Energy storage systems are required by applications for several reasons:
1) The application to supply is not connected to a grid
2) The consumption is not in phase with the production
3) An application on the network has a high peak power requirement
The choice of the energy storage technology depends on the following parameters: power, duration, volume, weight, price......Most of the applications depends on a compromise between the energy density and the power density of the energy storage system. The situation is well summarized with the help of a "Ragone plot".
Electrical energy storage
Batteries are electrochemical energy storage systems. Today different technologies are available. For example:
- Pb Lead
- NiMH Nickel Metal Hydrid
- Li Lithium ion or metal
Among the energy storage the batteries have the highest energy density after the different fuels (petrol, H2,...)
Capacitors unlike batteries have a very low energy density but a very high power density. They are used mainly in inverters, high-voltage switchgears, balast, etc, in AC and DC applications. Different capacitor technologies are available:
- Film foil (PP, PET, PA, PS,.... dielectric film between Al electrode
- Metallized film (same dielectric foil with a metallization on the surface
- Ceramic (Ferroelectric with very high permittivity)
The energy content is proportional to the capacitance and to the the square of the working voltage. To increase the energy density the research is oriented in the direction of thinner film with higher dielectric constant. Basically the high dielectric constant is accompanied by high dielectric losses (Kramers Kronig). In other words, the AC operation will be accompanied by heating. The dielectric strength is also an important limit for the capacitor operation. The higher the applied electrical field is, the shorter will be the lifetime. Above a determined field limit, the capacitor will breakdown.
When using capacitors, it is important to keep in mind that the properties depends on the temperature, the voltage and the stress duration. At higher temperature the dielectric strength will be reduced.
The capacitor technology choice is often performed considering the losses generation.
Capacitors unlike batteries have a low energy density but a very high power density. They are used mainly when load leveling or when high power peak supply are required. They are also in competition against the market when the applications require a maintenance free operation.
Vanadium redox flow battery is one of the well known example. The Redox Flow Cell is an electrochemical system which allows energy to be stored in two solutions containing different redox couples with electrochemical potentials sufficiently separated from each other to provide an electromotive force to drive the oxidation-reduction reactions needed to charge and discharge the cell. Unlike conventional batteries, the redox flow cell stores energy in the solutions, so that the capacity of the system is determined by the size of the electrolyte tanks, while the system power is determined by the size of the cell stacks. The redox flow cell is therefore more like a rechargeable fuel cell than a battery.
This technology can store a huge energy. It has already been demonstrated in windmill and solar installation, UPS system and load leveling substation as big as the 450 kW /1 MWh from Sumitomo Electric Industries in Japan.
Magnetic energy storage
Superconducting magnetic energy storage systems store energy in the field of a large magnetic coil with direct current flowing. It can be converted back to AC electric current as needed. Low temperature SMES cooled by liquid helium is commercially available. High temperature SMES cooled by liquid nitrogen is still in the development stage and may become a viable commercial energy storage source in the future. SMES systems are large and generally used for short durations, such as utility switching events.
Mechanic energy storage
The energy is stored in the rotational kinetic energy of a rotor. The energy content is proportional to the rotor mass and to the square of the rotational speed (pulsation). This system is in competition with supercapacitors. It can deliver its energy with a very high power. The maintenance of this system is delicate.
Pneumatic or compressed air
Compressed Air Energy Storage (CAES) uses pressurized air as the energy storage medium. An electric motor-driven compressor is used to pressurize the storage reservoir using off-peak energy and air is released from the reservoir through a turbine during on-peak hours to produce electrical energy. Ideal locations for large compressed air energy storage reservoirs are aquifers (water bearing rock formations), conventional mines in hard rock, and hydraulically mined salt caverns. Facilities are sized in the range of several hundred megawatts. Air can be stored in pressurized tanks for small systems. Small systems have also been used in demonstrator hybrid cars.
These systems are in competition with supercapacitors. They can deliver their energy with a very high power.
One of the oldest way of storing energy is to use springs. It may supply energy to a clockwork during days.
Hydro dam store water which is turbined in an electrical plant several 100 meters below. It is current to pump water again in the dam during the night, when the electricity cost is low, to reused it during the period of higher cost.
In a smaller energy range, the gravitation is also used to actuate big clockwork. Heavy lead masses are suspended at a string which pulls a mechanical system.
Thermal energy storage
Several techniques may be used to store heat in order to transform it in work in a thermodynamical cycle.
- liquid tank
- materials which undergo a phase transition
In both cases the materials are heated up to store the energy and a lower temperature "tank" is necessary to satisfy the 2nd law of thermodynamic.
Carbon-Hydrogen energy storage
This is today one of the biggest energy storage source we have at our disposal.
- Wood, other organic material, etc
The energy is recovered during a reaction with the oxygen. In the case of the carbon-based material, this reaction is problematic, because beside the heat generated, a lot of CO2 is produced.