Development of Open-Drive Scroll Expander for an Organic Rankine Cycle (ORC) Engine and First Test Results

Abstract

A novel open-drive scroll expander has been designed and included in a small-scale organic Rankine cycle (ORC) engine. According to the design, the heat input is 100 kWth with hot water temperature up to 100 °C and organic fluid pressure 40 bar. These conditions correspond to supercritical state of the vapour organic fluid (R-404a) with designed thermal efficiency of 6%, which is higher than similar operation at subcritical conditions. Due to supercritical operation, a new expander was necessary to be developed. An open-drive scroll expander has been designed with optimized scroll geometry, manufactured, and then integrated in an ORC engine with net capacity of 6 kW. The ORC has been installed at the laboratory for performance tests, which include the variation of heat input and hot water temperature. The tests presented here concern the first series of results for hot water temperature up to 80 °C and low expander speed (580 rpm@10 Hz), limited by severe vibrations at higher speeds. At these off-design conditions, the pump global efficiency reached was about 40%, the maximum thermal efficiency was 1.7% with expander isentropic efficiency almost 40% and volumetric efficiency about 30%, resulting to high internal leakages due to the low speed. These performance values are promising compared to part-load operation of similar set-ups, making it clear that the new expander can bring a superior performance compared to standard converted scroll expanders.