Elevated temperatures beneath cities: An enhanced geothermal resource

Abstract

Anthropogenic alteration in the urban environment causes an increase in atmospheric temperatures, which is known as the urban heat island (UHI) effect. However, this is not only an atmospheric phenolmenon. We also find significant warming of the urban subsurface and shallow groundwater bodies. Besides potential negative effects on groundwater quality, thermal anomalies in aquifers also represent attractive shallow geothermal energy reservoirs for space heating. In this study, we inspect aquifer temperatures in several German cities, such as Berlin, Munich, Cologne and Karlsruhe. A significant increase in groundwater temperature by more than 4 K was detected close to the city centre of all studied urban areas. Even locally, hot spots of up to +20 K can be found in the vicinity of insufficiently insulated power plants or reinjection sites of cooling water. This yields a highly variable spatial and temporal pattern of increased ground temperatures. Furthermore, the geothermal potential of the subsurface was calculated for the studied cities. In the city of Cologne, the calculation of the potential heat content in the 20 m thick aquifer shows that by decreasing the aquifer’s temperature by 2 K, the extractable geothermal energy could supply the space heating demand of the whole city for at least 2.5 years.