Energy is useful only if available when and where it is wanted. Carrying energy to where it is wanted is called distribution; keeping it available until when it is wanted is called storage.
Methods for energy storage:
Methods for energy storage may be classified according to the form in which energy is stored; the following categories appear to be the most important:
1. Mechanical energy storage
(i) Pumped hydroelectric storage
(ii) Compressed air
2. Electrical storage : the lead acid battery
3. Chemical energy storage
(iii) Reersible chemical reactions
4. Electromagnetic energy storage
5. Thermal (heat) energy storage
(i) Sensible heat
(ii) Latent heat
(iii) Chemical reactions
6. Biological storage
The primary electric-generating plant is continuously operated in a base load mode, which results in excess electricity production during the off peak periods. Electrical or electrical-mechanical energy storage is then used to hold this excess electricity for use during peak demand.
In thermal storage, all schemes deal with storing energy in a thermal form in a material during periods of low power demand and releasing it back during periods of high demand. The primary electric generating plant is operated to meet the real-time electrical demands during off-peak hours. The available thermal energy input to the plant may be essentially constant as is that from fossil or nuclear fuel, or varying as from solar incidence. The excess thermal energy is stored as such and withdrawn to be converted to meet peak electrical demands.
Energy storage systems like pumped hydro, compressed air and super conductive magnets are, or will be, suitable for large utility energy storage. Others, like flywheel and batteries, are in the development stages and will probably be suitable for medium utility energy storage.
Some of the uses of storage are:
Storage in solar devices can be used to provide energy during cloudy periods and at night. Storage can also be used to facilitate starting large solar turbine systems in the morning by preheating the equipment and to allow a gradual cooling of the system when it is closed at night. It can also mitigate the rapid, temperature changes which can result from sudden fluctuations in the Sun’s intensity during periods of partial cloudiness.
Storage equipment can be used to make more effective use of generating equipment. Without storage the output of generating equipment must be continuously adjusted to meet fluctuating demands. This means that generating equipment is either idle or operating at part capacity much of the time— resulting in higher electric costs since capital charges must be paid on the equipment, even if it is not used. Operating plants frequently be low full capacity also means that the generators are not operating at peak efficiency. Most large coal and nuclear plants cannot readily be adjusted to meet changing loads; demand fluctuations must be met with generating equipment which burns oil or gas—fuels which are becoming scarce and expensive.
Storage can improve the performance of heating, cooling, and other energy consuming equipment in much the same way that it can reduce the cost of generating electricity. Without storage, the equipment must be large enough to meet peak demands. This means that the equipment must be operated at less than its maximum capacity much of the time, increasing the need for generating equipment and decreasing system efficiency. Storage can have the additional benefit of permitting heat-pump devices and air-conditioning devices to operate when ambient conditions are most favorable. Air-conditioning systems, for example, are much more efficient at night when ambient air temperatures are relatively low.