SAN JOSE, Calif. - It's been five years since a study by Lawrence Berkeley National Labs outlined the potential for DC power distribution to cut data center energy use by 10 to 20 percent. But adoption of DC in data centers remains limited, even as the industry aggressively pursues a wide array of other energy savings strategies.
But advocates of DC power continue to make the case for direct current distribution in data centers. The recent Data Center Efficiency Summit featured a case study showing gains over AC systems, and discussion of whether global efforts to establish a standard for 380 volt systems might build momentum for DC power.
"It works, it's safe, it's reliable, and we'd like to take it worldwide," said Dennis Symanski, Senior Project Manager for the Electric Power Research Institute (EPRI).
Savings Seen in "Apples-to-Apples" Test
At the Summit, Symanski presented results of a DC implementation in a Duke Energy data center which balanced the load across AC and DC distribution systems in the same environment. "We were trying to get as close as we could to an apples-to-apples comparison," said Symanski, who said the DC system showed energy savings ranging between 14.9 and 15.6 percent versus AC distribution systems in multiple tests with different IT workloads. The test used HP and IBM servers and EMC storage arrays on the IT side, with a distribution system featuring Delta rectifiers, Starline busway and Dranetz-BMI metering.
The case study offers additional data points in the ongoing debate about the efficiency potential for DC power distribution. Most data centers use power distribution systems in which AC power from the grid is converted into DC power to charge the UPS batteries, and then converted back to AC for the equipment. The loss of power through multiple AC/DC conversions has been cited as an argument for using DC power distribution. "DC distribution can eliminate at least three conversions," said Symanski.
The potential advantages of DC power have been also been studied by The Green Grid and Intel (in a joint effort with HP and Emerson Network Power). But many data center professionals remain leery of DC power, and some vendors argue that high-voltage AC configurations would be a better approach than DC power distribution. Some cite safety concerns about DC, but others note that DC has been safely used in telecom facilities for years.
"It's really just waiting for the industry," said Bill Tschudi of Lawrence Berkeley National Lab. "This really has the opportunity to be a worldwide standard."
380V Advanced as Global Standard
An industry association for DC power adoption, The EMerge Alliance, has formed a new technical standards committee for data centers, and is advancing a 380-volt DC power standard. The alliance is currently working with the European Telecom Standards Institute and other standards bodies to align on 380V as a standard for data centers.
Symanski said the effort settled on 380V because it represented a "sweet spot" in the range of voltages and requirements, with the potential to work across telecom facilities and data centers, while requiring modest changes in equipment. "It requires a very small change to existing power supplies," said Symanski.
NTT Facilities has been using DC power distribution in its new data centers in Japan, according to Keizo Hoshijima, President and CEO of NTT Facilities USA. The company now has five facilities in Tokyo using 380V distribution - including one using data center containers.
Tschudi noted that a growing number of DC products have gained UL listing certification from the Underwriters Laboratory, a key step in gaining broader adoption. Commercialization of power supplies designed for DC is a key step in efforts to boost adoption for DC distribution, as is the development of a standard DC line cord and connector.
Over the past several years, the energy-saving potential for DC power has featured in several discussions at industry conferences, in which consultants and service providers have reported little or no customer interest in DC distribution.
ABB Champions DC Distribution
But the push for DC distribution has gained a major supporter in global power specialist ABB. In May ABB bought a controlling interest in Validus DC Systems, a leading provider of direct current (DC) power infrastructure equipment for data centers. ABB and Validus are installing an advanced DC power distribution system for green.ch, a leading service provider in Switzerland. ABB is also is partnering with IO on the development of a new data center module based on DC power.
The discussion at the Nov. 18 event, which was sponsored by the Silicon Valley Leadership Group, is the latest chapter in a debate over the relative merits of AC and DC power that dates all the way back to the 1880s and the “War of the Currents” between power pioneers Thomas Edison and Nikola Tesla.
"It's a paradigm shift," said Symanski. "It's a little bit radical, perhaps. Thomas Edison would be proud of us."