Super Capacitors

Discussion in 'OnBoard Electronics & Controls' started by Doug Lord, Jan 13, 2009.

  1. Doug Lord

    Doug Lord Guest

  2. Jimbo1490
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    Jimbo1490 Senior Member


    When I saw the first 1f (albeit only 5v) capacitor that was the size of a dime about 15 years ago, I wondered when caps would get this good. So here we are! Trouble with any electric cars is not the cars, per se, but the fact that the electric grid in the US is hopelessly outdated and just can't deal with more capacity (no pun intended). So if widely adopted, these would place strain on that grid that it just can't handle.

    On the plus side, capacitors suffer none of the horrid charging losses that even the best batteries exhibit, so unlike battery powered electrics, cap powered electrics do not represent a net loss in efficiency. Another cool thing about caps is that once charged, they stay charged until discharged; even sitting for years.

    There is still the issue of what powers the grid, which right now is still mostly fossil fuels.

  3. CDK
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    CDK retired engineer

    Two conducting surfaces, separated by an insulator, form a capacitor. The capacity is determined by the surface area and the distance between the surfaces.
    For the insulator all kinds of materials have been used, like glass, paper, mica and polyester film. A major breakthrough was the use of aluminium oxide in a liquid filled tube: the electrolytic capacitor was born! That was around 1935.
    This principle is still used for all kinds of electrolytic caps, first with aluminium foil and electrolyte soaked paper, later with more exotic metals like tantalum and vacuum deposited conductors on an oxide surface.

    One common characteristic is that the devices are unipolar: reversing the polarity destroys the oxide layer and causes a short circuit. Another is
    the fact that the maximum working voltage is very low because the oxide layer is very thin. For the gold caps or super caps, now used in equipment to retain data during a power interruption the working voltage is limited to just a few volts, because the technology is the same as is used in MOS-FET's, only with even thinner oxide layers.

    But how much energy can a super-cap store?
    By definition, a capacitor of 1 Farad can supply 1 Amp for 1 second. Not very spectacular if you compare it with a small car battery that can deliver 1 Amp for 50 hours ( 180000 seconds). You would need a 180.000 Farad capacitor to do that, provided that it could handle 12 volts. If a 1 F super-cap has the size of a small coin, 180.000 pieces wired together will take at least the volume of the car battery.

    You could argue that super-caps could be produced that have a much higher capacity, say 1000 Farads, so you only need 18 pieces to imitate the car battery. True, but for practical use we need a battery that can deliver not 1 Amp for 50 hours, but 50 Amps for 1 hour, plus 400 Amps for half a minute to start the engine. Such currents require massive conductors, like copper bars ending in the kind of terminal used for car batteries.
    Only if the super-cap could be cooled to near 0 degrees Kelvin and kept there indefinitely it could be significantly smaller than Ni-Mh, Li-ion or lead-acid batteries. (Disregarding the 1000+ lbs equipment to reach and maintain super-conductivity of course)

    To construct a power source for vehicle use, the 50 Ah example is of course insufficient. Instead of 50x12 = 600 Watts, we need at least 50 times more storage capacity for a modest city vehicle, 200 times more if it must be competitive with present day gas or diesel vehicles.
    That is a lot of electrical energy in a very small space, requiring all sorts of safety devices to keep it from turning into a power bomb in case of a less-than-a-micron-thick insulator brake-down.

    One last remark: that capacitors have 100% efficiency only holds true for a theoretical case where isolation is 100% non-conductive and conductors have 0 resistance. In real life however, nothing is perfect. The charge will slowly leak away because there are always impurities in such an enormous insulator surface and the internal resistance of the conductor grid will turn part of the electrical energy into heat.
    The only thing positive about super-caps is that there are only electrons moving around, no atoms, so the life expectancy exceeds that of any form of chemical energy storage.
  4. Fanie
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    Fanie Fanie

    Forget it !!

