You can't see it or hold it, but scientists estimate that wind has the potential to supply the world's electricity five times over while producing almost no greenhouse gas emissions. Yet the U.S. currently gets less than two percent of our electricity from wind power. Wind energy's share is set to increase with the approval of the Cape Wind project off the Massachusetts coastline, and the other offshore projects that may follow in its wake.

Large commercial wind farms are a relatively new idea. The wind turbine is a young technology with high costs, but those costs are falling as more turbines are built. In the US, the majority of wind farms located in the "wind belt" that stretches from Texas to Montana and North Dakota.

But the location of the wind belt brings another challenge. It is located far from either coast. To harness and utilize wind energy, new transmission lines would have to be built to connect the wind belt to the more populated areas on the East and West coasts. And wind doesn't blow all the time, which means either a back-up source of energy will be needed -- or a way to store wind power.

So while the costs of wind turbines are dropping, the problems of wind intermittency, remote locations, and an insufficient energy grid will need to be resolved before this clean and renewable energy source can become truly competitive.

Photo Credits: National Digital Library/Joshua Winchell