In-situ test and numerical investigation on the long-term performance of deep borehole heat exchanger coupled heat pump heating system

Given the substantial initial investment required by the drilling and implementation of the Deep Borehole Heat Exchanger (DBHE), it becomes imperative to quantitatively evaluate its long-term performance and sustainability. This work introduces a pilot DBHE project in Xi'an, along with the 500-h in-situ monitored data, which is used to validate the 3D numerical model established and simulated in the OpenGeoSys (OGS) software. Based on the validated model, a series of extended scenarios are executed to evaluate the influence of design and operational parameters on the long-term performance and sustainability of the DBHE system. The results show that the drilling depth is the most significant factor that influences long-term performance. On the contrary, pipe diameter and inner pipe thermal conductivity have a very limited impact. A comprehensive evaluation is suggested to determine operational flow rates in real projects while considering the energy consumption by the circulation pump. Moreover, the heat extraction performance and energy analysis of the DBHE coupled with a geothermal heat pump under intermittent operation are also investigated. With a fixed daily heating demand, a longer daily operating time results in lower circulating temperature drops, which is advantageous for the heat pump's energy efficiency in long-term operation.