A unified analytical model for vertical ground heat exchangers: non-uniform finite line source method (NFLS)

Deep borehole heat exchanger (DBHE) is in rapid development. Currently, two sets of different models used for shallow ground heat exchanger (GHE) and DBHE. The study presents a unified analytical model for both shallow and deep GHEs. The method adopts the linear distribution of borehole wall heat flux in depth direction and then derived a new finite line source model that can treat both shallow and deep borehole without considering segmentation. The method is used for various types of demonstrations from shallow single U-tube to deep coaxial bore-field heat transfer simulation, and all the cases are validated with experimental data that showing relative error lower than 5 % for most cases. With aid of the new models, some analyses are made for shallow and deep single BHE or bore-field. The results revealed the shallow coaxial GHE has better performance than shallow single U-tube but on the contrary when borehole is deeper; a new index for thermal performance declination of DBHE in long run; better layout arrangement of shallow and deep bore-field. In the end, some useful suggestions and limitations are provided for further development or applications of the new model.