Mechanism behind capacitor's high-speed energy storage discovered

Feb 23, 2012

Researchers at North Carolina State University have discovered the means by which a polymer known as PVDF enables capacitors to store and release large amounts of energy quickly. Their findings could lead to much more powerful and efficient electric cars.

Capacitors are like batteries in that they store and release . However, capacitors use separated electrical charges, rather than , to store energy. The charged enable energy to be stored and released very quickly. Imagine an electric vehicle that can accelerate from zero to 60 miles per hour at the same rate as a gasoline-powered sports car. There are no batteries that can power that type of acceleration because they release their energy too slowly. Capacitors, however, could be up to the job – if they contained the right materials.

NC State physicist Dr. Vivek Ranjan had previously found that capacitors which contained the polyvinylidene fluoride, or PVDF, in combination with another polymer called CTFE, were able to store up to seven times more energy than those currently in use.

"We knew that this material makes an efficient , but wanted to understand the mechanism behind its storage capabilities," Ranjan says.

In research published in Physical Review Letters, Ranjan, fellow NC State physicist Dr. Jerzy Bernholc and Dr. Marco Buongiorno-Nardelli from the University of North Texas, did computer simulations to see how the atomic structure within the polymer changed when an electric field was applied. Applying an electric field to the polymer causes atoms within it to polarize, which enables the capacitor to store and release energy quickly. They found that when an electrical field was applied to the PVDF mixture, the atoms performed a synchronized dance, flipping from a non-polar to a polar state simultaneously, and requiring a very small to do so.

"Usually when materials change from a polar to non-polar state it's a chain reaction – starting in one place and then moving outward," Ranjan explains. "In terms of creating an efficient , this type of movement doesn't work well – it requires a large amount of energy to get the atoms to switch phases, and you don't get out much more energy than you put into the system.

"In the case of the PVDF mixture, the atoms change their state all at once, which means that you get a large amount of energy out of the system at very little cost in terms of what you need to put into it. Hopefully these findings will bring us even closer to developing capacitors that will give electric vehicles the same acceleration capabilities as gasoline engines."

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More information: "Electric Field Induced Phase Transitions in Polymers: a Novel Mechanism for High Speed Energy Storage" by V. Ranjan, M. Buongiorno Nardelli and J. Bernholc, Center for High Performance Simulation and Department of Physics, North Carolina State University, Published online in Physical Review Letters.

Abstract
Using first-principles simulations, we identify the microscopic origin of the non-linear dielectric response and high energy density of PVDF-based polymers as a cooperative transition path that connects non-polar and polar phases of the system. This path explores a complex torsional and rotational manifold and is thermodynamically and kinetically accessible at relatively low temperatures. Furthermore, the introduction of suitable copolymers significantly alters the energy barriers between phases providing tunability of both the energy density and the critical fields.

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Scottingham
not rated yet Feb 23, 2012
The White Zombie may beg to differ concerning the acceleration of EVs compared to gas cars.
antialias_physorg
5 / 5 (1) Feb 23, 2012
And especially the Ultimate Aero EV
http://en.wikiped..._Aero_EV
0-60mph in 2.5 seconds.
MR166
1 / 5 (6) Feb 23, 2012
""Usually when materials change from a polar to non-polar state it's a chain reaction starting in one place and then moving outward," Ranjan explains. "In terms of creating an efficient capacitor, this type of movement doesn't work well it requires a large amount of energy to get the atoms to switch phases, and you don't get out much more energy than you put into the system."

Am I just being picky, or is academia getting more ignorant by the day? " You don't get out much more energy than you put in", give me a break. He should change fields and become a climate "scientist".
Callippo
not rated yet Feb 23, 2012
Polyvinylidene fluoride is piezoelectric and similar in this extent to high-k dielectric materials, like the barium titanate used in EEStor technology.

http://en.wikiped...electric
http://en.wikiped...i/EEStor
antialias_physorg
5 / 5 (5) Feb 23, 2012
Am I just being picky, or is academia getting more ignorant by the day?

I think you're being picky. They are referring to the energy needed to make the atoms switch phases - not the energy that is put into storage. It is preferrable to require little switching energy to get the stored energy out.
axemaster
4.4 / 5 (7) Feb 23, 2012
Am I just being picky, or is academia getting more ignorant by the day? " You don't get out much more energy than you put in", give me a break.

Given that you're wrong about what he said, I suspect... I think I detect the taint of quackery...

He should change fields and become a climate "scientist".

