RESEARCH AND DEVELOPMENT
Ambitions of All-Electric Navy Get Reality Check
The poster child for the Navy’s shift to all-electric propulsion is the DDG-1000 destroyer. Originally conceived as a fleet of 32 ships, the Zumwalt class now consists of only three ships to be built at Bath Iron Works in Maine. The class truncation has many implications for naval technology development.
Since the early days of the DDG-1000 program, the Navy has invested more than $100 million to develop a superconducting motor prototype to demonstrate its efficacy as the primary propulsion technology for all-electric ships.
American Superconductor Corp. and Northrop Grumman delivered the 36.5-megawatt high-temperature superconductor (HTS) ship propulsion motor in 2006. A few months ago they announced successful test results. This was the first test of a system sized for a large Navy combatant — and the sole motor among three competitors to make full power rating for DDG-1000.
So far, however, no ships will be outfitted with the advanced motors.
A study found that the DDG-1000 could double its missile count by using superconducting motors because they take up less space on the ship. “There are multiple interconnecting parts of a system on a naval ship that if converted to superconductor [technology], would have a very revolutionary impact on the combat capability of that ship,” says Dan McGahn, senior vice president and general manager of American Superconductor, based in Devens, Mass.
Superconductors are known for their magnetic field-generating properties and their ability to conduct electricity efficiently. For the Navy this means naval vessels would be able to transfer power from propulsion systems to weapons and other shipboard functions as necessary and also free up shipboard space for other uses.
The HTS motor is the largest superconducting machine in the world, McGahn says. And yet, it is far smaller and lighter than a comparable conventional motor with the same output of 49,000 horsepower.
The secret lies in the wire. Motors typically consist of coils of copper wire in the rotor. Superconductor wire has about 150 times more conducting capacity than traditional copper wire and creates a strong magnetic field in less space. It also yields stealth properties that are desirable in naval warship propulsion systems.
McGahn says that the HTS technology is fuel-efficient, especially at the low speeds Navy ships tend to operate. Depending on the price of oil, the service could save as much as $1 million a year, he says.
The full-power testing of the motor took place in December at the Navy’s land-based test site in Philadelphia. It doubled the service’s power rating test record, making it the most powerful electric propulsion motor evaluated.
“Testing completed to date has validated the technology concept and potential application for future programs,” says Kathleen Roberts, a Naval Sea Systems Command spokesperson. “Additional HTS test and development is not currently required to support any existing ship acquisition program.”
The technology has a drawback. The rotor — where the superconductor wire generates a strong magnetic field — requires cryogenic cooling, unlike in a conventional motor. The specialized refrigeration technology is more costly than the oil necessary to cool a conventional motor’s stator, the stationary part of the rotor.
Despite the slowdown in the program, contractors remain optimistic about the future of the technology. “We’ve demonstrated the motor already. It’s ready to go,” says American Superconductor’s Jason Fredette, director of investor and media relations. “We could start selling motors tomorrow if there were ships to put them on.”
The DDG-1000 motors have already been selected, so company officials are eyeing other Navy ships as potential candidates. The list includes the future cruiser class ships, the DDG-51 class destroyers and even the new littoral combat ship.
Powered by the same Rolls Royce MT-30 engines, the littoral combat ship steams with as much horsepower as planned for the DDG-1000. “There’s very tight machinery space on LCS,” John Ulliman, American Superconductor’s director of government relations, says. “So if ever there was going to be electric drive, I can’t imagine anything but HTS going in there.”
The company’s HTS wire and magnet cable technology have been applied to a degaussing coil system that was installed aboard the USS Higgins, a DDG-51 destroyer, for a two-year trial at sea. Degaussing systems can eliminate residual magnetic fields on a ship’s hull that could otherwise signal its presence in enemy waters and trigger mines.