Operational performance of a large-scale photovoltaic-thermal heat pump system with liquid overfeed method

Photovoltaic-thermal (PVT) heat pump systems commonly employ the direct-expansion liquid supply method. However, in large-scale applications, these systems face issues such as uneven refrigerant distribution and low heat transfer efficiency. In this study, the liquid overfeed supply method was adopted for refrigerant delivery to a large-scale PVT array comprising 84 modules. A novel PVT heat pump system integrated with a mechanical pump was developed and implemented in a practical project. Field tests were conducted to evaluate the refrigerant distribution characteristics in the PVT array and its heating/electrical performance during two daytime sessions in May 2025. The uniformity of refrigerant distribution was quantified using a return temperature inhomogeneity index, revealing inhomogeneities of 0.3 % across PVT groups and 0.2 % across modules. The mean module pressure loss was 26.1 kPa on a cloudy day and 25.9 kPa on a sunny day. These values represent only 24.6 %–32.5 % of those reported in existing studies. Additionally, power generation efficiency improved by 3.0 % and 4.81 %, respectively. During daytime continuous water heating, the coefficient of performance decreased over time. The heating capacity, which is influenced by both cooling capacity and solar irradiance, showed a sharp drop in the morning followed by a slow recovery. In the afternoon, it exhibited a rapid decline and then a gradual reduction. The results may provide an experimental basis for introducing the liquid overfeed supply method into PVT heat pumps, and offer guidelines for developing large-scale PVT heat pump systems.