Design of Borehole Heat Exchangers for Ground Source Heat Pumps: A Comparison between Two Methods

AbstractDifferent methods for the design of Borehole Heat Exchangers are available, as reported for example in the Italian standard UNI 11466. Therefore the question arises about the impact of the design methodology on the final result and its sensitivity to the main design and input parameters.In this paper two common design approaches, namely the ASHRAE analytical method by Kavanaugh and Rafferty and the GLHEPRO commercial tool, based on g-functions method by Eskilson, are taken into account. The two methods are used to design a BHE field for a GSHP system in two case studies, namely a small-scale residential and a medium-scale commercial building. Moreover, a sensitivity analysis for each method is carried out, considering the influence of the main design choices and uncertainties on the required inputs.The comparison between the two methods shows that ASHRAE tends to overestimate, up to 27%, the BHE size compared to GLHEPRO. Among the parameters investigated, the heat pump size and the BHE layout modestly affect the final BHE size. In turn, the thermal-vector fluid temperatures on the ground side of the heat pump, the single/double U pipe configuration, the distance among adjacent boreholes and the ground thermal conductivity result in the major influence. In particular it is shown that the uncertainty in the ground thermal conductivity and the choice of the fluid temperatures have a comparable impact on the final sizing as the choice of the sizing method