I'd guess that anybody wanting to put Li technology batteries aboard aircraft is likely to spend the next 10 years jumping through regulatory hoops, so unless rechargeable battery technology is "ready to deploy" right now on aircraft, I don't see any way this is happening within a decade.
Whereas you wouldn't think twice about jet fuel fires, because you're used to it rather than because fuel isn't a fire hazard :-)
Since current turnarounds include freight container swaps, I don't see why a battery swap wouldn't fit in it.
Looking at the empty weight, the weight of max fuel capacity and range[1], it seems realistic to fly 200-300mi on batteries without any kind of jettisoning.
So it seems feasible, the next question is where could it be in terms of commercial viability.
So, it presumably can't work now but they're putting their money on the idea that it will in the near future.
There's also the issue of the mass of batteries not going away during the flight, unlike fuel, which will require maximum landing weight to equal maximum takeoff weight - this will impose a structural penalty versus IC aircraft.
Excellent point; currently as fuel is burned off, planes can climb to higher, more efficient altitudes (known as a step climb). What's the performance penalty for not being able to do this as your batteries aren't getting lighter as the flight progresses.
One can imagine an electric aircraft relying on powered landing gear (as previously trialled by Airbus/Lufthansa Technik) which would further improve efficiency during taxiing.
The current turbofan engines are close to the size limits in relative fan size and bypass ratio without a gearbox, which adds cost and mass. (Only one manufacturer is in the process of introducing a geared turbofan.)
Because of no high speed jet exhaust and large area thrusters, these aircraft have the potential to be able to take off really steeply and to be really quiet.
Subtract again for the fact that you're not shedding half your takeoff weight in fuel over the course of a flight. You land with as much battery as you started with.
More if you're using metal-air batteries.
Imagine a system where the plan actually jettisons the spent batteries along the way. They could glide/parachute down to a collection depot to be recharged and sent back to the airport for installation.
The plane could take off with battery packs slung under the wings which are jettisoned and become drones, gliding down to a depot.
I'm now waiting for someone who actually knows what they're talking about to point out the flaws. One that springs to mind is the logistics of collecting the drone-batteries and transporting them to be reunited with the parent aircraft.
There's also the problem of mass-transfer. In general, aircraft should keep the center of mass behind the center of lift, and ... bad things happen when this isn't maintained. With liquid fuel, tanks are actively pumped to retain both forward-aft and left-right balance, something difficult to achieve with solid battery packs.
There's the problem of both energy consumption profiles and battery delivery/drain cycles. An aircraft generally needs maximum power to get off the ground (hence: all batteries delivering at or near their maximum output), but only partial power once airborne. What you'd like to do is to drain a few batteries completely in the take-off and ascent stages, then jettison them, but this doesn't match the batteries' own power delivery capabilities. You might be able to switch to jettisonable packs after TOaA, to completely drain those.
If fit between wing spars, you might end up with a roughly rectangular package which could be ejected aft of the aircraft from the wing, with a door sealing off the cavity. The battery itself would require some sort of deployable wing itself, as well as guidance and control systems and surfaces, possibly a small propulsion unit. A guided descent stage might actually be one of the more viable options.
It's also possible that jettisoning additional batteries on final approach would make landing dyanamics for the aircraft itself simpler.
On whole, though, I'm questioning the usefulness of this, particularly given coplexities, a likely low airspeed, and competition with ground-based alternatives (high-speed rail, Chunnel) which would bypass the power storage requirements entirely, and would likely operate at equivalent or greater speeds, direct to city centres.
Though it might be best-suited to short hops between islands, islands and mainlands, or across deep peninsulas. That's a somewhat limited set of markets.
For travel between heavily-populated continental points, ground-based rail would almost certainly be more efficient and practicable, and even in the case of the suggested London-Paris route, there is a ground-based alternative.
E.g. transmit energy wirelessly through microwave and just have enough fuel for emergencies.
Clearly only possible during the day though.
But for the sake of fun - For a Bombardier Q400(that was the only reliable data I could find)[0], the total surface area is ~213 square meters. That's obviously total surface area, so it includes both the top and bottom of the airplane. For simplicity sake, let's say only half is in full view of the sun - so 107 square meters.
Even if we assume magical solar panels that can actually convert 100% of the energy into electricity, and assume that the plane is flying in full daylight, that's only ~100kW.
The aircraft uses two PW100 engines, each producing....3700kW, so the total output of the aircraft is around 7400kW.
So even if the conditions were ideal, we had magical solar panels, and covered every inch of the aircraft, they would produce....less than 1% of power necessary to fly it. To be fair, a normal 2.0L petrol engine produces more power than what those solar panels would produce.
In reality, those panels could maybe power the onboard lights and computers? Just about?
[0]http://www.supersonicinstitute.org/docs/Q400SurfaceAreaRepor...
Doubtful. I don't have the exact numbers handy, but the power draw there is much higher than you think. That said, if you're unicorn farting skittles scenario was actually possibly (100kW solar per plane), which we know it isn't, then sure, it's enough power for that.