Performance of a small-scale regenerative Rankine power cycle employing a scroll expander

Abstract

A small scroll expander has been incorporated into a power cycle for performance evaluation. Using heat from a circulating hot oil supply, a working fluid (R123) was vapourized under pressure and fed to the inlet of the expander. Power generated was measured by a torque/rotation rate sensor as the power was delivered to a compressor. The exhausted working fluid was then sent through a regenerator to recover thermal energy, and then to an air-cooled condensation heat exchanger. To complete the cycle, the working fluid passed through a pump and was fed back to the boiler by way of the regenerator. The major components of the power cycle were monitored for performance, and from these values, overall cycle efficiency was determined. A model of the system was also developed in order to validate system state points and allow follow-on projections of cycle performance with better component efficiencies. Depending on the operating state points during testing, the power output from the cycle ranged from 187 to 256 W. At nominal ambient outside temperatures (22.5 °C), the system efficiency was 7.2 per cent. The critical component limiting the overall system efficiency was the expander. Operating within the power cycle, expander efficiency was measured at levels between 45 and 50 per cent, based on the isentropic ideal. The obvious path forward is to improve the efficiency of the scroll expander.