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I don't understand why hydrogen fuel cells are not investigated for electric planes - it's a mature enough technology that you can get it in consumer cars, it d

I don't understand why hydrogen fuel cells are not investigated for electric planes - it's a mature enough technology that you can get it in consumer cars, it doesn't suffer from the same weight penalty of batteries. If you put the powertrain of a hydrogen powered Honda Clarity in a Cessna 172, I'm sure it would work.

acdha 5y ago
They have been experimented with for decades (I remember reading about 1970s prototypes as a kid in the 80s) but I believe it's stalled on energy density, very similar to batteries. In aircraft, the big factor is storage: if you had a robust storage system you could get more power from burning the hydrogen in a turbine but hydrogen is hard to store safely without very heavy containers.
- torginus 5y ago
  I remember the Soviets converted a plane for hydrogen combustion: https://en.wikipedia.org/wiki/Tupolev_Tu-155 https://en.wikipedia.org/wiki/Tupolev_Tu-155 But I feel that's a bit of a distraction - turbines must use thermodynamic cycles to convert fuel into thrust - something whose maximum efficiency is less than the real world efficiency of a hydrogen fuel cell. I feel like hydrogen turbofans are not practical for small craft for a number of reasons - cost, maintenance, noise, limited start/stop cycles etc., while electrics have the ability to scale to every aircraft category.
  - baybal2 5y ago
    The maximum efficiency hydrogen to electricity conversion is around on around 50% for real world fuel cells. This is around efficiency of GE90. Your idea will not work. Hydrogen storage, cells, and motors will weights many times the jet engine, or a plane itself
- ncmncm 5y ago
  An LH2 tank hardly weighs anything. They even put them in rockets. But they take up more room than would fit in the wings. They probably would need to be in nacelles slung under the wings like the engines.
  - acdha 5y ago
    I'm aware that rockets have them but we have much stronger safety standards for passenger airplanes than we do for rockets. What I've heard is that it's very hard to build a hydrogen tank which is both lightweight and safe enough in the event of a crash or other incident to meet general civilian aviation standards. I'd love to be wrong on that but given the obvious appeal there must be some thorny engineering challenges for an idea which has been batted around since the 1950s.
    - ncmncm 5y ago
      You are hearing confused echoes. The high-pressure hydrogen tanks needed for H2-fueled cars and trucks are difficult, in a place where using LH2 instead would be wholly impractical. LH2 tanks of a size appropriate for transport aircraft are trivial: thin-walled aluminum shrouded in insulation. LH2 for transport aircraft suffers none what would make it impractical for cars: it is always used immediately, and would be synthesized on demand right at the airport. Risk of exploding leaks is easily eliminated by positive ventilation; if it can't build up, it can't ignite; if it doesn't ignite, it can't explode. In nacelles attached to the wing, alongside the engines, any extra safety concerns would be moot. Kerosene fuel is carried in wing tanks so that the wings shear off in a very bad landing and take the fuel away with them. A whole separate tank shears off much more easily than a wing.
adgjlsfhk1 5y ago
it doesn't suffer a weight penalty, but it does suffer a volume penalty. it's also not that light since you need a lot of metal to keep hydrogen at the type of pressure you need.
- torginus 5y ago
  I did a bit of googling at it seems that the 2016 Honda Clarity has a 141L tank, that gives it a range of 366 miles, while the Tesla Model 3 Standard Range Plus has a 334L battery, which gives it a range of 267 miles. So even if my numbers are off by a bit, the volume of hydrogen tanks is not a prohibitive factor.
hannob 5y ago
They are: https://www.airbus.com/en/innovation/zero-emission/hydrogen/zeroe https://www.airbus.com/en/innovation/zero-emission/hydrogen/... The thing is - all carbon free options for aviation come with their downsides. Producing hydrogen creates a lot more conversion losses than using batteries, so when you can get along with a battery you will. Hydrogen will likely be used in the mid-range sector though.
- torginus 5y ago
  True - but hydrogen is still immensely cleaner than the jet fuel powered planes we have today. And for aircraft the weight savings could lead to the aircraft carrying more useful cargo, which would probably offset the efficiency loss. Edit: The link you provided describes hydrogen-combustion engines, not fuel-cell airplanes, which is also a promising concept, but not what I'm proposing.
  - nradov 5y ago
    That's generally not true in practice today. Most hydrogen is produced from fossil fuels, so in most respects it's not any cleaner than just burning the fuel directly. In theory it is possible to produce hydrogen from clean energy without using fossil fuels. But we're a long way from doing that at scale. There is no real weight savings from hydrogen fuel once you account for the tankage, plus the more powerful engines needed to overcome the additional drag from those larger tanks.
    - torginus 5y ago
      "No weight savings" - In a reply to another commenter, if we compare the tank of the Honda Clarity FCEV with the Model 3s battery, it seems that hydrogen is more than 3x more as wrt. space for the same energy output as batteries - so if they can fit those, these will fit as well.
    - ncmncm 5y ago
      LH2 tanks hardly weigh anything. You must be talking about compressed gas, which of course would be obviously useless for aviation. So when anyone mentions hydrogen in connection with aviation, they do not mean compressed gas.

