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Comment by jerf | Hacker News Reader

I'd like to see some energy density numbers in units I can understand before I would jump up and down about this use.

(I'm not saying you're wrong so much as saying I can't provide evidence you're right, and I'm hoping someone else can. Or at least tell us what exactly "the heat storage capability of water" is such that something can have 4 times as much. Please hold wild-assed guesses, please, I can compute the energy difference between two temperatures of water for a given volume as well as the next guy, but what temperatures, or is that even the right question? I'm looking for someone who knows.)

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The term of art you're looking for is "specific heat".

https://en.wikipedia.org/wiki/Specific_heat

The relevant number for water at 25C is 4.184 joules per gram. But this isn't a strict apples to apples comparison: if you heat and cool zeolite in a vacuum chamber, it'll have a pathetic specific heat. (Like perlite, another foamed mineral, which has a specific heat of something like 0.1) That's because there's a chemical reaction taking place, not pure dumb-matter heating or cooling.

Meanwhile, the energy density of a lithium ion battery is 720 joules per gram, and the energy density of gasoline is 47,200 joules per gram. This does not "solve" energy storage, in any way, shape, or form.

Someone downmodded you because you messed up your units.

It's 4.184 joules per gram Kelvin. Meaning it stores that much energy for each dress of heat you add to it. If you take water from near freezing to boiling that's 100 degrees of storage - meaning 418.4 joules per gram - which is much more reasonable.

And there is no reason you have to stop at boiling. Storing something at 500 degrees is not impractical, so assuming starting at 20 degrees (room temperature) you can store over 2000 joules per gram. And there are plenty of materials that can handle even higher temperatures.

And that's a big part of my question... given that in the abstract water can store any amount of heat (between absolute 0 and the point at which we can no longer call it "water" due to being a plasma of some form), what exactly does it mean for this material to be able to store 4 times as much?

I'm sure there's an answer, because I'm sure the journalist got that number from somewhere, but I lack the connections to know where to begin finding this information.

Oh gosh, you're right.

The specific heat is the energy needed to raise the temperature per mass. So for water it takes 4.184 joules per gram to raise the temperature by 1 kelvin. So if you heated water to 75 degrees you would have put in about 205 joules per gram. You then have to deal with conductive losses and the efficiency of a heat engine.

This is not directly comparable to the energy density of gasoline or a lithium ion battery which use chemical reactions to store the energy. So you can turn almost 100% of the energy in a lithium ion battery into useful work but if you put the equivalent number of joules into heating an object you wouldn't get close unless you have a handy 0 kelvin object.

0. you must play the game

1. you can never win

2. you can only break even on a very cold day

3. it never gets that cold

Fair point. Although the exact energy per volume is a less important measure, given that we know the ballpark, than the energy stored per energy invested, and the maximum storage available when considered as a commodity in the market and resource availability.

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