 compressed air energy storage, CAS, has gained attention in recent years due to its potential to store energy from intermittent renewable sources and provide reliable electricity supply. However, existing CAS systems suffer from several drawbacks, including energy losses due to compression, the use of fossil fuels, and dependence on geologic formations. Innovative concepts of CAS, such as adiabatic CAS, isothermal CAS, micro CAS combined with air cycle heating and cooling, and constant pressure CAS combined with pumped hydrostorage, have been proposed to address these issues. By analyzing the energy and exergy of each concept, we can better understand the characteristics of each CAS system and compare them. This article was authored by Youngmin Kim, Jang Hee Lee, Sukjoon Kim, and others.