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Modern technology requires very large amounts of electrical power. This is normally generated at large power plants which convert some other kind of energy into electrical power. Each such system has advantages and disadvantages, but many of them pose environmental concerns.
The efficiency of some of these systems can be improved by cogeneration (combined heat and power) methods. When electricity is generated by by producing heat, the coolant can often be reused, after all the electrical power has been extracted, to heat nearby buildings.
Most electricity today is generated by burning fossil fuels. This produces high temperatures, which drive some sort of heat engine, often a steam turbine.
Such systems allow electricity to be generated where it is needed, since fossil fuels can readily be transported. They also take advantage of a large infrastructure designed to support consumer automobiles. However, the world's supply of fossil fuels is finite (but large); there has been some concern that it will run out. This would be a serious problem not only for fossil fuels as an energy source but also for the manufacture of plastics and many other things. Various estimates have been calculated for exactly when it will be exhausted, but new sources of fossil fuels keep being discovered.
More serious are concerns about the emissions that result from fossil fuel burning. Fossil fuels constitute a very large repository of carbon buried deep under the ground. Burning them results in the conversion of this carbon to carbon dioxide, which is then released into the atmosphere. This may result in an increase in the Earth's levels of atmospheric carbon dioxide, which might lead to the greenhouse effect and significant global warming. This linkage, carbon dioxide causing global warming, is very widely but not universally accepted, and its severity is not clear.
Depending on the particular fossil fuel and the method of burning, other emissions may be produced as well. Ozone, NO2 and other gases are often released, as well as particulate smoke. In the past, plant owners addressed this problem by building large smokestacks, so that the pollutants would be diluted in the atmosphere. While this helps reduce local contamination, it does not help at all with global issues.
Fossil fuels, particularly coal, also contain small amounts of radioactive material, and burning them in very large quantities releases this material into the environment, leading to low but real levels of local and global radioactive contamination.
Water on the Earth goes through a constant cycle, being evaporated from the oceans, raining out on land, and running back downhill to the oceans. Hydroelectric power extracts some of the energy of this downhill flow of water. Normal hydroelectric power plants consist of a dam which creates a large lake; when power is needed, water is allowed to flow out of the lake through large turbines, generating electicity.
Hydroelectric power is renewable, that is, it will not run out as long as the sun continues to shine. However, it does pose some environmental problems all the same.
When a new dam is constructed, it produces a new lake. The land that this lake covers is immersed, which renders it unavailable to animals and people and immediately kills all the vegetation. Many animal species are threatened simply because there is not enough undamaged land for them to live on, and a large hydroelectric project floods a large amount of this land. The lake itself may well not develop into a healty ecosystem; eutrophication is a danger, and the large mass of rotting vegetation under the water does not lead to a lake full of healthy species. These concerns are in a sense one-time problems; once the dam is built, it can continue to produce power indefinitely without worsening these problems.
Hydroelectrc dams can have problems with silt. Erosion naturally carries a certain amount of dirt downstream; this dirt will precipitate out in the holding lake for a hydroelectric dam. If enough silt accumulates, the power inakes will be blocked, so hydroelectric installations must remove the silt and dispose of it somehow without doing further ecological damage.
Hydroelectric dams also block migration routes for fish such as salmon which need to swim upriver to spawn. This has been partially addressed by building fish ladders, small streams the fish can swim up to circumvent the dam.
In regions such as the Bay of Fundy with very large tidal swings, tidal harnesses can be built to extract electrical power from the tidal motion.
Tidal power is also renewable, in the sense that it will contine for as long as the Moon orbits the Earth. However, it has environmental problems similar to those of hydroelectric power. A tidal harness usually requires a large dam, which can endanger ecosystems by restricting the motion of animals. Perhaps more seriously, a tidal harness reduces the tidal swing, which can severely disrupt ecosystems which depend on being periodically covered by water. Local economies which depend on the same thing will also be adversely affected.
