Air Energy Storage

“I have far too much storage space.”- said no one ever.

The advent of green technologies has revolutionized energy systems throughout the world. From connecting solar panels to home circuits for water heating to fusion reactors generating 500MW of energy, money worth billions has been put into developing sustainable renewable energy sources. This is required to generate electricity for meeting the demands of the ever-increasing population. However, developments in energy conversion technologies must be supplemented with feasible and cleaner stationary energy storage systems for a clean energy system. Many storage systems have been considered over the years including electrochemical storage, flywheels, pumped hydroelectric storage, compressed air energy storage etc.

In 1859, the French physicist Gaston Plantè invented a rechargeable lead storage battery known as a Lead-Acid battery. In this design, the anode and cathode were made of two spirals of lead foil separated with a sheet of cloth and were coiled up. Lead batteries in cars generally have a pasted plate construction. Each plate consists of a rectangular lead grid, the holes of which are filled with a paste of red lead and 33% dilute sulphuric acid. The porous plate allows the acid to react with the lead inside the plate, thus increasing the surface area. Once dried, the plates are stacked with suitable separators and inserted into the battery container. Usually, there is one additional negative plate as compared to the positive plate. The battery also contains barium sulphate (which acts as a seed crystal for the lead-to-lead sulphate reaction), lignosulphonate (which prevents the negative plate from forming a solid mass during the discharge cycle and instead enables the formation of long needle-like crystals) and Carbon black (to counteract the effect of inhibiting formation caused by the lignosulphonates.)

Although these batteries are reliable yet they have quite a few disadvantages. They have a limited lifetime, are difficult to ship, are greatly affected by temperature conditions, contain toxic materials that require unique removal methods at the end of their useful life as well as need maintenance to maximize efficiency.

Keeping the above facts in mind, Air Energy Storage has thus been identified as a potential electricity storage mechanism. Air can be compressed using excess electricity, and released during peak demand. In addition, these batteries are long-lasting, require low maintenance and are easy to set up. There are two major types of air energy storage - Liquid Air Energy Storage (LAES) and Compressed Air Energy Storage (CAES).

LAES:

Liquid Air Energy Storage is thermo-mechanical storage wherein energy is stored in the form of liquid air (or nitrogen) at cryogenic temperatures. There are three key sub-processes in the system - charge, storage and discharge. During charge, ambient air is first purified, compressed using excess electricity and finally cooled down to reach the liquid phase - Liquid air is then stored in vessels having a pressure near atmospheric pressure. While discharging, electricity is retrieved by pumping, evaporating and expanding of liquid air stream through a set of turbines, in a power recovery unit. LAES provides an energy density 1 to 2 orders of magnitude higher than its alternatives and has no site constraint limits in its development. Due to the cryogenic temperatures of liquid air, the power generation cycle can be driven largely by available heat sources at ambient temperatures. This eliminates the need for combustion, thus curbing the associated carbon emissions. However, it allows the recovery of low-temperature streams such as waste heat within the LAES process.

There have been several developments in LAES. A joint venture between Highview Power and the University of Leeds, UK led to the design and construction of the first fully integrated LAES plant with 350kW, 2.5MWh. It was commissioned in 2010 and successfully tested in 2013, after being relocated to Birmingham for further research and development. One more plant was set up by Highview generating 5MW and 15MWh in June 2018. A number of international projects such as the CyroHub project and IEA Energy Storage Task 36 have also been established.

CAES:

Compressed Air Energy Storage plants are similar to pumped hydro-power plants in terms of their applications. However, instead of pumping water from a lower to an upper pond during periods of excess in power, in CAES ambient air or another gas is compressed and stored under pressure in an underground cavern or container. When there is a requirement for electricity, the pressurized air is heated and expanded in an expansion turbine which drives a generator for power production. In general compression cycles, heat is evolved, thus raising the temperature of the system. To avoid this, the heat is either extracted during the compression process or removed by an intermediate cooler. This lost heat is compensated for during the expansion turbine power generation phase by heating the air in combustors using natural gas fuel, or heat of combustion gas turbine in a recuperator to heat incoming air before the expansion cycle. Sometimes, the heat of compression is thermally stored before entering the cavern and used for adiabatic expansion extracting heat from a thermal storage system. A diabatic system gives an efficiency of 42%-55% without and with waste heat utilization respectively, while an adiabatic system is expected to give an efficiency of 70%. Storage spaces include artificially constructed salt caverns in deep salt formations. It is also possible to make use of natural aquifers and natural gas fields for storing compressed air. Two commercially active CAES plants are located in Huntorf, Germany and McIntosh, Alabama, USA.

The current trend and ever-increasing need for electricity show immense potential for storage systems for both renewable and non-renewable sources of energy. Technological advancements in these allied fields have grown over the years. Newer battery types have been put forth to increase efficiency, effectiveness and rate of conversion. LAES and CAES systems can thus be called the emerging new techniques that can revolutionize the battery market with their capabilities.

- Ganpati S Nayak