Estimation method of layered ground thermal conductivity for U-tube BHE based on the quasi-3D model

Abstract

The distribution of ground thermal conductivity (λs) is a significant consideration for the design of ground source heat pump systems. This paper proposed a new method to estimate the layered λs for U-tube borehole heat exchanger (BHE) using particle swarm optimization (PSO) and distributed thermal response test (DTRT) data. Firstly, a quasi-3D heat transfer model considering geothermal gradient for U-tube BHE had been built to simulate fluid temperature profiles. And the experiment in DTRT was performed using a 45 m sandbox. Then, the layered λs were estimated through PSO and DTRT data based on the established model. The results showed that the convergent λs distribution could be reliably obtained by short-term DTRT. Furthermore, the estimation results are independent of the number of layers and data points. The mean difference in λs distribution between 9 layers and 45 layers is 0.007 W/(m∙K) and the layered λs also could be obtained when the number of layers is more than the number of temperature data points, and the root mean square error between 30 and 220 points is less than 0.3 W/(m∙K).