Influence of axial turbine efficiency maps on the performance of subcritical and supercritical Organic Rankine Cycle systems

The utilization of low-to-medium enthalpy geothermal resources for electricity generation is commonly done using the binary cycle power plant technology. The low temperature of the geothermal fluid necessarily results in low thermal efficiencies but it can be profitably used as a driver to reduce the equipment cost. In particular, single stage axial flow turbines, a cheaper option compared to multistage solutions, can be used due to the moderate enthalpy drop in the expansion process. In this work the design performance of ORCs (Organic Rankine Cycle systems) equipped with single stage axial flow turbines is optimized for utilization of a 150°C geothermal fluid. Accurate predictions of turbine efficiency are included in the optimization procedure to take into account the influence of the thermodynamic cycle parameters, fluid properties and size on the maximum achievable efficiency. A set of six fluids is considered to examine a wide spectrum of design conditions. Results show that supercritical ORCs outperform subcritical ORCs even taking into account the detrimental effect of high expansion ratios on turbine efficiency. Hydrofluoroolefins are found to exhibit a net power output approximately equal or even higher than the best hydrofluorocarbons.