Coupling effect of evaporation and condensation processes of organic Rankine cycle for geothermal power generation improvement

Organic Rankine cycle (ORC) is widely used for the low grade geothermal power generation. However, a large amount of irreversible loss results in poor technical and economic performance due to its poor matching between the heat source/sink and the working medium in the condenser and the evaporator. The condensing temperature, cooling water temperature difference and pinch point temperature difference are often fixed according to engineering experience. In order to optimize the ORC system comprehensively, the coupling effect of evaporation and condensation process was proposed in this paper. Based on the laws of thermodynamics, the energy analysis, exergy analysis and entropy analysis were adopted to investigate the ORC performance including net output power, thermal efficiency, exergy efficiency, thermal conductivity, irreversible loss, etc., using geothermal water at a temperature of 120 degrees C as the heat source and isobutane as the working fluid. The results show that there exists a pair of optimal evaporating temperature and condensing temperatures to maximize the system performance. The net power output and the system comprehensive performance achieve their highest values at the same evaporating temperature, but the system comprehensive performance corresponds to a lower condensing temperature than the net power output.