Thermal performance testing of shallow geothermal systems: Insights from field experiments and numerical modelling

Decarbonisation of the heating and cooling sectors is essential for achieving net zero targets, and geothermal systems provide a promising pathway to reduce reliance on fossil fuels. To design and optimise ground source heat pump (GSHP) systems, an understanding of subsurface thermal properties is critical. Traditionally, this is achieved through thermal response tests (TRTs), which measure subsurface behaviour under constant heat flux conditions. However, thermal performance tests (TPTs) offer an alternative approach by applying a constant inlet temperature, allowing not only the determination of effective thermal conductivity but also an assessment of the system’s thermal capacity under both heating and cooling scenarios. This study presents what is believed to be the UK’s first documented TPT and a new direct comparison of heat injection and extraction tests, aiming to demonstrate the method’s value for characterising geothermal systems in real-world applications. The experiments at the Cheshire Geoenergy Observatory consisted of two borehole heat exchangers (BHEs) injecting heat at a constant temperature of 30 °C and two BHEs extracting heat by circulating fluid at a set point of 1 °C within the Sherwood Sandstone Group. Results indicate that TPTs provided effective thermal conductivity estimates (2.43–3.65 W/mK) consistent with core measurements and numerical models. However, there are issues with heat extraction TPTs, which appear to under-estimate thermal conductivity. This could be due to (i) natural convection enhancing heat transfer in the subsurface during heat injection mode, or (ii) the oscillatory behaviours of heaters and chillers may introduce errors when estimating properties, potentially impacting this conclusion.