Batteries actually provide only a small part of today's utility-scale energy storage. The energy industry is using, or at least experimenting with, lots of other technologies, from simple (gravity storage involving trains and a hill) to exotic (superconducting magnetic coils).

The rise of solar and wind energy has focused attention (and research) on storage to smooth out the inherent variability of those two electricity sources. In some places, they're now generating more energy on sunny, breezy days than local grids can handle. (For details, click here.)

To store that excess energy for when it's needed (such as at night and on calm days), rechargeable batteries aren't even close to being the most popular large-scale technology. Here's a look at what is, along with some other storage methods:

  • Pumped-storage hydropower: You may not think of a reservoir created by a hydropower dam as an energy storage device, but it is. Pumped hydro operates on the same general principle, using two reservoirs at different elevations. When energy is abundant, pumps move water to the higher elevation. During times of high demand, gravity pulls the water back downhill, spinning power-generating turbines. Pumped hydro currently accounts for almost all utility-level storage. In Bath County, Virginia, the world's largest pumped-hydro plant can generate 3 gigawatts of electricity.
  • Other types of gravity storage: Options range from heavy electric trains to weights suspended from cranes or oceangoing barges. Excess energy runs the trains up a hill or lifts the weights. To turn the stored energy back into electricity, the trains roll back down the hill, or the weights drop.
  • Flywheels: Carbon-fiber flywheels powered by electric motors spin in a vacuum enclosure on frictionless magnetic bearings at up to 60,000 rpm. When needed, they provide power almost instantly. They can last for decades with virtually no maintenance.
  • Compressed air: Air pumped into a salt cavern or other underground chamber gets released when needed to drive a turbine. Sometimes the released air is heated, either by natural gas or, for greater efficiency, by waste heat generated and stored during the compression process.
  • Thermal: That's a blanket term covering a wide range of technologies. They capture or create heat or cold, store it in rocks, water, molten salt, or some other insulated medium, and then release it to generate electricity. Thermal energy can also be used to heat and cool buildings directly, without conversion to electricity.
  • Hydrogen: Electrical energy can produce hydrogen from water for later use in fuel cells. The fuel cells generate electricity by combining hydrogen and oxygen, emitting only water vapor and warm air.
  • Superconducting magnetic energy storage: A charged superconducting coil can store energy indefinitely without loss, then release it almost instantaneously when needed. Such coils now stabilize some power grids that are subject to sudden large changes in load. Drawbacks include high cost, small storage capacity, and high power requirements for cooling the coil below its superconducting critical temperature.

Other energy-storage options exist. New ones doubtless will emerge. Stay tuned.