Cities and companies around the world tap into geothermal energy — the thermal energy below the Earth’s surface — but there’s another type of man-made energy coursing through sewer pipes beneath cities and towns: sewage. The town of Brainerd, Minnesota plans to develop a unique alternative energy system that will tap into the  thermal energy in sewer pipes to heat and cool buildings.

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When people use hot water to wash clothes, take a shower or run the dishwasher, all of that heat energy ends up in sewers, which get quite hot. In 2010, the Minnesota-based company Hidden Fuels installed sensors in the city’s sewers so that they could measure water temperatures and create a thermal energy map. “It shows that there’s a significant amount of energy — literally enough to heat hundreds of homes — within the streets of the city of Brainerd,” Hidden Fuels’ Peter Nelson told NPR.

At one location, Hidden Fuels found that there was enough thermal energy to heat 229 homes. Brainerd isn’t heating homes with that energy just yet, but the town is getting closer. Next, the city is planning to heat the Brainerd police station and the local high school using energy drawn from the sewer, which will save the city an estimated $18,000 per year for the high school alone. The bigger challenge will be figuring out how to connect the entire town to a system that harvests energy from the sewers.

+ Hidden Fuels