5 ms·

>It doesn't seem like current fuels cell have much of a energy density / range advantage even though hydrogen itself is much more energy dense, the fuel cell we

>It doesn't seem like current fuels cell have much of a energy density / range advantage even though hydrogen itself is much more energy dense, the fuel cell weight and efficiency and high pressure liquid hydrogen tanks must be accounted for. Hydrogen has a better gravimetric scaling than batteries - as stored energy increases, the fixed weight of the fuel cell is amortised over more energy and the weight of high pressure tanks goes up with their surface area while the energy stores goes up with volume. So for applications where you want to store a lot of energy and are weight but not volume limited, hydrogen might make enough sense to pay the efficiency penalty required to turn electricity into hydrogen and back again. That's why it makes more sense for big trucks than it does for passenger cars where volume limitations matter a lot more and where total energy is less.

eternauta3k 5y ago
> the weight of high pressure tanks goes up with their surface area while the energy stores goes up with volume. Nope, tank mass scales with volume because bigger tanks need thicker walls: https://en.wikipedia.org/wiki/Pressure_vessel#Scaling https://en.wikipedia.org/wiki/Pressure_vessel#Scaling
- abakker 5y ago
  With hydrogen this is especially problematic due to hydrogen embrittlement. https://en.wikipedia.org/wiki/Hydrogen_embrittlement https://en.wikipedia.org/wiki/Hydrogen_embrittlement
  - semi-extrinsic 5y ago
    Nope, because you don't make hydrogen tanks out of materials that are susceptible to hydrogen embrittlement. In particular, pressurized automotive-size hydrogen tanks aren't made from metal at all, they are typically epoxy/kevlar/carbon fiber composites.
    - abakker 5y ago
      OOC, since you know what these tanks are made out of, what is the duty cycle/lifespan of those tanks? I was presented some info on Kevlar tanks for drones using fuel cells and remember thinking that the lifespan was very short.
      - samplatt 5y ago
        Anything bound by epoxy (or any polymer?) surely will have a comparably short lifespan...
      - semi-extrinsic 5y ago
        Toyota certify and guarantee their tanks for 15 years, in production cars that you can go out and buy today.
- Mvandenbergh 5y ago
  Right you are!
SigmundA 5y ago
Lithium batteries still seem to have a way to go in energy density currently at around 250-300 wh/kg and going up year over year with a theoretical maximum (Sulfur cathode) of above 2500 wh/kg. At around 2000 wh/kg you are equivalent to diesel drivetrain energy density both size and weight based on my rough math. Expensive large hydrogen tanks in a trailer with high pressure lines to tractor seems like it would cause safety and cost issues offsetting much of the benefit.
- emptyfile 5y ago
  I have no clue about batteries or hydrogen tanks. But I'm always amused to think that posters on HN give more credence to their back-of-the-napkin calculations over opinions of, say, folks at Toshiba, Volvo or Daimler.
  - daniellarusso 5y ago
    Are these humans mutually exclusive?
  - SigmundA 5y ago
    Yes I always like to do my own math it has sniffed out the BS many a time from the likes of Tesla, Nikola and so on. Not always right, but amazing how many times theirs is wrong especially if marketing something. You should alway try to understand the math / science and make your own judgements rather to falling for arguments from authority. This isn't always feasible but I think most people can do enough to weed out the snake oil to a much better degree than we seem to be doing as a society now. Note this will also make you a better engineer / programmer in my experience.
    - nick_kline 5y ago
      Do you see Tesla as a snakeoil company today when it comes to their batteries, drivetrain, range, reliability? I wouldn't put Nikola and Tesla in the same categories. Tesla has one serious issue imho that they can only fix with refunds of the cost, that their auto-driving cars probably won't get there in the next 5+ years, so all the people that paid up to 10k for that will have to get some compensation.
robertlagrant 5y ago
> So for applications where you want to store a lot of energy and are weight but not volume limited, hydrogen might make enough sense to pay the efficiency penalty required to turn electricity into hydrogen and back again. It's also not inconceivable that if we have more base load power generation (e.g. through more nuclear) then we could generate hydrogen at night with the excess.
semi-extrinsic 5y ago
For trucks and other heavy duty applications, liquid hydrogen starts to win out over pressurized. In LH2 tanks, the scaling becomes advantageous as you are optimizing against heat ingress, which goes down as 1/R when you increase the radius R.
- SigmundA 5y ago
  Double walled high vacuum tanks that vent hydrogen gas if left unused for less than day and empty themselves after 10 days. Liquid hydrogen at -253c that is more energy intensive to make than high pressure. Not sure I see that being practical.
  - semi-extrinsic 5y ago
    This information is just wrong. Air Liquide built automotive size LH2 tanks using multilayer insulation in the early 2000s that had 3 days of autonomy (time before first venting) and 3% per day boil-off after that. For trucks where you need bigger tanks, both autonomy and boil-off rate improve by themselves due to the square-cube-law. And you can use the boil-off to charge the batteries in your hybrid electric powertrain, run auxiliary systems etc. The LH2 trucks currently under planning have sophisticated fuel management systems where the driver inputs where she will stop and for how long, and the truck will optimize LH2 consumption and battery SoC before arriving at the stop point. As for LH2 versus compressed: do you actually know how energy demanding a hydrogen compression train is? Compressing a low molecular weight gas to 700 bar is just fundamentally inefficient. At the scale required for mass adoption of hydrogen fuels, both compression and liquefaction are talking 6-8 kWh/kg. For comparison the energy content in hydrogen itself is 33 kWh/kg. So naturally there is great interest in recovering as much as possible of that work before consumption.
    - SigmundA 5y ago
      This is the BMW tank I got those figures from which looks to be manufactured by Air Liquide [1]: "To stay a liquid, hydrogen must be cooled and maintained at cryogenic temperatures of, at warmest, −253 °C (20.1 K; −423.4 °F). When not using fuel, the Hydrogen 7’s hydrogen tank starts to warm and the hydrogen starts to vaporize. Once the tank’s internal pressure reaches 87 psi, at roughly 17 hours of non-use, the tank will safely vent the building pressure. Over 10–12 days, it will completely lose the contents of the tank because of this" At that temperature even sloshing around adds enough energy to heat and cause venting much sooner another reason its not great for mobile use. Looks like Air Liquide made a different one that you are quoting perhaps an improvement, still high vacuum double walled, so you lose all your fuel in a month instead, thats progress. This shows liquefaction to be about twice as energy intensive as compression (1.3 vs 3.4 kWh/kg theoretical 6 vs 13 kWh/kg actual) [2] So please point me to better info. 1. https://en.wikipedia.org/wiki/BMW_Hydrogen_7 https://en.wikipedia.org/wiki/BMW_Hydrogen_7 2. https://www.hydrogen.energy.gov/pdfs/9013_energy_requirements_for_hydrogen_gas_compression.pdf https://www.hydrogen.energy.gov/pdfs/9013_energy_requirement...

