CENTRAL RECEIVER SYSTEM (CRS) SOLAR POWER PLANT USING MOLTEN SALT AS HEAT TRANSFER FLUID J.Ignacio Ortega (1), J.Ignacio Burgaleta (2), Félix M. Téllez (3) (1) SENER, Severo Ochoa 4, P.T.M., 28760 Tres Cantos, Madrid, Spain Email: ignacio.obasagoiti@sener.es (2) SENER, Av. Zugazarte 56, 48930 Las Arenas, Vizcaya, Spain Email: ignacio.burgaleta@sener.es (3) CIEMAT, Av. Complutense 22, 28040 Madrid, Spain Email: felix.tellez@ciemat.es Abstract Of all the technologies being developed for Solar Thermal Power Generation, Central Receiver Systems (CRS) are able to work at the highest temperatures and to achieve higher efficiencies in electricity production. The combination of this concept and the choice of molten salts as the heat transfer fluid, both in the receiver and for heat storage, enables solar collection to be decoupled from electricity generation better than water/steam systems, yielding high capacity factors with solar-only or low hybridisation ratios. These advantages, along with the benefits of Spanish legislation on solar energy, were the reasons that the SENER company decided to promote the 17-MW e Solar TRES plant. It will be the first commercial CRS plant with molten-salt storage, and will help consolidate this technology for future higher-capacity plants. This paper describes the basic concept developed in this demonstration project, reviewing the experience accumulated in the previous Solar TWO project, and present design innovations, as a consequence of the development work performed by SENER and CIEMAT, and of the technical conditions imposed by Spanish legislation on Solar Thermal Power Generation. Keywords: solar power plant, CRS, central tower, molten salt, tube receiver, Solar TRES Background The Solar TRES demonstration project based on CRS technology inherited the lessons learned from the previous Solar TWO experimental project and takes advantage of the experience in molten salt receiver experiments carried out in the U.S. and Spain in the early nineties. The Solar TWO project[13] was a collaborative venture for the design, construction, testing and limited-time operation of a 10-MWe CRS power tower solar plant using molten salt as its heat transfer and storage medium (Figure 1). The engineering, manufacturing, and construction of Solar TWO lasted from 1992 into 1995, with initial startup and testing beginning in 1996. Solar TWO operated from April 1996 to April 1999. Despite its many successes, operation of Solar TWO was not without problems, mainly related to component startup issues, including heat tracing, piping, and the steam generator, that delayed routine operation of the plant for more than a 1.
year. At the end, all of the issues were essentially overcome with some combination of redesign and/or rework, improved operating procedures, or workarounds for fixes that could not be implemented at Solar TWO. Some of the key results of Solar TWO that constitute the starting point for Solar TRES were: Receiver efficiency was measured at 88% in low-wind conditions (and 86% in allowable operating winds), matching design specifications. Storage system efficiency was measured at over 97%, also meeting design goals. Gross Rankine-turbine cycle efficiency was at 34%, matching 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, the Plant operated around-theclock for 154 hours straight [13]. Plant reliability was also demonstrated. During one stretch in the summer of 1998, the plant operated for 32 out of 39 days (four days down because of weather, one day because of loss of offsite power, but only two days down for maintenance). Despite its short test and evaluation phase, which did not allow annual performance to be determined or operating and maintenance procedures to be defined, the project identified several areas for simplifying the technology and improving its reliability. Figure 1. Solar TWO molten-salt power tower system schematic diagram Besides the technological background in the U.S., both SENER and CIEMAT have a long experience in developing systems for solar power plants, between other in the heliostat design, construction and operation, since the 80 s. CIEMAT has well-known and reputated capabilities in CRS (design and operation) and in operation with molten salts systems. In addition to validating the design and technical characteristics of molten-salt receiver and storage technology, Solar TWO has also been successful in promoting commercial interest in power towers. Two of the project s key industrial partners, the Boeing Company and Bechtel Corporation, agreed with a Spanish company called GHERSA to pursue the commercial deployment of molten salt technology taking advantage of Spanish prices for renewable power (premiums and incentives). The initial project, called SOLAR TRES, was pre-designed according to the Spanish incentive framework applicable at that time, which did not allow hybridization to obtain high-capacity factors. The project was proposed to the EC (5 th R&D 2.
