1850 3820

Steam Generator Rating, MW 35 37 130 254

Annual Net Energy Production, MWh 16600 75500 302000 613000

Peak Net Efficiency 0.13 0.19 0.22 0.22

Annual Net Efficiency 0.08 0.14 0.15 0.16

Levelized Energy Cost, $/kWh N/A 0.16 0.12 0.08

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Solar Tres will take advantage of several advancements tomolten salt technology since Solar Two was designed and bThese include:

• a larger plant with a heliostat field approximately three timthe size of Solar Two,

• large-area glass-metal heliostats each 96 m2 which havehigher-reflectivity glass,

• a receiver system with higher flux capability and thus lowheat losses which also is immune to stress corrosion crack

• a larger thermal storage system~16 hours, 600 MWh!,• advanced pump designs that will pump salt directly from

storage tanks eliminating the need for pump sumps@21#,• a steam generator system that will have a forced-recircula

steam drum,• a more efficient, higher-pressure reheat turbine, and• a simplified molten-salt flow loop that reduces the number

valves.

With these advancements, the peak and annual conversionciency will improve over Solar Two’s design. Although the tubine will be only slightly larger than Solar Two’s turbine, thlarger heliostat field and thermal storage system will enableplant to operate 24 hours a day during the summer and havannual capacity factor of approximately 65% Fig. 4. The levelizenergy costs are estimated to be approximately $0.16/kWh.

To reduce the costs further for molten-salt power towers, tmust be scaled-up to take advantage of economies of scasummary of the characteristics of various sizes of moltenpower towers is shown in Table 3. Two steps are required tothe commercial module. A 50 MW plant represents an appromate three-and-a-half size increase in the collector field, recethermal storage, steam generator, and turbine. The performanthe turbine, thermal storage, and receiver improves relative tolar Tres. This plant would use larger 148 m2 heliostats. The lev-elized energy cost of electricity for this plant is expected to$0.12/kWh. The levelized cost reduction relative to Solar Tresbe attributed to the fact that as larger components are manutured ~e.g., turbines, steam generators, receiver, etc! their costsincrease at a rate less than linear. Likewise, the specific orcosts of heliostats are expected to be reduced because expe

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can be applied to the next generation design and the toolingquired for larger production volumes of the heliostats can amtize over more heliostats. The manpower required to operate aMW plant is slightly higher than Solar Tres, which attributeslower specific operating and maintenance costs because 3.5 tmore energy is produced. Similarly for the 100 MW commercmodule, a scale up of two relative to the 50 MW plant, furthspecific capital cost reductions are expected for the same reasin addition to the better solar resource in the United States veSpain, bringing the levelized energy costs down to approximat$0.08/kWh.

2.3 The Phoebus Scheme: From TSA to PS10.The use ofair as working fluid for CRS plants has been demonstrated sithe early 1980s. The selection of air is based on its intrinsic

Fig. 4 Influence of heat storage sizing on normalized energycosts for molten salt CRS plants. For annual capacity factors of65% the lowest cost is obtained „Source: G. Kolb-Sandia Na-tional Labs ….

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vantages such as available from the ambient, environment friencharacteristics, no troublesome phase changes, higher wortemperatures, easy operation and maintenance, and high dispability. Several project initiatives shifted to air-cooled receiveafter the operational problems identified at CRS plants when pducing superheated steam like Solar One, CESA-I or EureliosGerman-Spanish project named GAST coordinated by the comnies Interatom and Asinel proposed the construction of a 20-Mplant in southern Spain using a tubular panel air-cooled rece@22#. Several components of the plant such as heliostats andceiver panel were tested at the PSA, in particular a metallic-treceiver was tested in 1985-1986 producing 2.45 kg.s21 hot air at9.5 bar and 800°C outlet temperature. A second panel withramic SiC tubes was tested in 1987, with a mass flowrate of 0kg.s21 at 9.3 bar and 1000°C. The high estimated investment cand the low incident solar fluxes permitted by the tubes~lowerthan 200 kW.m22! made it unpractical to pursue the constructioof the plant.

