1.Electric power, like mechanical power, is represented by the letter P in electrical equations. The term wattage is used colloquially to mean 'electric power in watts'.In direct current resistive circuits, instantaneous electrical power is calculated using Joule's Law, which is named after the British physicist James Joule, who first showed that electrical and mechanical energy were interchangeable.whereP is the power (watt or W)I is the current (ampere or A)V is the potential difference (volt or V)For example:.Joule's law can be combined with Ohm's law to produce two more equations:whereR is the resistance (Ohm or Ω).For example:andIn alternating current circuits, energy storage elements such as inductance and capacitance may result in periodic reversals of the direction of energy flow. The portion of power flow that, averaged over a complete cycle of the AC waveform, results in net transfer of energy in one direction is known as real power (also referred to as active power). That portion of power flow due to stored energy, that returns to the source in each cycle, is known as reactive power.Power triangle The components of AC powerThe relationship between real power, reactive power and apparent power can be expressed by representing the quantities as vectors. Real power is represented as a horizontal vector and reactive power is represented as a vertical vector. The apparent power vector is the hypotenuse of a right triangle formed by connecting the real and reactive power vectors. This representation is often called the power triangle. Using the Pythagorean Theorem, the relationship among real, reactive and apparent power is:(apparent power)2 = (real power)2 + (reactive power)2The ratio of real power to apparent power is called power factor and is a number always between 0 and 1.2. In a fossil fuel power plant the chemical energy stored in fossil fuels such as coal, fuel oil, natural gas or oil shale is converted successively into thermal energy, mechanical energy and, finally, electrical energy for continuous use and distribution across a wide geographic area. Almost all large fossil fuel power plants are steam-electric power plants, except for gas turbines and utility-sized reciprocating engines that may run on natural gas or diesel.The burning of fossil fuel is summarized in the following chemical reaction:and the simple word equation for this chemical reaction is:All fossil fuels generate carbon dioxide, when combusted. Chemical side reactions also take place, generating, among others, sulfur dioxide (predominantly in coal) and oxides of nitrogen. Each fossil fuel power plant is a highly complex, custom-designed system. Present construction costs, as of 2004, run to US$1,300 per kilowatt, or $650 million for a 500 MWe unit. Multiple generating units may be built at a single site for more efficient use of land, natural resources and labor.Coal Power Station in Tampa FL[edit] Fuel transport and deliveryCoal is delivered by mass transport systems, truck, rail, barge or collier. A large coal train called a "unit train" may be two kilometers long, containing 100 cars with 100 tons of coal in each one, for a total load of 10,000 tons. A large plant under full load requires at least one coal delivery this size every day. Plants may get as many as three to five trains a day, especially in "peak season", during the summer months when power consumption is high. Modern unloaders use rotary dump devices, which eliminate problems with coal freezing in bottom dump cars. The unloader includes a train positioner arm that pulls the entire train to position each car over a coal hopper. The dumper clamps an individual car against a platform that swivels the car upside down to dump the coal. Swiveling couplers enable the entire operation to occur while the cars are still coupled together. Unloading a unit train takes about three hours. A large thermal power plant such as the one at Nanticoke Ontario stores several million tons of coal for winter use when the lakes are frozen.Shorter trains may use railcars with an "air-dump", which relies on air pressure from the engine plus a "hot shoe" on each car. This "hot shoe" when it comes into contact with a "hot rail" at the unloading trestle, shoots an electric charge through the air dump apparatus and causes the doors on the bottom of the car to open, dumping the coal through the opening in the trestle. Unloading one of these trains takes anywhere from an hour to an hour and a half. Older unloaders may still use manually operated bottom-dump rail cars and a "shaker" attached to dump the coal. Generating stations adjacent to a mine may receive coal by conveyor belt or massive diesel-electric-drive trucks.A collier (cargo ship carrying coal) may hold 40,000 tons of coal and takes several days to unload. Some colliers carry their own conveying equipment to unload their own bunkers; others depend on equipment at the plant. Colliers are large, seaworthy, self-powered ships. For transporting coal in calmer waters, such as rivers and lakes, flat-bottomed vessels called barges are often used. Barges are usually unpowered and must be moved by tugboats or towboats.For startup or auxiliary purposes, the plant may use fuel oil as well. Fuel oil can be delivered to plants by pipeline, tanker, tank car or truck. Oil is stored in vertical cylindrical steel tanks as large as 90,000 barrels (14,000 m³). The heavier no. 5 "bunker" and no. 6 fuels are steam-heated before pumping in cold climates.Plants fueled by natural gas are usually built adjacent to gas transport pipelines or have dedicated gas pipelines extended to them.