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entral Receiver System Solar Powerlant Using Molten Salt as Heatransfer Fluid

. Ignacio OrtegaENER,evero Ochoa 4,.T.M., Tres Cantos, 28760 Madrid, Spain-mail: ignacio.obasagoiti@sener.es

. Ignacio BurgaletaENER,venida Zugazarte 56,as Arenas, 48930 Vizcaya, Spain-mail: ignacio.burgaleta@sener.es

élix M. TéllezIEMAT,venida Complutense 22,8040 Madrid, Spain-mail: felix.tellez@ciemat.es

f all the technologies being developed for solar thermal powereneration, central receiver systems (CRSs) are able to work athe highest temperatures and to achieve higher efficiencies inlectricity production. The combination of this concept and thehoice of molten salts as the heat transfer fluid, in both the re-eiver and heat storage, enables solar collection to be decoupledrom electricity generation better than water/steam systems, yield-ng high capacity factors with solar-only or low hybridizationatios. These advantages, along with the benefits of Spanish leg-slation on solar energy, moved SENER to promote the 17 MWeolar TRES plant. It will be the first commercial CRS plant witholten-salt storage and will help consolidate this technology for

uture higher-capacity plants. This paper describes the basic con-ept developed in this demonstration project, reviewing the expe-ience accumulated in the previous Solar TWO project, andresent design innovations, as a consequence of the developmentork performed by SENER and CIEMAT and of the technicalonditions imposed by Spanish legislation on solar thermal powereneration. �DOI: 10.1115/1.2807210�

eywords: solar power plant, CRS, central tower, molten salt,ube receiver, solar TRES

ntroductionThe Solar TRES demonstration project based on central re-

eiver system �CRS� technology inherited the lessons learnedrom the previous Solar TWO experimental project and takes ad-antage of the experience in molten-salt experiments and testingsarried out in the U.S. and Spain in the late 1980s and early 1990s1–10�.

The Solar TWO project �11,12� was a collaborative venture forhe design, construction, testing, and short term operation of a0 MWe CRS power tower solar plant using molten salt as its heatransfer and storage medium �Fig. 1�. The engineering, manufac-

Contributed by the Solar Energy Engineering Division for publication in the JOUR-

AL OF SOLAR ENERGY ENGINEERING. Manuscript received October 31, 2006; finalanuscript received September 12, 2007; published online February 12, 2008. Re-

iew conducted by Manuel Romero Alvarez. Paper presented at the Solar PACES

006, Seville, Spain.

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turing, and construction of Solar TWO lasted from 1992 to 1995,with initial startup and testing beginning in 1996. Solar TWOoperated from April 1996 to April 1999. Despite its many suc-cesses, the operation of Solar TWO was not without problems,mainly related to component startup issues, including heat tracing,piping, and the steam generator, which delayed routine operationof the plant for more than a year. At the end, all of the issues wereessentially overcome with some combination of redesign and/orrework, improved operating procedures, or work-arounds for fixesthat could not be implemented at Solar TWO �13�.

Some of the key results of Solar TWO that constitute the start-ing point for Solar TRES �14� were as follows:

• Receiver efficiency was measured at 88% in low-wind con-ditions �and 86% in allowable operating winds�, matchingdesign specifications.

• Storage system efficiency was measured at over 97%, alsomeeting design goals.

• Gross Rankine-turbine cycle efficiency was at 34%, match-ing performance projections.

• Measured plant peak-conversion efficiency was 13.5%.• The plant successfully demonstrated its ability to dispatch

electricity independent of collection. On one occasion, theplant operated around-the-clock for 154 h straight.

• Plant reliability was also demonstrated. During one stretchin the summer of 1998, the plant operated for 32 days out of39 days �4 days down because of weather, 1 day because ofloss of off-site power, but only 2 days down for mainte-nance�.

