PVT-assisted heat pumps achieve up to 49.3% exergy efficiency in cold climates

A research team at China’s Xihang University has conducted an exergy analysis and sensitivity study of a photovoltaic-thermal (PVT) air-source heat pump (ASHP) coupled heating system for cold climates and has found that the system could achieve strong low-temperature adaptability.

“This study conducts a systematic exergy analysis of the proposed PVT-ASHP system,” the researchers said. “The main objectives include quantifying the distribution of exergy destruction among key components under typical heating conditions; investigating the effects of ambient temperature, relative humidity, and preheating temperature on system exergy efficiency; performing a sensitivity analysis to identify the most influential parameters; and proposing targeted optimization strategies based on the exergy analysis results.”

The researchers modeled the system in TRNSYS 18.0 for a six-story office building with a total floor area of 2,065 m² in Xi’an, China. The simulation covered the local heating season from Nov. 15 to March 15, with weekday operation from 8:00 to 18:00.

The system comprised two 90 kW low-temperature air-source heat pumps, 270 m² of PVT collectors with 57.5 kW of electrical capacity, a 200-liter thermal storage tank, and circulation pumps.

A water circulation loop recovered waste heat from the PVT panels, helping improve their electrical performance. The recovered heat was either stored for space heating or transferred through an air-to-water heat exchanger to preheat the air entering the ASHP evaporator. The higher inlet air temperature improved evaporator operating conditions, reducing frost formation and defrosting losses and enhancing the heat pump’s thermodynamic performance.

The ASHP switched on when the tank temperature fell below 40 C and stopped when it reached 50 C. The preheating heat exchanger operated when the tank temperature was at least 20 C higher than the ambient temperature.

The analysis considered ambient temperatures ranging from -10 C to 10 C, relative humidity of 40% to 90%, and preheating temperatures of 3 C to 15 C.

“The system achieves an 80.25% total useful energy ratio in the Xi’an heating season, with heat pump heating, PV/T thermal and electrical outputs as the main energy supply,” the researchers said. “In terms of exergy destruction, the compressor and evaporator are the dominant components, accounting for 38.6% and 22.55% of total exergy loss, respectively, which are identified as the core weak links for system optimization.”

The results also showed that the PVT waste-heat preheating strategy significantly reduced exergy destruction in the evaporator. The maximum reduction was 44.3% at -10 C, while a 38.0% reduction was recorded at 10 C.

System exergy efficiency ranged from 41.2% to 49.3% across the -10 C to 10 C ambient temperature range and increased with higher relative humidity. Exergy efficiency measures how effectively a system converts the maximum useful work available from an energy source into useful output while accounting for thermodynamic losses.

The researchers said the results indicate strong low-temperature adaptability and frosting suppression under cold and humid conditions.

“The system exergy efficiency reaches the peak of 48.1% at the preheating temperature of 9 C, indicating an optimal preheating interval,” the researchers said. “Sensitivity analysis shows that ambient temperature (0.82%/C) and preheating temperature (0.65%/C) are the most sensitive parameters, which provide clear control priorities and optimization targets for practical operation.”

The researchers presented their findings in “Exergy analysis and sensitivity study of a PVT-air source heat pump coupled heating system under cold climates,” published in Case Studies in Thermal Engineering.