[edit] Fuel processingCoal is prepared for use by crushing the rough coal to pieces less than 2 inches (50 mm) in size. The coal is transported from the storage yard to in-plant storage silos by rubberized conveyer belts at rates up to 4,000 tons per hour. A 400 ton silo may feed each coal pulverizer (coal mill) at a rate of up to 60 tons per hour. Coal fed into the top of the pulverizer and ground to a powder, the consistency of face powder, is blown, with air, into the furnace. A 500 MWe plant will have six such pulverizers, five of which can supply coal to the furnace at 250 tons per hour under full load.[edit] Feedwater heating and deaerationThe feedwater used in the steam boiler is a means of transferring heat energy from the burning fuel to the mechanical energy of the spinning steam turbine. The total feedwater consists of recirculated condensed steam, referred to as condensate, from the steam turbines plus purified makeup water. Because the metallic materials it contacts are subject to corrosion at high temperatures and pressures, the makeup water is highly purified before use. A system of water softeners and ion exchange demineralizers produces water so pure that it coincidentally becomes an electrical insulator, with conductivity in the range of 0.3–1.0 microsiemens per centimeter. The makeup water in a 500 MWe plant amounts to perhaps 20 US gallons per minute (1.25 L/s) to offset the small losses from steam leaks in the system.The feedwater cycle begins with condensate water being pumped out of the condenser after travelling through the steam turbines. The condensate flow rate at full load in a 500 MWe plant is about 6,000 US gallons per minute (0.38 m³/s).Diagram of boiler feed water deaerator (with vertical, domed aeration section and horizontal water storage sectionThe water flows through a series of six or seven intermediate feedwater heaters, heated up at each point with steam extracted from an appropriate duct on the turbines and gaining temperature at each stage. Typically, the condensate plus the makeup water then flows through a deaerator[1][2] that removes dissolved air from the water, further purifying and reducing its corrosivity. The water may be dosed following this point with hydrazine, a chemical that removes the remaining oxygen in the water to below 5 parts per billion (ppb). It is also dosed with pH control agents such as ammonia or morpholine to keep the residual acidity low and thus non-corrosive.[edit] Boiler operationDiagram of a steam power plant boiler.The boiler is a rectangular furnace about 50 ft (15 m) on a side and 130 ft (40 m) tall. Its walls are made of a web of high pressure steel tubes about 2.3 inches (60 mm) in diameter.Pulverized coal is air-blown into the furnace from fuel nozzles at the four corners and it rapidly burns, forming a large fireball at the center. The thermal radiation of the fireball heats the water that circulates through the boiler tubes near the boiler perimeter. The water circulation rate in the boiler is three to four times the throughput and is typically driven by pumps. As the water in the boiler circulates it absorbs heat and changes into steam at 700 °F (370 °C) and 3,200 psi (22.1 MPa). It is separated from the water inside a drum at the top of the furnace. The saturated steam is introduced into superheat pendant tubes that hang in the hottest part of the combustion gases as they exit the furnace. Here the steam is superheated to 1,000 °F (540 °C) to prepare it for the turbine.Plants designed for lignite (brown coal) are increasingly used in locations as varied as Germany, Victoria, and North Dakota. Lignite is a much younger form of coal than black coal. It has a lower energy density than black coal and requires a much larger furnace for equivalent heat output. Such coals may contain up to 70% water and ash, yielding lower furnace temperatures and requiring larger induced-draft fans. The firing systems also differ from black coal and typically draw hot gas from the furnace-exit level and mix it with the incoming coal in fan-type mills that exhaust the pulverised coal and hot gas mixture into the boiler.Plants that use gas turbines to heat the water for conversion into steam use boilers known as HRSGs, Heat Recovery Steam Generators. The exhaust (waste) heat from the gas turbines is used to make superheated steam that is then used in a conventional water-steam generation cycle.