• Despite its short test and evaluation phase, which did notallow annual performance to be determined or operating andmaintenance procedures to be defined, the project identifiedseveral areas for simplifying the technology and improvingits reliability.

Besides the technological background in the U.S., both SENERand CIEMAT have had a long experience in developing systemsfor solar power plants, in the heliostat design, construction, andoperation, since the 1980s. CIEMAT has well-known and reputedcapabilities in CRS �design and operation� and in operation withmolten-salt systems.

In addition to validating the design and technical characteristicsof molten-salt receiver and storage technology, Solar TWO hasalso been successful in promoting commercial interest in powertowers. Two of the project’s key industrial partners, the BoeingCompany and Bechtel Corporation, agreed with a Spanish com-pany called GHERSA to pursue the commercial deployment ofmolten-salt technology taking advantage of Spanish prices for re-newable power �premiums and incentives�. The initial project,called “Solar TRES,” was predesigned according to the Spanishincentive framework applicable at that time, which did not allowhybridization to obtain high-capacity factors. The project was pro-posed to the EC �Fifth R&D Framework Program� for partial fi-nancing and was approved. Nevertheless, during project develop-ment, some legal issues �changes in Spanish renewablelegislation� along with other issues related to the partners them-selves led to some reorientation of the project and the promoter’sconsortium. This process came to an end in the year 2005 andconcluded with an entirely European development team �Spanish,French, and German companies� and is now working under theleadership of SENER.

The final absence of contributions from the U.S. companies,participating in Solar TWO, multiplied the challenges for design-ing and building a new feasible molten-salt plant. The most deli-cate development components were identified as a reliable anddurable receiver and a low-cost heliostat.

To overcome these challenges, SENER and CIEMAT signed aparallel agreement for developing and testing a prototype receiver

module �about 4 MWth� as a component acceptance milestone in

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olar TRES construction, which also includes the interaction in-olved in developing a 120 m2 low-cost heliostat with their ownechnology.

Heliostats remain to be one of the crucial economic aspects ofhis technology, since they are the most significant cost compo-ent of a CRS plant, accounting for 30–40% of capital invest-ent, of which 40–50% is tied to the cost of the drive system

gears, motors, etc.�. However, there was a rather limited experi-nce in developing industrial programs for manufacturing theseomponents at a large scale. For that reason, SENER decided toake a significant effort to evaluate current technologies and de-

elop an innovative low-cost heliostat design solution. During theast years, SENER has designed and tested a new 120 m2 low-ost heliostat and drive system, shown in Fig. 2, at its location inlataforma Solar de Almería �PSA�, and thoroughly described inef. �15�.CIEMAT’s contribution to receiver development was based on

ts expertise in both development and testing of several tube16–21� and salt receivers �22–24� and in materials technology22–26�. Furthermore, the test facilities at PSA, which are wellnown for their expertise in the concentrating solar community,onstitute the natural place for receiver panel acceptance testingnd heliostat evaluation and performance diagnostics.

panish Legal Framework for Solar ElectricityDemonstration projects in Europe are conceived as semicom-ercial units, involving new technologies that are not yet fully

ommercial but that must operate under commercial conditionslifetime and annual availability� in order to show the commercialeadiness of the technology.

In the renewable energy field, these requirements imply that thelant must show its capability for uninterrupted, commercial-scalemegawatt-size� power generation that could be fed into the gridor a period equivalent to the usual lifetime of a power plant.

