Found this somewhere, is it still up to date?
> Jet fuel has an energy density of 9.6 kWh/L. What this means is that jet fuel, pound for pound, is nearly 50x more energy-dense than batteries. However, the inefficiency of internal combustion means that 1,000 pounds of jet fuel yields only about 14x more power than 1000 lbs of batteries.
But better chemistry lithium ion (lithium sulfur, lithium metal anode, etc) gets you to 2MJ/kg and the difference is only about a factor of 7.
A factor of 7 sounds like a lot until you realize jet fuel is better than it needs to be and conventional aircraft still leave a huge amount of efficiency on the table. (This is the same reason why gasoline is ~15 times better than lithium ion, and typical gas car ranges are 400 miles but you can get a Model S with the same 400 miles range). The Virgin Globalflyer jet went over 40,000km on a single fueling of jet fuel at Mach ~0.5, and there have been aerodynamic and structural/material improvements since then. One seventh of that is 5700km, enough for long haul battery electric flight.
Cars push on the ground, and the ground gives back all the force applied to it, so no losses here, planes that fly very close to the ground can also take advantage of the ground effect (that's the name) for more efficient lift.
To how much extent, I don't know, but one thing for sure, manufacturers strive to make their planes as light as possible.
As a side note, heavier gliders actually fly a bit better (faster) but unlike powered airplanes, they don't lift themselves.
Edit: added ground effect
I think the real reason is there is that no need for bigger gas tanks.
Still waiting for a ground vehicle that I can pilot* on ordinary roads and highways, go a bazillian range units without stopping, and thereby usually avoid the dramatic'ly worse flying aggravations compared to when I was grammar school. (^_~)
* not technic'ly "driving" when you're not behind the thing that you're causing to move. Although really, any linguist would acknowledge that in reference to ground vehicles, the verb "drive" has acquired the additional meaning of the verb "pilot", so I'm just being silly.
Going by your numbers, jet fuel maintains 46MJ/kg as it is depleted—the last kg has the same energy density as the first kg. I would exclude the weight of the enclosing tank because I think that’s usually the airframe itself.
Batteries start at 1MJ/kg for the first MJ and fall to 0MJ/kg when depleted (I don’t know what the curve looks like, but I’d expect it to be worse than linear).
What is the practical impact of this difference, if any?
Go on. I’m listening and I’ll bet Airbus and Boeing are too
Roughly speaking, you can increase the efficiency of an airplane in 2 ways: increase the thermodynamic efficiency of the engine, and make the engine push more air at a slower speed.
For the first one you need to make the engine burn hotter. Since metals melt at a certain temperature, you need to use ceramics if you want to exceed that temperature. Such as Silicon Carbide [1] which "melts" (or decomposes) at 2830 degrees Celsius. The efficiency of an ideal Carnot engine is 1-T_cold/T_hot, so if T_cold = 300K and T_hot = 3000K, you end up with 90% efficiency. Of course the jet engine is not quite the Carnot engine, but the idea stands that higher burning temperature leads to higher efficiency. Now, if manufacturers decide to use Hafnium Carbide one day, that one melts at 1000 degrees Celsius higher.
The second method is due to the relationship between momentum and energy. Energy is m v^2/2, which means that for a given energy (think quantity of jet fuel burned in the engine), you can either push a certain amount of air at a given speed, or 4 times as much air at half the speed. But momentum is m v, so the second choice gives you twice the momentum. In other words, twice the thrust. To push more air, you want to grab more air, so you want turbines with higher diameter, and a higher bypass ratio.
General Electric and Pratt&Whitney do exactly that with their engines. Engines with higher bypass ratios and using ceramics (more precisely, ceramic matrix composites). In fact GE's engine is so fantastic, that Boeing decided to put it on the 730-Max, and the rest is history ...
So, yes, Boeing and Airbus know what needs to be done, but it's just not that trivial.
[1] http://edition.cnn.com/travel/article/boom-supersonic-four-h...
Batteries weigh the same full or depleted. So after you deplete 50% of your battery power you’re still carrying 100% of the weight.
In cars this is less of an issue. But for planes, where you are defying gravity, it can make a large difference.
But the total impact is related to the full system, which in cars includes:
- The useful power at the interface of road and wheel.
- The weight of the engine you get rid of (including the safety precautions to stop this killing passengers).
- The location of the weight.
- high torque at low speeds feeling "peppy"
- The type of journeys (stop'n'go traffic vs long distance trucking)
- The ability to use regen to save brake weight and pollution.
And so on.
Some of these have unexpected impacts, in EV the low center of gravity, the ability to put the wheels closer to the corners, the quiter operation, the ease of home refueling, ease of building on a shared platform are among the things that emerged.
