An analytical method to simulate the dynamic performances of truncated cone helix ground heat exchangers

This paper proposes a dynamic analytical method to simulate the thermal performances of truncated cone helix ground heat exchangers (i.e., the so-called “energy baskets”). These ground-coupled devices are attractive solutions to reduce the initial cost of ground-coupled heat pump systems, as they require lower cost to be drilled and installed with respect to traditional boreholes. However, both design methodologies and performance assessment models are still not well developed, producing substantial uncertainties on final operative performances. This work presents a plain evaluation method based on the heat exchangers theory and the analytical solution of the truncated cone helix heat source in a semi-infinite medium. It can be advantageously used to simulate the thermal performance of truncated cone helix ground heat exchangers as a function of helix geometries and operative conditions evolution (e.g., inlet temperature, fluid flow rate, ground temperature…). Specifically, in this paper, we perform a sensitivity analysis of the thermal performances of a case study by varying the main geometrical parameters. Besides, we compare the heat transfer of the reference configuration with an equivalent cylindrical arrangment. The truncated coil configuration is more effective than cylindrical one as the cone aperture reduces the short-circuits between helix pitch and the equivalent thermal resistance with the ground surface. However, obtained results are notably affected by the assumption of an isothermal surface temperature, which leads to a shallow/plain helix/spiral as the best configuration: different conclusions are expected when a time dependent or adiabatic boundary condition will be accounted in the model.