With the transition towards electrification of our heating in our homes, there is much talk online around the efficiency of heat pumps, and rightly so.
For the uninitiated, heat pumps are machines that move heat from one place to another. Your fridge is a heat pump – it moves heat from a relatively cold place (inside the fridge) to a warmer space (usually the kitchen). Left to its own devices, heat tends to move from hotter to colder areas of matter – the second law of thermodynamics – so some work needs to be done to encourage it back in the opposite direction, and this is the task of the heat pump.
Heat pumps are shepherding heat energy that is already in the external environment that surrounds us, whether that is the air, the ground or a water source. Only a relatively small amount of energy is necessary to run the compressor and circulation pump that do most of the work in a heat pump. We can define the efficiency of a heat pump by stating the ratio of the amount of heat energy that is supplied to the space to be heated, to the amount of electrical energy used to run the heat pump – this ratio is called the Coefficient of Performance, or CoP. (CoP is not to be be confused with the climate change jamboree Conference of the Parties, or Cabal of Oil Producers as one wag suggested renaming it.) The CoP will vary throughout the year as heat pumps have to work harder in colder weather, so the industry tends to use the averaged Seasonal CoP or SCoP
Early heat pump installations had relatively poor SCoPs of 2-2.5 – this is because installers were still getting to grips with best practices for installing heat pumps… indeed in some areas of the industry this remains an issue, highlighting the importance of selecting a competent, well-trained installer. SCoPs for modern heat pump installations now average around 3, however SCoPs of 4 or higher are readily achievable with care.
We should not, however, consider the SCoPs in isolation, as it ultimately it is not the most important metric when it comes to tackling our carbon footprint or indeed minimizing our energy bills. An analogy would be a driver who is obsessed with maximising fuel economy when they may be missing out on the potentially much more impactful steps of eliminating unnecessary journeys by walking, cycling or taking public transport, and combining those chores that can only be carried out by car into a smaller number of trips.
There is an interesting irony with heat pump efficiencies when it comes to older buildings, as it is often the case that uninsulated properties achieve particularly high SCoPs. On the face of it this may seem odd as one would think a heat pump would have to work particularly hard in this scenario, however it is likely that the improved efficiencies arise in part from the heat pumps running for more of the heating season, i.e. spending a greater proportion of the time operating in milder conditions when it’s easier to extract the heat from the outside environment. In contrast, a heat pump in a highly-insulated Passivhaus may only have to operate in space heating mode for a few days a year, in sub-zero conditions when the CoP is relatively poor.
What really matters is not the SCoP but the carbon emissions from the energy consumed, and the running cost. Both of these can vary according to the time of day and year, and each are big topics in their own right which will be covered in future articles. Fundamentally, however, we should be aiming to reduce our overall energy consumption, rather than just maximise the CoP of our heat pumps, although of course the latter can play an important role in the former. Ultimately, our climate responds to carbon dioxide concentrations in the atmosphere, not the efficiency of our heat pump.