Global climate change causes air temperatures to rise continuously, particularly in cities, where sealed surfaces, high building densities, and scarce natural vegetation increase the impact. The air temperature difference between urban and unaffected rural areas is known as the urban heat island (UHI) effect. The UHI effect is a non-exclusive phenomenon in the atmosphere. The high air temperatures and warmed surfaces combined with underground anthropogenic heat sources lead to subsurface UHIs. Anthropogenic heat sources are building basements, district heating networks, sewage systems, and car as well as tram tunnels. These sources can raise urban groundwater temperatures by 3 K to 7 K above those in rural areas. Can this huge, untapped energy potential be harnessed to supply heating demand in cities?

Employing shallow geothermal systems, underground and groundwater heat is extracted and utilized for heating systems and domestic hot water. Geothermal systems can be run sustainably by taking the subsurface UHI into account. The annual average anthropogenic heat flux into the ground can be calculated by means of a spatially resolved heat transport model and a Monte Carlo simulation (Benz et al., 2015). Merging the anthropogenic heat flux and the energy stored underground result in the total geothermal potential. This potential is compared with the energy demand for heating system as well as domestic hot water. Based on the result, we can model the number of open and/or closed geothermal systems which have to be installed to meet the energy demand. This procedure is illustrated in figure 1 and can be applied to a city block, a neighbourhood or even a whole city.

Figure 1: Estimation of the geothermal potential, anthropogenic heat flux, energy demand and number of installed geothermal systems.

Shallow geothermal systems are even more efficient if they are used for both heating and cooling. The heat extracted from buildings in summer which is induced into the ground can be used for heating during winter.
To sum up, heating and cooling demand in cities can be met sustainably for several decades by including the UHI effect into the geothermal potential and applying shallow geothermal energy systems.