Get Heat from Thin Air

Edward Thompson, GSHPA

E.ON's posters and videos proudly announce that we can "Get heat from thin air" – using air source heat pumps. This is true and it is encouraging that E.ON is spreading the word and showing everyone that there are alternatives to burning fossil fuels for heating:

Get heat from liquid water

It is also true that you can get heat more efficiently from water using a ground source heat pump, because water is much more dense than air, and is capable of yielding far more heat from a given volume than thin air is.

A ground source heat pump can also transfer heat from water without having to spend energy (and create noise) by using a fan to suck thin air through a heat exchanger.

And the water from the ground will be warmer than the air in winter – when heating is most needed.

For these reasons a well designed ground source heat pump system uses less electricity to transfer the heat your building needs than an air source heat pump.

However, the key merit of an air source heat pump over a ground source heat pump is that it is cheaper to install, because it avoids the cost of making the connection with the ground and so E.ON is right to promote air source as an initial step to decarbonising heating.

Reducing carbonisation of heating to zero is possible with heat pumps if they are driven by a carbon-free electricity sources such as wind or photovoltaic cells.

In practice most people will use grid electricity. It is worth noting that the carbon emissions from grid electricity in the UK have fallen by over 50% since 2014, as coal fired generation is being phased out and wind and other renewables are providing a greater share of grid electicity.

BEIS predicts that the average carbon factor of grid electricity will be down to 205 grams CO2e/kWh in 2018. On this basis, heat transfer from an air source heat pump will save 66% of the CO2 that would have been emitted by a gas boiler (assuming a CoP of 2.8).

Heat transfer from a ground source heat pump will save 74% of the CO2 that would have been emitted by a gas boiler (assuming a CoP of 3.6).

The Future of Heating

We should all be grateful to E.ON for bringing the debate about the decarbonisation of heating into our living rooms, via the small screen, and onto our streets, via posters.

We all need to continue the debate about the electrification of heating (using heat transfer) and the electrification of transport (using electric vehicles) in order to realise the urgent shift away from combustion towards heat transfer that is needed to address the urgent issue of arresting the slide toward global warming.