[edit] Steam turbine generatorThe turbine generator consists of a series of steam turbines interconnected to each other and a generator on a common shaft. There is a high pressure turbine at one end, followed by an intermediate pressure turbine, two low pressure turbines, and the generator. As steam moves through the system and loses pressure and temperature energy it expands in volume, requiring increasing diameter and longer blades at each succeeding stage to extract the remaining energy. The entire rotating mass may be over 200 tons and 100 ft (30 m) long. It is so heavy that it must be kept turning slowly even when shut down (at 3 rpm) so that the shaft will not bow even slightly and become unbalanced. This is so important that it is one of only four functions of blackout emergency power batteries on site. They are emergency lighting, communication, station alarms and turbogenerator lube oil.Superheated steam from the boiler is delivered through 14–16 inch (350–400 mm) diameter piping to the high pressure turbine where it falls in pressure to 600 psi (4 MPa) and to 600 °F (315 °C) through the stage. It exits via 24–26 inch (600–650 mm) diameter cold reheat lines and passes back into the boiler where the steam is reheated in special reheat pendant tubes back to 1,000 °F (540 °C). The hot reheat steam is conducted to the intermediate pressure turbine where it falls in both temperature and pressure and exits directly to the long-bladed low pressure turbines and finally exits to the condenser.The generator, 30 ft (9 m) long and 12 ft (3.7 m) diameter, contains a stationary stator and a spinning rotor, each containing miles of heavy copper conductor—no permanent magnets here. In operation it generates up to 21,000 amps at 24,000 volts AC (504 MWe) as it spins at either 3,000 or 3,600 RPM, synchronized to the power grid. The rotor spins in a sealed chamber cooled with hydrogen gas, selected because it has the highest known heat transfer coefficient of any gas and for its low viscosity which reduces windage losses. This system requires special handling during startup, with air in the chamber first displaced by carbon dioxide before filling with hydrogen. This ensures that the highly flammable hydrogen does not mix with oxygen in the air.The power grid frequency is 60 Hz across North America and 50 Hz in Europe, Oceania, Asia (Korea and parts of Japan are notable exceptions) and parts of Africa.The electricity flows to a distribution yard where transformers step the voltage up to 115, 230, 500 or 765 kV AC as needed for transmission to its destination.[edit] Steam condensingDiagram of a typical water-cooled surface condenserThe lower is the pressure of the exhaust steam leaving the low pressure turbine, the more efficient is the train of turbine stages. The exhaust steam from the low pressure turbine is condensed in a shell and tube heat exchanger commonly referred to as a surface condenser. Cooling water circulates through the tubes in the condenser's shell and the low pressure exhaust steam is condensed by flowing over the tubes as shown in the adjacent diagram. Typically the cooling water causes the steam to condense at a temperature of about 32–38 °C (90–100 °F) and that creates an absolute pressure in the condenser of about 5–7 kPa (1.5–2.0 in Hg), a vacuum of about 95 kPa (28 mmHg) relative to atmospheric pressure. The condenser, in effect, creates the low pressure required to drag steam through and increase the efficiency of the turbines. The limiting factor is the temperature of the cooling water and that, in turn, is limited by the prevailing average climatic conditions at the power plant's location.From the bottom of the condenser, powerful pumps recycle the condensed steam (water) back to the feedwater heaters for reuse. The heat absorbed by the circulating cooling water in the condenser tubes must also be removed to maintain the ability of the water to cool as it circulates.3. Theoretical annual mean insolation, at the top of Earth's atmosphere (top) and at the surface on a horizontal square meter.Map of global solar energy resources. The colours show the average available solar energy on the surface (as measured from 1991 to 1993). For comparison, the dark disks represent the land area required to supply the total primary energy demand using PVs with a conversion efficiency of 8%.Solar radiation reaches the Earth's upper atmosphere at a rate of 1366 watts per square meter (W/m2).[1] The first map shows how the solar energy varies in different latitudes.While traveling through the atmosphere 6% of the incoming solar radiation (insolation) is reflected and 16% is absorbed resulting in a peak irradiance at the equator of 1,020 W/m².[2] Average atmospheric conditions (clouds, dust, pollutants) further reduce insolation by 20% through reflection and 3% through absorption.[3] Atmospheric conditions not only reduce the quantity of insolation reaching the Earth's surface but also affect the quality of insolation by diffusing incoming light and altering its spectrum.The second map shows the average global irradiance calculated from satellite data collected from 1991 to 1993. For example, in North America the average insolation at ground level over an entire year (including nights and periods of cloudy weather) lies between 125 and 375 W/m² (3 to 9 kWh/m²/day).