Multi-Variable Optimisation Of Wet Vapour Organic Rankine Cycles With Twin-Screw Expanders

A multi-variable optimization program has been developed to investigate the performance of Wet Organic Rankine Cycles for low temperature heat recovery applications. This cycle model contains a detailed thermodynamic model of the twin-screw expander, and the methods used to match the operation of the expander to the requirements of the cycle are described. The capability of the cycle model has been demonstrated for the case of heat recovery from a source of pressurized hot water at 120°C. There are two main findings from the paper. Firstly, power output can be increased by up to 50% by correct selection of the expander inlet and discharge conditions, compared to the case with dry vapour admission. Secondly, the maximum power output occurs at an expander inlet dryness of around 20%, but for lower dryness fractions the gain in net power output falls off due to the decreasing pressure ratio required to maintain good expander efficiency.