Substation - Civil & Structural

Environmental concerns with electricity generation

2007-09-16T11:16:00.000-07:00

Most electricity today is generated by burning fossil fuels and producing steam which is then used drive a steam turbine that, in turn, drives an electrical generator.

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.

Flue gas stack at GRES-2 Power Plant in Ekibastus, Kazachstan is 420 meters tall

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 matter. Sulfur and nitrogen oxides contribute to smog and acid rain. In the past, plant owners addressed this problem by building very tall flue gas stacks, 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 levels of local and global radioactive contamination, the levels of which are, ironically, higher than a nuclear power station as their radioactive contaminants are controlled and stored.

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 environment, 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.

According to Environment Canada:

"The electricity sector is unique among industrial sectors in its very large contribution to emissions associated with nearly all air issues. Electricity generation produces a large share of Canadian nitrogen oxides and sulphur dioxide emissions, which contribute to smog and acid rain and the formation of fine particulate matter. It is the largest uncontrolled industrial source of mercury emissions in Canada. Fossil fuel-fired electric power plants also emit carbon dioxide, which may contribute to climate change. In addition, the sector has significant impacts on water and habitat and species. In particular, hydro dams and transmission lines have significant effects on water and biodiversity."^[1]

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.

Nuclear power

Kewaunee Nuclear Power Plant, Kewaunee, Wisconsin

Main articles: Nuclear safety and 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 ton 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 (Taking 10,000 years to decay to activity levels below that of the original ore), 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 at an RBMK reactor, for example, released large amounts of radioactive contamination, killing many and rendering a large area 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, many citizens are still concerned about the use of nuclear power. But their concerns should be weighed against the need to address the threats posed by climate change and the opinions of the broader community. 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.

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).

Biomass

Main article: Biomass

Electrical power can be generated by burning anything which will combust. 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.

Wind power

Assembly of an Enercon E-70 wind turbine

Main articles: Wind turbine and Wind power

Wind power extracts electricity 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. A primary publicity issue regarding wind turbines are their older predecessor, such as the turbines located in California. These older, smaller, wind turbines are rather noisy and densely located, making them very unattractive to the local population. The turbines need constant maintenance, and result in bird deaths due to their high number of revolutions per minute. The downwind side of the turbine does disrupt local low-level winds. Modern wind turbines have overcome these constraints however, and have evolved in to a highly efficient and attractive energy source. Many homeowners in areas with high winds and expensive electricity set up small windmills to reduce their electric bills.

A modern wind farm, when installed on agricultural land, has one of the lowest environmental impacts of all energy sources:^[2]

It occupies less land area per kilowatt-hour (kWh) of electricity generated than any other energy conversion system, apart from rooftop solar energy, and is compatible with grazing and crops.
It generates the energy used in its construction within just months of operation.
Greenhouse gas emissions and air pollution produced by its construction are small and declining. There are no emissions or pollution produced by its operation.
The construction of offshore wind turbines has vastly increased the market.
Modern wind turbines are almost silent and rotate so slowly (in terms of revolutions per minute) that they are rarely a hazard to birds.^[3]

Landscape and heritage issues may be a significant issue for certain wind farms. However, when appropriate planning procedures are followed, the heritage and landscape risks should be minimal. Some people may still object to wind farms, perhaps on the grounds of aesthetics, but their concerns should be weighed against the need to address the threats posed by climate change and the opinions of the broader community.^[4]

Geothermal power

Main article: Geothermal power

Geothermal energy is the heat of the Earth, which can be tapped into to produce electricity in power plants.Warm water produced from geothermal sources can be used for industry, agriculture, bathing and cleansing. Where underground steam sources can be tapped, the steam is used to run a steam turbine. Geothermal steam sources have a finite life as underground water is depleted. Arrangements that circulate surface water through rock formations to produce hot water or steam are, on a human-relevant time scale, renewable.

While a geothermal power plant does not burn any fuel, it will still have emissions due to substances other than steam which come up from the geothermal wells. These may include hydrogen sulfide, and carbon dioxide. Some geothermal steam sources entrain non-soluable minerals that must be removed from the steam before it is used for generation; this material must be properly disposed. Any (closed cycle) steam power plant requires cooling water for condensors; diversion of cooling water from natural sources, and its increased temperature when returned to streams or lakes, may have a signifiant impact on local ecosystems.

Solar power

Main article: Solar Power

Solar power, which is a renewable source of energy, has been used as an alternative to fossil fuels, primarily in Germany (where the Government offers financial incentives) and in areas with an abundant amount of sun. Solar power works by converting the sun's radiation into DC power by use of photovoltaic cells. This power can then be converted into the more common AC power.

Solar power offers a viable alternative to fossils fuels for its cleanliness and supply. Its negative impact on the environment lies in the creation of the solar cells (which are made of primarily silicon and the extraction of this silicon requires the use of fossil fuels) and the storage of the energy (which usually requires Lead-Acid batteries). It should be noted that solar power carries an upfront cost to the environment via production, but offers clean energy throughout the lifespan of the solar cell.

Negawatt power

Main article: Negawatt power

Negawatt power is a way of supplying additional electrical energy to consumers without increased generation capacity at around half the cost of large scale generation. Whilst related to and utilising consumption efficiencies it differs in scale and market behaviour. This virtual generation method can supply decades of growth of supply in place of generation thus reducing environmental impacts of generation. Put simply it costs less to increase available supply by improving efficiency (and therefore reducing consumption) than by increasing plant generation capacity.

References

Coal as fuel

2007-09-16T11:13:00.000-07:00

Etymology

The word "coal" is of Aryan origin, and appears in many Germanic languages (German language Kohle, Swedish language kol, Hindi Language "Koyla"), ^[1] also giving the name for element carbon in those languages—charcoal is wood rendered to carbon and carbonic compounds by pyrolysis (charring).

Types of coal

As geological processes apply pressure to peat over time, it is transformed successively into:

Lignite - also referred to as brown coal, is the lowest rank of coal and used almost exclusively as fuel for steam-electric power generation. Jet is a compact form of lignite that is sometimes polished and has been used as an ornamental stone since the Iron Age.
Sub-bituminous coal - whose properties range from those of lignite to those of bituminous coal and are used primarily as fuel for steam-electric power generation.

Bituminous coal - a dense coal, usually black, sometimes dark brown, often with well-defined bands of bright and dull material, used primarily as fuel in steam-electric power generation, with substantial quantities also used for heat and power applications in manufacturing and to make coke.
Anthracite - the highest rank; a harder, glossy, black coal used primarily for residential and commercial space heating.
Graphite - technically the highest rank, but difficult to ignite and is not so commonly used as fuel.

Early use

Outcrop coal was used in Britain during the Bronze Age (2-3000 years BC), where it has been detected as forming part of the composition of funeral pyres.^[2] It was also commonly used in the early period of the Roman occupation. Evidence of trade in coal (dated to about AD200) has been found at the inland port of Heronbridge, near Chester, and in the Fenlands of East Anglia, where coal from the Midlands was transported via the Car Dyke for use in drying grain.^[3] Coal cinders have been found in the hearths of villas and military forts, particularly in Northumberland, dated to around AD400. In the west of England contemporary writers described the wonder of a permanent brazier of coal on the altar of Minerva at Aquae Sulis (modern day Bath) although in fact easily-accessible surface coal from what is now the Somerset coalfield was in common use in quite lowly dwellings locally.^[4]

However, there is no evidence that the product was of great importance in Britain before the High Middle Ages, after about AD1000. Mineral coal came to be referred to as "seacoal," probably because it came to many places in eastern England, including London, by sea. This is accepted as the more likely explanation for the name than that it was found on beaches, having fallen from the exposed coal seams above or washed out of underwater coal seam outcrops. These easily accessible sources had largely become exhausted (or could not meet the growing demand) by the 13th century, when underground mining from shafts or adits was developed.^[2] In London there is still a Seacoal Lane (off the north side of Ludgate Hill) where the coal merchants used to conduct their business. An alternative name was "pitcoal," because it came from mines. It was, however, the development of the Industrial Revolution that led to the large-scale use of coal, as the steam engine took over from the water wheel.