With the advent of volumetric receivers, the air-cooled sotowers gained real interest. Volumetric receivers use highly porstructures for the absorption of the concentrated solar radiatThe solar radiation is not absorbed on an outer surface but indepth~inside thevolume! of the structure. The heat transfer medium ~mostly air! is forced through the porous structure andheated by convective heat transfer. Figure 5 shows a comparof the two absorber principles. Common volumetric absorbersmade from thin heat resistant wires~as knittings or layered grids!or from open-cell matrix structures, either from metals or ceraics ~reticulated foams, matrix structures etc.!. Good volumetricabsorbers show a high open porosity, allowing the radiationpenetrate deeply into the structure. Thin substructures~wires,walls, or struts! ensure good convective heat transfer. A govolumetric absorber shows the so-calledvolumetric effectwhichmeans that the absorber temperature at the irradiated side is lthan the temperature of the medium leaving the absorber.

Under specific operating conditions volumetric absorbers teto show mass flow distribution instabilities. Receiver arrangments with mass flow adaption elements located behind thesorber~e.g., perforated plates! can reduce this tendency, as well aappropriate selection of the operating conditions and the absomaterial.

The relatively large number of volumetric prototypes tested hdemonstrated the feasibility to produce hot air at temperature1000°C and upwards and with aperture areas similar to thoseat molten salt or water/steam receivers@24,25#. Average fluxes of400 kW.m22 and peaks of 1000 kW.m22 have been proven, andtheir low-inertia and quick sun-following dispatchability are rmarkable. Even though long endurance experiences are mis

Fig. 5 Absorption and heat transfer of tubular and volumetricreceivers „after †22‡…

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radiation losses need further reduction and air recirculation rain open designs should be improved, several initiatives of plahave been promoted in the last 15 years.

The first study for a solar CRS plant with an atmosphericheat transfer circuit was METAROZ in the Swiss Alps at the ea1980s@26#, followed by a second study performed by the SwiSOTEL Consortium. This pioneer study served to define the afwards so-called PHOEBUS scheme@27#, where atmospheric air isheated up through a metal wire mesh receiver to temperaturethe order of 700°C and used to produce steam at 480–540°C35-140 bar, in a heat recovery steam generator with separateperheater, reheater, evaporator and economizer sections feedRankine turbine-generator system. The PHOEBUS scheme igrates several equivalent hours of ceramic thermocline therstorage able to work for charging and discharging modes byversing air flow with two axial blowers. Current heat storage cpacity restrictions lead to designs with a limited number of hou~between 3 and 6 hours maximum!, therefore for higher annuacapacity factors hybrid designs are proposed with the backup fa duct burner located in the downcomer of the receiver.

In 1986 under the initiative of SOTEL and DLR, the studof a 30 MW plant for Jordan was initiated. The internationPHOEBUS Consortium was formed by companies from GermaSwitzerland, Spain, and the USA and the feasibility study copleted in March 1990@28#. Unfortunately, the project could nocomplete the necessary grants and financial support and didcome to eventual construction. Technology development of kcomponents followed through the German TSA Consortiu~Technology Program Solar Air Receiver!, with the leadership ofthe company Steinmu¨ller. A 2.5 MWt air-receiver facility compris-ing the complete PHOEBUS-power plant cycle that included arecirculation loop, thermal storage and steam generator wassembled on top of the CESA-1 tower in Spain at the end of 19The plant was successfully operated by DLR and CIEMAT fortotal of nearly 400 hours between April and December 1993, afor shorter periods in 1994 and 1999, demonstrating that aceiver outlet temperature of 700°C could easily be achievwithin twenty minutes of plant start-up@29#.

TSA operational results have been decisive to convinceSpanish company Abengoa to promote the first commercial donstration plant with this technology. The project named PSstarted in 1999 and its goal is the construction and connectiothe grid of a 10 MW plant to be located in Seville~Spain!. It isexpected that PS10 will be starting civil works by November 20and operation by end of year 2004. A schematic and main desand performance characteristics are summarized in Fig. 6Table 4. A detailed description about the plant and project magement can be found in@30#.

