Large Capacitor

Maxwell Technologies has a 165F, 48V ultracapacitor that's 416x190x160mm and weighs 14.kg.

www.maxwell.com

Thanks, Bhankiii. I appreciate your help.

Try not to blow yourself up.

Good point. In the "electric shock from plastic" thread, we were talking about tossing charged capacitors to unsuspecting friends, and other pranks. This is a bit bigger than those. I don't think I'd want to discharge it with a pair of crossed screwdrivers.

Ken, couldn't you use a lead acid battery as a 'capacitor' to store this sort of charge?

Just a thought, John.

Yes I could use a battery, at a fraction of the cost.

Actually, this will be, potentially (no pun intended) in a small hybrid vehicle. The issue that make capacitors attractive is that during a stop (and even when going down a moderately steep hill), they can accept the inrush current from regen braking much more quickly than a battery can. With batteries the excess energy either goes off as heat from resistors, or directly off (more firmly applied) mechanical brakes. (The capacitor charge from regen would be fed into the battery as quickly as the battery can tolerate, and also used to directly the accelerate the vehicle, if the battery is already pretty well topped up.)

The capacitor Bankiii referenced is at the "sophisticated" end of the spectrum and is very close to the size I'm contemplating. Rounding to 240,000 watt seconds, then that's 4000 watt minutes, or 66 watt hours -- which, thinking in terms of car batteries, is only a 5AH 12v battery: tinier than a moped battery. (Could this be right? Maybe I'll actually have to do the math, rather than guess at it -- but I think that works out. But I also think the ultracapacitor people advertise battery storage density or better -- which, if my quasi math is right, they are well short of. If you have a minute, see what you come up with.)

There is a place over here that is promoting a huge capacitor bank (400#) as an alternative to batteries in EV's. Here's a link. Whadya think? Seems some of the claims that some people accept at face value are pretty wild -- but again, I haven't really thought this out yet.

(I'm about at the stage in this where I think "Why not go the the grocery store and buy 50 rolls of aluminum foil and 50 rolls of waxed paper, wrap the thing together, and stuff it into a trash can. Charge it up, see what happens. In the early concept stage, it's easy to say " I just need a big capacitor." Later, in the disillusionment stage, you realize how significant that word "just" can be.)

Ken:

The energy storage densities by weight of ultra (or super) capacitors is only a fraction (~1/10) of a conventional storage battery. Their value lies in their ability to supply and absorb large amounts of power much faster than a battery is capable of doing, while also being able to endure many thousands of times more charge/discharge cycles. They are also about 95% efficient, which is much higher than storage batteries. However, to be effective in your application, you have to charge them to a voltage much higher than the battery voltage because their output voltage is a factor of their amount of charge, as with regular capacitors, and once the removal of current causes the voltage to drop down to battery voltage, it can supply no more power regardless of the amount of charge remaining. This is not a problem when storing the energy from regenerative braking because you control the output voltage of the motor/generator. Therefore some additional electronic circuitry is required to enable it to work in conjuction with the battery, but having done that it is the ideal complement to it.

If charging an empty ultracapacitor for testing purposes, make sure you have a current limited circuit.

Greg

Hi Greg. Thanks for your comments. Your understanding and mine mesh pretty well re storage densities of capacitors vs lead acid batteries. Gut feel tells me that the EEstor claims are probably completely whacko – but I don't have hard evidence of that, right now. I pulled the following from one of the many web articles:

• According to the BusinessWeek article, the company was founded in 2001 by former senior managers at Xerox PARC and IBM. Little information is known about EEStor, which prefers to operate in stealth. In fact, to this day it still doesn't have a corporate Web site.
• However, BusinessWeek did learn that EEStor has developed a "parallel plate capacitor with barium titanate as the dielectric," and that it claims to make a battery at "half the cost per kilowatt-hour and one-tenth the weight of lead-acid batteries." It also learned that EEStor planned to build its own assembly line to prove the technology works, and following that, would license the technology to manufacturers for volume production.

So, they are claiming a roughly 100 fold improvement in storage density over ultracapacitors, for a tiny fraction of the cost (even less than the cost of lead acid). Seems ludicrous to me – I can't think of any area in which we have made gains like that in one "step": in other words, where we've had laser surgery and stone axes on the market at the same time. Honestly, I find it hard to even say "But who knows?"

If they could deliver on this promise, the world would change very quickly. My small hybrid would only need to fire up its IC engine when crossing the Himalayas. In ordinary usage, I'd just plug the thing in once a week.

