By Don Willlmott
Don Willmott is a New York-based journalist who writes about technology, travel, and the environment for a wide variety of publications and websites.
Wind turbines are a great way to generate electricity...until the wind stops blowing. Maybe what we really need is a way to manufacture wind to keep those turbines spinning all the time.
That's the idea that Maryland-based Solar Wind Energy is pitching to the alternative energy crowd, while armchair physicists and perpetual motion machine truthers speculate on whether a weather machine built on an epic scale could ever make sense.
The design of the company's Downdraft Tower is really quite simple. First you build an enormous silo more than 2,200 feet high and 1,200 feet across in a hot, dry place. (It would be around 500 feet shy of Dubai's Burj Khalifa skyscraper, the world's tallest tower.) Then you pump water to the top, and spray a fine mist inside. That mist cools the rising hot air, making it heavier and causing it to plummet down through the tower, creating a 50 mph wind that blasts through turbines positioned all around the base. The tower would essentially be creating the same kind of microbursts that flatten trees during severe thunderstorms, but it would be generating electricity instead of chewing up the landscape.
Would you like one in your town? The mayor of hot and sunny San Luis, Arizona would, so that's where Solar Wind Energy is collecting permits in the hope of turning its privately-funded, $1.5 billion dollar dream into a reality by 2018, on the site of a former citrus grove.
How much electricity are we talking about? While the company claims that the ultimate tower working optimally could generate a whopping 2,500 megawatt-hours per hour, with one third of the power diverted to run the water pumps, a more realistic annual average might be around 435 megawatt-hours per hour of sellable juice. Executives from Solar Wind Energy say that the output would be about the same as that of Hoover Dam, although skeptics have come to some less optimistic conclusions. One point of contention: wind speed. Would the falling air meet resistance inside the tower that would slow it down, delivering insufficient velocity for the turbines to work well?
The company states that similarly powerful solar or wind farms would require from 30 to 300 times as much land, cost four times as much to build, and would have half the usable life of a tower. Even better, it says, a tower can operate 24 hours a day, seven days a week, although it's unclear how much oomph it would lose during the cooler nighttime hours.
Also unclear is exactly how much water would be needed, where it would come from (perhaps desalinated water from the Sea of Cortez, about 40 miles away), how much power it would take to pump it almost half a mile straight up, and how much of it could be re-circulated (the company claims 75 percent). On this point, the company says only that the citrus grove the tower replaces used twice as much water. Critics also wonder if a simpler solar chimney design, in which hot air rises to drive turbines at the top of a tower, wouldn't be more feasible, if only because water wouldn't play a role.
So the Downdraft Tower is colossal, it's clever, it's clean, and it's carbon-neutral. But will it be looming on the Arizona horizon in just three years? That certainly seems like a tall order.
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