For a while it seemed like hybrid cars might make the most sense, but a thousand small changes added up to basically make fully BEV the logical next step.
In aircraft, it's all still a bit up in the air (pun totally intended) but things like operating in thinner air that give combustion engines trouble, the ability to have lots of propellers and rethink a bunch of design assumptions, quieter operation in urban areas are already being noted and explored.
I think building battery cells designed to fail safely in an accident is probably a more reasonable option than trying to dump them.
This is why, time permitting, planes will first burn off or drop fuel before doing an unplanned landing.
They _can_ land fully loaded but you are exceeding the maximum weight for landing which is neither advised by the manufacturer nor is it guaranteed that nothing will break. So if you it‘s not an emergency that requires immediate landing, you won‘t go for it.
1) Detaching heavy objects while in-flight is a tried and tested technology. We've been doing it ever since flight was invented. 2) UAVs are not making the news anymore and fully autonomous landing has been commercially available since the mid-60s, granted, for very different type of aircraft, but it's becoming more common for light aircraft too.
Dropping UAVs from airplanes is of course a whole different ball game, but wouldn't you say it's.. doable? Ultimately whether the added complexity is worth it depends on the proportion of energy used during taxi, takeoff and climbing compared to the total which varies wildly depending on distance and other factors, but tends to be highest for very short (but also very long) flights which is where electric aircraft currently stand a chance.
If so catapults like aircraft carriers maybe a good idea.
Or extension cords that pop out when it gets airborne?
Changing that industry is about ramping up the manufacturing and fuel infrastructure to the point where it is profitable. It's a very slow and expensive process.
The right question to ask though is why big heavy jets are interesting to begin with. They are big, heavy, expensive, and complicated machines. Why do we go through the trouble of building those at a cost of around 100M per vehicle for a small one?
This has to do with a lot of things but the two main ones that stand out are that fuel is expensive and pilots are expensive (and scarce). So the optimal conventional plane is one that moves around a lot of stuff with a minimum amount of fuel per kg of useful payload and the requisite two pilots. Hence the A380 briefly existed until it got to expensive to operate it.
An autonomous small battery operated plane would fix both those pain points. Electricity is comparatively cheap and autonomous vehicles means no expensive pilots are needed anymore. People that are talking about making electrical airbuses or boeings are missing the point: these form factors become a lot less relevant when we can fly dozens/hundreds of autonomous vehicles for orders of magnitude less money. Mostly small planes are pretty cheap to manufacture. So, are electrical engines and batteries. And the electronics needed to run them. You could feasibly build hundreds of them for the price of a single A320. And they'd be comparatively dirt cheap to maintain and operate. And a lot more flexible in where they could go. You are not going to cross ocean in one any time soon. But short haul flight is a different matter. Neither of the best selling jets (737 & A320) is commonly used for intercontinental flights. They are used for short haul flights.
A big and multi-faceted factor not considered here is airport capacity, in terms of terminal building footprint, aircraft ground handling, flight control and airspace.
Many large airports are already at capacity, so it's difficult to see how higher numbers of aircraft could work.
And that is of course before you consider VTOL.
Surely, if you took an existing jet engine and put some electric heaters in it would be very inefficient, but if you played to the strengths of electric heat, you could potentially make something interesting. For example, there are no potentially corrosive combustion products and in fact you don't even need to have oxygen flowing through it - it might as well be CO2 (hello martian electric jets).
The second benefit is that you can deliver electric heat into a closed system continuously without cooling it from fluid flow. It's more beneficial for furnaces and things, but maybe there's a design out there that can take advantage of it. Probably more relevant for some sort of closed cycle piston aircraft. You could reach higher internal temperatures as long as the materials can support it.
One could however make an electric jet engine. You "just" need to replace the fuel-based heating element with an electric-based one. Then the front compressor would be electric, and you'd have no use for the turbine, since its sole purpose in conventional jet engines is to extract energy from the combustion to power the compressor.
I have this recurring daydream of an electrodynamic jet engine. The inlet ionizes the airflow, the compressor is electromagnetic, the heating element inductive. Just one fancy-shaped tube with a bunch of complicated electronics around it. That would be really interesting from a whole lot of perspectives -- for example, noise reduction. It also would open up a lot more flexibility in terms of power density (which is more or less determined by the amount of heat you can dump into the engine, which is more or less limited by the melting point of the turbine blades).
Something else that's interesting about that idea is that it doesn't depend on oxygen in the air, which means you could use it on Mars and Venus. I've never looked into the numbers for Venus, but as I remember, the local speed of sound is a significant challenge for rotorcraft on Mars, so having a rotorless, purely-electric jet engine there could open up a lot of flexibility in terms of aircraft performance.