Surface–subsurface modelling of seasonal borehole thermal energy storage for a university building

Decarbonisation of heating and cooling is essential for achieving net-zero carbon emissions. At present, c. 80 % of residential heating in the UK is sourced from natural gas, with renewable alternatives sought after to meet the remaining heat demand. Solar energy can be used but its availability is seasonal, with supply periods (i.e., summer) not coinciding with when heating is needed (i.e., winter). Borehole thermal energy storage (BTES) can help to shift supply to meet demand, by storing solar thermal energy (captured by solar thermal collectors in summer) underground to be used in winter. This paper explores the technical potential to incorporate a solar BTES system notionally to the James Watt building, University of Glasgow. Results indicate that 318 MWh of excess heat is available from the modelled solar–thermal collectors (4920.295 m 2 ) after meeting the building’s heat demand in summer. This excess heat can be directed to charge a BTES system (with 192 boreholes, each 42 m deep spaced at 3 m between boreholes) which can discharge to fulfil 16 % of the building’s winter heat demand. The system operation was modelled for a one-year period, to explore the transient changes in the system and efficiency of the array in storing the heat. Lithological layering, frequency of discharge and pipe configuration of the BTES showed significant effect on BTES array response. After an annual charge–discharge cycle, there were 12.79 GJ of net energy stored for 6 months charge, 4 months of discharge and 2 months of recovery (rest). This reduced by 32.29 % for a homogeneous formation, increased by 15.40 % with a U-tube replacing a coaxial tube and increased by 10.79 % when the top insulation layer was removed. Discharging with or without rest showed the least significant impact — the net heat stored increased by 6.18 % when a reduced discharge rate was used over 6 months with no recovery period. It is noted that an increase in the amount of heat stored implies less heat recovery from the ground. Hence there is potential to incorporate solar-based BTES in the James Watt building but this would fulfil only 11 % of the building’s annual heat demand. • Potential of solar BTES to fulfil a university building’s heat demand. • Effect of pipe configuration and lithological layering on heat storage and recovery. • Atlite for solar–thermal system and OpenGeoSys-TESPy for BTES system. • Comparison of BTES array results between EED and OpenGeoSys.