Proposing stratified segmented finite line source (SS-FLS) method for dynamic simulation of medium-deep coaxial borehole heat exchanger in multiple ground layers

Extracting thermal energy from medium-deep ground layer for building heating is a promising means for pushing forward cleaner heating initiative. There is no available analytical model that can provide fast, accurate and effective simulation of both fluid and soil temperature field of deep borehole heat exchanger (DBHE) under complex geothermal gradient and subsurface stratifications. In this study, a stratified segmented finite line source method (SS-FLS) is proposed as an main contribution and novelty. The proposed model is validated by comparing with finite element method, model degradation example, and actual engineering project data of DBHE. The accuracy and robustness of the SS-FLS is fully checked. Then the effects of stratification on soil temperature field and fluid temperature field are investigated. It is found that the shape of temperature contour and temperature distribution have direct link with stratification as well as heat extraction rate on the heat pump side. Moreover, two critical model parameters are discussed in details which poses an important guide for both model application and system analysis.