[4] This represents the available power, and not the delivered power. At present, photovoltaic panels typically convert about 15% of incident sunlight into electricity; therefore, a solar panel in the contiguous United States on average delivers 19 to 56 W/m² or 0.45 - 1.35 kWh/m²/day.[5]The dark disks in the third map on the right are an example of the land areas that, if covered with 8% efficient solar panels, would produce slightly more energy in the form of electricity than the total world primary energy supply in 2003.[6] While average insolation and power offer insight into solar power's potential on a regional scale, locally relevant conditions are of primary importance to the potential of a specific site.After passing through the Earth's atmosphere, most of the sun's energy is in the form of visible and Infrared radiations. Plants use solar energy to create chemical energy through photosynthesis. Humans regularly use this energy burning wood or fossil fuels, or when simply eating the plants.A recent concern is global dimming, an effect of pollution that is allowing less sunlight to reach the Earth's surface. It is intricately linked with pollution particles and global warming, and it is mostly of concern for issues of global climate change, but is also of concern to proponents of solar power because of the existing and potential future decreases in available solar energy. The order of magnitude is about 4% less solar energy available at sea level over the timeframe of 1961–90, mostly from increased reflection from clouds back into outer space.[7][edit] Types of technologiesMany technologies have been developed to make use of solar radiation. Some of these technologies make direct use of the solar energy (e.g. to provide light, heat, etc.), while others produce electricity.[edit] Solar design in architectureMain article: Passive solar building designSolar design in architecture involves the use of appropriate solar technologies to maintain a building’s environment at a comfortable temperature through the sun's daily and annual cycles. It may do this by storing solar energy as heat in the walls of a building, which then acts to heat the building at night. Another approach is to keep the interior cool during a hot day by designing in natural convection through the building’s interior.[edit] Solar heating systemsMain articles: Solar hot water and Solar combisystemSolar water heaters, on a rooftop in Jerusalem, IsraelSolar hot water systems use sunlight to heat water. Solar hot water systems were used extensively in the United States up to the 1920s until replaced by relatively cheap and more reliable conventional heating fuels. The economic advantage of conventional heating fuels has varied over time resulting in periodic interest in solar hot water; however, solar hot water and heating technologies have yet to show the sustained momentum they lost in the 1920s. That being said, the recent spikes and erratic availability of conventional fuels has resulted in a renewed interest in solar heating technologies.On a technical level, solar water heating is particularly appropriate for low temperature applications (100-150F). This advantage has been successfully applied to heating swimming pools where solar water heating can economically increase pool use. Solar water heating is also used in stand alone or hybrid domestic water heating systems.Solar water heating systems are composed of solar thermal collectors, a storage tank and a circulation loop.[8] The three basic classifications of solar water heaters are:Batch systems which consist of a tank that is directly heated by sunlight. These are the oldest and simplest solar water heater designs, however; the exposed tank can be vulnerable to cooldown.[9]Active systems which use pumps to circulate water or a heat transfer fluid.Passive systems which circulate water or a heat transfer fluid by natural circulation. These are also called thermosiphon systems.A Trombe wall is a passive solar heating and ventilation system consisting of an air channel sandwiched between a window and a sun-facing wall. Sunlight heats the air space during the day causing natural circulation through vents at the top and bottom of the wall and storing heat in the thermal mass. During the evening the Trombe wall radiates stored heat.[10]A transpired collector is a perforated sun-facing wall. The wall absorbs sunlight and pre-heats air up 40F as it is drawn into the building's ventilation system. These systems are inexpensive and pay for themselves within 3-12 years in offset heating costs.[11][edit] Solar cookingMain article: Solar cookerSolar Cookers use sunshine as a source of heat for cooking as an alternative to fire.A solar box cooker traps the sun's energy in an insulated box; such boxes have been successfully used for cooking, pasteurization and fruit canning. Solar cooking is helping many developing countries, both reducing the demands for local firewood and maintaining a cleaner breathing environment for the community.The first known western solar oven is attributed to Horace de Saussure in 1767, which impressed Sir John Herschel enough to build one for cooking meals on his astronomical expedition to the Cape of Good Hope in Africa in 1830.