DC might be better bet now, but AC is firmly entrenched

Alternating current is the standard for electrical service the world over. Preference for it dates back to the early 20th century when Thomas Edison and Nikola Tesla were competing to establish the standard electrical grid for the United States.

Tesla’s AC won out because it can be transmitted over long distances with little loss. Simple dual winding transformers increase and decrease voltage, which allows wires to carry the energy at high voltage and low current.

The amount of energy lost is proportional to the square of the current measured in amperes, and the resistance of the wire measured in ohms. So reducing the current by a factor of 10 will lower the energy lost to resistance by a factor of 100 for any given size of conductor.

Transmitted power is the product of voltage and current. As voltage goes up for a given power, transmitted current goes down.

A simple example will illustrate. A typical U.S. household uses about 10,000 watts annually, which requires 83 amperes. Transmitted at home voltage of 120 volts, this would generate a tremendous amount of heat and a power loss of more than 1,200 watts per mile. That is an unacceptable 12 percent loss for each mile from the power station.

At a transmitted voltage of 300,000 volts, the resulting current is only three-tenths of an ampere, and the power loss is a negligible 0.18 watt per mile.

AC transmission has additional factors that complicate it. The changing voltage causes a fluctuating magnetic field outside the wires. This in turn causes an opposing current in parallel wires. To counteract this, transmission towers control the phase of the AC waves in the wires. This is not a problem with DC (direct current) transmission because the constant current creates an unchanging magnetic field surrounding the wires.

AC has other complications that involve the wires themselves. Together these make AC transmission much more complicated now than it was in Edison and Tesla’s time.

AC won that original competition because of the ease of changing the voltage. Transmission can be at high voltage and low current and then changed to household voltage using transformers at or near service connections. By contrast, Edison had proposed a DC power station in every block in New York to minimize the distances and the concomitant power loss.

Transmission and wiring are more straightforward with DC. Today’s lower-resistance wires and advanced electronic circuitry might make DC the better choice for the grid. Most electronic devices use a transformer and a DC converter to suit the sensitive electronics in solid-state circuits. DC works equally well for lighting and other applications, although electric motors would need upgrades to work with DC.

But the electrical grid of the United States suffers from serious infrastructure obsolescence.

Although DC might be a better choice in the modern world, the cost and inconvenience of converting transmission facilities and upgrading machinery make it unlikely that DC will become the preferred method of delivery anytime soon.