Denver has a plan to heat and cool buildings without fossil fuels. It involves … sewage?

Denver plans to heat and cool a cluster of downtown buildings with a system using water, geothermal energy and even heat from ... sewage.

Like many cities, Denver's largest source of greenhouse gas emissions is its buildings. Heating and cooling skyscrapers requires a lot of fossil fuels. Now, the city is trying a surprising solution.

(Image credit: Aaron Ontiveroz)