Uses today

Coal rail cars in Ashtabula, Ohio.

Coal as fuel

See also Clean coal and Fossil fuel power plant

Coal is primarily used as a solid fuel to produce electricity and heat through combustion. World coal consumption is about 5.3 billion tonnes annually, of which about 75% is used for the production of electricity. The region including the People's Republic of China and India uses about 1.7 billion tonnes annually, forecast to exceed 2.7 billion tonnes in 2025.^[5] The USA consumes about 1.0 billion tons of coal each year, using 90% of it for generation of electricity.

When coal is used for electricity generation, it is usually pulverized and then burned in a furnace with a boiler. The furnace heat converts boiler water to steam, which is then used to spin turbines which turn generators and create electricity. The thermodynamic efficiency of this process has been improved over time. "Standard" steam turbines have topped out with some of the most advanced reaching about 35% thermodynamic efficiency for the entire process, which means 65% of the coal energy is rejected as waste heat into the surrounding environment. Old coal power plants, especially "grandfathered" plants, are significantly less efficient and reject higher levels of waste heat. The emergence of the supercritical turbine concept envisions running a boiler at extremely high temperatures and pressures with projected efficiencies of 46%, with further theorized increases in temperature and pressure perhaps resulting in even higher efficiencies^[6] Approximately 40% of the world electricity production uses coal. The total known deposits recoverable by current technologies, including highly polluting, low energy content types of coal (i.e., lignite, bituminous), might be sufficient for 300 years' use at current consumption levels, although maximal production could be reached within decades (see World Coal Reserves, below).

A more energy-efficient way of using coal for electricity production would be via solid-oxide fuel cells or molten-carbonate fuel cells (or any oxygen ion transport based fuel cells that do not discriminate between fuels, as long as they consume oxygen), which would be able to get 60%–85% combined efficiency (direct electricity + waste heat steam turbine). Currently these fuel cell technologies can only process gaseous fuels, and they are also sensitive to sulfur poisoning, issues which would first have to be worked out before large scale commercial success is possible with coal. As far as gaseous fuels go, one idea is pulverized coal in a gas carrier, such as nitrogen. Another option is coal gasification with water, which may lower fuel cell voltage by introducing oxygen to the fuel side of the electrolyte, but may also greatly simplify carbon sequestration.

Coking and use of coke

Main article: Coke (fuel)

Coke is a solid carbonaceous residue derived from low-ash, low-sulfur bituminous coal from which the volatile constituents are driven off by baking in an oven without oxygen at temperatures as high as 1,000 °C (1,832 °F) so that the fixed carbon and residual ash are fused together. Metallurgic coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace. Coke from coal is grey, hard, and porous and has a heating value of 24.8 million Btu/ton (29.6 MJ/kg). Byproducts of this conversion of coal to coke include coal tar, ammonia, light oils, and "coal gas".

Petroleum coke is the solid residue obtained in oil refining, which resembles coke but contains too many impurities to be useful in metallurgical applications.

Gasification

High prices of oil and natural gas are leading to increased interest in "BTU Conversion" technologies such as gasification, methanation and liquefaction.

Coal gasification breaks down the coal into its components, usually by subjecting it to high temperature and pressure, using steam and measured amounts of oxygen. This leads to the production of syngas, a mixture mainly consisting of carbon monoxide (CO) and hydrogen (H₂).

In the past, coal was converted to make coal gas, which was piped to customers to burn for illumination, heating, and cooking. At present, the safer natural gas is used instead. South Africa still uses gasification of coal for much of its petrochemical needs.

The Synthetic Fuels Corporation was a U.S. government-funded corporation established in 1980 to create a market for alternatives to imported fossil fuels (such as coal gasification). The corporation was discontinued in 1985.

Gasification is also a possibility for future energy use, as the produced syngas can be cleaned-up relatively easily leading to cleaner burning than burning coal directly (the conventional way). The cleanliness of the cleaned-up syngas is comparable to natural gas enabling to burn it in a more efficient gas turbine rather than in a boiler used to drive a steam turbine. Syngas produced by gasification can be CO-shifted meaning that the combustible CO in the syngas is transferred into carbon dioxide (CO₂) using water as a reactant. The CO-shift reaction also produces an amount of combustible hydrogen (H₂) equal to the amount of CO converted into CO₂. The CO₂ concentrations (or rather CO₂ partial pressures) obtained by using coal gasification followed by a CO-shift reaction are much higher than in case of direct combustion of coal in air (which is mostly nitrogen). These higher concentrations of carbon dioxide make carbon capture and storage much more economical than it otherwise would be.

Liquefaction

Coal can also be converted into liquid fuels like gasoline or diesel by several different processes. The Fischer-Tropsch process of indirect synthesis of liquid hydrocarbons was used in Nazi Germany for many years and is today used by Sasol in South Africa. Coal would be gasified to make syngas (a balanced purified mixture of CO and H₂ gas) and the syngas condensed using Fischer-Tropsch catalysts to make light hydrocarbons which are further processed into gasoline and diesel. Syngas can also be converted to methanol, which can be used as a fuel, fuel additive, or further processed into gasoline via the Mobil M-gas process.

A direct liquefaction process Bergius process (liquefaction by hydrogenation) is also available but has not been used outside Germany, where such processes were operated both during World War I and World War II. SASOL in South Africa has experimented with direct hydrogenation. Several other direct liquefaction processes have been developed, among these being the SRC-I and SRC-II (Solvent Refined Coal) processes developed by Gulf Oil and implemented as pilot plants in the United States in the 1960s and 1970s.^[7]

Another direct hydrogenation process was explored by the NUS Corporation in 1976 and patented by Wilburn C. Schroeder. The process involved dried, pulverized coal mixed with roughly 1wt% molybdenum catalysis. Hydrogenation occurred by use of high temperature and pressure synthesis gas produced in a separate gasifier. The process ultimately yielded a synthetic crude product, Naptha, a limited amount of C₃/C₄ gas, light-medium weight liquids (C₅-C₁₀) suitable for use as fuels, small amounts of NH₃ and significant amounts of CO₂.^[8]

Yet another process to manufacture liquid hydrocarbons from coal is low temperature carbonization (LTC). Coal is coked at temperatures between 450 and 700°C compared to 800 to 1000°C for metallurgical coke. These temperatures optimize the production of coal tars richer in lighter hydrocarbons than normal coal tar. The coal tar is then further processed into fuels. The Karrick process was developed by Lewis C. Karrick, an oil shale technologist at the U.S. Bureau of Mines in the 1920s.

