4 ms·

Even in rockets, where weight is extremely critical, SpaceX chose to use kerosene/methane due to the problems of working with LH2 (very low density, very cold t

by plat12 6y ago

Even in rockets, where weight is extremely critical, SpaceX chose to use kerosene/methane due to the problems of working with LH2 (very low density, very cold temperatures, hydrogen embrittlement). Depending on energy density and power density requirements, I think a combination of batteries and traditional fossil fuels will win in the short term. In the long term, when we have stopped burning fossil fuels for electricity, then we can use electricity to manufacture them. Under this path, hydrogen will have limited importance.

elcritch 6y ago
Preventing explosions in the event of H2 leaks is difficult too. H2 has one of the lowest ignition energy curves of any gas. Electronics used in areas with H2 must follow very strict regulations (worse than medical requirements in many ways).
- upofadown 6y ago
  The explosion happens when you cross the ignition energy threshold. So having a low ignition energy threshold can actually be an advantage as then much less fuel is available to drive the explosion. Agreed that doing intrinsic safety for H2 is hell. I did once design a LED sidelight for the relevant class with significant current (redundant electronic energy regulation). So it is possible to do stuff, it just takes more work. The tendency for modern electronics to be very low voltage helps a lot.
  - dredmorbius 6y ago
    More likely: A small amount of the leaked hydrogen encounters an ignition point, igniting the primary mass. The ignition zone is larger, risk is higher.
    - Retric 6y ago
      Pure hydrogen can’t burn without oxygen, so you generally get a tiny flare and then a flame based the size of the crack. It’s really about the same risks as using gasoline which almost never causes a significant detonation.
      - dredmorbius 6y ago
        With vehicle, heating, rocket, or aviation applications, quantities involved are nontrivial. Hydrogen's flammable and explosive ratios with oxygen are large, and contrast starkly with other fuels; kerosene (jet/rocket fuel), bunker oil, and deisel, which ignite with difficulty, petrol, which ignites readily but not explosively, and even natural gas which deflagrates rather than explodes under most circumstances. Add in hydrogen's extreme tendency to leak, metal embrittlement, and high pressures and/or low temperatures, and the risks are immense. Particularly at scale, in widespread use, with poor maintenance and inspections.
        Retric 6y ago
        It’s technically explosive in many situations where the location or net energy released makes that irrelevant. Being significantly lighter than air you might hear a loud bang above the vehicle where gasoline’s foof ends up being significantly more deadly. It’s not strictly better or worse, just different. Diesel for example can be very dangerous in a large open topped container, hydrogen just doesn’t stick around in that environment. PS: Consider what it would take to make a large hydrogen fuel air bomb that’s as effective as it’s hydrocarbon equivalent.
akira2501 6y ago
> Even in rockets, where weight is extremely critical, SpaceX chose to use kerosene/methane due to the problems of working with LH2 (very low density, very cold temperatures, hydrogen embrittlement). That and they have very few missions where the increased ISP from LH2 would actually benefit them. The other upside of LH2 and LOX is that you can run it in engines, but you can also run it in fuel cells directly and generate a surprising amount of electricity from a very small package -- the other benefit there is that the Fuel Cells produce pure water as a byproduct. The Space Shuttle had three fuel cells which provided more than enough power for a full mission. Most missions could be run on two without any compromises. Also, when it was visiting the ISS, it was a convenient means of supplying water to the station. On non ISS missions, so much water is generated that they have to dump it overboard.

by plat12 6y ago

elcritch 6y ago

Preventing explosions in the event of H2 leaks is difficult too. H2 has one of the lowest ignition energy curves of any gas. Electronics used in areas with H2 must follow very strict regulations (worse than medical requirements in many ways).

upofadown 6y ago
The explosion happens when you cross the ignition energy threshold. So having a low ignition energy threshold can actually be an advantage as then much less fuel is available to drive the explosion. Agreed that doing intrinsic safety for H2 is hell. I did once design a LED sidelight for the relevant class with significant current (redundant electronic energy regulation). So it is possible to do stuff, it just takes more work. The tendency for modern electronics to be very low voltage helps a lot.
- dredmorbius 6y ago
  More likely: A small amount of the leaked hydrogen encounters an ignition point, igniting the primary mass. The ignition zone is larger, risk is higher.
  - Retric 6y ago
    Pure hydrogen can’t burn without oxygen, so you generally get a tiny flare and then a flame based the size of the crack. It’s really about the same risks as using gasoline which almost never causes a significant detonation.
    - dredmorbius 6y ago
      With vehicle, heating, rocket, or aviation applications, quantities involved are nontrivial. Hydrogen's flammable and explosive ratios with oxygen are large, and contrast starkly with other fuels; kerosene (jet/rocket fuel), bunker oil, and deisel, which ignite with difficulty, petrol, which ignites readily but not explosively, and even natural gas which deflagrates rather than explodes under most circumstances. Add in hydrogen's extreme tendency to leak, metal embrittlement, and high pressures and/or low temperatures, and the risks are immense. Particularly at scale, in widespread use, with poor maintenance and inspections.
      - Retric 6y ago
        It’s technically explosive in many situations where the location or net energy released makes that irrelevant. Being significantly lighter than air you might hear a loud bang above the vehicle where gasoline’s foof ends up being significantly more deadly. It’s not strictly better or worse, just different. Diesel for example can be very dangerous in a large open topped container, hydrogen just doesn’t stick around in that environment. PS: Consider what it would take to make a large hydrogen fuel air bomb that’s as effective as it’s hydrocarbon equivalent.

akira2501 6y ago

> Even in rockets, where weight is extremely critical, SpaceX chose to use kerosene/methane due to the problems of working with LH2 (very low density, very cold temperatures, hydrogen embrittlement). That and they have very few missions where the increased ISP from LH2 would actually benefit them. The other upside of LH2 and LOX is that you can run it in engines, but you can also run it in fuel cells directly and generate a surprising amount of electricity from a very small package -- the other benefit there is that the Fuel Cells produce pure water as a byproduct. The Space Shuttle had three fuel cells which provided more than enough power for a full mission. Most missions could be run on two without any compromises. Also, when it was visiting the ISS, it was a convenient means of supplying water to the station. On non ISS missions, so much water is generated that they have to dump it overboard.