Supercapacitor as Prius 'battery'

They mention the $4K to $5K price for a Prius battery, but they don't say what a supercapacitor equivalent would cost. They do say that it's a long way from being ready for use, and that Toyota considered and rejected capacitors.

 

The author of this article doesn't get it. Twice, he mistakenly refers to capacitors as a chemical storage device when in fact it IS purely physical. Even when an expert tries to explain it to him, he inserts a reference to chemicals INSIDE the quote.

For those of you who are uninformed, capacitors store energy by physically pushing against the sides of their container. When overloaded, they explode from the pressure. That's why high capacity capacitors are very large and always cylindrical (the cylinder can take the pressure) AND that's why capacitors can discharge so quickly; the pressure is released faster than any chemical process.

The point of the supercapacitor is that the special chemical coating allows more electrons to physically attach to the plate inside the capacitor.

Nate

 

The reason a combined system would be good is that the characteristics of Batteries and Capacitors complement each other.

Batteries are good for storing a lot of energy, and releasing it at a steady rate, but they have a limited charging rate, and discharge rate.

Capacitors generally store less energy, but they can charge and discharge very quickly, and at a very high rate. But, the discharge is non-linear and tapers off fairly quickly, not making them good for sustained release at a steady state.

So, the Batteries are good for steady assists, like on the highway. You charge the battery on the downhill, and then use the battery energy to lessen the load on the ICE on the uphill, with a very steady, long duration output.

Capacitors can charge quickly, so you can recover more energy than with a battery if you exceed the charge rate of the battery during regeneration, allowing you to recover more total energy in a short time.

They also can discharge very rapidly, and at a high energy output rate, so they are very good for assisting rapid acceleration.

So, if you floor it at a stoplight, or in a passing situation, a Capacitor could provide a transient burst of energy much higher than the battery max, but it can't sustain it for as long, and the amount it provides tapers off relatively rapidly.

Combining the two would be a very effective system, providing off the line performance the battery alone can't offer, while retaining the ability to unload the ICE with recovered energy during sustained operations.

 

Yup, so to speak.

 

This is just from the hip but how about a whole series of these with a computer control that activates the next in line when the prior is discharged. It could also start charging any dischaged ones. The Prius could use another computer. One more node in the LAN! There are probably great reasons why this can not be done, if for no other reason that I know nothing about it.

 

Where do I find details on the capacitor mod?

 

how capacitors work

The above explanation is incorrect. Capacitors don't involve any physical pressure. They involve storing energy via electric fields. Except for the parallel plate variable capacitors used in old AM radios (where there was just an air gap between the plates, with air being not much different from a vacuum in terms of dielectric properties, pretty much all capacitors use a strong dielectric which is a polarizable material that lets you store more energy for a given voltage between the capacitor plates.

Don't take my word for it; look it up in any college level physics book, or better yet a great text like Introduction to Electrodynamics by Griffiths.

 

Toyota will probably incorporate capacitors when they're ready. Potential is one thing. Cost is another. The article does not say how much they'd cost.

Evan, can you tell us how much additional capacity was installed via capacitors, and the cost, size, and weight compared to an equivalent amount of battery capacity?

 

<div class='quotetop'>QUOTE(altaskier\";p=\"32097)</div>

thank you thank you thank you for that clarification!
i was going to feel guilty if it were me, again, doing the EE class .......

capacitors store energy in the electric field in an insulator between two conductive "plates". if the insulator is vacuum, you can put a very high voltage across them without the voltage arcing across. if the insulator has a "dielectric constant" that is multiples of that of vacuum (which is nearly the same as air,) more energy can be stored for a given area of plates per volt, etc....

http://xtronics.com/reference/esr.htm

is a good place to read about some of the basic concepts.

now, if you really want to compare capacitors to batteries, do this math: how watts can you draw from the Prius main drive battery and for what length of time, before the battery voltage drops by "x" amount of volts? that number of watts times the average voltage, times the number of hours = the watt-hours you've drawn from the battery. use the equation from that link, p=c*(v squared)/2t to estimate how many farads © large the capacitor would have to be to provide a similar amount of power for a similar amount of time.

it's going to be very large, and more than very expensive.

by the way, "charging" a capacitor is the equiavalent of "pumping current into it and watching the voltage across it increase." if the source of the current has an inherent limit to its output voltage, you're darned right you can't "overcharge" the capacitor! it stops "charging" when the source of the current hits its maximum voltage! at that point, the charging current goes to zero and the voltage completely stops rising. if their were a source of CURRENT which could keep pumping into the capacitor, irrelevant of the voltage across the capacitor, the capacitor would eventually arc over internally and discharge all of its internally-stored energy pretty instantaneously. when this happens in real life, the capacitor literally explodes like a firecracker or small grenade. very dangerous.

also, by the way, since a capacitor is basically two parallel plates with opposite charges on 'em, the two plates actually are pulled TOWARDS each other by the electric field between them, not pushed apart. if anything, a failing capacitor implodes, rather than explodes, but it's really the energy release that makes it actually explode...


end of class/rant.....

 

postscript, after a bit more Googling...

http://www.batteryuniversity.com/print-partone-8.htm

also, from
http://www.csiro.au/index.asp?type=mediaRe...et=mediaRelease

"The supercapacitors have a number of advantages over batteries. For instance they are capable of providing a very high power level in a very short time. This is something that a battery cannot do," says CSIRO researcher Dr Tony Vassallo.

"They also can be charged and discharged almost indefinitely, the recharge rate is rapid, they require no maintenance and be made from non-toxic and relatively inexpensive materials."

"Batteries on the other hand are able to sustain their energy output for longer - if you need energy output for longer than 90 seconds you need to use a battery. So while supercapacitors won't replace batteries in all applications they work very well if you put them together," Dr Vassallo says.

please do not think of supercapacitors as replacements for batteries. at least yet, maybe never.

as you draw current from them, their voltage drops linearly with time (volts per second or minute or hour). batteries don't.

 

<div class='quotetop'>QUOTE(plusaf\";p=\"63873)</div>

That's what has been going through my mind. They can deliver constant current until depleted, but not constant voltage. You would use only a small percentage of the capacitor's capacity, based on your voltage tolerance. However, since most of the requirements from the inverter are really current, we may be able to go down to a much lower voltage than we realize, as long as we have the current.

 

I'm not sure if I'd want a supercapacitor under my rear seat anytime soon.

In industry, very high capacity capacitors are used in VFD's (Variable Frequency Drives) as their quick charge/discharge duty is suited to the fast Pulse Width characteristics. In that application - high current transients for brief time periods - the capacitor is ideal.

In research with high peak power levels, such as the research into practical fusion power, capacitors are used to store enormous amounts of power to be used for very brief time periods. You spend hours or days building up the charge in the capacitors, and they're discharged in milliseconds.

However, when one does eventually fail, it's a catastrophic failure. Capacitors in research are usually kept in a steel cage with proper grounding for personnel safety, while capacitors in industry are kept enclosed to minimise explosive effects.

 

Personally, I'm holding out for Doc Brown's Flux Capacitor. I heard he put one in a DeLorean once. Was that in the past, or back in the future?
;-)