All of these liquid fuel production methods release carbon dioxide (CO₂) in the conversion process, far more than is released in the extraction and refinement of liquid fuel production from petroleum. If these methods were adopted to replace declining petroleum supplies, carbon dioxide emissions would be greatly increased on a global scale. For future liquefaction projects, Carbon dioxide sequestration is proposed to avoid releasing it into the atmosphere, though no pilot projects have confirmed the feasibility of this approach on a wide scale. As CO₂ is one of the process streams, sequestration is easier than from flue gases produced in combustion of coal with air, where CO₂ is diluted by nitrogen and other gases. Sequestration will, however, add to the cost.

Coal liquefaction is one of the backstop technologies that could potentially limit escalation of oil prices and mitigate the effects of transportation energy shortage that some authors have suggested could occur under peak oil. This is contingent on liquefaction production capacity becoming large enough to satiate the very large and growing demand for petroleum. Estimates of the cost of producing liquid fuels from coal suggest that domestic U.S. production of fuel from coal becomes cost-competitive with oil priced at around 35 USD per barrel,^[9] (break-even cost). This price, while above historical averages, is well below current oil prices. This makes coal a viable financial alternative to oil for the time being, although production is not great enough to make synfuels viable on a large scale.^[10]

Among commercially mature technologies, advantage for indirect coal liquefaction over direct coal liquefaction are reported by Williams and Larson (2003). Estimates are reported for sites in China where break-even cost for coal liquefaction may be in the range between 25 to 35 USD/barrel of oil.

[edit] Cultural usage

Coal is the official state mineral of Kentucky and the official state rock of Utah. Both U.S. states have a historic link to coal mining.

Harmful effects

Coal mining

Coal mining causes a number of harmful effects. When coal surfaces are exposed, pyrite (iron sulfide), also known as "fool's gold", comes in contact with water and air and forms sulfuric acid. As water drains from the mine, the acid moves into the waterways, and as long as rain falls on the mine tailings the sulfuric acid production continues, whether the mine is still operating or not. This process is known as acid rock drainage (ARD) or acid mine drainage (AMD). If the coal is strip mined, the entire exposed seam leaches sulfuric acid, leaving the infertile subsoil on the surface and begins to pollute streams by acidifying and killing fish, plants, and aquatic animals who are sensitive to drastic pH shifts.

By the late 1930s, it was estimated that American coal mines produced about 2.3 million tonnes of sulfuric acid annually. In the Ohio River Basin, where twelve hundred operating coal mines drained an estimated annual 1.4 million tonnes of sulfuric acid into the waters in the 1960s and thousands of abandoned coal mines leached acid as well. In Pennsylvania alone, mine drainage had blighted 2,000 stream miles by 1967.

Coal burning

Combustion of coal, like any other fossil fuel, produces carbon dioxide (CO₂) and nitrogen oxides (NO_x) along with varying amounts of sulfur dioxide (SO₂) depending on where it was mined. Sulfur dioxide reacts with oxygen to form sulfur trioxide (SO₃), which then reacts with water to form sulfuric acid (see Acid anhydride for more information). The sulfuric acid is returned to the Earth as acid rain. Scrubbing systems, which use lime to remove the sulfur dioxide can reduce or eliminate the likelihood of acid rain.

Emissions from coal-fired power plants represent one of the two largest sources of carbon dioxide emissions, which are the primary cause of global warming. Coal mining and abandoned mines also emit methane, another cause of global warming. Since the carbon content of coal is higher than oil, burning coal is a more serious threat to the stability of the global climate, as this carbon forms CO₂ when burned. Many other pollutants are present in coal power station emissions, as solid coal is more difficult to clean than oil, which is refined before use. A study commissioned by environmental groups claims that coal power plant emissions are responsible for tens of thousands of premature deaths annually in the United States alone.^[11] Modern power plants utilize a variety of techniques to limit the harmfulness of their waste products and improve the efficiency of burning, though these techniques are not subject to standard testing or regulation in the U.S. and are not widely implemented in some countries, as they add to the capital cost of the power plant. To eliminate CO₂ emissions from coal plants, carbon capture and storage has been proposed but has yet to be commercially used.

Coal and coal waste products including fly ash, bottom ash, boiler slag, and flue gas desulferization contain many heavy metals, including arsenic, lead, mercury, nickel, vanadium, beryllium, cadmium, barium, chromium, copper, molybdenum, zinc, selenium and radium, which are dangerous if released into the environment. Coal also contains low levels of uranium, thorium, and other naturally-occurring radioactive isotopes whose release into the environment may lead to radioactive contamination.^[12]^[13] While these substances are trace impurities, enough coal is burned that significant amounts of these substances are released, resulting in more radioactive waste than nuclear power plants.^[14] Mercury emissions from coal burning are concentrated as they work their way up the food chain and converted into dangerous biological compounds that have made it dangerous to eat fish from many waterways of the world. Due to its scientifically accepted connection with climate change,^[15] the world's reliance on coal as an energy source, and health concerns in areas with poor air pollution controls, The Economist recently labeled the burning of coal "Environmental Enemy No. 1."^[16]. Coalization is the mass use of coal-fired power plants to produce electricity, as happens in China and USA.

Energy density

Main article: Energy value of coal

The energy density of coal is roughly 24 megajoules per kilogram.^[17]

The energy density of coal can also be expressed in kilowatt-hours, the units that electricity is most commonly sold in, to estimate how much coal is required to power electrical appliances. The energy density of coal is 6.67 kW-h/kg and the typical thermodynamic efficiency of coal power plants is about 30%. Of the 6.67 kW-h of energy per kilogram of coal, about 30% of that can successfully be turned into electricity - the rest is waste heat. Coal power plants obtain approximately 2.0 kW-h per kg of burned coal.

As an example, running one 100 watt computer for one year requires 876 kW-h (100 W × 24 h × 365 {days in a year} = 876000 W-h = 876 kW-h). Converting this power usage into physical coal consumption: .

It takes 438 kg (967 pounds) of coal to power a computer for one full year.^[18] One should also take into account transmission and distribution losses caused by resistance and heating in the power lines, which is in the order of 5 - 10%, depending on distance from the power station and other factors.

Relative carbon cost

Because coal is at least 50% carbon (by mass), then 1 kg of coal contains at least 0.5 kg of carbon, which is where 1 mol is equal to N_A (Avogadro Number) particles. This combines with oxygen in the atmosphere during combustion, producing carbon dioxide, with an atomic weight of (12 + 16 × 2 = mass(CO₂) = 44 kg/kmol). of CO₂ is produced from the present in every kilogram of coal, which once trapped in CO₂ weighs approximately .

This fact can be used to put a carbon-cost of energy on the use of coal power. Since the useful energy output of coal is about 30% of the 6.67 kW-h/kg(coal), we can say about 2 kW-h/kg(coal) of energy is produced. Since 1 kg coal roughly translates as 1.83 kg of CO₂, we can say that using electricity from coal produces CO₂ at a rate of about 0.915 kg(CO₂) / kW-h, or about 0.254 kg(CO₂) / MJ.

