# Theoretical ultimate supercapacitor?

Andrew Hamilton
Science 23 Dec '08 11:53
1. 23 Dec '08 11:533 edits
Is there a theoretical maximum specific energy (which is the energy density in terms of energy per unit mass rather than per unit volume) that a supercapacitor can store and, if so, what is it?

I would love to know this because I just wonder what the storage capacity of the best supercapacitors would be in the far future assuming unlimited advances in nano-engineering (so every atom is put in exactly the right place thus guarantee no micro faults or molecular faults in the material) and assuming unlimited understanding of quantum mechanics etc so that the best possible design is deduced.

-is it possible to, for example, to make a supercapacitor with the same specific energy storage as octane!?
-it would be wonderful if that was achieved! -supercapacitors have several advantages over batteries and such high energy dense supercapacitors would make storage of the energy from renewables extremely easy and practical and energy efficient for the national grid and also for vehicles and it would even make it reasonably practical for an airplane to be powered by nothing but a capacitor!

I tried searching the net for this info but I found nothing relevant for ages so given up.
2. 23 Dec '08 14:14
I don't know too much about capacitors, but I guess it's limited by breakdown.
3. 23 Dec '08 16:24
Originally posted by KazetNagorra
I don't know too much about capacitors, but I guess it's limited by breakdown.
I take what you mean by “breakdown” is the “working voltage” which is the maximum voltage that can be applied to the capacitor without the danger of the breakdown voltage being reached?
-yes, that is one of the major factors but it is more complicated than that because the specific energy of a capacitor with a given mass X is also effected by at least two other major factors which are:

2, the total amount of area of overlap between opposing plates within the capacitor.

3, the dialectic constant of the dialect that separates the plates.

But these 3 factors would relate to one another in such a complicated way in a capacitor that has been optimised that I do not quite know how to work out the theoretical maximum specific energy of such a “ultimate” capacitor.
4. 23 Dec '08 16:36
2. Yes, but the relevant factor for applications is the energy density per volume, which is unaffected by the area since the breakdown limit scales with length.

3. Yes, but this delivers some technical complications, although changing the dielectric constant is being investigated (for example I have worked on laser wakefield acceleration, which is based on this).
5. 23 Dec '08 20:284 edits
Originally posted by KazetNagorra
2. Yes, but the relevant factor for applications is the energy density per volume, which is unaffected by the area since the breakdown limit scales with length.

I think I understand what you mean -you mean that, for example, if you double the thickness of the dialect between opposing plates, you double the breakdown voltage and thus although doubling up the thickness of the dialect halves the number of plates you can squeeze in per volume (assuming negligible thickness of the plates in proportion to the thickness of the dialect between them) this is offset by the doubling of the breakdown voltage.

But I am curious; doesn’t that assume negligible thickness of the plates in proportion to the thickness of the dialect between them? What effect (if any) has the thickness of the plates have in how much electrical charge each given unit area of overlapping opposing plates can store?
6. 23 Dec '08 20:49
Originally posted by Andrew Hamilton
I think I understand what you mean -you mean that, for example, if you double the thickness of the dialect between opposing plates, you double the breakdown voltage and thus although doubling up the thickness of the dialect halves the number of plates you can squeeze in per volume (assuming negligible thickness of the plates in proportion to the thic ...[text shortened]... ave in how much electrical charge each given unit area of overlapping opposing plates can store?
Not sure on that one.
7. 26 Dec '08 20:24
Three-quarters down;

http://reference.findtarget.com/search/Electric%20double-layer%20capacitor/

It says:

“Carbon aerogel capacitors have achieved 325 J/g (90 Wh/Kg) energy density “

If this is correct, this works out as 0.325MJ/kg which is impressive because it compares with about 0.28MJ/kg for the current best lithium ion batteries!
-but surely there is a catch? -I mean, this should make this capacitor better than any battery in practically very way so why are they not currently used instead of batteries? -so what is the catch? Is it just the current cost of making them? -if so, I am sure it is just a matter of time before this is reduced.
8. 26 Dec '08 20:42
Lithium ion batteries are pretty cheap, so if the energy gain is that small I suppose it's only useful for applications where a small size is critical.
9. 27 Dec '08 19:087 edits
I have just calculated a very crude estimate that an electric car that is powered by a 0.325MJ/kg supercapacitor about the weight of a typical full car-tank of petrol would need recharging very roughly every 4 miles of travel.