For that reason, before any demonstration project in the solarhermal power �STP� sector could be developed, the Spanish En-rgy Authorities had to define the legal and economic frameworkor STP plants as part of a national renewable energy plan, sincehis technology, still regarded as being in the research and devel-pment stage, was not initially included in the legislation regulat-ng power generation in Spain since 1997 �Law 54/97�, althoughhere was a full section for power generation with renewable en-rgy and combined heat and power �CHP� units. In the year 2002,his was finally changed to include STP in the “feed-in tariff”cheme supporting renewable power plants.

ig. 1 Solar TWO molten-salt power tower system „schematiciagram…

Fig. 2 SENER heliostat and drive mechanism „detail…

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Further changes in legislation forced plant construction to bepostponed, as it posed some fundamental technical and economicuncertainties for the promoters. These issues were finally resolvedin the Spring of 2004 �Royal Decree 436/2004� and, hence, STPplants became a real alternative for renewable energy in Spain,provided they qualify as a “renewable energy producer” �and re-ceive an adequate price for the electricity produced� by meetingthe following conditions:

• Maximum installed power of 50 MW.• No hybrid plants.• A small percentage of natural gas �12–15% on primary en-

ergy basis� may be used in plants involving heat storagesystems only to maintain the thermal storage temperatureduring nongeneration periods. By Royal Decree 2531/2004,natural gas can also be used for power production duringnone or low-irradiation periods.

• Solar thermal electricity generators that deliver their produc-tion to a distributor may receive a fixed tariff of 300% of thereference price for the first 25 years after startup and 240%afterward. Solar thermal electricity generators that sell theirelectricity on the free market may receive a premium of250% of the reference price for the first 25 years after star-tup and 200% afterward, plus a 10% incentive. The averageelectric tariff or reference for the year 2004 was7.2072 c€ /kW h.

Following this regulation, the project design had to be re-viewed.

Molten-Salt Central Receiver Systems Compared toCompeting Technologies

According to SENER estimates, CRS power plants withmolten-salt storage are, even at the design stage, the winningchoice for STP plants in terms of energy efficiency, cost per unitproduced, and surface required for power production.

Moreover, high-capacity molten-salt storage makes it possiblefor the plant to provide dispatchable power, which, from the utili-ties’ point of view, is crucial for the deployment of these plants ascapable of secure, predictable, and programmable power supply,avoiding the problems for the national grid caused by other re-newable sources of power, such as wind or photovoltaic.

According to the European Concentrated Solar Thermal RoadMapping study entitled ECOSTAR �27,28�, cofunded by the EC,the U.S. 10 MW pilot plant experience has made the molten-salttechnology the best developed CRS today. Based on cost esti-mates provided by U.S. colleagues and the ECOSTAR evaluation,even small-scale �17 MWe� costs �leverized electricy cost �LEC�of 18–19 cents /kW h� look relatively attractive. This is mainlydue to very low thermal energy storage costs, which benefit froma three times larger temperature rise in the CRS compared to theparabolic trough systems. Furthermore, a higher annual capacityfactor than in parabolic trough systems is possible due to thesmaller difference between summer and winter performances. Thehighest risk is associated with expected plant availability, whichcould not be proven in the Solar TWO demonstration due to avariety of problems linked to the molten salt and the age of theheliostat field. However, technical solutions have been identifiedaddressing these issues. To further reduce molten-salt power towercosts, they must take advantage of economies of scale.

The plant availability risk can only be resolved in a demonstra-tion plant such as the Solar TRES now being designed. In the end,this risk could lead to additional costs not previously considered.

Published data from Refs. �27,28� and SENER studies lead tothe figures shown in Table 1.

Solar TRES Project Description

A schematic flow diagram of the plant is shown in Fig. 3.

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The Solar TRES project will take advantage of several advance-ents in the molten-salt technology since Solar TWO was de-

igned and built. These include the following:

• A larger plant with 2480 heliostat field, approximately threetimes the size of Solar TWO �120 m2 large-area glass-metalheliostats developed by SENER based on economic crite-ria�. Use of a large-area heliostat in the collector fieldgreatly reduces plant costs, mainly because fewer drivemechanisms are necessary for the same mirror area.

• A 120 MWth high-thermal-efficiency cylindrical receiversystem, able to work at high flux and lower heat losses. Thereceiver has been designed to minimize thermal stress and toresist intergranular stress corrosion cracking. High nickelalloy materials and an innovative integral header and nozzledesign developed by SENER, achieving the objectives ofhigh thermal efficiency, improved reliability, and reducedcost, will be used.

