How Reliable Will Facebook’s Windy New Iowa Home Be?

If you were looking for the latest indication in the changing ways that large data centers are being designed and powered, yesterday’s announcement from Facebook that it will use 100 percent wind power at its Altoona, Iowa, facility was a pretty good one.

In a corporate blog post yesterday, the social networking giant said the facility will, when completed, draw from an Iowa electrical grid enhanced by a wind farm that it said will add as much as 138 megawatts of new power.

And that sounds good — in theory. But will it be reliable power? Clean and environment-friendly as it seems, wind isn’t as predictable as, say, a steady supply of coal. Some days, indeed some hours, are windier than others. So how to think about this variability factor?

I did a little checking. First off, looking at 52 years worth of average wind speed data through the year 2001 for nearby Des Moines shows that, on an annual basis, the new wind farm can expect winds at any given time of the year of about 11 miles per hour. The area is located in the zone that the U.S. Government’s National Renewable Energy Laboratory rates as “fair” for wind power generally, but that’s better than a large portion of the country where the average wind speed is much lower. (See map.)

Second, the trick to dealing with the simple variability in wind is to build lots of wind farms over a wide area. In practice, there’s an 80 percent chance that a group of turbines in a small area will see their output change up or down by 10 percent within an hour. But it gets less severe when you scale out to five hours: There’s a 40 percent chance that you’ll see a shift of plus or minus 10 percent over five hours.

In order to deal with that variability, you need to distribute your turbines widely. Do that and those wild variations begin to smooth out and don’t swing quite so much. If one turbine is sitting still, another a few miles away may be spinning like a top.

And it turns out that Iowa is one of the better markets in the country to use wind-generated power. The state has a goal of building up 10,000 megawatts of capacity by the year 2020, and will probably exceed that by 2017. According to the Iowa Wind Energy Association, a trade group, the state already has 3,198 turbines in operation at 100 different sites.

And as of 2011, Iowa already led the U.S. in terms of the amount of wind power generated per kilometer: Nearly 30 megawatts, much higher than Illinois (home to the “Windy City” of Chicago), which generated less than 20. Clearly, Iowa has figured this wind power thing out better than most states, which probably has a lot to do with what attracted Facebook to the area in the first place. Microsoft and Google are already fans of the Hawkeye state themselves.