Construction and performance evaluation of an efficient CO2 heat pump for space heating over a wide temperature range

Replacing conventional coal-based heating with air-source heat pump is widely recognized as a key measure to reduce greenhouse gas emissions in space heating. In recent years, CO2 heat pump has attracted considerable attention owing to their environmental benefits and favorable thermophysical properties. However, current research on the configuration optimization of high-efficiency CO2 system relies heavily on expert knowledge. To address the limitation, this study extends a previously established generic design method to the supercritical region, enabling automated CO2 cycle optimization across both subcritical and supercritical regions. Using this extended approach, CO2 heat pump cycles were automatically constructed under various space heating conditions, with ambient temperature ranging from −30 °C to 10 °C and water temperature lifts between 5 K and 25 K. An efficient CO2 heat pump over a wide temperature range was developed by synthesizing results obtained across multiple operating conditions. The proposed system features a three-stage water heating process and integrates two thermodynamic operating modes, namely the transcritical cycle with dedicated mechanical subcooling (DMS cycle) and the subcritical cascade cycle (CAS cycle). Control strategies for adaptive water flow configuration and mode transition were developed for the proposed system. Simulation results indicate that the proposed system achieves heating seasonal performance factor improvements of up to 9.6% and 17.7% compared to DMS and CAS cycles, respectively. Although the total capital investment is 12.6% and 15.2% higher than these benchmarks, the lower operation and maintenance costs result in total levelized cost reductions reaching 3.3% and 5.2%, respectively.