Yes! There it is! *watches duck waddling into a pond*
Shootist
3 / 5 (2) Feb 23, 2012
And especially the Ultimate Aero EV
http://en.wikiped..._Aero_EV
0-60mph in 2.5 seconds.


"How fast did it go 'round our track?" - Jeremy Clarkson
that_guy
5 / 5 (5) Feb 23, 2012
Yeah, I think something is wrong with the statement -

Imagine an electric vehicle that can accelerate from zero to 60 miles per hour at the same rate as a gasoline-powered sports car.


Most fully electric cars do. it's hybrids, due to their energy moving strategies that have poor acceleration.

With the amount of energy needed to be stored, setting up the cells in a way that can provide that much spot power is not at issue.

Tesla Roadster 0-60: 3.6 seconds.
Leaf 0-60: 7 seconds

350z 0-60: 5-6 seconds
Enzo: 3.14 seconds

accord (V6): 7 seconds

and on and on. Releasing power is not at issue. It's about charging rapidly and efficiently at a high power density.

Most electric cars have no problem accelerating.

jshloram
not rated yet Feb 23, 2012
"...and you don't get out much more energy than you put into the system.

Am I just being picky, or is academia getting more ignorant by the day? " You don't get out much more energy than you put in", give me a break. He should change fields and become a climate "scientist".


This stop me, too. The most generous explanation would be either poor communications on the part of the researcher or poor science writing.
TabulaMentis
5 / 5 (2) Feb 24, 2012
This YouTube video "Killacycle battery powered drag bike's record-breaking run" about a drag racing motorbike should be a good example of how fast an electric motorbike can go:

http://www.youtub...pAZci9m0

Or check out the dragster video:

http://www.youtub...=related
Callippo
5 / 5 (1) Feb 24, 2012
The capacitors would be more useful in hybrid cars, which do need to recharge often, which decreases the life-time of the accumulator, but not of the capacitor. The batteries in hybrid cars are generally smaller and the requirement of high power during fast acceleration is more dangerous for smaller batteries, than for larger ones.
ab3a
not rated yet Feb 24, 2012
At the opposite end of the scale, do these ultra-capacitors have any potential use for Hi-Q applications in RF? I didn't read much about temperature stability.

Something like this might be useful for building very small and low phase noise Voltage Controlled Oscillators...
MR166
1 / 5 (1) Feb 24, 2012
Axmaster, do ignorant journalists make me angry? Do charlatans calling themselves "Climate Scientists" make me mad?

You Bet, Quack Quack

MR166
bredmond
not rated yet Feb 24, 2012
I am somewhat ignorant of electronics, thought it is one of my hobbies, albeit not a priority, but why wouldn't the capacitors be left with no energy once they discharge? like if i accelerate once, the capacitors would then be down to a fraction of the original charge, and then really ineffective. i dont understand that part. what am i missing?
antialias_physorg
5 / 5 (1) Feb 24, 2012
but why wouldn't the capacitors be left with no energy once they discharge?

When you break the circuit the discharge stops (the electrons/holes have nowhere to go). Discharge happens not linearly but logarithmically with a certain time constant. So there's always something left.

Capacitors have a few advantages and disadvantages compared to batteries:

Pro:
- can charge/discharge rapidly
- can take up/give off large amounts of energy quickly

Con:
- the voltage isn't constant (it depends on the charge held) as opposed to batteries which give you pretty much constant voltage until depleted. This is a problem for electronics that rely on constant voltages. to get constant voltages you have to transform the output (leading to losses). The further away from your target voltage the more the losses you incur.
- capacitors discharge of their own. They don't store energy nearly as long as batteries can
- energy density is significantly lower than for batteries
MR166
1 / 5 (1) Feb 24, 2012
If the capacitor is large enough the change in voltage for a given amount of power supplied is small enough that it begins to look like a battery but it can be recharged in much less time and over many more cycles. As things stand today they cannot be made large enough to power a car for more than a few seconds but are good at supplying the high peak currents needed for short bursts of acceleration. They are also very good at storing the power produced by regenerative braking.
bredmond
not rated yet Feb 25, 2012
- the voltage isn't constant (it depends on the charge held) as opposed to batteries which give you pretty much constant voltage until depleted. This is a problem for electronics that rely on constant voltages. to get constant voltages you have to transform the output (leading to losses). The further away from your target voltage the more the losses you incur.


this is what i mean.

As things stand today they cannot be made large enough to power a car for more than a few seconds but are good at supplying the high peak currents needed for short bursts of acceleration


and this.