Power station

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The Susquehanna Steam Electric Station, a boiling water reactor nuclear power plant.
St. Clair Power Plant, a coal-fired plant in Michigan.
The Three Gorges Dam, a hydroelectric dam.

A power station (also referred to as a generating station, power plant, or powerhouse) is an industrial facility for the generation of electric energy.[1][2][3]

At the center of nearly all power stations is a generator, a rotating machine that converts mechanical energy into electrical energy by creating relative motion between a magnetic field and a conductor. The energy source harnessed to turn the generator varies widely. It depends chiefly on which fuels are easily available and on the types of technology that the power company has access to.

Contents

[edit] History

The first power station was the Edison Electric Light Station, built in London at 57, Holborn Viaduct, which started operation in January 1882. This was an initiative of Thomas Edison that was organized and managed by his partner, Edward Johnson. A Babcock and Wilcox boiler powered a 125 horsepower steam engine that drove a 27 ton generator called Jumbo, after the celebrated elephant. This supplied electricity to premises in the area that could be reached through the culverts of the viaduct without digging up the road, which was the monopoly of the gas companies. The customers included the City Temple and the Old Bailey. Another important customer was the Telegraph Office of the General Post Office but this could not be reached though the culverts. Johnson arranged for the supply cable to be run overhead, via Holborn Tavern and Newgate.[4]

[edit] 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 that 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. Not all thermal energy can be transformed into 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 by-product heat for the desalination of water.

The efficiency of a steam turbine is limited by the maximum temperature of the steam produced and is not directly a function of the fuel used. For the same steam conditions, coal, nuclear and gas power plants all have the same theoretical efficiency. Overall, if a system is on constantly (base load) it will be more efficient than one that is used intermittently(peak load)

Besides use of reject heat for process or district heating, one way to improve overall efficiency of a power plant is to combine two different thermodynamic cycles. Most commonly, exhaust gases from a gas turbine are used to generate steam for a boiler and steam turbine. The combination of a "top" cycle and a "bottom" cycle produces higher overall efficiency than either cycle can attain alone.

[edit] Classification

CHP plant in Warsaw, Poland.
Geothermal power station in Iceland.
Coal Power Station in Tampa, United States.

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

[edit] By fuel

[edit] By prime mover

[edit] Cooling towers

Cooling towers evaporating water at Ratcliffe-on-Soar Power Station, United Kingdom.

All thermal power plants produce waste heat energy as a byproduct of the useful electrical energy produced. The amount of waste heat energy equals or exceeds the amount of electrical energy produced. Gas-fired power plants can achieve 50%* conversion efficiency while coal and oil plants achieve around 30-49%*. The waste heat produces a temperature rise in the atmosphere which is small compared to that of greenhouse-gas emissions from the same power plant. Natural draft wet cooling towers at many nuclear power plants and large fossil fuel fired power plants use 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. However, the mechanical induced-draft or forced-draft wet cooling towers in many large thermal power plants, nuclear power plants, fossil fired 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 typically rectangular, box-like structures filled with a material that enhances the contacting of the upflowing air and the downflowing water.[7][8]

In areas with restricted water use a dry cooling tower or radiators, directly air cooled, may be necessary, since the cost or environmental consequences of obtaining 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.

Water consumption by power stations is a developing issue.[9]

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.

[edit] 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.

A hydroelectric dam and plant on the Muskegon river in Michigan, United States.

[edit] Hydroelectricity

Main article: Hydroelectricity

Dams built to produce hydroelectricity impound a reservoir of water and release it through one or more water turbines, connected to generators, and generate electricity, from the energy provided by difference in water level upstream and downstream.

