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Semantics of "Can we store electricity?"

  1. Nov 15, 2016 #1
    I am working on "electrical energy" storage and I want to know why exactly can't we store "electricity" from a power plant.

    My understanding is that "electrical energy" storage actually refers to the conversion of electricity to other forms of potential energy (such as mechanical eg: dam, or chemical eg: batteries).

    When someone tells me that we can't store "electricity" I think they mean that we can't store electrical current (ie the flow of electrons).

    But, if one defines "electricity" as charges, do we have a way of storing charges (eg: capacitors) and if so could we do it to deal with outputs from a power plant ?

    But all that seems to imply that what we can't store is electrical power (Watts = Voltage x Current) because we can't store current.

    However, if we can store charges, surely we can store electrical energy according to : Energy = voltage × charge ?

    The bottom line being : If I say "and that jolly morning I stored electricity." Am I going to be pulled apart by the examiner?
     
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  3. Nov 15, 2016 #2

    phinds

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    Electric charge can be stored on a capacitor but that is not even remotely a good way to store electrical power since capacitors, even really big ones, are pretty constrained as to the amount they can store and come nowhere close to power station levels. You COULD store some current in a superconductive loop, but the cost would be ridiculous and you would not be storing power station quantities

    So yes, we store electrical power by converting it to something else and converting it back when it's needed. The most common is potential energy stored by moving water up above a hydroelectric generator and letting it turn the generator later to reproduce electrical energy.
     
  4. Nov 15, 2016 #3

    CWatters

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    Anytime you recharge your phone you are storing electrical energy from a power station....just not a lot of it.

    Some hydroelectric power projects can be used in reverse to store excess energy from wind or solar farms. Google pumped storage.
     
  5. Nov 16, 2016 #4
    But isn't that conversion of electrical energy to chemical energy (in a battery) rather than storing electricity itself ?
     
  6. Nov 16, 2016 #5

    CWatters

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    It is.

    The problem with storing charge on a capacitor is the way capacitors work.. There are three relevant equations..

    E = 0.5CV2......................................(1)
    where E = Energy, C = Capacitance, V = Voltage

    V = Q/C ......................................(2)
    where Q is the charge.

    C = ε0εr A/d ........(3)
    where A is the Area of the plates, d is the separation between them, εr is the relative permittivity of the insulator (dielectric) between the plates.

    Together these tell you that to storing a lot of energy or charge increases the voltage. However equation (3) tells you that you want thin insulator between the plates. These are contradictory objectives. High voltage leads to breakdown of thin insulator. You can try and increase the area of the plates but that makes them physically large/heavy. You can also try and increase the relative permittivity of the dielectrics but in many cases that seems to reduce the breakdown voltage..

    Quite a bit of research is going on but currently capacitors store a lot less energy weight for weight and volume for volume than batteries. I don't know if this is upto date but you can compare various ways of storing energy here...

    https://en.wikipedia.org/wiki/Energy_density

    From the first table the Specific Energy is..

    Super capacitors - 0.031 MJ/Kg
    Lithium Ion Battery - 0.36 to 0.875 MJ/Kg

    So Lithium Ion batteries can store ten to twenty times the energy per Kg than super capacitors.

    The main advantage of capacitors is that the charge/discharge current can be high and that they tolerate cycling better. So in some applications capacitors are being used in parallel with batteries.
     
  7. Nov 16, 2016 #6
    Every now and then people talk about powering things with capacitors and it's usually not very practical because of their low energy density. There's a big difference in the way batteries and capacitors store energy, one being in the form of an electric field and the other chemically. The practical difference is that capacitors can charge and discharge through many cycles without degradation where batteries have limited cycle life.

    If batteries could be developed with much higher cycle life and capacitors could be developed with much higher energy density there would be applications where they could be used interchangeably. There are capacitors now that are a hybrid of the two, though they don't have all the properties of a traditional capacitor; link

    What is stored is potential energy. As already mentioned there's a lot of ways to do that from electric fields such as capacitors to the gravitational potential of mass such as water for a hydro-electric plant.
     
    Last edited: Nov 16, 2016
  8. Nov 16, 2016 #7

    russ_watters

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    What examiner? What is the purpose/relevance of this question? Ie, who cares/why does it matter? At the user level, nobody cares if chemical energy is different from electricity and on the engineer level, nobody is confused about - so nobody should quibble about - the difference. Heck, you can even argue it as an issue of grammar if you want, but why bother?
     
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