Sensitivity Analysis on the Performance of a Ground Source Heat Pump Equipped with a Double U-pipe Borehole Heat Exchanger

Ground Source Heat Pumps (GSHP) are economically and environmentally advantageous for the heating and cooling of buildings, provided that the long-term sustainability of the thermal exploitation of the soil is ensured. In particular, the performance of a closed-loop Borehole Heat Exchanger (BHE) strongly depends on the geometrical and physical properties of its components and on the thermo-hydrogeological properties of the surrounding soil. In this work, we present the results of a series of simulations of a double U-pipe Borehole Heat Exchanger, carried out with the finite-element flow and heat transport modelling software FEFLOW to assess the relative influence of these parameters on the operation of a GSHP. The analysis confirms that the length of the borehole is the main design parameter, but the thermal conductivity of the grout, the pipe spacing, the heat carrier fluid and its flow rate also have an important effect on the energy efficiency of the system. The thermal conductivity of the soil is another fundamental variable in the design of a GSHP, and hence it is better to rely on site-specific data, rather than adopting values from the literature. Although most design methods neglect it, the presence of a subsurface flow results in an enhancement of the performance of the system. Thermal dispersion also enhances the efficiency of the system but, since it has not yet been adequately studied, relying on it is not advised for the design of BHE fields.