Thermal storage improves the dispatchability a nd marketability of parabolic trough power plants allowing them to produce electricity on demand independent of solar collection. One such thermal storage system, a thermocline, uses a single tank containing a fluid with a thermal gradient running vertically through the tank, where hotter fluid (lower density) is at the top of the tank and colder fluid is at the base of the tank. The thermal gradient separates the two temperature potentials. A lowcost filler material provides the bulk of the thermal capacitance of the thermal storage, prevents convective mixing, and reduces the amount of fluid required. In this paper, development of a thermocline system that uses molten-nitrate salt as the heat transfer fluid is described and compared to a two-tank molten salt system. Results of isothermal and thermal cycling tests on candidate materials and salt safety tests are presented as well as results from a small pilot-scale (2.3 MWh) thermocline.
NOMENCLATURE
INTRODUCTIONThermal storage for parabolic-trough power plants
The two cycles selected for final consideration are the UT-3 cycle and the Sulfur-Iodine (S-I) cycle. The UT-3 cycle was invented at the University of Tokyo and much of the early development was done there. This cycle has been studied extensively by the Japan Atomic Energy Research Institute (JAERI). After considering several different flowsheets making use of the UT-3 cycle, JAERI selected the so-called Adiabatic UT-3 process for further development. The predicted efficiency of the Adiabatic UT-3 process varies between 35% and 40% depending upon the efficiency of membrane separators which are under development. A 10% overall efficiency increase is projected if electricity is co-generated along with the hydrogen.
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.
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