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The hydrogen conversion of the Cessna Caravan seems a little more practical. Hydrogen is bulky, but light. Fuel cells aren't cheap though. However, hydrogen

The hydrogen conversion of the Cessna Caravan seems a little more practical. Hydrogen is bulky, but light. Fuel cells aren't cheap though. However, hydrogen is quick to refuel unlike battery charging. I think the weight to power is better with hydrogen than li-ion and weight is the most important thing for aviation. https://www.militaryaerospace.com/commercial-aerospace/article/14283551/textron-aviation-taps-zeroavia-to-develop-hydrogenelectric-powertrain-for-its-cessna-grand-caravan https://www.militaryaerospace.com/commercial-aerospace/artic...

joe_the_user 4y ago
If you can switch battery packs, refueling could be very quick.
- mikepurvis 4y ago
  And even though they're heavy and bulky, it's much more tractable to install that infrastructure at a few dozen key airports, and also to manage the tracing/ownership of "fleets" of batteries. Battery swapping has repeatedly faceplanted when it comes to cars, but it's a pretty different scope of challenge, and it seems like every major issue is more favourable to aircraft. On the other hand, planes are often on the ground for 2+ hrs between flights anyway, so maybe it could be realistic with enough power delivery capability to just charge a big pack in situ. Certainly simpler to plug in a big umbilical at the gate than having to have another ground vehicle reaching into the belly of the thing.
  - _hypx 4y ago
    Absolutely not. Airplanes have to be aerodynamic, and it would be incredibly challenging to make the entire bottom portion of the plane something that can open up and fit a giant battery. Never mind the weight of the whole system plus battery. The whole idea is absurd and completely impractical.
  - nradov 4y ago
    Batteries power will only be feasible for short-haul flights, and those have much shorter turn around times. Southwest Airlines has it down to 35 minutes. Recharging within that time limit might eventually become feasible but it's going to take some further battery innovations.
mattmaroon 4y ago
I’ve been wondering also why fuel cells aren’t used. Hydrogen is less energy dense than fossil fuels in terms of energy per unit of volume, but much more dense in terms of weight (almost 3) and far far more than batteries, which matters more in aviation. It seems like an obvious choice to me.
- sacrosancty 4y ago
  Squared-cubed law for tank weight and fuel weight. So hydrogen will have poor energy density at small scales. Not sure where it dips below that of batteries but rockets are OK with it, so smaller than that. Perhaps if you store it in a balloon? But then volume would bed a problem for an aeroplane.
  - mattmaroon 4y ago
    The square cubed law argues in favor of something that is more energy dense by weight but less by volume right? You make the wings a little bigger and the surface area increases a lot less than the volume. There’s also the potential of enlarging the fuselage and storing some there too.
    - sacrosancty 4y ago
      It's about the pressure vessel's weight. That goes up with the square of the linear dimension while the quantity of fuel goes with that cubed. I don't think finding space to fit it is the problem. In the case of normal fuel, there's no pressure vessel so it can be much lighter.
      - pfdietz 4y ago
        In a tank dominated by pressure loads, wall thickness increases in proportion to the linear dimensions of the tank, so no, tank mass is proportional to volume.
        sacrosancty 4y ago
VBprogrammer 4y ago
Hydrogen is light but tanks capable of 10,000psi don't tend to be. Nor particularly are cryogenic tanks but at least that improves the situation volume problem by an order of magnitude.
stareatgoats 4y ago
Hydrogen will likely win out ultimately, once they have figured out which high density solid-state hydrogen storage solution is the most viable for aviation. It's not going to be the most energy efficient (many big losses in the process of converting to water to solid-state hydrogen), but it is vastly safer than the alternatives and will likely at least enable long range green air travel, at long last.
- coderenegade 4y ago
  Solid state hydrogen storage typically only gets you to a maximum of 20% hydrogen by weight, which is equivalent to ammonia. Ammonia also lets you liberate all of the hydrogen, whereas hydrides don't. If you want to use hydrogen for aviation, LH2 is really the best answer, since the percentage hydrogen by weight is much higher. The next best thing would probably be an sofc using hydrocarbon fuel, if you really wanted to go electric, but still needed endurance.
chroma 4y ago
Hydrogen fuel cells won't happen. There are too many downsides. Hydrogen must be stored as a compressed gas or as a deep cryogenic liquid. Either way such tanks are expensive, heavy, and create safety concerns. Hydrogen is a very pernicious molecule, and leaks cannot be detected by human senses. All hydrogen vehicles and fueling stations need special sensors to detect leaks. Hydrogen is much easier to ignite than gasoline. Any concentration from 4-74% will explode in air, and the flame is nearly invisible. Refueling a hydrogen tank involves going from high pressure to low pressure, which causes the fuel line and nozzle to get extremely cold. Even in southern California, refueling a few Toyota Mirais causes the nozzle to freeze to the valve. This limits refueling speeds and duty cycles. Lastly, hydrogen is far less efficient as an energy storage medium. With a battery, you put electricity in and get electricity out. It's 80-90% efficient. With hydrogen, you use electricity to split water, then compress and liquefy the hydrogen, then run it through a fuel cell. The fuel cell itself is 40-60% efficient. At the end of the whole process, around 30% of your initial electricity comes out of the fuel cell.
- _hypx 4y ago
  Long-distance battery airplanes are literally impossible. If you don't agree with hydrogen powered aircraft, you are advocating for conventional airplanes forever.
  - jamiek88 4y ago
    That’s ridiculously short sighted to think out only options are hydrogen or the status quo. Off the top of my head I can think of biofuels plus carbon neutral recaptured co2 from excess renewable energy. Diesel electric motors for example with the ‘diesel’ being carbon neutral biofuel. This is actually being tested on large airbus jets currently. Hydrogen is one of the worst possible options for aircraft. Energy density is crap, tankage is heavy, failure modes catastrophic.
    - _hypx 4y ago
      Which would basically be conventional aircraft but running on biofuels or syn-fuels.
senectus1 4y ago
why not Ammonia? Ammonia has a higher energy density, at 12.7 MJ/L, than even liquid hydrogen, at 8.5 MJ/L. Liquid hydrogen has to be stored at cryogenic conditions of –253 °C, whereas ammonia can be stored at a much less energy-intensive –33 °C. And ammonia, though hazardous to handle, is much less flammable than hydrogen.
- jsjohnst 4y ago
  You sure of those numbers? While this [0] compares liquid ammonia to pressurized hydrogen rather than liquid hydrogen, I doubt the energy density would drop that much to give up the advantage of hydrogen over ammonia. [0] https://www.researchgate.net/publication/347258594_Operation_of_a_Solid_Oxide_Fuel_Cell_Based_Power_System_with_Ammonia_as_a_Fuel_Experimental_Test_and_System_Design/fulltext/609d6c3a458515c2658c9e30/Operation-of-a-Solid-Oxide-Fuel-Cell-Based-Power-System-with-Ammonia-as-a-Fuel-Experimental-Test-and-System-Design.pdf?origin=publication_detail https://www.researchgate.net/publication/347258594_Operation...
conradev 4y ago
When you get to the size of airliners, though – so much money, time and effort has gone into designing, producing and operating the existing airliner fleet that it makes sense to invest in biofuels rather than completely new airliners: https://en.wikipedia.org/wiki/Aviation_biofuel#Economics https://en.wikipedia.org/wiki/Aviation_biofuel#Economics

