Effect of Geological Formations on Thermal Performance Modelling of a Deep Borehole

Abstract

Deep borehole heat exchangers (DBHEs) are currently one of the hot research topics for geothermal heat pump (GHP) applications. There are several computational and experimental works on the topic. Because of the great depth, DBHEs are surrounded by vertically varying geological layers which have different thermal properties. A reliable prediction of time-dependent heat transfer rate (thermal performance) for a DBHE is an important issue for engineering design of a GHP application. Homogeneous or layered models can be established and used for predictions. The homogeneous model is based on the homogenization approach which considers all layers as a single layer having effective thermal properties. On the other hand, the layered model considers all layers separately and gives better accuracy in its predictions, but it is computationally more complicated and time consuming. In this study, thermal performance predictions are made by both the homogeneous and layered models and the results are compared to determine the difference between these two approaches. Furthermore, by considering the true geological formations, different thermal property profiles are used in these models to examine the dependency of thermal performance on formation characteristics. It is seen that the deviations of layered model from those of the homogenous one for energy predictions are proportional to the square of the coefficient of variation of thermal conductivity. The results help to understand the effect of geological formations on thermal performance modelling of a DBHE and as well as the necessity to use a layered model for performance predictions.