Researchers in Germany have modeled dozens of indirect-expansion solar-assisted heat pump systems using photovoltaic-thermal panels for single-family homes in Munich, Berlin, and Hamburg. Simulations accounted for local weather, electricity use, and thermal demand.

An international research group led by Germany’s Karlsruhe Institute of Technology conducted energy, economic, and environmental assessments of 54 photovoltaic-thermal (PVT) indirect-expansion (IDX) solar-assisted heat pumps (SAHPs), finding only two configurations delivered positive net present values and acceptable payback periods.

The group used MATLAB to simulate systems with three refrigerants – R32, propane, and R410A.

The researchers analyzed a parallel IDX SAHP setup in which the storage tank is heated by two separate loops: a submerged coil from the PVT collector and an external heat exchanger (HEX) from the heat pump loop. The tank provides both hot water demand (HWD) and space heating demand (SHD) via two additional HEXs.

All systems used 1.55 m² PVT panels rated at 260 W, with 16% electrical efficiency and 51% optical thermal efficiency. The 54 configurations tested included the three refrigerants, three design temperatures (-9.2 C, -8.3 C, -6.1 C), three pinch point temperature differences (3 C, 5 C, 10 C), and two heat output levels (5,500 W and 7,000 W).

From these, the team selected three representative systems per refrigerant: one high-performing, higher-cost system with the top mean coefficient of performance (COP); one lower-cost, lower-performing system with the lowest total cost; and one mid-range option offering an average COP and moderate price.

All systems were simulated for a single-family house in Munich, Berlin, or Hamburg, using location-specific weather, electricity demand, and thermal demand data. Each house had a floor area of 93.1 m², with 89.1 m² of heated flooring. The roof measured 82.4 m², with 32.9 m² covered in PVT panels to support the SAHP.

“From an energy perspective, for the considered household, the system allowed over one-third of the annual heat demand and around half of the annual appliance electricity demand to be covered by solar energy. However, a mismatch between solar energy generation and heat consumption limited the system’s ability to utilize even larger amounts of solar energy,” the academics explained. “During summer, the low heat demand was largely met by solar energy. In contrast, during winter, the larger heat demand was primarily covered by the HP running on grid-sourced electricity.”

The analysis showed that the SAHP system achieved the highest solar energy coverage in Munich, meeting up to 39% of heat demand among the cities tested. Across all configurations, the high-performance design using R32 refrigerant delivered the highest solar contribution.

“Economically and environmentally, designs showing higher demand coverage by solar energy had the best performance. The high-performance HP designs using R32 and propane were the only ones shown to be a viable investment with positive net present value (NPV) of €960 ($1,099) and €255, and payback time (PBT) lower than the technology lifetime (19.3 and 19.8 years),” the scientists said. “Comparing the SAHP to other technologies, a standalone HP or natural gas boiler could only match the levelized cost of energy (LCOE) of the SAHP at exceptionally low electricity or gas prices.”

Their findings appeared in “Comprehensive energy, economic, and environmental analysis of a hybrid photovoltaic-thermal (PVT) heat pump system,” published in Energy. Researchers from Germany’s Karlsruhe Institute of Technology, the University of Cyprus, and Egypt’s University of Science and Technology also participated in the study.