Borehole Heat Exchangers: sensitivity analysis of the most important factors affecting their performances

The performance modelling of Borehole Heat Exchanger fields is essential for their correct design. In this work, the influence of the BHE materials, the hydrogeological settings (e.g groundwater flow…) and the soil thermal parameters has been investigated, in order to assess their relative impacts on the fluid working temperatures and, therefore, on the heat pump energy consumption. Long-term simulations with a benchmark thermal load have been carried, using the dedicated interface of FEFLOW. The resulting fluid temperatures have been compared and the heat pump energy consumption has been estimated. The results show that thermally enhanced grouts, pipe spacers and low-viscosity heat carrier fluids can lead to significant energy savings and permit to reduce the BHE size. The most important soil parameter for BHE simulations is the heat conductivity: its value is site-specific and literature references report wide ranges for each lithology. The presence of groundwater flow can strongly enhance the performances of closed-loop vertical exchangers. Concluding, a thorough site characterization is necessary to avoid large errors in the GSHP dimensioning, which would impair the economic sustainability of this solution.