Perhaps if there was an electric terminal put on every street corner with the electric power coming from just renewables then we could learn to live with that slight inconvenience?

Not sure how such electric car travel would work on the main motorways and duel carriageways -imagine all the cars having to stop every ~4 miles to be recharged!
Perhaps on the main motorways the power for the cars could come continually from overhead electric cables as the cars move forward (like those that power certain electric trams) -just an idea.
What if there is a power cut? -the power supply would have to be made ultra-reliable.
10. 28 Dec '08 09:20
Electric cars already exist and they work fine with a radius of several hundred miles.
11. 28 Dec '08 11:19
Originally posted by KazetNagorra
Electric cars already exist and they work fine with a radius of several hundred miles.
But are those cars powered by just a capacitor or just by a lithium battery etc?
Or are they hybrids (thus still burn fuel) or do they use fuel cells?

I will be interested to know how many mile the best purely electric car to date powered just by a battery/capacitor can go on a fully charged battery/capacitor -so far I haven’t found any websites that tell me this.
12. sonhouse
Fast and Curious
29 Dec '08 01:34
Originally posted by Andrew Hamilton
But are those cars powered by just a capacitor or just by a lithium battery etc?
Or are they hybrids (thus still burn fuel) or do they use fuel cells?

I will be interested to know how many mile the best purely electric car to date powered just by a battery/capacitor can go on a fully charged battery/capacitor -so far I haven’t found any websites that tell me this.
Look at this link, about a company in Texas making a hybrid ultracap/battery in the same box. The problem with using batteries is they get very stressed when putting out max current for long periods of time, distorting electrodes, overheating and so forth, but using them in conjunction with ultracaps gives the best of both worlds. The discharge rate of the ultra's are many times that of the battery, both giving up and recharging a lot faster so when you gun it uphill say, the ultra cap gives the big power pulse needed for only ten seconds or so then when things quiet down power wise, the battery charges up the ultra cap for another round of acceleration, and during decel, power can be stuffed back into the ultracap a lot faster than the lithium Ion cell so power is managed a lot more efficiently resulting in much longer lifetime for the lithium Ion cell.

http://www.technologyreview.com/biztech/18086/

There is an interesting discussion at the end of this piece also.
13. 29 Dec '08 11:18
Originally posted by sonhouse
Look at this link, about a company in Texas making a hybrid ultracap/battery in the same box. The problem with using batteries is they get very stressed when putting out max current for long periods of time, distorting electrodes, overheating and so forth, but using them in conjunction with ultracaps gives the best of both worlds. The discharge rate of the ...[text shortened]... yreview.com/biztech/18086/

There is an interesting discussion at the end of this piece also.
http://www.technologyreview.com/biztech/18086/

That is great! -thanks for that ðŸ™‚
14. 29 Dec '08 17:37
Originally posted by Andrew Hamilton
But are those cars powered by just a capacitor or just by a lithium battery etc?
Or are they hybrids (thus still burn fuel) or do they use fuel cells?

I will be interested to know how many mile the best purely electric car to date powered just by a battery/capacitor can go on a fully charged battery/capacitor -so far I haven’t found any websites that tell me this.
I'm not sure, but I think they just consist of an array of lithium batteries.
15. sonhouse
Fast and Curious
29 Dec '08 23:09
Originally posted by KazetNagorra
2. Yes, but the relevant factor for applications is the energy density per volume, which is unaffected by the area since the breakdown limit scales with length.

3. Yes, but this delivers some technical complications, although changing the dielectric constant is being investigated (for example I have worked on laser wakefield acceleration, which is based on this).
A bit off topic but you mentioned wakefield accelerators, I found this link, 3 GEV in 3 1/3 CM! Had you seen this work? At Berkeley:

http://www.physorg.com/news78323567.html