Nuclear power has raised much public concern. Under normal operation, a nuclear power plant releases very little contamination of any sort to the environment. It does produce radioactive waste of several sorts. Moderate amounts of low-level waste are produced; this can be disposed of simply by placing it somewhere it won't be disturbed. However, a relatively small amount (perhaps a tonne a year from a large nuclear power plant) of high-level waste is produced, and this poses a significant disposal problem. It can be expected to be dangerous for tens or hundreds of thousands of years, so extremely secure disposal methods must be found. Currently, most such waste is stored in temporary storage facilities which require constant care and attention. Several methods have been suggested for final disposal of the waste, including deep burial in stable geological structures, transmutation, and removal to space.
Like any large facility, nuclear power plants are subject to accidents. Accidents at nuclear power plants pose a risk of severe environmental contamination. The Chernobyl accident, for example, released large amounts of radioactive contamination, killing many and rendering large amounts of land unusable for the next few centuries. However, the power plant a Chernobyl was built with minimal concern for safety; modern nuclear power plants are much less likely to have such problems. The potential for such an accident still exists, however, so many experts are still concerned about the use of nuclear power. This danger has received significant coverage in the popular press, so the public has a very strong fear of nuclear power (by contrast, the radioactive contamination due to coal burning is virtually unknown, as are most of the hazards of other methods of electrical power generation).
Nuclear power can also pose the risk of nuclear proliferation. Some designs of nuclear power plant can produce materials suitable for the construction of a nuclear weapon. Almost any design requires equipment and expertise that would greatly aid a nuclear weapons program. For example, the CANDU design of nuclear reactors is capable of producing weapons-grade material, so the Canadian government refuses to license the building of such a plant unless the government of the nation in question agrees not to develop nuclear weapons and permits inspections to verify that this is not occurring. However, it is believed that both India and Pakistan have produced nuclear weapons using reactors of very similar design.
Electrical power can be generated by burning anything which will burn. Some electrical power is generated by burning crops which are grown specifically for the purpose. Usually this is done by fermenting plant matter to produce ethanol, which is then burned. This may also be done by allowing organic matter to decay, producing biogas, which is then burned.
Burning biomass produces many of the same emissions as burning fossil fuels. However, growing biomass captures carbon dioxide out of the air, so that the net contribution of the cycle to global atmospheric carbon dioxide levels is zero.
The process of growing biomass is subject to the same environmental concerns as any kind of agriculture. It uses a large amount of land, and fertilizers and pesticides may be necessary for cost-effective growth.
Solar power extracts the energy of sunlight. This may be done directly, with photovoltaic cells, or it may be done by using fields of mirrors to concentrate sunlight on a target which then becomes extremely hot. This heat can then be dumped to the surrounding air, providing electricity. In either case, a large expanse of land receiving steady sunlight is needed. Deserts are the usual choices. Once a large solar collector is built, its environmental impact is not very severe. It occupies a great deal of land, denying its use to the fragile desert ecosystems, and the heated air may lead to slight changes in local weather systems. The mirrors or cells must also be kept clean, and this probably requires a certain amount of water. Where possible, solar power is generally quite environmentally friendly.
Some projects have been established to take advantage of relatively small solar cells by putting solr collectors on the roofs of individual houses. Generally, the electrical power generated is not sufficient to pay back the cost at current electricity prices, so when government subsidies are not present, this method is rarely used. It is generally more practical to use small solar collectors to heat water directly, satisfying household hot water needs.
Wind power extracts electicity from the flow of air over the surface of the earth. Wind power stations generally consist of large "wind farms", fields of large windmills in locations with relatively high winds. These windmills disrupt local low-level winds, and they can be very noisy. Both facts create problems for local bird populations. The windmills also need constant maintenance, as they have many moving parts exposed to the elements. Many homeowners in areas with high winds and expensive electricity set up small windmills to reduce their electric bills.
Geothermal power extracts electrical power from the temperature difference between hot rock deep in the earth and relatively cool surface air and water. This requires that the hot rock be relatively shallow, so it can only be applied in geologically active areas. In these areas, though, it is a very clean source of power. It may affect nearby geysers (since they are very delicate systems), but the only major environmental concern it heat pollution. Since electrical power is extracted from the flow of heat, the excess heat must be dumped into either air or water; in either case, it may interfere with local ecosystems. However, this sort of pollution is more or lees inevitable in all northern settlements.
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