Solar-Driven Steam Topping Cycle for a Binary Geothermal Power Plant

Abstract

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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“…This LCOE value compares favorably with reported values for solar photovoltaic plus battery energy storage (PV+BES) systems in the open literature, i.e. $0.148/kWhe for a PV+BES system with 4 hours of electrochemical battery energy storage capacity (McTigue et al, 2018a;McTigue et al, 2018b). Addition of battery energy storage with more hours of storage would further increase PV+BES system LCOE and increase the separation between GeoTES and PV+BES.…”

“…4. The LCOE for high capacity GeoTES systems is lower than the value of $0.148/kWh estimated by McTigue et al (2018aMcTigue et al ( , 2018b for a low capacity (4 hr) photovoltaic plus battery energy storage (PV+BES) system. Addition of increased BES capacity would result in significant increases in PV+BES LCOE.…”

Integration of a Concentrating Solar Steam Topping Turbine to an Existing Geothermal Binary Power Plant (CRADA CRD-17-700 Final Report)

“…We have successfully completed all four tasks. The detail work regarding the tasks were summarized in the 68-page technical report [1]. In addition, one short article was published on the GRC bulletin [2] and one journal article [3] was just published on the high-impact journal: Applied Thermal Energy.…”

Techno-Economic Analysis of Greenfield Geothermal Hybrid Power Plants using a Solar or Natural Gas Steam Topping Cycle

“…The current study concentrates on the development of a new-build geothermal-solar or geothermalgas hybrid plant [11], as opposed to the previous projects which largely considered retrofitting an existing geothermal plant with solar thermal.…”

“…Previous work by this group, including McTigue et al's 2020 article [7], considered starting from a known geothermal plant and adding solar in the form of a steam Rankine topping cycle which feeds additional heat into an ORC-bottoming cycle. McTigue et al [19] studied the different possible options for moving heat from the solar thermal system to the geothermal cycle and found that using a topping cycle to add heat to the bottoming cycle working fluid provided the best compromise between efficiency and complexity. [7], [20], [21], [22].…”