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2.2 Sources of Energy

Electricity is produced by converting energy from one form to electricity. The process used may be a direct conversion process, where the energy source is converted directly to electricity. An example of this is solar photovoltaic cells, which converts the energy found in solar radiation directly to electricity. An indirect conversion process consists of converting energy from one form, to an intermediate form, to electricity. Coal-fired generating plants are an example of the indirect process, as the chemical energy released as heat by burning the coal is changed to rotating kinetic energy by the steam turbine, and then the rotating kinetic energy is converted to electricity. The majority of the electricity today is produced through an indirect energy conversion process. Both indirect and direct processes use the following major sources of energy for the production of electricity: fossil fuels, nuclear energy, solar radiation, and hydroenergy.

Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are a finite, non-renewable resource. They remain the primary source for the production of electricity. The combustion of these fuels releases their chemical energy, which produces heat to power steam turbines. The steam turbines power rotating electric generators, which turn kinetic energy into electricity. No energy conversion process converts all the energy present in one form completely into the new form. Since the production of electricity from fossil fuels involves several energy conversion steps fossil fuel power plants inefficiently produce power (somewhere around 40%). Table 2.1 summarizes the energy content of fossil fuels and the estimate of fossil fuel reserves for the world. We must also consider the environmental effects of electricity generation from fossil fuels. For example, flyash, physical matter left after coal combustion, is harmful to human (animal and environmental) health. It demands environmentally friendly disposal. Combustion of fossil fuels also produces carbon monoxide, carbon dioxide, sulfur dioxide, and nitrous oxides gases. These "greenhouse gases" contribute to acid rain and global warming effects.

Table 2.1

Estimate of Fossil-Fuel Energy Content and World Fossil Fuel Reserve

Approx. Energy Content (Watts/yrs)
Estimated Reserves
7.6 trillion metric tons
2 trillion barrels
Natural Gas

Nuclear energy, just like fossil fuels, is a finite, non-renewable, energy source that uses an indirect conversion process to produce electricity. There are two basic forms of nuclear energy, fission and fusion. Since the fusion reaction has never been performed, only the fission reaction is used to produce electricity. The fission reaction involves the splitting of the nuclei of a heavy element. The heat output from this reaction powers a steam turbine, which drives a rotating electric generator, just as in a fossil fuel power plant. Nuclear fission produces a much greater energy content than fossil fuels. Uranium contains an energy content of approximately 1010 Btu/kg, which is about one million times the energy content of fossil fuels. There is an estimated reserve of approximately 426 million kg in the United States. Nuclear fission also comes with heavy environmental costs and risks. The fission process leaves the the nuclear fuel, the reactor vessel where fission occurs, and the steam pipes highly radioactive. Also, plant failures can lead to the release of radioactive steam into the atmosphere or worse.

Solar radiation includes energy used directly as intercepted solar radiation, or indirectly as wind and hydropower. Solar radiation is a renewable energy source. The average incident power at the earth's surface is 182 W/m2, which corresponds to a daily average energy of 4.4 kWh/m2. Direct use of solar power includes active types involving photovoltaic cells, and passive types using radiation to heat solar collectors. Photovoltaic cells directly convert sunlight into electricity. The best photovoltaic cells have efficiencies in the 14% to 17% range. Environmentally attractive, photovoltaics introduce an emissionless, wasteful by-product minimized, extractionless energy conversion process. However, the most efficient solar cells use gallium arsenide, a toxic material. It seems that solar cells are too new for an understanding of the disposal requirements and costs involved for worn out cells. In addition to this, photovoltaic solar cells generate direct current, hence requiring inverting equipment to obtain the desired alternating current for most large-scale operations. Solar collectors are normally incorporated into a solar thermal system, converting sunlight into heat for various forms of use, including space heating, water heating, industrial process steam, and electricity production. At present several factors limit large-scale utilization of solar energy, including the cost of solar cells and solar collector-heat exchanger systems, and the requirement of an adequate energy storage system to smooth out the daily variation. Yet, sunlight is available everywhere making the use of solar radiation for energy production non-site specific.

Wind energy is also a form of indirect use of solar radiation. Solar radiation produces wind by heating the air. During the day, the air over land is heated much faster than air over water bodies because the land absorbs much less sunlight, and the evaporation is less. The heated air over land expands, becomes lighter, and rises. The cooler, heavier air over large water bodies moves in to replace the lighter and warmer air, creating a horizontal motion of air. During the night the land cools faster than water, the cool air moves seaward to replace the warm air that rises from the surface of the water. Wind energy to electricity is an indirect energy conversion process. It requires turbine type wind generators to transform the kinetic energy of the wind into rotary-shaft motion and, in turn, this is converted to electrical energy. Wind turbine generators are site specific. They require a sustained wind speed of 20 km/hour. To meet this requirement the bulk of wind generators are located on mountain passes or the coast. Wind turbines have a maximum possible efficiency of 59.3%, with a more common efficiency of around 40%. The significant environmental problems associated with wind turbines are noise, aesthetics, and interactions with birds.

Hydropower is also an indirect means of using solar power to produce electricity, since hydropower uses the stream-flow part of the hydrological cycle. In a hydropower plant, the potential energy of a mass of water in a reservoir a distance above the stream bed is converted to kinetic energy by flowing through a hydraulic turbine. The resulting kinetic energy of the turbine drives an electric generator. Hydropower is available wherever a suitable site exists having enough stream flow, potential drop and area. Industrialized nations contain about 30% of all hydropower potential, and are responsible for about 80% of all electricity produced from hydropower. Asia accounts for 30% of hydropower potential, and produces only 7% of electricity such produced. Africa accounts for 20% of hydropower potential, yet produces only 2% of electricity such produced. Hydropower is very attractive because it is a non-polluting renewable resource. It can be very disruptive environmentally. The dam and reservoir effect the normal ecology of the stream and the surrounding habitat by altering water use, changing natural water flow cycles of the stream, and taking up land area for the reservoir. For example, hydroelectric operation in the northwest United States, within the Columbia River basin, is often constrained in order to protect the yearly river migration of salmon. Additionally, new dams may often require the relocation of people and buildings.

Tidal energy uses the tidal flow of oceans to run a hydropower plant to produce electricity. Basically, a dam encloses a tidal pool. The tidal pool fills during periods of high tide, then empties during periods of low tide. The water flow into and out of the pool drives a reversible hydraulic turbine. Since the turbine is reversible, the flow of water into and out of the bay produces electricity. Therefore, tidal power is available twice during each 12h 25min tidal period. The ideal sites for tidal energy have a large difference in tides.

Many energy sources produce this diverse range of potential electrical power. Non-renewable energy source include nuclear and fossil fuels. Renewable energy sources range from direct solar to the indirect solar sources and geothermal energy (although it was not discussed). Several of these are site specific, such as hydropower, wind power, and tidal power. Each source possesses both benefits to general well-being and costs to the environment and future burdens.

The next section describes the efficiencies of different electrical systems and the costs of electric production due these efficiencies.


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