Comparative study of upgraded CO2 transcritical air source heat pump systems with different heat sinks

Highlights: • Three improved CO₂ heat pump systems with two compressors are compared at three heat sinks by thermodynamic analysis. • Based on five domestic heating regions of China, the economic change of improved CO₂ heat pump systems is studied. • The most economical system in different application ranges is proposed. For improving the coefficient of performance of system for heating, CO₂ transcritical heat pump system is upgraded by using two compressors. In this paper, three CO₂ transcritical systems with two-compressors (the CO₂ cascade system, the CO₂ two-stage compression system and the CO₂/CO₂ mechanical subcooling system) are discussed and compared with CO₂ single-stage compression transcritical heat pump system under three different heat sinks. The three heat sinks are the small temperature difference fan-coil unit, the floor-coil radiator and the traditional designed radiator. And the optimal operation states of three upgraded systems are analyzed. When the thermodynamic performance of CO₂ transcritical heat pump system is optimized, the initial investment cost of its equipment often increases and the overall cost will be affected during the life cycle of the equipment. Thus, an objective function that comprehensively considers the initial investment cost and operating cost is defined, and the comparative analysis of four CO₂ transcritical heat pump systems is combined meteorological parameters of five typical regions in China for home heating. The results show that under nominal working conditions of three heat sinks, COPs of CO₂ cascade system, CO₂ two-stage compression system and CO₂/CO₂ mechanical sub-cooling system are all increased by more than 10% compared with that of single-stage system and the growth extent of COP increases with the rise of return water temperature of heat sink, but based on the economic analysis, it is implied that CO₂ cascade system, CO₂ two-stage compression system and CO₂/CO₂ mechanical sub-cooling system are not conducive to their application in home heating compared with single-stage system, and more suitable for the situations with low evaporation temperatures and large capacities. The analysis results show that the economy of single-stage system and the three upgraded systems are related to the evaporation temperature and heating capacity. Under the analysis conditions in this paper, CO₂ cascade system is the least economical. Single-stage system and the other two upgraded systems can become the most economical system in different application ranges under three heat sink conditions.