Batteries are the most common device used for storing electrical energy.
Lead-acid batteries are the most common type of battery. They are used in automobiles, and by both utilities and electricity consumers as a backup energy source for critical electricity needs.
The traditional lead-acid battery is made up of plates, lead, and lead oxide immersed in a solution consisting of 35% sulfuric acid and 65% water. This solution is called "electrolyte," and causes a chemical reaction that produces electrons. Various other elements are also used to change the density, hardness, and porosity of the plates.
A couple of variations on the traditional design have emerged:
Valve-regulated lead-acid (VRLA) batteries — are sealed and need no topping off with water, and so require less maintenance than regular lead-acid batteries.
Gel-type lead-acid batteries — are filled with a gel instead of liquid, making them much less likely to spill.
Advanced battery technologies include lithium-ion, lithium polymer, and sodium sulfur types. Advanced batteries offer much smaller "footprints" (i.e., they take up less space) than lead-acid batteries. They are just starting to be applied to large-scale utility applications, a scale-up of previous niche applications for power quality and backup purposes at manufacturing plants.
Lithium-based batteries are also used in consumer goods and automobiles. With lithium-ion battery packs now being deployed in hybrid and plug-in electric vehicles, automakers are even considering the reuse of old automotive battery packs for utility storage applications. In contrast, sodium sulfur batteries are heavy and operate at high temperatures, so they are more suitable for large-scale stationary applications, including utility-scale energy storage systems. Nickel metal hydride batteries, widely used in hybrid vehicles, are also considered to be advanced batteries, but they are not suitable for utility-scale energy storage applications.
Flow batteries work in a similar fashion to lead-acid batteries, but the electrolyte is stored in external containers and circulated through the battery cell stack as required. This external reservoir of rechargeable electrolyte can be as large as needed, and situated where convenient. Some flow batteries use two different kinds of electrolyte that are stored separately.
The great advantage to flow batteries is that their electrical storage capacity is limited only by the capacity of the electrolyte storage reservoirs. They provide very high power and very high-capacity batteries for load-leveling applications on the electricity grid. Zinc-bromine flow batteries are the most common type in use in the United States.