In 1903, as a last-ditch effort to maintain direct current as the standard for distributing electricity around the United States, Thomas Edison presided over a notorious event meant in part to demonstrate the danger of alternating current: the electrocution of Topsy, a circus elephant deemed a threat to humans, by a 6,600-volt AC charge. Edison’s stunt was pure fear-mongering (DC being equally dangerous at high voltage), and it failed: our grid today is primarily AC.
But a little over a century after Topsy’s collapse, it is AC that looks increasingly wobbly. Thanks to growing power consumption by digital devices of all kinds, DC power is making a comeback, this time on its own merits.
Anything that uses transistors relies on direct current, the flow of electricity in one direction. That explains why PCs, iPhones, and flat-screen TVs all have converter boxes to turn the alternating current in wall sockets (which reverses direction 120 times a second) into direct current.
Such digital consumer devices account for up to a fifth of total power consumption today, according to Greg Reed, director of the Power & Energy Initiative at the University of Pittsburgh. Reed says the steep growth curve of DC power is due not only to computers but also to the spread of devices such as LEDs and solar panels.
“Within the next 20 years we could definitely see as much as 50 percent of our total loads be made up of DC consumption,” he says. “It’s accelerating even more than we’d expected.”
With the growing number of devices generating and using direct current, Reed says, comes a big opportunity to save energy. By distributing DC power to DC devices instead of converting it to AC along the way, it’s possible to avoid substantial energy losses that occur every time electricity is converted.
Some electronics-heavy facilities are now developing all-DC “microgrids” to feed power to users. Consider the plans for a DC microgrid at China’s Xiamen University, announced in March. A self-contained electrical grid will span three campus buildings, linking a 150-kilowatt rooftop solar array to LED lighting systems and banks of computer servers.
The spread of electric vehicles could make DC even more important: electric cars charge on direct current and require substantial amounts of power. Dragan Maksimovic, an expert in power electronics at the University of Colorado in Boulder, estimates that solar-powered vehicle chargers his group is developing should cut power losses from 10 percent of what the panels produce to just 2 percent. Maksimovic is partnering with Satcon, a manufacturer of the power converters, and has funding from the Hawaii Renewable Energy Development Venture; the team plans to install solar chargers this spring at a resort on the Hawaiian island of Lanai.
Another driver for DC is the data centers that run the Internet and telecommunications networks. Large computer farms now consume more than 1.3 percent of electricity worldwide, and that figure is rising fast. The incoming power is AC and needs to be converted. Instead of having power converters on each computer, some companies are installing large centralized converters and distributing 380-volt DC power across their server farms. Japanese telecom giant NTT has four data centers in the Tokyo region operating on DC; last year it completed a DC-based server center in Atsugi City, southwest of Tokyo, that is its first to serve external clients.
Power savings are achieved largely by replacing the AC-to-DC converters attached to individual servers with more efficient centralized inverters. Making that switch and eliminating AC-DC converters on battery backup systems cut power consumption by 15 percent compared with conventional AC configurations, according to Keiichi Hirose, a senior research engineer at NTT Facilities in Tokyo. Intel has valued annual power savings for a medium-sized data center in the U.S. at $1.2 million, and the value should be considerably more in Japan and Europe, where power prices are higher.
Also catching on are DC lighting circuits. Emerge Alliance, a consortium based in San Ramon, California, that advocates for DC power distribution in commercial buildings, has established a standard for 24-volt DC ceiling circuits and says that running LED ceiling lights on DC lines uses up to 15 percent less energy than doing the AC-to-DC switch inside the fixtures. Emerge is now working on bringing DC power to employees’ desktops, letting them plug in computers or phones without the need for hot-running converter boxes.
Will the DC rebellion spread beyond buildings to take over the larger lines that feed neighborhoods, cities, and beyond, as Edison once hoped? Many power experts are skeptical. Alternating current is the standard for transmitting electricity around the grid, and many devices, such as electric motors, lend themselves to AC power. “I don’t think there’s going to be a wholesale transformation of the power system into DC,” says UC Boulder’s Maksimovic.
But others, such as Reed, see a DC takeover as inevitable. He notes that transmission lines increasingly use DC power, because high-voltage DC (HVDC) lines are easier to control and have lower losses than AC lines. Long-distance lines are often the key to tapping renewable resources located far from power-needy cities, such as wind and solar energy.
Expanding DC power distribution at the top and bottom levels of the electrical food chain creates an opportunity to close the gap with regional distribution in DC as well, just as Edison once imagined. Reed notes that converting high-voltage AC power into 120 volts for residential use leads to losses around 5 percent higher than with equivalent DC systems. “If you have HVDC on one end and DC consumption on the other, that becomes an enabler for medium-voltage DC between them,” he says.
With savings like that, Reed predicts that the first direct delivery of DC power from high-voltage line to end user may not be so far off, especially in rapidly developing economies that are building new power infrastructure. “I think we’re within 10 years of it here,” he says, “and within three to five years in China.”
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