The Wizard has been an avid paying subscriber of The Economist, for nearly 15 years. In the previous week's edition, the world's greatest news magazine featured a story on how advances in power electronics are enabling ships in the 21st century to handle massive flows of current. They are also being complemented by improvements in electrical motors which are smaller, more powerful, and more versatile. Amazing changes on how our ocean vessels move about are taking place far from the minds of your average everday John and Jane Doe.
Amongst the story's exerpts:
Galley slaves pulled on oars; river-boat steam engines turned paddles; and nuclear reactors boiled water to drive turbines connected to propellers on aircraft carriers and submarines. 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.
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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.
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But these early examples were large and unwieldy, and the idea was abandoned.
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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. They have also made possible the development of totally integrated power systems, which make energy fungible: instead of having one engine dedicated to driving the ship and another one devoted to generating shipboard power, electricity from multiple sources can simply be routed to wherever it is needed at the time.
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. As warfare has become more digital, the demand for electricity on board warships has increased. 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. Some navies are already testing rail guns, which use huge amounts of electricity to produce a magnetic field which then accelerates projectiles to many times the speed of sound.
Even more futuristic and power-hungry applications are within sight, such as “direct energy” weapons that zap enemy ships and “electric armour” that vaporises incoming missiles. With such demands for power, some of it only for a fraction of a second, warship designers are keen to have a single system doing all manner of things. Think of the Enterprise in “Star Trek”, where power is diverted to the shields, weapons or warp drive as needed. “We're going with electric drive because of warfighting need,” says Rear-Admiral Kevin McCoy of the American navy. “We are almost at the limits of technology and affordability in making improvements in mechanical drives.”
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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.
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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.
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The savings can be huge. America's Congressional Research Service reckons that installing electric drives on naval ships can cut fuel use by 10-25%. The American navy, which already has a handful of electric-drive support ships, expects savings of close to 20% for future warships using the technology.
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Perhaps surprisingly, many of these advantages also apply to cruise liners, which present designers with many of the same problems as warships.
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Much of this power is used to keep passengers happy, running air conditioners during hot Caribbean days, for instance, and powering discos and cinemas in the evenings. Then, once all the passengers have gone to bed, the power can be routed down to the propellers for a high-speed dash to the next port.
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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.
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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.
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These propellers could be driven by superconducting motors, which can generate three times the torque of a conventional motor of the same weight and power input, according to a paper published in August in the journal Superconductor Science and Technology by Philippe Masson and his colleagues at Florida State University. American researchers are working on superconducting technology for maritime propulsion, which would leapfrog the British electric-drive system.
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The World turns.
Wizard