Energy storage is not only batteries and hydrogen. Other systems exist that take energy from generating stations and store it for later use. Large storage plants can operate at the transmission grid level while the smallest can offer storage services to small commercial and residential consumers. Some examples are given below: Thermal storage system made of solid materials is normally used for a volumetric air or compressed-air system in which thermal energy is transferred to another medium that can be any solid material with high density and high thermal capacity. Technologies include energy storage with molten salt and liquid air or cryogenic storage. Pumped hydroelectric storage facilities store energy in the form of water in an upper reservoir, pumped from another reservoir at a lower elevation. During periods of high electricity demand, power is generated by releasing the stored water through turbines in the same manner as a conventional hydropower station. During periods of low demand (usually nights or weekends when electricity is also lower cost), the upper reservoir is recharged by using lower-cost electricity from the grid to pump the water back to the upper reservoir. Mechanical storage systems are arguably the simplest, drawing on the kinetic forces of rotation or gravitation to store energy. The main options are energy storage with flywheels and compressed air systems, while gravitational energy is an emerging technology with various options under development.