3 ms·

> you're looking at roughly a factor of 100, with ~40 MJ/kg for gasoline and 0.4 MJ/kg for a lithium ion battery If you only care about acceleration, energy de

> you're looking at roughly a factor of 100, with ~40 MJ/kg for gasoline and 0.4 MJ/kg for a lithium ion battery If you only care about acceleration, energy density doesn't matter very much. What matters is power: The rate at which you can move energy from the storage medium to the wheels. In that respect a capacitor-powered dragster could probably smoke a top fuel dragster, even though the energy density of capacitors is pretty lousy. No idea if anybody has tried to build a full-size capacitor-powered dragster. Or if any human would be brave enough to drive one.

perilunar 4y ago
Has anyone considered flywheels? Much higher energy density than capacitors, and higher power output than batteries.
- LawTalkingGuy 4y ago
  Have you tried spinning the batteries?
LeifCarrotson 4y ago
Having investigated some of these questions with respect to quadcopters and micro-mouse 'bots, the power density (both in mass and volume) of capacitors is pretty terrible. Whether you're talking about aluminum electrolytic traditional capacitors or double-layer supercapacitors, they're inferior in most cases to a lipo. That's especially true if you're open to high-discharge 50C or 75C lipos, which can dump their entire energy capacity in ~60 seconds. If you only need a few milliseconds of 'zap', yeah, a capacitor bank is great. It's the only option if you need to do that more than a couple hundred times in the life of the battery; a chemical battery will wear out but a capacitor can last for millions or billions of charge/discharge cycles. But even a time as short as 1.461 seconds is probably on the far side of the inflection point where capacitors make sense. If you like, run through the options on Digikey: https://www.digikey.com/en/products/filter/electric-double-layer-capacitors-edlc-supercapacitors/61 https://www.digikey.com/en/products/filter/electric-double-l... ex: https://www.eaton.com/content/dam/eaton/products/electronic-components/resources/data-sheet/eaton-xv%20supercapacitor-cylindrical-snap-in-data-sheet.pdf https://www.eaton.com/content/dam/eaton/products/electronic-... Also remember that an ultracapacitor is about as far in behavior from a physicist's ideal plate capacitor as you can get - they're not linear, they have highly significant internal series resistance...
- TheSpiceIsLife 4y ago
  Also remember that top fuel dragster engines only run for a handful of seconds before they need to be rebuilt, and will be destroyed in a non-recoverable way after more than a few handful of seconds of run time. I'm not sure if anyone is building electric dragsters with run-once components, or how that might change assumptions?
- dreamcompiler 4y ago
  I was thinking more in terms of ordinary capacitors than supercapacitors because the former are vastly superior in terms of power density. However, ordinary caps are so poor in energy density that it just might matter, even for a very short duration energy dump application. IOW I'm not sure there's enough volume in a dragster to meaningfully power it with ordinary caps. But if there is, it would be a very quick car.
- floxy 4y ago
  Superconducting Magnetic Energy Storage (SMES) might be the solution looking for this particular problem. Figures 2 & 3: https://snf.ieeecsc.org/sites/ieeecsc.org/files/CR5_Final3_012008.pdf https://snf.ieeecsc.org/sites/ieeecsc.org/files/CR5_Final3_0... ...if nothing else it adds the extra failure points and safety issues that come with cryogenics!
greggsy 4y ago
> Or if any human would be brave enough to drive one. From the perspective of being that close to being fried, or with respect to the acceleration? To be honest though, I still think it’s amazing that people casually handle extremely flammable and volatile hydrocarbons whenever they visit the gas station. The safety ecosystem is very well tuned, albeit after a hundred+ years of accidents and research.
- dreamcompiler 4y ago
  > From the perspective of being that close to being fried, or with respect to the acceleration? I was thinking the former. One does not casually fuck around with big capacitor banks and live to talk about it.
- bobthepanda 4y ago
  FWIW, gas stations are basically all environmental disasters, in that after decommissioning they need extensive remediation as the storage tanks have inevitably leaked.

perilunar 4y ago

Has anyone considered flywheels? Much higher energy density than capacitors, and higher power output than batteries.

LawTalkingGuy 4y ago
Have you tried spinning the batteries?

LeifCarrotson 4y ago

Having investigated some of these questions with respect to quadcopters and micro-mouse 'bots, the power density (both in mass and volume) of capacitors is pretty terrible. Whether you're talking about aluminum electrolytic traditional capacitors or double-layer supercapacitors, they're inferior in most cases to a lipo. That's especially true if you're open to high-discharge 50C or 75C lipos, which can dump their entire energy capacity in ~60 seconds. If you only need a few milliseconds of 'zap', yeah, a capacitor bank is great. It's the only option if you need to do that more than a couple hundred times in the life of the battery; a chemical battery will wear out but a capacitor can last for millions or billions of charge/discharge cycles. But even a time as short as 1.461 seconds is probably on the far side of the inflection point where capacitors make sense. If you like, run through the options on Digikey: https://www.digikey.com/en/products/filter/electric-double-layer-capacitors-edlc-supercapacitors/61 https://www.digikey.com/en/products/filter/electric-double-l... ex: https://www.eaton.com/content/dam/eaton/products/electronic-components/resources/data-sheet/eaton-xv%20supercapacitor-cylindrical-snap-in-data-sheet.pdf https://www.eaton.com/content/dam/eaton/products/electronic-... Also remember that an ultracapacitor is about as far in behavior from a physicist's ideal plate capacitor as you can get - they're not linear, they have highly significant internal series resistance...

TheSpiceIsLife 4y ago
Also remember that top fuel dragster engines only run for a handful of seconds before they need to be rebuilt, and will be destroyed in a non-recoverable way after more than a few handful of seconds of run time. I'm not sure if anyone is building electric dragsters with run-once components, or how that might change assumptions?
dreamcompiler 4y ago
I was thinking more in terms of ordinary capacitors than supercapacitors because the former are vastly superior in terms of power density. However, ordinary caps are so poor in energy density that it just might matter, even for a very short duration energy dump application. IOW I'm not sure there's enough volume in a dragster to meaningfully power it with ordinary caps. But if there is, it would be a very quick car.
floxy 4y ago
Superconducting Magnetic Energy Storage (SMES) might be the solution looking for this particular problem. Figures 2 & 3: https://snf.ieeecsc.org/sites/ieeecsc.org/files/CR5_Final3_012008.pdf https://snf.ieeecsc.org/sites/ieeecsc.org/files/CR5_Final3_0... ...if nothing else it adds the extra failure points and safety issues that come with cryogenics!

greggsy 4y ago

> Or if any human would be brave enough to drive one. From the perspective of being that close to being fried, or with respect to the acceleration? To be honest though, I still think it’s amazing that people casually handle extremely flammable and volatile hydrocarbons whenever they visit the gas station. The safety ecosystem is very well tuned, albeit after a hundred+ years of accidents and research.

dreamcompiler 4y ago
> From the perspective of being that close to being fried, or with respect to the acceleration? I was thinking the former. One does not casually fuck around with big capacitor banks and live to talk about it.
bobthepanda 4y ago
FWIW, gas stations are basically all environmental disasters, in that after decommissioning they need extensive remediation as the storage tanks have inevitably leaked.