Heat extraction performance and techno-economic analysis of a deep U-type borehole heat exchanger under intermittent operation

Abstract

The deep U-type borehole heat exchanger (DUBHE) has been extensively investigated in terms of its long-term heat extraction capacity and economic feasibility. In this work, a 3D numerical model of DUBHE was established based on the open-source numerical software OpenGeoSys (OGS). Three- and sixty-day experimental data from two pilot projects in Xi’an, China, were used for the model validation. Subsequently, the model was extended to the long-term operational period by introducing different operation-shutdown ratios. In the long-term operation, the extension of the daily operational time from 8 to 24 h resulted in a reduction of the soil temperature difference from 14.87 °C to 7.51 °C. Compared to the continuous operational scenario with an operation-shutdown ratio of 24:0, the total heat extraction for operation-shutdown ratios of 8:16, 12:12, and 16:8 was discovered to be 65.15 %, 76.14 %, and 84.85 %, respectively. Considering the system’s initial investment and the operational costs coupled with the heat pump and circulating water pump, a techno-economic analysis was also executed based on benchmark parameters. For the DUBHE heating system with an operation-shutdown ratio of 8:16, the investment payback period is approximately 8.2 years, which is the shortest among all scenarios. The related suggestions proposed in this work aim to provide insightful references for engineers and decision-makers in practice.