Are we flushing a heat source down the drain?

(Photo by Evan Long via Creative Commons)

A northern Minnesota start-up company wants to recover a waste heat source that’s currently flushed down the drain.

Hidden Fuels of Brainerd, Minnesota, is trying to develop a municipal sewer heat recovery system, which would heat buildings using warmth from the city’s sewers — minus the stench.

Jeff Aga, one of the company’s principals, said it would work similar to geothermal, but that sewers have an advantage of warmer starting temperatures than ground wells.

“We did some tests throughout their system and found where there’s good heat that can be captured,” Aga said.

The company placed temperature sensors throughout the city’s sewer system as part of a 16-month, federal stimulus-funded feasibility study that was completed in January. They found temperatures ranged from 38 to 78 degrees, but were mostly between 45 and 60 degrees. The warmest temperatures were found near a commercial laundry facility.

Aga said they’ve identified the police department as a good potential customer for the system, which might also be able to heat the local high school and a nearby apartment high rise.

“The fact that no one else has really thought about tapping into that until we did this study, I thought, was kind of fascinating,” says Scott Sjolund, technology supervisor at Brainerd Public Utilities, which sponsored the study after the city was approached by Hidden Fuels.

The company believes the system would be the first of its kind in the United States, though Vancouver built something like it as part of its 2010 Olympics village. The New York Times reported that the Vancouver project was the first district energy system in North America to draw heat from untreated wastewater, and that three others existed in Oslo and Tokyo.

The fact that so few systems exist is probably a sign of their challenging economics, said John Whitehouse, vice president of business development for Recycled Energy Development, an Illinois company that designs, builds, owns and operates cogeneration and waste heat recovery projects around the country.

“Unless you have really high energy prices, it’s going to be a hard sell,” Whitehouse said of the concept in general.

Low natural gas prices mean the payback time is likely to be long, he said. That’s especially true for projects that will involve retrofitting buildings rather than incorporating systems into new construction.

And while there is heat in sewers that’s technically recoverable, “there’s just not that much there,” Whitehouse said. His company normally looks for opportunities where the temperature is at least 180 degrees.

Hidden Fuels has presented the results of its feasibility study at public meetings in Brainerd. Next, it hopes to find funding to build a system at the police station.

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