What brown can do for you: The next big green energy idea

Local officials in Denver and across the US believe the next clean-energy breakthrough could come from your latest trip to the porcelain throne.

In the Mile-High City, a growing number of sewage pipes are heating and cooling buildings with hidden energy found in wastewater.

How it works: Sewage represents a largely untapped source of energy due to its stable temperature of ~70°F. Denver’s new systems are designed to extract that energy using a heat exchanger, a device which takes heat from one source (wastewater) and puts it into another (clean water).

• This energized clean water is sent into a pump that heats or cools the rooms in a building based on temperature.
• Meanwhile, the wastewater flows back into the sewer system once the thermal energy has been extracted.
• The dirty and clean water loops never come into contact with one another, meaning no odors are transferred—only heat.

The potential impact: America flushes away the energy equivalent of ~350 billion kilowatt-hours of hot water every year, or enough to power ~32 million US homes, according to 2005 data from the Department of Energy.

Proponents of wastewater heat recovery say it represents a cheaper and more reliable source of energy compared to wind or solar, and can be incorporated into cities with little or no new infrastructure.

Big picture: In addition to Denver, systems to extract wastewater heat have already been installed across California, Washington, New York, and Canada.