HEAT RECYCLING IN THE SHALLOW SUBSURFACE - HEAT AS A RESOURCE

Space heating makes up a significant share of our residential energy use – 40% in the United States and 60% in EU households. Conventional methods for heating typically rely on direct or indirect burning of fossil fuels. Consequently, the decarbonization of our heating systems is a key challenge in the combat against climate change. This study highlights subsurface heat recycling as a suitable method: geothermal energy systems can be installed to extract our waste heat from the shallow (here < 60 m depth) subsurface and groundwater. To assess the widespread feasibility of this method, accumulated waste heat as well as annual heat input at more than 8,000 locations primarily in Europe, Australia, and North America are quantified.

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.