Coal fires

There are hundreds of coal fires burning around the world.^[19] Those burning underground can be difficult to locate and many cannot be extinguished. Fires can cause the ground above to subside, combustion gases are dangerous to life, and breaking out to the surface can initiate surface wildfires. Coal seams can be set on fire by spontaneous combustion or contact with a mine fire or surface fire. A grass fire in a coal area can set dozens of coal seams on fire.^[20]^[21] Coal fires in China burn 109 million tonnes of coal a year, emitting 200 million tonnes of carbon dioxide. This amounts to 2-3% of the annual worldwide production of CO₂ from fossil fuels, or as much as emitted from all of the cars and light trucks in the United States.^[22]^[23] In Centralia, Pennsylvania (a borough located in the Coal Region of the United States) an exposed vein of coal ignited in 1962 due to a trash fire in the borough landfill, located in an abandoned anthracite strip mine pit. Attempts to extinguish the fire were unsuccessful, and it continues to burn underground to this day. The Australian Burning Mountain was originally believed to be a volcano, but the smoke and ash comes from a coal fire which may have been burning for over 5,500 years.^[24]

At Kuh i Malik in Yagnob Valley, Tajikistan, coal deposits have been burning for thousands of years, creating vast underground labyrinths full of unique minerals, some of them very beautiful. The only way to peek inside and survive for more than a few seconds is by wrapping yourself in a wet blanket. Local people once used this method to mine ammoniac. This place has been well-known since the time of Herodotus, but European geographers mis-interpreted the Ancient Greek descriptions as the evidence of active volcanism in Turkestan (up to the 19th century, when Russian army invaded the area).

The reddish siltstone rock that caps many ridges and buttes in the Powder River Basin (Wyoming), and in western North Dakota is called porcelanite, which also may resemble the coal burning waste "clinker" or volcanic "scoria".^[25] Clinker is rock that has been fused by the natural burning of coal. In the Powder River Basin approximately 27 to 54 billion tonnes of coal burned within the past three million years.^[26] Wild coal fires in the area were reported by the Lewis and Clark Expedition as well as explorers and settlers in the area.^[27]

Production trends

Coal output in 2005

In 2005, China was the top producer of coal with almost one-third world share followed by the USA and India, reports the British Geological Survey.

World coal reserves

US coal regions

In 2003 it was estimated that there was around one exagram (1 × 10¹⁵ kg or 998 billion tons) of total coal reserves accessible using current mining technology, approximately half of it being hard coal. The energy value of all the world's recoverable coal is 27 zettajoules,^[28] which is expected to last 200 years.^{[citation needed]} At the current global total energy consumption of 15 terawatt,^[29] there is enough coal to provide the entire planet with all of its energy for 57 years.

This article or section may contain original research or unverified claims.
Please help Wikipedia by adding references. See the talk page for details.

British Petroleum, in its annual report 2006, estimated at 2005 end, there were 909,064 million tons of proven coal reserves worldwide (9.236 × 10¹⁴ kg), or 155 years reserve to production ratio. This figure only includes reserves classified as "proven", exploration drilling programs by mining companies, particularly in under-explored areas, are continually providing new reserves. In many cases, companies are aware of coal deposits that have not been sufficiently drilled to qualify as "proven".

The United States Department of Energy uses estimates of coal reserves in the region of 1,081,279 million short tons (9.81 × 10¹⁴ kg), which is about 4,786 BBOE (billion barrels of oil equivalent).^[30] The amount of coal burned during 2001 was calculated as 2.337 GTOE (gigatonnes of oil equivalent), which is about 46 million barrels of oil equivalent per day.^[31] Were consumption to continue at that rate those reserves would last about 285 years. As a comparison, natural gas provided 51 million barrels (oil equivalent), and oil 76 million barrels, per day during 2001.

Of the three fossil fuels coal has the most widely distributed reserves; coal is mined in over 100 countries, and on all continents except Antarctica. The largest reserves are found in the USA, Russia, Australia, China, India and South Africa.

**Proved recoverable coal reserves at end-1999 (million tonnes)**^{[citation needed]}
Country	Bituminous (including anthracite)	Sub- bituminous	Lignite	TOTAL
United States of America	115891	101021	33082	249994
Russian Federation	49088	97472	10450	157010
People's Republic of China	62200	33700	18600	114500
India	82396		2000	84396
Australia	42550	1840	37700	82090
Germany	23000		43000	66000
South Africa	49520			49520
Ukraine	16274	15946	1933	34153
Kazakhstan	31000		3000	34000
Poland	20300		1860	22160
Serbia	64	1460	14732	16256
Brazil		11929		11929
Colombia	6267	381		6648
Canada	3471	871	2236	6578
Czech Republic	2114	3414	150	5678
Indonesia	790	1430	3150	5370
Botswana	4300			4300
Uzbekistan	1000		3000	4000
Turkey	278	761	2650	3689
Greece			2874	2874
Bulgaria	13	233	2465	2711
Pakistan		2265		2265
Iran (Islamic Rep.)	1710			1710
United Kingdom	1000		500	1500
Romania	1	35	1421	1457
Thailand			1268	1268
Mexico	860	300	51	1211
Chile	31	1150		1181
Hungary		80	1017	1097
Peru	960		100	1060
Kyrgyzstan			812	812
Japan	773			773
Spain	200	400	60	660
Korea (Democratic People's Rep.)	300	300		600
New Zealand	33	206	333	572
Zimbabwe	502			502
Netherlands	497			497
Venezuela	479			479
Argentina		430		430
Philippines		232	100	332
Slovenia		40	235	275
Mozambique	212			212
Swaziland	208			208
Tanzania	200			200
Nigeria	21	169		190
Greenland		183		183
Slovakia			172	172
Vietnam	150			150
Congo (Democratic Rep.)	88			88
Korea (Republic)	78			78
Niger	70			70
Afghanistan	66			66
Algeria	40			40
Croatia	6		33	39
Portugal	3		33	36
France	22		14	36
Italy		27	7	34
Austria			25	25
Ecuador			24	24
Egypt (Arab Rep.)		22		22
Ireland	14			14
Zambia	10			10
Malaysia	4			4
Central African Republic			3	3
Myanmar (Burma)	2			2
Malawi		2		2
New Caledonia	2			2
Nepal	2			2
Bolivia	1			1
Norway		1		1
Republic of China	1			1
Sweden		1		1
TOTAL	519062	276301	189090	984453

Major coal exporters

**Exports of Coal by Country and year (million tonnes)**^[32]
Country	2003	2004
Australia	238.1	247.6
United States	43.0	48.0
South Africa	78.7	74.9
Former Soviet Union	41.0	55.7
Poland	16.4	16.3
Canada	27.7	28.8
People's Republic of China	103.4	95.5
South America	57.8	65.9
Indonesia	107.8	131.4
Total	713.9	764.0

