Performance evaluation of a direct expansion air-solar-ground CO2 heat pump for water heating applications
The transition toward efficient and low-carbon heating technologies is a key requirement for sustainable development, with heat pumps playing a central role in residential applications. However, conventional heat pumps face intrinsic limitations: air-source units suffer from seasonal temperature fluctuations, ground-source units ensure stable performance but face high installation costs, and solar-assisted systems are constrained by irradiation availability. Hybrid heat pumps that combine multiple renewable sources offer a potential solution to mitigate these drawbacks and enhance overall system performance. This study develops a numerical model of an innovative direct-expansion multisource CO2 heat pump equipped with dedicated evaporators to utilize air, solar, and ground energy. Solar input is extracted from photovoltaicthermal (PV-T) collectors, ground energy from borehole heat exchangers (BHEs), and air energy from a finned-coil heat exchanger. The prototype can operate in two configurations, solar-air and ground-air modes, in which CO2 evaporates simultaneously in two heat exchangers: flooded operation in the solar or ground evaporator and dry expansion in the air evaporator. Simulations investigate the effects of air temperature, soil temperature, solar irradiance on system performance. The coefficient of performance (COP) shows strong sensitivity to environmental conditions, increasing by approximately 5.3% per 1 K rise in air temperature in both operating modes. In solar-air mode, COP increases by 3% per 100 W/m² of global tilted irradiance, whereas ground-air operation exhibits a 1.1% COP gain per 1 K rise in soil temperature. Compared to a reference air-source heat pump, the multisource configuration achieves a 6-15% improvement, depending on renewable energy availability. Performance can be further enhanced by increasing the number of BHEs or PV-T panels. In ground-air mode, using one to three BHEs leads to 8-21% COP increase, while in solar-air mode, deploying one to four PV-T collectors improves COP by 4-22%.