My interest in using capacitors is exactly as you have described – in other words, to use the capacitor to absorb short term high inrush currents, and to then feed the main storage system (batteries) from that charge at a rate that the batteries can tolerate. (I've thought a little about using voltage doubling, or even inverting/transforming, etc to deal with the otherwise inaccessible part of the charge.) Using the capacitors directly for acceleration would only be done when the batteries are fully charged. But even then, there would be times (e.g., stopping at the bottom of a long hill) when the cap would be fully charged, the batteries would be fully charged, and mechanical brakes would be used.

(But wait – I've got that covered too: I beam that extra energy through well known Tesla principals to my own wireless energy receiver atop my home.

The newest generation of lithium ion polymer batteries have much higher charge rates, so the utility of this capacitive charge intermediate step may not be worth the cost or effort. (But, of course, the LIP batteries are expensive.)

All of this actually strays a bit from my original design intent, which is to keep things simple and as "old tech" and low cost as possible. I'm planning to keep the whole system not too far removed from something you'd find in a golf cart (the theory being , that if everything else about the vehicle supports high fuel efficiency, then the whole hybrid storage system need not be too exotic – and if some braking energy goes off into the air, so be it.)

Ken

¿ May an ultra capacitor explode like an "electric" grenade?

The energy stored in a capacitor is E = V^2 * C/2

If C = 200, and V = 50 then E = 2.5E5 Joules

If after a component failure a short is established inside the capacitor, and this energy is dissipated in say 0.001 seconds, then during the explosion the power would be some 250 Mega Watts average, and that is dynamite, and your laboratory would become a crater.

It would be interesting to know how much serial equivalent resistance does have that capacitor, and if a current limiting device is already placed inside the capacitor to avoid that disaster.

Jaime Soto Figueroa

http://www.matharts.cl

Yes, I think these are real concerns -- especially in a vehicle application, where the capacitor could be deformed into significant short during a collision. But, I suppose this could be a good thing for reducing the manufacturer's product liability: How could anyone sue, if every time there is an accident there is no evidence left?

Ken, if the idea is to have a suitable storage system for regenerative braking etc... then yes I agree a lead acid battery would need to be a special design to accept the large currents involved...

So how about storing the energy in a flywheel?

Its been done before, I think the buses in Denmark(?) use huge flywheels under the floor to store energy when descending gradients or braking and then to transfer the energy derictly back to the wheels when accelerating - So there are minimal losses compared with converting to electricity and batteries etc...

I think the flywheels could easily store many Mega joules of energy... But this was 20 years ago, maybe they have re-engineered the system now?

John.

Hi John,

I like the idea of flywheels, and they can match (or better) batteries in storage density, although to do that they generally must be spinning in a vacuum at very high speeds. Large slow-turning ones end up being too heavy. If a high speed flywheel is mechanically-linked to low speed wheels, then the continuously variable transmission of 50,000 rpm flywheel to 100 rpm wheels (or less, when starting out) becomes a challenge to do economically.

Another possibility is to spin up and extract power from the flywheel with a motor- generator. This kind of thing can used for uninterpretable power supplies. "The word" seems to be that this won't make it to mobile applications, partly because of the support equipment required, and partly because the gyroscopic forces can be so high.

It will be interesting to see if EEstor (see response to Greg) comes up with anything close to their claims. Frankly, I simply can't believe that they can do what they claim, but I have been wrong before (as recently as 1982, if I recall correctly). Light weight, large capacity, high charge rate, long life -- the holy grail uncovered. (Even so, you have to deal with 3200 volts... )

It may be a good thing to connect the terminals of such a capacitor across a resistor when it is not in use, for safety...

Hi Ken,

I drive a 2006 Honda Civic Hybrid, and it's NiMH batteries charge quickly under the conditions you describe. I don't know the Ah rating, but it must be pretty high considering the performance I get out of the the car. I'll check for you. The batteries charge very quickly, from 10% to >80% in just a few minutes. You may wish to look into NiMH batteries for your application.

-e

Hi Europium,

Thanks for your comment. NiMH batteries are high on my list: they are much better than lead acid at accepting high charge rates, but not as good as capacitors, which live for the thrill of the "big inrush". Unfortunately, they are expensive, as compared to lead acid. LIP batteries are even better in every respect -- but even more expensive. (And of course ultracapacitors are no bargain, either.)

I'm trying to stay with my original design intent, which is to keep things as simple, inexpensive, and and old tech as possible -- but occasionally I am seduced by the sexy charge curves of some device that can really suck up and swallow those electrons like...

