A secret cache of renewable energy lurks in the sewers. The key question is how to bring it to the surface and put it to work in the fight against climate change.
There is no mystery about how excess energy ends up under cities. Showers, hot water, and sinks all add hot water to the sewers. In fact, the US Department of Energy estimates Americans wash 350 billion kilowatt-hours of energy in sewers each year. To a certain perspective, that’s enough power for about 30 million American homes.
The largest sewer heat recovery project in North America is currently under construction in Denver.
Over the next few years, a billion dollar renovation will transform the National Western Complex, which is home to the National Western Stock Show and Rodeo, into a center of art, education and agriculture.
The project will add approximately one million square feet of new interior space. All of this will be heated and cooled with a heavy dose of heat from the sewer pipes under the 250-acre campus.
Brad Buchanan, CEO of the National Western Center, said the project had already readjusted his thinking about the best location for real estate. Large chunks of sewerage infrastructure have long held back development. In the future, he imagines that manufacturers could research them to reduce energy costs and avoid greenhouse gas emissions.
âIt will be interesting to see if people start looking not only where the streetcar lines or the good schools are, but how close to a major sanitary sewer line is,â Buchanan said.
How to harness the heat from the sewers
The technology for harvesting heat from sewers is not complicated.
At the National Western Center, construction crews have already completed a pit exposing the main sewer line. According to the Denver Metro Wastewater Reclamation District, indoor wastewater stays soft at 55 to 75 degrees year-round, regardless of the weather outside. The constant temperature can then be used to heat and cool buildings above ground.
Officials at the National Western Center estimate the system will help the campus avoid about 2,600 tonnes of carbon per year, roughly the same as driving an average gasoline car around the equator 250 times.
The key to the trick is a massive heat pump, which will be housed in a central factory on campus. The device works as a reversible air conditioning unit. In winter, it will transfer energy from wastewater into a clean water loop connecting buildings, adding heat to interior spaces. The process can then be reversed to keep things cool in the summer.
Since sewage is never exposed, future occupants will not be affected by a wave of sewer stench.
Shanti Pless, a research engineer at the National Renewable Energy Laboratory in Golden, said the technology opens up a vast world of “renewable heat extraction.”
âWith the advent of large heat pumps, we can cost-effectively use, say, 70-degree wastewater to heat our buildings and our hot water systems,â Pless said.
Rethinking the way we heat and cool buildings
Pless said the biggest hurdle isn’t the technology – it’s helping developers rethink the size of their heating and cooling systems. Sewer heat recovery often works best as the heart of a neighborhood-sized energy system, where a central plant supplies power to an entire neighborhood or office complex.
Denver itself shows how this kind of centralized strategy has fallen out of favor in the United States.
A 141-year-old steam heating system is heating up many buildings in the city center, supplying power from a gas-fired power plant operated by Xcel Energy. As the costs of improving the system shifted to customers, many building operators have disconnected from steam service in favor of smaller, independent heating and cooling systems.