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Electrification History 3 - AC or DC?


Generation, transmission, and distribution—the same then as now. But back at the very beginning, transmission was a matter of intense debate. On one side were proponents of direct current (DC), in which electrons flow in only one direction. On the other were those who favored alternating current (AC), in which electrons oscillate back and forth. The most prominent advocate of direct current was none other than Thomas Edison. If Benjamin Franklin was the father of electricity, Edison was widely held to be his worthy heir. Edison's inventions, from the lightbulb to the electric fan, were almost single-handedly driving the country's—and the world's—hunger for electricity.

However, Edison's devices ran on DC, and as it happened, research into AC had shown that it was much better for transmitting electricity over long distances. Championed in the last 2 decades of the 19th century by inventors and theoreticians such as Nikola Tesla and Charles Steinmetz and the entrepreneur George Westinghouse, AC won out as the dominant power supply medium. Although Edison's DC devices weren't made obsolete—AC power could be readily converted to run DC appliances—the advantages AC power offered made the outcome virtually inevitable.

With the theoretical debate settled, 20th-century engineers got to work making things better—inventing and improving devices and systems to bring more and more power to more and more people. Most of the initial improvements involved the generation of power. An early breakthrough was the transition from reciprocating engines to turbines, which took one-tenth the space and weighed as little as one-eighth an engine of comparable output. Typically under the pressure of steam or flowing water, a turbine's great fan blades spin, and this spinning action generates electric current.

Steam turbines—powered first by coal, then later by oil, natural gas, and eventually nuclear reactors—took a major leap forward in the first years of the 20th century. Key improvements in design increased generator efficiency many times over. By the 1920s high pressure steam generators were the state of the art. In the mid-1920s the investor-owned utility Boston Edison began using a high-pressure steam power plant at its Edgar Station. At a time when the common rate of power generation by steam pressure was 1 kilowatt hour per 5 to 10 pounds of coal, the Edgar Station—operating a boiler and turbine unit at 1,200 pounds of steam pressure-generated electricity at the rate of 1 kilowatt-hour per 1 pound of coal. And the improvements just kept coming. AG&E introduced a key enhancement with its Philo plant in southeastern Ohio, the first power plant to reheat steam, which markedly increased the amount of electricity generated from a given amount of raw material. Soon new, more heat-resistant steel alloys were enabling turbines to generate even more power. Each step along the way the energy output was increasing. The biggest steam turbine in 1903 generated 5,000 kilowatts; in the 1960s steam turbines were generating 200 times that.


     1- Early Years
     2- Rural Electrification
     3 - AC or DC?
     4 - Power Generators
     5- Looking Forward
     Essay - E. Linn Draper, Jr.

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