Making Waves

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ACROSS the road from a golf course and next to a verdant, cow-filled field in Whetstone, a village about as far from the sea as it is possible to get in England, there is a ship’s engine-room in a barn. The area is dripping with history—Frank Whittle, one of the inventors of the jet engine, used a neighbouring shed for his project—but this is not some clanking historical curiosity, such as a steam engine rebuilt by an amateur enthusiast. The whirring gas turbine and whining motor being put through their paces in bucolic Leicestershire are at the cutting edge of maritime engineering. The electric drive being tested there could represent the next leap forward in ship design, as significant a technological shift as the one from sail to steam power in the 19th century.

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What makes the experimental engine room in Leicestershire so special is that it leaves out the bit that usually links the engine and propeller. Instead of a propulsion shaft connecting the two, the all-electric drive being tested uses the ship’s engines (turbine or diesel) to burn fossil fuels to generate electricity, which is then routed down thick cables to an electric motor that drives its propellers.

The idea of using electricity to drive ships is not new. Almost a century ago, around the time of the emergence of modern ship propulsion, electric drives were seen as viable contenders to compete with the then-rising mechanical drives. […] Some electric-drive warships used 20% less fuel than conventional geared-turbine vessels. But these early examples were large and unwieldy, and the idea was abandoned.

Its recent rebirth has been almost as rapid as its fall, helped by two related developments: power electronics capable of handling huge flows of current, and smaller, more powerful electric motors. These advances have allowed shipbuilders to reduce the size and weight penalties associated with electric drives. […]

And that is why advanced navies such as Britain’s and America’s are now among the most enthusiastic and earliest adopters of electric-drive ships. […] Radar, computers and combat systems now account for as much as 30% of the fuel burned on modern warships. And the demand for power could be about to jump dramatically. […]

Electric drives can help to cut costs. Although they are more expensive, bigger, heavier and in theory less efficient than mechanical drives, they use much less fuel. This is because the diesel engines and gas turbines commonly used to power ships are most efficient when buzzing away constantly at close to their maximum output. Throttle them back even a little, and the amount of energy obtained for each barrel of fuel burned falls sharply. That does not matter for ships that potter along all day and night at the same speed from one port to another; their engines can be run at their most efficient setting all the time. But in the navy, few ships have that luxury.

By some estimates, American navy ships spend 80% of their time travelling at half speed, which requires barely one-eighth of the power needed to propel a ship at top speed. But this requires them to burn almost as much fuel as they would when going much faster. By using an electric drive of the sort that is purring away in Leicestershire, ships can fire up their engines one at a time and run each at its most economical throttle setting.

Perhaps surprisingly, many of these advantages also apply to cruise liners, which present designers with many of the same problems as warships. Cruise ships need huge amounts of power. Stephen Payne, the chief designer of the Queen Mary 2, reckons the ship—the world’s biggest passenger liner when it was launched in 2003, and fitted with an electric-drive system—could supply enough electricity for a town of 700,000 people.

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Now that electrical propulsion is being taken seriously in ships, not to mention trains and cars, can electric drives defy gravity and remake air travel? In some ways they are already doing so. Manufacturers of civil and military jets are collaborating on projects to add more electrically powered devices to aircraft, such as landing gear and flaps. These are lighter and more reliable than the hydraulic and mechanical systems now in use and could draw power as needed from non-essential systems such as in-flight entertainment. Military jets, meanwhile, need ever more power for their radar systems or to jam and dazzle those of their enemies. And airborne lasers are on the way.

Electrical propulsion is much more difficult. But some small experimental aircraft are already flying with electric motors driving their propellers. They are generally powered by high-discharge lithium-polymer batteries, which are also being used in some electric cars. Fuel cells are another option. Boeing is testing an electrically powered light aircraft which uses both batteries and a fuel cell as power sources. Some gliders also have small electric motors as auxiliary propulsion systems. Just a small amount of electrical propulsion, carefully applied, can dramatically increase the amount of time a glider can stay in the air. And many unmanned aerial vehicles, not to mention remote-control aircraft flown by hobbyists, are electrically powered.

Yet all these electrical aircraft are small and have limited range. What of larger aircraft? Retrofitting a large airliner with electric motors instead of engines would not be feasible because the power-to-weight ratio of an electric motor cannot compete with that of a jet engine, and storing and generating the energy needed for a long-haul flight would not be possible given the shape and size constraints of existing aircraft. But a “blended wing”—an aircraft in which the fuselage is a flat, tail-less structure resembling a giant wing—could provide huge efficiency gains and may form the basis of future airliners.

From The Economist print edition

Dec 6th 2007

The Economist

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