For electricity flowing all the way from power plants to the wall socket, alternating current rules the roost. For the short transmissions inside those computers, though, DC power prevails. The search for ways to convert AC to DC more efficiently is leading some data center companies to consider a DC-centric engineering approach.

The DC sales pitch, most notably from server market newcomer Rackable Systems, puts the once-ignored issue of electrical efficiency at center stage. However, while Sun Microsystems and IBM might agree with DC backers that computer power and cooling is a major problem, those server incumbents generally disagree that DC is part of the solution.

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DC advocates argue that plugging servers into AC power is inefficient, and switching systems to DC would cut down on waste heat and component failure.

Proponents argue that using DC outside the server removes some of the inefficiencies of power supplies that convert AC electricity to DC. Servers without such power supplies don’t have to contend with as much waste heat and attendant component failure.

"It keeps the units considerably cooler within the chassis themselves and saves us somewhere between 10 and 20 percent over the AC-powered alternative," in terms of electrical costs, said Charles Taylor, a senior systems engineer at the University of Florida. Taylor installed an $840,000 cluster of 200 Rackable servers as a mini-supercomputer at the university in October.

By using the DC-powered cooler servers, Taylor said he sidestepped a problem with insufficient air conditioning. "We didn’t have the cooling capacity to house the cluster we thought we were going to be putting in there," he said.

However, some believe the DC argument is a marketing snow job.

"One of our competitors has created this myth that DC power is more efficient. In fact, the opposite is true," said Andy Bechtolsheim, a top Sun server designer and a respected engineer.

"One of our competitors has created this myth that DC power is more efficient. In fact, the opposite is true." –Andy Bechtolsheim, server designer, Sun Microsystems

And Bob Sullivan, a data center expert for research consultancy the Uptime Institute, said that even some in the DC stronghold– telecommunications companies that put switching equipment in data centers called central offices–are moving to AC equipment.

"I think it’s a niche," Sullivan said of DC-powered data centers. "The telecommunications central office looks more like a computer room now than it does a switch room…Much of the equipment going into those facilities is AC-powered."

Rackable declined to comment for this story because it is in a stock exchange quiet period, after announcing a plan to sell 3.3 million shares in a secondary offering to raise new financing.

However, its marketing materials don’t mince words. "Rackable Systems’ DC power solutions help alleviate the cooling and power efficiency challenges that burden large-scale data," according to a company white paper. "By distributing redundant direct current power to each server–and replacing the standard AC power supply with a far more reliable and efficient DC power supply…server reliability is increased by as much as 27 percent, and monthly power costs are reduced by up to 30 percent."

AC versus DC
With AC, the flow of electrons in a wire switches back and forth rapidly–60 times per second in the U.S. electricity grid. With DC, the same type of current that batteries supply, electrons travel only one direction.

For physics reasons, it’s easier to transmit AC over long distances; DC requires thick copper cables or bars, instead of comparatively lightweight wires. But DC becomes a more serious possibility for power once AC reaches a building.

Telecommunications companies historically have used mainstream AC power to charge their own large battery banks, which then distribute DC power to servers at a 48-volt level. Sun, IBM, HP and others supply special versions of their servers, such as IBM’s BladeCenter T, to these customers.

But distributing DC power throughout a data center is a difficult task, Sullivan said. The current travels through massive copper bus bars that are bolted together, but joints must be inspected regularly. Loose joints are a big problem.

"We have been involved in a number of cases where one joint failed catastrophically," he said. "The explosion kicked out the entire power distribution system. It wasn’t maintained, because everything was packed in so tight that it wasn’t accessible."

The newer efforts employ a much more local approach, distributing DC power via a copper bus bar placed within a rack of computing gear. "I think distributing DC power in a single rack or cabinet is a viable alternative," Sullivan said.