Compressed gas energy storage system

A compressed air energy storage system comprising:

• a) an accumulator disposed underground and having an interior configured to contain compressed gas at a storage pressure that is at least 10 bar when in use;
• b) a gas compressor/expander subsystem comprising at least a first compression stage having a first gas inlet and a gas outlet in fluid communication with the accumulator interior via a gas flow path for conveying compressed gas at the storage pressure to the accumulator when in a charging mode and from the accumulator when in a discharging mode;
• c) a thermal storage subsystem comprising:
• i) a cold storage chamber for containing a supply of granular heat transfer particles at a cold temperature and cold storage pressure;
• ii) a hot storage chamber for containing the supply of granular heat transfer particles at a hot temperature and hot storage pressure;
• iii) at least a first mixing chamber in the gas flow path and having an interior in which the compressed gas contacts the granular heat transfer particles at a mixing pressure that is greater than the cold storage pressure and the hot storage pressure;
• iv) a conveying system operable to selectably move the granular heat transfer particles from the cold storage chamber, through the first mixing chamber and into the hot storage chamber, and vice versa;
• wherein:
• when the compressed air energy storage system is in the charging mode i) granular heat transfer particles are conveyed from the cold storage chamber into the first mixing chamber, ii) thermal energy is transferred to the granular heat transfer particles within the mixing chamber from the compressed gas stream being conveyed into the accumulator via the first mixing chamber thereby heating the granular heat transfer particles and cooling the compressed gas, and iii) the heated granular heat transfer particles are conveyed from the first mixing chamber toward the hot storage chamber for storage and the cooled compressed stream is conveyed from the first mixing chamber toward the accumulator for storage;
• and
• when the compressed gas energy storage system is in the discharging mode i) granular heat transfer particles are conveyed from the hot storage chamber into the first mixing chamber, ii) thermal energy is transferred from the granular heat transfer particles within the mixing chamber to the compressed gas stream being conveyed out of the accumulator via the first mixing chamber thereby cooling the granular heat transfer particles and heating the compressed gas and iii) the cooled granular heat transfer particles are conveyed from the first mixing chamber toward the cold storage chamber for storage and the heated compressed stream is conveyed from the first mixing chamber toward the compressor/expander subsystem for expansion.