Embodiments of a system for storing and providing electrical energy are disclosed. Also disclosed are embodiments of a system for purifying fluid, as well as embodiments of a system in which energy storage and fluid purification are combined. One disclosed embodiment of the system comprises a latent heat storage device, a sensible heat storage device, a vapor expander/compressor device mechanically coupled to a motor/generator device, a heat-exchanger, and a liquid pressurization and depressurization device. The devices are fluidly coupled in a closed-loop system, and a two-phase working fluid circulates therein. Embodiments of a method for operating the system to store and generate energy also are disclosed. Embodiments of a method for operating the system to purify fluid, as well as embodiments of a method for operating a combined energy storage and fluid purification system are disclosed.