We often need to consume energy at a different time or place from where the energy is generated. We therefore store energy in the interim. My subjective ranking from least to most interesting modes of energy storage is:
- Batteries. When (or if) most people think of energy storage, they think of batteries. Batteries store energy chemically; this energy can be converted into electricity pretty efficiently, and then into mechanical energy moderately efficiently. The drive to smaller computing devices and to electric cars has produced an explosion in battery research.
- Biomass and petroleum. Fusion in the sun produces lots of energy; a small fraction of that energy arrives at the Earth as sunlight and by plants to grow. We can liberate that energy by burning the plants, but this usually requires large volumes of material: think of the huge stack of firewood outside a house that’s heated with a wood-burning stove. Over very long times, some buried plant material is converted into petroleum. Petroleum is more energy dense than wood, is easier to transport (since it is a liquid), and is “free” (since it represents energy harvested from sunlight in the distant past, it doesn’t require any acreage be set aside today). Petroleum used to be abundant, but the most accessible sources have already been depleted; the first oil well in the U.S. was in Titusville, PA, at a depth of 69 feet (!) and Pennsylvania dominated worldwide oil production for the next 40 years; the giant refinery complexes in northern New Jersey are a legacy of those early days of when the mid-Atlantic states dominated oil production. Other combustible liquids, such as ethanol, share some of the advantages of petroleum.
- Hydrogen. Molecular hydrogen can be combusted or used to generate electricity in fuel cells. If technical and safety hurdles can be overcome, it could function as a relatively high-energy-density, clean, transportable form of stored energy. Despite its proponents’ careless rhetoric, hydrogen is not a source of energy: since there are no preexisting pools of molecular hydrogen, we have to make the stuff, which requires more energy as input than is present in the product.
Gravitational potential energy. Flowing water can become a source of energy. As it runs downhill, the gravitational potential energy of water is turned into kinetic energy, which can be harvested by a turbine. Historically, mills and factories were situated next to streams and millwheels provided energy for everything from grinding grain to turning lathes (except in Holland, which everyone knows is flat: since the Dutch don’t have fast-moving streams they had to use windmills). Nowadays we don’t rely on natural streams for energy: we build dams to store potential energy in artificial lakes, and extract the energy by draining the lake through a turbine in the base of the dam. During the night, when they have spare generating capacity, the British pump water into an old quarry at the top of Electric Mountain in Wales; during peak demand (for instance, at halftime on soccer game day, when everyone turns on the electric kettle to make tea), they release the water and recover its energy through a turbine as electricity.- Mechanical energy. The kinetic energy of a rotating flywheel can be used as storage. On a small scale, flywheels are used as reservoirs to accumulate bursts of energy and release the energy slowly. A spinning wheel contains a large flywheel what is pumped intermittently by a foot pedal; in a car, combustion in the cylinders spins a flywheel which then delivers it to the transmission; some versions of regenerative braking systems (typically used on buses) shunt translational kinetic energy (of the bus) into rotational kinetic energy (of a flywheel). On a larger scale, I once worked in the Princeton Plasma Physics Laboratory, which uses giant flywheels to store energy to be used to initiate fusion reactions. Drawing the necessary power directly off the grid would brown out central New Jersey, so instead they gradually spin up a pair of enormous flywheels over several hours, and then harvest the entire accumulated energy within minutes. The rim of each flywheel weighs 600 tons, and at full charge the outer edge spins at 329 mph, producing a centripetal acceleration of 800 g.