Comment by hwillis | Hacker News Reader

Comment by hwillis | Hacker News Reader

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> lithium ion battery will surpass both energy and power density of gasoline, which seems impossible even from physics point of view.

Sure, I can do the math on that.

Gasoline has about 45 MJ/kg[1]. 1 kg of lithium contains 868x10^23 atoms, or 13.9 megacoulombs if each atom donates a single electron. Since a joule is one volt-coulomb, every electron must average out (nominal) to 3.24 volts. Todays li-ion batteries have a nominal voltage of ~3.65 V, and the nominal voltage of li-air batteries are 2.91 V.

The actual voltage of li-air is ~90% lower than the required voltage to reach raw parity with gasoline, which makes sense given that the theoretical limit of li-air batteries is 40.1 MJ/kg (.9 * 45 = 40.5).

Reaching the same power density as gasoline in vehicle engines (=<9 MJ/kg) isn't too bad (very close to 10x best current consumer batteries, closer to weirder chemistries), but parity with the real power of gasoline is a very hard ask. Donating more than one electron per lithium would require some serious re-evaluation of what we know about chemistry. If you only have one electron per atom, you have almost no weight left over for other materials to use to develop a voltage difference with. A battery that exceeds gasoline's full heat energy would need some very exotic materials- either lighter atoms[2], different ways of storing electrons[3], or ways to store energy besides electrons[4].

[1] divide by 5 for a realistic engine efficiency. At that rate, a gallon of gas provides 9.4 kWh- enough to get a Tesla model S over 35 miles.

[2] there are none, besides hydrogen, which itself isn't really helpful. It's not very good for electrochemistry.

[3] preferably, in a different state than me. Electrons hate each other. A 3 Ah battery has 11,000 hateful coulombs in it, each electron within a less than a nanometer of the others around it. That's a 10^9 on top and a 10^-9 on the bottom. Not positive on the math on that but I'm reasonably sure it's in the thousands of megatons. It is rapidly trying to change chemical and then geographical state.

[4] preferably on another planet from me. If you're storing energy with a different force than the electromagnetic force, you're pretty much guaranteed to be doing some real fuckery to subatomic particles, and they are not going to be happy.

Lol. Fabulous answer to the math, with nice humour! Unless the Gas industry learns how to make new Gas on the fly, Lion will continue to shine also because of the amazingly different ways that it's energy losses can be replenished (wind, water, sun, vibration...), even while it's doing its thing.

So, GP's comment of "develop the tech that seems impossible as the biggininng" (typo included) is the important part. It might be that in the future, because the motors and other moving parts become increasigly efficient (& are already much better than a combustion engine) then less energy is required for the same resulting work. Even more so, the ingenious ways that energy can be 'reharvested' to flow back into the system could in fact allow us to develop something that seems impossible...simply because we're harvesting energy and pouring it into our Lion dependant system better than Gas ever can.

> because the motors and other moving parts become increasigly efficient (& are already much better than a combustion engine) then less energy is required for the same resulting work

The resulting work is more important than the efficiency. Teslas have exceptionally low air drag, which is great for driving range.

However when you go from 80 to 90% system efficiency, you only increase range by 12%. Once youre at 99%, you really cant improve any more. 99% to 99.5% means the motor outputs half as much heat but goes hardly any farther.

You could make gasoline from the air. A bit inefficient, but if we somehow get to the fabled "free fusion" power plants, it could be a neat trick to have synthetic gas in pipelines flowing from fusion CO2 atmospheric scrubbers.

You don't need fusion. PV works too; you just need a lot of it. Making liquid fuel from air is currently the only viable solution for carbon-neutral aircraft, which is why the military likes the idea.

What about the weight of the entire drive system? Still not the same thing, as the weight of batteries doesn't change as you drive. Not sure, as a quick search doesn't give me the right data, but it seem like the ICE setup requires more engine/exhaust etc. weight, so it seems likely the amount of efficiency required for batteries is less than gas at the current tech level.

Last time I tried to estimate that it seemed with a passenger car you get some win there that offsets the battery weight. Something around 300lbs. My guess is if batteries got 30% more power dense the weight penalty would disappear. Part of that is as the weight of battery drops you can make the rest of the car lighter as well.

Thanks for the numbers, which I was aware of, but not exactly. The constant supply of money in Research can do impossible magic. If you look at it from social perspective, all the semiconductor progress in last 20-30 years happened just because people want to send their photo to another people. Finger crossed, but I'm hopeful.

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