Energy sources
Kategorie: Biológia (celkem: 966 referátů a seminárek)
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- Přidal/a: anonymous
- Datum přidání: 01. července 2007
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Energy sources
Energy is the lifeblood of modern industrial society. Important as it is, energy was taken for granted for many years.Most of the electricity we use comes from four sources: coal, hydroelectric power, oil, and nuclear power.
Most experts agree that at the current rate of consumption, the proven reserves of crude oil will last only about 60 years. Making matter worse, any increase in the rate of oil consumption would drastically cut the lifespan of oil. At the current rate of usage, proven global reserves of natural gas would last about 100 years.
Coal is the world’s most abundant nonrenewable fossil fuel. World proven reserves will last about 600 years at the current rate of consumption. Coal will probably play a large role in the near term, for it can also be used to make liquid fuels to power our transportation systems and can be converted into a synthetic natural gas for home heating.
Oil and natural gas supplies are fast on the decline and in need of a substitute. The world is also suffering the early symptoms of global warming caused by air pollution. Both trends suggest the need for alternative energy sources. Two major groups exist to pick from -nonrenewables and renewables.
Some nonrenewable energy sources may have a place in the immediate future. Nuclear reactors are fueled by uranium-235, whose nuclei split when they are struck by neutrons. This process, fission, releases an enormous amount of energy. Nuclear fuels pass through a complex cycle from mining to waste disposal, at each stage of the cycle radioactive materials can escape, either by accident or through normal operations.
Nuclear power offers many advantages over coal and oil. It produces very little air pollution, less land is disturbed by mining, the cost of transporting nuclear fuels is lower than of coal.
The major problems with nuclear power are disposal of radioactive wastes, contamination of the environment, thermal pollution, health impacts from radiation, limited supplies of uranium ore, low social acceptability, high construction costs, questionable reactor safety, lack of experience with the technology.
The proposal of breeder reactor is getting around the problem of limited fuel supply. Besides producing electricity, the breeder reactor makes fissionable plutonium-239 from the abundant uranium-238.
Another proposed energy system is fusion power.
Fusion is the uniting of two or more small nuclei to form a larger one, a process accompanied by the release of energy. But the research of fusion is still a long way from being commercially available. The forms of hydrogen needed to fuel the fusion reactor are abundant and could provide energy for millions of years. Fusion reactions occur at extremely high temperatures, the emissions of highly energetic neutrons from the fusion reaction would weaken metals and necessitate their replacement every 5 years. Metal fatigue might lead to rupture of the vessel and the release of radioactive materials or highly reactive lithium.
Solar energy is abundant, but it provides only a fraction of our energy needs. Passive solar systems are the simplest and most cost-effective. Buildings are designed to capture sunlight energy and store it within thermal mass, walls and floors. The stored heat is gradually released into the structure. Active solar systems rely on collectors that absorb sunlight and convert it into heat, which is then transferred to water or air flowing through them. Pumps generally move water or air to a storage unit, where heat can be drawn off as needed. Photovoltaics are made of silicon or other materials that emit electrons when struck by sunlight, thus producing electricity.
Solar systems provide many advantages over conventional power sources: The fuel is free, nondepletable, and clean. When operating, systems produce no pollution and pay back energy invested in their production. The major limitations are that the source is intermittent, making it necessary to store energy overnight or on cloudy days.
Winds can be tapped to generate electricity. The potential of wind energy is enormous. Wind energy offers many of the advantages of direct solar energy : It’s clean and renewable, uses only a small amount of land, and is safe to operate. Moreover, wind technologies do not preclude other land uses - wind farms can be grazed and planted. The disadvantages to wind systems are that the wind does not blow all the time, so backup systems and storage are needed. The other weaknesses are the visual impact, the noise and impairing of television reception and telephone communications.
Biomass, a form of indirect solar energy, has some potential. Biomass is the organic matter contained in plants. Useful biomass includes wood, wood residues, crop wastes, industrial wastes, manure, and urban waste.
The simplest way of getting energy from biomass is to burn it, but many believe that a more sensible strategy would be to convert it to gaseous and liquid fuels and chemicals needed by the chemical industry.
Hydroelectric power, another indirect form of solar energy, is renewable, creates no air pollution, and is relatively inexpensive. However, sediments fills in reservoirs, giving them an average lifespan of 50 to 100 years.
The earth harbors a great deal of energy from the decay of naturally occurring radioactive materials in the crust and from magma, molten rock. The most useful geothermal resource is the hydrothermal convection zone, where magma penetrates into the crust and heats rock formations containing large amounts of groundwater. The heat pressurizes the groundwater and drives it to the surface through fissures. Currently these zones are exploited for space heating and electricity. Such systems produce steam and hot water laden with toxic minerals, salts, metals, and hydrogen sulfide. Noise pollution is also a problem.
Hydrogen fuel is produced by heating or passing electricity through water in the presence of a catalyst. Water breaks down into hydrogen and oxygen. Hydrogen is a clean-burning fuel that could replace gaseous and liquid fuels. It is easy to transport but is explosive, has a low energy yield.
Conservation is one of the key untapped energy resources for tomorrow. By reducing energy waste in homes, factories, and transportation, we could inexpensively unleash an enormous supply of energy. The largest gain can be had when new homes and offices are built with passive and active solar systems and heavy insulation. Mass transit by bus and train have to replace much of the automobile traffic, both in cities and between them.
A smooth transition from nonrenewables such as oil, natural gas, coal, and nuclear power to renewable energy resources can be made into a sustainable future, but it will require an immediate investment in renewable energy resources by governments and individuals.