What is Compressed Air Energy Storage (CAES)?

"Compressed air energy storage (CAES) is based around the gas turbine cycle. Surplus power is used to compress air using a rotary compressor and then store it, often in an underground chamber. When the power is required, it is released from the chamber and passed through an air turbine that generates electricity from the flow of high pressure air."
https://www.sciencedirect.com/topics/engineering/compressed-air-energy-storage

"One such storage system uses compressed air to save electricity for when it is needed. The idea of energy storage using compressed air has been around for decades but is recently being explored more as a solution to augment renewable energy systems."
https://www.quincycompressor.com/compressed-air-energy-storage-renewable/

"Compressed air energy storage (CAES) offers a method for storing compressed air within a sealed enclosure. Storage in a compressed air system allows users to supplement energy usage during high-demand periods, enhances air quality, and maintains system stability. The energy is recovered by allowing the air to decompress through a turbine. Heat that is released during expansion can be reused for added energy efficiency."
https://www.compressedairsystems.com/blog/5-benefits-compressed-air-energy-storage/

"One of the major issues with compressed air energy storage is that when you compress air it heats up. When the electricity is required it needs to be expanded, which requires heat. In addition the cooler the air, the more you can store. Companies are therefore trying to find ways to best store the heat generated during compression, so it can then be used to heat the air for the expansion helping drive more efficiency in the overall process."
https://www.thegreenage.co.uk/tech/compressed-air-energy-storage/

"Currently, the energy storage is dominated by banks of batteries, but other forms of energy storage are beginning to appear alongside them. CAES is one of them."
https://interestingengineering.com/compressed-air-energy-storage-caes-systems

"Compressed Air Energy Storage (CAES) is usually regarded as a form of large-scale energy storage, comparable to a pumped hydropower plant. Such a CAES plant compresses air and stores it in an underground cavern, recovering the energy by expanding (or decompressing) the air through a turbine, which runs a generator.

Unfortunately, large-scale CAES plants are very energy inefficient. Compressing and decompressing air introduces energy losses, resulting in an electric-to-electric efficiency of only 40-52%, compared to 70-85% for pumped hydropower plants, and 70-90% for chemical batteries."
https://www.lowtechmagazine.com/2018/05/ditch-the-batteries-off-the-grid-compressed-air-energy-storage.html

"The compressed air energy storage market is expected to grow at a CAGR of more than 42% over the forecast period of 2020-2025."
https://www.globenewswire.com/news-release/2020/05/07/2029163/0/en/Compressed-Air-Energy-Storage-CAES-Market-Growth-Trends-and-Forecast-2020-2025.html

"The market for CAES systems remained underdeveloped, due to several reasons, such as lack of investment and lack of technology, which prevented the development of new CAES plants. However, factors, such as renewable integration with CAES systems and implementation of demonstration projects, coupled with technological developments in the CAES systems, are expected to drive the market studied during the forecast period."
https://www.businesswire.com/news/home/20191223005498/en/Compressed-Air-Energy-Storage-CAES-Market--