It has been established that the development of a storage option and increasing the operating temperature for parabolic trough electric systems can significantly reduce the levelized electricity cost compared to the current state of the art. Both improvements require a new heat transfer fluid that must have a very low vapor pressure at the hot operating temperature and combined with a high thermal stability, higher than 450°C. Further, the piping layout of trough plants dictates that the fluid not be allowed to freeze, which dictates the use of extensive insulation and heat tracing unless the fluid has a freezing point near 0°C. At present, it seems likely that this “ideal” fluid will have to be found among organic rather than inorganic salts. We are, therefore, investigating the chemical and thermal properties of “room temperature ionic liquids” that hold much promise as a new class of heat transfer or storage fluids.

Solar Two was a collaborative, cost-shared project between 11 U. S. industry and utility partners and the U. S. Department of Energy to validate molten-salt power tower technology. The Solar Two plant, located east of Barstow, CA, comprised 1926 heliostats, a receiver, a thermal storage system, a steam generation system, and steam-turbine power block. Molten nitrate salt was used as the heat transfer fluid and storage media. The steam generator powered a 10-MWe (megawatt electric), conventional Rankine cycle turbine. Solar Two operated from June 1996 to April 1999. The major objective of the test and evaluation phase of the project was to validate the technical characteristics of a molten salt power tower. This report describes the significant results from the test and evaluation activities, the operating experience of each major system, and overall plant performance. Tests were conducted to measure the power output (MW) of the each major system, the efficiencies of the heliostat, receiver, thermal storage, and electric power generation systems and the daily energy collected, daily thermal-to-electric conversion, and daily parasitic energy consumption. Also included are detailed test and evaluation reports.

It has been established that the development of a storage option and increasing the operating temperature for parabolic trough electric systems can significantly reduce the levelized electricity cost (LEC) compared to the current state of the art. Both improvements require a new heat transfer fluid that must have a very low vapor pressure at the hot operating temperature and combined with a high thermal stability, higher than 450°C. Further, the piping layout of trough plants dictates that the fluid not be allowed to freeze, which dictates the use of extensive insulation and heat tracing unless the fluid has a freezing point near 0°C. At present, it seems likely that this “ideal” fluid will have to be found among organic rather than inorganic salts. We are therefore investigating the chemical and thermal properties of ‘room temperature ionic liquids’ (RTILs) that hold much promise as a new class of heat transfer or storage fluids.

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The integrated solar plant concept was initially proposed by Luz Solar International [1] as a means of integrating a parabolic trough solar plant with modern combined cycle power plants. An integrated plant consists of a conventional combined cycle plant, a solar collector field, and a solar steam generator. During sunny periods, feedwater is withdrawn from the combined cycle plant heat recovery steam generator, and converted to saturated steam in the solar steam generator. The saturated steam is returned to the heat recovery steam generator, and the combined fossil and solar steam flows are superheated in the heat recovery steam generator. The increased steam flow rate provides an increase in the output of the Rankine cycle. During cloudy periods and at night, the integrated plant operates as a conventional combined cycle facility. Two studies on integrated plant designs using a General Electric Frame 7(FA) gas turbine and a three pressure heat recovery steam generator are currently being conducted by the authors. Preliminary results include the following items: 1) the most efficient use of solar thermal energy is the production of high pressure saturated steam for addition to the heat recovery steam generator; 2) the quantity of high pressure steam generation duty which can be transferred from the heat recovery steam generator to the solar steam generator is limited; thus, the maximum practical solar contribution is also reasonably well defined; 3) small annual solar thermal contributions to an integrated plant can be converted to electric energy at a higher efficiency than a solar-only parabolic trough plant, and can also raise the overall thermal-to-electric conversion efficiency in the Rankine cycle; and 4) annual solar contributions up to 12 percent in an integrated plant should offer economic advantages over a conventional solar-only parabolic trough power plant.