joe_the_user 4y ago

If you can switch battery packs, refueling could be very quick.

mikepurvis 4y ago
And even though they're heavy and bulky, it's much more tractable to install that infrastructure at a few dozen key airports, and also to manage the tracing/ownership of "fleets" of batteries. Battery swapping has repeatedly faceplanted when it comes to cars, but it's a pretty different scope of challenge, and it seems like every major issue is more favourable to aircraft. On the other hand, planes are often on the ground for 2+ hrs between flights anyway, so maybe it could be realistic with enough power delivery capability to just charge a big pack in situ. Certainly simpler to plug in a big umbilical at the gate than having to have another ground vehicle reaching into the belly of the thing.
- _hypx 4y ago
  Absolutely not. Airplanes have to be aerodynamic, and it would be incredibly challenging to make the entire bottom portion of the plane something that can open up and fit a giant battery. Never mind the weight of the whole system plus battery. The whole idea is absurd and completely impractical.
- nradov 4y ago
  Batteries power will only be feasible for short-haul flights, and those have much shorter turn around times. Southwest Airlines has it down to 35 minutes. Recharging within that time limit might eventually become feasible but it's going to take some further battery innovations.

mattmaroon 4y ago

I’ve been wondering also why fuel cells aren’t used. Hydrogen is less energy dense than fossil fuels in terms of energy per unit of volume, but much more dense in terms of weight (almost 3) and far far more than batteries, which matters more in aviation. It seems like an obvious choice to me.

sacrosancty 4y ago
Squared-cubed law for tank weight and fuel weight. So hydrogen will have poor energy density at small scales. Not sure where it dips below that of batteries but rockets are OK with it, so smaller than that. Perhaps if you store it in a balloon? But then volume would bed a problem for an aeroplane.
- mattmaroon 4y ago
  The square cubed law argues in favor of something that is more energy dense by weight but less by volume right? You make the wings a little bigger and the surface area increases a lot less than the volume. There’s also the potential of enlarging the fuselage and storing some there too.
  - sacrosancty 4y ago
    It's about the pressure vessel's weight. That goes up with the square of the linear dimension while the quantity of fuel goes with that cubed. I don't think finding space to fit it is the problem. In the case of normal fuel, there's no pressure vessel so it can be much lighter.
    - pfdietz 4y ago
      In a tank dominated by pressure loads, wall thickness increases in proportion to the linear dimensions of the tank, so no, tank mass is proportional to volume.
      - sacrosancty 4y ago

