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Long-term model-based performance of ground heat exchangers

Ground-coupled heat pumps are highly efficient HVAC systems that provide heating and cooling for buildings. The high initial cost of drilling boreholes is one of the primary reasons that limit the widespread application of a ground-coupled heat pump. It is very important to develop numerical models that are capable of predicting and evaluating the performance of the ground heat exchanger throughout its entire life span. In this article, heat transfer inside the borehole is simplified as a one-dimensional (vertical) quasi-steady-state model. The equivalent circuit method is used to solve for the constant heat flux at the borehole wall, which becomes the boundary condition for heat transfer outside the borehole. A two-dimensional (vertical and radial) transient model is developed to analyze heat transfer outside the borehole. The thermal resistances and capacitances of the ground are corrected when interference from adjacent boreholes is taken into consideration. These coupled equations are solved using the bisection method. This numerical model is applied to a theoretical office building using a ground-coupled heat pump located in Chicago as a case study. The results show that the temperature of the ground between two adjacent boreholes rises about 3°C (5.4°F) after a 20-year operation, which results in an increase of the overall annual energy consumption for the ground-coupled heat pump of about 1.4%.