Compensation Liquid for a Compressed Gas Energy Storage System

A hydrostatically compensated, compressed gas energy storage system comprising:

• a) an accumulator disposed underground and comprising an interior for containing a layer of compressed gas above a layer of compensation liquid, the layer of compressed gas being at an accumulator pressure that is between about 20 bar and about 90 bar and the compensation liquid having a density of at least 1500 kg/m3;
• b) a compressor and expander subsystem in fluid communication with the accumulator interior via a gas flow path and configured to selectably convey compressed gas into the accumulator and to extract gas from the accumulator;
• c) a compensation liquid reservoir spaced apart from the accumulator and a compensation liquid flow path extending between the compensation liquid reservoir and the layer of compensation liquid within the accumulator;
• and
• the system being operable in at least a charging mode in which the compressor and expander subsystem conveys gas into the layer of compressed gas thereby displacing a corresponding volume of compensation liquid from the layer of compensation liquid within the accumulator out of the accumulator via the compensation liquid flow path thereby maintaining the layer of compressed gas at substantially the accumulator pressure during the charging mode.