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The urban heat island effect and climate change have not only caused surface temperature increase in most urban areas, but during the last hundred years also enhanced the subsurface temperature by several degrees. This phenomenon yields aquifers with elevated temperature, which are attractive though underestimated thermal energy reservoirs. Detailed groundwater temperature measurements in Cologne (Germany) and Winnipeg (Canada) reveal high subsurface temperature distributions in the centers of both cities and indicate a warming trend of up to 5 °C. The case-specific potential heat content in urban aquifers and available capacities for space heating are quantified. The results show, for example, that, by decreasing the 20 m thick urban aquifer's temperature by 2 °C, the amount of extractable geothermal energy beneath Cologne is 2.5 times the residential heating demand of the whole city. The geothermal potential in other cities such as Shanghai and Tokyo is shown to supply heating demand even for decades.
Zhu et al. (2010) studied this question.