A new analytical model for short-time analysis of energy piles and its application

Abstract

An energy pile is a special form of vertical ground heat exchanger that couples the roles of structural support and heat transfer. Modeling the transient heat transfer process inside an energy pile has importance; however, available analytical models either have insufficient calculation accuracy or are computationally demanding. Based on three existing models, this paper proposes a novel short-term hybrid composite-medium line-source (HCMLS) model, which is not only efficient in computation but also more accurate than most traditional analytical models. The model is suitable for ground heat exchangers of various radii. Comparisons between the hybrid analytical model and a numerical model are made for energy pile cases with different parameters, including the thermal properties, borehole radii, relative positions of tubes, and number of tubes. In general, the hybrid composite-medium line-source model gives credible prediction after 100 min. The new model is further validated by the infinite composite-medium line-source (ICMLS) model, which is currently the most theoretically complete short-term model. Moreover, the new model is applied to thermal response tests (TRTs). The least dimensionless test duration for interpretations based on the modified hybrid composite-medium line-source (C-HCMLS) solution is Fo > 1.7. This study renders the application of in situ TRTs to energy piles with large diameters feasible.