Quantifying vertical borehole heat exchanger sustainability from a geothermal resource perspective in the UK

Abstract

Low-temperature geothermal energy is viewed as a renewable resource that could contribute to the decarbonisation of residential heating in the UK. Here we constrain the geothermal potential of the shallow subsurface to provide the heating requirements of a typical single-family house via a vertical Borehole Heat Exchanger (BHE), to provide a preliminary framework for the heat resource licensing of such systems. Through analytical and numerical models, we calculate the heat balance for a typical house with a garden located in Carboniferous sedimentary basins, which represent a large proportion of the UK land mass and the regions where energy poverty is more common. Neglecting the heat recharge via groundwater flow, our results indicate that geothermal, solar heat flux and radiogenic heat production can only replenish up to 5% of the heat extracted annually by a 100 m BHE in heat extraction mode only. The lack of natural recharge causes the continuous expansion of the thermal footprint at an average rate of 18 m² yr -1 over 30 years, considering a ground thermal conductivity of 2.2 W (m°C) -1 , with the area cooled by 1°C extending beyond the current minimum BHE spacing guidelines of 6 m. In Scotland, reinjecting c. 35% of the yearly heat requirements, which could be accomplished through borehole thermal energy storage, is shown to safeguard against resource over-exploitation and interference risks between neighbouring users in the majority of the high-demand areas. In view of a large deployment of BHEs, it is crucial to incorporate such reinjection strategies into a regulatory framework to ensure a resilient and sustainable utilisation of shallow geothermal resources.