An electric utility typically provides electric power to a large group of customers located in its service area. The electric power demand caused by the customers typically varies widely. Some of these variations in electric power demand are predictable. For example, electric power demand during the summer may be expected to peak on weekdays during late afternoon or early evening as residential customers return home and switch on their air conditioning systems. Other load demands, however, may not be predictable. For example, a utility may experience a sudden drop in electric power demand due to tripping of a circuit breaker controlling a major feeder circuit.
Electric power demand variations are problematic to electric utilities. For example, it may be impossible for an electric utility's electric generation equipment (e.g., a coal fired power plant) to respond quickly to a change in demand for electricity, resulting in undesired voltage and/or frequency fluctuations. As another example, an electric utility may not have adequate generation capacity to meet a peak electric power demand, and the utility may therefore be forced to purchase electric power at a high price from another party to cover the shortfall.
Accordingly, energy storage systems have been proposed to compensate for changes in electric power demand. Such systems may provide electric power to an utility's electric power grid (“regulation up”) when additional electric power is needed, such as during times of peak electric power demand. As another example, such systems may absorb or store electric power from the utility's electric power grid (“regulation down”) when required or when desired, such as when electric power demand suddenly drops.