CAES refers to the compression of air to be used later as energy source.It can be stored during periods of low energy demand (off-peak),inorder to meet periods of higher demand (peak-load). The first CAES systems were constructed by Victor Popp in 19th century which was in 1870.

Off-peak (low-cost) electrical power is used to compress air into an underground air-storage vessel and later the air is used to feed a gas-fired turbine generator set to generate electricity during on-peak (high-price) times.
High pressured air can be obtained by placing the pressurized air under water in flexible containers (e.g. plastic bags) Due to the limited number of suitable locations, this will becomes expensive, Here the charge and discharge pressures are always constant.

Below attached is a detailed presentation on CAES Compressed Air Energy Storage