Direct-Current Networks Gain Ground
Efficiency tempts computer centers to go DC
Photo: Tom DeFanti/Project Greenlight
DC Data: A test of DC distribution at a data center at the University of California, San Diego, could show big energy savings.
Nobody thinks twice about unplugging an AC device, but with DC there's a danger. With 60-Hz AC, the voltage zeros out 120 times per second, nipping potential arcs in the bud. But DC's continuous current doesn't have that inherent safety. So manufacturers are building it into the plug itself. Taiwan's Rong Feng Industrial Co., which expects safety certification early this year, adds a short data pin whose early disconnection signals the attached device to shut off. "By the time you unplug the actual power wires, there's no current going through them," says Dennis Symanski, a senior project manager at the Electric Power Research Institute.
Manufacturers and energy efficiency gurus are joining forces to battle the black bricks multiplying in offices and homes, each one providing a dribble of direct current for a distinct electronic or battery-driven device. Their chosen remedy, DC power distribution, promises simpler equipment and significant energy savings. After more than a dozen beta installations worldwide, DC wiring is going commercial as manufacturers start selling the first products challenging AC power's 120-year dominance of electrical distribution.
Standards efforts are coalescing to accelerate DC's commercial adoption, notes Bill Tschudi, an energy efficiency expert at Lawrence Berkeley National Laboratory, in Berkeley, Calif. In November the EMerge Alliance, in San Ramon, Calif., representing more than 70 manufacturers of power equipment, electronics, and building components, certified the first commercial products meeting its standard for 24-volt DC circuits—aimed initially at overhead lighting systems.
And by this month EMerge and the European Telecommunications Standards Institute (ETSI) both expect to issue draft standards for 380-V DC wiring for building-wide power distribution. DC supporters hope to merge the standards in the months ahead. "Our goal is to develop a worldwide standard," says Tschudi.
DC distribution's primary attraction—the promise of simpler equipment and significant energy savings—stems from the increasing proportion of electrical loads with DC-based electronic components or batteries. Distributing DC enables replacement of AC-DC converters within individual devices with a smaller number of larger, more efficient converters. LED-lighting installations that run on 24-V DC lines, for example, will require up to 15 percent less energy than the same lights running on fixture-level rectifiers, according to the EMerge Alliance.
Losses in the lines limit 24-V DC distribution to 10 meters, so manufacturers are developing 380-V DC wiring to extend similar benefits to entire buildings. Telecommunications firms and data centers are the likely early adopters of 380-V DC.
Today's data centers generally take 480-V AC power from the grid and convert that to DC to charge up a battery-based uninterruptible power supply, or UPS. The secure DC stream is then converted back to AC and transformed to 208-V AC for distribution, only to be rectified back to 380-V DC by the first stage of each server's power supply to charge up power-smoothing capacitors.
DC distribution offers a comparatively simple scheme, whereby a single rectifier turns 480-V AC into 380-V DC that can both charge the UPS and supply the servers. The University of California, San Diego, began testing a 380-V DC data center last year, and in November, the Electric Power Research Institute (EPRI) and Duke Energy Corp. measured a 15 percent reduction in power consumption in a test of 380-V DC distribution at the utility's Charlotte, N.C., data center. Net energy savings could be twice that, they claim, once the cooler-running equipment's reduced air-conditioning burden is factored in.
The U.S. Environmental Protection Agency's Energy Star program and similar initiatives to drive up the efficiency of AC power supplies should narrow DC's advantage. However, a 2008 analysis by Intel predicts that, even compared with premium high-efficiency AC systems, DC distribution will use 7 percent less power.
Though right now up-front costs are about equal, DC systems' simpler components should also provide a cheaper, more reliable power supply. "The price isn't lower right now, simply because of volume," says Dennis Symanski, EPRI's senior project manager and chairman of EMerge's 380-V DC standards committee.
That volume might come from a combination of solar panel installations and battery-powered vehicles. DC distribution is an efficient means of combining these inherently DC devices, according to Dragan Maksimovic, a power electronics expert at the University of Colorado at Boulder. "PV/DC chargers have a target efficiency of 98 percent. Compared to 90 percent for the round-trip efficiency of inverters, that's a 5-to-1 difference in losses," says Maksimovic. Intel Labs is incorporating a 10-kilowatt solar array and electric vehicle charging stations into a microgrid at its New Mexico Energy Systems Research Center, in Rio Rancho, and the enterprise software firm SAP is doing much the same in Palo Alto, Calif.
Symanski predicts that EMerge and ETSI could harmonize their respective drafts to forge a worldwide 380-V DC standard before the end of the year. If that happens, equipment for 380-V DC power could be available within months.
This article originally appeared in print as "Edison Vindicated".