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STEVE INSKEEP, HOST:

A city in Minnesota is tapping into an unusual energy source. Sewers apparently have huge potential for heating and cooling buildings. Who knew? Minnesota Public Radio's Conrad Wilson did.

CONRAD WILSON, BYLINE: Everything from the water in the dishwasher you ran last night to the hot shower you took this morning, or even the...

(SOUNDBITE OF TOILET FLUSHING)

WILSON: ...organic material you flushed down the toilet. For the most part it all ends up in the sewer, which not surprisingly can be a pretty warm place. Heat can generate energy and it's that energy some in the central Minnesota city of Brainerd are trying to capture.

Scott Sjolund heads up technology for Brainerd Public Utilities. He's standing on the corner of 6th Avenue and College Drive in Brainerd, where sewage is rushing through underground pipes.

SCOTT SJOLUND: Everybody heats water up. All that gets drained down to the sewer, and that's potential energy that could be extracted. That's part of the equation.

WILSON: What's the other part of the equation?

SJOLUND: Actually extracting it in an economical fashion.

WILSON: The idea for this project comes from Hidden Fuels. Hidden Fuels' Peter Nelson says the first phase of the project involved installing sensors in the city's sewers to create a thermal energy map. For more than a year they measured the temperature and the amount of sewage running through them.

PETER NELSON: It shows that there is a significant amount of energy - literally enough to heat hundreds of homes - within the streets of the city of Brainerd.

WILSON: That data helped the team match energy sources with buildings where they could install new heating and cooling systems. Nelson says now that they know how much energy is there, the next step is converting it into usable energy, something that's challenging, but doable.

Hidden Fuels will rely on technology already in use. It's similar to the way geo-thermal heating and cooling systems work. By circulating water through a heat-pump, energy can be extracted to either heat or cool a building.

NELSON: We're not dealing with clean fluids. We're dealing with contaminated fluids. And so that's really the challenge, is to be able to operate efficiently in that contaminated environment.

WILSON: Experts in renewable energy circles say they're impressed by the potential of this idea. John Lund is a professor of civil engineering at Oregon Institute of Technology and says going into the sewer for energy could cut the cost of installing geothermal heating and cooling systems in half.

JOHN LUND: Using sewer water, I think it's a natural. You're just digging in and tapping into the sewer line and just pulling the heat or rejecting it.

WILSON: In Brainerd, two public buildings may be the first to get energy from the sewer. In addition to the police station, the Brainerd School District is interested. Earl Wolleat is the director for buildings and grounds with the district and says there's enough energy running in one of the sewer pipes to heat the entire high school. That could save tens of thousands of dollars every winter.

EARL WOLLEAT: If you stop and think about it, it's not surprising that the energy is there. But I think it all comes down to the practical application, to find that way to get that heat out of the sewer.

WILSON: Wolleat is confident that the technology will work. But with natural gas prices at historic low prices, it could take years before the district amortizes the costs of switching over. Meanwhile though, Hidden Fuels hopes to have the Brainerd police station fully heated by sewer power by the end of this year.

For NPR News, I'm Conrad Wilson.

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