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Is it easy to store energy ?

september 2003 - last modified: August 2013

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Question of no relevance, some might think : of course it is easy to store energy. It just requires to have a tank full of fuel oil at home, a gasoline tank filled up in a car, a pile of coal in the basement, or a battery : all these effectively allow to have a stock of energy we can draw on whenever we like.

Indeed, what constitutes the interest of "energy", it's not only to have some : it's to have some when we desire it. Sailsboats or windmills, that rely on an energy source (wind) that is not necessarily available when we wish it were, illustrate pretty well this problem. As soon as we cannot store the energy from a given source, the actions that are enabled by this energy source are not necessarily done when we whish to perform them, but just when we can perform them. It most certainly is a determining factor for the whole organization of humane societies.

Presently, 85% of the commercial energy consumed in the world (commercial energy is energy that somebody sells to somebody else, like gasoline you buy, but not like fire wood you pick up in a forest) is constituted of fossil fuels (oil, coal, natural gas) that naturally fulfill the storage function : we can store them and then pick in them at any desired rate, without particular constraints. It is then just a matter of money.

Breakdown by nature of primary energy consumed in the world in 2012. Electrical energies are converted on the primary energy equivalent

Data from BP Statistical Review, 2013.

Why shoud we bother about storage, then, since mother nature did it for us, through the "manufacturing" of fuels that are so easy to transport and store ? Because, alas :

their usage disturbs the climate system, and therefore the vanishing of their use might be the result of our will not to use them any more (or not as much as today),

the reserves are finite, and even though the above mentionned problem could find a satisfying solution (which is very far from being the case today), we will have to "do without" one day.

Hence if we wish to keep an organization of the world where 80% of the energy we use is available on demand, and not only when the sun shines or the wind blows, we will need a a way to store the energy produced when nature decides it is, to give it back when we have a use for it. That's were troubles begin, because storing energy is not always a simple business :

We can store energy under a mechanical form : a moving object, for example, has some energy called "kinectic". Why not put large wheels - or other large objects - in motion, that would store energy and give it back on demand ? But we need VERY large and heavy objects to store energy amounts that become significant compared to what we consume : a 10 tonnes truck, going at 100 km/h (62 mph), own a kinetic energy that amounts to "just" 1 kWh. If we use a wheel, it is therefore necessary to set up an intallation weighting several tonnes to store a couple kWh at best : not fabulous....

Mechanical energy can also be "potential", which means that some available enery has been used to create a situation where movement can be produced on demand. This can designate water pumped up in a dam reservoir, air compressed into a tank, etc. Water stored in an dam reservoir, for example, can be flown down on demand, and when it goes down it will acquire a speed, and therefore a kinetic energy, that it is possible to transform into electricity with an alternator.

The national french electricity company already uses this possibility to store a little part of the electricity produced off peak hours by nuclear plants, that it is not easy to stop quickly at night. This electricity is used to pump up water in dam reservoirs where it will be used afterwards. 3,6 tonnes of water must fall from 100 m to provide one kWh of kinetic energy.

Energy can be stored as heat. For example the earth crust, heated by the geothermal enery freed by the center of the earth (the goethermal energy comes from the natural radioactivity of rocks), has stored considerable quantities of heat over the ages, and has constituted a "stock" that we could partly recuperate. A hot water balloon at home contains also energy stored as heat. In the future we might consider storing heat collected in the summer to use it during the winter, or heat collected during the day to use it at night.

We need to increase the temperature of 86 kg of water by 10°C to "store" 1 kWh of energy.

Energy can be stored under a chemical form. Any fuel is energy stored under the chemical form. We just have to burn the stored coumpound to get back the energy under the form of heat.

If we want to store energy under a chemical form without using oil, gas or coal, it means that we have to recreate quickly a fuel from something else. A first way of doing so is to use photosynthesis, that also created the fossil fuels, that merely restitute a small fraction of solar energy captured several million to hundred million years.

Wood fire and biofuels allow us to store energy under the chemical form, with various efficiencies. To store 1 kWh with biofuels we need about 10 cl of these (about 0,04 US gallon), or about 200 grams of wood. To produce these fuels, we need about 1 (biofuels) to about 0,5 (wood) m2 of land per kWh (but biofuels "include" a fossil fuel spending for their production).

We can also store chemical energy under the form of hydrogen (for which we should recall that it is not a native coumpound on earth, for which we just have to dig a hole to find it at the bottom). There are quantities of solutions, on the paper, to use hydrogen as a storage means, but today only compression and liquefaction are technically fit to put non ridiculous quantities of hydrogen into a given volume. Hydrogen compressed at 200 bars (that is 200 times the atmospheric pressure) stores 0,4 kWh per litre (one litre then weights 30 grams d'hydrogène), and if we assume that the weight of the tank is 50 to 100 times the weight of the stored hydrogen (it's what technology is able to offer right now), the weight required to store the equivalent of a kilogram of oil (11,6 kWh) is 17 to 35 kg roughly. Liquefaction allows to store much more energy by volume unit, but to liquefy one must accept to sacrifice more than 50% of the energy of the initial hydrogen (before liquefaction) : not fabulous !

