Environmental Engineering Reference
In-Depth Information
consumers, the voltage is increased by electrical transformers and the power is merged into a
network of high-voltage transmission lines that joins the outputs of many power plants to supply
the myriads of end-users who are connected to the transmissions lines by a network of distribution
lines at lower voltage. More recently, electric power is being generated in smaller plants, often
employing gas turbine engines, either alone or in combination with steam turbines (called combined
cycle power plant), as the mechanical power source. In addition, some electricity is generated in
industrial or commercial plants that utilize the waste heat from the power production process for
process or space heating (called cogeneration). The transmission and distribution systems that
connect all these sources to each other and to the users of electric power is usually owned and
managed by public utility companies.
2
Some electric power is produced from nonthermal sources of energy. The most prominent of
these is hydropower, where mechanical power is produced by hydroturbines supplied with high-
pressure water from a reservoir impounded by the damming of a river. The energy source is the
difference in gravitational energy of the higher-level water behind the dam compared with the lower-
level water downstream of the dam, this difference in level being called the head.
3
Hydropower
may be generated as well from the rise and fall of ocean tides. Some electric power is now generated
by the mechanical power of wind turbines extracting energy from the wind.
Solar insolation is being used to generate electric power either directly, utilizing photovoltaic
cells that convert the energy of solar photons to electric power, or indirectly by supplying heat to
thermal steam or vapor engines that drive electric generators. Solar insolation provides the energy
needed to grow plants and may thereby be utilized indirectly to generate electric power when
biomass crops are used as fuel in thermal power plants.
Nonthermal and solar sources of electric power are termed renewable, in contrast to mineral
fuels, such as fossil and nuclear, that are extracted from the earth or ocean. Renewable energy
systems are discussed in a Chapter 7.
Thermochemical systems, such as batteries and fuel cells, convert the chemical energy of
reactant molecules directly to electricity without an intermediate step where mechanical power is
generated. While they currently contribute very little to the amount of electric power generation,
they are obviously important in such applications as portable communication devices and to the
development of electric drive road vehicles.
4
The physical principles of electricity and magnetism, which explain how stationary and moving
electric charges generate electric and magnetic fields, how mechanical forces are exerted on electric
currents flowing in electrical conductors in the presence of magnetic fields, and how electric current
is induced to flow by electric fields in conducting materials, provide the basis for the mechanism
by which electrical power is generated, transmitted, and utilized.
5
These interactions are those of
thermodynamic work, using the parlance of the science of thermodynamics explained in Chapter 2.
2
In many U.S. states, the ownership of transmission and distribution lines is being separated from the ownership
of generation facilities as a part of the government's deregulation of electric utility industry.
3
The ultimate source of this power is solar insolation that evaporates ocean water which is subsequently
precipitated to the land drainage basin of the reservoir.
4
See Section 8.6 for a discussion of electric drive vehicles.
5
Electric communication systems consume only a small proportion of electrical power in modern economies.
Despite their great importance, we will not explore here their interesting technologies.
