A secret cache of clean ener­gy is lurk­ing in sew­ers, and there are grow­ing efforts to put it to work in the bat­tle against cli­mate change.

The U.S. Depart­ment of Ener­gy esti­mates Amer­i­cans wash enough ener­gy down the drain every year to pow­er about 30 mil­lion homes. The sources are often every­day items inside homes. Think hot show­ers, wash­ing machines and sinks. Evolv­ing tech­nol­o­gy is mak­ing it eas­i­er to har­ness that most­ly warm water.

Den­ver is now con­struct­ing what is like­ly the largest sew­er heat-recov­ery project in North Amer­i­ca, accord­ing to Enwave, a Cana­di­an ener­gy com­pa­ny set to oper­ate the system.

Over the next few years, a $1 bil­lion remod­el will turn the 250-acre site, home to the Nation­al West­ern Stock Show and Rodeo, into a hub for art, edu­ca­tion and agri­cul­ture. The revamped Nation­al West­ern Cen­ter will include about a mil­lion square feet of new indoor space, all of which will be heat­ed and cooled with ener­gy from the sew­er pipes below.

Brad Buchanan, the CEO lead­ing the rede­vel­op­ment, said the project has already changed how he thinks about the best loca­tion for real estate. Big pieces of sew­er infra­struc­ture have long repelled devel­op­ment. Now he imag­ines they might be sought out as a way to save ener­gy costs and avoid green­house gas emissions.

The Nation­al West­ern Cen­ter esti­mates the project will help it annu­al­ly avoid the car­bon equiv­a­lent of dri­ving an aver­age gas-pow­ered car around the equa­tor 250 times.

It’ll be inter­est­ing to see if folks start to look at not just where light rail lines or good schools are locat­ed, but what’s the prox­im­i­ty to a large san­i­tary sew­er line,” Buchanan said.

The tech­nol­o­gy to har­vest sew­er heat isn’t complicated.

At the Nation­al West­ern Cen­ter, con­struc­tion crews have already com­plet­ed a pit expos­ing the main sew­er line. The waste­water inside stays a mild 55 to 75 degrees year-round, local offi­cials say, no mat­ter the weath­er out­side. That con­sis­tent tem­per­a­ture can be tapped to heat and cool above­ground buildings.

The key is a mas­sive heat pump, which will be housed in a cen­tral plant on the cam­pus. The device works like a reversible air con­di­tion­ing unit. In the win­ter, it will trans­fer ener­gy from the sewage into a clean-water loop con­nect­ing the build­ings, adding heat to indoor spaces. The process can then be flipped to keep things cool in the summer.

Shan­ti Pless, a research engi­neer with the Nation­al Renew­able Ener­gy Lab­o­ra­to­ry in Gold­en, Colo., said the tech­nol­o­gy opens up a vast world of “renew­able heat mining.”