Of course, not everyone can or wants to build or operate a Facebook/Open Compute style data center, which is hyper-focused on ultra-low PUE numbers, but does not include a high level of facility-based redundancy. In point of fact, these are effectively only Tier-1 data centers, yet they operate and deliver reasonably robust and reliable services, primarily because of their multi-site redundancy and their IT architecture’s inherent ability to failover to an alternate site relatively seamlessly, just as internet firms such as Google and Yahoo are able to do.
Facebook’s release of the WUE numbers for its Prineville, OR, data center could shake up the industry far more than breakthrough 1.0x PUE claims.
Water usage may become the next inconvenient truth for the data center industry. Drinking water is mostly taken for granted in the U.S. However globally, the shortages of potable water have the long-term potential to make it as valuable as oil, hence my choice of the title of this column, which is quoted from “The Rhyme of the Ancient Mariner” (with apologies).
Facebook’s “presumably low” WUE numbers (0.22 liters per kWh annualized), are primarily because of its use of fresh-air cooling in conjunction with its directmisting adiabatic system (Of course, the energy-saving benefit of “fresh-air” cooling is an important but different and highly debated issue currently). A conventional data center, which typically has a water-cooled chiller system (with the required evaporate cooling tower), may use significantly more water. Of course since no other data center has offered to disclose actual water usage, we can only presume Facebook’s WUE to be low.
To put all this into perspective, data centers are far from the only large users of water. There are many industrial processes that use large quantities of water for cooling, including in most cases the production of power at the utility itself (“source” energy water usage). According to The Green Grid WUE white paper, the national average is 1.8 liters of water per kilowatt-hour. although the rate varies from 1.4 to 1.9 for different geographic locations and 0.8 to 3.3 for different power generation system types (figures were based on the U.S. Department of Energy 2006 report to Congress). So even though we are only now beginning to look at WUE “site” water usage, “source” water consumption (as well as carbon), by power utilities is a global and long standing issue by comparison.
There are many anecdotal reports of water usage figures from data centers having conventional evaporative cooling towers, but none that seem to have been officially reported. It may well be that when examined more closely, that the use of water, even for traditional water-based cooling systems, may still be greener overall, than the closed-loop air-cooled systems, that use more power (requiring the use of far more “source” water), but does not use any “site” water.
Up until now, water-cooled chillers have been the preferred choice (for data centers with inside-air closed-loop cooling), mostly because they have a simple and basic advantage over most traditional air-cooled DX based systems (in-room CRAC, or roof top unit “RTU,” as well as air-cooled chillers): lower energy usage.
Of course, most of this water is not actually “consumed,” it is “lost” makeup water. Nonetheless, it does take energy to process waste water, as well as pumping energy required to deliver fresh (and processed potable water) to the data center (or any other users).
I have long been a proponent of recognizing the use of water as part of the true operational (energy) efficiency of data centers. In fact, I wrote a tongue-in-cheek article in 2009 titled, “Water Usage in the Roman Empire’s Data Centers,” in which I had proposed a water usage factor (WUF) as a modifying/equalizing factor to PUE.
In hindsight, my thinking may have prompted, in part, The Green Grid to create the WUE metric.
THE BOTTOM LINE
While it has become easy to point to the use of power, and now water, by data centers, it is important to put that usage into perspective. The average person (including me) uses (and wastes) drinking water in relatively huge quantities for everyday tasks, like running the hot water until it is warm enough to wash, without thinking much about it. Unfortunately, even basic necessary tasks such flushing toilets use potable water. These everyday uses of water add up to very significant amounts. According to the U.S. Environmental Protection Agency, each American uses an average of 100 gallons of water a day at home. This amount of water could be used to cool 70 kW of data center load, based on FaceBook’s WUE at its Prineville facility.
This column is meant to be less about Facebook and more about the ever-evolving design challenges and environmental impact of the data center. You might recall that the industry took a while to accept PUE when it was first introduced in 2008. Now that metric helps focus and improve the data center facility power efficiency, despite misuse by some organizations.
Since its introduction in 2011, the WUE metric has been somewhat ignored. Facebook’s release of its water usage may also start a new WUE hype and boasting cycle, but more importantly it will help bring awareness and hopefully bring a better understanding of the types of cooling systems and their true energy and water utilization, as well as a clearer understanding of their relationship and tradeoffs in the data center.
So like the Ancient Mariner, we all need to consider navigating a new course, lest we too may wind up uttering “Water, water everywhere, nor any drop to drink.” n