Coupled hydrothermal modelling of subsurface heating in the vicinity of high-power power cables under natural conditions

In Germany, electricity from the wind power plants in the north must be transported to the south. This is done via underground cables that act as a heat source. We numerically simulated the coupled heating and drying by the cables in the bedding and the surrounding soil under natural boundary conditions. For this purpose, we used a modified version of the Hydrus-2D/3D software code, which enables the coupled modelling of water, vapour and heat flow. The water transport is described with the Richards equation. For heat transport, the processes of heat conduction, convective heat transport and the transport of latent heat are considered. The simulations were carried out in a 2D depth profile oriented orthogonally to the main direction of the cables. The simulations were carried out under atmospheric boundary conditions for the period 2016-2018, the latter being one of the driest years in Germany in recent history.  

We found that warming leads to thermal vapour fluxes that are directed away from the cable, which can significantly reduce the water content of the material near the cable. The lowering of the soil water content reduces the thermal conductivity of the soil and can therefore lead to overheating of the cable with the risk of technical failure. However, the simulations indicate that even under the conditions of 2018, overheating of the cables is unlikely if the bedding material has sufficient thermal conductivity and the spacing between the individual cables is chosen wisely. Crucial for adequate modelling of the water and heat flow was the correct representation of the water retention curve in the dry soil, as the water head in the soil near the cable reaches values down to -10⁵ J kg^-1 and soil thermal conductivity changes rapidly at low water contents.