Satellites find sustainable energy in cities

 By Benjamin Plackett

New research from a German university shows that most cities are sitting on a decent amount of thermal energy, caused by the very existence of the city itself. In cities, temperatures are usually higher than their rural surroundings. This comes from population density, surface sealing, heat emitted by buildings, industry, and transport and a lack vegetation. This phenomenon is well known to affect the atmosphere of the city, but perhaps less known is that it also heats up the ground below city’s surface. While these temperature pockets have their disadvantages, they also have the potential for energy supply—and by virtue of their location close to the source of demand, the supply chain could be fairly simple…


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Cities are generally warmer than the countryside that surrounds them. The density of people, vehicles and buildings gives off warmth, which contributes to this urban heat island effect.

According the the Environmental Protection Agency in the United States, cities with more than one million residents are usually between 1.8–5.4°F (1–3°C) warmer, but the difference can reach 22°F (12°C) in the evening when the pattern is more pronounced.

Engineers at the Karlsruhe Institute of Technology in Germany wanted to know if cities create enough heat to warm the earth below their foundations—and if so, is the effect great enough to explore as a source of energy? “The idea just came to me,” says one of the researchers Philipp Blum, “Why not use satellites to help imagine the surface below?”

In a research paper published recently in the journal, Environmental Science & Technology,Blum and his colleagues outlined their findings, which indeed suggest urban areas warm the groundwater enough to consider geothermal energy as a localized source of power for those cities.

The researchers used infrared scanners on satellites that pass over Europe twice a day to get surface temperature readings, which they use to infer the temperature below ground. They gathered data on four cities; Berlin, Munich, Cologne, and Karlsruhe. “We estimate our measurements to have an error rate of 0.9 Calvin degrees,” says Blum, “We can consider this a success.”

After comparing the surface heat islands measured by satellite with readings from the subterranean groundwater, Blum found that the pattern is correlated and actually more marked below the surface than at street level; 95 percent of the areas Blum scanned have a higher groundwater temperature than surface temperatures. For this reason, Blum says that satellites are practical and cost effective way to get a preliminary estimation of a city’s underground energy potential.

Unsurprisingly, the larger metropolises like Munich and Berlin have warmer groundwaters than smaller cities like Karlsruhe. “The pattern starts with populations at about 200,000,” estimates Blum, “But the bigger the city the better.”

Blum says these subterranean heat pockets can provide a real source of energy and estimates that geothermal pumps designed to take advantage of the warm groundwaters could meet as much as 32 percent of a city’s household energy demands. Though he stresses that’s a best case scenario figure.

He imagines a system where new office buildings and houses are built with pumps 100 m (328 ft) below their foundations, powered by the warm waters. “It’s a very decentralized system,” he says, “The payback time for such an investment could be about 20 years, but that’s a conservative guess.”

He adds that larger commercial buildings would see a shorter payback period, perhaps as short as five years. Assuming oil prices will bounce back in the future, Blum says this system could become more financially competitive.

Regardless, he thinks it’s worth pursuing, “This is a sustainable source because this energy will always be there,” he says. ” When you think about it, cities will only get warmer in the future.”

about the author
Benjamin Plackett
I’m a journalist based in London. I report on all things science, tech, and health for a number of different publications. My work has been published by The Daily Dot, Inside Science and CNN among others. I earned my M.A. in Journalism at New York University and my B.Sci in Biology from Imperial College, London.