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Figure 1 Specific power and energy for several storage systems.
Figure 1 Specific power and energy for several storage systems.

Not all the energy produced by electric generators can be used immediately after it is generated. Electrical utilities take advantage of lower demand during off-peak periods by storing excess electricity and using it during peak periods, when load exceeds the generator’s capacity. This reduces the load on the system during peak hours, allowing the main generators to work at constant power, near full capacity, and under optimal conditions, thus reducing the average capital and operating costs by cutting the number of power generating stations. The important parameter in evaluating energy storage devices is their energy storage densities per unit mass (kJ/kg) and per unit volume (kJ/m3).

The most common methods of storing energy are pumped storage plants, storage batteries, capacitors, and flywheels. In addition, electricity can be stored by producing hydrogen that can be recovered when it is burned or used to operate fuel cells. Figure 1 shows a comparison between energy and power densities for various energy storage devices.


Pumped Storage Plants

In pumped storage plants, the water downstream of a dam or hydropower generating station is pumped back into a storage tank at a higher elevation to be used to generate electricity when it is needed. Pumped storage plants were discussed in detail in hydro energy.

Compressed Air

Figure 2 Alabama Electric Cooperative in McIntosh, uses air compressed and stored in an underground cavern to supplement electrical generation capacity during peak periods.
Figure 2 Alabama Electric Cooperative in McIntosh, uses air compressed and stored in an underground cavern to supplement electrical generation capacity during peak periods.

Instead of pumping water as is used in pumped storage plants, it is possible to compress air to a high pressure and store it in sealed underground caverns. During peak hours, air is mixed with natural gas in a conventional gas turbine combustion process to generate electricity. The McIntosh plant in Alabama has the largest compressed air storage facility in the United States (Figure 2).


Figure 3 Lead-Acid Battery.
Figure 3 Lead-Acid Battery.

Batteries are devices that store electric energy as chemical energy (during charging) and release it as electric energy during discharge. A battery consists of a number of voltaic cells. Each cell produces a certain voltage that depends on the type of cell and materials used. When several cells are arranged in a series configuration in a plastic casing, they are collectively called a battery.

Each cell consists of two unlike metal electrodes immersed in an electrolyte which reacts with them. The positive electrode (cathode) produces negative ions (molecules with extra electrons in their outer shells), while the negative electrode (anode) produces positive ions (molecules which lack electrons in their outer shells). If the two ends of the electrodes are connected through a wire and external load, an electric current path is established that allows electrons to flow from the negative to the positive electrode (Figure 3). The circuit is closed by a flow of ions through the electrolytes. In small batteries, electrodes are small rods inserted in a pool of electrolyte. In larger batteries, electrodes are in the shape of thin metal plates. The most widely used battery is the lead-acid battery, where the electrolyte is a solution of sulfuric acid and water and electrodes are pure lead and lead-dioxide.

Depending on whether cells can be recharged or not, batteries are classified as primary and secondary. In primary batteries (such as those used in most flashlights) the chemical reactions that supply current are irreversible. Once these batteries are discharged, they must be replaced. In secondary batteries (such as car batteries) chemical reactions are reversible; by supplying current in the opposite direction, the chemical reactions are reversed and depleted electrode materials are restored. Depending on their application, batteries can be classified as deep-cycle or shallow-cycle. Deep-cycle batteries, such as those used for golf-carts, electric vehicles, backup power, or renewable energy systems, are allowed to deplete up to 80% of their charge many hundreds of times. Automotive batteries (also called SLI, for starting, lighting, and ignition) are shallow-cycle batteries and cannot be allowed to discharge more than 2-5% of their capacity without being damaged. In addition to deep- and shallow-cycle batteries there are marine (hybrid) batteries used on boats and other marine applications which can be discharged up to 50%.

Fuel Cells

Another electrochemical device capable of storing energy is a fuel cell. The principle of operation of fuel cells is given in transportation and will not be repeated here.


Figure 4 Ultracapacitor.
Figure 4 Ultracapacitor.

Unlike batteries which store energy by chemical reactions, ultracapacitors (also called super capacitors) store energy as static energy (Figure 4). A capacitor is made of two conducting plates filled with an insulator called the dielectric. The major differences between a battery and an ultracapacitor are their energy and power densities. Typically, batteries are best suitable for storing a lot of energy and little power, whereas capacitors can provide large amounts of power but store little energy. Because of their ease of operation, low maintenance requirements, lack of negative environmental impacts, and virtually indefinite number of cycles (charging and discharging), ultra capacitors have been of intense practical interest in hybrid and electric vehicles for capturing the energy produced during regenerative braking, which improves fuel efficiency by 10-15%.


Flywheels are large wheels that store energy by rotating at very high speeds. Traditionally, flywheels have been used for energy balance in internal combustion engines. Recent advances in material technology have made it possible to design flywheels from lighter composite materials that can run at much higher speeds - up to 100,000 rpm - making them ideal for storing surplus energy from power plants to be delivered later during peak electrical demand. The storage capacity of flywheels can reach those of batteries with the advantage that they can be energized and de-energized faster; the cost is higher however.

Flywheels have also found applications in fuel cells, gas turbines, and other prime movers. Flywheels are advantageous over conventional lead-acid batteries because they have high efficiencies, much longer life, operate at any ambient conditions, and can deliver energy over a much shorter time, i.e., they have a very high specific power. If desired, flywheels can be used to charge batteries, making them suitable for providing steady power to electric and hybrid vehicles or to produce bursts of energy during acceleration and on steep grades (1).


Strictly speaking, hydrogen is not a fuel source, but a carrier and a means of energy storage. To produce hydrogen, one requires at least as much energy as is produced when it is burned in a combustion process, or the electricity it generates when operating a fuel cell. In other words, we store the energy contained in other energy sources in the form of hydrogen, and then use the hydrogen to produce power.


(1) Rosen, H., and Castleman, D. R., “Flywheels in Hybrid Vehicles,” Scientific American, p. 75-77, October 1997.

(2) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

Further Reading

Bureau of Naval Personnel, Basic Electricity, Dover Publishing Company.

The Environmental Effects of Electricity Generation, IEEE, 1995.

The Electricity Journal, Direct Science Elsevier Publishing Company, This journal addresses issues related to generating power from natural gas-fired cogeneration and renewable energy plants (wind power, biomass, hydro and solar).

International Journal of Electrical Power and Energy Systems, Direct Science Elsevier Publishing Company.

Home Power Magazine (

External Links

Federal Energy Regulatory Commission (

Energy Information Agency, Department of Energy (

California Energy Commission (

National Council on Electricity Policy (

Southern California Edison (

Pacific Gas and Electric (