Estimation method of soil thermal conductivity distribution of coaxial vertical borehole heat exchanger based on distributed thermal response test

Soil thermal conductivity (λ_s) is a vital parameter for sizing vertical borehole heat exchanger (VBHE) of ground-source heat pump, which may vary dramatically with the depth. To measure the λ_s distribution of coaxial VBHE, this study proposes an estimation method based on distributed thermal response test (DTRT). Firstly, a 3D numerical model is built and validated, which is utilized to simulate some DTRTs under various conditions. A 1D model is then developed for any layer of coaxial VBHE, which is discovered to agree better with the 3D model than the traditional infinite line source model. The proposed estimation method utilizes the 1D model to match DTRT data to identify the λ_s distribution, and then it is compared with the traditional direct method based on the simulated DTRT data. The results show that the traditional method probably has great errors, and that the proposed method could accurately estimate the λ_s of nearly all the soil layers, the errors of which are basically smaller than 4%. The precision of the proposed method is slightly affected by the DTRT duration, number of split layers and errors of soil heat capacities. The proposed method is simple and concise, which is convenient for practical application.