Field Test and Numerical Investigation on Long-term Performance of Deep Borehole Heat Exchanger Coupled Heat Pump Heating System Based on Pilot Projects in Xi’an, China

Abstract

The ground source heat pump is by far the most widely applied technology in geothermal energy utilization. To meet the heating demand in densely populated urban areas, the Deep borehole heat exchanger (DBHE) coupled geothermal heat pump is increasingly being applied in northern China. In this paper, field test data collected from two pilot projects with two types of DBHE (coaxial and U-type) are reported and analyzed. Based on the Dual-continuum Finite Element Method, the heat transfer processes both within the DBHE and in the surrounding subsurface are simulated by the open-source software OpenGeoSys and validated against the field monitoring data. Long-term energy analysis of DBHE systems shows that the amount of heat extracted by the DBHE is mainly supplied by the thermal energy stored within the surrounding subsurface. Furthermore, based on the short-term thermal performance test and long-term thermal response analysis, the long-term performance of the DBHE heating system equipped with coaxial DBHE is evaluated. Last but not least, considering the official electricity price and dynamic operational consumption of the heat pump and circulation pump, the optimal design of the drilling depth for the coaxial DBHE is determined by conducting an entire lifespan economic analysis. The Levelized cost of heating and net present values are discussed in detail. The related work and recommended design value can serve as a reference for the corresponding decision-makers, researchers, and users in the geothermal community.