Solar PV/Thermal Collectors for Cold-Climate Heat Pumps

Abstract

Meeting climate targets will require deploying heat pumps at a rate 10 times higher than today, implying not only accelerating installations in existing markets but also unlocking their potential where adoption is constrained by social, technical, or economic barriers. Conventional heat pump solutions, such as ground source systems, may be limited by land availability and high investment, whereas air-source units may face noise or aesthetic constraints and reduced performance at low outdoor temperatures. In this context, photovoltaic-thermal (PVT) collectors are of particular interest, as they provide both electricity and heat from the same surface area and can be integrated either as supplementary and regenerative support to ground source heat pumps (GSHPs), or as the sole heat source.

This doctoral thesis investigates how different PVT collector designs should be integrated with heat pumps (HP) under Nordic climatic conditions, focusing on multi-family buildings where borehole drilling is limited or not possible. The work combines numerical modelling, outdoor experimental testing, dynamic system simulation, and techno-economic analysis to address four questions: i) the design features governing low-temperature collector performance, ii) the experimental comparison of different PVT concepts, iii) the design of PVT+GSHP systems under constrained borehole capacity, and iv) the techno-economic feasibility of PVT-sourced heat pumps.

The component-level results show that low-temperature PVT performance is governed primarily by the thermal contact area between the PV module, the circulating fluid, and surrounding air. Designs with limited contact area and no extended surfaces exhibit similar performance despite differences in material, whereas rear-side fins significantly increase ambient heat capture, leading to 2-3x greater heat transfer coefficients compared to conventional collectors.

At system level, the role of a PVT array depends on its function within the HP system. In PVT-assisted GSHP systems with reduced borehole capacity, flow rate and array size have the greatest impact on techno-economic performance. The cost optimum is reached at collector fields of about 0.7-1.2 m² per kWth of heat pump capacity and flow rates of about 60-80 l/h-m². Differences between collector designs are less important; while finned collectors increase annual thermal yield by 20-30 %, this translates into less than 3 % difference in seasonal performance factor at system level. Across the investigated electricity-price cases, the optimized PVT+GSHP system achieves lower lifetime costs than district heating (DH), air-source heat pump (ASHP), and solar PV+GSHP.

For heat pump systems using PVT as the sole heat source, the design logic changes fundamentally because the collector field must supply sufficient instantaneous thermal power without the buffering effect of the ground. Collector design therefore becomes more important, and finned concepts outperform unfinned designs by maintaining higher source temperatures and reducing auxiliary heating demand. Below about 2.5 m²/kWth, auxiliary heating exceeds 40 % and seasonal performance factor (SPF4+) remains below 1.8. At around 3.5 m²/kWth, auxiliary heating falls below 30 % and SPF4+ reaches approximately 2.3-2.4 for the best performing designs. The results confirm that PVT-sourced heat pumps are technically feasible in cold climates, with best-performing configurations achieving TLCC values of approximately 515-530 k€. However, while they provide a lower-cost alternative to district heating, they remain above the ASHP, PV+GSHP, and PVT+GSHP benchmarks.

Overall, this thesis establishes a techno-economic design logic for unglazed PVT collectors used as low-temperature heat sources for heat pumps in cold climates. It demonstrates that PVT can enable or strengthen heat pump solutions under drilling constraints, and that the preferred collector design depends strongly on whether PVT supports a GSHP or acts as the sole heat source. These findings provide practical guidance for collector manufacturers, system designers, building owners, and decision-makers working on the electrification of heating in dense urban areas.

Link to DiVA