New scenario of CO2 heat pump for space heating: Automatic mode switch between modified transcritical and cascade cycle in one system

Hot water production and space heating consume huge amounts of energy. CO2 heat pump has been proven more efficient in hot water production, but less efficient in space heating. In the paper, we proposed a new scenario of CO2 heat pump for space heating. Wherein the same system can automatically switch between transcritical cycle with dedicated mechanical subcooling (DMS) and cascade cycle with desuperheating heat recovery (DHR), making it possible to combine the advantages of transcritical and cascade system in different ambient conditions and heating terminals. With the aid of modeling and simulation, main application concerns on system design and operation are discussed. Recommendations for matching design of key components are made. A correlation for automatic mode switch is developed. At last, the proposed scenario is numerically evaluated with weather data of six typical cities in China, verifying the improvement of at most 8.7% and 19.4% in the whole heating season compared to transcritical system with DMS and cascade system, respectively.