Model-Based Performance Analysis of a Single Borehole in Ground Heat Exchanger

Ground Source Heat Pumps (GSHP) are high efficient HVAC equipment that provide heating and cooling for buildings. The high initial cost of drilling boreholes is one of the main reasons that hinder the widespread application of GSHP. It is very important to develop proper design methods to predict and evaluate the performance of the Ground Heat Exchanger (GHE) throughout its entire life span. In this paper, heat transfer inside the borehole is simplified as a one-dimensional (vertical) quasi-steady state model. The equivalent circuit method is used to solve the constant heat flux at the borehole wall that becomes the boundary condition for heat transfer outside the borehole. A two-dimensional (vertical and radial) transient model was developed to analyze heat transfer outside the borehole. These highly coupled equations are solved by the bisection method. This numerical model is used for a building located in Chicago as a case study. The results show that the ground temperature rises about 2.0 °C after 10 years operation at a location 5.0 meters away from the borehole. A hybrid GHE system that integrates cooling towers may be considered in this case to balance the net energy input to the GHE system.