AS A CONCEPT, ENERGY STORAGE HAS IMMEDIATE APPEAL. IT transfers energy through time, from generation to consumption, just as energy transmission transfers energy from one place to another. Indeed, in the early days of electricity, the Leyden jar and Volta's columnar apparatus were very much cutting-edge technology. However, after the invention of the dynamo, ac generation, and the spectacular spread of continental electric grids, the storage of electricity became relatively unimportant-with one exception. The lead-acid battery became an essential ingredient of the automobile. Due to large production volume, this battery has become relatively inexpensive and fairly reliable. More recently, energy storage has also become a critical component of electronic equipment such as portable computers and mobile phones. In this market, where cost is less important, the need for greater energy density, zero maintenance, and long cycle life has allowed advanced batteries such as lithium-ion (Li-ion) and nickel-metal-hydride batteries to find wide application.By contrast, energy storage (other than pumped hydro) has found very little application in utility or large-scale industrial applications. A large 20-MW/15-min storage facility in Puerto Rico kept the entire island grid stable for several years. Because the battery was designed primarily as an energy source for spinning reserve but was used as a rapidly fluctuating power source performing frequency regulation, it reached end-of-life prematurely and did not meet predicted life expectations. However, recognizing its vital importance to the island, the facility has recently been repowered with 20 MW of tubular plate lead-acid batteries. An ©DIGI TAL VISIO N, LTD. even bigger 27-MW/15-min facility was commissioned in Fairbanks, Alaska, in 2003. It utilizes nickel-cadmium batteries and does an excellent job providing voltage support for the long transmission line from Anchorage. Even when not discharging, it can supply about 10 Mvar. Similarly successful is a 250-kW/8-h vanadium redox facility that was put into service on a long distribution line in Utah. In Wisconsin, a system of six 1-MW/1-s superconducting magnetic energy storage (SMES) devices was effectively used to inject power into a collapse-prone transmission loop. A number of demonstrations in the 100-kW to 1-MW range have been field-tested with the U.S. Department of Energy's support and in cooperation with major utilities. Technologies include lead acid, zinc bromine (ZnBr), Li ion, and sodium sulfur (NaS). In addition, a considerable number of megawatt-sized lead-acid uninterrupted power supply (UPS) systems are in operation throughout the United States for financial institutions, server farms, airports, and the like. Japan, on the other hand, boasts over 70-MW installed capacity for NaS technology alone-the largest facilities at 8 MW/8 h.Meanwhile, analytical studies have shown the applicability of energy storage for voltage support and frequency stability, for peak shaving, renewables firming, transmission upgrad...