Framework Program) for partial financing and was approved. Nevertheless, during project development, some legal issues (changes in Spanish Renewable legislation) along with other issues related to the partners itself led to some reorientation of the project and the promoter s consortium. This process came to an end in the year 2005, and concluded with an entirely European development team (Spanish, French, and German companies), is now working under the leadership of SENER. The final absence of contributions from the U.S. companies, participating in Solar TWO, multiplied the challenges for designing and building a new feasible molten saltplant. The most delicate development components were identified as a reliable and durable receiver and a low-cost heliostat. To overcome these challenges, SENER and CIEMAT signed a parallel agreement for developing and testing a prototype receiver module ( about 4 MW th ) as a component acceptance milestone in Solar TRES construction, which also includes the interaction involved in developing a 120 m 2 low-cost heliostat with own technology. Heliostats remain to be one of the crucial economic aspects of this technology, since they are the most significant cost component of a CRS plant, accounting for 30% to 40% of capital investment, of which 40% to 50% is tied to the cost of the drive system (gears, motors,...). However there was a rather limited experience in developing industrial programs for manufacturing these components at a large scale. For that reason SENER decided to make a significant effort to evaluate current technologies and develop an innovative low-cost heliostat design solution. During the last years, SENER has designed and tested a new 120 m 2 low-cost heliostat and drive system. CIEMAT s contribution to receiver development was based on its expertise in both development and testing of several tube [1-7] and salt receivers [8-10] and in materials technology ([11, 12]). Furthermore, the test facilities at the Plataforma Solar de Almería, which are well-known for its expertise in the concentrating solar community, constitute the natural place for receiver panel acceptance testing and heliostat evaluation and performance diagnostics. Spanish Legal Framework for Solar Electricity Demonstration projects in Europe are conceived as semi-commercial units, involving new technologies that are not yet fully commercial, but that must operate under commercial conditions (lifetime and annual availability), in order to show the commercial readiness of the technology. In the Renewable Energy field, these requirements imply that the plant must show its capability for uninterrupted, commercial-scale (MW-size) power generation that could be fed into the grid, for a period equivalent to the usual lifetime of a power plant. For that reason, before any demonstration project in the Solar Thermal Power (STP) sector could be developed, the Spanish Energy Authorities had to define the legal and economic framework for Solar Thermal Power plants as part of a national renewable energies plan, since this technology, still regarded as being in the R&D stage, was not initially included in the legislation regulating power generation in Spain since 1997 (Law 54/97), although there was a full section for power generation with Renewable 3.
Energy and CHP units. In the year 2002 this was finally changed to include STP in the feed-in tariff scheme supporting renewable power plants. Further changes in legislation forced plant construction to be postponed, as it posed some fundamental technical and economic uncertainties for the promoters. These issues were finally resolved in Spring 2004 (Royal Decree 436/2004), and hence Solar Thermal Power Plants became a real alternative for Renewable Energy in Spain, provided they qualify as a Renewable Energy Generator (and receive an adequate price for the electricity produced) by meeting the following conditions: Maximum installed power 50 MW. No hybrid plants. A small percentage of natural gas or propane (12-15% on primary energy basis) may be used in plants involving heat storage systems only, to maintain the thermal storage temperature during non-generation periods. By Royal Decree 2531/2004, natural gas can also be used for power production during none or low-irradiation periods. Solar thermal electricity generators who deliver their production to a distributor may receive a fixed tariff of 300% of the reference price for the first 25 years after startup and 240% afterwards. Solar thermal electricity generators, which sell their electricity on the free market, may receive a premium of 250% of the reference price for the first 25 years after startup and 200% afterwards, plus a 10% incentive. The average electric tariff or reference for the year 2004 was 7.2072 c /kwh In view of this regulation, the project design had to be reviewed. Molten-salt CRS systems compared to competing technologies According to SENER estimates, CRS Power plants with molten-salt storage are, even at the design stage, the winning-choice for solar thermal power plants, in terms of energy efficiency, cost per unit produced, and surface required for power production. Moreover, high-capacity molten-salt storage makes it possible for the plant to provide dispatchable power, which, from the utilities point of view, is crucial for the deployment of these plants as capable of secure, predictable and programmable power supply, avoiding the problems for the national grid caused by other renewable sources of power, such as wind or photovoltaic. According to the European Concentrated Solar Thermal Road Mapping study entitled ECOSTAR [14, 15], co-funded by the EC, the US 10-MW pilot plant experience has made the molten salt technology the best developed central receiver system today. Based on cost estimates provided by U.S. colleagues and the ECOSTAR evaluation, even small-scale (17 MWe) costs (LEC 18-19 cents/kwh) look relatively attractive. This is mainly attributable to very low thermal energy storage costs, which benefit from a three times larger temperature rise in the CRS compared to the parabolic trough systems. Furthermore, a higher annual capacity factor than in parabolic trough systems is possible, due to the smaller difference between summer and winter performance. The highest risk is associated with expected plant availability, which could not be proven in the Solar TWO demonstration due to a variety of problems 4.