[12] Today, there are many different designs in use around the world.[13][edit] Solar lightingMain articles: Daylighting and Light tubeDaylighting is a passive solar method of using natural light to provide illumination. Daylighting directly offsets energy use in electric lighting systems and indirectly offsets energy use through a reduction in cooling load.[14] Although difficult to quantify, the use of natural light also offers physiological and psychological benefits compared to conventional lighting.Daylighting features include building orientation, window orientation, exterior shading, sawtooth roofs, clerestory windows, light shelves, skylights and light tubes.[15] These features may be incorporated in existing structures but are most effective when integrated in a solar design package which accounts for factors such as glare, heat gain, heat loss and time-of-use. Architectural trends increasingly recognize daylighting as a cornerstone of sustainable design.Hybrid solar lighting (HSL) is an active solar method of using natural light to provide illumination. Hybrid solar lighting systems collect sunlight using focusing mirrors that track the sun. The collected light is transmitted via optical fibers into a building's interior to supplement conventional lighting.[16]Daylight saving time (DST) can be seen as a method of utilising solar energy by matching available sunlight to the hours of the day in which it is most useful. In 2001 this was estimated to reduce peak demand in California by 35–70 MW (0.08%–0.16%) in June through August, though total electricity use was unaffected.[17] However, there is some question whether these estimates are valid. In 2000 when parts of Australia began DST in late winter, overall electricity consumption did not decrease, but the peak load increased.[18][edit] Solar electricityMain article: PhotovoltaicsKyocera headquarters. PV cells on the side of the building generate electricity from sunlight.The solar panels (photovoltaic arrays) on this small yacht at sea can charge the 12 V batteries at up to 9 A in full, direct sunlightSolar cells, also referred to as photovoltaic cells, are devices or banks of devices that use the photovoltaic effect of semiconductors to generate electricity directly from sunlight. Until recently, their use has been limited because of high manufacturing costs. One cost effective use has been in very low-power devices such as calculators with LCDs. Another use has been in remote applications such as roadside emergency telephones, remote sensing, cathodic protection of pipe lines, and limited "off grid" home power applications. A third use has been in powering orbiting satellites and spacecraft.To take advantage of the incoming electromagnetic radiation from the sun, solar panels can be attached to each house or building. The panels should be mounted perpendicular to the arc of the sun to maximize usefulness. The easiest way to use this electricity is by connecting the solar panels to a grid tie inverter. However, these solar panels may also be used to charge batteries or other energy storage device. Solar panels produce more power during summer months because they receive more sunlight. The cost payback time may take over 10 years depending on the cost of grid electricity and tax rebates.Total peak power of installed PV is around 3,700 MW as of the end of 2005.[19] This is only one part of solar-generated electric power.Declining manufacturing costs (dropping at 3 to 5% a year in recent years) are expanding the range of cost-effective uses. The average lowest retail cost of a large photovoltaic array declined from $7.50 to $4 per watt between 1990 and 2005.[20] With many jurisdictions now giving tax and rebate incentives, solar electric power can now pay for itself in five to ten years in many places. "Grid-connected" systems - those systems that use an inverter to connect to the utility grid instead of relying on batteries - now make up the largest part of the market.In 2003, worldwide production of solar cells increased by 32%.[21] Between 2000 and 2004, the increase in worldwide solar energy capacity was an annualized 60%.[22] 2005 was expected to see large growth again, but shortages of refined silicon have been hampering production worldwide since late 2004.[23] Analysts have predicted similar supply problems for 2006 and 2007.