Environmental Engineering Reference
In-Depth Information
Insofar as electromagnetic physics deals with forces on charges and currents, it may be regarded
as explaining a form of work or power, reasonably called electrical work or power, to distinguish
it from mechanical work or power, in which the forces are caused by mechanical contact or a
gravitational field. Correspondingly, we may regard that the work required to move a charge or
current-carrying conductor in an electric or magnetic field results in a change of electric or magnetic
energy of the field, so that energy may be said to be stored in the field, in analogy with the change
in potential energy of a mass in the earth's gravity field.
There are circumstances where it is desirable to store mechanical or electrical work generated
from mineral fuel consumption for use at later times. A common example is the storage of electrical
energy in an automobile's electric storage battery to supply the power needed to start the engine in
a subsequent use. In the automobile engine itself a flywheel stores rotational energy produced by
the power stroke of the pistons, returning it during the compression stroke.
One of the most prominent uses of electric energy storage is that employed by electric utility
systems to even out diurnal variations in the demand for electric power by storing energy during
nighttime hours when demand is low and excess power is available and then restoring this energy
to the system during daytime hours of peak demand. This permits the central electric generating
plants to operate at constant power and best efficiency all day long, lowering the cost of electricity
generation.
The principle involved is illustrated in Figure 4.1 depicting the diurnal variation of electric
power demand in a typical electric utility system. Starting at midnight, the demand declines to a
minimum in the early morning hours and then increases to a daily maximum in the late afternoon or
early evening hours, after which it declines again to its midnight level. The utility power suppliers
usually match this demand by turning on individual plants during the rising portion of the demand
and then taking them off line as the demand declines. However, a sufficient number of plants
must be run continuously to supply the minimum demand, called the base load. Supplying the
base load can be accomplished more efficiently than supplying the variable load between the
minimum and peak load where plants operate at less-than-optimal conditions, both economically
Base
load
Average
demand
Peak
demand
0
6
12
18
24
Hour
Figure 4.1
A typical diurnal demand for electric power has an early morning minimum and a late afternoon
maximum, with the former defining the base load demand that is met by continuously operating plants.
