2.2.1 Energy Sources A brief definition of terminology that will be used throughout this section will help the reader understand the ensuing discussions. Primary energy refers to energy that is used and converted directly by an end user. Secondary energy refers to energy that has gone through a previous conversion process before it reaches the end user (e.g. electricity generated at a power plant that then goes to your house). Three categories are used for classification of primary energy sources. (1) Fossil Fuels (oil, coal, gas): organic matter subjected to high temperature and pressure were formed by natural processes on a geological time scale. Energy contained in fossil fuels is usually released by burning. (2) Nuclear (fission, fusion): when atomic nuclei decay or combine they produce high energy particles which can sustain a chain reaction. Heat produced by these processes is captured and used to produce electricity. (3) Renewable (solar, biomass, wind, tidal, river, geothermal): the forces of nature can be harnessed as energy sources. Radiation from the sun can be a heat source or can drive photovoltaic energy conversion. Biomass accumulates solar energy in the form of plant matter using photosynthesis. Plants can provide several different sources of energy; oil produced by plants such as sunflowers or soybeans, alcohol produced by fermentation of plant material, or simply the plant material itself which can be burned. The heating and cooling of the global atmosphere generates wind that can be used to drive a wind turbine. Interactions with the moon, sun and other planets cause movement of the liquid surface of our planet. This kinetic energy, as well as that of flowing rivers, can be used to drive generators. And lastly, the internal energy of the Earth's magma comes close enough to the surface in places so that we can tap this natural heat source. The current world energy supplied by each category is: Fossil Fuels provide 90% (coal; 30%, oil; 40%, gas; 20%), Nuclear 5%, and Renewables about 5% (hydroelectric provides most of this with a negligible contribution from solar, wind, etc.). [Sci. Amer., 1990] Characteristics of primary energy sources vary greatly and can have a significant effect on suitability for satisfying particular energy needs. Two broad categories of characteristics will be discussed. The first are inherent properties of the energy source and include physical properties (e.g. energy density), storability and future availability. Table 2.1 summarizes the inherent characteristics of primary energy sources. The future availability estimates are for depletable (fossil fuel and nuclear) and continuous (renewable) supplies. [Scientific American Inc., 1971] Depletable reserves are an estimate of the quantities available at no more than twice present costs. In general, fossil fuels have high energy density, are easily stored and have finite reserves which will be exhausted at some point in the future depending upon use rates. Nuclear power has the highest energy density per mass, is storable with special handling requirements and also has finite, exhaustible reserves. Renewable energy sources are usually low density, are dynamic sources and therefore not storable, but are inexhaustible as long as the sun continues to shine and the Earth continues to exist. The value given in the table for solar energy, which is an enormous amount of energy, is only for the fraction that falls on land. If the solar energy falling on a few percent of the land area in the U.S. was converted to electricity at an efficiency of 12%, it would satisfy most of the countries energy needs in the year 2000. The second category includes those characteristics which are related to energy production, such as resource location compared to point of use location and potential environmental impact. Fossil fuels are found in discrete locations and require transportation to use point locations. It is significant that resource locations are within national boundaries and are therefore available to outsiders at the discretion of the controlling country. Since energy is released from fossil fuels by burning, there are corresponding environmental consequences. Also, the mining and transportation of fossil fuels can have a serious impact on the environment. Nuclear fuel is found in discrete locations, must be transported to processing and use point locations, requires very controlled and complete processing to be useful, and has potential environmental impact which has caused significant public concern. Safety regulations and strict adherence to established operating procedures minimize the potential of nuclear accidents, but the question of safe disposal of radioactive waste is still unresolved. All of the renewable energy sources, except solar, have restricted appropriate locations which are often not near population (and therefore energy use) areas. Environmental impact tends to be a disruption of natural ecosystems rather than atmosphere contamination. Table 2.2 provides a summary of characteristics which will influence choice of energy source, operation of energy production facilities, and ultimately the cost of energy.
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