What could better fit a month of discussing UPS systems in the data center than a controversial design topic like AC versus DC power? Over the past two weeks, the Data Center Journal has reviewed the basics of UPS systems as well as broad guidelines for selecting an appropriate design. But what if you could use an electrical system that would simplify both your UPS system and other aspects of your electrical infrastructure? Proponents of DC (direct current) power believe this is possible, all with a significant increase in efficiency. Naysayers dismiss these benefits as resting on inaccurate comparisons with AC (alternating current). So, who’s right?
AC Proponents Poisoning the Well?
If the history of science shows anything, it shows that people who are quick to call one thing science and another pseudoscience are just as likely in the wrong as they are in the right. Although science is supposedly neutral, unbiased and based largely on empirical evidence, scientists are often the most dogmatic, biased and committed individuals on the planet. The debate over AC versus DC in the data center—although not as rancorous as, say, the matter of climate change—seems to bring out the dogmatist in many people.
Case in point: in a company blog post, Kevin Brown (Vice President, Data Center Global Offer for Schneider Electric) consistently uses scare quotes around the word “study” when referencing publications on DC efficiency benefits. The tell is that he does this for a paper he admits to not having thoroughly investigated: “We are digging into this one but science is science. . . It’ll be interesting to understand the details of this ‘study’ to see if they tipped the scales in favor of DC.” Even the blog title says much: “Great Hoaxes: Bigfoot, UFO’s, and DC vs. AC efficiency studies.” (One wonders if even the capitalization, or lack thereof, is significant.)
Disagree with the findings of a paper? Fine. But labeling your position as science and the other guy’s position as a hoax, pseudoscience, nonscience or whatever is often just unprofessional.
To be sure, not all AC proponents are dogmatic, and not all DC proponents are detached and objective, and sorting through the hype to find the nuggets of truth can be difficult. But that’s the case for any topic subject to heated debate, whether it’s evolution, climate change or DC power in the data center.
Maybe DC Really Is a Little Better in the Data Center
To his credit, Brown does admit the potential for DC to be more efficient than AC, albeit reluctantly: “there is really very little difference (~2-4% at most) in the efficiency of DC versus a well designed AC system.” So, maybe the arguments for DC power in the data center do have some validity—at least they’re not as unscientific as some might suggest, even if their claims are a little exaggerated. Even if the efficiency improvement isn’t 20–30% but is low single digits, that’s potentially tremendous savings in the long term. And as data centers max out their benefits from virtualization, consolidation and free cooling, they’ll begin looking elsewhere to reduce power consumption: isn’t even a few percent efficiency improvement worth considering?
As always, however, some restraint is needed when facing wild claims about a new or different technology. Every year, some development is touted in the press as the next (fill in the blank), or as something that will revolutionize the (fill in the blank) industry. Most of these do not pan out. Thus, a new strategy or technology should be viewed with caution, and sometimes even skepticism. Nevertheless, one should also keep an open mind.
What DC Has Going for It
Consider the double conversion UPS design: incoming power to the data center is converted from AC (what the electric generator produces) to DC to charge the battery and remove spikes, dips and other power quality anomalies. It must then be converted back to AC for distribution over the typical data center’s infrastructure—which is designed for AC. The conversion back to AC bears some inefficiency, as with any power conversion step. So what’s the advantage of DC in this case? The reconversion to AC can be eliminated, saving some waste energy. Furthermore, since IT equipment generally uses DC (most products convert input AC power to usable DC power internally), another conversion stage could be eliminated: conversion from the “cleaned” AC power (from the UPS/PDU) back to DC once more. The elimination of several power conversion stages is the most widely touted benefit of DC over AC.
Furthermore, eliminating equipment (power conversion stages, for instance) increases available space (a commodity in data centers) and generally improves the reliability of systems (one less thing to break). And it reduces both capital and operating expenses. Some quibbling over other elements of the system, such as reduced losses in cabling and such, takes place as well.
What DC Has Going Against It
Now, the cons. Perhaps the biggest concern for companies building data centers is the lack of equipment designed to run on DC power. The problem here is not so much a technical one as it is a matter of product features. On the other hand, some equipment is designed to use either AC or DC (or just DC), and as more data centers implement DC infrastructure, more products will become available.
Another concern is safety, with some opponents suggesting that high-voltage DC poses a greater risk to data center personnel compared with high-voltage AC (for instance, charge could build in areas of a cable, leading to arcing). The size and weight of cabling for DC relative to AC may also be a concern, although the amount of cabling could be less in a DC design.
But the need for a solid reputation is what really hampers DC. This problem will ease as more data centers adopt DC and as more publications (unbiased and down to earth, preferably) review good DC designs compared with good AC designs, thereby giving companies solid data regarding potential savings for each approach.
And the Winner Is...
Perhaps the biggest impediment to adoption of DC power is the contentious nature of the subject, with each side making claims that either misrepresent the other side or that simply are not accurate regarding its own champion. If some design technique is controversial, a conservative company—and how many are willing to take a big risk in today’s struggling economy?—is likely to pursue the status quo rather than a unique approach.
DC seems to offer some efficiency improvement over AC, and this will no doubt drive increasing adoption of DC power distribution in data centers. Whether it will become a tsunami, however, remains to be seen. Unless vastly superior efficiency can be demonstrated, AC and DC will probably remain competing options rather than one taking the lion’s share of the market (unless AC remains dominant simply owing to historical momentum). In the meantime, a healthy debate is wonderful—but a little respect to and from both sides would be nice.
Photo courtesy of Philippe Put.