I think there is a market for a simple, small vehicle that holds two, is reasonably (but not wonderfully) crash-worthy, that can keep up with traffic, and that can consistently squeeze better than 100 mph from a gallon of gas. It is possible to achieve that goal even without going to a hybrid -- but for around town use, hybrids make a great deal of sense. The Aptera is not dramatically different than my design, but it pushes the envelope out a little further than necessary, I think. The Aptera may end up at pretty close to $20,000, at which price many people will simply go with a car like yours or a Prius.

Ken,

This is a bit off thread, but, it's puzzled me for some time. Wouldn't a small car be better if it was long and thin instead of short and wide? You could use stabilisers/outriggers for parking, and a flywheel/gyroscope (which could double up to store braking energy) for stability once the engine is running. Think about the total amount of road area needed when you include the safe gap between a commuter and the one in front. I realise that there would need to be some changes to the ways that roads are used but the benefits seem worth it.

Hi Randall:

I agree. A long thin shape is likely to have not just a lower coefficient of drag, but also less frontal area. So, without too much work, you can come up with a shape that has 1/4 the aerodynamic drag of a conventional small car.

In fact, when I started building the prototype, it was on two wheels, and as you said, had outriggers (although no gyroscope). In a way, it was like an inexpensive version of the Ecomobile, but with the focus on very good fuel efficiency, rather than on going fast. The Ecomobile has computer-controlled outriggers, and apparently it still swerves a little when you are starting out, as you transition from very low speed to normal speeds.

I found it hard to get the knack of driving the two wheeler: I've done quite a bit of motorcycling, including some roadracing, and the "feel" of a fully enclosed vehicle makes you want to drive it like a car. Although there are still many motorcyclists unaware of how a motorcycle corners (who ride without really thinking about it), in fact, when you bank to the left, you must first turn the handlebars to the right. This causes the gyroscopic effect of the front wheel to bank the bike to the left and also steers the front wheel out from under the bike, also causing the bike to bank left. When you are on top of a motorcycle, you are generally unaware of this -- until you panic -- and then too many people turn the bars away from the thing they are trying to avoid, causing them to bank into it. In an enclosed two wheeler, the transition from outriggers down (steering like a car) to outriggers up (counter steering) is a little tricky to do smoothly. You have to learn to drive it, and I didn't want to sell something that would feel squirrelly on a test drive -- or something that people might steer incorrectly in a panic.

The Ecomobile suffers from this same problem, even with its computer-controlled outriggers. However, the people who buy it are real enthusiasts willing to put up with this odd feature on an $80,000 motorcycle. (It's a little like putting up with quirks in a $250,000 Ferrari that you wouldn't accept on a Honda Civic.) The people I am planning to sell my vehicle to simply want to get from a to b, inexpensively and with little environmental impact. So my prototype is on three wheels, and operates just like a car.

Many years ago, there was a gyro-stabilized two wheeler like you described.

Thanks Ken,

I can see that the problems probably outweigh the benefits:

• People don't want to learn to ride a two wheeler.
• I had realised that the axis of the gyro was a problem (when you try to go up a hill it throws the car on its side).
• Maybe someone should go the whole hog, incorporate Segway technology, and, go for just one wheel.

You'd need 2000 of the 70 Farad 2V version of these: not really a possibility. I just thought that the life expectancy discussion in the data sheet might be more honest/revealing than normal.

http://www.cde.com/catalogs/EDL.pdf

Thanks, Randall. Yes, the data sheet is interesting. These ones seem to pack a lot of capacitance into a small package, but they sacrifice traditional capacitor benefits (near instant charge and discharge, extraordinarily long life) to do so.

Ken,

I have spent years, off on on remembering a story about some collage or university which did a field experiment with very Large Capacitor. I heard about it 25 years ago and back then it was said to have been done about 20 years before that.

The construction had to do with two level of plates an upper one and and a lower one out in a field or parking lot because the natural wind blowing between the plates would build up a charge. The charge became to great and fear of the potential, they discharge the energy to ground and disassembled it.

All I know of it is from the above and that the plates may have been to separate types of material. The voltage was getting near to that of a lighting strike.

Well this may sound strange, but the source I was told from was good. Who knows what actual voltage it was or what materials were used and if it is not exactly perfect telling of the facts it is still something worth looking into if it could be done. Or maybe someone might know what truth that story stemmed from.

Think about driving down the road making a source of power from a spoiler or a wind generation plant with no moving parts.