Thermal decline mitigation in a geothermal plant by hybridization with a concentrating solar power system

Geothermal resources are subject to thermal decline during the lifetime of geothermal power plants. The decline of the fluid temperature significantly affects the thermal energy available for electric conversion and in turn plant efficiency and power output. Furthermore, the performance of geothermal power plants based on air-cooled Organic Rankine Cycles (ORC) is diminished at higher ambient temperatures because of the higher turbine exhaust pressures. In this study a new hybrid Geothermal - Concentrating Solar Power (CSP) plant is modelled, which enables a better utilization of geothermal energy and improves the performance of the geothermal ORC system during the whole lifespan of the power plant. Solar energy is used to heat up the geothermal fluid entering the heat exchanger of the ORC. The CSP plant is equipped with a thermal energy storage system which accumulates solar energy and releases it at low ambient temperature when the ORC system efficiency is higher. The addition of the parabolic trough CSP system without storage to the geothermal-only plant brought about an increase of 2.8% in the total annual electrical energy production. This power gain reached 5.3% when the CSP solar field was oversized with a SM equal to 1.6. The further addition of a TES unit to the oversized system resulted in an increase of about 6.3% in the total annual energy production. The storage system alone was responsible for an additional gain of 20% of the electrical power output contribution to the geothermal-only production. The addition of the TES system was particularly effective during summertime when it markedly improved the efficiency of the entire hybrid plant. The overall result is a mitigation of the thermal decline of the geothermal source and a consequent performance and productivity gain.