PS10 is a demonstration project looking for a single intermeate step before commercial modules are systematically produThe size of the plant has been selected because of the restrictia maximum investment of $28 M adopted by the IPP society ait is therefore not fully optimized. Legal constraints for hybripower plants in Spain and tariffs policy have made PS10 confiration as a solar-only PHOEBUS-type technology with a therm

Fig. 6 Process flow diagram of the PS10 solar tower powerplant

MAY 2002, Vol. 124 Õ 103

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storage system sized just for transients and short cloudy periodis an essentially conservative project regarding technology wthe following distinctive features as compared to the previousdan 30 MW PHOEBUS project:

• PS10 has developed its own glass-metal almost squH9.57m3W9.67m, 91-m2 heliostats Sanlucar @31#. Thecompact design of the reflective surface and the light strture ~moving structure less than 17 kg.m22! lead to the costgoal of $109-131/m2 for 1000 units, representing a significaimprovement in investment costs.

• Wire-mesh volumetric receiver will make use of the sammodular absorber already qualified at the 2.5 MWt TSA re-ceiver but the aperture shape is sectional cylindrical. Asorber material lifetime risks will be limited by reducing thoutlet temperature of the air from 700 °C to 680 °C.

• Parametric materials analyses showed that it is possibltake into consideration to design and manufacture a stosystem in a relative economical way, based on Al2O3 ceramicsaddles geometry for the storage core material. The sizthe heat storage has been reduced to half an hour~18 MWhtotal capacity with a useful storage capacity of 14.4 MWh! by

Table 4 PS10 Solar Plant Design Parameters

Annual Irradiation@kWh/m2# 2063Design Point Day 355~noon!

Design Point Irradiance@W/m2# 860

Design Point Power@MWe# 10

Solar Multiple/Heat storagecapacity@MWh#

1.15/18

Tower Height@m# 90

Heliostats Number/HeliostatReflective Surface@m2#

981/91

Heliostat Annual AverageReflectivity

0.90

Focal Length@m# 500

Receiver Shape Half cylinde

Receiver diameter@m#/ReceiverHeight @m#

10.5/10.5

Design Point Annual Balance

Power/Energy onto ReflectiveSurface

75.88 MW 183.50 GWh

Heliostat Field Optic Efficiency 0.729 0.647

Gross Power/Energy onto Receiver 55.27 MW 118.72 GW

Receiver and Air Circuit Efficiency 0.740 0.614

Power/Energy to Working Fluid 40.92 MW 72.90 GWh

Power/Energy to Storage 5.34 MW

Power/Energy to Turbine 35.58 MW 72.90 GWh

Thermal→Electric Efficiency 0.309 0.303

Gross Electric Power/Energy 11.00 MW 22.09 GWh

Parasitic Losses 1.00 MW 2.89 GWh

Net Electric Power/Energy 10.00 MW 19.20 GWh

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running from storage at a reduced air mass flow rate cosponding to approximately 70% of the nominal value.

Total project costs amount to $28 million~Table 5! and lead to atypical LEC of $0.18/kWh. The small size of the plant and thsolar-only design require economic support through public graAt present a $4.55-million investment subsidy has been obtaifrom the EC RTD ENERGIE Program. The plant will be paabove-market rates of $0.11/kWh according to the tariffs estlished by the Spanish Administration for solar-only STPP lethan 50 MW.

2.4 Solgate: From REFOS to the new Generation of CRSPlants. Introducing solar energy into the gas turbine of Combined Cycle systems~CC! offers significant advantages over othesolar hybrid power plant concepts. A very promising way to itroduce solar is solar preheating of the compressor dischargebefore it enters the combustor of the gas turbine. A scheme ofconcept is shown in Fig. 7.

Solar air preheating offers superior performance, as the senergy absorbed in the heated air is directly converted withhigh efficiency of the CC plant. For a certain annual solar shathis results in reduced heliostat field size and thus less ove

Fig. 7 Solar air preheating system

Table 5 Breakdown of PS10 project costs „1 EuroÄ$0.91 rateas of September 2001 …

INVESTMENT PS10 „Thousands $…

General Coordination 162.0Definition Basic Engineering 99.2

Plan O&M 62.8Civil Works 597.9Engineering 69.2Safety plan 61.9

Works 465.9Heliostats 11717.7

Engineering 391.1Fabrication 7577.2

Transport and assembly 3631.Acceptance tests 118.7

Tower 1707.2Engineering 337.6

Transport and erection 1369.6Receiver¿Storage¿Steam Gen. 8718.7

Engineering 3968.5Fabrication 3929.4

Transport and assembly 695.2Acceptance tests 125.6

EPGS 4370.7Engineering 1266.7Fabrication 1225.8

Transport and erection 1809.1Acceptance tests 69.2

Control 710.7Engineering 584.2

Implementation 126.5TOTAL 27984.8

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investment cost for the solar part as compared to solar stgeneration. Solar air preheating has a high potential for cosduction of solar thermal power. In addition, this concept couldapplied to a wide range of power levels (1 – 100 MWe). At thesmaller power levels, highly efficient recuperated gas turbcycles can be used instead of CC. The solar share can be chquite flexibly by the receiver outlet temperature, it could be snificantly higher than with other hybrid concepts~e.g., integratedsolar combined cycle system with parabolic troughs!.

