Environmental Engineering Reference
In-Depth Information
4.3.1
AC/DC Conversion
Although nearly all electric power is generated, transmitted, and utilized in AC form, there are
important uses for DC power that usually require the conversion of an AC power input to DC form.
The most prominent of these are communication systems, such as the telephone and computer,
where digital circuits use DC power. Another common DC system is that of the automobile, where
alternating current generated in the engine driven alternator is converted to 12-volt DC power that
charges the battery and supplies power for lights, fans, radio, and so on.
A
rectifier
is an electrical circuit device that converts AC to DC power. It consists of diodes
that permit current to flow in one direction only, thereby transforming the AC current to DC form.
As in most electrical devices, some power is lost in this transformation. Every computer has an
internal or external power supply that converts household AC to the DC power needed for digital
circuits.
There are some sources of electric power that are DC in nature, most notably photovoltaic
cells and fuel cells. Where this power is fed to electric utility AC transmission or distribution
lines, the DC power first must be transformed to AC form. The electric device that accomplishes
this conversion is called an
inverter
. While it is possible to achieve this conversion mechanically
by employing a DC motor to power an AC generator, inverters are generally electrical circuit
components that accomplish the same purpose. There is some power loss that accompanies this
conversion.
4.4
ENERGY STORAGE
There is very little energy stored in the electric utility system that supplies electric power to
consumers. The electric power must be supplied at the same rate that it is being utilized by the
utility's customers. Although there is some energy temporarily stored (and removed) each half-
cycle in the transmission and distribution lines and transformers, it is not available for supplying
power when demand exceeds supply during a sustained period. However, there are systems that
will fulfill this need and that of other applications, such as storing energy that could be used for
electric drive vehicles. In this section we will describe the principles that form the basis for the
construction of such devices.
4.4.1
Electrostatic Energy Storage
A capacitor is a device for storing electric charge at an elevated potential. As sketched in Fig-
ure 4.7(a), it consists of two electrically conducting plates, of area
A
, separated by an electrically
insulating dielectric medium of equal area and thickness
h
. Positive and negative charges, in equal
amounts
Q
, stored on the plates induce an electric potential difference
between them. The
charge and potential difference are related by Coulomb's law for this configuration,
φ
A
h
Q
=
φ
≡
C
φ
(4.8)
where
is the
electric permittivity
of the dielectric medium and
C
≡
A
/
h
is the
capacitance
of
