This paper presents an experimental study on heat pump system for hot water heating using CO_2 as a refrigerant. From the viewpoint of the sector emitting CO_2, CO_2 emissions of residential housing are large. And, heating purposes hold a large ratio on the whole energy consumption of housing. It is remarkable on cold European cold climates. High temperature radiators are used mainly for space heating on cold European climates. High temperature radiators are mainly heated by a fossil fuel-burning boiler emitting a lot of CO_2. We noticed the characteristics of CO_2 refrigerant, which has a high heat capacity and can produce high-temperature water. With the aim of greatly reducing current CO_2 emission levels by converting their fossil-fuel-burning boilers to heat pumps that use CO_2 as a refrigerant, we began the development of a high-efficiency heat pump system for hot water heating using CO_2 as a refrigerant. We carried out the following: 1) The development of power recovery technology using an expander, 2) The construction of a high-efficiency injection control system for responding to changes in ambient temperature, 3) The verification of the efficiency improvement and the reliability through field test in a cold European climate. As a result, we carried out a low-temperature intermittent defrost operation over 1,000 hours in a cold European climate, and achieved the performance improvement of 15%, by constructing the power recovery type heat pump system using a compressor combined with expander.
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