Pumped Heat Electricity Storage

Energy storage is likely to become increasingly important as intermittent production technologies like wind and solar are added to the grid. Isentropic energy, a British company, proposes an innovative solution to energy storage, using insulated tanks filled with gravel as heat reservoirs and compressed air as the energy-transference medium in a large-scale Carnot heat engine. For those not familiar, the Carnot heat engine is a thermodynamic system through which a thermal energy gradient between hot and cold reservoirs can be exploited to do (mechanical) work, as an energy-carrying medium is allowed to flow from the hot side to the cold side of the system. The maximum efficiency of such a system is dictated by the difference in temperatures of the two reservoirs: the greater the difference, the greater the potential efficiency. In the Isentropic engine, the cold reservoir is maintained at -150 C, boosting efficiency over a more typical ambient-temperature reservoir. Also notable is their claim that one of their storage systems occupies a physical footprint 1/300 the size of a pumped-hydro plant of similar capacity (see figure in linked site).