Many green energy sources only generate power in a narrow range of conditions. Solar panels won't work when it's dark, for instance, and wind turbines are useless when everything is still. If Solar Wind Energy Tower has its way, though, we'll soon get clean electricity around the clock. It recently received permission to build a tower in San Luis, Arizona that produces power through hot air downdrafts; water injected at the top of the tower cools the desert winds, dragging them toward turbines at the bottom. Since it's almost always hot in the area, the plant should run all day and night for much of the year. An ideal summer day could have it churning out a healthy 1,250 megawatts per hour.