Comment by Veserv | Hacker News Reader

Comment by Veserv | Hacker News Reader

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That is not even comparing apples to oranges, that is comparing apples to steel. You are correct in that energy density does not matter very much for weight-insensitive generation such as grid-scale generation, but energy density matters for weight-constrained applications such as airplanes and rockets as the poster mentioned.

However, assuming that the renewable generation cost curve continues to improve exponentially then the most likely outcome for a carbon-neutral or carbon-negative future will be using electricity to manufacture high density combustible fuels out of atmospheric carbon, effectively using it as a high density "battery" for use cases that demand high energy density.

To the extent that your analysis is relevant to the concerns of the poster, all it means is that batterys are actually ~4x better than the raw energy density would indicate. As to the specifics, Wikipedia claims petrol is ~12,888 W*h/kg or ~24x the battery energy density in the article, so ~6x better with respect to car motion. Note that the current curve has only gone from ~100 W*h/kg to ~500 W*h/kg, so we would need to see density growth comparable to the last 30 years to happen again.

What are the theoretical limits of electrical energy density?

Excellent question! A sufficiently advanced battery can theoretically beat gasoline.

Any given energy storage technology can store a maximum amount of energy in a fixed volume or mass. Behold one of my favorite plots: [1]

From lowest to highest energy density:

- springs, which use mechanical elastic potential energy, are kinda horrible

- capacitors, which use electric permittivity, aren't great

- next are both batteries and combusted fuels, which both use chemical reactions.

- nuclear gets us another few orders of magnitude

- finally, antimatter (E=mc^2) is a ways beyond that

Both batteries and fuels rely on the energy difference between unreacted molecules, so their theoretical energy density is the same. Well, actually, fuels are burnt to create heat which is converted to energy, and this heat->energy conversion is fundamentally thermodynamically inefficient (only ~tens of percent), whereas batteries are the same sorts of reaction but much more controlled. A sufficiently clever battery, which moves atoms around to react in the right places at the right time, is thus more efficient and thus energy-dense than fuel. However, moving atoms around like this to make a more efficient battery is much more advanced nanotech than what we currently have. But it's theoretically possible.

This is what biology does: us humans are powered by chemical storage (sugar/fat/glucose), which is used more efficiently than current batteries but without combustion. (lithium-ion is ~0.8 MJ/kg, glucose is ~16 MJ/kg, gasoline ~46 MJ/kg)

[1] https://en.wikipedia.org/wiki/Energy_density

That is indeed a fun chart.

One thing I wanted to add is that fat (lipids) are much more energy dense than glucose. ~38 Mj/kg, though I am not sure what fraction of that the body recovers. Which makes sense, you want to maintain your long-term storage in a denser format.

Fuel(gasoline, carbohydrates, fats, etc. but not rocket one) does use oxygen from the atmosphere.Batteries are self contained (like rocket fuel).

So the density of chemical reactions is by definition higher.

Side note: energy density should apply to volume, not weight, but we'll - it is too common now.

What makes one chemical more able to store a greater energy per unit mass than another? Wouldn't the theoretical limit be a volume of pure electrons compressed in the densest unit volume possible? Say, stored in a magnetic field?

Compressing neon gas won't do much, aside storing energy as compressed fluid.

My point was the traditional fuels (incl. the edible ones) use more material/weight than their own. So it is very likely they'll be more efficient. The batteries require a reversible action by just applying current - this is quite the climb compared to most chemical reactions.

We have not done much since the li-ion inception, using FePO4 instead of cobalt is more sensible from an economic point of view but the energy density is even lower.

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