To provide a system capable of converting volume change caused by the temperature of a refrigerant into mechanical energy and generating electric power by using the mechanical energy.

An ammonia refrigerant in the liquid state is made to enter from a compressor 3 to a condenser 1 and is cooled by the condenser 1 by approximately 5°C, and gas moisture separation is performed in a liquid receiver 5. The ammonia refrigerant is decompressed by an expansion valve 4, and by rotating a rotor of an expanding device 6 by the expanded pressure, a shaft is also rotated.