You could be forgiven for thinking solar assisted heat pumps were simply standard heat pumps powered by solar, but you'd be wrong. Find out exactly what they are and if they're worth considering
Although solar panels can be used to help power an air source heat pump, solar assisted heat pumps are a different product in their own right. But how do they differ from normal heat pumps and is there any value in considering one in your plans?
In this article I take a look at exactly what they are, how they differ from other types of heat pump and consider the pros and cons to help you decide if installing one is a good idea for you.
I'll also provide some details on budgeting for a solar assisted heat pump so you can factor this into your potential home renovation or extension plans, should you consider it a viable option.
What is a solar assisted heat pump?
Solar assisted heat pumps, also known as thermodynamic water heaters, are effectively a small heat pump that does not have a fan like an air source heat pump, or a ground loop like a ground source heat pump, but instead has a black metal panel (or 2 panels) that are approximately 1.5m2 each and are either mounted on the roof or on an external wall.
The panels contain refrigerant and are connected to the heat pump body in the home via a refrigerant pipe. The idea is that the heat from the sun, and ambient air, will be absorbed by the refrigerant and be transferred to a water cylinder by the heat pump.
The heat pump itself is also a very small capacity. Usually the compressor rating is around 600W and the manufacturers expectation is that it should produce around 1800W of heat. Solar assisted heat pumps were originally developed, and are traditionally used in hotter climates, but due to the low power output, are best suited to hot water preparation rather than as a means of heating your home.
Solar thermal panels absorb heat from the sun to provide hot water to your home(Image credit: Kangestudio/Getty Images)
Standard solar thermal panels are glazed and have pipework and insulation and are around 2m2. The solar thermal panels usually have treated water in them that is pumped through the panel and back to a coil in a water cylinder. As the water travels through the panel it absorbs heat from the sun and then deposits it into the water in the cylinder.
By having a refrigerant in the panel, the theory is that the super cool refrigerant can absorb more heat than water and then it can be compressed to enhance the temperature so that more of the energy and latent heat can be transferred to the home.
Pros and cons of solar assisted heat pumps
Advantages
The advantages of a solar assisted heat pump are that they are compact, live in the home, have no outside condenser like you would have on an air source heat pump, and have no disruptive groundworks like you would encounter when installing a ground source heat pump.
Unlike solar thermal systems they can still produce heat when there is no direct sunlight on the panels – albeit the efficiency and resultant output can be lower.
Disadvantages
The disadvantages of solar assisted heat pumps are that they require a specialised refrigeration engineer to install them and they also require an area of roof or wall to mount the panels.
If they are mounted on the roof then care also needs to be taken in making sure that if any snow or rain were to freeze on the panels that it could not dislodge as the ambient conditions improved and resulted in a block, or sheet of ice descending to the ground where it could, at best damage property, and at worst damage animals or people.
The Energy Savings Trust website also notes that because the system works by continuously producing a low volume of hot water, if you use all the hot water up, it takes time for the hot water cylinder to heat up again. Potentially not ideal in a larger household with small children, running multiple baths each day.
A newer concern, and one for the future, is the use of flammable refrigeration gas. Currently the solar assisted heat pump unit does not use flammable refrigerants, but many non-flammable refrigerants are being phased out in heat pumps and flammable products replacing them.
This poses a problem for solar assisted heat pump units as the refrigerant goes into the home and will need to be safely managed. Only time will tell how that will be accomplished.
You may not be able to generate as much hot water as you need(Image credit: Burwell Photography/Getty Images)
How efficient are solar assisted heat pumps?
The efficiency of solar assisted heat pumps is dependent on a number of factors. If there is direct sunlight on the panels, the ambient temperature is reasonably high, and the water temperature you are trying to produce is also not too hot, then the efficiency will be good.
If any of these factors change then the efficiency will drop and the ability to produce heat will be reduced. It is reasonable to assume that at night and in winter the light will be less and the ambient temperatures much lower.
If there is likely to be snow settling on the panels, then it is also vitally important to ask for the empirical data from the manufacturers in order to be able to specify the technology to meet your requirements, and your expectations.
If the empirical data is not available or seems to have been based on the weather conditions and energy demands in other countries, then it is best to avoid that particular product and find one with UK reference.
Is my home suitable for a solar assisted heat pump?
The installation of a solar assisted heat pump can be more complicated than installing an air source heat pump. Both need a hot water cylinder, but the solar assisted heat pump will also need enough space inside the home for the actual unit which is around the size of a washing machine.
A number of metal panels (usually around 1 to 3) are installed on a south facing roof or vertical wall and are connected back to the solar assisted heat pump unit inside the home via two refrigerant pipes that are around 28mm in diameter, including insulation.
In the same way as you need to check your roof suitability for solar panels, you'll need to check if your roof is suitable for the panels used with a solar assisted heat pump.
You'll need to check the suitability of your roof for a solar assisted heat pump(Image credit: EDF Energy)
How much will a solar assisted heat pump cost?
A solar assisted heat pump will cost anywhere from around £500 to £2000 for the materials. Installation cost could also be between £2000 and £4000, depending on whether you need to include scaffolding costs, and how much plumbing work and disruption needs to take place in the home.
Final verdict
My final thoughts? Solar assisted heat pumps are mainly designed to produce hot water for use in the home for showers and washing. Most of the systems are not powerful enough to heat the central heating systems of most UK homes.
The ideal scenario for a solar assisted heat pump unit is for it to have direct sunlight on the panels. Although many manufacturers and installers state that they will work in shade and darkness, these technologies were developed in warmer climates and the fact remains that if there is no incident sunlight, and lower ambient temperature around the panel, then the energy resource is lower and the heat output is less.
Although the technology and physics of the system are absolutely sound, very often the systems can struggle in cold weather and do not meet the expectations of the homeowner.
Need your air source heat pump to provide more than just hot water but keen to integrate some additional renewable technology into your home heating plans? Follow our guide to using solar panels and heat pumps together and check out the full range of off-grid heating options that are available to you.
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David is a renewables and ventilation installer, with over 35 years experience, and is a long-standing contributor to Homebuilding and Renovating magazine. He is a member of the Gas Safe Register, has a Masters degree in Sustainable Architecture, and is an authority in sustainable building and energy efficiency, with extensive knowledge in building fabrics, heat recovery ventilation, renewables, and also conventional heating systems. He is also a speaker at the Homebuilding & Renovating Show.
Passionate about healthy, efficient homes, he is director of Heat and Energy Ltd. He works with architects, builders, self builders and renovators, and designs and project manages the installation of ventilation and heating systems to achieve the most energy efficient and cost effective outcome for every home.