eternauta3k 5y ago

> the weight of high pressure tanks goes up with their surface area while the energy stores goes up with volume. Nope, tank mass scales with volume because bigger tanks need thicker walls: https://en.wikipedia.org/wiki/Pressure_vessel#Scaling https://en.wikipedia.org/wiki/Pressure_vessel#Scaling

abakker 5y ago
With hydrogen this is especially problematic due to hydrogen embrittlement. https://en.wikipedia.org/wiki/Hydrogen_embrittlement https://en.wikipedia.org/wiki/Hydrogen_embrittlement
- semi-extrinsic 5y ago
  Nope, because you don't make hydrogen tanks out of materials that are susceptible to hydrogen embrittlement. In particular, pressurized automotive-size hydrogen tanks aren't made from metal at all, they are typically epoxy/kevlar/carbon fiber composites.
  - abakker 5y ago
    OOC, since you know what these tanks are made out of, what is the duty cycle/lifespan of those tanks? I was presented some info on Kevlar tanks for drones using fuel cells and remember thinking that the lifespan was very short.
    - samplatt 5y ago
      Anything bound by epoxy (or any polymer?) surely will have a comparably short lifespan...
    - semi-extrinsic 5y ago
      Toyota certify and guarantee their tanks for 15 years, in production cars that you can go out and buy today.
Mvandenbergh 5y ago
Right you are!

