Environmental concerns with electricity generation

From Academic Kids

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Assembly of an Enercon E-70 wind turbine

Modern technology uses large amounts of electrical power. This is normally generated at 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. Process steam can be extracted from steam turbines. Waste heat produced by thermal generating stations can be used for space heating of nearby buildings. By combining electric power production and heating, less fuel is consumed, thereby reducing the environmental effects compared with separate heat and power systems.

Contents

Fossil fuels

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A crude oil power station in Iraq

Main article: Fossil fuels

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. The world's supply of fossil fuels is large, but finite. Exhaustion of low-cost fossil fuels will have significant consequences for energy sources as well as 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 significant 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 results in an increase in the Earth's levels of atmospheric carbon dioxide, which enhances the greenhouse effect and contributes to global warming. The linkage between increased carbon dioxide and global warming is nearly universally accepted, though fossil-fuel producers vigorously contest these findings.

Depending on the particular fossil fuel and the method of burning, other emissions may be produced as well. Ozone, sulfur dioxide, NO2 and other gases are often released, as well as particulate smoke. Sulfur and nitrogen oxides contibute to smog and acid rain. 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 dilute 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.

Coal also contains traces of toxic heavy elements such as mercury, arsenic and others. Mercury vaporized in a power plant's boiler may stay suspended in the atmosphere and circulate around the world. While a substantial inventory of mercury exists in the environent, as other man-made emissions of mercury become better controlled, power plant emissions become a significant fraction of the remaining emissions. Power plant emissions of mercury in the United States are thought to be about 50 tons per year in 2003, and several hundred tons per year in China. Power plant designers can fit equipment to power stations to reduce emissions.

Coal mining practices in the United States have also included strip mining and removing mountain tops. Mill tailings are left out bare and have been leached into local rivers and resulted in most or all of the rivers in coal producing areas to run red year round with sulfuric acid that kills all life in the rivers.

Hydroelectric power

Main article: Hydroelectric power

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 reservoir; when power is needed, water is allowed to flow out of the reservoir through turbines, generating electicity.

Hydroelectric power is renewable, that is, it will not run out as long as the water continues to flow. Environmental effects stem from the operation of the reservoirs, and the associated alteration of pre-existing water flow conditions.

When a new dam is constructed, it produces a new lake (reservoir). The land that this lake covers is immersed, which renders it unavailable to animals and people and immediately swamps 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 may flood a large amount of this land. The lake itself may not develop into a healthy eco-system; eutrophication is a danger, and the large mass of rotting vegetation under the water does not lead to a healthy eco-system. These concerns are in a sense one time problems; once the dam is built, it can continue to produce power indefinitely while allowing a new eco-system to develop.

Hydroelectric dams may accumulate silt due to suspended solids falling out of the water in the plant reservoir. If enough silt accumulates, the water intake's to the turbines will become blocked, so hydroelectric installations must remove the silt and dispose of it somehow.

Hydroelectric dams also block migration routes for fish 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.

It is rarely possible to construct a large hydroproject close to a city or industrial factory that will use all the power produced. It is therefore necessary to construct long power transmission lines to carry the electric power to its destination. Such construction requires clearing and keeping clear a very long narrow corridor of forest (or other terrain). Such construction can affect migration routes and erosion. There are also concerns about the effects of corona discharge and extremely low-frequency electromagnetic radiation near the lines, although it remains unclear what the effects actually are on people, animals, and plants near long power lines. Long access roads are also necessary, and such roads (and their constant maintenance) can have a negative environmental effect as well.

For certain projects with relatively large reservoirs (less than 100 watts of plant output per square metre of forebay), significant emissions of methane may occur as a result of anaerobic decay of submerged plant matter. This is especially true for reservoirs in tropical areas where the forebay has not been cleared before innundation. Some calculations suggest that in the extreme case the greenhouse effect due to these emissions would be of the same order as those due to a fossil-fuel plant producing the same annual energy.

The World Commission on Dams in 2004 released a report outlining a consensus on best practice for large dams, considering environmental, social and economic factors. See the report (http://www.dams.org/), including discussion of greenhouse gas emissions.

Tidal power

Main article: Tidal power

In regions such as the Bay of Fundy with very large tidal swings, tidal power plants can be built to extract electrical power from the tidal motion.

Tidal power is also renewable, in the sense that it will continue for as long as the Moon orbits the Earth. However, it has environmental problems similar to those of hydroelectric power. A tidal power plant usually requires a large dam, which can endanger ecosystems by restricting the motion of marine animals. Perhaps more seriously, a tidal power plant reduces or increases the tidal swing, which can severely disrupt ecosystems which depend on being periodically covered by water; resulting changes in fisheries or shellfish beds may result in adverse economic effects. Certain proposed tidal power plants in the Bay of Fundy would increase the tidal swing by an estimated 50 cm as far south as the coast of Maine (where the tidal swing is not particularly large now).

Nuclear power

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Kewaunee Nuclear Power Plant, Kewaunee, Wisconsin

Main article: Nuclear power

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 for a few years. 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. Some nuclear reactors, in particular the Integral Fast Reactor, have been proposed that use a different nuclear fuel cycle that avoids producing waste containing long-lived radioactive isotopes.

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 at 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. Fission products can be reprocessed out of spent reactor fuel and diverted to a weapons program, or a reactor can be used to produce weapons materials through transmutation by direct irradiation by neutrons.

Biomass

Main article: Biomass

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. Also, when burned, wood is a form of biomass fuel.

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. Biomass that is produced as a by-product of agriculture shows some promise, but most such biomass is currently being used, for plowing back into the soil as fertilizer if nothing else.

Solar power

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Small-scale solar power from photovoltaic cells on the roofs of Berlin buildings

Main article: Solar power

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. Manufacture of solar cells results in environmental releases of pollutants and the emissions associated with the energy input to make and install the cells. 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 solar 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

Main article: Wind power

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 farms are generally considered unattractive. In addition the 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

Main article: Geothermal power

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 is 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 less inevitable in all northern settlements. Geothermal plants may also emit salts, hydrogen sulfide, or radon carried to the surface by the geothermal steam.

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