Field test and long-term heat extraction performance evaluation of the deep U-type borehole heat exchanger system

The concept of the deep U-type borehole heat exchanger (DUBHE) has been proposed and implemented in recent years to achieve higher heat extraction capacity and efficiency. Given the substantial initial investment required for implementation, it becomes imperative to quantitatively assess the long-term thermo-techno-economic performance of the DUBHE system. In this study, the pilot DUBHE project in Xi'an, China, is first introduced in detail, along with 60-day in-situ monitoring data. The monitoring data are used for the validation of a heterogeneous 3D numerical model that is established in OpenGeoSys (OGS) software. Based on the validated model, a series of extended scenarios of the DUBHE system are simulated to show that the maximum sustainable heat extraction rate of the pilot DUBHE system is around 480 kW. The heat extraction capacity of the DUBHE can be significantly improved by a larger depth rather than a longer horizontal length. The thermally affected distance extends more than 35 m away from the borehole. The techno-economic analysis shows that the lowest value of the Levelized Cost of Energy (LCOE) of the DUBHE system is 0.584 Yuan/kWh, which corresponds to the scenario with a vertical depth of 2800 m and a horizontal length of 600 m.