Study on control strategy optimization and performance analysis of a PV/T-coupled transcritical CO2 heat pump system with ejector and vapor injection based on environmental mode switching

• A PV/T-coupled ejector–vapor injection transcritical CO 2 heat pump (PV/T-EIHP) with multi-mode operation is proposed. • A thermodynamic model with a stratified storage tank and mode-specific optimal pressure correlations is developed. • A mode-switching strategy based on evaporating temperature is proposed (SEJM ≥ 5 ℃, otherwise EVIM). • Compared with PV/T-DSHP, the COP of PV/T-EIHP increases by 5.32%–14.03%, reaching 2.87 on a sunny day. • Sunny day, PV/T module achieves 31.40% overall efficiency, PV covers 67.74% of heat pump’s electricity during irradiation. PV/T (photovoltaic/thermal) coupled heat pump systems can enhance overall energy utilisation by cascading solar energy from “light-to-electricity-to-heat”. This paper proposes a PV/T-coupled ejector–vapor-injection enhanced transcritical CO 2 heat pump with multi-mode operation (PV/T-EIHP) to improve heating performance under low ambient temperatures. A steady-state thermodynamic model incorporating a stratified thermal storage tank was developed for the dual-source PV/T system, and mode-dependent optimal high-side pressure correlations were fitted and embedded to facilitate consistent system calculations. Based on a comparative evaluation of three operating modes—Ejector Vapor Injection Mode (EVIM), Sole Ejector Mode (SEJM), and the baseline air-source mode (BASM)—a practical mode-switching strategy was derived for low-temperature operation. Typical winter-day simulations were then conducted for Luntai, Xinjiang to assess both thermal and electrical performance. Results show that applying the calculated switching rule (SEJM for evaporating temperature ≥ 5°C and EVIM otherwise) increases the COP of PV/T-EIHP by 5.32%–14.03% relative to the PV/T-DSHP system. The daily average COP reaches 2.87 on a typical sunny day and 2.33 on an overcast day. On sunny days, the PV/T subsystem achieves a daily average overall conversion efficiency of 31.4%, and PV power covers 67.74% of the heat pump electricity demand during irradiation hours. The findings provide guidance for performance-oriented design and operation of PV/T dual-source transcritical CO 2 heat pump systems under cold-climate conditions.