Performance analysis and city applicability evaluation of a transcritical CO2 heat pump system integrated with expander-compressor unit subcooling for space heating

CO2 air source heat pump is a crucial and environmentally friendly technology for space heating. However, the low performance due to the larger throttling loss and the sharp deterioration at higher return water temperature is a challenge. To enhance its performance, a transcritical CO2 heat pump system integrated with expander-compressor unit (E-C) subcooling is proposed, which can achieve subcooling while reducing throttling loss. Mathematical model and heating season performance model considering the effect of the gas cooler pinch point are developed and evaluated for three Chinese cities using different heating terminals. The result demonstrates a 21.9 % coefficient of performance (COP) improvement compared to the basic system. It also outperforms conventional mechanical subcooling CO2 heat pump system. The E-C unit can significantly reduce the exergy destruction of the expansion valve and compressor. Under the return and supply water temperature of 40/75 °C, with the varying of ambient temperature, the exergy efficiency of E-C system is 72.8 %–74.5 % which is better than the basic system of 64.5 %–65.8 %. The heating terminal using floor-coil radiator has the largest COP improvement about 33 % among the three cities. In conclusion, the novel system effectively enhances the CO2 heat pump performance and provides a solution for its application.