Experimental characterization of a hermetic scroll expander for use in a micro-scale Rankine cycle

This paper presents the results of an experimental study carried out on a prototype of a hermetic scroll expander, integrated into a gas cycle test rig, whose working fluid is HFC-245fa. This system is designed to test only the performance of the expander. It is made up mainly of a scroll compressor, a scroll expander, a heat exchanger, and a by-pass valve. The latter is used to adjust the pressure ratio imposed to the expander. The expander was originally a compressor designed for heat pump applications and is characterized by a nominal power input of 2.5 kWe. Performance of the expander is evaluated in terms of isentropic effectiveness and filling factor as functions of the main operating conditions. The study also investigates the impact of oil mass fraction on the expander performance. Maximum overall isentropic effectiveness of 71.03 per cent is measured, which is partly explained by the good volumetric performance of the machine. Using the experimental data, parameters of a semi-empirical simulation model of the expander are identified. This model is used to analyse the measured performance of the expander. Finally, a polynomial empirical model of the expander is proposed for fast and robust simulations of organic Rankine cycle systems.