The geothermal potential of cities

What is the heat beneath our feet? There is a growing interest in the geothermal resources available at shallow depth beneath cities. However, there exists no general procedure for quantifying the low-temperature geothermal potential in urban ground and groundwater. This review categorizes previous work based on different definitions of the geothermal potential and compares the different assessment methods used. It is demonstrated that the theoretical potential of the available heat at a shallow depth is enormous, especially when not only the heat in place, but also compensating heat fluxes are considered. The technical potential describes the extractable heat by a specific technology. The methods to evaluate the extractable heat are manifold, including the use of technical performance standards, analytical and numerical simulation tools and mathematical regression procedures. These are different for groundwater well based open-loop systems and heat-exchanger-based closed loop systems, and the results depend on variable local factors, the density of systems applied and whether heat and/or cold is utilized. We contrast the published findings based on the power density and the relative contribution to the demand of a city. The broad span of the results highlights the need for a more consistent framework that distinguishes between the conceptual assumptions for calculating the technical geothermal potential and the local city-specific factors. This will be the basis for a reliable analysis of the economic geothermal potential of low-temperature geothermal applications on a local, district or city scale. This will also enhance the reliability and the trust in these technologies, and thus the public acceptance reflected in the acceptable geothermal potential.