If a clean energy project in the Arizona desert goes forward, the second tallest structure on Earth will be a 2,600-foot solar updraft tower, which could last 80 years and generate 200 MW of electricity each day — using only hot air. (Insert your own joke about how we could power Cleveland with Bill O’Reilly.)

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The tower works on the principle that hot air rises. In this case, it rises through the tower, turning turbines as it goes. The tower uses no water, and it works pretty much all the time, unlike wind and solar projects. (At night, the ground is still letting off the heat it captured during the day, so there's still hot air available to float upward.)

The tower is so tall because the stronger air flow in taller towers creates more energy. This one will be made of concrete and pay back its carbon debt in 2.5 years, says EnviroMission, the company that's building it. It will also cost $750 million to build, but as CNN reports, "because air is free, operating costs will be minimal," according to the company.

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