Notes

^ Oxford English Dictionary 1989 edition
^ ^a ^b Britannica 2004: Coal mining: ancient use of outcropping coal.
^ Salway, Peter (2001): A History of Roman Britain. Oxford University Press.
^ Forbes, R J (1966): Studies in Ancient Technology. Brill Academic Publishers, Boston.
^ International Energy Outlook. Retrieved on September 9, 2005.
^ [http://www.powergeneration.siemens.com/download/pool/PGE2005_BalancingEconomics.pdf Balancing economics and environmental friendliness - the challenge for supercritical coal-fired power plants with highest steam parameters in the future] (PDF). Retrieved on 2006-10-23.
^ Cleaner Coal Technology Programme (October 1999). "Technology Status Report 010: Coal Liquefaction". Department of Trade and Industry (UK). Retrieved on November 23.
^ Phillip A. Lowe, Wilburn C. Schroeder, Anthony L. Liccardi (1976). "Technical Economies, Synfuels and Coal Energy Symposium, Solid-Phase Catalytic Coal Liquefaction Process". The American Society of Mechanical Engineers.
^ Diesel Fuel News: Ultra-clean fuels from coal liquefaction: China about to launch big projects - Brief Article. Retrieved on September 9, 2005.
^ Welcome to Coal People Magazine. Retrieved on September 9, 2005.
^ Deadly power plants? Study fuels debate. Retrieved on September 4, 2006.
^ Coal Combustion. Retrieved on September 9, 2005.
^ Radioactive Elements in Coal and Fly Ash, USGS Factsheet 163-97. Retrieved on September 9, 2005.
^ Coal Combustion: Nuclear Resource or Danger. Retrieved on October 16, 2006.
^ [http://www.realclimate.org/index.php/archives/category/climate-science/greenhouse-gases/
^ Environmental enemy No. 1. Retrieved on September 4, 2006.
^ Fisher, Juliya. Energy Density of Coal. The Physics Factbook. Retrieved on 2006-08-25.
^ A similar result, using a lightbulb instead, see
How much coal is required to run a 100-watt light bulb 24 hours a day for a year?. Howstuffworks. Retrieved on 2006-08-25.
^ Sino German Coal fire project. Retrieved on September 9, 2005.
^ Committee on Resources-Index. Retrieved on September 9, 2005.
^ http://www.fire.blm.gov/textdocuments/6-27-03.pdf. Retrieved on September 9, 2005.
^ EHP 110-5, 2002: Forum. Retrieved on September 9, 2005.
^ Overview about ITC's activities in China. Retrieved on September 9, 2005.
^ Burning Mountain Nature Reserve. Retrieved on September 9, 2005.
^ North Dakota's Clinker. Retrieved on September 9, 2005.
^ BLM-Environmental Education- The High Plains. Retrieved on September 9, 2005.
^ http://www.wsgs.uwyo.edu/Coal/CR01-1.pdf. Retrieved on September 9, 2005.
^ International Energy Outlook 2007 Chapter 5 Coal
^ BP2006 energy report, and US EIA 2006 overview
^ International Energy Annual 2003: Reserves. Retrieved on September 9, 2005.
^ IEA Publications Bookshop. Retrieved on September 9, 2005.
^ World Steam Coal Flows

References

(2005) The Face of Decline: The Pennsylvania Anthracite Region in the Twentieth Century. Cornell University Press. ISBN 0-8014-8473-1.
Rottenberg, Dan (2003). In the Kingdom of Coal; An American Family and the Rock That Changed the World. Routledge. ISBN 0-415-93522-9.
Robert H. Williams and Eric D. Larson (December 2003). "A comparison of direct and indirect liquefaction technologies for making fluid fuels from coal" (PDF). Energy for Sustainable Development VII: 103-129.
Outwater, Alice (1996). Water: A Natural History. New York, NY: Basic Books. ISBN 0-465-03780-1.
Smith, Duane A. (May 1993). Mining America: The Industry and the Environment, 1800-1980 (in English). Lawrence, KS: University Press of Kansas, 210. ISBN 0870813064.

Thermal power stations

2007-09-16T11:10:00.000-07:00

Thermal power stations

Rotor of a modern steam turbine, used in power station

Main article: Thermal power station

In thermal power stations, mechanical power is produced by a heat engine, which transforms thermal energy, often from combustion of a fuel, into rotational energy. Most thermal power stations produce steam, and these are sometimes called steam power stations. About 86% of all electric power is generated by use of steam turbines.^{[citation needed]} Not all thermal energy can be transformed to mechanical power, according to the second law of thermodynamics. Therefore, there is always heat lost to the environment. If this loss is employed as useful heat, for industrial processes or district heating, the power plant is referred to as a cogeneration power plant or CHP (combined heat-and-power) plant. In countries where district heating is common, there are dedicated heat plants called heat-only boiler stations. An important class of power stations in the Middle East uses byproduct heat for desalination of water.

Classification

CHP plant in Warsaw, Poland

Geothermal power station in Iceland

Coal Power Station in Tampa FL

480 megawatt GE H series power generation gas turbine

Thermal power plants are classified by the type of fuel and the type of prime mover installed.

By fuel

Nuclear power plants^[4] use a nuclear reactor's heat to operate a steam turbine generator.
Fossil fuelled power plants may also use a steam turbine generator or in the case of natural gas fired plants may use a combustion turbine.
Geothermal power plants use steam extracted from hot underground rocks.
Renewable energy plants may be fuelled by waste from sugar cane, municipal solid waste, landfill methane, or other forms of biomass.
In integrated steel mills, blast furnace exhaust gas is a low-cost, although low-energy-density, fuel.
Waste heat from industrial processes is occasionally concentrated enough to use for power generation, usually in a steam boiler and turbine.

By prime mover

Steam turbine plants use the dynamic pressure generated by expanding steam to turn the blades of a turbine.
Gas turbine plants use the dynamic pressure from flowing gases to directly operate the turbine. Natural-gas fuelled turbine plants can start rapidly and so are used to supply "peak" energy during periods of high demand, though at higher cost than base-loaded plants.
Combined cycle plants have both a gas turbine fired by natural gas, and a steam boiler and steam turbine which use the exhaust gas from the gas turbine to produce electricity. This greatly increases the overall efficiency of the plant, and most new baseload power plants are combined cycle plants fired by natural gas.
Internal combustion Reciprocating engines are used to provide power for isolated communities and are frequently used for small cogeneration plants. Hospitals, office buildings, industrial plants, and other critical facilities also use them to provide backup power in case of a power outage. These are usually fuelled by diesel oil, heavy oil, natural gas and landfill gas.
Microturbines, Stirling engine and internal combustion reciprocating engines are low cost solutions for using opportunity fuels, such as landfill gas, digester gas from water treatment plants and waste gas from oil production.

Cooling towers

Coal power plant in China with a hyperbolic cooling tower

Because of the fundamental limits to thermodynamic efficiency of any heat engine, all thermal power plants produce waste heat as a byproduct of the useful electrical energy produced. Natural draft wet cooling towers at nuclear power plants and at some large thermal power plants are large hyperbolic chimney-like structures (as seen in the image at the left) that release the waste heat to the ambient atmosphere by the evaporation of water (lower left image).

A Marley mechanical induced-draft cooling tower

However, the mechanical induced-draft or forced-draft wet cooling towers (as seen in the image to the right) in many large thermal power plants, petroleum refineries, petrochemical plants, geothermal, biomass and waste to energy plants use fans to provide air movement upward through downcoming water and are not hyperbolic chimney-like structures. The induced or forced-draft cooling towers are rectangular, box-like structures filled with a material that enhances the contacting of the upflowing air and the downflowing water.^[5]^[6]

Cooling towers evaporating water at Ratcliffe Power Plant, UK

In desert areas a dry cooling tower or radiator may be necessary, since the cost of make-up water for evaporative cooling would be prohibitive. These have lower efficiency and higher energy consumption in fans than a wet, evaporative cooling tower.

Where economically and environmentally possible, electric companies prefer to use cooling water from the ocean, or a lake or river, or a cooling pond, instead of a cooling tower. This type of cooling can save the cost of a cooling tower and may have lower energy costs for pumping cooling water through the plant's heat exchangers. However, the waste heat can cause the temperature of the water to rise detectably. Power plants using natural bodies of water for cooling must be designed to prevent intake of organisms into the cooling cycle. A further environmental impact would be organisms that adapt to the warmer plant water and may be injured if the plant shuts down in cold weather.