    Super capacitors are not even close yet to provide the energy supply (even small) batteries are capable of. Although new caps have impressive farrad ratings, the amount of energy stored is not good enough for propulsion.

    Unless they meant you drive 500 miles to pay $9 for a cap you can charge to rated voltage. Whole different meaning :D

    There are new batteries currently being developed they claim can charge in 90 seconds to full capacity, and although it may well be possible, I haven't seen any of them. The expected life span (charges before replace) is also a factor in batteries.

    We are still looking for the ideal energy source, while some near ideal ones exist (radio active source, hydrogen gas) the deveplopment of these have not yet been commercialized and will not happen until they can find a way of safely work with /dispose of the sources, or find a way to rip you for a liter of water.
  5. ancient kayaker
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    ancient kayaker aka Terry Haines

    These things are what matter when it comes to electrical power storage for autos:

    KWH/cu ft
    overall efficiency at high currents
    charge/recharge cycles and durability
    memory effects
    charge retention

    As far as I know, lithium ion batteries are the current leader in most of these areas and are rapidly catching up in areas where they are deficient such as cost. Not sure about safety aspects: storing that much energy in a small space is unlikely ever to be completely safe, no more than a tankful of gas when the tank splits. There are several lithium ion technologies, some better than others.

    Concerning super-capacitors, my understanding is that they are currently able to power a toy car for a minute or so if that.

    And then there's fuel cells.

    I look forward to be proven wrong!
  6. Fanie
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    Fanie Fanie

    The Ancient meets the future :D

    Batteries are usually designed for a specific type of application, so in most cases I cannot replace the two 105AH SLA batteries for my 24V trolling motor. Well, I can but the financial impact would be considerable, and then also, would it really be worth my while to do so ?

    It is always so that if something really good and affordable becomes available, it replaces the old stuff, and that becomes the new 'standard'.
  7. Doug Lord

    Doug Lord Guest

    So if you guys are right the Zenn car story is a complete ripoff. We'll see.
    Personally, I hope you are dead wrong-well, not the "dead" part!
    Just found this-seems like the eestor(Zenn) system would really be terrific-if it's really possible. But this seems encouraging as well:
  8. mark775

    mark775 Guest

    They have been having amazing press releases for years. It´s how this type of company obtains venture capitol. Believe it when you see it.
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    FAST FRED Senior Member

    So if you guys are right the Zenn car story is a complete ripoff. We'll see.
    Personally, I hope you are dead wrong-well, not the "dead" part!

    If you like "stories" look up Ballard and fuel cells, million$ and million$ in research bucks , for NOTHING.

    so far

  10. Fanie
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    Fanie Fanie

    Fast Fred,

    There are two problems with caps. The first is the voltage drops as you discharge it, unlike a battery that tends to maintain it's voltage. Using a cap then requires a higher voltage that gets regulated to your working voltage.

    The second problem is the size. A 1 farrad cap that can charge safely to ie 24 V is a sizeable item. To be able to run a trolling motor for say 10 mins off a cap, the size required will probably be much bigger than the boat if at all possible for that long a time at some current.

    Capacitors lend themself excellent to supply high currents for very short periods of time, like in stud spot welders, but as a power source they are not suitable.

    As for research, there are too many who gets the money and use it to live off and well too, while no real research is done. We have the same here. Some guys have really rediculous ideas that will never work, but it seems a slick tongue gets them the green.
  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    Fanie: valid point about voltage drop.

    I hope my pessimism is wrong too, so OK, I did the math:

    9 min charge, lets say it can absorb the entire 200A/220V available from a house service and everything else in the house is turned off. (assumptions: ability to handle the energy level, 100% charging efficiency)

    That makes max 6.6kWH energy available.

    500 km range? Lets assume 50 klick speed, so it will have to drive 10 hours at 0.66 kW which is about 1 hp. (implications and/or assumptions: 100% efficiency, single occupant, prone position bike, fully-enclosed streamlined shell, extremely lightweight). Hardly suitable for shopping or going to work.

