This report concludes that there is a cost-effective strategy for seasonal storage of heat that will provide firm, but dispatchable, electrical generating capacity in times when other renewable energy is not available to meet demand. Deployment of the technology appears to require no new technology, but instead combines solar, geothermal, and conventional oil and gas drilling technologies in a novel way. The study basis is the use of sedimentary geologic formations as a medium for thermal energy storage (TES), specifically for heat collected in concentrating solar collectors.
Executive SummaryGreater deployment of moderate-temperature geothermal binary power plants (GBPPs) and concentrating solar power (CSP) can be limited in some markets by their respective generation characteristics and/or levelized cost of electricity (LCOE). Worldwide, both geothermal power and CSP are under intense pressure from the rapidly dropping cost of solar photovoltaic (PV) and wind energy. In the United States, there is further pressure from the ongoing era of low natural gas prices. CSP can generate high-temperature thermal energy that enables high cycle efficiency and low-cost thermal storage systems, and GBPP can produce baseload power generation. Hybridization may take advantage of technical merits of both technologies to produce energy at a levelized cost of energy (LCOE) competitive in the future energy market. New approaches that can lower the LCOE of both geothermal power and concentrating solar can therefore help to increase adoption of both technologies.In this technical report, we explore several methods of combining heat generated by a concentrating solar field with an existing geothermal binary power plant. The Raft River geothermal power plant in southern Idaho operates below its rated capacity because the reservoir is unable to deliver the design flow and temperature of geothermal energy. The GBPP is a twopressure cycle with independent high-pressure and low-pressure isopentane turbines. Solar heat may be added in several ways to this cycle and we perform a screening study and then more detailed calculations for several configurations, including:1. Geothermal production brine directly heated by solar energy; 2. A fraction of geothermal brine extracted from some part of the binary plant, heated with solar energy, and mixed with the production fluids; and3. The addition of a steam topping cycle, the hot exit flow of which is used to:a. Heat brine, as in points 1 and 2, and b. Vaporize the binary-cycle working fluid after the fluid has been preheated by the geothermal brine.The initial screening study was performed on a large number of different cycle configurations at a single operating condition. Subsequently, a more detailed study investigated annual cost and performance. Both the screening study (using one set of modeling software) and the detailed analysis (using another software package) were calibrated against one or more aspects of the Raft River operating condition. The combined results of the screening and detailed study are shown in Table A (although the results from one model should not be compared to those of the other). vii This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications.Table A: Comparison of different hybrid plant configurations from the screening study and detailed study. '-' indicates that the configuration was not studied.
Design-Point Cycle EfficiencyAnnual Efficiency
Cycle and DescriptionScreening StudyDetailed Study
Detailed StudyTwo solar steam turbines: Use high-temperature solar steam in two turbines, which...
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