Numerical modelling of deep coaxial borehole heat exchangers in the Cheshire Basin, UK

Few deep geothermal wells have been drilled in the Cheshire Basin resulting in high geological and financial risks of such developments. In recent years the focus of exploiting deep geothermal resources in the UK has moved towards lower risk strategies such as deep coaxial borehole heat exchangers (BHEs) for spatial heating. A model of a deep coaxial BHE was designed within MATLAB using the finite-difference method. The model produces accurate results in comparison to an analytical solution with a fast computational time. Results indicate sustainable heat loads in excess of 298.7 kW can be produced from deep coaxial borehole heat exchangers at a depth of 2.8 km over the duration of a 20 year operational cycle. The thermal gradient, depth of borehole, volumetric flow rate and thermal conductivity of the surrounding rock all impact the heat load and outlet temperature of a deep coaxial borehole heat exchanger. The coefficient of system performance decreases with increased volumetric flow rates due to an increase in power consumption within the borehole heat exchanger. For an optimal flow rate of 4 l/s, the coefficient of system performance was 5.29. The thermal performance and efficiency of the system proves the geothermal resource of the Cheshire Basin has significant potential to be developed via deep coaxial borehole heat exchangers. Regression analysis from this study can also be used to predict heat loads and outlet temperatures at the end of a heating season without the need for complex numerical modelling