Modelling of alternative borehole configurations for geo-exchange

Abstract

During the operation of a ground source heat pump (GSHP), the ground acts as a heat sink and heat source in cooling and heating modes, respectively. When the heating and cooling loads are extremely unbalanced, ground temperature can slowly migrate up or down in the long term, diminishing the GSHP system's performance, and eventually causing the system to fail. This failure occurs when the ground can no longer accept or provide more heat for a building. Therefore, a method to mitigate thermal imbalance is needed. Previous studies in the literature examine the effects of borehole configurations in geo-exchange. However, no study has been done to analyze the effects of varying borehole lengths in a bore field. The objective of this study is to examine the effects on thermal performance from changing the length of individual boreholes while retaining the same total borehole length. In this paper, the four centre boreholes in a 4x4 borehole system were shortened and the length of the remaining boreholes was recalculated to meet the total required ground loop length. A 20 year operation was simulated for a school building model with centre borehole lengths of 100 m, 80 m, and 50 m and separation distances of 3 m, 4 m, and 6 m, to study the benefits of shortening the centre boreholes. The results demonstrate that by adjusting the length of the centre boreholes, separation can be reduced.