In the UK, we have made enormous progress in decarbonising the electricity sector, however this will only get us so far. Arguably, the biggest challenge we face is the decarbonisation of the heat sector. This sector currently is mainly dependent on burining natural gas, which while convenient, does not make much sense from a thermodynamic viewpoint. This is because the second law of thermodynamics states that energy has a quality, and low-grade heat, i.e. heat close to ambient temperatures like that we use to heat our homes or deliver hot water, is very low quality. In fact for heat, the quality is directly determined by the temperature and it is higher for higher temperatures. Since gas can be used to create much higher temperatures than those we require for space heating or hot water, then from a thermodynamic perspective this is a lot more efficient. Of course, even more important are the fact that reliance on gas leaves many countries (the UK included) at the mercy of highly volatile gas markets and burning gas in a small boiler also leads to CO2 emissions, which are practically impossible to capture at such a small scale.

Improving heat pump performance

Heat pumps are currently a polarising technology, with proponents arguing they already make sense and opponents arguing that they are only suitable in well-insulated houses with efficient heat emitters (i.e. large surface area radiators or underfloor heating). The reality is somewhere in between - in some homes heat pumps perform very well while in others their performance is not as good as we expect it should be. The reason behind this disparity is not currently understood and could include a multitude of mechanisms, including poor control settings, poor temperature choices, poor heat pump sizing, unnecessary cycling and suboptimal installation (i.e. uninsulated pipe runs or poor weather compensation setup). Fig. 2 shows a distribution of the seasonal performance factors (SPF) for homes in the electrification of heat demonstration project (along with a fitted normal distribution). Given that these homes are experiencing quite similar external temperatures, this variation in performance is not simply explained.

In our group, we are currently undertaking analysis, modelling and simulation work to understand this varation. We are exploring heat pump data from the electrification of heat demonstration project, along with other datasets, in order to understand this disparity and find low cost methods of bringing the performance of all heat pumps in line with the best performing systems. The benefit of doing this would be considerable in a UK where heat pumps are the dominant heating technology. To illustrate this, consider a scenario in which 75% of homes in 2050 are heated by heat pumps, with moderate improvements to building fabric, such that the mean UK annual heat load decreases from 12 MWh to 10 MWh per home. Assuming the performance represented by the distribution in Fig. 2, bringing the performance of all heat pumps in line with the 75th percentile of performance (SPF≥3.1) would reduce the UK’s annual electricity demand by ~14TWh (~4.5% of current electricity demand).

Thermal energy storage

For both low and high temperature heat applications, it is clear that thermal energy storage (TES) will play a key role in future. It will be required to make massive heat demands for space heating and industrial processes flexible, so that they can be powered by temporally abundant (and therefore cheap at certain times) renewable energy. Our work in thermal storage development involves experimental testing of new thermal energy storage materials and simulating their use in building level and energy system models.