Experts debate power distribution options

It was last century that inventor Nikola Tesla came up with a rival to Thomas Edison's direct current design, the alternating current network, which trumped to become the standard for electric power distribution networks

AC won because it was more practical. It was the more efficient and economical way to transmit power over long distances. Now the debate has risen anew, and passions are again running high. But this time, the battleground is over which is most energy efficient -- and practical -- as the distribution network for increasingly power-constrained data centres.

Edison may finally have his revenge. Or will he?

"You'll find people who are religiously convinced in both directions," says John Pflueger, technology strategist at Dell and a board director at The Green Grid consortium. Many of those debates are raging within companies that are building new data centre equipment, and The Green Grid is at the center. It launched a power distribution options study, due by year's end, which is investigating seven alternatives. Three of those specify DC voltages.

IT equipment power supplies convert incoming AC power to the various DC voltages the subcomponents require. While some equipment also accepts DC input power, most data centres distribute 208-volt AC out to the racks. At least 10 per cent of the AC power coming into data centres is lost to AC/DC and DC/AC conversion inefficiencies before it reaches the IT equipment. "Every conversion step is maybe 90 per cent efficient on a good day," says HP senior technologist and Green Grid board director Roger Tipley. That wasted energy creates heat that must be removed, increasing air conditioning loads.

DC returned to the forefront with the 2006 release of a paper by Lawrence Berkeley National Laboratory researcher William Tschudi. It showed a 20 per cent overall efficiency gain in the data centre by moving to a 380-volt DC power distribution scheme. "Distributing DC would be relatively easy," Tschudi says.

That has some IT executives excited. "We're studying it for a new data centre we're looking at building in the next two and a half years," says Bob Culver, senior vice-president and manager of technology information group facilities at Wells Fargo. Joseph Hedgecock, senior vice president and head of platform and data centres at Lehman Brothers, says he is also taking a hard look at DC.

"There's a lot of buzz, and a lot of interest in using DC power in the data center," says Peter Gross, CEO at EYP Mission Critical Facilities, which designs data centres.

But DC is far from a slam-dunk. Of the three proposed DC voltages, only the smallest -- the --48-volt telco facility standard -- is in use today. But such low voltage requires high current, and thus large cables, to deliver the kind of power that large data centers need. The distribution rails required could resemble an overhead train trestle. A proposed 380-volt standard would use lower current and thus smaller conductors. It also exactly matches the uninterruptible power supply's operating voltage, making it very efficient. A proposed 550-volt DC standard would use even smaller cables. But using either 380- or 550-volt DC would require the development of an entirely new infrastructure.

Meanwhile, conversion efficiencies for AC infrastructures have been increasing. American Power Conversion claims 96 per cent efficiency for its newest UPSs, and the latest enterprise-class servers have moved power supply efficiencies from the 70 per cent-to-80 per cent range to over 90 per cent, even when running at low utilization levels.

There still may be efficiency benefits from DC power, but they won't be enough to get DC off the ground. Adopting DC would mean maintaining two power distribution infrastructures, since there will always be equipment in the data centre that needs AC. And without a major financial payback, organizations are unlikely to invest the time and money to move into new and uncharted territory.

Even if DC wins the battle for energy-efficiency supremacy, it will still lose the war. Tesla can rest easy.