Why not use high potential capacitors?

[dE_logics]

I mean like...20KV or above...I've seen their energy density and specific energy is pretty high compared to batteries (I hope I'm right)...so why not use them for battery replacement in vehicles?

 

[uart]

I mean like...20KV or above...I've seen their energy density and specific energy is pretty high compared to batteries (I hope I'm right)...so why not use them for battery replacement in vehicles?

Why not you post a link to a currently available 20kV+ capacitor that has energy storage capacity anything close to current battery technology.

Here's a link to some available high voltage capacitors. http://www.hivoltcapacitors.com/page1.html

Let's look at the largest in their 25kV range, part number PMR 250-504. It's volume is over 3L and its rating is 0.5uF 25kV DC working voltage.

Max energy storage is 1/2 C V^2 = 156 Joules.

For comparison LiPo batteries currently have volume energy densities around 300 W-h / L, so you'd store over 3000000 Joules in the same volume as the above capacitor!

Sure lower voltage supercapacitors will have higher energy density than the 25kv capacitor I linked, but even those currently have only about one tenth the energy density of what's available in batteries. The higher charge/discharge rate of the supercapacitors does give them some advantages which makes then useful in conjunction with a battery (as in a battery/supercapacitor hybrid).

Any dE_logics, please post a link to the currently available 20kV+ capacitor that you claim to have such good storage capacity compared to batteries. You've made the claim so please substantiate it. Personally I'll believe it when I see it.

 

[gnurf]

I mean like...20KV or above...I've seen their energy density and specific energy is pretty high compared to batteries (I hope I'm right)...so why not use them for battery replacement in vehicles?

Perhaps you were thinking of high power density? http://www.nytimes.com/2008/01/13/a...ed=1&ref=automobiles&oref=slogin&oref=slogin" describes a hybrid electrical car that combines lithium ion batteries with ultracapacitors:

The ultracapacitors take up about as much space as the lithium ion batteries, although they store a total of less than one kilowatt hour. [...] In use, the capacitors function much like the water tank on a toilet. That reservoir let's the toilet get by with a small supply pipe, yet still delivers a large volume at once for flushing.

 

[mgb_phys]

I mean like...20KV or above...I've seen their energy density and specific energy is pretty high compared to batteries (I hope I'm right)...so why not use them for battery replacement in vehicles?

The dielectric used for capacitors, especially supercaps, breaks down at high voltages.
So unfortunately you can't increase the capacity (by CV^2) by simply upping the voltage

 

[dE_logics]

Ok...we don't have such capacitors...I was wrong.Thanks for clarifying.

 

[mgb_phys]

Worse than that - it's pretty much a fundamental feature of dielectrics that as the dielectric constant goes up the breakdown voltage goes down.