IN-24-C051 – Performance Analysis of Vertical Borehole Ground Heat Exchanger Design Methods Using Modelica-Based Modeling and Simulation

The emerging need for the widespread adoption of ground source heat pumps (GSHPs) in the United States is faced with challenges such as the high installation cost of vertical borehole ground heat exchangers (VBGHE). This installation cost constitutes a significant portion of the overall cost of GSHPs. It is important to accurately size the VBGHE to avoid operational problems of GSHP while making it economically feasible. This study aims to evaluate different design methods by evaluating the performance of a VGBHE system designed by the different design methods using Modelica-based modeling and simulation. Firstly, the VBGHE length was evaluated using different design methods which include the rule of thumb (M0), two pulses (M1), ASHRAE (M2), and GLHEPro software (M3). The VBGHE system model was then developed, and the performance of the different design methods was evaluated through the comparison of the entering fluid temperature to the heat pump and the electrical energy consumed by the heat pump. Finally, the developed model was used to evaluate the influence of ground load on the soil temperature for different years of operation of the GSHPs. The VBGHE lengths vary over a range of 74m with a minimum length of 223m (M1) and a maximum length of 297m (M0). A range of 74m between the minimum and maximum VBGHE length gives an increased electrical energy consumption of the heat pump by 102kWh (2.2%) for a year operation period and 1100kWh (2.4%) for a ten-year operation period. The electricity consumed by heat pump for M2 and M0 are 1.4% and 1.6% lower than the electricity consumed by M3 while the electricity consumed by the heat pump for M1 is 0.6% higher than the electricity consumed by M3 method. Compared to the reference method (M3), underestimation of VBGHE length led to about 1% increase in the electrical energy consumed by the heat pump for the test case.