Benefits from Subsurface Urban Heat Islands to Shallow Geothermal Applications – an Example from the City of Cologne, Germany

Abstract

In urbanized areas subsurface temperatures are often elevated in comparison to surrounding rural areas. These anthropogenic distortions of the thermal field are usually refered to as subsurface urban heat islands (SUHI) that typically show a positive offset of several Kelvin. Especially shallow geothermal heating systems benefit from this nonuniform but often large scale temperature rise. Therefore, SUHIs have an elevated geothermal potential. Accessing this potential yields an eco-friendly and cost-efficent option to meet the enormous local heating demand in cities. Recycling the long-term anthropogenic thermal discharge by geothermal use also reduces the thermal disturbance in the subsurface and brings subsurface conditions a step closer to initial natural conditions. Besides the ecological compatability, shallow geothermal applications have to be competitively viable in economic terms. Currently, there exists no universal approach for quantifying the potential of low-temperature geothermal systems, which hinders comparison to alternative technologies of urban heat supply. The shallow geothermal potential can roughly be classified in theoretical, technical, acceptable and economic potential. As the economic suitability varies with the heat-in-place, compensating heat fluxes, local demand, regulatory requirements and hydrogeological settings, the geothermal potential of an urban area is highly site-specific. In the present study, the subsurface of Cologne is introduced as a well studied regime that serves as a reference city to apply and assess different low-temperature geothermal potential classfications.