Solar preheating of the air can be realized by molten saltceivers~up to 560°C! @32# or with pressurized volumetric receivers @33,34#. A schematic of a pressurized volumetric receivershown in Fig. 8. Due to the limited size of the quartz windownumber of receiver modules are placed on the tower of a stower plant. Each module consists of a pressurized receiverand a secondary concentrator in front. The secondary concentwith a hexagonal entrance aperture~located in the focal plane othe heliostat field! reconcentrates the solar radiation to the apture of the pressure vessel which is closed by a domed quwindow to maintain pressure. After passing through the wind

Fig. 8 REFOS receiver module

Table 6 Cost estimate for solar-hybrid CC power plant

Type of Plant solar-hybrid combined cycle plant, 30 MWe,with solar air preheating to 1000°C max.

Heliostats Sanlucartype, mirror area: 91 m2, $131/m2

1047 heliostats, total mirror area: 95277 m2

Receiver REFOS type, 94 modules, 120 m2

aperture area, max.receiver outlet temperature:1000°C

Power block Combined Cycle, 30 MWe, based on WR-21 gasturbine with intercooling, 4000 full load hours peyear

Site specification Barstow, direct normal insolation: 2373 kWh/m2aInvestment cost land: $682,500

heliostats: $12,481,287receiver: $2,730,000tower: $2,347,800power block: $13,650,000additional cost factor: 1.15total cost: $36,675,325specific cost: $1,222/kWe

Performance analysis annual field efficiency: 58.1%annual receiver efficiency: 77.1%annual solar share: 38.5%annual solar to electric efficiency: 20.6%

LevelizedElectricity cost

O&M cost ~2% of investment!: $685,000/apersonnel cost: $682,500/afuel cost~$11.83/MWh!: $1,915,282/atotal LEC: $0.061/kWhsolar LEC: $0.082/kWh

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the radiation is absorbed in the volumetric absorber which trafers the heat by forced convection to the air stream flowthrough it.

Power scale-up is achieved by installing multiple modules la honeycomb to cover the complete focal spot~see Fig. 9!. Themodules are then interconnected in parallel and serial connec

In 1996, the REFOS project was initiated to demonstratetechnical feasibility of the required receiver technology@34#. Theaim of the REFOS project is to develop, build and test modupressurized volumetric receivers under operating conditionsresentative for the coupling with gas turbines. Emphasis istesting of solar air preheating, accompanied by basic researcmaterials. A cluster of three receiver modules will be tested wan absorbed thermal power~design conditions! of 1 MWt ~designefficiency: 80%!, a maximum air outlet temperature 800°C andpressure of 15 bar.

The project is led by DLR and is carried out in cooperation wCIEMAT, Spain, and G1H, Germany. The REFOS test systeminstalled in the CESA-1 solar tower test facility of the PlataformSolar de Almerı´a ~PSA!, Spain. In 1999, the design conditionwere demonstrated with a single module operating at air outemperatures of 800°C at 15 bar, at power levels up to 400 kt .Currently, the scale-up to 1 MWt is under preparation by addintwo new modules to the test system.

In 2001, a related project started with the goal of integratingreceivers with a gas turbine to a complete solar-hybrid powsystem of about 250 kWe. The project~called SOLGATE! is co-funded by the EC and includes also further development onreceiver technology, especially to increase the receiver tempture to 1000°C and to reduce receiver cost. As a result ofproject a detailed layout for industrial prototype plants will bavailable.

