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I'm not so sure. For very long haul flights, I see hydrogen making sense. There are 2 things going for liquid hydrogen: the square-cube law and the rocket equa
by credit_guy 4y ago
I'm not so sure. For very long haul flights, I see hydrogen making sense.
There are 2 things going for liquid hydrogen: the square-cube law and the rocket equation.
The square-cube law: the mass of the hydrogen in the tank goes up with the cube of the size, while the heat loss rate by the square of the size. Which means for bigger tanks, the refrigeration overhead goes down.
The rocket equation: the actual equation doesn't matter, but the idea is that you need fuel to carry the plane, then you need fuel to carry the fuel, than fuel to carry that one, etc. It all converges, but the lowest the density energy of the fuel, the more fuel you need.
Hydrogen has an energy density 3 times as high as kerosene, by mass. By volume, it's about 8 times lower. But mass matters more than volume.
A long range airplane, like the Dreamliner, starts with a lot of fuel at takeoff, about 45% of its total mass. If you replace that fuel with a fuel that's 3 times as energy dense, then you don't need only 3 times less fuel, but about 3.7 times less fuel. Which means the takeoff weight gets reduced by about a third. Which means you can reduce the structural weight of the airplane by a lot. Maybe not a third, but by maybe 20%.
The compounding benefits of having a fuel that's 3 times more energy dense are considerable. Will they be enough to overcome the need for cryogenic tanks and much larger volume of fuel? I don't know, I think it's a close call. But it doesn't sound absolutely crazy to me.
- deleted 4y ago[deleted]
- janef0421 4y agoThe issue is that you're going from a fuel that produces net energy to a fuel that loses net energy. No amount of efficiency gains can overcome that.
- adgjlsfhk1 4y agowhat do you mean by "loses net energy"?
- carbonguy 4y agoPresumably that it costs more energy to produce hydrogen than you get back when you use it as a fuel, which is strictly true for all fuels; however, with fossil fuels, the energy cost of production has mostly been "paid" already.
- ZeroGravitas 4y agoBut it's still not true as stated. Well to Wheel analysis reliably shows that hydrogen FCEV are more energy efficient than ICE vehicles. (Battery EV are better again, because Hydrogen productiom does use some energy). So efficiency improvements make FCEVs more energy efficient overall, despite the Hydrogen production step replacing fossil fuels.
- janef0421 4y agoI don't think a "well-to-wheel" analysis is relevant here, for two reasons. First, because it involves electrical energy as an intermediate product, which could be converted to kinetic energy in ways that are orders of magnitude more efficient. Second, because I'm analysing this based on the amount of energy that is initially invested, not the total quantity of energy that exists in nature. EDITED because I said converted to electrical energy when I meant kinetic energy.
- jonititan 4y agoI'm actually on a project looking at this and it's not so rosy. My area is trying to predict maintenance costs for the hydrogen PEM fuel cells. The cells get contaminated pretty fast. Regarding hydrogen suitability while it has excellent gravimetric efficiency it has terrible volumetric efficiency. You either need huge tanks and high pressure or cryogenic tanks to store it in its liquid phase. Like anything else its doable we just need to accept higher airliner prices and slower speeds/shorter ranges.
- R0b0t1 4y agoThere are iirc substances like graphite(?) sponge that will take in more H2 at a given pressure. It's still high, but it's better. The tanks can be made long and thin and stacked. Is the extra tank material too much in either cost or mass?
- jonititan 4y agoI haven't looked at the various sponge or chemical binding concepts personally. To be used in an aircraft to be delivered say in 10 years time they would already need to have been approved by EASA or FAA or have a clear and straightforward approval pathway. Otherwise they represent a large schedule risk and new airliner types cost several billion to launch so they are pretty risk averse. That's not to say those concepts won't work. Regarding long thin tanks wings don't actually have that much suitable internal volume for that. Lots of ribs, stringers, etc. That's why most hydrogen aircraft concepts use under wing pods or fuselage tanks to store the hydrogen even though it reduces safety margin. Civil aircraft regulators are allergic to storing fuel in the fuselage near the passengers for good reason.
- R0b0t1 4y agoThe containers for the hydrogen can be made to vent safely for a lot of impacts. Fiberglass, carbon fiber, or other composites.
- credit_guy 4y ago> You either need huge tanks and high pressure or cryogenic tanks According to Airbus all 3 demonstrator planes will have liquid hydrogen [1]. As for the fuel cells, they will basically not be used. The engines will be regular engines that burn hydrogen instead of jet fuel. Fuel cells will only be used for the electricity needs of the airplane, but not for moving the airplane through the air. They are probably going to use less than 1% of the liquid hydrogen. [1] https://www.airbus.com/en/innovation/zero-emission/hydrogen/zeroe https://www.airbus.com/en/innovation/zero-emission/hydrogen/...