Freezing of geothermal borehole surroundings: A numerical and experimental assessment with applications

This study examines the thermal consequences of freezing the ground in the immediate vicinity of geothermal boreholes. First, a one-dimensional radial numerical heat transfer model is developed to evaluate heat transfer from the borehole wall to the ground. The model can account for multiple ground layers in the radial direction and phase change is handled using the effective capacity method. The results obtained from the model are in excellent agreement with the results given by analytical solutions for simple cases. A small-scale experimental set-up has also been built to validate the numerical model using temperature measurements. The apparatus mimics the behavior of a geothermal borehole and uses a homogeneous saturated laboratory-grade sand to reproduce unsaturated and saturated conditions. It is shown that the results of the numerical model are in good agreement with the experimental results.In the application section, the numerical ground model is combined with a borehole model to examine various scenarios involving typical heat pump operation. Results show that the borehole wall temperature remains around 0 °C for several days when the ground freezes while it would drop to much lower values in non-freezing conditions. Freezing is restricted to a few centimetres around the borehole. If solar energy is available, and a 4-pipe borehole with two independent circuits is used, then it is possible to melt the ice and recharge the ground for the next freezing cycle. Using this approach, borehole depth can be reduced by as much as 38% in small thermal conductivity grounds.

► We propose a new geothermal borehole configuration equipped with a saturated sand ring. ► Heat transfer in the ground and the ring is modeled using a numerical model. ► The numerical model is coupled to a recently developed analytical borehole model. ► We use the model to examine the effect of freezing the ring on the borehole depth. ► The results indicate that the borehole depth can be reduced by 38% in some cases.