Facebook passed another milestone in the green data center arms race today with the announcement that its Altoona, Iowa data center will be 100 percent powered by wind power when it goes online in 2015.

This will Facebook's second data center – after its Lulea, Sweden location – to run on all renewable power.

The electricity for the new data center will come from a nearby wind project in Wellsburg, Iowa, according to a blog post from Facebook. Both the wind project, which will be owned and operated by MidAmerican Energy, and the data center are currently under construction.

Green data centers have come a long way since environmental advocacy organization Greenpeace began railing against Facebook in 2010. Following criticism of their energy use patterns, companies like Facebook and Apple vowed to clean-up their acts. Since then, Greenpeace has upgraded its ratings of Apple's environmental practices and praised Facebook's energy use transparency.

But that's not to say that these data centers are actually environmentally friendly as of yet. Reaching 100 percent renewable energy is tough to pull off. Apple claims that its data centers are powered by 100 percent renewable sources, but it's using renewable energy credits to "offset" its use of coal and nuclear power. Google is working with regulators to find ways to buy more clean energy directly. Facebook has settled on a goal of powering its data centers on 25 percent renewable energy by 2015. eBay and Microsoft, meanwhile, have unveiled plans to use natural gas-based fuel cells to power new data centers. Natural gas burns far cleaner than coal, but still emits some carbon.

But one of the biggest impacts of these sorts of projects is a boost in overall availability of renewable energy, says Greenpeace IT analyst Gary Cook. "When Facebook said back in spring that they were going to Iowa, the utility company in Iowa, MidAmerican Energy, announced that they were shelving plans to build a new nuclear facility and then filed plans to build a wind plant instead," Cook told us earlier this week. "If you look at the regulatory filing, this was because they have new customers, namely Facebook, that want more renewable energy."

That fact is not lost no Facebook. "When we settled on Altoona as the location for our fourth data center, one of the deciding factors was the opportunity to help develop a new wind project in the state," the Facebook blog says.

"The project will add up to 138 MW of new renewable wind capacity to the grid in Iowa – more than what our data center is likely to require for the foreseeable future."