The novel designs for increasing heat transfer in ground heat exchangers to improve geothermal heat pump efficiency

Heat transfer between ground heat exchangers (GHE) and the soil is highly important for the overall efficiency of geothermal heat pump systems. Therefore, the present study investigates how adding materials with high thermal conductivity influences the efficiency and outlet temperature of horizontal heat exchangers. To this end, novel methods were developed and evaluated. Initially, metal sheets were used under pipes as fins or blades to enhance the heat transfer rate (HTR) between GHE and the ground and involve a larger portion of the soil. Due to soil moisture effects on thermal conductivity, the boxes used in this study were filled using two distinct methods. Given its higher moisture content, the soil (backfill material) within the box demonstrated a higher thermal conductivity than the soil surrounding the box. In the first method, the box was connected to the ground so that water could enter it when irrigating the green space, thereby increasing the soil's moisture content and the thermal conductivity of the backfill material. In the second method, the boxes were equipped with aluminum and steel blades to enhance heat transfer through pipes. The simulation was performed using the 1D-3D method and COMSOL software was used. After confirming the efficiency of the proposed methods, their dimensions and thermal properties were altered to produce optimal modes. The temperature of the outlet fluid was reduced by 3.8% in cooling mode using only one aluminum sheet, and the overall efficiency of the GHE increased by 37%. Moreover, the reduction in the outlet temperature of the GHE when the blade was connected to the box filled with the proposed backfill materials was reduced by 10%, which is a very significant result.