Thermal Performance Analysis of Deep Borehole Heat Exchanger Based On Field Test and Numerical Method

Geothermal energy has considerable development potential in space heating. As a method of heat extraction, the deep borehole heat exchanger has superior thermal performance to that of the conventional borehole heat exchanger. The existing related researches are primarily focused on theory and modeling, via which the temperature characteristics and thermal processes cannot be truly reflected. Therefore, in this study, a field test on deep borehole heat exchanger is performed. A distributed optical fiber temperature sensor is used for the real-time monitoring of the system operation. The thermal performances of the long-term and intermittent operation are analyzed. Moreover, the temperature distribution characteristics, thermal process and impact scope of the formation are discussed. The results show that the temperature of the bottom borehole is measured as 107.3 ºC, and the average geothermal gradient is determined as 0.0507 ºC/m. The thermal process between the soil and fluid is enhanced close to the borehole. The fluid temperature evolutions with depth exhibit exponential and linear responses for the heating and non-heating periods, respectively. The main impact scope at the bottom borehole can reach 40m. The average value of the coefficient of system performance is able to 6.14 in the intermittent operation period. The results can provide a reference for optimum design of DBHEs for practical engineering.