Geothermal / solar hybrid designs: Use of geothermal energy for CSP feedwater heating

This paper examines a hybrid geothermal / solar-thermal plant design that uses geothermal energy to provide feedwater heating in a conventional steam-Rankine power cycle deployed by a concentrating solar power (CSP) plant. The geothermal energy represents approximately 11% of the annual thermal input to the hybrid plant. The geothermal energy allows power output from the hybrid plant to increase by about 8% relative to a stand-alone CSP plant with the same solar-thermal input. Geothermal energy is converted to electricity at an efficiency of 1.7 to 2.5 times greater than would occur in a stand-alone, binary-cycle geothermal plant using the same geothermal resource. While the design exhibits a clear advantage during hybrid plant operation, the annual advantage of the hybrid versus two standalone power plants depends on the annual operating hours of the different plants. Annual net power from the hybrid plant matches the combined output from a stand-alone CSP plant and stand-alone geothermal plant when the assumed stand-alone geothermal plant availability is 74% or less. The cost implications are not covered in this study, but the hybrid plant avoids the need for an ORC power block and produces more power during hot afternoons that generally correspond to peak demand periods.