Evaluation of heat pump efficiency within an energy pile group: effect of heat cluster, underground seepage and seasonal cycles

Energy geostructures are a cost-effective solution for low enthalpy geothermal installations. Heat exchanger pipes are attached to the reinforcing cages of geotechnical structures to combine structural and geothermal roles. However, the installation of a group of multiple energy structures, such as a pile group, can lead to a heat cluster effect, which tends to decrease the efficiency of the heat pump (COP factor). This phenomenon is amplified by the number of grouped energy geostructures. However, energy geostructures are often installed in areas with groundwater flow. This induces a thermal washing effect, which decreases the risk of multi-year thermal shifts and preserves heat pump efficiency. In this paper, we evaluate the heat cluster effect in a group of piles, its influence on heat pump COP, the benefit of thermal washing due to underground water flow, and the effect of various seasonal scenarios. For this purpose, we present and use a specific numerical process that allows for modeling energy geostructure installations and interactions between groundwater flow models and thermal models. The heat pump COP is then estimated according to variable heat load, which depends on soil temperature.