Another similar project, CONSOLAR, was started by an Israconsortium in 1995 and is supported by the Israeli Ministry

Fig. 9 Modular receiver arrangement

Fig. 10 Reflective Tower concept †35‡

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Industry and Trade. Part of the project is the development ofReflective Tower concept~Fig. 10!, intended for solar powerplants with introduction of solar energy into gas turbines. Thconcept foresees an additional reflector at the top of a tower whredirects the concentrated solar radiation to a receiver locateground level. The receiver also uses a modular arrangemenpressurized units with secondary concentrators in front@33#. Thevolumetric absorber is composed of thin ceramic pins whichconvectively cooled. In a prototype receiver with 50 kWt tempera-tures up to 1200°C were achieved at pressures up to 20 bar. Twith a 500 kWt receiver and a gas turbine at the WIS Solar Towfacility are planned for 2002. Tests with the full loop incl. gaturbine are expected to be performed during the first half of 20

A cost estimate for a solar-hybrid tower plant of 30 MWe isgiven in Table 6~data is based on a exchange rate of $0.91/Eu!.Although the power level of 30 MWe is relatively small, the colevel is attractive. Simulation results for other cases can be foin @33,36,37#. Future developments and cost reductions from mproduction of components~especially heliostats! can bring costdown even further.

2.5 Solar Fuel Upgrading. Another option to introduce so-lar energy into high efficiency CC power plants is solar fuel ugrading. A schematic of the process is shown in Fig. 11. Inconventional CC process, the fuel~usually natural gas! is fed di-rectly to the gas turbine combustor. In a solar fuel upgradsystem, the natural gas~i.e., methane! is mixed with steam andintroduced into a solar reformer where the mixture is converteda synthesis gas~syngas! with high content of H2 and CO. Aftercooling down, the syngas is fed to the combustor instead of nral gas.

Advantages of this concept are that it needs only minor mofications of conventional CC technology~mainly a replacement ofthe burner!, has no direct interaction between fossil and soplant, and the solar plant can be added subsequently to an exiCC power plant. It has a storage capability for solar energystoring the cold syngas, and the fossil and solar plant can be band operated at different sites. The main restriction is thatsolar contribution is limited to about 25%, since fuel is alwarequired as basic feed component. The reaction requires opertemperatures in the range of 850–1000°C to achieve good mane conversion levels.

In 1998, a joint project was started under the acronySOLASYS, aiming at the demonstration of a complete solarforming power system in the size of 300 kWe @38#. The project issupported by the European Commission. The overall technicalindustrial objective is to demonstrate feasibility of the SOLASYconcept and to get confidence in the new technology. The specomponents of the test plant are the solar concentrating eqment, the solar driven receiver-reformer for the chemical conv

Fig. 11 Scheme of a solar reforming power plant

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sion of hydrocarbons to an upgraded fuel, and the adaptedturbine unit, which can be operated with conventional fuel awith syngas. The results of the SOLASYS project are neededidentify possible weak points of the design or operation produres and to achieve sufficient operational experience for futsteps towards commercialization and market introduction.

Even though reforming is a well established process in checal industry some new components had to be developed due toutilization of solar energy. A direct absorption concept withvolumetric absorber is used to perform the chemical reaction. Treceiver concept is very similar to the above described REFconcept. The main difference is that the absorber consists ocatalytically active porous structure. A ceramic foam structureused as absorber, with a catalyst coating to drive the reacusing the absorbed solar power.

The complete system will be installed and operated at the stower test facility of the Weizmann Institute of Science, IsraWith the given system layout under nominal solar conditionsmethane conversion of 84% will be achieved, the resulting syncontains 49% H2 and 13% CO.

Most of the SOLASYS equipment was constructed during tyear 2000, integration of the solar reformer and the gas turbwas completed in February 2002, and start-up of the plant isseen for April 2002.

3 Technological, Economical and Social Measures forthe Deployment of CRS

It is generally agreed that with current investment costsSTPP technologies do require a public support strategy for thdeployment into the market. An independent study promotedthe World Bank@39# confirms STPP as the most economical tecnology to produce bulk electricity from solar energy. Its diagnoassigns, however, the direct capital costs for STPP in 2.5–times the one of a fossil-fueled power plant and the electricproduced reaches generation costs 2–4 times higher. For theof solar tower power plants, the degree of uncertainty on thfigures and feasibility studies is relatively high since there islack of information concerning mass production costs of socomponents and O&M costs. Many performance assumptionscost trends are taken into account that should be proven withfirst generation of commercial plants scaling up to the 10–50 Mrange.