In recent years, recycled wastewater, or grey water, has been used in cooling towers. The Calpine Riverside and the Calpine Fox power stations in Wisconsin as well as the Calpine Mankato power station in Minnesota are among these facilities.

Other sources of energy

Other power stations use the energy from wave or tidal motion, wind, sunlight or the energy of falling water, hydroelectricity. These types of energy sources are called renewable energy.

Hydroelectricity

A hydroelectric dam and plant on the Muskegon river in Michigan

Main article: Hydroelectricity

Hydroelectric dams impound a reservoir of water and release it through one or more water turbines to generate electricity.

Pumped storage

A pumped storage hydroelectric power plant is a net consumer of energy but decreases the price of electricity. Water is pumped to a high reservoir during the night when the demand, and price, for electricity is low. During hours of peak demand, when the price of electricity is high, the stored water is released to produce electric power. Some pumped storage plants are actually not net consumers of electricity because they release some of the water from the lower reservoir downstream, either continuously or in bursts.

Solar

Main article: Solar power

A control room of a modern waste incineration power plant

A solar photovoltaic power plant converts sunlight into electrical energy, which may need conversion to alternating current for transmission to users. This type of plant does not use rotating machines for energy conversion. Solar thermal electric plants are another type of solar power plant. They direct sunlight using either parabolic troughs or heliostats. Parabolic troughs direct sunlight onto a pipe containing a heat transfer fluid, such as oil, which is then used to boil water, which turns the generator. The central tower type of power plant uses hundreds or thousands of mirrors, depending on size, to direct sunlight onto a receiver on top of a tower. Again, the heat is used to produce steam to turn turbines. There is yet another type of solar thermal electric plant. The sunlight strikes the bottom of the pond, warming the lowest layer which is prevented from rising by a salt gradient. A Rankine cycle engine exploits the temperature difference in the layers to produce electricity. Not many solar thermal electric plants have been built. Most of them can be found in the Mojave Desert, although Sandia National Laboratory, Israel and Spain have also built a few plants.

Wind

Main article: Wind power

Wind turbines can be used to generate electricity in areas with strong, steady winds. Many different designs have been used in the past, but almost all modern turbines being produced today use the Dutch six-bladed, upwind design. Grid-connected wind turbines now being built are much larger than the units installed during the 1970s, and so produce power more cheaply and reliably than earlier models. With larger turbines (on the order of one megawatt), the blades move more slowly than older, smaller, units, which makes them less visually distracting and safer for airborne animals. However, the old turbines can still be seen at some wind farms, particularly at Altamont Pass and Tehachapi Pass.

References

^ British Electricity International (1991). Modern Power Station Practice: incorporating modern power system practice, 3rd Edition (12 volume set), Pergamon. ISBN 0-08-040510-X.
^ Babcock & Wilcox Co. (2005). Steam: Its Generation and Use, 41st edition. ISBN 0-9634570-0-4.
^ Thomas C. Elliott, Kao Chen, Robert Swanekamp (coauthors) (1997). Standard Handbook of Powerplant Engineering, 2nd edition, McGraw-Hill Professional. ISBN 0-07-019435-1.
^ Nuclear Power Plants Information, by IAEA
^ J.C. Hensley (Editor) (2006). Cooling Tower Fundamentals, 2nd Ed., SPX Cooling Technologies.
^ Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants, 4th Edition, John Wiley and Sons. LCCN 67019834. (Includes cooling tower material balance for evaporation emissions and blowdown effluents. Available in many university libraries)

High voltage AC transmission pylons

2007-09-16T10:56:00.000-07:00

High voltage AC transmission pylons

Three-phase electric power systems are used for high and extra-high voltage AC transmission lines (50 kV and above). The towers must therefore be designed to carry three (or multiples of three) conductors. The towers are usually steel lattices or trusses (wooden structures are used in Germany in exceptional cases) and the insulators are either glass or porcelain discs assembled in strings whose length is dependent on the line voltage and environmental conditions. One or two earth conductors (alternative term: Ground conductors) for lightning protection are often added at the top of each tower.

In some countries, towers for high and extra-high voltage are usually designed to carry two or more electric circuits. For double circuit lines in Germany, the "Danube" towers or more rarely, the "fir tree" towers, are usually used. If a line is constructed using pylons designed to carry several circuits, it is not necessary to install all the circuits at the time of construction.

Medium voltage circuits are often erected on the same towera as 110 kV lines. Paralleling circuits of 380 kV, 220 kV and 110 kV-lines on the same towers is common. Sometimes, especially with 110 kV circuits, a parallel circuit carries traction lines for railway electrification.

History

Electricity transmision towers have been used since at least the 1910s.

High voltage DC transmission pylons

HVDC Distance Pylon near the terminus of the Nelson River Bipole adjacent to Dorsey Converter Station near Rosser, Manitoba — August 2005

High voltage direct current (HVDC) transmission lines are either monopolar or bipolar systems. With bipolar systems a conductor arrangement with one conductor on each side of the tower is used. For single-pole HVDC transmission with ground return, towers with only one conductor can be used. In many cases, however, the towers are designed for later conversion to a two-pole system. In these cases, conductors are installed on both sides of the tower for mechanical reasons. Until the second pole is needed, it is either grounded, or joined in parallel with the pole in use. In the latter case the line from the converter station to the earthing (grounding) electrode is built as underground cable.

Railway traction line pylons

Tension tower with phase transposition of a powerline for single phase AC traction current (110 kV, 16.67 Hertz) near Bartholomä in Germany

Towers used for single phase AC railway traction lines are similar in construction to those towers used for 110 kV-three phase lines. Steel tube or concrete poles are also often used for these lines. However, railway traction current systems are two-pole AC systems, so traction lines are designed for two conductors (or multiples of two, usually four, eight, or twelve). As a rule, the towers of railway traction lines carry two electric circuits, so they have four conductors. These are usually arranged on one level, whereby each circuit occupies one half of the crossarm. For four traction circuits the arrangement of the conductors is in two-levels and for six electric circuits the arrangement of the conductors is in three levels.

With limited space conditions, it is possible to arrange the conductors of one traction circuit in two levels. Running a traction power line parallel to a high voltage transmission lines for three-phase AC on a separate crossarm of the same tower is possible. If traction lines are led parallel to 380 kV-lines, the insulation must be designed for 220 kV, because in the event of a fault, dangerous overvoltages to the three-phase alternating current line can occur. Traction lines are usually equipped with one earth conductor. In Austria, on some traction circuits, two earth conductors are used.

Assembly

Lattice towers can be assembled horizontally on the ground and erected by push-pull cable, but this method is rarely used because of the large assembly area needed. Lattice towers are more usually erected using a crane or, in inaccessible areas, a helicopter.

Testing of mechanical properties

There are tower testing stations for testing the mechanical properties of towers.

Sign markings

Tower Identification Tag on HVDC anchor pylon at Dorsey Converter Station near Rosser, Manitoba — August 2005

Hyperboloid pylon in the suburb of Nizhniy Novgorod, Russia.

One of the Pylons of Cádiz

Besides the obligatory high voltage warning sign, electricity towers also frequently possess a sign or circuit identification plate, with the names of the line (either the terminal points of the line or the internal designation of the EVU^[vague]) and the tower number. This makes it easier identifying the location of a fault to the power company that owns the tower.

