THERMAL Energy demand
Heating and cooling demand in buildings
Heating and cooling are required to keep the interior temperature of buildings within a comfortable range. The decarbonisation of heating and cooling is a major challenge for the energy transition especially in countries, such as the UK, where the majority of heating systems rely on gas. It is currently estimated that about 37% of greenhouse gas emissions in the UK come from heating, and 17% is associated with space heating and cooling.
Sources of UK carbon emissions
Heat pumps
Some gas heating systems are currently being replaced by electrical heat pumps coupled with a heat source. This shift allows the carbon emission of a gas boiler to be replaced by the carbon emission of the local electricity grid.
The technology underpinning heat pumps decreases the amount of electricity needed to provide space heating and cooling compared to a traditional boiler. The magnitude of the decrease depends on the thermal energy of the heat source supplied to the heat pump. This has driven the development of subsurface closed- and open-loop thermal storage systems which act as the heat source.
Thermal energy storage
Storage of thermal energy can reduce the need for heat or cooling generation. There are a variety of thermal energy storage technologies that may play a substantial role in the energy transition. These include both above-ground and underground storage systems, with underground systems being more suitable for larger-scale storage of thermal energy. The temperature at which the energy is stored impacts its value, as well as the amount of thermal energy which can be stored, and the rate of charging and discharging the heat; these influence the benefits of different options.
Very large-scale storage of thermal energy presents a challenge in terms of the volume of material needed. Thermal energy can be stored in the subsurface, either in the ground around boreholes, drilled to depths of about 100 – 250 m, or in larger-scale aquifer thermal energy stores.
Boreholes
Storage of thermal energy using boreholes relies on heat transfer to a geological formation via thermal conduction. This method is suitable for inter-seasonal storage of thermal energy due to slow rates of thermal conduction in the subsurface, which stops the heat from dissipating. There are many installations of this for space heating and cooling, especially in northern Europe.
The potential for such systems is significant, especially in locations of favourable geology where sub-surface groundwater flow is small. However, there are also challenges associated with heating the subsurface, as this could change the solubility of minerals in the subsurface water and impact the subsurface biosphere. The temperature of storage is also an important consideration: in the UK, there are currently no high-temperature underground thermal energy storage systems, in part due to concerns around potential environmental impacts. Adjacent sites may impact the performance of one another, and this should be considered when designing a borehole system.
Larger-scale heat storage
Highly populated areas benefit from large-scale underground heat supply systems which can distribute heated water through heat networks. The advantage of a large thermal battery lies in its ability to capture waste industrial heat, cooling loads from buildings, or solar energy in the summer, and therefore provide an energy efficient centralised heat source for the winter. As such, an aquifer thermal energy storage system (ATES) can store and provide more thermal energy to the heat pump system compared to boreholes.
Large-scale storage of thermal energy in aquifers