The always-excellent Economist blogger Babbage posts about Facebook's new Prineville, Oregon, datacenter. Why did Facebook opt for a location where most of its electricity will be generated by coal? Google, for example, built on the Columbia River and has access to cheap, renewable hydropower.
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The answer, apparently, is that Facebook prefers the desert environment of Prineville because it allows ambient outside air to do most of the cooling without the need for energy-hungry air conditioning. The desert air is cool at night and in the winter, of course. But even in the summer, when the air is hot, it is very dry and can be cooled economically with evaporative coolers that spray water through the airstream. (Study up on the reason this works, the high latent heat of water, here.)
Facebook claims a power usage effectiveness of 1.07 at the new facility. Generally corporate datacenters achieve around 2, and Google claims a weighted average PUE of 1.16 for all its datacenters for the 12 months ending in March.
The Babbage post has some additional interesting info about Facebook's datacenter. There is also the Prineville Data Center's Facebook page, of course.
My guess is that their siting evaluation didn't include thinking very much about whether the electricity came from coal or not--either that or they just didn't care. They were willing to emit a lot more carbon to get some economic advantage available from the Prinevile location.
Facebook's next datacenter will be built in Rutherford County, western South Carolina, a location not noted for its desert conditions. (See Charlotte Observer article.) The electricity there will come from Duke Energy. Electricity is cheap there (Google and Apple are also in South Carolina), but much of it comes from coal. According to this Wikipedia article, half of Duke's Carolinas power comes from nuclear. The rest would be from coal and natural gas.