MICHAEL KANELLOS “From Edison’s Trunk, Direct Current Gets Another Look” http://www.nytimes.com/2011/11/18/business/energy-environment/direct-current-technology-gets-another-look.html

“War of the Currents”という言葉は知らなかった。トーマス・エディソンが採用したものの、交流との争いに敗れた直流が復権しつつあるという話。

Today, AC is still the standard for the electricity that comes out of our wall sockets. But DC is staging a roaring comeback in pockets of the electrical grid.

Alstom, ABB, Siemens and other conglomerates are erecting high-voltage DC grids to carry gigawatts of electricity from wind farms in remote places like western China and the North Sea to faraway cities. Companies like SAP and Facebook that operate huge data centers are using more DC to reduce waste heat. Panasonic is even talking about building eco-friendly homes that use direct current.

In a DC grid, electrons flow from a battery or power station to a home or appliance, much as water flows downhill to a lake. In AC, electrons flow back and forth between generators and appliances in a precisely synchronized manner ― imagine a set of interlocking canals where water continually surges back and forth but the water level at any given point stays constant.


Direct current was the electrical transmission technology when Edison started rolling out electric wires in the 19th century. Alternating current, which operated at higher voltages, was later championed by the Edison rivals Nikola Tesla and George Westinghouse.

The AC forces won when Tesla and Westinghouse figured out how to fine-tune AC transmission so that it required far fewer power plants and copper cable.


But those constant conversions cause power losses. For example, in conventional data centers, with hundreds of computers, electricity might be converted and “stepped down” in voltage five times before being used. All that heat must be removed by air-conditioners, which consumes more power.

In a data center redesigned to use more direct current, monthly utility bills can be cut by 10 to 20 percent, according to Trent Waterhouse, vice president of marketing for power electronics at General Electric.

“You can cut the number of power conversions in half,” Mr. Waterhouse said.