PROBLEM TO BE SOLVED: To provide a natural refrigerant heat pump system improved in economic efficiency and operability.;SOLUTION: In this natural refrigerant heat pump having a cycle for sending a natural refrigerant such as CO2 evaporated by an evaporator 11 to a heat exchanger 14 in a supercritical state by a compressor 12, and expanding the natural refrigerant kept in a low temperature and high pressure state through the heat exchanger 14, a reciprocating piston engine 15 is used as a mechanism for expanding the natural refrigerant. By introducing the natural refrigerant in the low temperature and high pressure state to a cylinder of the reciprocating piston engine 15 to thermally insulating and expanding the same, a crank shaft 30 connected with a piston is rotated and driven, and a power generator 41 is driven by its rotational output to generate AC electric power. The electric power is converted into the electric power of commercial frequency by an invertor 42, and supplied to a motor 13 as a power source of the compressor 12.;COPYRIGHT: (C)2007,JPO&INPIT
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