linked to the molten salt and the age of the heliostat field. However, technical solutions have been identified addressing these issues. To further reduce molten-salt power tower costs, they must take advantage of economies of scale. The plant availability risk can only be resolved in a demonstration plant like the Solar TRES now being designed. In the end, this risk could lead to additional costs not previously considered. Published data from [14, 15] and SENER studies leads to the figures showed in Table 1. Technology Mean gross efficiency (as % of direct radiation, without parasitics) Parabolic Trough + oil CRS + steam CRS+molten salts 15.4 14.2 18.1 Mean net efficiency 14 13.6 14 Specific power generation (kwh/m 2 -y) 308 258 375 Capacity factor (%) 23-50 % 24 % Up to 75 % Unitary Investment ( /kwh-y) 1.54 1.43 1.29 Operation and Maintenance (c /kwh) 3.2 4.1 3.7 Levelized Electricity Cost ( /kwh e ) 0.16-0.19 0.17-0.23 0.14-0.17 Table 1. Technology assessment for 50 MWe plants Solar TRES Project Description A schematic flow diagram of the Plant is shown in Figure 2. Figure 2. Solar TRES flow schematic 5.
The Solar TRES project will take advantage of several advancements in the molten salt technology since Solar TWO was designed and built. These include: Larger plant with 2480 heliostat field approximately three times the size of Solar TWO, 120 m² large-area glass-metal heliostats developed by SENER based on economic criteria. Use of a large-area heliostat in the collector field greatly reduces plant costs, mainly because fewer drive mechanisms are necessary for the same mirror area. A 120-MWth high-thermal-efficiency cylindrical receiver system, able to work at high flux, and lower heat losses. The receiver has been designed to minimize thermal stress and to resist intergranular stress corrosion cracking. High nickel alloy materials will be used, and an innovative integral header and nozzle design developed by SENER, achieving the objectives of high thermal efficiency, improved reliability and reduced cost. An improved physical plant layout with a molten-salt flow loop (Figure 3) that reduces the number of valves, eliminates dead legs and allows fail-safe draining that keeps salt from freezing. Figure 3. Solar TRES 3D view (SENSOL output) A larger thermal storage system (15 hours, 647 MWh, 6250 t salts) with insolated tank immersion heaters. This high-capacity liquid nitrate-salt storage system is efficient and low-risk, and high-temperature liquid salt at 565ºC in stationary storage drops only 1-2ºC/day. The cold salt is stored at 45ºC above its melting point (240ºC), providing a substantial margin for design. Advanced pump designs that will pump salt directly from the storage tanks, eliminating the need for pump sumps, and high-temperature multi-stage vertical turbine pumps to be mounted on top of the thermal storage tanks, using a long-shafted pump with salt-lubricated bearings. This pump arrangement eliminates the sump, level control valve and potential overflow of the pump sump vessels. A 43-MW steam generator system that will have a forced-recirculation steam drum. This innovative design places components in the receiver tower structure at a height above the salt storage tanks that allows the molten-salt system to drain back into the tanks, providing a passive fail-safe design. This simplified design improves plant availability and reduces O&M costs. 6.
