The outside air cooling system at Facebook's Prineville, Oregon data center that the North Carolina system was based on.
Facebook published a blog post today over at its Open Compute Project discussing the success of the outdoor air cooling system at its Forest City, North Carolina data center. The company had utilized the same system in place of mechanical chillers at its Prineville, Oregon data center, but the climate there is much more mild.
Here's what the blog post has to say about the different approach required in North Carolina:
When we started looking at Forest City, the bin weather data suggested that refrigeration might not be required, but ASHRAE 50-year design weather maximums suggested otherwise. We ultimately decided to install a direct expansion (DX) coil system in the facility, just in case it might be needed, but it was important to us to find a way to make the free cooling system work — the potential efficiency gains to be found in keeping those DX units switched off were just too great to ignore.
To try to make the free cooling system work in Forest City, we expanded the maximum allowable temperature and humidity conditions for our server environment. Because wet bulb temperatures are higher in western North Carolina than they are in central Oregon, we set the upper end of the server inlet temperature range at 85°F, instead of at 80°F. And because of the higher humidity in North Carolina, we expanded the relative humidity (RH) maximum from 65% RH to 90% RH.
And guess what? Despite July being the second hottest month on record in North Carolina and the past summer being the third hottest in U.S. history, Facebook didn't have to run the DX coils at all. Luckily, on the days that saw record heat, the humidity was relatively low so a misting system was able to provide necessary cooling and when relative humidity was high, the dry bulb temperature tended to be lower so the outside air didn't need to be cooled before being sent into the data hall.
So, here's the kicker. Not only did the outside cooling system work just as well in the hotter and more humid environment in North Carolina, but that data center actually achieved a lower PUE this summer than the Prineville, Oregon data center. Forest City was able to hit 1.07 while Prineville hit 1.09. Both of those are very efficient PUE scores, or Power Usage Effectiveness -- a measurement of how much energy going into the data center is used on IT machinery instead of other power loads like heating, cooling and other facility management. The lower the PUE number, the more energy efficient the data center, with 1.0 being a perfect PUE.
You can see a couple of the temperature and humidity charts Facebook compiled from this past summer below.