Long-term thermal performance analysis of deep coaxial borehole heat exchanger based on field test

Abstract Geothermal energy is an abundant and clean energy source that is abundant and widely and naturally distributed in the earth. Deep coaxial borehole heat exchanger (DCBHE) can efficiently extract heat from rock-soil for building heating. And DCBHE systems have superior application performance to that of the traditional BHEs. However, the existing related researches are primarily focused on theory and modeling, via which the temperature characteristics and thermal processes cannot be truly reflected, thus field testing is necessary. In this study, the long-term thermal performance of a DCBHE is developed based on field test with a distributed optical fiber temperature sensor (DOFTS). The numerical model is validated by field test data while considering power failure. The effects of the heat load mode, flow rate, heat load, radius ratio and soil thermal conductivity on the fluid and soil are analyzed and discussed. Moreover, the thermal process in the formation is analyzed. It is demonstrated that the results of the numerical simulation fit well with the test data. The temporal and spatial distributions of the fluid temperature and thermal process can be affected by two heat load modes, for which thermal compensation should be applied. A higher flow rate, lower heat load and larger radius ratio can promote the outlet fluid temperature. The thermal equilibrium points in the formation will move downward with the increase of the soil thermal conductivity, and main impact scope can reach 40m. A higher flow rate and larger radius ratio will also obviously strengthen the pressure drop, and the power consumption will increase accordingly. Furthermore, two methods respectively called “Intercept” and “Guide” are proposed to reply the reverse heat conduction in shallow soil for DCBHE. The results can provide a reference for optimum design of DCBHEs for practical engineering.