Investigation on a direct expansion multisource carbon dioxide heat pump to maximize the use of renewable energy sources

Traditional heat pump systems typically rely on a single low-temperature source, such as air, ground, or solar energy, each with intrinsic limitations. Air-source heat pumps are highly sensitive to temperature fluctuations across seasons and daily cycles, while the efficiency of solar-source heat pumps is constrained by the intermittent availability of solar radiation. Ground-source heat pumps, in contrast, deliver consistent seasonal performance but involve higher costs for the installation of borehole heat exchangers. Multisource heat pumps offer a promising technology to overcome these challenges and maximize the use of renewable energy. This paper presents a numerical investigation of an innovative multisource heat pump using CO2 as a low GWP refrigerant, that can exploit three different thermal sources through dedicated evaporators: air source with a finned coil heat exchanger, solar source with photovoltaic-thermal (PV-T) collectors, and ground source with a U-tube borehole heat exchanger (BHE). Two modes of operation are foreseen: solar-air mode (SA-mode) and ground-air mode (GA-mode). The novelty of this system lies in the concept of a multisource direct expansion heat pump in which the CO2 directly vaporizes in flooded mode in the solar or ground evaporators, while the finned coil works in dry expansion mode. Differently from all other systems, the solar and ground evaporators operate simultaneously with the air evaporator. Simulations are performed to assess the performance of the multisource heat pump under varying environmental conditions: air temperature, solar irradiance, and soil temperature. The results demonstrate that while air temperature influences the performance of both SA-mode and GA-mode, each mode exhibits distinct sensitivities to the other environmental parameters. SA-mode performance is significantly affected by solar irradiance, with a 100 W/m2 increase in irradiance corresponding to a 2.8 % enhancement in the coefficient of performance (COP). Conversely, GA-mode performance shows a notable response to soil temperature variations, where a 1 K increase in soil temperature results in a 0.9 % improvement in COP. The results compare SA-mode and GA-mode with air-source heat pump mode under varying thermal loads for space heating (SH) and domestic hot water (DHW) production, showing up to 22 % COP increase for GA-mode and SA-mode. As a further step, the study investigates the effect of varying the number of PV-T modules and borehole heat exchangers on the heat pump performance. The simultaneous use of two energy sources always results in improved system performance even with limited PV-T or BHE heat transfer areas.