Electrical energy is a constant flow of electrons that move within a conductor. To want to store it in that form is as unrealistic as wanting to store wind. So to do it, you have to convert the electricity into another form (chemical, for example, like batteries) and turn it back into electricity when you need it.

Depending on the system used, this transformation can involve losses, difficulties and variable storage costs.

Electricity storage in the form of chemical energy

Batteries

Battery storage is based on what is known as a 'reversible' chemical reaction, as it can take work in both directions.

• In one direction, the reaction makes it possible to convert electricity into chemical energy so it can be stored.
• In the other, it generates an electric current.

In order to increase performance and reduce the impact on the environment, new types of battery (salt water, redox or sodium-sulphur) are being developed. Lithium-ion batteries are currently the most efficient.

Capacitors

Capacitors store electrical energy on two metal plates separated by a semiconductor, and they restore it at the moment of discharge.

Capacitors can:

• charge and discharge very quickly
• provide high currents (although they are time-limited)
• allow an electric vehicle to be charged very quickly.

Hydrogen

Hydrogen (which can be recovered by electrolysis or by burning CH4) and oxygen, generate electricity when combined.

The combustion of hydrogen in a motor makes it possible to supply an electric generator. Used in a fuel cell, it can produce electricity directly.

Electricity storage in the form of potential energy

Pumped-storage hydroelectricity

Pumped-storage hydroelectricity involves pumping water from a low-level lake to an accumulation pond higher up.

When there is demand for electricity, the water in the upper reservoir is released to the lower basin, turning a turbine which drives an alternator that generates an electric current.

Gravity wells

A very heavy piston is raised from the bottom of a well 500 m deep using an electric motor. The body is then released. As it descends, it compresses the well water with its weight. The water is pushed back under the pressure, making it possible to turn an electric generator.

Inclined trains

Using a principle similar to old-fashioned clocks (where the mechanism had to be raised to make them work), a train climbs a 10 km ramp with a 7% gradient. It goes back down under the influence of its own weight, and the mass of the train turns a generator that in turn produces electricity.

Electricity storage in the form of kinetic energy

Flywheels

A very heavy mass (a wheel, cylinder, etc.) is rotated by a motor, driving a generator that produces electricity by gradually slowing down.

Silent electric buses running on this principle drove around in Belgium in the nineteen-sixties. They could drive several kilometres with the kinetic energy accumulated in their flywheel.

Electricity storage in the form of heat energy

It is possible to store electricity by turning it into heat (by heating a water tank for central heating, for example).

In a domestic context, transforming it back into electricity would not be of interest because the yield would be low: it is better to use it for heating. This is therefore energy storage in a broad sense.