the accumulation of energy consists in preserving a quantity of energy produced for subsequent use, which allows to balance production and consumption by limiting losses. Storage is at the heart of many economic, environmental and strategic issues.
It is important to understand the reasons for the development of electrical energy storage. Here are three main reasons.
A problem linked in particular to the rise of renewable energies
With the renewable energies, energy production is not necessarily continuous throughout the day. Wind turbines have intermittent production depending on the wind. Solar panels produce energy only when there is sunlight, so during the day but not in the evening. However, it is at the end of the day that the energy consumption is highest.
Therefore, in order for these low-carbon energy sources to be truly used to fully meet the energy needs of the population, it is necessary to be able to store the energy produced when it is produced for later use.
Avoid shortages and surpluses
A massive blackout occurred in the UK on 9 August 2019. In particular, it paralyzed the capital, London. Two power generation sites, a gas plant and a wind farm, failed at the same time.
To avoid this type of situation, the grids have reserves of electricity production, but these have proved insufficient. The frequency of the power grid has therefore decreased, causing the interruption. To prevent this type of problem, the storage of electricity is therefore essential.
So that the eletricity grid it works correctly, its frequency must be stable and it must be identical in all points of the network. But real-time imbalances between energy production and consumption produce variations in grid frequency. If we produce more electricity than we consume, the frequency increases. If we consume more electricity than we consume, the frequency decreases.
There are mechanisms to maintain the frequency of the electricity grid. To bring the system back into balance, TENthe French electricity grid operatorin particular, it has an electricity reserve from various energy producers.
The development of renewable energies such as solar and wind power is one of the solutions recommended by the IPCC to reduce CO2 emissions. However, their growth may be slowed by the lack of large-scale storage solutions.
The penetration of renewable energies in the energy mix network represents a major challenge for network stability. The development of renewable energies has occurred in Europe at a faster rate than the growth in electricity consumption.
It is estimated that the need for storage could reach 60 GW (Gigawatts) by 2050, according to the International Energy Agency (IEA). To confront, according to CRE, the average annual electricity consumption of a French household in 2020 was estimated at 0.004 GW. 60 GW would therefore be the equivalent of the annual consumption of 15,000 households.
In France, renewable energies represent 19.1% of the gross final consumption of energy (2020 data). They should take up more space in the energy mix. The storage capacity being weak today, there is interest in developing new solutions. However, it should be taken into account that the need for storage is not essential in the short term.
To store electricity, it must be transformed. Electrons as such are too difficult to store.
They are thus transformed into another resource, then transformed again into electrical energy or are used in their new form: thermal, mechanical, chemical energy, etc.
There are two storage systems:
- the stationary storage of energy : it is a question of keeping the energy produced in excess, in order to restore it later. We will simply create a reserve of energy. Fixed storage systems are stationary and usually have large capacities.
- the storage on board : these are small capacity systems integrated in a mobile system (eg car) or in an electronic device (telephone batteries, etc.).
The different storage technologies:
STEPs and CAES – Energy transformation stations by pumping and accumulating compressed air energy (energy storage by compressed air)
These two electricity storage techniques use the potential mechanical energy of a fluid, water and air, respectively.
With STEPs, two basins are designed which are located at different altitudes. When the electricity requirement is low, the water from the lower basin is pumped to the upper basin. When the demand is high, the water flows back into the lower basin, passing through a turbine, which generates electricity.
The installation of STEP is quite complex and is only used to adjust the network. The maneuver loses 30% of the energy and currently very few sites are available. This last element makes it an insufficient solution to cover all storage needs.
CAES work in exactly the same way, but using compressed air, stored in cavities. This solution is still under development today.
Batteries and cells
Cells and batteries are a universal technique for storing electricity. There are different sizes and capacities, corresponding to different needs. They offer a good storage density, giving the possibility to store a lot of electricity in a small space. Thanks to a very high reactivity, the latter offer the best response to the whims of the electricity grid.
Today the lithium-ion batteries are market standards. These batteries are the components of large-scale storage units. Mature and efficient, this solution meets the needs of the electricity grid generated by the massive development of renewable energies. Battery technology is also found in many electric cars and plug-in hybrid vehicles.
Finally, hydrogen tanks are used to power fuel cells. By increasing the pressure and therefore decreasing the volume, we can now store 5 kg of hydrogen in a 125 liter tank.
Storage units have different sources of income to give visibility to their developers and make the battery market in France attractive:
Focus on AOLT
RTE, manager of the public electricity transmission network, has the task of guaranteeing the balance between electricity production and consumption and of resolving congestion on the transmission network.
With this in mind, RTE organizes every year a multi-annual call for tenders, called “AOLT”, whose regulatory framework is defined by Climate and Resilience Act of 2021. It is intended for new flagship production capacities and has been launched since Ministry of Ecological Transition in 2019. AOLT’s goal is to provide visibility at a stable price for these capabilities and facilitate new investments.
At the end of each tender, a guaranteed price is thus defined, and the selected candidates benefit from a contract for difference which guarantees them a stable remuneration equal to the “guaranteed price”, for a period of 7 years. During this period, if the guaranteed price is higher than the market price, the winner will receive the difference. If not, he will pay the difference into a dedicated fund.
Candidates undertake in return to make part of their capacity available during peak days known as PP2 (capacity market).
Article L. 352-1-1 of the Energy Code defines the storage categories for which these calls can be organized: electric power pumping stations (STEP), batteries and hydrogen. Other types of storage are possible on a case-by-case basis.
Like any technology, renewable energy storage involves human activity, equipment manufacturing, and the release of carbon emissions. in any case, theecological impact it can be mastered, especially by developing recycling channels that recycle the waste produced. Especially since storage systems also have advantages: they generate little noise pollution and take up little space.
It remains to democratize the current technologies, the rise of renewable energies that raises needs in terms of storage. Many innovations are emerging to optimize this storage.