In some countries, electricity towers of lattice steel have to be equipped with a barbed wire barrier approximately 3 metres above ground in order to deter unauthorized climbing. Such barriers can often be found on towers close to roads or other areas with easy public access, even where there is not such a requirement.

Special designs

Antennas for low power FM radio, television, and mobile phone services are sometimes erected on pylons, especially on the steel masts carrying high voltage cables.

To build branches, quite impressive constructions must occasionally be used. This also applies occasionally to twisting masts that divert three-level conductor cables.

Sometimes (in particular on steel framework pylons for the highest voltage levels) transmitting plants are installed. Usually these installations are for mobile phone services or the operating radio of the power supply firm, but occasionally also for other radio services, like directional radio. Thus transmitting antennas for low-power FM radio and television transmitters were already installed on pylons. On the carrying pylon of the Elbe Crossing 1 there is a radar facility belonging to the Hamburg water and navigation office.

For crossing broad valleys, a large distance between the conductor cables must be maintained to avoid short-circuits caused by conductor cables colliding during storms. Sometimes a separate pylon is used for each conductor. For crossing wide rivers and straits with flat coastlines very high pylons must be built, because a large height clearance is needed for navigation. Such masts must be equipped with flight safety lamps.

Two well-known crossings of wide rivers are the Elbe Crossing 1 and Elbe Crossing 2. The latter has the highest overhead line masts in Europe (height: 227 meters). The pylons of the overhead line crossing of the bay of Cádiz, Spain have a particularly interesting construction. They consist of 158-meter-high carrying pylons with one cross beam atop a frustum framework construction. The largest spans of overhead lines are the crossing of the Norwegian Sognefjord (span between two masts of 4,597 meters) and the Ameralik span in Greenland (span width: 5,376 meters). In Germany the overhead line of the EnBW AG crossing of the Eyachtal has the largest span in the country, a width of 1,444 meters.

In order to drop overhead lines into steep, deep valleys, inclined pylons are occasionally used. An example of this type of pylon is located at the Hoover dam in the USA. In Switzerland a NOK pylon inclined around 20 degrees to the vertical is located near Sargans. Highly sloping masts are used on two 380 kV pylons in Switzerland, the top 32 meters of one of them being bent by 18 degrees to the vertical.

Power station chimneys are sometimes equipped with crossbars for fixing conductors of the outgoing lines. Because of possible problems with corrosion by the flue gases, such constructions are very rare.

Types of pylons

Specific functions

anchor pylons (or strainer pylons) or terminal towers utilize tension insulators and occur at the endpoints of conductors.
pine pylon — an electricity pylon for two circuits of three-phase AC current, a which the conductors are arranged in three levels. In pine pylons the lowest crossbar has a wider span than that in the middle and this one a larger span than that on the top.
Transposing pylons are anchor or terminal pylons at which the conductors are "transposed" so that they exchange sides of the pylon.
long-distance anchor pylon
branch pylon
anchor portal
termination pylon

Materials used

Conductor arrangements

Specific locations

roof stand
crossing pylon
bridge mounted structures, as on Storstrøm Bridge

Specific purposes

Substation Design and Layout

2007-09-16T10:48:00.000-07:00

Substation Design and Layout

Earthing and Bonding

The function of an earthing and bonding system is to provide an earthing system connection to which transformer neutrals or earthing impedances may be connected in order to pass the maximum fault current. The earthing system also ensures that no thermal or mechanical damage occurs on the equipment within the substation, thereby resulting in safety to operation and maintenance personnel. The earthing system also guarantees eqipotential bonding such that there are no dangerous potential gradients developed in the substation.

In designing the substation, three voltage have to be considered.

1. Touch Voltage: This is the difference in potential between the surface potential and the potential at an earthed
equipment whilst a man is standing and touching the earthed structure.

2. Step Voltage: This is the potential difference developed when a man bridges a distance of 1m with his feet
while not touching any other earthed equipment.

3. Mesh Voltage: This is the maximum touch voltage that is developed in the mesh of the earthing grid.

Substation Earthing Calculation Methodology

Calculations for earth impedances and touch and step potentials are based on site measurements of ground resistivity and system fault levels. A grid layout with particular conductors is then analysed to determine the effective substation earthing resistance, from which the earthing voltage is calculated.

In practice, it is normal to take the highest fault level for substation earth grid calculation purposes. Additionally, it is necessary to ensure a sufficient margin such that expansion of the system is catered for.

To determine the earth resistivity, probe tests are carried out on the site. These tests are best performed in dry weather such that conservative resistivity readings are obtained.

Earthing Materials

1. Conductors: Bare copper conductor is usually used for the substation earthing grid. The copper bars themselves
usually have a cross-sectional area of 95 square millimetres, and they are laid at a shallow depth
of 0.25-0.5m, in 3-7m squares. In addition to the buried potential earth grid, a separate above ground
earthing ring is usually provided, to which all metallic substation plant is bonded.

2. Connections: Connections to the grid and other earthing joints should not be soldered because the heat generated
during fault conditions could cause a soldered joint to fail. Joints are usually bolted, and in this case, the
face of the joints should be tinned.

3. Earthing Rods: The earthing grid must be supplemented by earthing rods to assist in the dissipation of earth fault
currents and further reduce the overall substation earthing resistance. These rods are usually made of
solid copper, or copper clad steel.

4. Switchyard Fence
Earthing: The switchyard fence earthing practices are possible and are used by different utilities. These are:

(i) Extend the substation earth grid 0.5m-1.5m beyond the fence perimeter. The fence is then
bonded to the grid at regular intervals.
(ii) Place the fence beyond the perimeter of the switchyard earthing grid and bond the fence to its
own earthing rod system. This earthing rod system is not coupled to the main substation earthing
grid.

Layout of Substation

The layout of the substation is very important since there should be a Security of Supply. In an ideal substation all circuits and equipment would be duplicated such that following a fault, or during maintenance, a connection remains available. Practically this is not feasible since the cost of implementing such a design is very high. Methods have been adopted to achieve a compromise between complete security of supply and capital investment. There are four categories of substation that give varying securities of supply:

Category 1: No outage is necessary within the substation for either maintenance or fault conditions.
Category 2: Short outage is necessary to transfer the load to an alternative circuit for maintenance or fault conditions.
Category 3: Loss of a circuit or section of the substation due to fault or maintenance.
Category 4: Loss of the entire substation due to fault or maintenance.

Different Layouts for Substations

Single Busbar

The general schematic for such a substation is shown in the figure below.

With this design, there is an ease of operation of the substation. This design also places minimum reliance on signalling for satisfactory operation of protection. Additionally there is the facility to support the economical operation of future feeder bays.

Such a substation has the following characteristics.

Each circuit is protected by its own circuit breaker and hence plant outage does not necessarily result in loss of supply.
A fault on the feeder or transformer circuit breaker causes loss of the transformer and feeder circuit, one of which may be restored after isolating the faulty circuit breaker.
A fault on the bus section circuit breaker causes complete shutdown of the substation. All circuits may be restored after isolating the faulty circuit breaker.
A busbar fault causes loss of one transformer and one feeder. Maintenance of one busbar section or isolator will cause the temporary outage of two circuits.
Maintenance of a feeder or transformer circuit breaker involves loss of the circuit.
Introduction of bypass isolators between busbar and circuit isolator allows circuit breaker maintenance facilities without loss of that circuit.

