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Does anyone know a general answer to these questions? (I've asked them together because they're all pretty related, it seems.)

  • Why is it that we find electrical energy so difficult to store? Do we just find energy difficult to store generally? (...surely not, we can store energy in a block by sending it to the top of a hill.)

  • is there something in particular about charge/electricity that makes effective batteries difficult to produce, and, if so, what?

  • Is the problem that we're having with storing the energy just an artefact of our use of the energy, or is it difficult to store electrical energy per se?

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we actually don't store electricity we use electricity to rvarse a chemical reaction which when needed can be followed in forward direction to produce electricity like in batterise –  Akash Mar 8 '13 at 15:01
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Difficult relative to what? –  Michael Brown Mar 8 '13 at 15:12
    

3 Answers 3

As pointed out by Akash in a comment: when a battery is charged electric energy (potential difference) is converted to potential chemical energy.

In an ideal battery only reversible chemical reactions occur, so that it can deliver many discharge/recharge cycles. However, it's not possible to design a battery in such a way that no unwanted reactions occur at all. Over time products of unwanted reactions foul up the mix. Battery design is about trade-offs. The lead-acid batteries that are used in cars are good for many cycles (indicating that very little capacity is lost to irriversible chemical reactions), but the lead-acid chemistry is not suitable for portable devices.


But there are also forms of storage of electric energy that do not convert it. A capacitor stores electric energy directly.

In a capacitor some regions of its interior get a surplus of electrons, and other regions (separated by an insulation with special properties) become proportionally electron depleted.

The electric force is mind-bogglingly strong, and it's a long-range force. That long range is the big problem. To avoid concentration of electric force the electron-enriched and electron-depleted regions must have a very large surface area, and all of that surface must be very close-by to each other. (A common way to fabricate a capacitor is to stack ribbons of foil and then roll up that ribbon to a cylinder.)

When a capacitor fails internally there is a runaway effect. A capacitor must have very strong safeguards against failing, because if it does it's catastrophic failure.


The reason that fuel such as gasoline is so efficient as a means of high-density stored energy is that the chemical force between the atoms of a molecule is very short-range.

Once fuel is burning a lot of heat becomes available. That heat is generated by the attraction between the oxygen atoms from the air and the carbon and hydrogen atoms in the fuel. But even a mixture of air and fuel vapor is still very stable; the oxygen is bound in oxygen molecules, the carbon and hydrogen are bound in the fuel molecules. It takes a pretty strong trigger to start combusting. Fuel is so stable (comparitively) because the force of chemical attraction between atoms is very short-range.

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Yes, electrical energy is difficult to store.

In my opinion for the following reasons:

It dissipates fast with explosive reactions in specific situations since it depends crucially on conductivity which can easily be affected by weather or accident. The more electrical energy is stored, the greater the possibility of breakdown of insulation. It is as if one built a dam and the water could easily find a hole on the floor or break the dam.

We are frail handlers and subject to death once meeting a strong electric current, which means that there should be a lot of fall back solutions, for example small energy scales and voltages.

Batteries are getting better as time goes on, but not for bulk energy storage.

For bulk electric energy storage pumping water to higher level and using it as hydroelectric power can be considered. This problem will have to be solved when (or if) solar and wind power become dominant.

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First, electricity is the flow of electric charges. That is, by definition it is not a stored form of energy but a flux. What you store is always internal energy: energy in the nucleus, electronic energy, bond energy within molecules (a multi-electron form of electronic energy), and inter-molecular energy (again essentially electronic energy),or bulk external energy such as gravitational potential energy, electrical potential energy, or kinetic energy

That brings us to the next issue: how do we convert electrical charges to internal/external energy of something and more specifically what kind of internal/external energy

External energy

1) Capacitors: Storage as actual separated electrical charges.

2) Pumped hydro storage, ball on the top of a hill: storage as gravitational potential energy

3) A spinning flywheel : macroscopic kinetic energy

Internal energy

1) A phase-change storage: Convert water to steam or ice, i.e., store energy as intermolecular energy), adsorb hydrogen on a storage medium, etc.

2) A chemical/electrochemical battery: Bond energy between atoms in a molecule (intramolecular) e.g., storage by converting water it back to a hydrocarbon fuel. Electrochemical, reducing ions back to non-charge molecules.

3 Create a nuclear fuel maybe.

So where is the difficulty?

1) Depending on which form you choose you are always making two transitions (Electricity--to another form---back to electricity) that are lossy: Say you want to convert electricity into chemical fuel by converting water and CO2 into methane and burn methane to get back electricity when you want. In going to methane you conserve energy but degrade its work potential (the useful part of the energy or exergy). This happens because you generate entropy. I can explain this in detail but this is not the main focus of this question. Essentially there is a thermodynamic loss of useful fraction of energy such that in going from electricity to methane and back to electricity again, you get a smaller fraction back.

2) The storage could be leaky or degrading: Say you were to store as internal energy in a battery. You are adding electrons to this electrochemical system and changing its composition. Every time you go back and forth a few active molecules remain in their more stable state such that after several cycles there are not enough left to store much. Essentially the reason is the same as before entropy generation.

3) There could be a limitation to the capacity of storage: Say you were storing energy by pushing balls uphill or storing water in a tank, there are only so many balls you can take after which your hill has no space left. Similarly if too many charges are pushed on a capacitor there could be dielectric breakdown of the capacitor. If you spin a flywheel too fast (to store more energy) you could shatter it because of rotational stresses.

Each of these three problems exist in each of the energy storage methods.

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What's with this "ball on the top of a hill" thing? Pumped hydro storage is by definition water - that's what hydro means in this context. –  EnergyNumbers Mar 9 '13 at 11:28
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You can store gravitational potential energy in any manner... You can pump water, you can push balls up the hill. I know water is hydro! Please pause before you make comments such as this. –  Sankaran Mar 11 '13 at 23:56

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