[edit] Pumped storage

A pumped-storage hydroelectric power plant is a net consumer of energy but can be used to smooth peaks and troughs in overall electricity demand. Pumped storage plants typically use "spare" electricity during off peak periods to pump water from a lower reservoir or dam to an upper reservoir. Because the electricity is consumed "off peak" it is typically cheaper than power at peak times. This is because the "base load" power stations, which are typically coal fired, cannot be switched on and off quickly so remain in service even when demand is low. During hours of peak demand, when the electricity price is high, the water pumped to the high reservoir is allowed to flow back to the lower reservoir through a water turbine connected to an electricity generator. Unlike coal power stations, which can take more than 12 hours to to start up from cold, the hydroelectric plant can be brought into service in a few minutes, ideal to meet a peak load demand. Two substantial pumped storage schemes are in South Africa, one to the East of Cape Town (Palmiet) and one in the Drakensberg, Natal

[edit] Solar

Main article: Solar power

Solar energy can be turned into electricity either directly in solar cells, or in a concentrating solar power plant by focusing the light to run a heat engine.

Nellis Solar Power Plant in the United States.

A solar photovoltaic power plant converts sunlight into direct current electricity using the photoelectric effect. Inverters change the direct current into alternating current for connection to the electrical grid. This type of plant does not use rotating machines for energy conversion.

Solar thermal power plants are another type of solar power plant. They use either parabolic troughs or heliostats to direct sunlight onto a pipe containing a heat transfer fluid, such as oil. The heated oil is then used to boil water into steam, which turns a turbine that drives an electrical generator. The central tower type of solar thermal 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 that drive electrical generators.

There is yet another type of solar thermal electric plant. The sunlight strikes the bottom of a water pond, warming the lowest layer of water which is prevented from rising by a salt gradient. A Rankine cycle engine exploits the temperature difference in the water layers to produce electricity.

Not many solar thermal electric plants have been built. Most of them can be found in the Mojave Desert of the United States although Sandia National Laboratory (again in the United States), Israel and Spain have also built a few plants.

[edit] Wind

Main article: Wind power
Wind turbine in front of a thermal power station in Amsterdam, the Netherlands.

Wind turbines can be used to generate electricity in areas with strong, steady winds, sometimes offshore. Many different designs have been used in the past, but almost all modern turbines being produced today use a three-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. Old turbines are still used at some wind farms, for example at Altamont Pass and Tehachapi Pass.

[edit] Typical power output

The power generated by a power station is measured in multiples of the watt, typically megawatts (106 watts) or gigawatts (109 watts). Power stations vary greatly in capacity depending on the type of power plant and on historical, geographical and economic factors. The following examples offer a sense of the scale.

Many of the largest operational onshore wind farms are located in the USA. As of 2011, the Roscoe Wind Farm is the largest onshore wind farm in the world, producing 781.5 MW of power, followed by the Horse Hollow Wind Energy Center (735.5 MW). As of November 2010, the Thanet Offshore Wind Project in United Kingdom is the largest offshore wind farm in the world at 300 MW, followed by Horns Rev II (209 MW) in Denmark.

As of 2011, the [[List of photovoltaic power stations|largest photovoltaic (PV) power plants in the world]] are rated up to 97 megawatts. [10] Larger power stations are under construction, some proposed will have a capacity of 150 MW or more.[11] A planned installation in China will produce 2000 megawatts at peak. [12]

Solar thermal power stations in the U.S. have the following output:

The country's largest solar facility at Kramer Junction has an output of 354 MW
The planned Blythe Solar Power Project will produce an estimated 968 MW

Large coal-fired, nuclear, and hydroelectric power stations can generate hundreds of Megawatts to multiple Gigawatts. Some examples:

The Three Mile Island Nuclear Generating Station in the USA has a rated capacity of 802 megawatts.
The coal-fired Ratcliffe-on-Soar Power Station in the UK has a rated capacity of 2 gigawatts.
The Aswan Dam hydro-electric plant in Egypt has a capacity of 2.1 gigawatts.
The Three Gorges Dam hydro-electric plant in China will have a capacity of 22.5 gigawatts when complete; 18.2 gigawatts capacity is operating as of 2010.

