Energy storage :
Energy supplying and handling is more than ever central to our daily concerns and represents a major socio-economic priority. The future electric energy storage for large volume applications together with electric transportation and network applications will be inextricably linked to energy sources alternative to fossil fuels (oil, gas and coal).
If one wants to meet the energy demand coming from the ever growing population of the 21st century, the energies with low carbon footprint i.e. wind, solar, geothermal and from the sea, will have to be mastered and stored. Electric energy storage will keep on playing a more and more vital part in other domains such as transportation (hybride vehicles and all electric), medicine, defence and aerospace, telecommunications, etc. Storage and production of electric energy has become essential in any new energy paradigm. It has become an important and strategic stake for our industry and our country as underlined by the Grenelle de l’environnement
How do we store energy?
One form of energy can convert to another according to various process/machineries. Some of these conversions are routine such as combustion, (boiler, our body), Joule effect (toaster, electric engines, solar panel), others are far less common (mechanical --> chemical). Some of these forms are not intrinsically storable (radiative energy); others are in a hardly efficient way or to a less extent: heat (Carnot efficiency) electric energy (capacitor). The others have stable forms of intrinsic storage: atoms (nuclear energy), molecules/bonds (chemical energy), dams/wheel flyers (mechanical energy).
It is among these last 2 forms (chemical, mechanical) that storing proves to be the most efficient and massive (dams to store the excess of electric production and electrochemical devices to store electricity in its chemical form). As for numerous applications, the purpose is to store energy in order to return it in the form of electricity, and the best way to do this is to convert chemical energy in electricity since their vector is the same: the electron. The discovery of the electrochemical generation of energy is due to Alessandro Volta in 1787 as he was trying to explain why the muscle of a frog’s leg was contracting when in contact with a couple of different metals. This simple production of electricity led to great discoveries in the field of electromagnetism by Michael Faraday who might be seen today as the founder of electrochemistry. He was the first to realize water electrolysis and to introduce the notions of cathode and anode.
Many electrochemical devices resulted from Volta’s discovery, and besides Daniell and Leclanché alkaline cells, are fuel cells, supercapacitors and batteries.
The working principle of a fuel cell is opposite to that of water electrolysis i.e. electricity is produced thanks to the oxidation of di-hydrogen (H2) on one electrode coupled with the reduction of one oxidant (air oxygen) on the other; their recombination produces water. It is an “open” system, i.e. externally fed therefore not directly electrically rechargeable.
As far as supercapacitors are concerned, they are based on the capacitive properties of a double layer of electrons-ions forming at the electrolyte-electrode interfaces with a capacity, per surface unit, up to millions times bigger than that of the usual capacitors.
Finally, batteries can deliver/store electric energy based on reversible oxydo-reduction reactions that can occur inside the materials forming their electrodes.
We have Pb batteries, Li batteries (metal-Li when the negative electrode is made of metal-Li, Li-ion when the Li stays in its ionic state, Polymer-Li when the electrolyte is a polymer), Ni-batteries (nickel-cadmium, Ni-Metal hydride), Na-ion batteries, Magnesium batteries, Metal-air batteries. Lastly midway between batteries and fuel cells are the REDOX-flow systems which separate the place where reactants are stored from the place where electricity is produced. High temperature (>250°C) operating batteries such as Na-S or ZEBRA (which relies on the NiCl2/Na electrochemical chain) do also exist and are mainly use for mass storage applications, namely the former which is solely produce by NEK.
Batteries and supercapacitors are central to the activities of the RS2E, the aim is to develop more performing storage systems in order to favour the use of renewable energies, such as wind, solar and ease up the progressive transition of the thermal vehicle to the electric vehicle.