[24][edit] Solar thermal electric power plantsSolar Two, a concentrating solar power tower (an example of solar thermal energy applied to electrical power production).Main article: Solar thermal energySolar thermal energy can be focused on a heat exchanger, and converted in a heat engine to produce electric power or applied to other industrial processes.4. Hydropower is the capture of the energy of moving water for some useful purpose. Prior to the widespread availability of commercial electric power, hydropower was used for irrigation, milling of grain, textile manufacture, and the operation of sawmills.The energy of moving water has been exploited for centuries; In India, water wheels and watermills were built; in Imperial Rome, water powered mills produced flour from grain, and in China and the rest of the Far East, hydraulically operated "pot wheel" pumps that raised water into irrigation canals. In the 1830s, at the peak of the canal-building era, hydropower was used to transport barge traffic up and down steep hills using inclined plane railroads. Direct mechanical power transmission required that industries using hydropower had to locate near the waterfall. For example, during the last half of the 19th century, many grist mills were built at Saint Anthony Falls, utilizing the 50 foot (15 metre) drop in the Mississippi River. The mills contributed to the growth of Minneapolis. Today the largest use of hydropower is for electric power generation, which allows low cost energy to be used at long distances from the watercourse.Types of water powerThere are many forms of water power:· Waterwheels , used for hundreds of years to power mills and machinery· Hydroelectric energy, usually referring to hydroelectric dams or run-of-the-river setups.· Tidal power, which captures energy from the tides in horizontal direction· Tidal stream power, which does the same vertically· Wave power, which uses the energy in waves[edit] Hydroelectric powerMain article: HydroelectricityHydraulic turbine and electrical generator.Hydroelectric power now supplies about 715,000 MWe or 19% of world electricity (16% in 2003). Large dams are still being designed. Apart from a few countries with an abundance of it, hydro power is normally applied to peak load demand because it is readily stopped and started. Nevertheless, hydroelectric power is probably not a major option for the future of energy production in the developed nations because most major sites within these nations are either already being exploited or are unavailable for other reasons, such as environmental considerations.Hydropower produces essentially no carbon dioxide or other harmful emissions, in contrast to burning fossil fuels, and is not a significant contributor to global warming through CO2.Hydroelectric power can be far less expensive than electricity generated from fossil fuels or nuclear energy. Areas with abundant hydroelectric power attract industry. Environmental concerns about the effects of reservoirs may prohibit development of economic hydropower sources.The chief advantage of hydroelectric dams is their ability to handle seasonal (as well as daily) high peak loads. When the electricity demands drop, the dam simply stores more water (which provides more flow when it releases). Some electricity generators use water dams to store excess energy (often during the night), by using the electricity to pump water up into a basin. Electricity can be generated when demand increases. In practice the utilization of stored water in river dams is sometimes complicated by demands for irrigation which may occur out of phase with peak electrical demands.[edit] Tidal powerMain article: Tidal powerHarnessing the tides in a bay or estuary has been achieved in France (since 1966), Canada and Russia, and could be achieved in other areas with a large tidal range. The trapped water turns turbines as it is released through the tidal barrage in either direction. Another possible fault is that the system would generate electricity most efficiently in bursts every six hours (once every tide). This limits the applications of tidal energy.[edit] Tidal stream powerA relatively new technology, tidal stream generators draw energy from currents in much the same way that wind generators do. The higher density of water means that a single generator can provide significant power. This technology is at the early stages of development and will require more research before it becomes a significant contributor.Several prototypes have shown promise. In the UK in 2003, a 300 kW Periodflow marine current propeller type turbine was tested off the coast of Devon, and a 150 kW oscillating hydroplane device, the Stingray, was tested off the Scottish coast. Another British device, the Hydro Venturi, is to be tested in San Francisco Bay.The Canadian company Blue Energy has plans for installing very large arrays tidal current devices mounted in what they call a 'tidal fence' in various locations around the world, based on a vertical axis turbine design.[edit] Wave powerMain article: Wave powerHarnessing power from ocean surface wave motion might yield much more energy than tides. The feasibility of this has been investigated, particularly in Scotland in the UK. Generators either coupled to floating devices or turned by air displaced by waves in a hollow concrete structure would produce electricity. Numerous technical problems have frustrated progress.A prototype shore based wave power generator is being constructed at Port Kembla in Australia and is expected to generate up to 500 MWh annually. The Wave Energy Converter has been constructed (as of July 2005) and initial results have exceeded expectations of energy production during times of low wave energy. Wave energy is captured by an air driven generator and converted to electricity. For countries with large coastlines and rough sea conditions, the energy of waves offers the possibility of generating electricity in utility volumes. Excess power during rough seas could be used to produce hydrogen.5.An estimated 1% to 3% of energy from the Sun that hits the earth is converted into wind energy. This is about 50 to 100 times more energy than is converted into biomass by all the plants on Earth through photosynthesis.