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We have been studying capacitors, devices that allow the temporary storage of energy in an electric circuit. This is another thing that physics is good for: understanding what energy is, it allows us to improve the ways in which we obtain energy, store it, and use it.
Why is important to be able to store energy? There are several reasons.
Power companies need to store large quantities of energy because they generate energy all night at the same rate that they do during the day. However, there is much less demand at night. Thus, some of the energy generated at night must be stored and released during the day. This process is known as "load shifting."
Another reason to store energy is to use it at a remote location. For instance, it takes electricity to start your car, but you don't want to have to plug it in to do so. Thus, you use an energy storage device (battery!) to carry a small amount of electrical energy around in
A third reason to store energy is in case of emergency. Let's say an electric generating plant has to shut down rapidly due to an accident, or it becomes isolated from the consumers due to a problem with the distribution system. If service is to be restored quickly, the power company has to have some reserve generating capability that can be brought on-line quickly.
What are the important characteristics of an energy storage system? Here are a few:
Power companies use several strategies to store energy during the day. A common example is "pumped hydroelectric power". In these systems, the power company pumps water up hill to a reservoir during low load periods (night and weekend) and runs the water back down through the turbines during peak load hours. These systems can store a lot of energy, and they can be started up quite quickly. However, they occupy a lot of land and they require a considerable change in height (usually at least 100 meters) between the upper and lower reservoirs.
A second way that the power company can shift loads to off peak hours is to get each customer to shift their use of electricity a bit. Many power companies offer discount rates for nighttime power usage. Of course, consumers can take advantage of these discounts by changing their habits (doing the wash at night for example). However, there are also systems that the consumer can buy that shift significant amounts of energy usage to the evening hours. An example is a thermal storage system which heats ceramic bricks, masonry, or water during the night then releases this energy to heat the house during the day. Systems like this are best used as a backup for a heat pump or in homes that rely on electric baseboard heat.
One of the largest forms of energy storage is the dam. In this case, water is stored at high elevation, and turns a turbine/generator on its way to low elevation. We may think of this as storing energy in the gravitational field, just as a capacitor stores charge at high voltage, keeping energy in the electrical field.
The Glen Canyon dam (shown at right) has a "hydraulic height" of 583 feet. It currently operates its generators at a discharge rate of 20,000 cubic feet per minute. Lake Powell, the reservoir above the dam, has a storage capacity of 27,000,000 acre-feet of water. (The second largest in the country) This dam stores energy that would normally be unusable, because the energy of the water flowing downstream is released too slowly to drive turbines. A dam (or natural waterfall) is required to get the gravitational energy converted to electrical energy efficiently.
Other examples of energy storage include:
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