Compressed Air Energy Storage
By Jack Goodwin
The world is slowly but surely adjusting to the needs of the climate crisis. While renewable energy use is rising around the globe, halting climate change is more complicated than building more solar plants and wind farms. For the world to fully change to renewable energy generation, significant changes need to be made to the electrical grid as a whole. Firstly, the distributed nature of wind and solar geographic potential necessitates a greatly changed transmission network to handle new loads and to bring energy from where it is most efficiently generated to where it is needed. Secondly, the unreliability of wind and solar requires a system of energy storage to gather energy when it is in abundance for use when generation is lacking. More commonly known methods of energy storage include batteries and hydroelectric pumping. To support a diverse and expansive grid of renewables, other forms of energy storage must be used. This article explores an alternative: Compressed Air Energy Storage (CAES).
CAES is not a particularly new form of energy storage. Over a hundred years ago, it was used in a number of powered mining machines and other industrial operations. The city of Paris even built a large compressed air network in 1881, akin to modern electrical systems, that served in driving clocks, industrial machines, and the workshops of artisans around the city. Pneumatic systems like these continue to be in use today.
But how does CAES work as a form of grid electrical storage? When storing energy a pump works to compress air either in pressurized tanks or in salt caverns. To extract energy, this air is expanded through a gas turbine, generating electricity. While initially sounding simple, this process is complicated by the fact that air greatly rises in temperature when compressed and greatly cools when expanded. When run at these large pressure differences, this temperature change reduces the efficiency of storage and can even damage the plant by overheating or causing vapor to freeze within the turbine. There is a need to cool the compressed air being stored and a need to heat the air going through the turbine. Managing these thermal systems is critical for effective energy storage and is the primary distinction between different CAES systems.
The first full scale CAES plant was constructed in 1978 in Huntorf, Germany as a load balancer (stabilizing electrical grid) rather than full energy storage. This plant uses diabatic storage meaning that when storing energy, excess heat is dissipated to the air. When extracting energy however, natural gas is used to heat the air to a workable temperature. It uses 0.8kWh of electricity and 1.6kWh of gas to produce 1kWh of electricity. The amount of natural gas used per kWh is greatly reduced compared to a standard natural gas plant. Nevertheless, for a world trying to wean itself off fossil fuels entirely, this approach is not sufficient for our needs.
Recent developments in the field focus on creating a near-adiabatic (no heat loss in system) process. The heat generated by compression is stored in some medium. During extraction, this heat is reintroduced to the air. With a perfect adiabatic process, the efficiency of the system is 100%. One company exploring this technology is Hydrostor, from Canada. They use a thermal management system to store the heat from compression and a large bed of water to provide a constant extraction pressure which is beneficial to turbine lifetime. With this technology, CAES can achieve an energy efficiency of greater than 70%.
With the technology we have today, CAES holds several advantages over other forms of energy storage. First, it does not rely on rare materials whose extraction poses ethical and environmental concerns like large-scale battery storage. Secondly, unlike pumped hydroelectric, it is relatively geographically independent: high pressure containers can be constructed anywhere, above or below ground. Additionally, salt caverns can be found globally, even repurposing those used to store natural gas. As the technology for adiabatic CAES is in its infancy, more research is needed to improve efficiency and cost effectiveness. CAES promises to be an efficient and clean method of energy storage, contributing further to a future without fossil fuels.