Geothermal helical heat exchangers: Comparison and use of two-dimensional axisymmetric models

• A thermal model is given for the subsoil.
• 2D axisymmetric models of helical geothermal heat exchangers are compared.
• The necessity of taking into account the earth freezing is shown.
• A comparison between the model and experimental results is done.

This study concerns near-surface geothermic heat pumps applied to the heating or the cooling of buildings, and more especially the modeling of new helical heat exchangers buried in the subsoil between 1 and 4 m depth. Two 2D axisymmetric models are considered for the exchangers: a horizontal rings model and an annular cylindrical conduit model. The models are described, and successfully compared with literature results applied to ground thermal energy storage. The 2 models give comparable results. The simulations run significantly faster when modeling the exchanger as an annular cylindrical conduit. The ability of the thermal model of the subsoil to simulate the earth freezing is validated by comparison to experimental results. The influence of the liquid fraction included into the ground on the thermal response of the geothermic heat pumps is analyzed. It has a significant impact on the return temperature from the geothermal heat exchangers because of the phase change phenomena and the increase of the conductivity due to the soil freezing.