Fig. 12 Strategy for penetration in the market of the solartower power plants. The figure represents predicted LEC „inhybrid plants only the solar portion … versus time.

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In Fig. 12, a number of breakthrough plants are depicted widecisive intermediate milestone of $0.08/kWh for the solar eltricity produced by the year 2010 as a Key Action formulatedEC Energy Program. This first period until 2010 is basically suported on awarded electricity tariffs and subsidies to the invment required by the European Commission to all Member Stthrough a specific Directive developing the recommendationsout in the White Paper on renewable energy sources and endoby the Energy Council in May 1998 where 12% of energy cosumption of the European Union is foreseen by the year 2based on the deployment of renewables. Regarding STPP, ththe goal of Spain and Italy. The introduction measures are oriein Spain by the Promotion Plan of the Renewable Energy, whan attainable objective of 200 MW STPP installed before the y2010 is desired. In a first phase before 2005, CRS plants inrange of 10–15 MW should demonstrate in Spain the commerfeasibility of air and molten salt schemes. PS10 with 1.15 SoMultiple and 10 MW and Solar Tres with 15 MW and 3 SolMultiple will be the first ones. They will be followed in 2006 an2007 by two or more new plants of 50 MW size~PS50 and SolarCuatro!, optimized in that case for the maximum unitary sizelowed by the Spanish Administration. Once technology is fuproven the LEC should come close to the objective of $0.08/kfor new plants in the range of 80–100 MW that are foreseen eifor the US in higher annual irradiance sites or within the WoBank GEF ~Global Environmental Facility! initiatives forMorocco, Mexico, Iran, or others@39#. Other Green Marketsmight arise at that time as well. The first projects using moltenand air cooled receivers will be solar-only plants motivatedthe restrictions stemming from the funding programs and envirmental subsidies. It is feasible that hybrid projects will alcome to reality before 2010 using SOLGAS, CONSOLAR, aSOLGATE schemes provided restrictive legal requirementscome more flexible and that the technology of hybrid receivgains experience and scaling up feasibility. Because of the loinvestment required and higher power conversion efficiency thplants may achieve LEC of $0.08/kWh for smaller units. A tentive project Solgate30 is depicted for reference in Fig. 12.

It is estimated that by the year 2015 solar towers may enterinitial competitive markets for intermediate loads~about 4500hours! that could lead to additional reductions of LEC betwe$0.04/kWh and $0.06/kWh. However, to reach this goal willquire significant efforts in component improvements and escially cost reductions from mass production of components.

Apart from technology improvements and size increase, thetroduction into the first competitive markets requires equitataxation policies. The capital-intensive character of the solar Cplants penalizes them greater than fossil-fueled power plants@40#.This is a crucial fact since taxes may account for up to one thirdLEC value for a solar CRS plant and has more impact than tenology improvements on its competitiveness.

On the other hand, the technological developments must foon the improvement of the efficiencies of the different compnents, the evaluation of optimal integration with the thermodnamic cycle, the reduction of costs and the increase of theirability and durability. For power tower plants, the first keobjective is to demonstrate actual annual capacity factors andficiencies in commercial plants and compare these values to tpredicted from the experiences in pilot plants. Also the costjectives of the solar components, specifically for the helios~$100–130/m2!, must be verified, since serial production expeence is lacking@7,31,40#. Today we may claim that heliostats havachieved maturity and reliability with commercial designs liSanlucar, ATS or ASM150 guaranteeing beam qualities in trange of 2–2.6 mrad that are more than enough for CRS plaTherefore, R&D on heliostats is basically focusing on corefining designs of the most cost-sensitive elements like tracksystems and facets, and the introduction of new power supplycommunication systems like wireless communication and auto

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mous PV operation to reduce cabling and trenching costs@41#.Regarding solar receivers a wide variety of designs and prototyhave been tested and reported both for tubular and volumereceivers@42#. ~See also information related to Solar Tres, PSand Solgate in previous sections.! Solar receivers should demonstrate the efficiencies predicted in small prototypes~from 75 up to90%! and the durability of the absorber. Today’s experience wmolten salt and volumetric receivers and their integration intopower block is limited to short periods of time gathered durisystem or component tests and is one of the crucial componenbe qualified in the first generation of 10–15 MW grid-connectplants. Those first plants will probably have relatively consertive performance or reliability guarantees from manufacturerssuppliers, except for the Solar Tres project where performaguarantees will be required for at least 10 years to secure bloans. In addition, the tower power stations require a high leveautomation and integration of the control systems to assureperformance and reduce manpower. The control system at thelar Two plant showed that transitions between state and procontrol of each subsystem could be successfully automated dustartup, in steady operation, through transient weather condit~clouds! @42#, and during shutdown. Better automation~e.g., au-tomated real-time heliostat aiming and biasing of heliostats! is oneanother aspect that requires further technological improvem@43,44#.

