Performance analyses of a hybrid geothermal-fossil power generation system using low-enthalpy geothermal resources

Low-enthalpy geothermal heat can be efficiently utilized for feedwater preheating in coal-fired power plants by replacing some of the high-grade steam that can then be used to generate more power. This study analyzes a hybrid geothermal-fossil power generation system including a supercritical 1000 MW power unit and a geothermal feedwater preheating system. This study models for parallel and serial geothermal preheating schemes and analyzes the thermodynamic performance of the hybrid geothermal-fossil power generation system for various geothermal resource temperatures. The models are used to analyze the effects of the temperature matching between the geothermal water and the feedwater, the heat losses and pumping power during the geothermal water transport and the resource distance and temperature on the power increase to improve the power generation. The serial geothermal preheating (SGP) scheme generally generates more additional power than the parallel geothermal preheating (PGP) scheme for geothermal resource temperatures of 100-130 °C, but the SGP scheme generates slightly less additional power than the PGP scheme when the feedwater is preheated to as high a temperature as possible before entering the deaerator for geothermal resource temperatures higher than 140 °C. The additional power decreases as the geothermal source distance increases since the pipeline pumping power increases and the geothermal water temperature decreases due to heat losses. More than 50% of the power decrease is due to geothermal water temperature decreases along the pipeline since less higher pressure extracted steam is replaced by the geothermal water. For geothermal resource temperatures of 140-160 °C, the additional power generated by the hybrid geothermal-coal power generation system is about 90% (at a geothermal source distance of 0 km) or 39-49% (at a distance of 20 km) greater than the power generated by an optimized organic Rankine cycle system using isopentane (R601a), the hybrid power generation system has little benefit over the ORC system when the distance increases to 40 km. However, the additional power generated by the hybrid power generation system is less than the power generated by the ORC system at distances over 20 km for geothermal resource temperatures of 100 °C.