Considering the depletion of fossil resources and global warming, the use of renewable energy resources such as wind turbines, photovoltaics, biomass, hydropower and geothermal energy appears to be the most promising alternative to fossil fuels in the future. short and medium term by providing cleaner and more efficient electricity.
In France, 13,6% of the energy produced comes from the renewable energy sources mentioned above, of which hydropower represents the largest share, ie around 9,8%. By comparing, worldwide, 11,4% of energy comes from renewable energy sources, but energy production remains dominated by fossil resources, around 84%. This is why it is vital to “decarbonize” our current energy production system through the integration of low carbon energy sources.
Availability of renewable energy sources
The main drawback of renewable energy sources of the wind and photovoltaic type is their dependence on weather conditions, which leads to a discontinuity in energy production. This production discontinuity compromises their integration into the electricity grid in order to be able to meet energy needs in real time.
In this context, to cope with the intermittent phenomena of these energy sources, energy storage plays a primordial role. Indeed, it makes it possible to store the surplus energy in periods of low energy demand and to restore them in the event of high energy demands, in order to compensate for the lack of production of these renewable energy sources.
Unfortunately, it should be noted that electricity is difficult to store in large quantities in its own form (in the form of electricity): it must be converted into another form (chemical or mechanical energy for example) to allow storage.
What technologies to store electricity?
At present, different systems exist for the storage of electricity: "gravity" storage of water mass, where water is pumped up the dam when electricity is in surplus and it is recovered. the energy thus stored by turning turbines; “thermodynamic” storage with compressed air storage systems, where air is compressed when there is surplus electricity and the stored energy is recovered by turning turbines; storage of kinetic energy with "flywheels", which rotate with little or no loss of energy during storage; and “electrochemical” storage with batteries, of which there are several types (sodium-sulfur, lithium-ion, sodium-ion, graphene), or with electrolysers, where electricity is stored in the form of hydrogen.
In comparison, the storage of kinetic energy with flywheels and electrochemical storage via batteries (lithium-ion, sodium-sulfur) are aimed in particular at portable electronics, transport, but also to renewable energy sources, like photovoltaics. Massive battery installations to store electricity produced by wind or solar farms have already been deployed and represent megawatt storage capacities.
Other battery technologies such as sodium-ion batteries et graphene are still in the development phase and will address some current issues - including optimization of charging time and storage capacity, as well as the use of alternative materials to avoid toxic materials such as lead and hazardous materials and harmful to the environment such as lithium.
Storage of electricity in the form of hydrogen: in the deployment phase
Chemical storage in the form of hydrogen presents itself as an attractive and promising solution for large-scale energy storage on the one hand, and for electric vehicles on the other. Indeed, compared to current electrical storage technologies such as batteries, hydrogen has a very high specific energy density: around 120 megajoules per kilogram.
Gasoline or diesel are other methods of “chemical” energy storage which have a high specific energy density, but their combustion releases greenhouse gases while the combustion of hydrogen emits only carbon dioxide. the water. However, hydrogen is a light gas, characterized by a low energy density by volume (about 10,8 megajoules per cubic meter), which makes it less favorable. to its storage and transport. To overcome this problem, hydrogen can be compressed in pressurized gaseous form (around 700 bars), in liquid form (at a temperature of - 253 ° C) or in solid form at low pressure (thanks to the use of materials capable of adsorbing hydrogen) such as metal hydrides.
Hydrogen can be produced through the electrolysis of water, which involves using electricity (ideally produced by renewable energy sources) to separate pure water (H2O) to hydrogen (H2) and oxygen (O2).
Currently, the storage of electricity in hydrogen form is in the midst of a demonstration and experimentation phase with many numerous experimental platforms in Germany, Canada, Denmark, France, Norway, Thailand and New Zealand. Different applications are envisaged.
Among them, the storage of electricity in hydrogen makes it possible to contribute to the decarbonization certain sectors responsible for global warming such as air, sea and land transport.
Personal electric vehicles (Kangoo ZE Hydrogen, Toyota Mirai, Hyundai Nexo, and Honda Clarity) and trains (Alstom's Coradia iLint running in Germany) running on hydrogen are already deployed. In this type of vehicle, hydrogen tanks are on board in order to supply a fuel cell, making it possible to transform the hydrogen into electricity. To optimize the life of the fuel cell and extend the autonomy of the vehicle, electric batteries are also present (frequently lithium-ion). Research is underway to also feed the ships and planes in hydrogen.
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Another application of hydrogen storage is the concept of "Conversion of electricity to gas" consists of converting the surplus renewable electricity into hydrogen and then the hydrogen into “green” natural gas. The methanation process is used for this. The natural gas can then be injected into existing underground natural gas pipelines and facilities, and be used as needed. However, the use of carbon dioxide in the methanation process poses problems in terms of decarbonation: it remains crucial to capture and enhance carbon dioxide emissions from industry to make this process more environmentally friendly.
In addition, but we are leaving the storage of electricity to enter the world of industrial production, hydrogen can be used in various industrial processes such as chemistry (synthesis of ammonia, production of methanol), metallurgy ( metalworking, carbon steels) and electronics (semiconductor manufacturing).
This article is republished from The Conversation under a Creative Commons license.
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