Re: Compulsory El Reg commentary moan
> Nobody claimed that it was a closed system or that you could recover 100% of the energy.
My elderly relative did, and that's who I was discussing ;)
Though also, the term "run in reverse" at least somewhat implies that you can recover 100% of the energy. And that's definitely untrue, not least because we don't just run an electric motor "in reverse", but instead use another system which is specifically designed to convert kinetic energy into electricity.
Aka regenerative braking.
> The simple fact remains that an electric motor being spun from an external force generates electricity, so a plane descending can recover some charge (maybe 10%, but that's still 10% that doesn't need topping up on the ground)
The problem is that this still isn't "free" energy. Because the plane had to use energy to rise up to the level it was at before it started descending. All it's doing is reclaiming /some/ of the energy it used when rising.
(It looks like a Tesla can theoretically recover up to 64% of it's energy via regenerative braking, but I genuinely don't know how that would translate for a plane, as a quick glance just turned up a few theoretical papers and discussions of things like applying regenerative braking to the wheels when landing, which is interesting, but presumably a relatively small percentage of the energy used for the trip)
> An ICE cannot generate more fuel under any circumstances (oil seeping through into the fuel doesn't count!)
But it can use /less/ fuel by using engine braking or by taking advantage of gravity. And as such, while the process differs slightly, the outcome is the same for both types of engine:
ICE engine: burns fuel during climb, reduces fuel burn during descent. Result: less fuel used
e-Engine: uses electricity during climb, reclaims some electricity during descent. Result: less electricity used
To be fair, it's difficult to say how the efficiencies of the two processes compare. But it's also worth noting that the ICE engine also has a secondary advantage of sorts (over and above the current 20:1 weight ratio between batteries and petroleum fuel): because it's consuming the fuel, the plane's weight will reduce as it travels.
This doesn't make a significant difference when it comes to cars - after all, they generally only lug around up to 70l (60kg) of fuel, which is a tiny fraction of their weight.
(A full take would be about 3% of my old Mondeo's curb weight, apparently!)
But for a commercial plane, the fuel is a significant percentage of the weight. And the lighter the plane gets, the less fuel it has to use.
This 747 model weighs 220 tonnes, and can weigh up to 448 tonnes when fully fueled and loaded. Which gives it a carrying capacity of approx. 230 tonnes, which can be split between fuel (max 200 tonnes) and passengers/cargo (max. 76 tonnes).
Interestingly, it also has a maximum landing weight of 312 tonnes.
So potentially, a 747 can be anywhere up to around 50% lighter when it lands, depending on how far it's flown and how much cargo it's carrying.
E.g. for the London -> NY trip, it'd burn around 70 tonnes of fuel and be up to 25% lighter.
And that's going to make a difference to the overall fuel economy...