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Superconducting motors could propel all-electric aircraft

05 July, 2007

Researchers in the US are predicting that superconducting motors could be used to propel a new generation of all-electric passenger aircraft which, they say, will be quieter, more efficient, less polluting, more controllable, and cheaper and easier to maintain than today’s jet aircraft. The researchers, funded by NASA and the US Department of Defense, suggest that future aircraft could be fuelled using liquid hydrogen which could also be used to cool the propulsion motors down to the cryogenic temperatures needed to achieve superconductivity.

For several years, Philippe Masson and Cesar Luongo from Florida State University (FSU) have been working with Gerald Brown from NASA and Danielle Soban from the Georgia Institute of Technology, to examine the feasibility of all-electric aircraft as a way of cutting emissions and noise levels. They have now published a report on their findings to date in the Institute of Physics’ journal, Superconductor Science and technology.

"We could potentially build a superconducting motor and generator smaller than a gas turbine," predicts Masson, the team leader. "The idea is to reduce the emissions from the aircraft and airports," he adds, suggesting that the necessary hydrogen could be generated by solar or wind powered plants. The all-electric craft would also eliminate the need for hydraulic actuators, and their heavy maintenance demands.

Jet

Current jet aircraft propulsion systems are based on gas turbines that are extremely reliable, but operate at relatively low efficiencies (typically less than 60%) and require high levels of maintenance. Equipment such as flaps and landing gear are actuated using relatively inefficient and heavy hydraulic systems which are responsible for around 70% of the maintenance faults on aircraft.

However, conventional electric motors and actuators are too heavy to power aircraft, typically delivering just 1kW of power per kg, with little prospect of substantial improvements in this figure. Superconducting motors operating with almost no losses could achieve power densities of 10–20kW/kg (and perhaps even higher) and torque densities of more than 35Nm/kg (compared to 10Nm/kg for the best conventional motors), the US researchers say.

Liquid hydrogen would be the ideal fuel for these electric aircraft, they argue, with an energy density about four times higher than aviation fuel and an order of magnitude better than batteries. Hydrogen can be converted cleanly into electrical energy via fuel cells or high-speed turbogenerators (or a combination of the two). And because the hydrogen would be stored cryogenically it could be used to cool superconducting systems and high-power-density converters at temperatures of around 20K (-253°C).

Fuel cell systems could convert the hydrogen into electricity with an overall efficiency of around 55%, but their power density is, at best, about 1kW/kg, making them relatively heavy. The alternative energy conversion technology, the gas-turbine-driven generator, can achieve much higher power densities – but at the cost of a lower efficiency than the fuel cell systems.

The US researchers suggest that one way of making the most of both conversion technologies would be to use a combined-cycle hybrid power plant in which the heat from a gas turbine would be harnessed to operate high-temperature solid oxide fuel cells at optimum efficiency.

There are several possible ways that superconducting motors could be used to propel an aircraft. One would be for the motor to replace the gas turbine in a ducted fan jet engine. In a conventional two-stage turbine design, the tip speed of the blades in the low-pressure stage limits the turbine’s speed, and thus its efficiency. A motor-driven-fan would not suffer from this limitation and could achieve a similar power density to a conventional ducted fan engine, the researchers say.

Cryogenically cooled switched reluctance and wound-rotor synchronous machines show promise, they add.

Another area where electrical technologies could have benefits would be as replacements for the heavy, unreliable hydraulic actuation systems used on conventional aircraft. Boeing is already moving in this direction in its latest "more electric" aircraft which combine hydraulic actuators with local electrical compressors, reducing the amount of pipework and moving towards "power-by wire".

If electrical actuators are to replace hydraulic systems, they will need to deliver high force densities than are possible using standard technologies. The researchers believe that this is achievable and point to linear motors developed by Oswald Elektromotren in Germany which use a combination of rare-earth magnets and superconductors to generate high forces. Such actuators could operate wing flaps and would save space in the wing compared to hydraulic actuators.

Much more powerful devices will be needed to retract the aircraft landing gear, so FSU’s Centre for Advanced Power Systems has been designing a linear motor that would be able to lift a 1-tonne undercarriage. The motor uses rare-earth permanent magnets distributed along a 1m shaft for excitation, and coated superconductor windings in the armature. The calculated force density of this actuator is some 300N/kg – eight times higher than is possible using conventional technologies. The developers say that the AC losses in such an actuator will be manageable because it would only be needed for short periods after take-off.

The final element, linking the power sources to the motors and actuators, would be a power management and distribution system, including power converters. Usually power electronics exhibits relative low power densities because of the space needed for heatsinks and large capacitors, but the US researchers have several ideas for overcoming this limitation. For example, superconducting motors are relatively insensitive to harmonics because of their large air gaps, thus allowing smaller capacitors and inductors to be used for filtering. And the on-board cryogenics help to remove heat from the semiconductors, thus boosting power densities by a factor of three.

For comparison purposes, the researchers cite the example of the venerable Boeing 737-200 airliner, which is propelled by two high-bypass turbofans, each weighing 1.58 tonnes and producing the equivalent of 10MW of shaft power. For structural and aerodynamic reasons, the weight under each wing should not increase if the plane was to be converted to electrical propulsion.

According to the researchers’ calculations, a superconducting propulsion motor will weigh about the same as the gas turbine that drives a conventional turbofan. The power management system would weigh around a tonne using off-the-shelf technology, but could be reduced to around 300kg by using cryogenic technology.

Superconducting turbogenerators would add a further tonne each, making the total weight of the electric propulsion system just over 2.5 tonnes. Although this is 60% heavier than the original jet engine, the system would be more efficient, allowing less fuel to be carried to achieve the same flight range. But, the researchers caution that "no conclusions can be drawn in terms of weight at this point" and say that an accurate weight assessment is still needed.

The US team is now looking for an industrial partner to help it build a prototype superconducting turbofan. "The technology is there," says Masson. "It’s a matter of finding a source of funding."




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