Improvements relating to energy storage

A pumped storage system 100 comprises upper and lower reservoirs 102, 104, a working fluid 120, and a conduit 106 between them which comprises a turbine generator 110 arranged to be driven by the flow of the working fluid 120 to generate energy. The turbine 110 can be driven as a pump to drive fluid to the upper reservoir 102 to store energy. A heat transfer device 130, 132 is arranged to transfer heat to and/or from the upper or lower reservoir 102, 104. The fluid may be a slurry comprising a suspension of mineral participles and a surfactant in water, with a specific gravity of between 1.4 and 3.0. The heating or cooling may maintain the slurry at an optimum temperature. An agitator may be provided to keep the particles suspended in the water.