Comparing the Performance of Single U-tube, Double U-tube and Coaxial Medium-to-Deep Borehole Heat Exchangers

The UK heating sector must be decarbonised to achieve net zero targets by 2050; and geothermal energy can help to provide low carbon heat. Closed-loop geothermal systems can provide low-risk solutions via the repurposing of ex-hydrocarbon and unused geothermal exploration wells. Closed-loop, medium-deep borehole heat exchangers (MDBHEs), at depths defined from 500-1000 m, could potentially use a variety of configurations, including single U-tube, double U-tube and coaxial. U-tube MDBHEs involve one or more pipes being inserted in the borehole with heat being transferred to a chilled circulating fluid within the U-tube(s) via conduction through any grout and the borehole wall. Coaxial MDBHEs consist of a concentric central pipe being inserted within an outer pipe (often, the borehole casing). Cool fluid is circulated down the annular space, gaining heat by conduction through the outer pipe wall, before being pumped to the surface through the central pipe. This study addresses the thermal and hydraulic performance of these different configurations under a range of geological and engineering conditions. Simulations were undertaken using OpenGeoSys software to evaluate optimal configurations by minimising hydraulic (and thus parasitic power) losses, while maximising the thermal output.
Under the base case scenario at 800 m, assuming water as the circulation fluid, it was observed at the end of the 25-year simulation, with a flow rate of 5 L/s and inlet temperature of 5 °C, that single U-tube, double U-tube and coaxial configurations provide specific heat extraction rates of 32.8, 36 and 39.1 W/m, respectively. These correspond to pressure losses of 1.46 MPa, 423 kPa and 85 kPa. From the analyses, it was observed that the coaxial configuration led to the lowest pressure losses and generally maximised the thermal output. The coaxial system was then applied to a case study for the Newcastle Science Central Deep Geothermal Borehole, where there are plans to repurpose the well to c.920 m in 2024 for geothermal testing. Results indicate that a thermal power of 50 kW could be attainable for a geothermal gradient of 33.4 °C/km, rock thermal conductivity of 2.5 W/(m.K) and flow rate of 5 L/s.