The new design will use a forced recirculation evaporator configuration to move molten salt through the shell side of all heat exchangers, reducing risk of nitrate salt freezing. A more efficient (39.4% at design point and 38% annual average), higherpressure reheat turbine, very high steam pressure and temperature conditions for relatively low size compared to conventional power plants. Can be started up and stopped daily, and responds well to load changes, assuring a 30-year lifetime with good efficiency. Improved instrumentation and control systems for heliostat field and high temperature nitrate-salt process. Improved electric heat tracing system for protection against freezing of salt circuits, storage tanks, pumps, valves, etc. These advancements improve the peak and annual conversion efficiency over the Solar TWO design. Although the turbine will be only slightly larger than Solar TWO s, the larger heliostat field and thermal storage system will enable the plant to operate 24 hours a day during the summer and have an annual solar capacity factor of approximately 64% and up to 71%, including 15% production from fossil backup [6]. An example of SOLAR TRES dispatchability is illustrated in Figure 4, which shows the load-dispatch capacity from the 14 th to the 18 th of August. The figure shows the solar intensity (power on receiver), energy stored in the hot tank, and power output as a function of the time of day Figure 4. Solar TRES power dispatch capacity SOLAR TRES Sensitivity Analysis Several plant configuration studies [16] taking into consideration economic profitability and plant investment cost were performed using the SENSOL code, developed by SENER for solar plant optimization. The following factors were analyzed: Number of heliostats: different heliostat field configurations, ranging from 1800 to 3500. 7.
Optimum heliostat mirror-surface, reflectivity and cleaning-factor performance. Tower height: from 90 m to 150 m. Receiver dimensions: Diameter (8 to 10 m), height ( 9 to 11 m), number of panels. Storage size: from 10 to 20 hours. Turbine power: from 10 to 20 MW e. Annual use of natural gas, ranging from 10% to 15%, with different applications for maintaining the hot-salt temperature. Storage during electricity generation and non-generation, supporting solar energy during startup. For each of these factors, several plant configurations have been evaluated with SENSOL, predicting the global investment and the economic profitability of each particular design. Global plant production and consumption were calculated, as well as other operating costs (maintenance, cleaning, etc.) for each configuration. As an example of the different plant configurations studied, Figure 5 shows the SENSOL output for number of heliostats, turbine power and cost per kwh produced. Figure 5. Solar TRES sensitivity analysis (heliostat/turbine power/energy cost) The analysis concluded that, based on RD 436/2004 and RD 2531/2004, the best combination of profitability and minimum investment leads to the basic Solar TRES Plant configuration defined in Table 2. Number of heliostats 2480 Surface covered by heliostats 285 200 m² Surface covered by heliostats 142.31 Ha Tower height 120 m Receiver Power 120 MWth Turbine Power 17 MWe Storage size 15 hours Natural Gas Boiler capacity 16 MWth Annual electric production (min.) 96 400 MWhe CO2 mitigation (best available technology) 23 000 tons/year CO2 mitigation (coal power plant) 85 000 tons/years Table 2. Solar TRES keys figures 8.
Present Status of CRS molten salt technology The Solar TRES project is now in the last stages of technical verification (testing of receiver modules, heliostats, molten-salt pilot plant and control system), and sitting (final definition, licensing and permitting, and final cost estimation). Outside of Spain, South Africa s ESKOM utility has undertaken a feasibility study for a 100 MW e molten-salt central receiver solar power plant. Conclusions The Central Receiver System (CRS) technology and molten salt storage proven experimentally in Solar TWO offers the following advantages over other solar technologies: high-capacity thermal storage, good availability for dispatchable power and least-cost kwh produced. These advantages, along with the benefits of Spanish legislation on solar energy were the reasons that SENER decided to promote the construction of a 17 MW e solar plant, named Solar TRES. It will be the first commercial CRS solar power plant with molten salt storage and will help to consolidate this technology for future plants with higher power. Acknowledgements The engineering and testing activities of the Solar TRES Power Plant are being partly funded by the European Commission (EC) (Contract NNE5-2001-369). Acronyms CHP CIEMAT CRS CSP DNI EC DOE CHP NREL SENER SNL STP RD Combined Heat and Power Center for Energy, Environment and Technological Research (Spain) Central Receiver System Concentrating Solar Thermal Power Direct Normal Insolation European Commission (E.U.) Department of Energy Combined Heat and Power National Renewable Energy Laboratory (U.S.) Engineering, Consulting and Integration Company (Spain) Sandía National Laboratory (U.S.) Solar Thermal Power Royal Decree References 1. Becker, M. and M. Boehmer, GAST: The Gas Cooled Solar Tower Technology Program. 1989: Springer Verlag, Berlin Heidelberg. 2. Schiel, W.J.C. and M.A. Geyer, Testing an external sodium receiver up to heat fluxes of 2.5 MW/m 2 : Results and conclusions from the IEA-SSPS high flux 9.
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