Geothermal heat islands

In urban environments, subsurface heat builds up during hot summers. Heat accumulates more strongly in urban undergrounds than in rural areas. A study by ETH researchers demonstrates that there is great potential for energy generation from underground urban heat islands.

At a depth of 20 metres, the subsurface temperature under Zurich is several degrees higher than above ground. In an article published in the scientific journal “Renewable Energy”, the research team directed by Peter Bayer and Jaime Rivera reports that these subterranean heat islands harbour an as yet untapped potential source of energy production.

According to model calculations carried out by the researchers, the potential for utilisation of geothermal heat in urban areas is 40 per cent greater than in rural areas thanks to this “island effect”. On Monday, the ETH researchers announced that this potential could be tapped with systems already used for the collection of geothermal heat.

More geothermal energy

In urban areas, heat accumulates on the earth’s surface and dissipates into the ground through streets, cellars, or sewer tunnels. This supplements the heat that already exists naturally in underground heat reservoirs. In Zurich, the underground temperature at a depth of 20 metres is several degrees higher than above ground. This surplus indicates that more energy could be extracted from the ground, and that boreholes for geothermal probes do not have to be as deep, reports the ETH.

„Each additional degree Celsius at the urban ground surface means the borehole required to collect the same amount of energy can be four metres shorter,“ explains Rivera. Normally drilling goes 150 metres deep for geothermal heating systems and a heat exchanger is installed in the borehole. A fluid circulating in the heat exchanger extracts heat from the ground and transports it to the surface.

The findings from model calculations could help in planning geothermal energy systems in areas with elevated soil temperatures, says study director Peter Bayer. However, other heat sources, such as tunnels, sewers and district heating systems also contribute to soil warming in cities. This is a concern for the researchers. „… since all these heat sources are artificial, strictly speaking we’re not exploiting a naturally renewable energy source.“