Distributed geothermal response test on a 750 m deep borehole thermal energy storage system

• A geothermal response test dataset was obtained over 42 days. • Thermal properties of the subsurface and the borehole heat exchanger were investigated. • Simulations based on depth-resolved temperature profiles yield accurate results. • Heat loss from the inner pipe to the annulus of the heat exchanger reduces overall system efficiency. Medium-deep borehole thermal energy storage (MD-BTES) systems are a promising solution for large-scale thermal energy storage, but their optimization requires a detailed understanding of subsurface thermal properties. In this study, a distributed geothermal response test (dGRT) was conducted on one of three 750 m coaxial boreholes of an MD-BTES demonstrator system using fiber optic temperature measurements. Local thermal conductivity was found to vary significantly, ranging from 2.0 to 2.8 W m −1 K −1 , reflecting the transition from fractured basalt to granodiorite within the storage reservoir. The borehole thermal resistance decreased steadily with depth, from 0.04 to 0.01 m K W −1 , reflecting better thermal coupling at greater depths and demonstrating the effectiveness of a low thermal conductivity grout in the upper borehole section. Numerical simulations based on the GRT results yielded high accuracy, particularly when based on depth-resolved temperature profiles. However, the analysis also revealed challenges, including significant heat loss from the inner pipe to the annulus at elevated temperatures, which likely reduces MD-BTES system efficiency. These findings highlight the capability of dGRT to provide high-quality thermal property data for medium-depth systems and offer insights into design optimizations for future MD-BTES installations.