This study is intended to develop the waste heat recovery generation system using steam-Rankine cycle for co-generation system with a small capacity prime mover in the range of about 500kW or less. In our past reports, a scroll expander has attracted attention as a power recovery device which has high efficiency potential even in the case of small power capacity, and the performance characteristics of it are investigated through the experiments. Then, it was confirmed that the leakage loss in the beginning of expansion process and the thermal loss are caused the performance of the scroll expander to decrease. Therefore, in this paper, the shape of the orbiting scroll was redesigned to prevent the leakage, and also the lubricant oil supplied to the expander was heated up to the same temperature of steam as working fluid to reduce the thermal loss. As a result, the expander performance is improved by the reduction of the leakage at the beginning of expansion process, and the expander efficiency reaches up to 59.0% at the maximum in the experimental conditions that the suction pressure of working fluid is 590kPa and the revolution speed is 1850rpm.
