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.
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," saidand a respected engineer.
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, 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.
--large rooms filled with the computing, networking and storage equipment at the heart of large-scale information technology operations.
Processors throw off vastly more waste heat than before, a situation aggravated by the way they're packed in more densely. On top of that, the appetite for equipment shows no signs of abating, as computerization penetrates ever more deeply into people's lives. The problem has become severe enough that installing computers now often requires discussions not just with IT staff but also with those in charge of the power and air conditioning for a building.
"There's a merge coming between IT and facilities. You can't just talk to the IT guy. You have to talk to facilities as well," said Ron Mann, director of engineering in HP's Infrastructure Group.
Historically, a standard 6-foot-tall rack of computing gear consumed 2 to 4 kilowatts of power and threw off a corresponding amount of waste heat, Mann said. Today, 7 to 10 kilowatts is more common, and 15 to 30 kilowatts can be found. For perspective, 15 kilowatts is the same amount of power used by 150 100-watt light bulbs.
Using DC doesn't help the problem of hot, power-hungry processors, but it does get rid of power supplies and their cooling fans in a server. They don't go away entirely, of course, but typically are relocated at the top of a rack where their hot waste heat will rise and not be drawn into server cooling intakes.
Although HP isn't as aggressive a DC advocate as Rackable, it does use the approach to power its blade servers, using bulk power supplies within a rack. Putting a larger power supplies inside the rack is more efficient than having smaller ones in each server, Mann said. For example, four 5.25-inch-thick power supply units can power 96 blade servers with dual Opteron processors from Advanced Micro Devices, he said.
Sun's Bechtolsheim is unconvinced of the merits of DC, though. The crux of his argument is that DC requires two conversions: one from outside AC to 48-volt DC for distribution within the building, and a second, within servers, from 48 volts to 12 volts. Even if each conversion is 90 percent efficient, wasting 10 percent of power as heat, the combination makes the overall process 81 percent efficient.
By contrast, a single AC-DC conversion with the server uses power supplies rated at 90 percent efficiency, Bechtolsheim said.
Tim Dougherty, director of blade strategy at IBM, doesn't always agree with Sun, but this time is an exception.
"It's one of those things that always looks better on paper than it turns out to be," he said. Sure, there are DC advocates, but they can't sidestep power conversion problems altogether, he said: "All they do is move the problem somewhere else in your computer room."