Theoretically, it seems like a crazy, crazy idea - for now:
Lithium-ion batteries provide around 0.36 MJ/kg ( source:
Lithium-ion battery - Wikipedia )
Jet A aviation fuel provides around 43 MJ/kg (source:
Energy Density of Aviation Fuel - The Physics Factbook )
Once Li Ion gets within one order of magnitude of Jet A energy density, its on.
Something like 4MJ/kg will be enough to start with a serious contender.
I found Tesla 2170 already at 250Wh/kg = 0.9MJ/kg. So it still needs something like a four fold energy density boost.
Electric fan jets would be about 3x more efficient than Jet A turbofans in converting MJs in the battery to thrust, at the same altitude. But flying 10k ft higher shifts that to at least 5x more efficient.
Today's greatest twin turbofans can fly for 16 hours at Mach 0.855.
If you give up 18 hour autonomy to 2 hours, that accommodates almost a whole order of magnitude less in energy density.
What's needed to start competing is a Mach 0.80 jet that can fly to 2 hours + reserves, enough to fly from Miami to Boston or Seattle to San Diego (and from Dallas/Chicago to ~50% or airports in the USA) against 99% worst head winds. With seating for 150 pax minimum.
The key is it must cost half as much in electricity costs and half as much in maintenance and about the same to purchase.
All it takes is an aircraft that can do 25% of the missions a similar size B737/A320 would be tasked with.
It needs to be an aircraft that can cruise with the fancy biz jets at FL510 or even higher. Electric jets only problem with altitude is that the thin air reduces how much thrust is produced per revolution, as long as it can just turn the fan faster that's mostly a non issue. Fossil turbofans takeoff and start their climb with a ton of thrust even after throttling down to max continuous thrust, as they climb they keep loosing thrust.
An electric aircraft would keep producing the same continuous thrust from climb to end of cruise, able to climb until the wings observe Mach buffet effects, the higher they go, the faster the true airspeed. I'm assuming there would be some extra thrust for take off and initial climb, limited by battery and motor thermal limits.
The key is the aircraft's normal cruising altitude. The higher it can cruise, the better. At some point, the lowest raw energy density of Lithium is fully offset by flying through thinner air. Assuming it would have the same design technology as a B787 or A350 (aerodynamics, lightweight carbon fiber), it would be mostly a battle between the battery's weight vs its lift.
Once the first design hits the market, for every 5% improvement in lithium energy density, the lighter weight would result in even higher cruise and something like a 20% improvement in range.
After that it would be some 20 years of design refinements after each small improvement in lithium energy density until intercontinental 300 pax type electric behemoths are in the market.
Being able to fly at FL510 means you'd literally be above ALL weather once in high altitude cruise.
Above FL450 aircraft are currently allowed to fly point to point as there's so little traffic up there. That could mean another 10% energy savings.
Converting an existing aircraft is not the solution. In order to get maximum benefit from flat thrust at any altitude we need a wing that's optimized towards producing more lift total but that's also able to go high subsonic. The ultimate limit is how fast it can go before the wing starts to get that huge drag increase due to transonic effects.
If a design that produces a lot of lift is produced, say with a 80 knot stall and 240 KIAS structural limit, which would make it quite slow below FL400, but faster and faster as it climbs.
Being the slowest aircraft in the air means you're never speed limited, you never have to slow down.
PS: I sound like I know what I'm talking about, but I'm not an aerospace engineer. But being a pilot/computer guy and interested in the subject for several decades I keep learning the concepts/issues and above all the fundamental give and takes involved in designing aircraft. I appreciate any and all criticism, good or bad.