[citation needed] Most of this wind energy can be found at high altitudes where continuous wind speeds of over 160 km/h (100 mph) occur. Eventually, the wind energy is converted through friction into diffuse heat throughout the Earth's surface and the atmosphere.The origin of wind is complex. The Earth is unevenly heated by the sun resulting in the poles receiving less energy from the sun than the equator does. Also the dry land heats up (and cools down) more quickly than the seas do. The differential heating powers a global atmospheric convection system reaching from the Earth's surface to the stratosphere which acts as a virtual ceiling.[edit] Wind variability and turbine powerA Darrieus wind turbine.The power in the wind can be extracted by allowing it to blow past moving wings that exert torque on a rotor. The amount of power transferred is directly proportional to the density of the air, the area swept out by the rotor, and the cube of the wind speed.The power P available in the wind is given by:The mass flow of air that travels through the swept area of a wind turbine varies with the wind speed and air density. As an example, on a cool 15°C (59°F) day at sea level, air density is 1.225 kilograms per cubic metre. An 8 m/s breeze blowing through a 100 meter diameter rotor would move almost 77,000 kilograms of air per second through the swept area.The kinetic energy of a given mass varies with the square of its velocity. Because the mass flow increases linearly with the wind speed, the wind energy available to a wind turbine increases as the cube of the wind speed. The power of the example breeze above through the example rotor would be about 2.5 megawatts.As the wind turbine extracts energy from the air flow, the air is slowed down, which causes it to spread out and diverts it around the wind turbine to some extent. Albert Betz, a German physicist, determined in 1919 (see Betz' law) that a wind turbine can extract at most 59% of the energy that would otherwise flow through the turbine's cross section. The Betz limit applies regardless of the design of the turbine.Distribution of wind speed (red) and energy (blue) for all of 2002 at the Lee Ranch facility in Colorado. The histogram shows measured data, while the curve is the Rayleigh model distribution for the same average wind speed. Energy is the Betz limit through a 100 meter diameter circle facing directly into the wind. Total energy for the year through that circle was 15.4 gigawatt-hours.Windiness varies, and an average value for a given location does not alone indicate the amount of energy a wind turbine could produce there. To assess the climatology of wind speeds at a particular location, a probability distribution function is often fit to the observed data. Different locations will have different wind speed distributions. The distribution model most frequently used to model wind speed climatology is a two-parameter Weibull distribution because it is able to conform to a wide variety of distribution shapes, from Gaussian to exponential. The Rayleigh model, an example of which is shown plotted against an actual measured dataset, is a specific form of the Weibull function in which the shape parameter equals 2, and very closely mirrors the actual distribution of hourly wind speeds at many locations.Because so much power is generated by higher windspeed, much of the average power available to a windmill comes in short bursts. The 2002 Lee Ranch sample is telling; half of the energy available arrived in just 15% of the operating time. The consequence is that wind energy does not have as consistent an output as fuel-fired power plants; additional output can only be made to compensate for load increase by utilizing advanced wind storing technologies (e.g. giant compressed air storage-tank facilities). — Since wind speed is not constant, a wind generator's annual energy production is never as much as its nameplate rating multiplied by the total hours in a year. The ratio of actual productivity in a year to this theoretical maximum is called the capacity factor. A well-sited wind generator will have a capacity factor of about 35%. This compares to typical capacity factors of 90% for nuclear plants, 70% for coal plants, and 30% for oil plants.