Environmental Engineering Reference
and thermodynamically. If electric storage systems are used to store energy during times of less-
than-average demand and return this energy during times of above-average demand, then fewer
power plants will be needed to satisfy the diurnal demand and those fewer plants may be operated
at optimal conditions.
The most common form of storing energy for use in central electric power systems is the
pumped storage system where water is pumped from a lower- to a higher-level reservoir, with
the energy being stored in the earth's gravitational field. This energy is later recovered when the
stored water flows through a turbogenerator to the lower-level reservoir. Other schemes have been
proposed as well, including the use of electric storage batteries, electric capacitors and inductors,
and flywheels. These latter are also possible sources of emergency electric power for essential
purposes, like hospital operating room power and computer power.
Electric power produced from some sources of renewable energy, such as photovoltaic cells,
wind turbines, and tidal power systems, may not be available at times that synchronize with electric
power demand, as illustrated in Figure 4.1. Energy storage may be necessary for the successful use
of these systems, especially where they cannot be tied into an electric power grid that will provide
power at times when the renewable source is absent.
In recent years a renewed interest in electrically propelled automobiles and other vehicles has
arisen as a consequence of a desire to improve vehicular energy efficiency and reduce air pollutant
emissions from ground transportation vehicles. Most of the proposed systems incorporate, in part,
some energy storage in the form of electric storage batteries, flywheels, or electric capacitors. For
such applications, the weight, volume, and cost of sufficient energy storage to provide accept-
able vehicle performance is generally much greater than that for conventional hydrocarbon-fueled
vehicles, posing obstacles to their widespread use. Compared to stationary energy storage systems,
mobile ones must meet much more stringent requirements.
This chapter treats the physical principles that determine how electrical power is generated,
transmitted, and stored. We first consider how mechanical work is converted to electrical work in an
electrical generator, along with the inverse of that process in an electric motor. We next show how
electric power flows in a transmission line that connects the electric generator with the end-user of
the electric power, and we explain how some of that power is lost in resistance to the flow of current
in electric wires. Finally, we describe the kinds of energy storage systems that are used to provide
electrical or mechanical power for use at times when it is not otherwise available. Emphasis is
placed on the energy and power per unit mass and volume of these systems, the cost per unit energy
stored, and the efficiency of recovery of the stored energy, because these are the parameters that
determine their use to replace the conventional systems where mineral fuel is consumed as needed
to supply electrical or mechanical power and heat.
ELECTROMECHANICAL POWER TRANSFORMATION
The electric generator and electric motor are the principal devices by which mechanical and elec-
trical power are converted from one form to the other. In the United States, almost all utility electric
power is generated by steam, gas, hydro, or wind turbines driving an electric generator. About 60%
of this electric power is converted by electric motors to mechanical power for residential, commer-
cial and industrial use. Fifty percent of all mechanical power produced by fossil and nuclear fuel
consumption is used to generate electric power.