VBprogrammer 4y ago

Hydrogen is light but tanks capable of 10,000psi don't tend to be. Nor particularly are cryogenic tanks but at least that improves the situation volume problem by an order of magnitude.

stareatgoats 4y ago

Hydrogen will likely win out ultimately, once they have figured out which high density solid-state hydrogen storage solution is the most viable for aviation. It's not going to be the most energy efficient (many big losses in the process of converting to water to solid-state hydrogen), but it is vastly safer than the alternatives and will likely at least enable long range green air travel, at long last.

coderenegade 4y ago
Solid state hydrogen storage typically only gets you to a maximum of 20% hydrogen by weight, which is equivalent to ammonia. Ammonia also lets you liberate all of the hydrogen, whereas hydrides don't. If you want to use hydrogen for aviation, LH2 is really the best answer, since the percentage hydrogen by weight is much higher. The next best thing would probably be an sofc using hydrocarbon fuel, if you really wanted to go electric, but still needed endurance.

chroma 4y ago

Hydrogen fuel cells won't happen. There are too many downsides. Hydrogen must be stored as a compressed gas or as a deep cryogenic liquid. Either way such tanks are expensive, heavy, and create safety concerns. Hydrogen is a very pernicious molecule, and leaks cannot be detected by human senses. All hydrogen vehicles and fueling stations need special sensors to detect leaks. Hydrogen is much easier to ignite than gasoline. Any concentration from 4-74% will explode in air, and the flame is nearly invisible. Refueling a hydrogen tank involves going from high pressure to low pressure, which causes the fuel line and nozzle to get extremely cold. Even in southern California, refueling a few Toyota Mirais causes the nozzle to freeze to the valve. This limits refueling speeds and duty cycles. Lastly, hydrogen is far less efficient as an energy storage medium. With a battery, you put electricity in and get electricity out. It's 80-90% efficient. With hydrogen, you use electricity to split water, then compress and liquefy the hydrogen, then run it through a fuel cell. The fuel cell itself is 40-60% efficient. At the end of the whole process, around 30% of your initial electricity comes out of the fuel cell.

_hypx 4y ago
Long-distance battery airplanes are literally impossible. If you don't agree with hydrogen powered aircraft, you are advocating for conventional airplanes forever.
- jamiek88 4y ago
  That’s ridiculously short sighted to think out only options are hydrogen or the status quo. Off the top of my head I can think of biofuels plus carbon neutral recaptured co2 from excess renewable energy. Diesel electric motors for example with the ‘diesel’ being carbon neutral biofuel. This is actually being tested on large airbus jets currently. Hydrogen is one of the worst possible options for aircraft. Energy density is crap, tankage is heavy, failure modes catastrophic.
  - _hypx 4y ago
    Which would basically be conventional aircraft but running on biofuels or syn-fuels.

senectus1 4y ago

why not Ammonia? Ammonia has a higher energy density, at 12.7 MJ/L, than even liquid hydrogen, at 8.5 MJ/L. Liquid hydrogen has to be stored at cryogenic conditions of –253 °C, whereas ammonia can be stored at a much less energy-intensive –33 °C. And ammonia, though hazardous to handle, is much less flammable than hydrogen.

jsjohnst 4y ago
You sure of those numbers? While this [0] compares liquid ammonia to pressurized hydrogen rather than liquid hydrogen, I doubt the energy density would drop that much to give up the advantage of hydrogen over ammonia. [0] https://www.researchgate.net/publication/347258594_Operation_of_a_Solid_Oxide_Fuel_Cell_Based_Power_System_with_Ammonia_as_a_Fuel_Experimental_Test_and_System_Design/fulltext/609d6c3a458515c2658c9e30/Operation-of-a-Solid-Oxide-Fuel-Cell-Based-Power-System-with-Ammonia-as-a-Fuel-Experimental-Test-and-System-Design.pdf?origin=publication_detail https://www.researchgate.net/publication/347258594_Operation...

conradev 4y ago

When you get to the size of airliners, though – so much money, time and effort has gone into designing, producing and operating the existing airliner fleet that it makes sense to invest in biofuels rather than completely new airliners: https://en.wikipedia.org/wiki/Aviation_biofuel#Economics https://en.wikipedia.org/wiki/Aviation_biofuel#Economics