acdha 5y ago

They have been experimented with for decades (I remember reading about 1970s prototypes as a kid in the 80s) but I believe it's stalled on energy density, very similar to batteries. In aircraft, the big factor is storage: if you had a robust storage system you could get more power from burning the hydrogen in a turbine but hydrogen is hard to store safely without very heavy containers.

torginus 5y ago
I remember the Soviets converted a plane for hydrogen combustion: https://en.wikipedia.org/wiki/Tupolev_Tu-155 https://en.wikipedia.org/wiki/Tupolev_Tu-155 But I feel that's a bit of a distraction - turbines must use thermodynamic cycles to convert fuel into thrust - something whose maximum efficiency is less than the real world efficiency of a hydrogen fuel cell. I feel like hydrogen turbofans are not practical for small craft for a number of reasons - cost, maintenance, noise, limited start/stop cycles etc., while electrics have the ability to scale to every aircraft category.
- baybal2 5y ago
  The maximum efficiency hydrogen to electricity conversion is around on around 50% for real world fuel cells. This is around efficiency of GE90. Your idea will not work. Hydrogen storage, cells, and motors will weights many times the jet engine, or a plane itself
ncmncm 5y ago
An LH2 tank hardly weighs anything. They even put them in rockets. But they take up more room than would fit in the wings. They probably would need to be in nacelles slung under the wings like the engines.
- acdha 5y ago
  I'm aware that rockets have them but we have much stronger safety standards for passenger airplanes than we do for rockets. What I've heard is that it's very hard to build a hydrogen tank which is both lightweight and safe enough in the event of a crash or other incident to meet general civilian aviation standards. I'd love to be wrong on that but given the obvious appeal there must be some thorny engineering challenges for an idea which has been batted around since the 1950s.
  - ncmncm 5y ago
    You are hearing confused echoes. The high-pressure hydrogen tanks needed for H2-fueled cars and trucks are difficult, in a place where using LH2 instead would be wholly impractical. LH2 tanks of a size appropriate for transport aircraft are trivial: thin-walled aluminum shrouded in insulation. LH2 for transport aircraft suffers none what would make it impractical for cars: it is always used immediately, and would be synthesized on demand right at the airport. Risk of exploding leaks is easily eliminated by positive ventilation; if it can't build up, it can't ignite; if it doesn't ignite, it can't explode. In nacelles attached to the wing, alongside the engines, any extra safety concerns would be moot. Kerosene fuel is carried in wing tanks so that the wings shear off in a very bad landing and take the fuel away with them. A whole separate tank shears off much more easily than a wing.

adgjlsfhk1 5y ago

it doesn't suffer a weight penalty, but it does suffer a volume penalty. it's also not that light since you need a lot of metal to keep hydrogen at the type of pressure you need.

torginus 5y ago
I did a bit of googling at it seems that the 2016 Honda Clarity has a 141L tank, that gives it a range of 366 miles, while the Tesla Model 3 Standard Range Plus has a 334L battery, which gives it a range of 267 miles. So even if my numbers are off by a bit, the volume of hydrogen tanks is not a prohibitive factor.

hannob 5y ago

They are: https://www.airbus.com/en/innovation/zero-emission/hydrogen/zeroe https://www.airbus.com/en/innovation/zero-emission/hydrogen/... The thing is - all carbon free options for aviation come with their downsides. Producing hydrogen creates a lot more conversion losses than using batteries, so when you can get along with a battery you will. Hydrogen will likely be used in the mid-range sector though.

torginus 5y ago
True - but hydrogen is still immensely cleaner than the jet fuel powered planes we have today. And for aircraft the weight savings could lead to the aircraft carrying more useful cargo, which would probably offset the efficiency loss. Edit: The link you provided describes hydrogen-combustion engines, not fuel-cell airplanes, which is also a promising concept, but not what I'm proposing.
- nradov 5y ago
  That's generally not true in practice today. Most hydrogen is produced from fossil fuels, so in most respects it's not any cleaner than just burning the fuel directly. In theory it is possible to produce hydrogen from clean energy without using fossil fuels. But we're a long way from doing that at scale. There is no real weight savings from hydrogen fuel once you account for the tankage, plus the more powerful engines needed to overcome the additional drag from those larger tanks.
  - torginus 5y ago
    "No weight savings" - In a reply to another commenter, if we compare the tank of the Honda Clarity FCEV with the Model 3s battery, it seems that hydrogen is more than 3x more as wrt. space for the same energy output as batteries - so if they can fit those, these will fit as well.
  - ncmncm 5y ago
    LH2 tanks hardly weigh anything. You must be talking about compressed gas, which of course would be obviously useless for aviation. So when anyone mentions hydrogen in connection with aviation, they do not mean compressed gas.