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for comparison between battery and energy density: typical lipo as used in a short flight endurance hobby quadcopter (5-7" prop size) is 155Wh/kg the best lit

for comparison between battery and energy density: typical lipo as used in a short flight endurance hobby quadcopter (5-7" prop size) is 155Wh/kg the best lithium ion cylindrical cells are around 255Wh/kg right now. quite a bit more limited in instantaneous amperage draw per cell than high C rate lipo. hydrogen fuel cell tank+PEM+piping+DC apparatus for large octocopters comes in somewhere around 1500Wh/kg There's a south korean company that recently hovered a large octocopter with hydrogen power source for 10.5 hours. Same system with lithium ion battery power would be approximately a 1 hour endurance. note that 1500Wh/kg is considerably less than the energy density of jet-a or diesel or ordinary 87/89 octane petrol, BUT, you have to account for 50% of it being lost to waste heat in an internal combustion engine, and the weight of the engine and drivetrain. or weight of jet turbine+generator vs hydrogen tank + fuel cell PEM apparatus.

_hypx 4y ago
Note that a full sized battery pack will also need piping, cooling, cables, etc. So it won't actually reach 255 Wh/kg.
Robotbeat 4y ago
255Wh/kg? That’s not true. About 300Wh/kg has been available for over a decade. The next Gen lithium metal anode and lithium sulfur batteries can do 400-500Wh/kg, up to 650Wh/kg in the lab.
- walrus01 4y ago
  I'm referring to li ion cylindrical cells in the 18650, 20700 and 21700 formats which are commercially available right now. Which I can buy online in 5 minutes with my visa card from a battery wholesaler. If you can find me the pdf datasheet for one that's above that I'd love to see it! And even better a vendor link. Note I am also referring to something like the actual usable watt hours from a battery before you discharge it below "permanently damaged" state, in the range starting from 100% SOC standard full voltage and manufacturer's do-not-exceed floor voltage. Such as on the NCR18650GA. I don't doubt there are specialty rare "ask us for a price" low volume 300Wh/kg in circulation if you have the right contacts.
  - Robotbeat 4y ago
    Okay, but you can’t get a non-science-kit fuel cell of decent size online in 5 minutes with my visa card from a battery wholesaler. You need the right contacts.
    - allisdust 4y ago
      We can: https://www.fuelcellstore.com/ https://www.fuelcellstore.com/ Their max fuel cell stack goes up to 6kw so its no longer in hobby territory.
      - Robotbeat 4y ago
        Thanks! Still pretty expensive and heavy for just a 6kW fuel cell stack alone! $27,000 is the base MSRP for a Chevy Bolt, before incentives.
- henearkr 4y ago
  So we're down to only a 50% energy density compared to a fuel cell. But the fuel cell uses hydrogen, which leaks inevitably (it slowly diffuses through all materials), and is an extremely potent greenhouse effect gas.
  - ema 4y ago
    How long does it take for hydrogen floating in the atmosphere to react with the oxygen?
    - henearkr 4y ago
      Your question would make sense only if it was a direct greenhouse effect gas, but H2 effect is indirect (but still huge), precisely in the way it disappears by combining with other molecules. The greenhouse effect of H2 is by reacting with the HO naturally present in the atmosphere (giving water vapour: 2 HO + H2 -> 2 H2O), thus reducing the concentration of HO, and thus modifiying the dynamics of atmospheric methane and ozone. From my understanding, in the atmosphere, HO has an important role in the decay of CH4. http://agage.mit.edu/publications/global-environmental-impacts-hydrogen-economy http://agage.mit.edu/publications/global-environmental-impac...
  - semi-extrinsic 4y ago
    No, that is factually incorrect, hydrogen is not an "extremely potent" greenhouse gas in this context. Hydrogen has a GWP of 11, meaning it is 11 times worse than emitting CO2. Methane, for example, has a GWP of 34, which is starting to get kind of bad, but "extremely potent" is something like SF6 wich has a GWP of 22800. Furthermore, GWP is calculated on a mass basis, which skews things a lot for a light molecule like hydrogen. I'm kind of astonished at the fear mongering on this issue, because it means people cannot be arsed to spend 20 minutes on Google with a calculator to check the numbers before they spread something as a truth. Let's do the actual math. If you look at a Toyota Mirai, it needs 5 kg of hydrogen to get a 300 mile range. A comparable diesel car needs 28 kg diesel to get that range, and will emit 88 kg of CO2 when that diesel is burned (the O's in CO2 comes from the air). Even if the hydrogen car had an absurdly high leakage rate of 50% during that trip and the period it stood still before next trip, it would still emit only the equivalent of 27.5 kg of CO2. So at insanely high leakage rate, diesel is still 3x worse than hydrogen. If we are realistic about leakage rate, published numbers from actual measurements with the Toyota Mirai placed in a sealed container gives us 2 mL/min at 1 bar, so the time to leak the entire contents of the tank is more than 3000 days. So let's say this is a hobby vehicle (like a small aircraft) seeing infrequent use, you consume two tanks of hydrogen per month. That puts your leakage rate at 0.5%, giving an emissions equivalent of 0.56 kg CO2, while the diesel (avgas) powered equivalent would emit 166 kg CO2 - 300x worse. Note that this is all based on technology that is already here - you can go buy it today - not some hypothetical developments.
ericbarrett 4y ago
> BUT, you have to account for 50% of it being lost to waste heat in an internal combustion engine What's the efficiency of a battery-powered system in flight? E.g. disregarding ground charging and grid supply. (edit) Wikipedia says ~95% for a generic electric motor and ~85% for a generic lithium-ion battery for a very rough estimate of about 80% efficiency in toto.
- walrus01 4y ago
  Also depends how you have many batteries paralleled and the amp draw per cell, for instance a single Sony vtc6 cell might be ok for 30A draw for short times but will heat up while doing it. And needs cooling/thermal dissipation solution. The lower amp draw per cell the less is lost to heat.
- deleted 4y ago
  [deleted]

_hypx 4y ago

Note that a full sized battery pack will also need piping, cooling, cables, etc. So it won't actually reach 255 Wh/kg.

