The effect of borefield geometry and ground thermal load profile on hourly thermal response of geothermal heat pump systems

Vertical BHE (borehole heat exchangers) are a common solution for GCHP applications (ground coupled heat pump). Correct BHE design and sizing are mandatory to either assure long term GCHP performance or reduce the payback period. Most models for performing the time varying analysis of complex borefields are based on the solution of the conduction equation, through the calculation of proper temperature response factors. The DST (Duct Storage Model) is often referred as the benchmark analysis tool, even if it is based on a simplified description of the borefield geometry. In this paper, DST predictions, in terms of hourly fluid temperatures along 20 years, are compared with the corresponding results obtained by implementing the MLAA (Multiple Aggregation Algorithm) approach by Bernier et al. into a model able to employ suitable g-functions generated starting from the Finite Line Source solution. This paper discusses some aspects of the improvements here introduced to the original MLAA method. The study is devoted to the comparison between the predicted fluid temperature values by the DST and MLAA models, with special attention to the influence of the BHE geometry (matrix like vs in line configurations) and to the shape of the yearly hourly load profiles (balanced vs unbalanced ground loads).