Compressed air electricity storage (CAES)

Alongside STEPs (and dams), compressed-air electricity storage is the only sustainable, large-scale means of storing mechanical energy. Its limited efficiency (40-50%) is being improved with adiabatic or isothermal storage, which recovers the heat from compression and reinjects it when the energy is released. This technology is still in the experimental phase.

The principle of compressed air electricity storage (CAES)

In addition to conventional compressed-air energy storage (CAES), there are two other technologies under development: adiabatic storage (AA-CAES) and isothermal storage.

Conventional compressed-air electricity storage

The principle of CAES electricity storage is very simple: when you have too much electricity, you use it to inject pressurized air into an underground cavity (usually a salt cavern). Then, when electricity is needed, you let this pressurized air out onto a turbine, which will restore part of the energy initially invested. Storage takes place at very high pressure, from 100 to 300 bars, which is required to operate a gas turbine.

The major problem with this system is its low efficiency: 40-50%. Indeed, a large part of the energy is lost in the form of heat during compression, and then again during restitution (the air has to be heated in a combustion chamber before it can be used). Another difficulty is choosing the right cavity. It needs to be large enough, but also leak-proof and able to withstand pressure. Some projects call for the use of tunnels or shafts. Some systems are designed to overcome this limitation and operate with a surface reservoir.

The system’s response time would be of the order of a few minutes.

Adiabatic storage(Advanced Adiabatic CAES, “AA-CAES”).

Adiabatic CAES (= not exchanging heat with the outside of the system) involves recovering the heat lost during compression in a reservoir, then reinjecting it during decompression. This considerably improves efficiency (70%), leaving only the energy lost during compression.

Nevertheless, this process has its own challenges. The heat must be conserved between compression and release, i.e. for the duration of storage! Stored heat can reach 600-700°C. (Thoraval 2016)

Easier said than done … There is research into suitable materials. They should be able to absorb a lot of heat in relatively little volume, allow good exchange with the heat transfer fluid, release the heat as desired and, of course, be durable and inexpensive.

Isothermal CAES storage

CAES energy storage using isothermal compressed air follows the same logic as adiabatic storage, except that instead of recovering the heat after compression, it is recovered uniformly and constantly. During compression, water would be injected into the compressor, enabling the heat generated to be recovered. This heat would then be stored and re-injected when it is released.

According to connaissancedesenergies, this system would have an efficiency of around 95% “for the expansion valve/compressor system”, but this figure doesn’t seem to me to be sufficiently sourced or to describe the energy required for compression. Another source mentions an efficiency of 70-80%, which seems to make more sense to me. I’d need more documentation on this subject.

Summary of the advantages and disadvantages of compressed-air storage

The overall advantages of compressed-air storage are that it’s fairly inexpensive and can store large quantities of energy. The disadvantages of these systems are the need for hermetic space. There may also be problems of corrosion due to the humidity in the air, but there are solutions. Finally, in the case of salt caverns, there is the problem of salt contamination, which can damage the turbines.

The disadvantages of “conventional” CAES are a relatively low overall efficiency (40-50%), even if it is much higher than hydrogen (20-25%), and, in addition, it generates CO2, using fossil energy to heat the gas before passing through the turbine. Adiabatic storage, on the other hand, has no disadvantages and a respectable final yield (70%). However, these systems have yet to be tested on a large scale. The problem of storage time and the corresponding heat retention time is not insignificant.

CAES energy storage systems

Existing installations

Although this is the most mature storage method, it is not widely used. In 2016, there were just 2 active, “classic” facilities in the world:

• Huntorf in Germany, 1979: The first facility, uses salt caverns totalling 310,000m3 at 650-800m depth. With a capacity of 290 MW. Air is stored at between 43 and 70 bar, and efficiency was 50%. Discharge time (at full capacity and full power, I assume) is 3 hours.
• McIntosh, in the USA, in 1991: with a maximum output of 110 MW, also used a salt cavern, 538,000m3 at a depth of 450-800m. Gas was stored at 45-76 bar, and efficiency was 54%. Discharge time was 26h. (Thoraval 2016)

A 2MW test had been started in Hokkaido, Japan, but was stopped.

Companies and projects around CAES energy storage

connaissancedesenergies.org mentions several projects, which I’ll have to check out:

• sustainX project in the USA (New Hampshire) 2011 1MW [To be verified] Hydrostor project in Canada 2013 1MW [To be verified
• hydrostor project in Canada 2013 1MW [To be verified]
• general Compression project in the United States (Texas) 2014 2MW [To be confirmed]
• The ADELE project in Germany was the first adiabatic (underground) CAES project. Construction began in 2013 and the project was worth 10 million euros. 2018 90MW [To be confirmed]
• pG&E project in the United States (California) 2021 300MW [To be confirmed]
• norton project in the United States (Ohio) 2700MW [To be confirmed]

• A 4-6 GWh capacity, 500MW adiabatic CAES storage facility is under construction in Rosamond, California. This is the system of startup Hydrostor, which has teamed up with Pattern Energy of the USA and Meridiam of France to exploit the technology.
• Gaelectric is developing a 330MW facility in the Irish town of Larne.

• A comprehensive article: https://www.connaissancedesenergies.org/fiche-pedagogique/caes-stockage-par-air-comprime
• Alain Thoraval, “Stockage souterrain de l’air comprimé dans le contexte de la transition énergétique”, INERIS, study report DRS-16-149645-00148A, January 11, 2016