Robotbeat 4y ago

255Wh/kg? That’s not true. About 300Wh/kg has been available for over a decade. The next Gen lithium metal anode and lithium sulfur batteries can do 400-500Wh/kg, up to 650Wh/kg in the lab.

walrus01 4y ago
I'm referring to li ion cylindrical cells in the 18650, 20700 and 21700 formats which are commercially available right now. Which I can buy online in 5 minutes with my visa card from a battery wholesaler. If you can find me the pdf datasheet for one that's above that I'd love to see it! And even better a vendor link. Note I am also referring to something like the actual usable watt hours from a battery before you discharge it below "permanently damaged" state, in the range starting from 100% SOC standard full voltage and manufacturer's do-not-exceed floor voltage. Such as on the NCR18650GA. I don't doubt there are specialty rare "ask us for a price" low volume 300Wh/kg in circulation if you have the right contacts.
- Robotbeat 4y ago
  Okay, but you can’t get a non-science-kit fuel cell of decent size online in 5 minutes with my visa card from a battery wholesaler. You need the right contacts.
  - allisdust 4y ago
    We can: https://www.fuelcellstore.com/ https://www.fuelcellstore.com/ Their max fuel cell stack goes up to 6kw so its no longer in hobby territory.
    - Robotbeat 4y ago
      Thanks! Still pretty expensive and heavy for just a 6kW fuel cell stack alone! $27,000 is the base MSRP for a Chevy Bolt, before incentives.
henearkr 4y ago
So we're down to only a 50% energy density compared to a fuel cell. But the fuel cell uses hydrogen, which leaks inevitably (it slowly diffuses through all materials), and is an extremely potent greenhouse effect gas.
- ema 4y ago
  How long does it take for hydrogen floating in the atmosphere to react with the oxygen?
  - henearkr 4y ago
    Your question would make sense only if it was a direct greenhouse effect gas, but H2 effect is indirect (but still huge), precisely in the way it disappears by combining with other molecules. The greenhouse effect of H2 is by reacting with the HO naturally present in the atmosphere (giving water vapour: 2 HO + H2 -> 2 H2O), thus reducing the concentration of HO, and thus modifiying the dynamics of atmospheric methane and ozone. From my understanding, in the atmosphere, HO has an important role in the decay of CH4. http://agage.mit.edu/publications/global-environmental-impacts-hydrogen-economy http://agage.mit.edu/publications/global-environmental-impac...
- semi-extrinsic 4y ago
  No, that is factually incorrect, hydrogen is not an "extremely potent" greenhouse gas in this context. Hydrogen has a GWP of 11, meaning it is 11 times worse than emitting CO2. Methane, for example, has a GWP of 34, which is starting to get kind of bad, but "extremely potent" is something like SF6 wich has a GWP of 22800. Furthermore, GWP is calculated on a mass basis, which skews things a lot for a light molecule like hydrogen. I'm kind of astonished at the fear mongering on this issue, because it means people cannot be arsed to spend 20 minutes on Google with a calculator to check the numbers before they spread something as a truth. Let's do the actual math. If you look at a Toyota Mirai, it needs 5 kg of hydrogen to get a 300 mile range. A comparable diesel car needs 28 kg diesel to get that range, and will emit 88 kg of CO2 when that diesel is burned (the O's in CO2 comes from the air). Even if the hydrogen car had an absurdly high leakage rate of 50% during that trip and the period it stood still before next trip, it would still emit only the equivalent of 27.5 kg of CO2. So at insanely high leakage rate, diesel is still 3x worse than hydrogen. If we are realistic about leakage rate, published numbers from actual measurements with the Toyota Mirai placed in a sealed container gives us 2 mL/min at 1 bar, so the time to leak the entire contents of the tank is more than 3000 days. So let's say this is a hobby vehicle (like a small aircraft) seeing infrequent use, you consume two tanks of hydrogen per month. That puts your leakage rate at 0.5%, giving an emissions equivalent of 0.56 kg CO2, while the diesel (avgas) powered equivalent would emit 166 kg CO2 - 300x worse. Note that this is all based on technology that is already here - you can go buy it today - not some hypothetical developments.

ericbarrett 4y ago

> BUT, you have to account for 50% of it being lost to waste heat in an internal combustion engine What's the efficiency of a battery-powered system in flight? E.g. disregarding ground charging and grid supply. (edit) Wikipedia says ~95% for a generic electric motor and ~85% for a generic lithium-ion battery for a very rough estimate of about 80% efficiency in toto.

walrus01 4y ago
Also depends how you have many batteries paralleled and the amp draw per cell, for instance a single Sony vtc6 cell might be ok for 30A draw for short times but will heat up while doing it. And needs cooling/thermal dissipation solution. The lower amp draw per cell the less is lost to heat.
deleted 4y ago
[deleted]