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.

► We propose the main aspects of ORC expander design.
► Target powerplant is a 50 kWe ORC fed with low enthalpy geothermal + solar.
► Radial and backswept inflow turbines are considered.
► Thermodynamic data of real organic fluids in the expansion are used.
► Different working fluids are compared and the most efficient are selected.