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The geothermal heat pump system based on deep borehole heat exchanger (DBHE) is one of the important systems to realize cleaner heating. It is key to ensure that DBHE can make full use of the dynamic characteristics of deep heat extraction and recovery (HER) in a long period of time to maintain the stability of geothermal field as well as sustainable development of DBHE and geothermal energy. This study mainly has made contributions in the following four aspects: (1) Through numerical analysis with a "stratified segmented finite line source method" (S2-FLS) and its model analytical expression, the impacts of heating duration and heating load distribution on the long-term operation performance of DBHE is revealed, which provides a theoretical basis for the optimal operation of DBHE. (2) The numerical analysis explained how complex geological conditions affect the dynamic heat extraction and heat recovery of DBHE in intermittent operation mode, which provides a theoretical basis for DBHE drilling site selection. (3) The model robustness is fully checked and an initial investigation on DBHE array with HER is presented, which may lead to massive application of deep geothermal systems. • A robust, efficient and easy implemented model is proposed for deep borehole heat exchanger. • The long-term heat extraction/recovery ability of deep borehole heat exchanger is verified. • Analysis on geological conditions can provide guides for drilling site selection. • The deep borehole array with heat extraction and recover is demonstrated. [ABSTRACT FROM AUTHOR]

• 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]