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What Advantages Does Electric Propulsion Offer Over Gas Turbine Engines?
Executive Editor, Technology, Graham Warwick responds:
Executive Editor, Technology, Graham Warwick answers: There are several advantages claimed for electric motors over gas turbines, but motors are only one part of an electric propulsion system. Just as turbine engines need fuel tanks, pumps, pipes and other systems, electric propulsion needs energy storage, power electronics, distribution buses and cooling systems. It is at the system level that electrified propulsion faces challenges.
One advantage is noise. An electric motor is quieter than an engine that combusts fuel. It still has to drive a propulsor—rotor, propeller or fan—and that produces noise on takeoff and climbout. But electric propulsion will be quieter when taxiing and cruising. Electric motors also enable distributed propulsion systems with multiple smaller, quieter rotors or fans.
Efficiency is another advantage. Electric drivetrains can be more than 90% efficient, compared with 55% for today’s large turbofans and 35% for small turboprops. That disparity in efficiency between large and small turbines is one reason why the electrification of propulsion is beginning with the modification of regional aircraft powered by turboprops such as the Pratt & Whitney PT6.
Another advantage is scalability. Whether you use one or two large motors or many small motors in a distributed electric propulsion architecture, performance is about the same. That is not the case with turbines. Development of electric motors for aircraft is still in its early days with many different topologies to pursue—both conventional and superconducting—so time will tell.
But the biggest challenge in electrifying propulsion is energy storage. Current batteries have a fraction of the energy density of aviation fuels. That is why all-electric propulsion is starting with small, short-range air taxis—with typically an hour’s flight endurance. Even hybrid-electric aircraft are starting with shorter-range regional aircraft.
There are higher-performing battery chemistries than today’s lithium-ion designs, but they have yet to be commercialized. Other forms of energy storage, such as hydrogen fuel cells, are being fielded by the automotive industry. New ways of storing energy are in early development, such as the flow battery, which NASA is adapting to aircraft propulsion under the Aquifer project.
Battery limitations are why initial applications of electric propulsion are targeting markets that are new to aviation, such as urban air mobility and regional air logistics, now the domain of road and rail transportation. It will take longer to electrify the short- and medium-range aircraft that make up the bulk of commercial aviation, while long-haul aircraft are considered likely to remain reliant on liquid fuels.
But there are signs that short- and -medium-range propulsion will become, if not all-electric, at least more electric. Integrating a megawatt-class motor/generator into a turbofan would allow power to be added as well as extracted. This could be used to boost takeoff power, allowing use of smaller, more efficient turbine engines. Using stored energy to manage the engine cycle could improve efficiency.
The road to propulsion electrification has a long way to go, but the European Union Aviation Safety Agency says the type certification of Pipistrel’s Velis Electro trainer in June has laid the first regulatory bricks on that path.
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