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The value of DC power in data centers still in question

New initiatives among data centers will weigh whether direct current power is more energy efficient -- and worth the risk – than alternating current power.

Using direct current (DC) power in data centers has long been an option, but the jury is still out on whether the downsides are worth it.

For more on direct current and alternating current in the data center:
Study predicts 20% savings for DC-powered data centers

DC power sparks interest
IBM's Power7 big iron will have DC power inputs

This year, the industry is studying whether a direct current power topology is more energy efficient than traditional alternating-current (AC) power, and if so, whether the potential risks and expenses associated with it can be mitigated enough to make DC a truly viable data center power choice. The Green Grid, a nonprofit focused on data center energy efficiency, for example, has taken a closer look at the use of a DC power, and Syracuse University has begun to use DC-powered computing in a data center facility.

Direct current power has become more attractive because it can improve data center energy efficiency.

Direct current power has become more attractive because it can improve data center energy efficiency. DC is a kind of electrical current that travels through a circuit in only one direction, whereas alternating-current power is an electrical current that frequently reverses direction. In a DC-powered system, there is only one conversion: from AC to DC. And with fewer conversions, there are fewer opportunities for power and energy loss.

DC-powered servers also don't need power supplies for extra conversion and thus save data center space. But the downside of DC power is that it can require much larger wires to carry the current, thus creating power buildup and arcing that can be endanger IT equipment and staff.

DC power has long been used in the telecommunications industry because high-end PBX equipment runs on DC. But until recently, other enterprise data centers haven't considered running on DC power. Now, however, the thinking has begun to change.

"One of the things that is driving renewed interest is that you're seeing more of the telecom companies getting into the IT space and the data center space," said John Pflueger, a technical committee chairman at the Green Grid and a technology strategist at Dell Inc. "So you're seeing a natural wish for telecoms to extend their architectures into the data center. I think that's helping to keep some of the issues on the table in reference to direct current."

Putting DC power in the data center
For Syracuse University, going to DC power hasn't been as much of an adjustment as its CIO, Christopher Sedore, initially expected. The university has carved out a portion of its new data center facility for DC-power computing and plans to measure its use. Last year, the school built a 4,000-square-foot data center with 500 kilowatts of power capacity. It carved out 150 kilowatts of that for a DC-powered distribution. Right now the center's IBM System z10 mainframe -- used for teaching and research -- runs on DC power. A rectifier converts the AC power to DC power, and the facility uses power distribution equipment from Validus, a DC-power components company.

"It doesn't look dramatically different from AC," Sedore said, adding that the university has in-house electricians who know how to deal with a DC power infrastructure.

The university is still installing all various instruments that will measure the facility's energy efficiency. In addition to having a DC power area, the facility is testing back-of-the-rack liquid cooling, microturbine generation, and other cutting-edge designs.

A lack of DC-powered data center gear
But despite the benefits of DC power, Sedore acknowledges a hindrance -- in addition to safety risks-- to running a DC-powered data center.

"Frankly, right now there isn't a lot of DC equipment available."

Studying the efficiency of data center DC power traces back nearly four years, when Lawrence Berkeley National Laboratory (LBNL) set up a demonstration project with Sun Microsystems Inc. Researchers discovered energy savings of up to 28% compared with traditional, older AC topologies and as much as 7% savings compared with so-called best-in-class AC topologies that included highly efficient uninterruptible power supplies (UPSes) and server power supplies. In March 2008, LBNL released its final report on DC power in the data center.

The Green Grid then did a peer review of the LBNL study and concluded that energy savings were in the 4% to 6% range, which it also asserted would not be worthwhile for retrofit situations. Last year the organization released a second paper researching the efficiency of different power distributions in the data center. It considered 11 power topologies, including three DC lines, and concluded that none was the most efficient over the whole data center load range.

Roger Tipley, a Green Grid board member and an engineering strategist at Hewlett-Packard Co., added that moderately high-voltage DC power poses some safety concerns, where the power can build up and arc. But, he said, these concerns can be addressed through preventative measures such as shielding.

"Four-hundred volts DC may be more dangerous than 400-volts AC," he said.

A Green Grid technical committee will look at general issues, such as how to assess whether DC power is a good choice, to more nuanced details such as what kind of electrical connector to use.

"You will find people anywhere in the spectrum, from 'AC is fantastic' to 'DC is the next big thing,' " Pflueger said. "You can find people at both extremes and anywhere in between. In some circumstances, it can save some energy, but it's still a question of how much and whether the cost of change is recoverable."

Mark Fontecchio can be reached at

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