Multi-criteria techno-economic and environmental optimization of PVT-assisted CO2 heat pump system for sustainable energy buildings in cold climates

While heat pump technologies provide sustainable heating and cooling, their performance depends on system design, energy storage, and economic feasibility. In cold climate regions, maintaining high heating efficiency while minimizing electricity consumption remains a critical challenge. This study presents a multi-criteria techno-economic and environmental optimization of three system configurations: CO2 heat pump (CO2 HP) alone, ground source heat pump (GSHP) alone, and a Hybrid CO2 HP + GSHP system. The system serves a multi-apartment smart building in Beijing, China, using a 50 kW CO2 heat pump, a 60 kW GSHP, and up to 700 m2 of PVT panel area. The configuration includes thermal storage with 6–9 m3 hot and 80–100 m3 cold storage capacity, delivering up to 1.92 GWh of annual energy. The optimization process is conducted using an iterative finite difference method (FDM)-based approach, solving time-dependent energy balance equations for heat pump operation, thermal storage dynamics, and PVT energy generation. The model evaluates different configurations to identify the optimal system setup that minimizes operational costs (OPEX), maximizes renewable energy utilization, and enhances CO2 emissions reduction. The results indicate that the Hybrid CO2 HP + GSHP system achieves the highest return on investment (ROI) of 13.27 % per year, with a payback period of approximately 6.8 years and a CO2 saving ratio of ∼1.05. The optimal configuration consists of 250 m2 PVT, 6 m3 hot storage, and 80 m3 cold storage, significantly reducing grid dependency and improving energy efficiency.