Comprehensive evaluation and optimization of IHX in a modified double-evaporator transcritical CO2 heat pump with ejector

Although there are some studies focusing on the individual integration of ejector and internal heat exchanger (IHX) in transcritical CO2 systems, research on the simultaneous integration of both components remains limited. In this paper, a modified double-evaporator transcritical CO2 heat pump system with ejector (DE-TCHSE) is proposed. Meanwhile, the energy and exergy analyses are conducted at different IHX effectiveness and operating conditions. Simulation results show that integration of IHX enhances system heating COP at low discharge pressure but degrades at high discharge pressure, driven by gas cooler CO2 outlet temperature variations. Although increasing ejector throat area reduces optimal COP, it amplifies the COP improvement effect by IHX. Specifically, at IHX effectiveness of 60 %, the COP improvement reaches 5.4 % with ejector throat area of 1 mm2, compared to only 1.9 % with 0.4 mm2. Parametric analyses further reveal that integrating IHX significantly enhances optimal COP at high ambient temperature, low water inlet temperature or high water outlet temperature, but may degrade at low ambient temperature due to increased exergy destruction of gas cooler. A novel index is proposed to assess comprehensive effects of IHX. Based on this index, for each operating condition, the optimal IHX effectiveness and corresponding optimal discharge pressure are obtained. The correlations for optimal IHX effectiveness and optimal discharge pressure have also been established. Furthermore, an optimal IHX design method is developed to ensure superior comprehensive performance of DE-TCHSE throughout annual operation. Taking Shanghai as a case, the proposed method achieves a maximum annual revenue of 2138.8 CNY.