    Substitute some sensible efficiency, weight and drag values that could translate into a useable vehicle and you may get 30 km. Actually, even that would be an impressive achievement for a 9 min charge. Reduce my assumption on charge rate to something that standard house wiring can handle and insist on minimum range of 100 km and you're looking at a charge time around 20 hours, but that could be reduced to 5 hours with a suitable electrical service.

    I think supercapacitors of this kind may have a place in future transportation through their contribution to overall system efficiency, by levelling power variations, easing surge demands on the battery so the battery technology can be optimized for capacity and efficiency.
  12. ancient kayaker
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    ancient kayaker aka Terry Haines

    Not quite nothing but certainly less than we all anticipated from all the hoopla back in the 90's. I understand that Ballard is still working actively with Ford, maybe Chrysler too.

    Two problems with the hydrogen fuel cell is fuel distribution and fuel handling safety. Some would say that, with modern attitudes to safety gaining acceptance for the gasoline engine would have faced similar challenges. The people best positioned to distribute hydrogen fuel is the oil industry ...
  13. Ilan Voyager
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    Ilan Voyager Senior Member

    As a very long time ago I ceased to believe in Santa Claus, I'll wait to see usable cap/batteries without shortcomings. Technically, it's very doubtful that works in a practical way.
    EEStor announces that for mid-2009...we'll see.
  14. marshmat
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    marshmat Senior Member

    EEStor actually announcing something? Well, that'll be the day.... lol

    Their notorious secrecy aside, they may be on to something. There was a rumour circulating a few years ago that EEStor was doing some experiments with barium titanate in the electrolyte; it's been documented that BaTiO3 can have a permittivity of 1200 or more if you grow the crystals just right. That's one hell of a dielectric.

    Ballard.... well, thats another story. I think the production challenges proved to be somewhat greater than they had expected. They never did manage to get away from the need for a platinum catalyst adjacent to the PEM itself. There's lots of work going on right now about that issue, probably the single biggest reason why the cost of fuel cells is so high, but the current research is being done with somewhat less media fanfare, and minus the stock market woes that plagued Ballard.

    I've worked on the design and construction of two cars that have crossed continents at 80-100 km/h on about a kilowatt. It's quite possible, but you need to give up on some conventional notions of what a car should look like.

    The claim of 500 km range on a $9, five minute charge- that sounds more like a 3rd-party blogger getting a bit exuberant. A $9 charge is about 82 kWh at current prices of about 11 c/kWh (incl. transmission, etc. fees) in North America. That's reasonable- 16.4 kW or 22 hp for 5 hours to drive a small car 100 km/h. (Again, neglecting inefficiencies for the purposes of simple illustration for now.)

    But to transfer 82 kWh in 5 minutes would require a 984 kW supply- ie, nearly a thousand amps at 1 kV. That's not something you would want in your garage!

  15. Fanie
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    Fanie Fanie

    Hang or Matt, a capacitor is formed when two surfaces are oposite one another, the capacity is determined by the surface area and the distance between the plates. Dielectric and quality of insulation between plates determines the ESR rating, which is the impedence of the device. The lower the ESR, the better the cap, but you are still haunted by the surface area restriction and distance between plates for a certain voltage.

    Cap charge is static, but you can only charge the cap to a certain voltage. Any load will cause a drop in the voltage. You cannot have a cap, like a battery, 'store' energy that will break the charge down over a period of time at a constant supply.

    I have heard of barium titanite before, but I doubt the association was with capacitors, or it was something similar to do with hydrofusion which in my humble opinion is more of a future fuel than anything else.

    If they do succeed to cross the capacitor with a battery and specially growed crystals we are back to a battery or cell and not a capacitor.

    Question is, while everyone complains about fuel cell and battery prices, I doubt these cap-bat cells are going to be cheaper. Similar to solar, growing crystals is a slow process and remains expensive hence the shortage on solar panel cells and their still bloody expensive and unefficient.

    My opinion if there was something that exciting someone would have started it in the 1960's already :D
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