Electricity storage is one of the great challenges of the ecological transition. It’s a question at the heart of ecomobility and, above all, of electricity generation in general: how can we manage the intermittent nature of renewable energies? Not only do we need short-term storage, but also seasonal storage.
Here, we’ll take a look at all electricity storage systems. Overall, we need to pay attention to a number of factors:
- Storage capacity (how many kWh?)
- Power (= how fast does the storage fill and empty)
- Inertia (= how long does it take for the storage to reach full power?)
- Storage durability (are there any losses over time?)
- Price and other practical aspects.
This is a very active sector of innovaiton: the increase in the number of patents in energy storage has been more than 3 times the average since 2012. (IAE 2020)
Part 1. Conventional electricity storage
There are currently two widely used and mature electrical storage systems: hydraulic storage and batteries. In terms of stationary storage, around 200GWh of storage is installed, 91% of which is STEP and 5% for batteries. (IAE 2020, p.31)
Today, the main form of electricity storage is in hydraulic form, behind dams or in reversible pumping stations: the STEP.
Hydraulic dams hold back phenomenal quantities of water, which is stored potential energy. They can thus generate hydroelectricity to absorb peaks in consumption or troughs in production, or when their capacity is exceeded.
Pumped storage power stations: STEPs
Pumped-storage power stations (P.S.T.P.s) are storage systems very similar to dams, except that they can lift the water they release. Unlike dams, which may be primarily dedicated to power generation (for example, if there is a very strong flow of water from upstream to downstream), STEPs are essentially systems for managing the electrical load on the grid. Dams are designed to capture a flow, whereas WWTPs are not.
In general, you’ll have an upstream basin that’s connected to a downstream basin: when electricity is needed, water is allowed to flow from the top to the bottom, then, when there’s a surplus, water is pumped in the opposite direction.
Batteries: chemical energy storage
Energy can also be stored in chemical form: that’s what batteries do. This is the second most important form of electricity storage, but it represents a much bigger market. Indeed, it is very practical for portable applications, such as the batteries in our smartphones or cars. Today, the market is dominated by lithium-ion batteries, ideal for just such applications. Even for stationary storage, they account for 93% of installed batteries. However, new battery models are being developed that could compete with this hegemony in certain applications: lithium-air, sodium-ion and, above all, redox flow batteries, which could be very interesting for large-scale, long-term stationary storage.
Lithium-ion also dominates innovation: in 2018, 2,547 patents were filed for lithium-ion cells, compared with 462 for all other cell types. However, innovation also focuses on other aspects, such as cell engineering (1406) or cell manufacturing (1526). (IAE 2020, p.6)
The main problems with batteries are their low storage capacity relative to their price, and their consumption of materials. It seems difficult to imagine them competing with STEPs and CEAS for seasonal electricity storage.
Part 2: “New” modes of electricity storage
In addition to these classic modes of electricity storage and energy management, there are many systems that are less mature or more “niche”:
- Power to gas, gaseous storage
- Thermal storage, notably using molten salts
- Compressed air (CEAS)
- Gravity towers
Power to gas: storage using hydrogen or methane
Power to gas” has been a major ecological theme for several decades. The idea is to transform electricity into hydrogen by electrolysis, and then eventually into methane (less difficult to store than hydrogen). This method has three major problems:
- the power of the equipment needed to capture surplus electricity, which must be too high compared with its overall use (=costs a lot of money), in the absence of any other storage method (e.g. thermal);
- disastrous efficiency (<25%).
- gas storage difficulties (especially true for hydrogen, less troublesome for methane).
For methane, efficiency is even more disastrous, and for hydrogen, storage is very complicated. To date, there is no proven large-scale method for storing it. Overall, this solution seems highly hypothetical.
There is, however, a potential exception: Airthium has developed a system that would make the whole thing viable. The French startup claims to be able to efficiently transform excess renewable electricity production into hydrogen, then ammonia (which is very easy to store), and then back into electricity. A molten-salt thermal storage system would smooth out electricity production and limit the power required from electrolyzers. The heart of this process is a Stirling engine, which will not only be used in combination with molten salts, but will also generate electricity from the combustion of ammonia.
