What’s next in energy innovation and policy?

Air source heat pumps are a fairly well established technology, but their efficiency can drop significantly if they have to draw heat from particularly cold air, or provide heat at a particularly high temperature. A hybrid solar heat pump uses a solar thermal collector to preheat the air before it enters the heat pump, so the heat pump can then produce water at a relatively high temperature without having to work too hard. This is ideal for producing domestic hot water.

The principle is entirely sound, and the heat pump should operate at a high efficiency and so use only a small amount of electricity. But we don’t have any independent performance data to estimate what the running costs would be and how this would compare to the cost of buying and installing a system.

You should bear in mind that the average household bill for providing hot water is around £80 a year, so if you buy a hybrid solar heat pump that only produces domestic hot water it can’t save more than this on your fuel bills.

These are different to hybrid solar heat pumps. They are conventional heat pumps, but with a secondary heat source alongside, usually a gas or oil boiler. The idea is that you can switch to using the boiler instead whenever the heat pump is unable to provide all the heating requirement, or to operate at a reasonable efficiency.

Some people describe these as bivalent systems, but that name can be used for any heating system that uses two different fuel sources.

We can’t currently advise whether there are any situations where it would be better to fit a hybrid heat pump rather than a heat pump on its own or a gas or oil boiler on its own.

PVT is another hybrid system, but this time it’s a solar PV panel (to generate electricity) with a solar thermal panel (to produce hot water) underneath. The theory says that the solar thermal panel draws heat from the PV panel, which cools it down and makes it more efficient, so you get more electricity generated while also getting some hot water. The hot water will have to be relatively low temperature though if it’s going to affect the PV efficiency (50 degrees Celsius instead of 70 degrees like in conventional systems). You’d then need to have a backup system (such as a boiler, a heat pump or both) to get hotter water, for washing or cleaning for example.

Solar PVT systems aren’t eligible for either Feed-in Tariff or Renewable Heat Incentive payments. They are usually used in very energy efficient homes aiming for zero energy bills, especially when roof space is relatively small, as this system makes the most of the solar energy to create both heat and hot water. Another common use is pool heating.