A quasi-two-stage trans-critical CO2 heat pump with in-cycle thermal storage for performance enhancement

• A thermal energy storage is introduced to trans -critical CO2 cycle for heat recovery. • In-cycle heat recovery and reuse improve the heat pump cycle’s performance. • The propose flexible trans -critical CO2 cycle enables quasi-two-stage operation. • The flexible trans -critical CO2 cycle has a comparable COP to two-stage cycles. • It has a comparable COP to trans -critical CO2 cycles with expander or ejector. To decarbonise the heating sector, there is a strong demand for environmentally friendly and cost-effective heating technologies. Trans-critical CO 2 heat pumps offer a sustainable alternative to traditional vapor compression cycles that rely on environmentally harmful synthetic refrigerants. However, a significant challenge within trans -critical CO 2 heat pumps is their high throttling loss during expansion, leading to substantial flash gas production and subsequently higher recompression power consumption. In this paper, we apply our Flexible Heat Pump cycle concept to trans -critical CO 2 heat pumps to address this issue. A thermal storage unit is introduced into a traditional trans -critical CO 2 heat pump cycle to recover heat from the hot super-critical CO 2 leaving the gas cooler during charging mode. Once it is fully charged, the stored heat is then used as a temporary heat source for the heat pump’s operation during the discharging mode. Since the recovered heat has a higher temperature than ambient. The resultant flexible trans -critical CO 2 heat pump (FlexiCO2-HP) reduces throttling losses and the compressor’s power consumption, leading to a higher average COP (Coefficient of Performance). The obtained results of the FlexiCO2-HP are compared against a single-stage trans -critical CO 2 heat pump. For gas cooler temperatures ranging from 35 °C to 60 °C and across various discharge pressures, its COP is potentially 10 % to 40 % higher than the standard cycle under ideal conditions. In addition, the proposed system demonstrates a lower optimal gas cooler pressure across different gas cooler temperatures. Furthermore, this study compares the FlexiCO2-HP with other trans -critical CO 2 systems, including those with expansion work recovery and two-stage configurations. The results demonstrate that the FlexiCO2-HP offers improvements in COP for most conditions, though not all. It has the potential to achieve a comparable COP when compared with ejector-based and expander-based heat pumps or two-stage systems, but it is much simpler in design without introducing an extra expander or compressor.