Edison was a believer in DC, or direct current. DC flows in one direction from the plus pole to the minus pole. We often get it from batteries and use it to power virtually all computers, cell phones, microchips and other electronic devices. Westinghouse and Tesla backed the big winner, AC, or alternating current. It comes out of power plants and its direction reverses, or alternates, 60 times per second in the U.S. (50 times in Europe). AC is the current in the wall socket and utilities favor it because it can be easily transformed and moved over long distances more efficiently than DC.
But DC has been recently opening a new offensive in large industrial applications. Ship builders, for example, have started exploring DC for marine propulsion. Engineers are looking at using DC current to build better and more efficient “green” ships.
Next-generation ships like the Royal Navy’s HMS Dragon could be soon using a DC propulsion system. Cargo ship and cruise ships could be next. Top image HMS Defender. Credit: Royal Navy
The technology allows them to shrink the overall size of the propulsion machinery, equipment and cables that power the propellers. Besides boosting efficiency, it also leaves more space for cargo.
Oliver Simmonds, lead naval engineer at GE Power Conversion, says that “DC architecture offers greater flexibility and allows designers to maximize the potential for using efficient drives that can run at variable speed. “Imagine a garden hose,” he says. “I can run the pump at full speed and then throttle the hose to reduce the flow, but that wastes energy. But what if I could reduce flow by slowing down the pump? I can do that now with power electronics, save fuel and get the same effect.”
That’s where direct current comes in. DC power electronics and architecture allow engineers to transmit as much as 23 percent more power than an equivalent AC system.
The system is still using AC generators to produce power (they are less complex than their DC cousins). But it converts AC to DC for efficient transmission around the ship. This design leads to another benefit: it allows engineers to store DC power in batteries and use it as a back-up in emergency. “Harbor safety regulations often require ships to run more than one power generator at a low load, which is inefficient, even though they use just a fraction of the electricity they generate,” Simmonds says. “That’s just like the throttle and the water pump, a lot of waste. But with a DC system, you could use just one generator and use an integrated battery system if something goes wrong. This represents potentially large fuel savings.”
Simmonds’ team is now working with the Royal Navy in the U.K. to develop the technology for next-next generation ships. But the system could find applications on cargo vessels as well as cruise ships.
Edison may have lost the war, but a century later he’s winning a battle.