This study numerically compares the seasonal heating performance (SCOP) of three 15 kW heat pumps using natural refrigerants: two with carbon dioxide and one with propane. The propane system is an air-source heat pump (R290-AHP). The carbon dioxide systems include an air-source heat pump (R744-AHP) and a dual-source solar-air heat pump (R744-SAHP) equipped with finned-coil and photovoltaic-thermal evaporators that can work simultaneously. In the case of a transcritical carbon dioxide cycle, the use of several low-temperature sources is a promising solution to improve the performance of the system by enhancing the exploitation of renewable energy sources. Since the efficiency of air and solar-based systems is related to weather conditions and location, there is the need for accurate models to evaluate the SCOP. In this work, a numerical model has been developed to design the three 15 kW heat pumps and assess the SCOPs under variable space heating and domestic hot water demand profiles, using climatic data for Rome and Strasbourg. The results indicate that the R290-AHP consistently achieves the highest SCOP, while the R744-AHP performs the lowest. The R744-SAHP overperforms the R744-AHP by approximately 4 % regardless of heating demand characteristics. In particular, the results show that the performance of the three heat pumps is significantly influenced by the distribution of the thermal load throughout the day. Specifically, when the thermal load is concentrated during daylight hours, the heat pumps can operate at a higher SCOP, especially for the R744-SAHP, and also increase, up to 44 %, the self-consumed photovoltaic energy produced.

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.

Dual-source solar-air heat pumps represent a promising solution for overcoming the limitations associated with single-source utilization, thereby enhancing heat pump performance. However, running the heat pump by alternatively employing the more advantageous source requires the integration of a controller capable of continuously monitoring and predicting the heat pump's performance in response to dynamic environmental and operational variables. Even so, a selective alternate operation does not allow to get the maximum possible performance from the use of the two heat sources. A different approach to address this challenge is the simultaneous utilization of the two sources, by properly combining two evaporators in the CO₂ circuit. This paper presents an experimental investigation of a dual-source heat pump using CO₂ as refrigerant, which can operate in three different evaporation modes: air-mode (using a finned-coil evaporator), solar-mode (using a photovoltaic-thermal PV-T evaporator), and simultaneous-mode (using both the evaporators simultaneously). The novel solution presented here does not require to split the refrigerant flow rate between the two evaporators and at the same time it solves the problem of possible maldistribution at the inlet of the evaporators. Experimental data indicate that the heat pump operating in simultaneous-mode allows to increase the evaporation pressure and the coefficient of performance compared to operation in air-mode or solar-mode. The measurements have been employed for validating a model of the system, capable of predicting steady-state and dynamic performance under various environmental and operational conditions. Simulation results show that the simultaneous-mode operation can be outperformed by the solar-mode only at high irradiance and low air temperature, when the evaporation temperature gets higher than the air temperature. Finally, the impact of the number of PV-T collectors and solar irradiance on the heat pump performance has been simulated and discussed. On this regard, the simultaneous use of the two heat sources adds more flexibility to the system and its design, because even the availability of a small solar area can contribute enhancing the performance over the mere air source heat pump.