consumption. Off grid electrical use was a niche market in the 20th century, but in the 21st century, it has expanded. Portable devices are in use all over the world. Solar panels are now common in the rural settings worldwide. Access to electricity is now a question of economics and financial viability, and not solely on technical aspects. Electric vehicles are gradually replacing combustion-engine vehicles. However, powering long-distance transportation without burning fuel remains in development. Methods Outline The following list includes a variety of types of energy storage: Fossil fuel storage Mechanical Spring Compressed air energy storage (CAES) Fireless locomotive Flywheel energy storage Solid mass gravitational Hydraulic accumulator Pumped-storage hydroelectricity (pumped hydroelectric storage, PHS, or pumped storage hydropower, PSH) Thermal Expansion Electrical, electromagnetic Capacitor Supercapacitor Superconducting magnetic energy storage (SMES, also superconducting storage coil) Biological Glycogen Starch Electrochemical (Battery Energy Storage System, BESS) Flow battery Rechargeable battery UltraBattery Thermal Brick storage heater Cryogenic energy storage, Liquid air energy storage (LAES) Liquid nitrogen engine Eutectic system Ice storage air conditioning Molten salt storage Phase-change material Seasonal thermal energy storage Solar pond Steam accumulator Thermal energy storage (general) Chemical Biofuels Hydrated salts Hydrogen storage Hydrogen peroxide Power to gas Mechanical Energy can be stored in water pumped to a higher elevation using pumped storage methods or by moving solid matter to higher locations (gravity batteries). Other commercial mechanical methods include compressing air and flywheels that convert electric energy into internal energy or kinetic energy and then back again when electrical demand peaks. Hydroelectricity Hydroelectric dams with reservoirs can be operated to provide electricity at times of peak demand. Water is stored in the reservoir during periods of low demand and released when demand is high. The net effect is similar to pumped storage, but without the pumping loss. While a hydroelectric dam does not directly store energy from other generating units, it behaves equivalently by lowering output in periods of excess electricity from other sources. In this mode, dams are one of the most efficient forms of energy storage, because only the timing of its generation changes. Hydroelectric turbines have a start-up time on the order of a few minutes. Pumped hydro Worldwide, pumped-storage hydroelectricity (PSH) is the largest-capacity form of active grid energy storage available, and, as of March 2012, the Electric Power Research Institute (EPRI) reports that PSH accounts for more than 99% of bulk storage capacity worldwide, representing around