A solar-assisted heat pump (SAHP) combines solar collectors with a heat pump so solar energy either raises the heat pump's source temperature or powers its compressor. In the most common indirect design, PVT or solar thermal collectors pre-warm the source circuit; each 1 °C of extra source temperature improves efficiency by roughly 2–3%, giving a higher seasonal COP than the same heat pump running on cold outdoor air alone.
What does "solar-assisted" actually mean?
A solar-assisted heat pump is any system in which solar collectors and a heat pump are engineered to work as one machine rather than as two separate appliances. The sun's contribution can enter on three sides: as heat into the evaporator (source side), as heat into the storage cylinder (load side), or as electricity into the compressor. National efficiency schemes treat SAHP as its own category — the UK's Energy Saving Trust, for example, lists solar-assisted heat pumps as a distinct water-heating technology — because the combined seasonal performance differs measurably from a stand-alone unit.
The defining benefit is thermodynamic: a heat pump's COP rises as the gap between source and delivery temperature shrinks. Feeding the evaporator 15–30 °C solar-warmed fluid instead of 0 °C winter air cuts that gap dramatically.
What are the main SAHP architectures?
| Architecture | How solar helps | Typical use |
|---|---|---|
| Direct-expansion (DX) SAHP | Refrigerant evaporates inside the solar collector itself | Compact DHW units; niche |
| Indirect PVT-source | PVT panels warm a brine loop feeding the heat pump evaporator | Homes wanting electricity + heat from one roof field |
| Dual-source | Controller switches between air and solar source, whichever is warmer | Cold climates; maximum seasonal COP |
| PV-powered | PV electricity drives the compressor; thermally the unit is a normal ASHP | Simple pairing, widely subsidised |
| Solar thermal + heat pump on one cylinder | Collectors heat the top of the store directly; the heat pump covers the rest | Retrofits with existing solar thermal |
The indirect PVT-source layout is the fastest-growing configuration in Europe, because PVT panels feeding a heat pump solve two problems at once: the panel is cooled (raising electrical yield by a few per cent a year) while the heat pump receives an elevated source temperature.
How much efficiency does solar assistance add?
The gain scales with source temperature. As a working rule, every 1 °C added to the source improves COP by about 2–3%, so lifting a winter source from 0 °C air to a 15 °C solar-warmed loop can raise instantaneous COP by a third or more. IEA SHC Task 60 field reports on PVT-heat pump systems document seasonal performance factors well above equivalent air-source baselines, with the largest margins in sunny shoulder seasons when collectors run warm but heating demand continues.
Properly sized collector fields can even act as the sole source: PVT panels can be the only heat source for a heat pump in moderate climates, removing the outdoor fan unit — and its noise and defrost cycles — entirely. Solimpeks has built PV-T hybrid panels since 2008, and its MPPT solar-assisted heat pump solution feeds PV power directly into the compressor circuit.
When is a SAHP worth it over a standard air-source heat pump?
A SAHP earns its extra complexity in four situations. First, when roof area exists but outdoor siting is difficult — dense urban plots, strict noise limits, or listed façades where a fan unit is unwelcome. Second, in cold-but-sunny climates where winter air is a poor source while irradiation remains strong. Third, when both electricity and heat are wanted from limited roof space, which points to PVT rather than separate fields. Fourth, when an existing solar thermal system is already on the roof and can be combined with a heat pump on one cylinder instead of being scrapped.
For a straightforward suburban home with easy outdoor siting and no roof constraint, a plain air-to-water unit remains the simpler, cheaper install — the SAHP case should be argued from numbers, not fashion.
How is a SAHP controlled and stored?
The controller decides, interval by interval, where solar energy is most valuable: direct to the cylinder when collectors run hot, to the heat pump source when they run lukewarm, or to a buffer for later. Storage matters more than in a simple system because solar supply and heating demand rarely coincide; good stratification in the cylinder preserves the temperature layers each source needs. Expect a twin-coil or combination store and a controller with dedicated SAHP hydraulic schemes rather than improvised relay logic.
Frequently asked questions
Is a solar-assisted heat pump the same as a heat pump plus solar panels?
No. Simply owning PV and a heat pump on the same meter is parallel operation. A SAHP integrates them hydraulically or electrically under one control strategy — solar heat raises the source temperature or charges the store, which changes the heat pump's operating point and seasonal efficiency.
Does a SAHP work at night and in winter?
Yes. Dual-source and PVT-source designs fall back to ambient air (or run the collectors as air absorbers — unglazed PVT still harvests heat from wind and sky) whenever solar input is absent. The heat pump always guarantees full output; solar assistance improves efficiency rather than availability.
Which collectors suit a SAHP: PVT, flat plate or evacuated tube?
For source-side assistance, PVT and unglazed collectors are ideal because the heat pump only needs 10–30 °C fluid and the PV yield adds value. For load-side assistance direct to the cylinder, glazed flat plates deliver higher temperatures. High-temperature evacuated tubes are usually oversized for heat pump source duty.
Are solar-assisted heat pumps eligible for subsidies?
In most countries the heat pump and the solar system each qualify under their own scheme, and some programmes pay explicit combination bonuses. Rules change annually, so check the current national scheme conditions before ordering.