4 ConclusionsThree concepts of power tower technology are on the verg

commercialization. Each technology has specific advantawhich can meet the needs of the power producer. Moltenpower towers offer cost-effective thermal storage that alloplants with high solar-only capacity factors to be built. The ficommercial plant of this type, Solar Tres, is planned to beployed in Spain by 2004. Closed loop volumetric air technolocan be integrated into combined cycle plants and achieve vhigh solar-to-electric conversion efficiencies. Two system demstrations of this technology are planned for Israel~Consolar! andSpain~Solgate!. Open-loop volumetric air technology offers simplified modular receivers designs with high outlet temperaturesintegration into Rankine cycle plants. A commercial plant of thtechnology, PS10, is planned for deployment in Spain in 20These plants, when fully commercialized, are expected to delelectricity at a levelized energy cost of $0.04–$0.06 per kWh.reach full commercialization, governmental grant, subsidies,tax equity are required.

References@1# Becker, M., Macias, M., and Ajona, J. I., 1996, ‘‘Solar Thermal Power S

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@18# Kelly, B., and Singh, M., 1995, ‘‘Summary of the Final Design for the 1MWe Solar Two Central Receiver Project,’’Solar Engineering: 1995, ASME,1, p. 575.

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@20# Zavoico, A. B., Gould, W. R., Kelly, B. D., Grimaldi, I., and Delegado, C2001, ‘‘Solar Power Tower~SPT! Design Innovations to Improve Reliabilityand Performance-Reducing Technical Risk and Cost,’’Proc. of Forum 2001Conf., April, Washington, DC.

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@25# Hoffschmidt, B., Ferna´ndez, V., Konstandopoulos, A. G., Mavroidis, IRomero, M., Stobbe, P., and Te´llez, F. 2001, ‘‘Development of Ceramic Volumetric Receiver Technology,’’Proc. of 5th Cologne Solar Symp., June 2001;K.-H. Funken and W. Bucher, eds. Forschungsbericht 2001-10, DLR-ColoGermany, pp. 51–61.

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@32# Price, H. W., Whitney, D. D., and Beebe, H. I., 1996, ‘‘SMUD Kokhala PowTower Study,’’Proc. of 1996 Int. Solar Energy Conf., San Antonio, TX.

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@41# Garcıa, G., Egea, A., Romero, M., and Ga´zquez, J. A., 2000, ‘‘The Stand-Alone Heliostat First Operation Results,’’Proc. Solar Thermal 2000—Renewable Energy for the New Millennium Conf., Sydney, Australia, March,H. Kreetz, K. Lovegrove, and W. Meike, eds., Australian and New ZealaSolar Energy Society, Sydney, pp. 165–170.

@42# Becker, M., and Vant-Hull L. L., 1991, ‘‘Thermal Receivers.’’Solar PowerPlants, C. J. Winter, R. L. Sizmann, L. L. Vant-Hull, eds., Springer-VerlaBerlin, pp. 163–198.

@43# Garcıa-Martin, F. J., Berenguel, M., Valverde, A., and Camacho, E. F., 19‘‘Heuristic Knowledge-Based Heliostat Field Control for the Optimizationthe Temperature Distribution in a Volumetric Receiver,’’ Sol. Energy,66~5!,pp. 355–369.

@44# Kolb, G. J., and Saluta, D., 1999, ‘‘Automatic Control of the Solar Two Rceiver,’’ Proc. of Renewable and Advanced Energy Systems for the 21sttury RAES’99, April, Maui, Hawaii, pp. RAES99-7707~CD-Rom!, R. Hogan,Y. Kim, S. Kleis, D. O’Neal, and T. Tanaka, eds., ASME, New York, 1999

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