Numerical investigation on the capacity and efficiency of a deep enhanced U-tube borehole heat exchanger system for building heating

Deep geothermal energy has become widely exploited in recent years through the use of closed loop systems for building heating. Intended to meet high heating demand in densely populated neighbourhoods, an enhanced U-tube borehole heat exchanger (EUBHE) system, in which a deviated deep borehole is connected with another vertical one to form a closed loop, is introduced in this work. For capacity and efficiency analysis of applying EUBHE systems to extract deep geothermal energy, a 3D numerical model is implemented and established based on the OpenGeoSys software. Through evaluation by thermal performance tests and thermal response tests on the EUBHE system, the maximum sustainable heat extraction rate is found to be 1.2 MW in a single heating season and 1.1 MW in 10 years, which can provide heating to more than 35,000 m2 of residential buildings located in northern China. Moreover, the 10-year system thermal performance and efficiency are evaluated when coupled with a ground source heat pump (GSHP), and compared with the two deep borehole heat exchanger (2-DBHE) array system that has the same total borehole length as the EUBHE system. Results show that GSHP-coupled EUBHE system is more efficient than the 2-DBHE array system, as it consumes 27% less electricity.