Numerical and experimental analysis of a photovoltaic-thermal evaporator integrated with a R-744 heat pump

The integration of photovoltaic–thermal (PV-T) collectors with heat pumps is a promising pathway for decarbonizing heating and electricity production. This study investigates the thermal behavior of a PV-T collector operating under winter conditions as a direct-expansion evaporator for a CO₂ (R-744) heat pump installed at the ENEA Casaccia Research Centre (Rome, Italy). A coupled three-dimensional Computational Fluid Dynamics (CFD) and thermal–energy model was developed and validated using outdoor experimental data, showing relative deviations below 10% for the refrigerant outlet temperature. The model was used to analyse heat transfer between the PV module and the refrigerant circuit under variable ambient temperature, solar irradiance and wind speed. Parametric simulations were conducted by varying ambient temperature (0–20 °C), solar irradiance (23–750 W/m 2 ), wind speed (0–10 m/s), refrigerant inlet temperature (0.8 °C and − 10.23 °C) and cooling-pipe density. The results show that internal cooling reduces the PV operating temperature by up to ∼10 °C compared to the uncooled case, leading to electrical gains of 1–3% on average and exceeding 5% under high irradiance (>500 W/m 2 ). However, this benefit decreases as wind speed increases, with gains dropping below 2% for wind speeds above 5 m/s. The findings highlight that the performance of PV-T evaporators is strongly dependent on environmental conditions and the need to optimize cooling geometry to balance thermal performance, electrical gain and system complexity.