Molten-salt storage is already commercially available for concentrating solar power (CSP) plants, allowing solar power to be produced on demand and to Bbackup[ variable renewable sources such as wind and photovoltaics. The first CSP plants to operate commercially with molten-salt storage utilized parabolic trough concentrators, for example, the Andasol-1 plant. A new type of storage plant has now reached commercial status, with the 19.9-MWe Torresol Gemasolar power tower, featuring 15 h of molten-salt storage, having come online in Spain in May 2011. Advantages of the power tower storage system include the elimination of heat transfer oil and associated heat exchangers, a lower salt requirement, higher steam cycle efficiency, better compatibility with air cooling, improved winter performance, and simplified piping schemes. Near-term advances in molten-salt power tower technology include planned up-scaling, with SolarReserve due to begin constructing a 110-MWe plant in Nevada by August 2011. Other advances include improvements to the thermal properties of molten salts and the development of storage solutions in a single tank. With these developments at hand, CSP will continue to provide dispatchable solar power, with the capacity to provide energy storage for 100% renewable electricity grids in sun-belt countries.
Concentrating solar power (CSP) can both generate and store renewable energy all in the one plant, delivering dispatchable powerVan enticing combination in the eyes of a grid operator. Parabolic mirrors concentrate the sun’s energy to a hot focus. This heat can be used to produce steam for immediate electricity generation, or alternatively it can be stored prior to electricity generation using molten salt –, sensible heat storage in solids –, phase change salts , or thermochemical storage cycles – . When required, this stored energy can be used to produce steam and drive a turbine. In this way, variable renewable energy sources such as wind and photovoltaics can be dispatched to the grid first, and the Bbackup[ provided by concentrating solar plants with storage.
Of the CSP storage methods listed, molten-salt storage is the only storage currently used in commercial CSP plants. Molten-salt CSP storage has been commercially proven since the end of 2008, when the 50 MWe (MW electric) Andasol-1 trough plant (Fig. 1) began power production with 7.5 h of molten-salt storage, near Guadix in the province of Granada, Spain . As of July 2011, seven similar 50-MWe parabolic trough plants, each with 7.5 h of molten-salt storage have come online in Spain, bringing the total to eight: Andasol-1, Andasol-2, Extresol-1, Extresol-2, Manchasol-1, Manchasol-2, La Florida, and La Dehesa . Andasol-2 is located adjacent to Andasol-1 (see Fig. 1), while Manchasol-1 and Manchasol-2 are located in the province of Ciudad Real, and the latter four plants are all located in the province of Badajoz. Another 17 trough plants with molten-salt storage are in advanced stages of construction in Spain , and more are planned. But there is a new technology entering the CSP storage market, and it comes with some advanced features.
At the beginning of May 2011, the 19.9-MWe Torresol Gemasolar power tower shown in Fig. 2 began selling power into the grid near Fuentes de Andalucı´a in the province of Seville, Spain. Test runs of the Gemasolar power tower had been carried out since March 2011. Gemasolar thus became the first commercial power tower to operate with dispatchable storage.1 The developer and operator, Torresol Energy based in Vizcaya, Spain, is a joint venture between the Spanish engineering firm SENER (60%), also headquartered in Vizcaya, and Abu Dhabibased Masdar (40%) from the United Arab Emirates. Plant construction has been carried out by a joint venture between SENER and Madrid-based Cobra Energı´a.
Gemasolar has a gross turbine capacity of 19.9 MWe, and a net capacity of 17 MWe during daylight hours. This net capacity can increase above the 17 MWe overnight when there are lower parasitic loads, as at night it is not necessary to pump salt up the tower to the receiver, and there is no mirror field operation. Like the trough plants discussed above, Gemasolar uses molten salt to store energy, but in this case, enough storage is provisioned for 15 h of operation after dark at the full 19.9-MWe gross capacity. This will allow Gemasolar to operate at an annual capacity factor of 74% from solar alone . Given a net plant capacity of 17 MWe, an annual capacity factor of 74% means that Gemasolar will produce 110 000-MWh/y net, out of a possible total of 148 920 MWh/y if it operated at 17-MWe net output, 24 h a day, 365 days a year. In contrast, the aforementioned trough plants with storage have a capacity factor of around 41% . From a technical point of view, the capacity factor of a trough plant could be increased by increasing the size of the mirror field and storage compared to the turbine. However, as discussed in this paper, with current trough technology this is a less attractive option economically than constructing a power tower with high capacity factor.
This review presents a history of molten-salt power tower development, the unique features of this technology, the case-study of the Gemasolar plant, and the near-term advances that can be expected in this field.
II. HISTORY OF MOLTEN-SALT POWER TOWERS
The first power towers to directly heat molten salt were the 2.5-MWe THEMIS tower in the French Pyre´ne´es, and the 1-MWe Molten-Salt Electric Experiment (MSEE/Cat B) Project in the United States, both of which began operation in 1984 , . These were followed by the 10-MWe Solar Two power tower near Barstow, CA, which featured 3 h of molten-salt storage, and operated from 1996 to 1999 (see Fig. 3) , . The cost of the Solar Two project was shared between the U.S. Department of Energy, and various industry partners, with technical support from Sandia National Laboratories and the National Renewable Energy Laboratory (NREL). A full list of project participants is given by Pacheco et al. . The Solar Two project retrofitted molten salt, both as heat transfer fluid and storage technology, to the existing Solar One power tower concentrator. Solar One operated with a steam receiver, and oil/rock storage from 1982 to 1988. Solar Two, on the other hand, demonstrated molten-salt power tower technology at a large scale, and resulted in practical recommendations for the commercialization of the technology.
Due to a lack of policy incentives, no molten-salt power towers were constructed from 1999 until November 2008, when construction began on the Torresol Gemasolar power tower. Originally conceived as the Solar Tres project , VSpanish for BSolar Three[Vthe Gemasolar tower builds on the experiences gained during the operation of the Solar One and Solar Two research facilities in the United States, with project-specific engineering completed by SENER.
III. THE PRINCIPLE OF MOLTEN-SALT STORAGE
One of the most important characteristics of using a thermal storage system is the very high efficiency of the storage, with an annual efficiency of 99% possible for commercial plants . The only losses come from:
• slow heat loss through the tank walls, which is kept to a minimum via insulation;
• the heat exchange process between mediums, i.e., salt to steam for towers, or oil to salt, salt to oil, and then to steam, in the case of a trough system.