Re: Just a matter of timing
@Vic,
The numbers I did the other day came out at 4.3TJ in a 787's tanks. Think about the batteries you'd need to save 10% of that - 430GJ. That's a lot of battery; the large Tesla pack holds 85KWh, which equates to 300MJ. So you nominally need 1400 of those to save 10% fuel - in practice, rather less, since a heat engine cannot be 100% efficient. But I doubt you'd get the effect with fewer than 500 packs[1] - and they're 544Kg each. That's 272t of the carrying capacity taken up in batteries; the 787-9 only has 126t to start with...
It's worse even than that. Fuel that's burnt gets blown out the back of the engine as H2O, CO2, etc. The aircraft weight decreases during flight, and becomes significantly more efficient as the fuel is burnt. For example, Concorde would drift higher and get faster without increasing the thrust setting as the fuel load burned off.
Whereas the batteries remain on board. The aircraft efficiency does not improve as the batteries are drained. So their contribution is even less. Worse still is that the aircraft landing weight has gone up, every landing would be at maximum take off weight, and there's very few aircraft out there that can do that now. So the airframe and undercarriage have to be stronger so that it can land whilst still carrying all those tons of batteries. And to avoid putting dents in runways they'd start needing to put extra undercarriage legs on to spread the weight (like a 747), adding even more weight. This makes the aircraft even less efficient. And the brakes would be heavier - so they're going to have a harder time. And the wing would need to be bigger with larger flaps /slats to make the landing performance reliable in all weather conditions, adding yet more weight. And all this extra weight would need more powerful engines to get the whole lot into the air in the first place, so they're going to be heavier. And there's the whole question of how do electricity get turned into hot exhaust. A burning fuel spray is very good at doing that, electric heaters are not. It's a question of surface area. Plus there's no heater element that's going to be light weight (tungsten is heavy) and get as hot as burning fuel. So it'd have to be a motor driving the fan, which is going to be heavier than the equivalent gas generator (lots of copper and iron, not titanium, aluminium, steel and the small amount of nickel used in the turbine blades at the hot end).
Every pound of 'fuel' remaining at the end of the flight adds a disproportionate number of pounds to the weight of the airframe.
So if batteries are ever going to compete against kerosene, they have to beat the energy density of kerosene by quite a large margin. It's still not going to work if they just reach parity with hydrocarbons.
Something like the A380 carries approx 250 tons of fuel, and can land (under max weight conditions) with 70 tons remaining. The equivalent batteries would have to weigh 70 tons, not 250 tons. So the energy density of the battery would have to be 350% that of kerosene.
It's not that bad because the maximum "fuel" load is then also 70 tons, 180 tons less. So quite a lot of weight could be trimmed (smaller wings, etc). I don't feel like doing the maths model this time of the morning, but it boils down to battery energy density having to be significantly higher than kerosene's before there is any benefit whatsoever.
Hydrogen would be a better bet I think. If only we could store that in a tank efficiently.
Biting the hand that feeds IT
