A solar-assisted heat pump (SAHP) is any system in which solar energy improves a heat pump's performance — feeding its evaporator as the heat source (PVT or unglazed collectors), working alongside it as a parallel generator, or powering its compressor with PV electricity. Because compressor efficiency rises roughly 2–3 % for every kelvin of warmer source temperature, solar-boosted sources translate directly into higher seasonal performance.
How does a solar-assisted heat pump work?
A heat pump's efficiency is set largely by the temperature gap it must bridge between source and sink. A solar-assisted heat pump attacks the source side of that gap: solar collectors — most often uncovered PVT hybrid panels or other WISC collectors — deliver fluid warmer than the ambient air would be, so the evaporator starts from a better temperature and the compressor works less per unit of heat. The rule of thumb is that COP improves by roughly 2–3 % per kelvin of warmer source, so a source running 10 K above a cold winter airstream is worth on the order of a quarter more efficiency. The IEA Solar Heating & Cooling Programme's Task 44 established the standard classification of these systems into parallel, series and regenerative concepts.
What are the main SAHP configurations?
| Configuration | Role of solar | Typical hardware |
|---|---|---|
| Parallel | Solar heats the store directly; heat pump covers the rest | Glazed collectors + air/water heat pump |
| Series (solar source) | Collectors feed the heat pump's evaporator | Uncovered PVT or WISC + brine/water heat pump |
| Dual-source | Unit switches or blends between air and solar source | PVT + air coil with changeover valves |
| Regenerative | Solar recharges a ground source | Collectors + borehole/ground loop |
| PV-assisted | PV electricity drives the compressor | PV array + any heat pump, often SG-Ready |
Parallel systems are the simplest and remain common for DHW. Series systems are the technically distinctive family: with PVT as sole source, the outdoor fan unit disappears entirely — the array is silent, has no defrost fan, and the PV side generates electricity at the same time, cooled by the evaporator loop. The full architecture walk-through is in What is a solar-assisted heat pump?
Why use PVT panels as the heat pump source?
Three reasons. First, performance: the thermal side of a PVT panel harvests solar radiation, ambient air heat and condensation gains, lifting source temperature above plain outside air for most of the year — the mechanism detailed in How does a PVT panel feed a heat pump? Second, siting: with no outdoor unit there is no fan noise and no visual box on the wall, which resolves real planning and neighbour constraints in dense housing. Third, the electrical bonus: evaporator cooling holds the PV cells below their free-standing temperature, and cooler cells convert sunlight better. Solimpeks has produced PV-T collectors since 2008 and also offers an MPPT solar-assisted heat pump that runs directly on PV power.
What performance can you expect from a SAHP?
It depends on which configuration and climate, which is why published field results span a wide band rather than one number. IEA SHC Task 44 field and simulation studies showed solar-source and parallel systems outperforming their air-source reference cases across Central European climates, with the advantage largest where winters are sunny and cold. Practical design questions decide the outcome: collector area per kW of evaporator demand, whether a backup source exists for long dark cold spells, and the control strategy that decides when solar heat goes to the store directly versus through the heat pump. Whether an array alone can carry a house through winter is examined in Can PVT panels be the only heat source for a heat pump?
Is a SAHP worth it over separate solar and heat pump systems?
When the integration solves a real constraint — no outdoor unit allowed, limited roof area needing both electricity and heat, or a ground loop that needs regeneration — the combined system earns its added design complexity. Where none of those constraints bite, a well-sized conventional heat pump plus separate solar can be the simpler investment. The honest comparison, including costs, is worked through in Is a hybrid solar + heat pump system worth the investment?
Frequently asked questions
What is the difference between series and parallel solar-assisted heat pumps?
In a parallel system, solar collectors and the heat pump both heat the storage tank independently. In a series system, the collectors feed the heat pump's evaporator as its heat source, raising source temperature and therefore COP. Series systems typically use uncovered PVT or WISC collectors.
Can a solar-assisted heat pump work at night and in winter?
Yes. Uncovered collectors keep absorbing heat from ambient air and condensation even without sun, so the evaporator stays supplied at night. In winter the source runs colder and output falls, which is why sizing and — in harsh climates — a backup strategy matter.
Does a PVT-source heat pump need an outdoor fan unit?
No — that is one of its main attractions. The collector array replaces the fan-coil outdoor unit, so the system is effectively silent outdoors and has no defrost fan. The heat pump itself is a brine/water unit installed indoors.
Is a heat pump running on PV electricity a solar-assisted heat pump?
In the broad sense, yes — PV-assisted operation is one recognised SAHP category. It improves running cost and self-consumption rather than the thermodynamic cycle itself; SG-Ready controls let the PV system tell the heat pump when to run.