[4] When comparing the size of wind turbine plants to fueled power plants, it is important to note that 1000 kW of wind-turbine potential power would be expected to produce as much energy in a year as approximately 500 kW of coal-fired generation. Though the short-term (hours or days) output of a wind-plant is not completely predictable, the annual output of energy tends to vary only a few percent points between years. — When storage, such as with pumped hydroelectric storage, or other forms of generation are used to "shape" wind power (by assuring constant delivery reliability), commercial delivery represents a cost increase of about 25%, yielding viable commercial performance.[5] Electricity consumption can be adapted to production variability to some extent with Energy Demand Management and smart meters that offer variable market pricing over the course of the day. For example, municipal water pumps that feed a water tower do not need to operate continuously and can be restricted to times when electricity is plentiful and cheap. Consumers could choose when to run the dishwasher or charge an electric vehicle, making it very convenient.[edit] Turbine placementMap of available wind power over the United States. Color codes indicate wind power density class.As a general rule, wind generators are practical where the average wind speed is 10 mph (16 km/h or 4.5 m/s) or greater. Usually sites are pre-selected on basis of a wind atlas, and validated with wind measurements. Obviously, meteorology plays an important part in determining possible locations for wind parks, though it has great accuracy limitations. Meteorological wind data is not usually sufficient for accurate siting of a large wind power project. Site Specific Meteorological Data is crucial to determining site potential. An 'ideal' location would have a near constant flow of non-turbulent wind throughout the year and would not suffer too many sudden powerful bursts of wind. An important turbine siting factor is access to local demand or transmission capacity.The most crucial step in the development of a potential wind site is the collection of accurate and verifiable wind speed and direction data as well as other site parameters.[6] To collect wind data a Meteorological Tower is installed at the potential site with instrumentation installed at various heights along the tower. All towers include anemometers to determine the wind speed and wind vanes to determine the direction. The towers generally vary in height from 30 to 60 meters. Some meteorological towers are much taller and more permanent like the Obninsk Meteorological Tower in Russia at 315 meters. The towers primarily used in determining site feasibility for potential wind farms are guyed steel-pipe structures which are left to collect data for one to two years and then usually disassembled. Data is collected by a data logging device which stores and transmits data to a server where it is analyzed.The wind blows faster at higher altitudes because of the reduced influence of drag of the surface (sea or land) and the reduced viscosity of the air. The increase in velocity with altitude is most dramatic near the surface and is affected by topography, surface roughness, and upwind obstacles such as trees or buildings. Typically, the increase of wind speeds with increasing height follows a logarithmic profile that can be reasonably approximated by the wind profile power law, using an exponent of 1/7th, which predicts that wind speed rises proportionally to the seventh root of altitude. Doubling the altitude of a turbine, then, increases the expected wind speeds by 10% and the expected power by 34% (calculation: increase in power = (2.0) ^(3/7) – 1 = 34%).Wind farms or wind parks often have many turbines installed. Since each turbine extracts some of the energy of the wind, it is important to provide adequate spacing between turbines to avoid excess energy loss. Where land area is sufficient, turbines are spaced three to five rotor diameters apart perpendicular to the prevailing wind, and five to ten rotor diameters apart in the direction of the prevailing wind, to minimize efficiency loss. The "wind park effect" loss can be as low as 2% of the combined nameplate rating of the turbines.Utility-scale wind turbine generators have minimum temperature operating limits which restrict the application in areas that routinely experience temperatures less than −20°C. Wind turbines must be protected from ice accumulation, which can make anemometer readings inaccurate and which can cause high structure loads and damage. Some turbine manufacturers offer low-temperature packages at a few percent extra cost, which include internal heaters, different lubricants, and different alloys for structural elements, to make it possible to operate the turbines at lower temperatures. If the low-temperature interval is combined with a low-wind condition, the wind turbine will require station service power, equivalent to a few percent of its output rating, to maintain internal temperatures during the cold snap. For example, the St. Leon, Manitoba project has a total rating of 99 MW and is estimated to need up to 3 MW (around 3% of capacity) of station service power a few days a year for temperatures down to −30°C. This factor affects the economics of wind turbine operation in cold climates.[citation needed]
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