A slightly inclined deep borehole heat exchanger array behaves better than vertical installation

The geothermal energy is abundant deep underground for sustainable heating, while the common way of drilling vertical deep borehole requires larger area on the surface. Currently, there are few reported studies on inclined deep borehole heat exchangers (DBHEs). This paper establishes a semi-analytical model of inclined DBHE arrays. The new model is verified through comparison with benchmarks. Subsequently, the long-term operation of this system is simulated and analyzed. The results show that a slightly inclined angle can greatly improve the thermal performance. With the increase in borehole length, the heat extraction per meter of inclined arrays increases faster than vertical arrays. Under the same heating load, the inclination of boreholes can reduce the borehole length, thereby cutting the operational cost of the heat pump and the initial investment of BHEs substantially by 9.3 %. Moreover, the inclined DBHE array can accommodate a greater heating load of 1280 kW, which is 31.3 % larger than that of the vertical array. Finally, a fast prediction model is proposed for inclined DBHEs, with a maximum relative error of 1.60 %. This work offers technical support for the design and operation of inclined DBHE arrays.