Empirical Correlation for Optimal Turbine-Inlet Temperature and Pressure for Geothermal Sub- and Supercritical Organic Rankine Cycles (ORC)

Abstract

Geothermal ORCs are classified as low temperature power-plant which has relatively low thermal efficiency. Hence, it is important to design such systems on its optimum operating condition, e.g. turbine inlet temperature and pressure (TITP) to maximize the energy recovery. In the present study, empirical correlations are developed to calculate the optimal TITP of sub- and supercritical ORC for geothermal applications, i.e. bottoming cycle from flash power-plant, medium-enthalpy wells. In geothermal ORCs, injection temperature parameter is an important design prerequisite due restriction to mineral scaling. Therefore, the ratios of injection temperature and hot-brine temperature (or equivalently, the heat input) to critical temperature of the working fluids, pinch-point, and condensation temperature are used to correlate the optimal TITP. The correlations are derived from the optimal TITP data of 6 typical working fluids at maximum specific net-power resulted from GeSi (Geothermal Simulation, an in-house program). In order to evaluate the accuracy, the correlations are tested by using the simulation results of other 15 pure working fluids at brine temperature of 120 °C – 180 °C and injection temperature of 70 °C – 160 °C. The prediction of the optimal TITP using the correlations is within 5% error. These correlations are very convenient for pre-design in fast and robust manner, especially to predict performance of new working fluids in specific working conditions. As a case study, bottoming cycle application in Indonesia is used. With average brine temperature of 453.15 K (180 °C) it is found, for n-pentane, the optimal turbine inlet temperature/pressure is 434.15 K (161 °C) / 1.9 MPa, which yields thermal efficiency of 15.8 %.