HEAT RECYCLING IN THE SHALLOW SUBSURFACE - HEAT AS A RESOURCE
Accumulated waste heat is defined as the difference in local groundwater temperatures and rural background groundwater temperatures. By multiplying the accumulated heat with the heat capacity of the soil at these locations the theoretical geothermal potential is determined. It ranges between 1 and 200 MJ per m2 and scales with population density – densely populated regions simply produce more waste heat. Comparing this potential to heating demand we gain insight into how long heating demand could be satisfied by simply recycling our waste heat. We find that it is higher in North America and Australia than in Europe. First results in the US also indicate that heat recycling is most feasible in areas that are currently most reliant on heat energy from fossil fuels.
Besides accumulated heat we also quantify the annual heat input through conductive heat transport from the surface and buildings into the underground. Previous studies of selected cities concluded ongoing heat input into (and hence ongoing warming of) the subsurface. In contrast, our large-scale analysis indicates that – disregarding additional local heat sources such as tunnels and basements - subsurface heat resources are currently in thermal equilibrium with adjacent heat sources in many locations, particularly where waste heat has already accumulated. However, we project that, once the waste heat has been recycled, the annual heat input has the potential to sustainably provide a significant percentage of annual heating demand.