The candidates exude power, supporters are campaigning hard, the issues are framed as vital, and the outcome is ultimately uncertain. That's the 2008 presidential campaign in a nutshell, but it also describes the contest shaping up between AC and DC for the hearts and minds of data center operators — and it's almost as political.

More than a century after DC bowed to AC as the most efficient method of electrical distribution, DC is getting a second look. But this time around, the ambitions of DC supporters are more narrowly focused. They're touting DC over AC as a way to make the facilities that house massive and power-ravenous data computing, storage, and communications systems more energy efficient. In a replay of the original AC-DC fight, however, AC supporters counter that tried-and-true AC, especially if it's optimized for efficiency, still reigns superior.

The battle is taking place against a backdrop of the surging “green” movement and an increased awareness of how much energy data centers use (and waste) because of how they're configured. Electrical engineering conferences, white papers, and demonstration projects that assess the comparable benefits of AC and DC for data centers have been proliferating. They've accelerated since 2006, when Congress directed the Environmental Protection Agency and the Department of Energy to oversee efforts to mitigate spiraling data center power consumption.

Has AC met its match?

DC has emerged as one possible fix, primarily because it would eliminate one of the biggest sources of energy loss and waste with AC — the multiple back-and-forth transformations and conditioning needed to step voltage down for use by IT equipment. By converting high-voltage AC to DC earlier, keeping it in DC form, and delivering it directly to rack-based servers, energy loss from conversion and the resultant heat that must be removed with cooling that also requires energy could be reduced. In fact, some studies peg energy savings as high as 30%.

But AC's defenders, including those who supply essential AC-system components like uninterruptible power supplies (UPSs) and power distribution units (PDUs), dispute findings that suggest even modest energy savings. They argue that many comparisons with AC don't factor in components that improve the efficiency of standard AC-fed systems, and fail to acknowledge additional infrastructure costs and a dearth of commercially available IT gear capable of working with a DC-based distribution system.

Electrical contractors and engineers serving the data center market are watching the AC-DC debate play out and formulating positions on the relative merits of both. Whether DC emerges as the solution or AC continues to dominate, both will be affected by the need to design and install electrical infrastructures that deliver greater energy efficiency without affecting performance and reliability.

One that's bullish on DC is Rosendin Electric. The San Jose, Calif.-based contractor, which does up to 15% of its business in the data center market, helped stage a 2006 DC-data center demonstration project sponsored by Lawrence Berkeley National Laboratory that put a positive spin on the technology. Based on that study and other research, Vice President of Engineering Bill Mazzetti says he sees DC as at least part of the solution for making data centers more energy efficient.

“DC is very nascent in the industry now, but we like it for a lot of reasons,” he says. “You're dealing with maybe three levels of power conversion compared to five for AC, plus it's a more stable source of power. It can improve system efficiency, and also improve the availability of power and uptime for a data center.”

Another contractor that also works in the data center market takes a more practical view of DC's prospects, however. Gary Schaeffer, president of Guarantee Electrical Co., St. Louis, believes that while DC could theoretically reduce energy consumption, it would be hard to implement on a broad scale.

“We don't have the infrastructure in place for DC, and at high voltages the design criteria aren't as readily familiar to engineers. Plus, there's no maintenance criteria that's been developed by groups like IEEE,” he says. “Data centers have a tremendous amount of operating experience with AC voltages, but there's not even five years experience with something like 380V or 550V DC.”

Data centers poised to move

Still, engineers who design and operate data centers have been taking a closer look at DC and other solutions because of the rising costs of operation with AC systems. Many prominent companies have been either participating in or closely watching prototype projects, and are deploying representatives to energy conferences featuring DC-data center technology presentations and roundtables. Some are coming away convinced DC is at least worth a closer look.

Gannett Co., Inc., a McLean, Va.-based media company, is evaluating high-voltage DC for its data centers. Michael Beard, special project manager in facilities management for the firm, says the emergence of new technologies has made DC appear more viable. He cites advances in direct DC-powered servers and high-voltage DC distribution systems as cause for optimism that DC can work.

One option being considered for new Gannett data centers, Beard says, is a system that would convert AC to 600VDC at the service entrance and transfer it to power conversion units, which would then convert it to usable 48V power. For an existing Web-hosting data center, he says the company is considering scrapping a 480VAC system that steps down voltage to 208V/120V and then to 48V. It would be replaced with the 48VDC system commonly used in telecom applications, which converts 480VAC to 48V in one step with a DC-UPS/rectifier.

“We're interested in moving to DC because, like other data centers, we have density issues,” he says. “It would enable us to operate in a denser environment because we wouldn't have the heat dissipation problems that AC systems present. A lot of the distribution problems with DC are being solved, and it's now in a form factor where it looks deployable.”

Those projects, however, would require substantial infrastructure work by electrical contractors, Beard concedes. In the retrofit, power distribution units would have to be disconnected, and a new system would have to be built to take 480V power to the racks. A 600V system would be even more involved, requiring the placement of a separate inverter unit to transform the AC power and installation of an extensive conduit infrastructure to carry power to the IT equipment racks.

Other data centers, however, are sticking with AC but are hoping to improve its efficiency. Guarantee Electrical is helping design and build a new data center for Sisters of Mercy Health System in St. Louis that will use a higher-voltage AC architecture endorsed by a leading supplier of AC-based power distribution gear. The design incorporates a 230/400V distribution system, generating what the supplier says are substantial savings compared to the standard 120/208V architecture.