Gas turbine power plants can generate tens to hundreds of megawatts. Some examples:

The Indian Queens simple-cycle peaking power station in Cornwall UK, with a single gas turbine is rated 140 megawatts.
The Medway Power Station, a combined-cycle power station in Kent, UK with two gas turbines and one steam turbine, is rated 700 megawatts.[13]

The rated capacity of a power station is nearly the maximum electrical power that that power station can produce. Some power plants are run at almost exactly their rated capacity all the time, as a non-load-following base load power plant, except at times of scheduled or unscheduled maintenance.

However, many power plants usually produce much less power than their rated capacity.

In some cases a power plant produces much less power than its rated capacity because it uses an intermittent energy source. Operators try to pull maximum available power from such power plants, because their marginal cost is practically zero, but the available power varies widely—in particular, it may be zero during heavy storms at night.

In some cases operators deliberately produce less power for economic reasons. The cost of fuel to run a load following power plant may be relatively high, and the cost of fuel to run a peaking power plant is even higher—they have relatively high marginal costs. Operators keep them turned off ("operational reserve") or running at minimum fuel consumption[citation needed] ("spinning reserve") most of the time. Operators feed more fuel into load following power plants only when the demand rises above what lower-cost plants (i.e., intermittent and base load plants) can produce, and then feed more fuel into peaking power plants only when the demand rises faster than the load following power plants can follow.

[edit] Operations

The power station operator has several duties in the electricity-generating facility. Operators are responsible for the safety of the work crews that frequently do repairs on the mechanical and electrical equipment. They maintain the equipment with periodic inspections and log temperatures, pressures and other important information at regular intervals. Operators are responsible for starting and stopping the generators depending on need. They are able to synchronize and adjust the voltage output of the added generation with the running electrical system without upsetting the system. They must know the electrical and mechanical systems in order to troubleshoot problems in the facility and add to the reliability of the facility. Operators must be able to respond to an emergency and know the procedures in place to deal with it.

[edit] See also

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Wikimedia Commons has media related to: Power plants

[edit] References

  1. ^ British Electricity International (1991). Modern Power Station Practice: incorporating modern power system practice (3rd Edition (12 volume set) ed.). Pergamon. ISBN 0-08-040510-X. 
  2. ^ Babcock & Wilcox Co. (2005). Steam: Its Generation and Use (41st edition ed.). ISBN 0-9634570-0-4. 
  3. ^ Thomas C. Elliott, Kao Chen, Robert Swanekamp (coauthors) (1997). Standard Handbook of Powerplant Engineering (2nd edition ed.). McGraw-Hill Professional. ISBN 0-07-019435-1. 
  4. ^ Jack Harris (14 January 1982), "The electricity of Holborn", New Scientist, http://books.google.co.uk/books?id=bfVKt7UzjnEC&pg=PA89 
  5. ^ Nuclear Power Plants Information, by International Atomic Energy Agency
  6. ^ SWEB's Pocket Power Stations
  7. ^ J.C. Hensley (Editor) (2006). Cooling Tower Fundamentals (2nd Ed. ed.). SPX Cooling Technologies. 
  8. ^ Beychok, Milton R. (1967). Aqueous Wastes from Petroleum and Petrochemical Plants (4th Edition ed.). John Wiley and Sons. LCCN 67019834.  (Includes cooling tower material balance for evaporation emissions and blowdown effluents. Available in many university libraries)
  9. ^ AAAS Annual Meeting 17 - 21 Feb 2011, Washington DC. Sustainable or Not? Impacts and Uncertainties of Low-Carbon Energy Technologies on Water. Dr Evangelos Tzimas , European Commission, JRC Institute for Energy, Petten, Netherlands
  10. ^ Denis Lenardic. Large-scale photovoltaic power plants ranking 1 - 50 PVresources.com, 2010.
  11. ^ Mark Z. Jacobson (2009). Review of Solutions to Global Warming, Air Pollution, and Energy Security p. 4.
  12. ^ http://blogs.worldbank.org/climatechange/will-china-and-us-be-partners-or-rivals-new-energy-economy
  13. ^ CCGT Plants in South England, by Power Plants Around the World

[edit] External links

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