Field test and numerical investigation on deep coaxial borehole heat exchanger based on distributed optical fiber temperature sensor

摘要

The thermal performance of DCBHE is superior to conventional borehole heat exchanger (BHE) systems. In the current study, the field test using a DCBHE (2044 m) is conducted, based on a distributed optical fiber temperature sensor (DOFTS). The temperature of the circulating fluid in terms of depth and operation time is monitored in real-time. The thermal extraction performance of the DCBHE is evaluated over an operation period of 60 d. Moreover, the temperature distribution characteristics, thermal process and impact scope of the formation are analyzed. The results demonstrate that the temperature of the bottom borehole is measured as 107.3 °C via DOFTS, and the average geothermal gradient is determined as 0.0507 °C/m. The thermal power and the coefficient of system performance are able to reach 238.0 W/ m and 7.40 following 60 d of operation, respectively. The inner pipe made of polyethylene has exhibits a high level of thermal insulation for the outlet fluid. The thermal process between the soil and fluid is enhanced close to the borehole. And the soil temperature profiles exhibit fluctuations around the borehole within 1 m due to power failure. Furthermore, the formation impact scope is mainly within radius 5 m for operation 60 d.