Principle and capacity quantification of deep-borehole heat exchangers

Deep-Borehole Heat Exchanger (DBHE) is generally installed as a co-axial pipe in deep boreholes. The DBHE can obtain heat without extracting groundwater. Literature review shows that the specific heat extraction rate of DBHE is less than 200 W·m⁻¹, which is much smaller than that by conventional hydrothermal exploitation systems. Using both analytical and numerical methods, we evaluated the capacity of DBHE with representative parameters in northern China. Three modeling scenarios were constructed using the simulator OpenGeoSys, including (1) short-term (4 months) continuous heat extraction; (2) short-term heat extraction with half-day intervals and (3) long-term (30 years) continuous heat extraction. Results show that the sustainable heat extraction rates in all continuous extraction scenarios are less than 150 W·m⁻¹. While in the intermittent scenario, the rate could be doubled, but will lead to larger fluctuations in the outflow temperature. Sensitivity analysis indicates that the depth of borehole is not a significant influencing factor to the heat extraction rate, while the heat conductivity of surrounding formation is critical. Finally, we point out that the basic need in order to increase the specific heat extraction rate of a DBHE is to increase the convection in the surrounding formation, or in other words, the contact area between circulation water and formation.