[caption id="attachment_3420" align="aligncenter" width="600"] How Facebook cools its Prineville, Ore. data center. (Source: Facebook)[/caption] As part of a commitment to openness within its data centers, Facebook has published its first “WUE” metric, which measures the effectiveness of using water to cool its data centers. Typically, data center operators focus on PUE, or Power Usage Effectiveness. Facebook's first Water Usage Effectiveness (WUE) number acts as a both a benchmark for itself and other members of the Open Compute Project, designed to lower data center costs through an open approach. Facebook’s first WUE number? 0.22 L/kWh, measured within the first building in its Prineville, Ore. data center (Prineville 1), which the company called a “great result." WUE measures water used for cooling a data center only, not the additional water used for plumbing or the company’s offices. By definition, WUE is an annualized calculation; however, Facebook said it will report results on a quarterly basis, and the numbers will eventually become a 12-month trailing metric. Facebook’s first WUE number reflects a year’s worth of data. As per its commitment to openness, Facebook has put the specifications of its water-cooling system online, with a related blog post describing the Prineville WUE number. In a nutshell, Facebook summarizes the Prineville system as “a built-up penthouse that utilizes 100% outside air economization with a direct evaporative cooling and humidification (ECH) misting system”. Facebook has been one of the more outspoken champions of using natural cooling methods to cut energy costs. A short time ago, Facebook outlined its progress toward sourcing 25 percent of its energy use from renewable sources by 2015. Its facility in Lulea, Sweden, will be cooled via the frigid air surrounding the facility. “It’s like using a window-mounted air conditioner to cool a room instead of putting a fan in a window when the outside temperatures are cooler than the temperature in the room,” Facebook’s Daniel Lee described the process. The Prineville facility does not use either a chilled water plant or cooling towers, instead blowing naturally cool outside air over the racks of servers, where it is either exhausted outside or recirculated. As the cool air is pulled from outside, a direct ECH misting system lowers the temperature still further via evaporative cooling. The water used for cooling is filtered and purified, to avoid clogging the array of microscopic nozzles with just 150-micron orifices. In all, Facebook estimated that just 25 percent of the water is “blown down” into storage tanks. Even then, the blown-down water is recaptured, filtered again, and treated with an ultraviolet (UV) lamp to purify it further. Hopefully, other data-center operators will be encouraged to report WUE numbers as well, although they’ll certainly vary depending upon the types of cooling used. Facebook also said that it expects to report WUE numbers for its second Prineville facility next year, as well as its Forest City N.C. data center as soon as enough data is collected.