Thermal performance analysis on the composition attributes of deep coaxial borehole heat exchanger for building heating

Abstract Geothermal energy has considerable development potential in space heating. As a method of heat extraction, the deep coaxial borehole heat exchanger has superior thermal performance to that of the conventional borehole heat exchanger. However, the composition attributes (inner pipe, outer pipe, grout, length) of the deep coaxial borehole heat exchanger greatly affect the thermal performance, and most investigations on coaxial borehole heat exchangers lack the support of a field test. Therefore, in this study, a field test on deep coaxial borehole heat exchanger with the depth of 2044 m was performed. And a distributed optical fiber temperature sensor is used for the real-time monitoring of the system operation. The geothermal conditions and the evolution of the fluid temperature with depth and the operation time in field test are determined. The effects of the composition attributes on the thermal process and thermal performance are analyzed and discussed based on a validated numerical model. The results show that the constant zone of subsurface temperature is determined to be at a depth of 0–100 m. The fluid temperature at the bottom borehole can reach 44.5 °C over an operation period of 60 d, and the temperature drop in the inner pipe is less than 2 °C/2044 m. The thermal equilibrium point is developed at a shallow depth, and the thermal process and the impact scope are enhanced for a deeper bottom borehole. The smaller radius and lower thermal conductivity of the inner pipe can increase thermal resistance. Then, the larger radius and higher thermal conductivity of the outer pipe are more conducive to heat production. Moreover, the thermal performance is enhanced with the increase of the thermal conductivity of the grout and the length of the borehole. The results can provide a basis for the thermal performance optimization of the deep coaxial borehole heat exchangers.