Researchers in China developed a large-scale PV thermal heat pump system using a liquid overfeed method that reportedly improves both thermal and electrical efficiency. Field tests showed a 4.81% increase in energy generation and significantly reduced pressure losses compared to conventional systems.
A research team from China’s Dalian University of Technology (DUT) has investigated the operational performance of a large-scale photovoltaic-thermal (PVT) heat pump system using a liquid overfeed method.
Unlike conventional direct expansion systems, where the expansion valve sends a mixture of liquid and vapor refrigerant directly to the PVT modules, the overfeed design separates the gas and liquid phases. Only the liquid refrigerant is sent to the PVT modules, reducing the load on the compressor and improving both thermal and electrical performance.
“The overfeed system provides better wall wetting and maximizes the use of the heat exchanger surface,” the team explains. “This allows the compressor to draw vapor with a higher density, increasing the refrigerant flow rate and heat transfer capacity of the system. In addition, the lower vapor mass flow in the evaporators minimizes pressure loss, further optimizing compressor operation.
Unlike the direct expansion system, where the compressor draws in superheated vapor, the overfeed method allows the compressor to take in saturated vapor at a lower temperature. This higher density vapor improves the indicated efficiency of the compressor and results in a steeper compression process.
The system was implemented in a TU Delft student house in 2023, as part of a project that introduced centralized shower facilities for students. On the roof of the dormitory, 84 PVT modules, each with a power of 450 W, were installed at an angle of inclination of 40°, facing south. The modules were divided into six groups of 14, connected in parallel via supply and return branch pipes. The setup also includes a heat pump with four scroll compressors, each with a cylinder capacity of 37.7 m³/h and a power of 7.3 kW.
The heat pump’s condenser has a capacity of 54 kW, supplemented with a 100 liter reservoir and a 460 liter receiver. The system uses refrigerant R134a. A centrifugal oil separator and a mechanical pump with a capacity of 10.0 m³/h and 3.0 kW are also installed. On the first floor, the heat storage station has a 32.0 m³ water tank that stores the hot water from the roof system. This hot water is supplied to the showers via two pumps, while a 40 kW string inverter provides the electrical conversion.
During operation, the PVT panels generate both electricity and heat. The R134a refrigerant circulates through the panels and absorbs heat before flowing to a collection tank, where the vapor separates and rises to the compressor, leaving the liquid at the bottom. The compressor pressurizes the vapor, which then flows through a condenser to transfer heat to the water used for showers. After releasing heat, the refrigerant condenses back into liquid form, which the mechanical pump returns to the PVT panels. Electricity produced by the PV layer powers system components such as the pump and compressor. Field tests were conducted during two daytime sessions in May 2025.
“The power generation efficiency of the PVT modules improved by approximately 3% on cloudy days and by 4.81% on sunny days,” the researchers said. “However, during the early morning and late afternoon periods of high humidity, efficiency decreased due to condensation on the PVT surface, scattering sunlight and reducing effective radiation. It is recommended that PVT heat pump systems avoid operation during these times.”
The team also assessed the uniformity of refrigerant distribution using a return temperature inhomogeneity index, finding inhomogeneities of 0.3% among PVT groups and 0.2% among modules. “The average module pressure loss was 26.1 kPa on a cloudy day and 25.9 kPa on a sunny day – only 24.6% to 32.5% of the values reported in existing studies,” the researchers noted.
Their findings are available in “Operational performance of a large-scale photovoltaic-thermal heat pump system with liquid overfeeding method”, published in Energy. Scientists from China’s Dalian University of Technology, China Northwest Architecture Design and Research Institute and Tsinghua University participated in the study.
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