SigmundA 5y ago

Lithium batteries still seem to have a way to go in energy density currently at around 250-300 wh/kg and going up year over year with a theoretical maximum (Sulfur cathode) of above 2500 wh/kg. At around 2000 wh/kg you are equivalent to diesel drivetrain energy density both size and weight based on my rough math. Expensive large hydrogen tanks in a trailer with high pressure lines to tractor seems like it would cause safety and cost issues offsetting much of the benefit.

emptyfile 5y ago
I have no clue about batteries or hydrogen tanks. But I'm always amused to think that posters on HN give more credence to their back-of-the-napkin calculations over opinions of, say, folks at Toshiba, Volvo or Daimler.
- daniellarusso 5y ago
  Are these humans mutually exclusive?
- SigmundA 5y ago
  Yes I always like to do my own math it has sniffed out the BS many a time from the likes of Tesla, Nikola and so on. Not always right, but amazing how many times theirs is wrong especially if marketing something. You should alway try to understand the math / science and make your own judgements rather to falling for arguments from authority. This isn't always feasible but I think most people can do enough to weed out the snake oil to a much better degree than we seem to be doing as a society now. Note this will also make you a better engineer / programmer in my experience.
  - nick_kline 5y ago
    Do you see Tesla as a snakeoil company today when it comes to their batteries, drivetrain, range, reliability? I wouldn't put Nikola and Tesla in the same categories. Tesla has one serious issue imho that they can only fix with refunds of the cost, that their auto-driving cars probably won't get there in the next 5+ years, so all the people that paid up to 10k for that will have to get some compensation.

robertlagrant 5y ago

> So for applications where you want to store a lot of energy and are weight but not volume limited, hydrogen might make enough sense to pay the efficiency penalty required to turn electricity into hydrogen and back again. It's also not inconceivable that if we have more base load power generation (e.g. through more nuclear) then we could generate hydrogen at night with the excess.

semi-extrinsic 5y ago

For trucks and other heavy duty applications, liquid hydrogen starts to win out over pressurized. In LH2 tanks, the scaling becomes advantageous as you are optimizing against heat ingress, which goes down as 1/R when you increase the radius R.

SigmundA 5y ago
Double walled high vacuum tanks that vent hydrogen gas if left unused for less than day and empty themselves after 10 days. Liquid hydrogen at -253c that is more energy intensive to make than high pressure. Not sure I see that being practical.
- semi-extrinsic 5y ago
  This information is just wrong. Air Liquide built automotive size LH2 tanks using multilayer insulation in the early 2000s that had 3 days of autonomy (time before first venting) and 3% per day boil-off after that. For trucks where you need bigger tanks, both autonomy and boil-off rate improve by themselves due to the square-cube-law. And you can use the boil-off to charge the batteries in your hybrid electric powertrain, run auxiliary systems etc. The LH2 trucks currently under planning have sophisticated fuel management systems where the driver inputs where she will stop and for how long, and the truck will optimize LH2 consumption and battery SoC before arriving at the stop point. As for LH2 versus compressed: do you actually know how energy demanding a hydrogen compression train is? Compressing a low molecular weight gas to 700 bar is just fundamentally inefficient. At the scale required for mass adoption of hydrogen fuels, both compression and liquefaction are talking 6-8 kWh/kg. For comparison the energy content in hydrogen itself is 33 kWh/kg. So naturally there is great interest in recovering as much as possible of that work before consumption.
  - SigmundA 5y ago
    This is the BMW tank I got those figures from which looks to be manufactured by Air Liquide [1]: "To stay a liquid, hydrogen must be cooled and maintained at cryogenic temperatures of, at warmest, −253 °C (20.1 K; −423.4 °F). When not using fuel, the Hydrogen 7’s hydrogen tank starts to warm and the hydrogen starts to vaporize. Once the tank’s internal pressure reaches 87 psi, at roughly 17 hours of non-use, the tank will safely vent the building pressure. Over 10–12 days, it will completely lose the contents of the tank because of this" At that temperature even sloshing around adds enough energy to heat and cause venting much sooner another reason its not great for mobile use. Looks like Air Liquide made a different one that you are quoting perhaps an improvement, still high vacuum double walled, so you lose all your fuel in a month instead, thats progress. This shows liquefaction to be about twice as energy intensive as compression (1.3 vs 3.4 kWh/kg theoretical 6 vs 13 kWh/kg actual) [2] So please point me to better info. 1. https://en.wikipedia.org/wiki/BMW_Hydrogen_7 https://en.wikipedia.org/wiki/BMW_Hydrogen_7 2. https://www.hydrogen.energy.gov/pdfs/9013_energy_requirements_for_hydrogen_gas_compression.pdf https://www.hydrogen.energy.gov/pdfs/9013_energy_requirement...