24-09-2012, 02:33 PM
Electric Energy and Electric Generators
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Introduction
Energy is defined as the capacity of a body to do mechanical work. Intelligent harnessing and control of
energy determines essentially the productivity and, subsequently, the lifestyle advancement of society.
Energy is stored in nature in quite a few forms, such as fossil fuels (coal, petroleum, and natural gas),
solar radiation, and in tidal, geothermal, and nuclear forms.
Energy is not stored in nature in electrical form. However, electric energy is easy to transmit at long
distances and complies with customer’s needs through adequate control. More than 30% of energy is
converted into electrical energy before usage, most of it through electric generators that convert mechanical
energy into electric energy. Work and energy have identical units. The fundamental of energy unity
is a joule, which represents the work of a force of a Newton in moving a body through a distance of 1 m
along the direction of force (1 J = 1 N × 1 m). Electric power is the electric energy rate; its fundamental
unit is a watt (1 W = 1 J/sec).
Major Energy Sources
With the current annual growth in energy consumption, the fossil fuel supplies of the world will be
depleted in, at best, a few hundred years, unless we switch to other sources of energy or use energy
conservation to tame energy consumption without compromising quality of life.
The estimated world reserves of fossil fuel [1] and their energy density are shown in Table 1.1. With
a doubling time of energy consumption of 14 years, if only coal would be used, the whole coal reserve
would be depleted in about 125 years. Even if the reserves of fossil fuels were large, their predominant
or exclusive usage is not feasible due to environmental, economical, and even political reasons.
Alternative energy sources are to be used increasingly, with fossil fuels used slightly less, gradually, and
more efficiently than today.
The relative cost of electric energy in 1991 from different sources is shown in Table 1.2.
Wind energy conversion is becoming cost-competitive, while it is widespread and has limited environmental
impact.
Electric Power Generation Limitations
Factors limiting electric energy conversion are related to the availability of various fuels, technical
constraints, and ecological, social, and economical issues.
Ecological limitations include those due to excess low-temperature heat and carbon dioxide (solid
particles) and oxides of sulfur nitrogen emissions from fuel burning.
Low-temperature heat exhaust is typical in any thermal energy conversion. When too large, this heat
increases the earth’s surface temperature and, together with the emission of carbon dioxide and certain
solid particles, has intricate effects on the climate. Global warming and climate changes appear to be
caused by burning too much fossil fuel. Since the Three Mile Island and Chernobyl incidents, safe nuclear
electric energy production has become not only a technical issue, but also an ever-increasing social (public
acceptance) problem.
Even hydro- and wind-energy conversion pose some environmental problems, though much smaller
than those from fossil or nuclear fuel–energy conversion. We refer to changes in flora and fauna due to
hydro–dams intrusion in the natural habitat. Big windmill farms tend to influence the fauna and are
sometimes considered “ugly” to the human eye.
Consequently, in forecasting the growth of electric energy consumption on Earth, we must consider
all of these complex limiting factors.
Shifting to more renewable energy sources (wind, hydro, tidal, solar, etc.), while using combined
heat–electricity production from fossil fuels to increase the energy conversion factor, together with
intelligent energy conservation, albeit complicated, may be the only way to increase material prosperity
and remain in harmony with the environment.
Electric Power Generation
Electric energy (power) is produced by coupling a prime mover that converts the mechanical energy
(called a turbine) to an electrical generator, which then converts the mechanical energy into electrical
energy (Figure 1.3a through Figure 1.3e). An intermediate form of energy is used for storage in the
electrical generator. This is the so-called magnetic energy, stored mainly between the stator (primary)
and rotor (secondary).