Table 1 Technology ass

Technology

Mean gross efficiency �as percentage of directradiation, without parasitics�

Mean net efficiencySpecific power generation �kw h /m2 yr�

Capacity factor �%�Unitary investment �€/kw h yr�

Operation and maintenance �c€/kW h�Levelized electricity cost �€ /kW he�

Fig. 3 Solar TRES

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• An improved physical plant layout with a molten-salt flowloop �Fig. 4� that reduces the number of valves, eliminates“dead legs,” and allows fail-safe draining that keeps saltfrom freezing.

• A larger thermal storage system �15 h, 647 MWh, 6250 tsalts� with insulated tank immersion heaters. This high-capacity liquid nitrate-salt storage system is efficient andlow risk, and high-temperature liquid salt at 565°C in sta-tionary storage drops only by 1–2°C /day. The cold salt isstored at 45°C above its melting point �240°C�, providing asubstantial margin for design.

• Advanced pump designs that will pump salt directly fromthe storage tanks, eliminating the need for pump sumps, andhigh temperature multistage vertical turbine pumps to bemounted on top of the thermal storage tanks, using a long-shafted pump with salt-lubricated bearings. This pump ar-

ment for 50 MWe plants

Parabolicrough+oil

CRS+steam

CRS+moltensalts

15.4 14.2 18.1

14 13.6 14308 258 375

23–50 24 Up to 751.54 1.43 1.293.2 4.1 3.7

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rangement eliminates the sump, level control valve, and po-tential overflow of the pump sump vessels.

• A 43 MW steam generator system that will have a forcedrecirculation steam drum. This innovative design placescomponents in the receiver tower structure at a height abovethe salt storage tanks that allows the molten-salt system todrain back into the tanks, providing a passive fail-safe de-sign. This simplified design improves plant availability andreduces operation and maintenance costs. The new designwill use a forced recirculation evaporator configuration tomove molten salt through the shell side of all heat exchang-ers, reducing risk of nitrate-salt freezing.

• A more efficient �39.4% at design point and 38% annualaverage�, higher-pressure reheat turbine and very high steampressure and temperature conditions for relatively low sizecompared to conventional power plants. A can be started upand stopped daily, and responds well to load changes, assur-ing a 30 yr lifetime with good efficiency.

• Improved instrumentation and control systems for heliostatfield and high temperature nitrate-salt process.

• Improved electric heat tracing system for protection againstfreezing of salt circuits, storage tanks, pumps, valves, etc.

These advancements improve the peak and annual conversionfficiency over the Solar TWO design. Although the turbine wille only slightly larger than Solar TWO’s, the larger heliostat fieldnd thermal storage system will enable the plant to operate4 h /day during the summer and have an annual solar capacityactor of approximately 64% up to 71%, including 15% produc-ion from fossil backup.

An example of Solar TRES’ dispatchability is illustrated in Fig., which shows the load-dispatch capacity from the 14th to the

Fig. 4 Solar TRES 3D view „SENSOL output…

Fig. 5 Solar TRES pow

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18th of August. The figure shows the solar intensity �power onreceiver�, energy stored in the hot tank, and power output as afunction of the time of day.

Solar TRES Sensitivity AnalysisSeveral plant configuration studies �29� taking into consider-

ation economic profitability and plant investment cost were per-formed using the SENSOL code, developed by SENER for solarplant optimization �30�.

The following factors were analyzed:

• Number of heliostats: different heliostat field configurations,ranging from 1800 to 3500.

• Optimum heliostat mirror-surface, reflectivity, and cleaning-factor performance.

