June 30, 2017, ©. Leeham Co: One of the areas I explored while at Paris Air Show last week was electric aircraft technology. With electric aircraft, we mean an aircraft where the propeller/fan is driven my an electrical motor.
We don’t talk about “more electrical aircraft systems” like for the Boeing 787.
One of the companies investing in technology for electric aircraft is Siemens. A Siemens-engined aerobatic aircraft, the Extra 330LE, made daily flight displays during the show.
When discussing electrically driven aircraft, one has to understand some basics:
Weight is a larger problem for an aircraft than space. Therefore, the difference between jet fuel as an energy source for the engines compared with a battery will be compared on weight.
A kg of jet fuel stores 11,900 Watt Hours (Wh) of energy. A kg of Lithium-Ion battery stores 170 Watt Hours of energy.
The next important step is, with what efficiency can this stored energy be used by the engine to produce power to a propeller or a fan?
A modern gas turbine core has an efficiency of transferring Jet fuel energy into shaft work of around 55%.
The modern electrical motor as produced by Siemens for the Extra aircraft has an efficiency of 95%. Add to that a power converter (called an inverter) from battery DC power to electrical motor AC power of 90% efficiency.
If we now put together the components we need from the energy store to a propeller or fan, we have:
We can see that one kg of battery gives us a tiny power per hour. To get to the necessary power density to drive an aircraft for some time, we need to add thousands of kilos of battery.
The consequence is a battery-driven aircraft can only be used for short-range operations. The demonstration aircraft from Siemens at the Paris Air Show, the Extra 330LE, had an operational time of 30 minutes. The battery used replaced the forward pilots seat.
It will remain like this until more efficient battery technology has come onto the market. Research is ongoing, but it will take decades before batteries with, say, 1,000 Watt-hours per kg would be available. This then gives us a motor shaft power of 1.2hp during one hour for each kg of battery.
To fly longer, we need another solution.
Hybrid electric aircraft
By now, it’s clear battery powered aircraft will be special cases, used for local purposes. The more promising electric aircraft is the hybrid one, where a combustion engine and alternator is combined with a battery which cover top power needs. The combination drives our electrical motor.
This is the subject of our next Corner.