“In most cities, with a variety of populations and climates, the large amount of geothermal energy stored in the urban local subsurface is capable of fulfilling the annual space-heating demand for years and potentially decades,” Ke Zhu of the University of Tübingen, Germany told environmentalresearchweb.

Together with colleagues from Karlsruhe Institute of Technology, Germany, St Francis Xavier University, Canada, and ETH Zürich, Switzerland, Zhu measured groundwater temperatures in Cologne, Germany, and Winnipeg, Canada. Both cities had extensive underground warming, with temperatures 3–5°C higher than surrounding rural areas. Similarly, the subsurface beneath urban green spaces was cooler than that below business districts.

The urban heat-island effect arises because of factors such as buildings preventing heat from leaving the ground at night, changes in the properties of the ground surface and the absence of plants that provide cooling by evapotranspiration.

“Urban aquifers with elevated temperature are attractive shallow geothermal-energy reservoirs, and meanwhile there is high energy demand just above,” said Zhu. “In our opinion, it is important to study the geothermal potential of urban heat islands before planning large geothermal projects.”

As well as carrying out fieldwork in Cologne and Winnipeg, the researchers estimated the geothermal potential of the groundwater beneath London, Tokyo, Shanghai, Istanbul and Prague.

Lowering the temperature of the 20-m thick aquifer beneath Cologne by 2°C would provide enough energy to cover residential heating needs in the city for 2 to 25 years, according to the team’s calculations. Cologne has around one million inhabitants and an average annual air temperature of 11°C.

In Shanghai and Tokyo, meanwhile, geothermal potential was enough to supply homes with heat for decades. And, because the energy of the subsurface is continually replenished, the researchers believe that the energy source could even last hundreds of years.

“The results show us that there is high geothermal-energy potential beneath urban areas,” said Zhu. “We should promote shallow geothermal-energy use in cities because extracting this large amount of thermal energy will not only fulfil part of the energy demand in urban areas, but also play a positive role in reducing greenhouse gas emissions.”

Now, the researchers are modelling heat transport beneath Cologne in detail to study the dynamics of energy fluxes in urban heat islands. “Our further study will focus on how to sustainably and efficiently extract the geothermal energy in urban aquifers with minimal detrimental environmental impact,” said Zhu.

The researchers reported their work in Environmental Research Letters.