Analytical modeling and thermal analysis of deep coaxial borehole heat exchanger with stratified-seepage-segmented finite line source method (S3-FLS).

• A new analytical model is proposed for deep borehole heat exchanger. • The model takes full consideration of seepage, stratification, and geothermal gradient. • The model is featured as high computational speed, robust, and flexibility. • An analysis on heat extraction and recover of single and multiple DBHE is offered. Deep geothermal energy can be used as space heating as an important sustainable, clean and efficient method through deep borehole heat exchanger (DBHE). However, the heat transfer of DBHE is much more complex than conventional shallow borehole and there is no available analytical modeling tool. This study fully considered the complex geological conditions of seepage, stratification, and geothermal gradient in the modeling. A S3-FLS with full name of "stratified-seepage-segmented finite line source method" is proposed and then verified through multi-facets comparison with numerical models and real project data. In addition, the unsteady heat transfer of water flow inside coaxial tube is established and incorporated with the S3-FLS model. The new model is featured as high computational speed, robustness, and flexibility. Those features are supported by the model analysis in this study, via investigating the impact of borehole segmentation, comparison between unsteady and steady water heat transfer and impact of complex geological conditions on system simulation. In addition, it is shown that the model can be easily used for dynamic heat extraction and recovery analysis, as well as DBHE array analysis, through some initial illustrations. [ABSTRACT FROM AUTHOR]