An analytical heat-transfer model for coaxial borehole heat exchanger with segmented method

Abstract

ABSTRACT In order to meet increasing demand for building energy, the coaxial borehole heat exchanger (CBHE) is proposed as an efficient and environmentally friendly geothermal energy-utilization device, but there is no suitable analytical model for its heat exchange analysis. Hence, an analytical model of CBHE with segmented method that can consider the non-uniformity of the thermal conductivity of outer soil is proposed. First, the heat-transfer models inside and outside the borehole are introduced, respectively; then, on the basis of time superposition and the segment cylinder-source method, using borehole-wall temperature as an intermediate quantity, the two models were coupled as a three-dimensional model; then, the corresponding calculation principle and calculation process of the model are introduced. After that, through a comparison between experiment- and numerical-analysis results, the rationality of the analytical model was verified on the basis of thermal-response-test (TRT) results and temperature distribution along with the depth. In addition, it can conclude that the analytical model has higher calculating efficiency than the numerical analysis on the premise of maintaining accuracy. Lastly, the effect of the thermal conductivity of the inner and outer pipes, backfill material, and outer soil on the heat-exchange efficiency of CBHE was studied, and the results showed that, for the inner pipe, as thermal conductivity increased, the heat-transfer efficiency of CBHE gradually decreased; for the outer tube, backfill material, and outer soil, as thermal conductivity increased, the heat-transfer efficiency of CBHE increased. The influence of material thermal conductivity on the heat-transfer effect of CBHE was in the descending order of outer pipe, backfill material, inner pipe, and outer soil. Besides, we concluded that a CBHE with an annular region as inlet had better heat-exchange efficiency than that with a round region as inlet. The most optimized design of CBHE (considering heat-exchange and economic efficiency) is that with an inner pipe with low thermal conductivity, thermal conductivity of the outer pipe larger than that of the backfill material, thermal conductivity of the backfill material larger than that of the outer soil, and the outer soil is in the area being rich in groundwater.