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Could capacitors ever rival best batteries?

  1. Feb 15, 2013 #1
    If I understand it correct,energy of atomic electronic shells fusion cannot exceed energy of their breakdown.For example you need to spend more energy to break down water molecule than you recieve energy when you burn hydrogen.Does that mean that energy stored in capacitors could potentially exceed energy stored in batteries?What we have in battery is fusion of atomic shells and obtain energy.What we have in capacitor is deformation of atomic shells (under breakdown limit) and store energy.
  2. jcsd
  3. Feb 15, 2013 #2


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    To be clear to anyone else who may read this, the only reason you expend more energy separating hydrogen from oxygen when you split water than you get back when burning it is because of losses in the machine that splits the water. Chemically the energy required to split water and the energy you get back when it burns is exactly the same.

    Also, I'd like to point out that capacitors and batteries act quite differently on discharge. A battery is typically putting out a very steady amount of voltage that drops only slightly as the battery discharges. A capacitor's voltage follows a time constant curve on charging and discharging, and if you discharge the capacitor to 50% the output voltage has dropped by 50% as well.

    Current super-capacitors have about 10% the energy density of conventional batteries, but have 10-100 times the power density due to their ability to discharge extremely quickly, whereas batteries are limited by their chemical reactions and internal resistance and charge and discharge much slower. So super caps have only 10% the storage capacity of a battery, but can apply that energy as power at a much greater rate.

    I think asking if capacitors will rival batteries is like comparing apples to oranges. Batteries and capacitors are simply two energy storage devices with different uses and strengths and weaknesses. Even if we do get capacitors to comparable energy densities the differences between batteries and capacitors means that batteries will probably never die out.

    However, here's a link to a prototype capacitor with an energy density of about 300 Wh/kg. For comparison, typical batteries you use around the house have an energy density of about 100-300 Wh/Kg. I don't know if these are being manufactured yet or not.

  4. Feb 15, 2013 #3
    It seems I read about such things before.What they mean is most likely a ``pseudocapacitor`` the type of energy storage which relies simultaneously on both charge storage and chemical reactions (like battery) to store energy.That thing will have some disadvantages of battery (faster degradation) and in completely packaged form will have much lower energy density than 300 Wh/kg maybe even 30 Wh/kg more realistic.
    I think the best way to increase energy density of capacitor could be using some material with extremely high dielectric constant.For example it could be metal nanoparticles embedded in ceramic matrix.Metal have dielectric constant close to infinity.In such package you would be able to obtain both high dielectric strenghts (ceramic matrix) and infinite dielectric constant.
  5. Feb 16, 2013 #4


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    There was the old idea of Abram Ioffe http://en.wikipedia.org/wiki/Abram_Ioffe
    to make super accumulator by polarising dielectric.

    There was much mess about it in the world (New York Times, Pravda,...).

    The stuff ended by nothing. Frenkel criticised that real accumulator did not exist...

    Ioffe have got nick name "dreamer Ioffe".

    I found only russian story in russian about this event:

    You can googletranslate this url
  6. Feb 16, 2013 #5
    What the eyewash.Majority of dielectric capacitors which used now in the World rely on this effect.Higher dielectric constant provides you higher energy density and up to now limit to dielectric constant was not found.I do not know if someday they will create material with diel. const. equal to billions,if yes than Ioffe dreams may become fulfilled...
  7. Feb 16, 2013 #6


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    Some companies have been trying to build super capacitors that rival batteries for years.


    One difficulty is evident from the basic equation for a capacitor....

    C = ε0εr A/D

    where A is the plate area and D is the plate separation.

    To maximise C you want D to be very small but that causes problems for the voltage rating of the capacitor. You need to a dielectric material that has a good breakdown voltage. Due to the small dimensions it also needs the right physical properties.

    Seek independant financial advice before investing any money in companies claiming to have got the technology working.
  8. Feb 16, 2013 #7


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    You are surely right.
  9. Feb 20, 2013 #8
    To maximise C you want D to be very small but that causes problems for the voltage rating of the capacitor.
  10. Feb 20, 2013 #9
    If you go a way by increasing dielectric constant it could be not necessarily.Scientists discovered superinsulators.
    Also near Curie point dielectric permittivity of some crystals approach infinity.
  11. Feb 20, 2013 #10


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    The problem (as others on this thread have said) with energy storage using capacitors is the voltage drop as charge is used. At 50% of stored charge used the voltage in a capacitor would be 1/2 the fully charged voltage, in a battery at 50% 'charge' the voltage would be close to the original voltage.

    Battery E = VQ
    Cap E = (1/2)QV


    A battery with a peaking capacitor that's only used to deliver burst power and to supply the first 20% of total system power in a circuit can be useful because it reduces total battery charge cycles (increases battery life) and usually most batteries have the lowest charge efficiency near 80-100% SOC so recharging a capacitor first might increase overall efficiency by keeping the battery at 80% SOC when power in being cycled rapidly.
    Last edited: Feb 20, 2013
  12. Feb 20, 2013 #11


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    On my boat, I have a 12 v 50 amp alternator driven by a 28 kw propulsion diesel. When the boat is not moving and the engine not used for propulsion, it takes hours to charge my batteries, while using only 5-10% of the rated horsepower. It is very costly and inefficient.

    I dream of a capacitor assisted charging system. It would let me use a larger capacity generator to charge a capacitor. While charging the cap, I could run my diesel at near 100% power for 5 minutes rather than at 5% for hours. Then I could shut off the engine and let the capacitor slow charge the batteries as it discharges.

    System cost & weight should be reduced if the generator and capacitor voltages were many times higher than 12 volts; say 600 volts. The charge controller limits the voltages and currents seen by the batteries. The total energy of a battery bank recharge is on the order of 2-3 kwh.

    If I could do that with a capacitor/generator/controller on the order of 100% of the size, weight, and cost of the lead-acid batteries, it would be a winner.
  13. Feb 20, 2013 #12


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  14. Feb 20, 2013 #13


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    I do have an aux generator, a 1 kw gasoline powered one. I use it to reduce wear and tear on the much more expensive diesel engine. However the aux generator also needs to run for hours to recharge the batteries.

    I could also buy a diesel genset, dedicated to battery charging. But they are very big, difficult to install, and they cost up to $10,000.

    Of course on some days solar panels can charge the batteries better. On other days wind generators can do it better. That still leaves days when charging by primary engine or aux engine is necessary. Capacitor assisted chargers could still be attractive.
  15. Feb 21, 2013 #14


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    Last edited: Feb 21, 2013
  16. Apr 26, 2013 #15
    Could somebody calculate what would be energy density of typical dielectric capacitor if dielectric constant is equal to 1 million? Lets say it voltage is 50.000 V.
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