Researchers from China and Spain have developed a three-source heat pump system designed for cold climates. The system integrates an air source heat pump (ASHP), a water source heat pump (WSHP), and a ground source heat pump (GSHP), and was evaluated through both simulations and experimental testing.

“Compared with existing studies that primarily focus on the integration of multi-source heat pump architectures or qualitative evaluations of system performance, this study aims to provide a quantitative thermodynamic assessment of multi-source heat pump systems operating under cold-climate conditions,” the researchers said.

“Multidimensional sensitivity analyses were conducted to quantify the influence of key operating parameters on system pressure ratio, phase-change temperature, and coefficient of performance (COP), thereby identifying the thermodynamic constraints associated with different low-grade heat sources,” they added.

The research team developed a test system combining a photovoltaic-thermal (PVT) collector with a single vapor-compression heat pump capable of switching between three heat sources: ambient air, heated water, and a simulated ground loop. The PVT collector served a dual purpose: its PV modules generated electricity to directly power the heat pump compressor, while its thermal component recovered heat from the modules and transferred it to circulating water used as the heat source for the WSHP.

The experimental setup included a rotary compressor using 1,1,1,2-Tetrafluoroethane (R134a) refrigerant, a 4.5 m² plate heat exchanger for the WSHP, a 4.5 m² fin-and-tube heat exchanger for the ASHP, a 1.0 m² simulated ground-source heat exchanger, an electronic expansion valve, a gas-liquid separator, and a monitoring system.

Although the test rig was designed to operate in all three configurations, only the PVT-assisted WSHP mode was experimentally validated and used to calibrate the Aspen Plus simulation model. The ASHP and GSHP configurations were assessed solely through simulation.

The initial Aspen Plus model, an industry-standard chemical process simulator used to model, design, and optimize complex industrial processes, was based on idealized assumptions, leading to discrepancies between simulated and experimental results. The researchers therefore calibrated the model using WSHP measurements recorded at an inlet water temperature of 30 C and a refrigerant mass flow rate of 18 kg/h. Corrections were subsequently applied to account for evaporator pressure losses, compressor efficiency variations, and environmental heat losses.

Using the calibrated model, the team evaluated system performance under weather conditions representative of Haidian District in Beijing. The analysis showed that the ASHP configuration achieved a COP ranging from 1.95 to 2.74, the WSHP configuration delivered a COP of 1.93 to 2.43, and the GSHP configuration reached a COP between 2.02 and 2.26.

Based on these results, the researchers proposed an operating strategy for the multi-source heat pump system. The WSHP mode would be prioritized during sunny winter days when the PVT outlet temperature exceeds 15 C. The GSHP mode would operate during winter nights and extreme cold events, when solar energy is unavailable and outdoor temperatures fall below -5 C. The ASHP mode would be used during spring and autumn periods when outdoor temperatures remain above 10 C.

“By prioritizing sensitivity weights, this study has identified the control priorities for multi-heat-source systems: in actual operation, the temperature of the heat source on the evaporation side can be stabilized by switching between multiple heat sources,” the researchers said.

“Combined with variable-frequency drive technology to achieve an optimal match between refrigerant flow and heat load, the system ensures an optimal balance in the utilization of cascaded energy across a wide range of operating conditions. These research findings provide a scientific theoretical basis and technical support for achieving the ‘dual carbon’ goals in the building sector,” they concluded.

The hybrid system was presented in “Simulation and experimental study on the performance of a solar-powered PVT electric-thermal hybrid air-water-soil three-source heat pump system,” published in Next Energy. Scientists from China’s Beijing University of Civil Engineering and Architecture, Tsinghua University, and Spain’s Charles III University of Madrid conducted the study.