Comparison of sub- and supercritical Organic Rankine Cycles for power generation from low-temperature/low-enthalpy geothermal wells, considering specific net power output and efficiency

Abstract

Electrical power production at low-enthalpy (∼150 °C) geothermal sites is usually realized using an Organic Rankine Cycle (ORC process). This paper presents our analysis of sub- and supercritical processes using propane, carbon dioxide and ten other refrigerants as working fluids. The impact of crucial indicators for optimization, such as specific net power, thermal efficiency and heat input is discussed in detail.The focus was to optimize the thermodynamic loop and the influence of other parameters, such as condensing temperature, minimal temperature difference in the heat exchanger, and internal heat recovery. Simulations showed that at a geothermal fluid temperature of 150 °C, a suitable working fluid such as propane or R143a can increase specific net power output up to 40%.Furthermore, systematic simulations on brine temperatures of 130–170 °C from subcritical to supercritical operation are discussed.Results from this research may also be applicable for electricity generation using waste heat from combined heat and power (CHP) plants or other technical processes.