Compressed Air Energy Storage (CEAS)
Akin to “Power to gas”, it is also possible to store energy in the form of compressed air(Compressed Air Energy Storage, CEAS): a gas is compressed and injected into a cavity (e.g. a salt cavern), then, when electricity is required, released to drive a turbine. One problem is the loss of heat during compression, then the need for heat during release. In the conventional version, the air has to be heated in a “combustion chamber” (often using fossil fuels), before it can pass through the turbine. Two new methods, adiabatic storage and isothermal storage, propose recovering the heat of compression, storing it, then reinjecting it before release into the turbine.
The question arises as to whether it might not be possible to combine the two approaches, recovering the mechanical energy of compression on the one hand, and the chemical energy of the gas on the other.
There are systems that combine thermal storage with electricity generation, creating de facto thermal storage of electricity. This is what we saw with Airthium: the French start-up used molten-salt heat storage in combination with an advanced industrial stirling engine to dampen variations in solar energy. The latter transformed excess electricity into heat, then heat into electricity to power an electrolyzer. This kind of system can also be seen in small nuclear reactors, such as TerraPower: there’s a molten-salt storage system which, if required, can generate more current for the turbines. The Terrapower system, for example, boosts reactor power from 375 to 500MW for 6 hours.
I won’t deal with this subject here, as we’ll be looking at heat, which is another form of energy.
Flywheels: kinetic storage
The flywheel is a cyclinder that turns very quickly around a pivot and stores energy in kinetic form. This system is already mature on a small scale, and can be found in many machines. Their main advantage is their reactivity: they ramp up very quickly. However, they don’t store much energy, and certainly not for long.
This technology already has a number of highly specific applications, but these correspond only to a limited functional logic, such as acting as a “shock absorber” to allow time for another, more durable device to rise in load.
Gravity power and concrete blocks
A few years ago, we saw a lot of these large concrete block towers, where cranes would raise and lower large blocks of a kind of concrete as the electricity was to be stored or destored. This is the Energy Vault project. They are now being marketed. Three towers with a total capacity of 1.6 GWh have been ordered for 2021. These systems have fairly obvious problems, such as mechanical wear and, above all, the need for materials. Comparisons with water-based storage systems are hard to make..
Another system, Gravicity, is perhaps more promising: a block is lowered and raised along an old borehole. They are still in the industrialization phase.
Part 3. The regulatory issue
The European energy market is not necessarily suited to energy storage. Here’s a case in point: https://www.nfp-energie.ch/fr/dossiers/191/cards/295
This is an important point that needs to be explored.
- An interesting summary: How to store energy https://www.drgoulu.com/2012/10/07/comment-stocker-lenergie/
- A summary thread by an engineer: https://twitter.com/Kako_line/status/1433151907820974080
- A complete summary by IFPEN: https://www.ifpenergiesnouvelles.fr/enjeux-et-prospective/decryptages/climat-environnement-et-economie-circulaire/stockage-denergie-accompagner-deploiement-des-energies-renouvelables
- ENEA Consulting, Study on the potential of energy storage, 2013
- A rich dossier on compressed air storage: https://www.nfp-energie.ch/fr/dossiers/191/
- A very interesting overview: https://www.canarymedia.com/articles/long-duration-energy-storage/long-duration-storage-roundup-news-players-and-technology
- 2014, Technology Roadmap – Energy Storage, https://www.iea.org/reports/technology-roadmap-energy-storage
- 2018, Will pumped storage hydropower expand more quickly than stationary battery storage? https://www.iea.org/articles/will-pumped-storage-hydropower-expand-more-quickly-than-stationary-battery-storage
- 2020, Innovation in Batteries and Electricity Storage, https://www.iea.org/reports/innovation-in-batteries-and-electricity-storage
- 2021, Very large thermal energy storage for renewable districts, https://www.iea.org/articles/very-large-thermal-energy-storage-for-renewable-districts
- 2021, Proving the viability of underground hydrogen storage, https://www.iea.org/articles/proving-the-viability-of-underground-hydrogen-storage
- 2021, How rapidly will the global electricity storage market grow by 2026? https://www.iea.org/articles/how-rapidly-will-the-global-electricity-storage-market-grow-by-2026
- 2022, Grid-Scale Storage, https://www.iea.org/reports/grid-scale-storage