Different types of
energy storage systems are described, in particular hydro-pneumatic storage systems. In one, energy is stored by compressing gas in a chamber (44,45,54,55) with a
liquid piston and released by
gas expansion. A spray head or grid at the top of the chamber (44,45,54,55) supplies liquid as a
shower through the gas being compressed or expanding in the cylinder (11,12) to maintain an isothermal condition. In another, energy is stored from an array of solar cells connected to an array of supercapacitors forming an auxiliary storage, and a main
energy storage device such as a hydro-pneumatic storage
system, for supply to an AC or DC network. The efficiency is improved by connecting the solar cells via the array of supercapacitors to the AC or DC network. An immersed hydro-pneumatic storage device for off-
shore / on-
shore power generation systems comprises a cylinder that is immersed in a liquid
mass, wherein energy is stored by compressing gas with a
liquid piston and energy is released by
gas expansion. The
mass of liquid maintains an isothermal condition in the cylinder during compression and expansion.