A novel solar-coupled CO2 transcritical heat pump system for building heating and hot water supply: Comparative study and multi-objective optimization

Solar-coupled CO 2 heat pumps provide an efficient and clean energy solution for building energy supply. A novel solar-coupled CO 2 transcritical quasi-two-stage compression heat pump with an ejector system (STQHPE) is proposed and compared to two alternatives: the solar-coupled CO 2 transcritical heat pump with an ejector system (STHPE) and the solar-coupled CO 2 transcritical quasi-two-stage compression heat pump (STQHP). Simulation models for these systems are developed using TRNSYS software, focusing on building heating and hot water supply. Three evaluation indicators were analyzed: energy efficiency, economy and environment. Increasing the collector area and tank volume enhances STQHPE performance, with its heating seasonal performance factor (HSPF) 1.5 % and 16.5 % higher than STQHP and STHPE, respectively, as the collector area grows. Adjusting the tank volume reduces CO 2 emissions. Based on sensitivity analyses found that increasing cooling water flow boosts heat production, with the system's comprehensive performance coefficient (ζ) improving by up to 39.8 %, averaging 65.5 % and 72.1 % higher than STQHP and STHPE across a 6 °C–10 °C temperature difference. With a collector area of 40–120 m 2 , the STQHPE's ζ is 25.0 % and 27.5 % higher than STQHP and STHPE. Optimization via the NSGA-II algorithm achieves a Pareto optimal solution , with HSPF of 5.215, annual cost value of 24,902.4 yuan, and CO 2 emissions of 19,796.43 kg for collector area of 40 m 2 , cooling water flow rate of 6 m 3 /h, and start-stop temperature difference of 5 °C. These findings highlight STQHPE as a superior option for building energy supply. • A novel solar coupled CO 2 transcritical heat pump system is proposed. • Quasi-two-stage compression heat pump with an ejector are used in the novel system. • The design parameters and operating parameters of the novel system are analyzed. • The performance of the novel system was compared with two other similar systems. • The operating conditions of the system are derived by multi-objective optimization.