A Modelling Study of Seasonal Borehole Thermal Energy Storage in Scotland: Integrating Surface Demand and the Subsurface Heat Store

Abstract

In the UK, 80% of heat is currently supplied by combustion of natural gas, which significantly contributes to the country’s greenhouse gas emissions. This makes it imperative to explore alternative energy sources in the drive towards a net-carbon-zero economy. Geothermal energy is a renewable energy source that can provide reliable low-carbon energy baseload for direct heat use. In addition to the UK’s large demand for direct heat use in the winter, significant cooling demand exists in the summer. Hence, seasonal underground thermal energy storage should be considered. Borehole thermal energy storage (BTES), in which the subsurface serves as the storage medium via borehole heat exchanger (BHE) arrays, is particularly attractive since it can technically be applied anywhere provided there is little significant groundwater flow.
In this work, the James Watt building at the University of Glasgow is taken as the basis for a modelling study. A low-temperature BTES system is employed to meet the building’s seasonal heating/cooling demands by numerical modelling on OpenGeoSys software coupled to TESPy, incorporating the subsurface BHE and a heat pump. BTES is chosen due to the absence of a suitable shallow aquifer for aquifer thermal energy storage and lack of surface footprint for pit or tank thermal energy storage. The change in BHE fluid inlet and outlet temperatures as well as ground temperature throughout one year of operation have been studied. Results indicate that thermal energy can be stored during summer and extracted during winter. A solar-thermal system is used to meet the demand of the James Watt building in the summer (1030 MWh) while a surplus of 318 MWh is stored for use in the winter. The stored heat is extracted in the winter, and it contributes to 16% of the building’s winter demand (i.e., 318 MWh of a total winter demand of 1981 MWh).