Compressed and liquid air for long duration & high capacity

Above: Air compressor for CAES and LAES service

At present, pumped hydro is the highest capacity power storage technology. Much has been said about the use of lithium-ion batteries for power storage, but the timescale over which they can be depleted is relatively short and between 4 and 8 hours. This is not sufficient to support many requirements, such as continuous power delivery from a solar farm.

Liquid air energy storage (LAES) involves compression and liquefaction of air for mid-term storage. The stored cryogen is pumped, vaporised, and released through a turbine to generate power as required.

The world’s first LAES demonstration plant was built by Highview Power at the Pilsworth landfill waste management site close to Manchester in the UK and commenced operation in 2018. Highview Power has since announced two commercial scale units for Vermont in the USA and Carrington in the UK. Each will have the capability to generate 50 MW of power and will have energy storage capacity exceeding 250 MWh equivalent to five hours of continuous discharge.

The start-up time of a LAES system cannot compete with ultra-capacitors or batteries, which react within milliseconds or seconds. However, the Highview Power plants can ramp up to be fully operational within less than 10 minutes from a “cold-start” or only a few minutes from a “warm-start”.

Compressed air energy storage (CAES) works in a similar way to LAES, but instead of the air being converted to a liquid, it is contained in a large underground storage cavern. When the electricity grid needs a power top-up, the high-pressure air is released through a turbine to generate power.

CAES was first implemented at scale in Germany more than 40 years ago. The Huntorf CAES facility was constructed in 1978. Since then, CAES has evolved to integrate thermal energy storage to eliminate natural gas usage in the system and improve the overall cycle efficiency.

The LAES compression, liquefaction, vaporisation and expansion cycle

Within the LAES cycle, there are four requirements for high-speed rotating machinery. The first step is to compress air to around 15 bar. A 4-stage centrifugal compressor, as might be used as the main air compressor on an air separation unit, would be ideal. A second 2- or 3-stage centrifugal air compressor then boosts the main inlet air to around 60 bar. Liquefaction of the air is then achieved using an expansion turbine.

Power generation is achieved by pumping the liquid air to around 160 bar pressure and then vaporising it against ambient air. The resultant high pressure air is expanded on a power generation turbine.

The selection of the expansion turbine depends on the size of the LAES facility. Up to around 20 MW calls for a multistage radial expansion turbine rotating at speeds of up to 10 000 rpm. Such turbines have been used for many years for energy recovery in natural gas pipeline let-down stations and non-condensing axial steam turbines.

For larger liquid air energy storage facilities, an axial turbo expander would be more appropriate.

These machines can be direct coupled to the synchronous generator and can run at a synchronous speed (3000 or 3600 rpm), without the need for a gearbox.

Huntorf: gas-fired peaker with integrated CAES

50 years ago, the introduction of pipeline natural gas supplies and LNG imports gave rise to the concept of gas-fired power generation to meet peaks in demand. In contrast to nuclear power or coal fired generation, gas fired power plants can start up from cold very quickly. In Europe, North America and Asia many open circuit and closed circuit gas-fired power plants were built to balance regional power grids.

The combustion turbine on a gas-fired power plant operates at a pressure of around 30 bar. About 65% of the energy produced by burning natural gas is used to compress the combustion air up to the operating pressure. The compression energy requirement is parasitic to the overall system efficiency. This issue seeded the idea of using stored compressed air to mix with natural gas to avoid the air-compression energy losses during power generation.

Huntorf power plant followed this design concept. The wide spread between power prices during peak and off-peak periods was deemed sufficient to justify investment in such technology. During periods of low demand on the grid, low-cost power is used to compress air into two underground salt caverns. When power generation is required and high prices can be achieved, the compressed air is released and blended with natural gas in the turbine to generate up to 321 MW of power. Start up from cold to 50% generation capacity can be achieved within three minutes, with full capacity reached after seven minutes.

The expansion turbines at Huntorf were manufactured by ABB. The high-pressure turbine is like a steam expansion turbine in a thermal power plant and the low-pressure unit is a typical gas fired power generation turbine.

Underground salt cavern storage for CAES, hydrogen and natural gas

Salt caverns are man-made cavities in naturally occurring salt deposits below ground and are created using a technique called ‘solution mining’. During this process, a bore hole is drilled from the surface to the underground salt layer and water is then injected to dissolve the rock salts. When the cavern reaches the desired volume, gas is injected to displace the brine.

Underground geological salt formations in northern Germany allowed the creation of salt caverns for air storage at Huntorf. The two Huntorf caverns are 140 000 and 170 000 cubic metres in capacity. The tops of the caverns are 650 m below the surface of the earth. They are about 150 m tall and up to 60 m wide. To prevent geological shifts, they have a minimum operational pressure of 20 bar. However, the normal duty cycle is between 43 and 70 bar. The amount of air released during decompression can operate the turbine for about three hours.

Underground storage of hydrogen (UHS) in salt caverns is effective when terawatt hours of energy storage is required for release on a seasonal basis. Whilst UHS in salt caverns offers low-cost, low-carbon, seasonal energy storage at utility scale, it has a low round-trip efficiency (MW power required for electrolysis compared to MW power generated by a turbine or fuel cell) of the order of 40%.

Similarly, underground storage of natural gas (UGS) is used for seasonal energy demand balancing. The UGS facility operated by NAM at Norg in the NL stores 7 billion cubic metres of natural gas in a depleted gas field and can release that gas at a rate of 3 million cubic metres per hour at periods of peak demand, such as during a cold winter’s day when heating demand is at its highest.

Advanced, adiabatic CAES

The future of CAES lies in energy efficient power storage to support variable renewable power generation and grid balancing. To increase the round-trip efficiency of CAES, various modifications have been made by innovators such as Hydrostor. One such development is to capture the heat of compression in a thermal energy storage (TES) unit. This heat is then given back to the air as it expands across the power generation turbine to avoid excessive cooling of the air. The innovative combination of TES and CAES is known as advanced, adiabatic CAES, or AA-CAES.

A further enhancement in some AA-CAES systems is that the pressure of air in the underground storage remains constant. This is achieved by the air displacing water in the underground cavern to an above-ground reservoir. In this mode of operation, the underground gas cavern must be mined into rock. The use of a salt cavern for such an application would not be appropriate.

Hydrostor is planning to develop several AA-CAES facilities worldwide. One in Cheshire, in the UK will use a salt cavern for compressed air storage and the pressure of the stored air will rise and fall through the cycles of operation. Another project planned for Kern County, California, will use a rock cavern for the compressed air storage. In this project, water will be used to operate the cavern at a constant pressure.

Author: Stephen B. Harrison sbh4 consulting, Germany