[geeks] Super/ultra capacitors for energy storage

[Jonathan Groll]

On Tue, 25 Oct 2011 23:29:59 +0200, Jochen Kunz <jkunz at unixag-kl.fh-kl.de> wrote:
> Many years, if it happens at all.
> Keep in mind that battery technology is evolving very fast too. Think
> of all the LiIon battery technology developed in the last 10..15 years
> along with "supercaps".

The relevant Wikipedia page gives me optimistic hope [1]:

"The specific energy of existing commercial EDLCs ranges from around
0.5 to 30 WB7h/kg including lithium ion capacitors, known also as a
"hybrid capacitor". Experimental electric double-layer capacitors have
demonstrated specific energies of 30 WB7h/kg and have been shown to be
scalable to at least 136 WB7h/kg. For comparison, a conventional
lead-acid battery stores typically 30 to 40 WB7h/kg and modern
lithium-ion batteries about 160 WB7h/kg. Gasoline has a net calorific
value (NCV) of around 12,000 WB7h/kg...."

Damn. Don't we have a good thing going with petroleum?

> "Supercaps" can't store as much energy as batteries per volume /
> weight. But they are easier to charge and withstand more charge /
> discharge cycles. Both technologies have there speciffic use and
> complement one another. E.g. in electric cars. Energy is drwan from
> batteries to accelerate. Energy from braking is stored in supercaps
> (recuperation) and reused at next acceleration.

>From a computing point of view I would certainly consider a capacitor
based UPS solution, even if it was say 3-4 times the volume/mass of a
battery offering. Those lead-acid batteries don't seem to last many
years, plus at least for the admittedly (cheap) UPSes that I've seen
the batteries weren't deep-cycle batteries. 

Of course, it all boils down to costs, doesn't it?

Here's a quick back-of the-envelope calculation:

The wikipedia EDLC page [1] says that costs had fallen to $2.85 per kJ
by 2006 (which admittedly was five years ago! For eBay comparison see
[3]). If say your typical 15kg lead-acid car battery has an energy
content of 2.6MJ [2], then at the above 2006 price the cost of a
capacitor that could replace your car battery would be 2.85 * 2600 =
$7436.00. 

(This is obviously excluding other concerns, such as the cost of
'balancing' components should one want to put the capacitors in series
etc.)

Cheers,
Jonathan

[1] http://en.wikipedia.org/wiki/Electric_double-layer_capacitor
[2] http://en.wikipedia.org/wiki/Energy_density
[3] How much do eBay capacitors cost? For a fully charged 3000F, 2.7V
capacitor the energy calc is W= B=*C*V2 = 3Wh (10.94 kJ). I've seen
them USED on eBay BIN for $23. That works out to $2.10 per kJ. 

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