Authors: Wu Gao, Meysam Qadrdan, Modassar Chaudry, Jianzhong Wu, Cardiff University
This technical note provides an overview of borehole thermal energy storage technologies and considers the status of the technology in the UK.
Significant amounts of heat can be stored in ground materials like soils, rocks, and pore water due to their high volumetric heat capacity. Borehole thermal energy storage technologies use an array of boreholes (narrow shafts bored in the ground, either vertically or horizontally) to store excess heat in shallow geological environments and can provide seasonal energy storage capability. The performance and feasibility of borehole thermal energy storage depend on several factors, including the design and arrangement of boreholes, material properties, ground properties, and operating parameters.
Borehole thermal energy storage is particularly advantageous for the heating demand of commercial and residential buildings in winter and cooling requirements in summer due to the typical ground storage temperatures of 30-50 ℃ in the core of the borehole field and approximately 10 ℃ at the borehole field’s periphery.
There are other geologic-based energy storage alternatives all of which differ from a technical and economic standpoint. In comparison to borehole thermal energy storage, tank thermal energy storage and pit thermal energy storage have greater heat storage density and heat recovery efficiency, but at much higher storage volume and capacity cost; aquifer thermal energy storage exhibits equivalent storage volume and storage capacity cost but require special geological conditions.
The design and arrangement of boreholes, material properties, ground properties, and operating parameters are important factors in the performance and feasibility of borehole thermal energy storage. Heat loss can be reduced by a suitable design with a small surface-to-volume ratio, top insulation, and consideration of local geological conditions. In addition, the heat charging and discharging process should be operated considering the synergy between the network components. A borehole thermal energy storage project requires a significant initial investment predominantly owing to the high expense of drilling boreholes, which can be 50% of the initial investment. Reduction in drilling costs can be achieved by developing advanced drilling technologies to decrease borehole expenditure, and therefore the project’s initial investment.
Although the deployment of borehole thermal energy storage in the UK is lagging compared to countries, such as Denmark, Germany, and Sweden, borehole thermal energy storage operated at low-temperature is feasible in the UK as a result of natural ground temperature and suitable geological conditions. Additionally, the deployment of district heating and cooling networks with other renewable energy and heat pumps is a positive drive for the growth of borehole thermal energy storage technologies in the UK.