Thermo-fluid dynamics preliminary design of turbo-expanders for ORC cycles

Selecting the best design options for turbo-expanders to be used in Organic Rankine Power Cycle applications is a difficult task, with special reference to the low-temperature applications (T<150°C) which are relevant for specific fields in rapid development (geothermal binary plants, solar thermal energy conversion systems). A wide literature exists on the optimal selection of the working fluid from the thermodynamic point of view; while the fluid dynamics design is often based on the direct application of CFD techniques, without a preliminary screening on the issues of application of a specific working fluid. In practice, however, the selection of the working fluid has relevant effects both on the cycle thermodynamics and on the fluid dynamic losses and turbo-expander efficiency. Referring to a radial-type turbo-expander, a comparison of different working fluids is presented and discussed.