Why is electrical energy so difficult to store?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

A general answer which is not of any particular use is that electrical energy, and the forms in which we store it, are typically very low entropy systems. The lower the entropy the more they "want" to dissipate and the harder it is to stop that tendency to turn into (ultimately) heat. Same way that it is a lot easier to store water that is 10 degC above ambient than 100 degC.

Answer 5

All energy is difficult to store, not just eletrical. Indeed, electrical energy is quite easy to store once you consider the big picture.

If you look at a tank of gasoline, you can see "wow, what a great storage for energy!". But while gasoline is great once you have it, consider how it was created in the first place:

• Fusion reactions inside of the Sun liberated some of the potential energy of a couple of protons
• The released energy propagated throughout the Sun (this already takes a very long time) until it reached the surface...
• ... where it formed into photons that started on their trip to the Earth
• Just to be partially absorbed and scattered by the atmosphere and water etc.
• With just a rather small part being absorbed by plants

Now the second part starts:

• The plants absorb a tiny part of the incident energy and store it in the form of sugars and fats. The efficiency of this process is something that's quite complicated to measure, but the typical number used is somewhere between 3-6% - that's absurdly horrible compared to anything human machines do.
• If you're lucky, when those plants die, they drop into a bog of some kind, where they can decompose slowly without oxygen.
• And then they are pushed deeper into the ground, increasing pressure and heat just to what's needed to form oil.

And the final, human part:

• The oil is mined (requiring more energy input as the well progressively depletes - most wells are abandoned long before they are drained simply because this becomes too expensive).
• ... and a small part of the original oil is refined to gasoline, the awesome fuel. We also find use for most of the rest, but looking at just the gasoline and diesel, you still throw away most of the refined oil which isn't the right composition for those.

The efficiency of the whole process is horrible. Gasoline is a great energy source today, but that's only because it's already here. Recreating it the way it formed in originally would be a huge waste of energy and time. We only retrieve something like 1-2% of the energy required in the first place. And once you have gasoline, it does actually decompose - if you don't purify it, it will no longer be good enough to power a car in a year or two. How does electricity compare?

A typical lithium-ion battery has an efficiency around 80-90% in reasonable conditions, along with a discharge of about 8% per month. A hydro-electric capacitor (basically two lakes joined by a pump that can either pump the water up using electricity or produce electricity as it goes back down) gets around 70-80%. This is where most of electricity is stored worldwide - reportedly around 99% (!) of spare electricity to deal with differences in day-night demand.

Gasoline is cheaper, but only because it was already made over millions of years in the past. We're reclaiming millions of years of work.

Safety is another tricky thing. Energy is dangerous - and efficient energy storage is also very good at discharging that energy by accident. Dams break. Gasoline burns. Batteries explode (and slowly self-discharge). Nuclear power seems almost trivial in comparison, if it were not for how concentrated (and thus "visible") their effects are on failure - using the typical technologies, one kilogram of fuel-uranium is used to produce as much energy as 1500 tonnes of coal. Of course, the uranium was first created in a supernova somewhere, using just tiny portion of the energy released in the explosion - if you wanted to use supernovae for "manufacturing" uranium, the efficiency would be even worse than coal :)

The thing is, neither gasoline nor diesel ignite easily, so it's relatively safe (and simple) to store them in large quantities. But while this makes them relatively safe, it also means that it's trickier to release the stored energy - compare an electric engine to a diesel engine. You can make electric engines as small as necessary (down to just a couple of individual atoms, it turns out!) - in fact, you could consider our own ATP-synthase ("the powerplant of the human cell") to be a tiny electric engine. The battery in your cell-phone likely has about one tenth the energy capacity of the same volume of gasoline - but it's vastly more practical for powering your cell-phone.

All in all, there are many competing characteristics of any energy storage:

• Charge/discharge efficiency - electrical storage is very good at this (usually using chemical potential as the "actual" storage mechanism)
• Long-term storage - great for non-rechargeable batteries, not so good for rechargeables - but still comparable to common gasoline.
• Environmental conditions - gasoline needs large machinery and high temperatures to be efficient, batteries work great at room temperature (and often lose efficiency when it's too cold or too hot) and are self-contained.
• Safety - gasoline doesn't burn easily and we have plenty of relatively safe ways to store it. Some batteries are safe, but the highest capacity/efficiency batteries are deathtraps. I hope you buy your LiPols from reputable manufacturers - mechanical damage, overcharging, overdischarging, temperature, improper voltage/current drain... there's plenty of ways to make them fail catastrophically, especially if their electronic safeguards are missing or not up-to-par :)
• Cost - gasoline is much cheaper than the same energy in a battery (though the comparison isn't simple at all - it's not very fair comparing an AA battery, considering the scaling issues; one gram of gasoline has about as much energy as a single AA battery). Rechargeable batteries are even more expensive, but quickly overtake gasoline over many charge/discharge cycles due to their high efficiency and relatively long life. And of course, gasoline becomes vastly more expensive if you consider making it "at home" from scratch (water + carbon dioxide), rather than from the fossil deposits.

Of course, the most direct storage of electric energy is a capacitor. They are incredible at both charge/discharge efficiency and peak power, but only deal with tiny amounts of energy density - the reason for that is that they fundamentally work with disbalanced charges, so there are pretty tight limits on how to hold those electrons that try very hard to push themselves apart (electromagnetic force is rather strong) together. There have also been some experiments with storing electricity directly in superconductive loops, though that has some obvious drawbacks :) As I said before, for bulk grid energy storage, pumped water is by far the most practical - at least for now. For consumer devices, chemical energy wins for the moment.

Answer 6

We need to create a battery that would instantly store a large amount of electricity at one time. Ex. When a bolt of lighting strikes it gives off a very large amount of power. However a battery needs time to take that energy and change it over to a chemical for storage.

Lets say a bolt of lightning is 500 gallons of water and the battery that we presently use is a 5 gallons bucket. If we take that 500 gallons of water and pour it fast into a 5 gallon bucket, it would instantly over pour over to the floor. Therefore we need a large enough bucket or a bolt of lightning in this case that battery that can hold all that engergy at one time