Leaving Pumped Hydro Energy Storage (PHES) aside, there is only one other long-duration energy storage technology that have been proven for a significant period of time and that’s Compressed Air Energy Storage (CAES).

This technology uses excess electricity (e.g. from an electricity grid, or from renewable generation) to compress air to high pressure – 70bar approximately- and stores it in reservoirs. When power is required, compressed air is drawn through the expander to power a generator. Because compressed air is not very energy-dense, it needs large volumes and therefore geological storage is used; such as caverns and salt mines.

Compression of air creates heat and expansion removes heat. If the heat generated during compression can be stored and used during expansion, the efficiency of the storage improves considerably. There are several ways in which a CAES system can deal with heat:

Traditional CAES Or Diabatic

When compressing air to 70bar, it heats up to ~ 650°C. But air cannot be stored in salt caverns above ~ 42°C. So traditional CAES (diabatic storage) dissipates much of the heat of compression with intercoolers into the atmosphere, essentially wasting the energy used to perform the work of compression.

However, expansion from 70bar at roughly ambient temperature cools down the air to ~ -150°C. This would freeze the equipment, so the heat needs to be put back in. Traditional CAES puts in the heat of expansion by burning gas. This degrades the efficiency of the cycle, adds an extra cost, and compromises the ecological benefits associated with most renewable energy sources.

The first two commercially deployed CAES plants are sited in Huntorf in Germany (290Mh using a salt dome to store 580 MWh) and McIntosh in the USA (110MW with a capacity of 2860MWh / 26 hours of storage) and are in operation since 1978 and 1992 respectively. They both operate with diabatic process, not utilizing the heat from the compression process and leading to a 42% overall efficiency in Huntorf and 54% in McIntosh.

Isothermal CAES

Isothermal CAES (I-CAES) is done by compressing the air at constant temperature to achieve higher efficiencies. Therefore compressors that extract heat at around 40°C are used. As the gas is compressed, the heat of compression is rapidly transferred to a thermal mass, so the gas temperature is stabilized. An external cooling circuit is then used to maintain the temperature of the thermal mass. However, this only considers half the cycle: the extracted heat is not usable within the system, so it is wasted.

Companies working with these technology were Lightsail, SustainX and General Compression all three no longer existing today. The latter was able to complete 2 MW project in Gaine, Texas and is also still under operation.

Adiabatic CAES

Adiabatic CAES (A-CAES) balances the heat over the whole cycle of compression and expansion, storing the heat of compression to re-use during expansion. Heat can be stored in a solid such as concrete or stone, or in a fluid such as hot oil or molten salt solutions.

Hydrostor was the first company in the world to have built an operating commercial A-CAES plant (2019) in Goderich, Ontario, accomplishing an efficiency of 60%.

Hydrostor’s A-CAES technology uses off-peak or renewable generated electricity to compress air, which is stored in a large cavern underground. Heat produced by the compression process is stored inside a proprietary thermal store preserving the energy for use later in the cycle. The compressed air is maintained at a constant pressure using hydrostatic compression via water reservoirs. When needed, the hydrostatic pressure forces air to the surface, where it is combined with the heat and passed through turbines to generate electricity on demand.

By 2021 the company has projects under development in Chile, Australia, California and a new one in Ontario.

Another similar design, the CRYOBattery by Highview Power, stores the compressed air as a liquid (LAES) in a supercooled chamber, rapidly heating it up to convert it back into a gas when energy is needed.

Storelectric has also developed its own proprietary adiabatic technology claiming to reach an efficiency near 62% at 40MW, rising to ~67% at 500MW but have not yet demonstrated their technology through commercial deployments. Storelectric aims at providing flexible solutions and easily adapt to its client’s needs, locations and environmental aspects by providing adiabatic, diabatic and hybrid solutions. Storelectric has two main technologies and hybrid versions of them:

• TES (Thermal Energy Storage) CAES, zero emissions, 70% efficient grid-to-grid.
• CCGT CAES retro-fittable to a suitably located gas-fired power station, reducing emissions by a third and adding storage-related services.

Other startup companies working with adiabatic CAES technologies are Alacaes and Apex. Apex has planned a CAES plant for Anderson County, Texas, but its completion has not yet been announced.

Other competitors of CAES technologies

After years of research and diverse technology implementations CAES is one of the promising long duration storage technologies to help de-carbonze the grid. The development of this technology has also lead to new approaches of compressed air systems. The company Cheesecake Energy also stores energy in the form of heat and pressurized air and does not use diesel or any fuel. But, the system would be a modular, portable, containerized package with no geological conditions other than firm flat land to be sited.

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