Mesh Substation

The general layout for a full mesh substation is shown in the schematic below.

The characteristics of such a substation are as follows.

Operation of two circuit breakers is required to connect or disconnect a circuit, and disconnection involves opening of a mesh.
Circuit breakers may be maintained without loss of supply or protection, and no additional bypass facilities are required.
Busbar faults will only cause the loss of one circuit breaker. Breaker faults will involve the loss of a maximum of two circuits.
generally, not more than twice as many outgoing circuits as infeeds are used in order to rationalise circuit equipment load capabilities and ratings.

One and a half Circuit Breaker layout

The layout of a 1 1/2 circuit breaker substation is shown in the schematic below.

The reason that such a layout is known as a 1 1/2 circuit breaker is due to the fact that in the design, there are 9 circuit breakers that are used to protect the 6 feeders. Thus, 1 1/2 circuit breakers protect 1 feeder. Some characteristics of this design are:

There is the additional cost of the circuit breakers together with the complex arrangement.
It is possible to operate any one pair of circuits, or groups of pairs of circuits.
There is a very high security against the loss of supply.

Principle of Substation Layouts

Substation layout consists essentially in arranging a number of switchgear components in an ordered pattern governed by their function and rules of spatial separation.

Spatial Separation

Earth Clearance: this is the clearance between live parts and earthed structures, walls, screens and ground.
Phase Clearance: this is the clearance between live parts of different phases.
Isolating Distance: this is the clearance between the terminals of an isolator and the connections thereto.
Section Clearance: this is the clearance between live parts and the terminals of a work section. The limits of this work section, or maintenance zone, may be the ground or a platform from which the man works.

Separation of maintenance zones

Two methods are available for separating equipment in a maintenance zone that has been isolated and made dead.

1. The provision of a section clearance
2. Use of an intervening earthed barrier

The choice between the two methods depends on the voltage and whether horizontal or vertical clearances are involved.

A section clearance is composed of a the reach of a man, taken as 8 feet, plus an earth clearance.
For the voltage at which the earth clearance is 8 feet, the space required will be the same whether a section clearance or an earthed barrier is used.

HENCE:

Separation by earthed barrier = Earth Clearance + 50mm for barrier + Earth Clearance

Separation by section clearance = 2.44m + Earth clearance

For vertical clearances it is necessary to take into account the space occupied by the equipment and the need for an access platform at higher voltages.
The height of the platform is taken as 1.37m below the highest point of work.

Establishing Maintenance Zones

Some maintenance zones are easily defined and the need for them is self evident as is the case of a circuit breaker. There should be a means of isolation on each side of the circuit breaker, and to separate it from adjacent live parts, when isolated, either by section clearances or earth barriers.

Electrical Separations

Together with maintenance zoning, the separation, by isolating distance and phase clearances, of the substation components and of the conductors interconnecting them constitute the main basis of substation layouts.

There are at least three such electrical separations per phase that are needed in a circuit:

1. Between the terminals of the busbar isolator and their connections.
2. Between the terminals of the circuit breaker and their connections.
3. Between the terminals of the feeder isolator and their connections.

Components of a Substation

The substation components will only be considered to the extent where they influence substation layout.

Circuit Breakers

There are two forms of open circuit breakers:
1. Dead Tank - circuit breaker compartment is at earth potential.
2. Live Tank - circuit breaker compartment is at line potential.

The form of circuit breaker influences the way in which the circuit breaker is accommodated. This may be one of four ways.

Ground Mounting and Plinth Mounting: the main advantages of this type of mounting are its simplicity, ease of erection, ease of maintenance and elimination of support structures. An added advantage is that in indoor substations, there is the reduction in the height of the building. A disadvantage however is that to prevent danger to personnel, the circuit breaker has to be surrounded by an earthed barrier, which increases the area required.
Retractable Circuit Breakers: these have the advantage of being space saving due to the fact that isolators can be accommodated in the same area of clearance that has to be allowed between the retractable circuit breaker and the live fixed contacts. Another advantage is that there is the ease and safety of maintenance. Additionally such a mounting is economical since at least two insulators per phase are still needed to support the fixed circuit breaker plug contacts.
Suspended Circuit Breakers: at higher voltages tension insulators are cheaper than post or pedestal insulators. With this type of mounting the live tank circuit breaker is suspended by tension insulators from overhead structures, and held in a stable position by similar insulators tensioned to the ground. There is the claimed advantage of reduced costs and simplified foundations, and the structures used to suspend the circuit breakers may be used for other purposes.

Current Transformers

CT's may be accommodated in one of six manners:

Over Circuit Breaker bushings or in pedestals.
In separate post type housings.
Over moving bushings of some types of insulators.
Over power transformers of reactor bushings.
Over wall or roof bushings.
Over cables.

In all except the second of the list, the CT's occupy incidental space and do not affect the size of the layout. The CT's become more remote from the circuit breaker in the order listed above. Accommodation of CT's over isolator bushings, or bushings through walls or roofs, is usually confined to indoor substations.

Isolators

These are essentially off load devices although they are capable of dealing with small charging currents of busbars and connections. The design of isolators is closely related to the design of substations. Isolator design is considered in the following aspects:

Space Factor
Insulation Security
Standardisation
Ease of Maintenance
Cost

Some types of isolators include:

Horizontal Isolation types
Vertical Isolation types
Moving Bushing types

Conductor Systems

An ideal conductor should fulfil the following requirements:

Should be capable of carrying the specified load currents and short time currents.
Should be able to withstand forces on it due to its situation. These forces comprise self weight, and weight of other conductors and equipment, short circuit forces and atmospheric forces such as wind and ice loading.
Should be corona free at rated voltage.
Should have the minimum number of joints.
Should need the minimum number of supporting insulators.
Should be economical.

The most suitable material for the conductor system is copper or aluminium. Steel may be used but has limitations of poor conductivity and high susceptibility to corrosion.

In an effort to make the conductor ideal, three different types have been utilized, and these include:

Flat surfaced Conductors
Stranded Conductors
Tubular Conductors

Insulation

Insulation security has been rated very highly among the aims of good substation design. Extensive research is done on improving flashover characteristics as well as combating pollution. Increased creepage length, resistance glazing, insulation greasing and line washing have been used with varying degrees of success.

Power Transformers

EHV power transformers are usually oil immersed with all three phases in one tank. Auto transformers can offer advantage of smaller physical size and reduced losses. The different classes of power transformers are:

o.n.: Oil immersed, natural cooling
o.b.: Oil immersed, air blast cooling
o.f.n.: Oil immersed, oil circulation forced
o.f.b.: Oil immersed, oil circulation forced, air blast cooling

Power transformers are usually the largest single item in a substation. For economy of service roads, transformers are located on one side of a substation, and the connection to switchgear is by bare conductors. Because of the large quantity of oil, it is essential to take precaution against the spread of fire. Hence, the transformer is usually located around a sump used to collect the excess oil.

Transformers that are located and a cell should be enclosed in a blast proof room.

Overhead Line Terminations

Two methods are used to terminate overhead lines at a substation.

Tensioning conductors to substation structures or buildings
Tensioning conductors to ground winches.

The choice is influenced by the height of towers and the proximity to the substation. The following clearances should be observed:

VOLTAGE LEVEL	MINIMUM GROUND CLEARANCE
less than 66kV	6.1m
66kV - 110kV	6.4m
110kV - 165kV	6.7m
greater than 165kV	7.0m