New efficiency charts for the optimum design of axial flow turbines for organic Rankine cycles

Turbine efficiency plays a key role in the design optimization of ORCs (organic Rankine cycles) and should be properly evaluated for an accurate estimate of the real power production. Its value is in general assumed as given in the design optimization procedure, without a check that it can be really achieved in the resulting working conditions. The peculiar properties of high molecular weight fluids markedly influence turbine design and ask for turbine design criteria specifically tailored to ORCs. In this work a meanline design procedure for single stage axial flow turbines is developed to find optimum turbine geometry and efficiency in a wide range of operating conditions. Unlike previous literature, real fluid properties and very recent loss models are implemented. The variation of the predicted turbine efficiency with loading coefficient, flow coefficient, specific speed and specific diameter is shown through new general maps that explicitly take into account the strong influence of compressibility and turbine size through the volumetric expansion ratio and size parameter, respectively. All these maps can be included in a general design optimization procedure of the ORC system to help select the optimum design point, overcoming any arbitrary assumptions on turbine efficiency.