“It looks like a solution that will save weight, space, and cabling and deliver better efficiency,” says Schaeffer. “And it looks like there might be a big savings in operating expenses over a long period of time.”

Stakeholders make their case

The architecture, based on one widely used in data centers outside of North America, is one that this particular manufacturer is touting in a bid to counter research that hails the benefits of DC over AC for data centers. In numerous white papers published on the subject, the supplier says DC's value, propriety, and feasibility are overrated. They argue that new AC system designs can deliver similar savings and performance at lower costs.

For instance, the company concludes in “Quantitative Comparison of High-Efficiency AC vs. DC Power Distribution For Data Centers” that a 380VDC architecture might only yield a 1% reduction in data center power consumption, compared to an optimized AC system, which would be a better choice.

“Because the efficiency of the most recent generation of correctly designed high-efficiency AC power distribution systems are so high to begin with, there is simply very little room for a DC alternative to provide a meaningful improvement,” it reads. “Most customers can and should specify high efficiency into their new AC designs, and solutions are available today to achieve very high power distribution efficiency.”

Nevertheless, DC proponents remain stalwart in their belief that it can deliver real savings. White papers published jointly by consulting firms, product manufacturers, and data center designers lay out equally detailed arguments for DC's superiority.

“DC Power for Improved Data Center Efficiency,” published in January 2007 as part of the Lawrence Berkeley National Laboratory research project into high-performance high-tech buildings that included a DC demonstration project, argues high-voltage DC for data centers is a sound concept.

“We were also able to conclusively demonstrate to the data center industry that DC delivery systems are viable, can be 20% or more efficient than current AC delivery systems, be more reliable, and potentially cost less in the long run,” it states.

Meanwhile, a paper presented at the recent Texas Power Electronics Technology Conference by DC-backer Richard Baugh, a Locust Grove, Va., engineering consultant, offered details on savings with a 400VDC power architecture.

“Servers equipped with DC power supplies, instead of AC power supplies, operate with 20% to 40% less heat, reduce power consumption by up to 30%, increase server reliability, offer flexibility to installations and decrease maintenance requirements,” it says, adding systems can use available off-the-shelf components.

As stakeholders continue to choose up sides in the AC-DC theoretical debate, issues of practicality, inertia, and fear of the unknown may ultimately determine whether data centers begin migrating to DC. More dispassionate observers, like Rajan Battish, an engineer and principal with the Applied Technology Group of RTKL Associates, Inc., a Baltimore data center designer, say it's unclear whether data centers will take the plunge. Without a critical mass, an impetus for change might not emerge, and the cost of DC architecture might remain too high.

“The problem for data centers is how do they know this is not just a fad,” Battish says. “Data center power usage is definitely an issue, but most operators are risk averse — and aren't likely to take a lot of chances. Some of the DC prototypes we're looking at would require an ungodly amount of power, two-story facility solutions, and massive cabling and busways. DC may be more efficient, but at the end of the day the juice may not be worth the squeeze.”

Zind is a freelance writer based in Lee's Summit, Mo.


Sidebar: The Different Flavors of DC

Installing a DC architecture in a data center involves more than just understanding fundamental differences between distributing AC and DC. It also requires understanding different DC solutions.

Three main configurations dominate discussion of DC for data centers.

  1. 48VDC

    This involves use of a DC-UPS rectifier to transform AC to DC. In a typical configuration, 480VAC is quickly stepped down to 48VDC in the rectifier and then shuttled to PDUs and outputted to rack power supplies.

    Common in telecommunications applications, its advantages include minimal AC-DC conversions, translating to high efficiency, scalability, reliability due to a lack of static switches that can fail, and a high degree of familiarity. Downsides for large data centers include the need for large copper runs, large-gauge power cables and/or bus bars to minimize distribution losses, and an infrastructure designed for short voltage transport distances.

  2. 500V to 600VDC

    In a 550V design, 480VAC would be converted to 550VDC in a rectifier, moved to a PDU, and stepped down to 48VDC in a DC-DC converter before being distributed to power supply units.

    A relatively new concept, this solution may be more efficient than either 48VDC or some configurations of 480VAC, the most common architecture used in modern data centers. With fewer components needed (static switches aren't required), it may be more reliable than 480VAC. Main distribution buses are smaller than in 48VDC systems, and long distances are possible between the power room and IT gear because distribution losses are minimized. It is also highly compatible with 48VDC power distribution and IT gear.

    Drawbacks for data centers include: a need for multiple converters to keep transport voltages high; costly and less widely available specialty components like rectifiers; and widespread unfamiliarity and a lack of safety standards.

  3. 380VDC

    In this configuration, current would enter a rectifier at 480VAC and exit at 380VDC, where it would remain as it moves through PDUs and power supply units.

An architecture being hailed as possibly the best, most-efficient DC solution, it offers fewer power conversion stages and less and smaller equipment than 480VAC. Small distribution buses translate to ease of transporting power over long distances, and paralleling UPS units may be less complex than in a 480VAC design.

Drawbacks include: a lack of commercially available gear such as power supply connectors and rectifiers designed to work in such a system; a lack of product safety or building electrical standards; and the need for extensive modification of computing equipment.