First results from a ground source heat pump monitoring project at the British Geological Survey Headquarters in Nottingham, United Kingdom

The heating systems of two buildings at the British Geological Survey’s campus in Nottinghamshire, UK, have been retrofitted using ground source heat pump technology. Low grade heat is extracted from the ground using 28 vertical closed-loop borehole heat exchangers installed to c.225 m. Two buildings are served by separate ground arrays with one coupled to two 60 kW heat pumps and the other to three 60 kW heat pumps, providing up to 300 kW peak heating capacity. Six of the boreholes are monitored using fibre optical distributed temperature sensing (FO-DTS) cables. The cable is also capable of distributed acoustic sensing (DAS) and active heating for formation characterisation including Enhanced Thermal Response Testing (E-TRT). Five of the wells have electrical resistivity tomography (ERT) capability using BGS’s PRIME monitoring system designed in-house. This enables time-lapse cross-hole tomography for 4D inversions to characterise geological heterogeneity, track natural seasonal processes and induced thermal changes in heat exchanger wells across a rock volume of approximately 360,000 m3. The thermal energy extracted from the ground and produced from the heat pump is measured with heat flow meters. Electrical consumption of heat pumps and circulation pumps is monitored enabling calculation of COP’s and Seasonal Performance Factors, to enable breakdown of OPEX costs for each system. One borehole within the array was cored to 238.5 m and the hole was logged with a comprehensive suite of geophysical tools to confirm the geological sequence and vertical variations in geophysical properties. We found that modelled and observed results of the thermal conductivity of the bedrock also differed depending on how the assessment was made. Specifically, TRT gave an effective ground thermal conductivity of 2.33 W/mK vs laboratory analysis of core material which gave variable but generally higher values. The presence of gypsum in Mercia Mudstone formations probably reduces bulk thermal conductivity. The impacts of groundwater flow, concurrent well installation during monitoring and subtle variations in bedrock competence are also discussed. This data feeds into the site thermogeological model and supports numerical models and public outreach resources.