Performance Prediction and Optimisation of Twin-Screw Expander

Positive displacement machines have been identified as appropriate expanders for small scale power generation systems such as ORCs. Screw expanders can operate with good efficiency for working fluids under both dry and two-phase conditions. Detailed understanding of the fluid expansion process is required to optimise the machine design and operation for specific applications, and accurate design tools are therefore essential. Using experimental data for R245fa expansion, both CFD and chamber models have been applied to investigate the accuracy of computed performances. Both models are shown to match experimental power output and mass flowrate with good accuracy. Finally, the validated chamber model is used to evaluate the maximum isentropic efficiency map for the chosen expander. This performance map can be used with ORC optimisation tools to identify deign built-in volume ratio of twin-screw expander and the required rotational speeds to operate at the maximum isentropic efficiency.