• Tower height: from 90 m to 150 m.• Receiver dimensions: diameter �8–10 m�, height �9–11 m�,

number of panels.• Storage size: from 10 h to 20 h.• Turbine power: from 10 MWe to 20 MWe.• Annual use of natural gas, ranging from 10% to 15%, with

different applications for maintaining the hot-salt tempera-ture.

• Storage during electricity generation and nongeneration,supporting solar energy during startup.

For each of these factors, several plant configurations have beenevaluated with SENSOL, predicting the global investment and theeconomic profitability of each particular design. Global plant pro-duction and consumption were calculated, as well as other oper-ating costs �maintenance, cleaning, etc.� for each configuration.

As an example of the different plant configurations studied, Fig.6 shows the SENSOL output for the number of heliostats, turbinepower, and cost per kW h produced.

The analysis concluded that, based on RD 436/2004 and RD2531/2004, the best combination of profitability and minimuminvestment leads to the basic Solar TRES plant configuration de-fined in Table 2.

Present Status of Central Receiver System Molten-SaltTechnology

The Solar TRES project is now in the last stages of technicalverification �testing of receiver modules, heliostats, molten-saltpilot plant, and control system� and sitting �final definition, licens-ing and permitting, and final cost estimation�.

Regarding the technical development, testing at PSA on a pro-totype receiver module, now under development, will allowSENER to determine the safety limits and the life of the receiver

er dispatch capacity

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nder critical conditions, as well as to confirm relevant parameterse.g., receiver efficiency� to minimize risks and to increase opera-ional experience with molten-salt system.

Project schedule is also pending on the administrative issues ofhe project, already mentioned. Construction and assembling isxpected to last for 24 months after having all those issues solvednd properly defined.

The Solar TRES project represents the demonstration step forRS molten-salt technology, since this technology has not yet

hown its real potential in a long term and continuous operation.or that reason, this project involves some technical risk, regard-

ng mainly the operation, on real conditions of a large molten-saltystem, including receiver, pumps, valves, long pipes, and tanks.

Further development of the technology should led to biggerlants, in the 50–100 MW range, that must reduce technologyosts. Feasibility studies being performed by SENER for sometilities show the cost reduction potential for a 100 MWe molten-alt central receiver solar power plant under different configura-ions.

onclusionsThe CRS technology and molten-salt storage proven experi-entally in Solar TWO offers the following advantages over other

olar technologies: high-capacity thermal storage, good availabil-ty for dispatchable power, and least-cost kW h produced. Thesedvantages, along with the benefits of Spanish legislation on solarnergy, were the reasons that SENER decided to promote the con-truction of a 17 MWe solar plant, named Solar TRES. It will behe first commercial CRS solar power plant with molten-salt stor-ge and will help consolidate this technology for future plantsith higher power.

ig. 6 Solar TRES sensitivity analysis „heliostat/turbineower/energy cost…

Table 2 Solar TRES key figures

umber of heliostats 2480urface covered by heliostats 285,200 m2

urface covered by heliostats 142.31 haower height 120 meceiver power 120 MWthurbine power 17 MWetorage size 15 hatural gas boiler capacity 16 MWthnnual electric production �min.� 96,400 MWhe

O2 mitigation �best available technology� 23,000 tons /yrO2 mitigation �coal power plant� 85,000 tons /yr

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AcknowledgmentThe engineering and testing activities of the Solar TRES power

plant are being partly funded by the European Commission �EC��Contract No. NNE5-2001-369�.

Nomenclature

AcronymsCHP � combined heat and power

CIEMAT � center for Energy, Environment and Techno-logical Research �Spain�

CRS � central receiver systemCSP � concentrating solar thermal powerDNI � direct normal insolationEC � European Commission �E.U.�

DOE � Department of EnergyNREL � National Renewable Energy Laboratory �U.S.�

SENER � Engineering, Consulting and Integration Com-pany �Spain�

SNL � Sandía National Laboratory �U.S.�STP � solar thermal powerRD � Royal Decree

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