At last a car battery is another way to store energy under the chemical form, the energy exchanges with the outside being nevertheless done under the form of electricity. With "standard" lead batteries used in cars, we need about 30 kg of batteries to store 1 kWh. We also need to take into account the energy reqquired to produce the battery, which represents about 10% of the energy that will be stored by the battery over its whole life.

Energy can be stored into the nucleus of a fissile atom. This is why uranium "contains" energy, with the precision that the device required to "extract" this energy (a nuclear power plant) is just a little bigger than a coal stove. We need a tenth of milligram of uranium to store one kWh : the amount of energy per mass unit is the highest among all the forms of energies that we know. This is why its exploitation requires very sophisticated devices : it is this form of energy that frees the highest powers per mass unit, and we ought to be able to manage it.


Let's summarize

All the energy forms described above can be stored, but more or less easily, and with efficiencies that can greatly vary. I have reproduced below the masses that are necessary to store the equivalent of one kg of oil.

Mass required to store the equivalent of one kg of oil (11,6 kWh - 1,3 litre - 0,55 US gallon in rough figures)


Lead-acid batteries

Compressed hydrogen

Moving object

Water in altitude



2,22 kg

exclusive use of a couple of m2 of land over a year

more than 300 kg of batteries

a tank weighting 15 to 30 kg, with an inside capacity of 30 litres

2 trucks of 40 tonnes going at 116 km/h (72 mph)

43 tonnes of water able to fall from 100 m

1 milligram

10 °C of temperature rise for 1 tonne of water, or 50 °C of temperature rise for 200 kg of water

A conclusion cannot not be drawn when reading this table : for fossil fuels, the storage function that they naturally fulfill will be very difficult to replace for the same amount of energy consumed when the use of these fuels has decreased (what will happen one day no matter what because the world is finite), except for wood, which remains close in terms of mass and space required. Nuclear power is an intermediate case : uranium is not renewable and would be depleted quite fast with the present technologies, but nuclear energy can become almost renewable with breeders, and otherwise it is very easy to store uranium but much harder to store the electricity produced with the power plants.


Eventually, is one kWh a large or a small amount of energy ?

1 kWh, it is therefore a lot when we have to store it, since we need 30 kg of batteries to perform such an operation, but it is not much when it comes to consuming it : in average, and all energies taken into account, a French person consumes about 45.000 kWh per year, that is 5 kWh per hour ! If all the northern european countries are close to this amount, an American is close to twice that amount while the world average is "only" 15.000 kWh per year.

Knowing that a working man represents a power of about 200 Watts, it means that any French has, through his energy consumption, the equivalent of 25 slaves all day round : these slaves of modern times are called car, boiler, washing machine, lifts, automated factories, etc. And this number of 25 does not apply only to the slaves of M. Bill Gates or of M. Silvio Berlusconi (converted into "slave equivalent", their consumption would be way over !) but well to "Mr(s). anybody" in France.

Nevertheless a large fraction of this energy may seem "invisible" to us, because we are not directely billed kWh, but we consume it through various goods and services that we buy. So, to consume 1 kWh, we "just" have to do one of the following :

drive 1 km with a car that consumes 8 litres per 100 km (that is 29 miles to the US gallon). As the average annual distance driven by a car is 14.000 km (in France), it means each car owner consumes roughly as many kWh per year through driving.

or travel 5 km by train,

or drive a 40 tonnes truck over...200 metres,

or fly 1 to 2 km (we do talk about the energy spending per passenger, and not for the whole plane !),

or plug in a european fridge for a full day, or plug in a european freezer for half a day,

or plug in a clothe dryer for 15 minutes,

or light up an average house for an average evening (average french consumption),

or heat a square meter of house for half a day to a day in the winter,

or produce 200 to 500 grams of steel (depends on the proportion or recycled steel in the materials used) or cardboard, 100 to 200 grams of plastic, or a couple tenths grams of aluminium,

or eat 30 grams of beef or 300 grams of pork (the energy "included" in the meat is essentially the energy that was spent to grow the cereals that fed the animal, what requires both gasoline in the tractor and the production of fertilizers, and agrochemistry is very energy intensive),

or buy 600 grams of litchees or pinaples brought from the islands by plane,

or use half an hour of an office clerck (in France an employee in an office requires an energy spending of 1,5 tonne oil equivalent per year, for heating, commuting to work, electricity, etc).


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