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Wind power
Wind power is the conversion of wind energy into a uful form of energy, such as using wind turbines to make electricity, wind mills for mechanical power, wind pumps for pumping water or drainage, or sails to propel ships.
At the end of 2009, worldwide nameplate capacity of wind-powered generators was 159.2 gigawatts (GW).(By June 2010 the capacity had rin to 175 GW.) Energy production was 340 TWh, which is about 2% of worldwide electricity usage; and has doubled in the past three years. Several countries have achieved relatively high levels of wind power penetration, such as 20% of stationary electricity production in Denmark, 14% in Ireland and Portugal, 11% in Spain, and 8% in Germany in 2009. As of May 2009, 80 countries around the world are using wind power on a commercial basis.
Wind power: worldwide installed capacity 1996-2008
Large-scale wind farms are connected to the electric power transmission network; smaller facilities are
ud to provide electricity to isolated locations. Utility companies increasingly buy back surplus electricity produced by small domestic turbines. Wind energy, as an alternative to fossil fuels, is plentiful, renewable, widely distributed, clean, and produces no greenhou gas emissions during operation. However, the construction of wind farms is not universally welcomed becau of their visual impact but any effects on the environment are generally among the least problematic of any power source. The intermittency of wind ldom creates problems when using wind power to supply a low proportion of total demand, but as the proportion ris, incread costs, a need to upgrade the grid, and a lowered ability to supplant conventional production may occur. Power management techniques such as exporting and importing power to neighboring areas or reducing demand when wind production is low, can mitigate the problems.
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Burbo Bank Offshore Wind Farm, at the entrance to the River Mery in North West England.
History
Humans have been using wind power for at least 5,500 years to propel sailboats and sailing ships. Windmills have been ud for irrigation pumping and for milling grain since the 7th century AD in wh
at is now Afghanistan, India, Iran and Pakistan.
In the United States, the development of the "water-pumping windmill" was the major factor in allowing the farming and ranching of vast areas otherwi devoid of readily accessible water. Windpumps contributed to the expansion of rail transport systems throughout the world, by pumping water from water wells for the steam locomotives. The multi-bladed wind turbine atop a lattice tower made of wood or steel was, for many years, a fixture of the landscape throughout rural America. When fitted with generators and battery banks, small wind machines provided electricity to isolated farms.
Medieval depiction of a wind mill
In July 1887, a Scottish academic, Professor James Blyth, undertook wind power experiments that culminated in a UK patent in 1891. In the United States, Charles F. Brush produced electricity using a wind powered machine, starting in the winter of 1887-1888, which powered his home and laboratory until about 1900. In the 1890s, the Danish scientist and inventor Poul la Cour constructed wind turbines to generate electricity, which was then ud to produce hydrogen. The were the first
of what was to become the modern form of wind turbine.
Small wind turbines for lighting of isolated rural buildings were widespread in the first part of the 20th century. Larger units intended for connection to a distribution network were tried at veral locations including Balaklava USSR in 1931 and in a 1.25 megawatt (MW) experimental unit in Vermont in 1941.
The modern wind power industry began in 1979 with the rial production of wind turbines by Danish manufacturers Kuriant, Vestas, Nordtank, and Bonus. The early turbines were small by today's standards, with capacities of
20–30 kW each. Since then, they have incread greatly in size, with the Enercon E-126 capable of delivering up to 7 MW, while wind turbine production has expanded to many countries.
Windmills are typically installed in favourable windy locations. In the image, wind power generators in Spain near an Osborne bull
Wind energy
The Earth is unevenly heated by the sun, such that the poles receive less energy from the sun than the equator; along with this, dry land heats up (and cools down) more quickly than the as do. The differential heating drives a global atmospheric convection system reaching from the Earth's surface to the stratosphere which acts as a virtual ceiling. Most of the energy stored in the wind movements can be found at high altitudes where continuous wind speeds of over 160 km/h (99 mph) occur. Eventually, the wind energy is converted through friction into diffu heat throughout the Earth's surface and the atmosphere.
The total amount of economically extractable power available from the wind is considerably more than prent human power u from all sources. The most comprehensive study as of 2005 found the potential of wind power on land and near-shore to be 72 TW, equivalent to 54,000 MToE (million tons of oil equivalent) per year, or over five times the world's current energy u in all forms. The potential takes into account only locations with mean annual wind speeds ≥ 6.9 m/s at 80 m. The study assumes six 1.5 megawatt, 77 m diameter turbines per square kilometer on roughly 13% of the total global land area (though that land would also be available for other compatible us such as farming). The authors acknowledge that many practical barriers would need to be overcome to reach this theoretical capacity.
Map of available wind power for the United States. Color codes indicate wind power density class
The practical limit to exploitation of wind power will be t by economic and environmental factors, since the resource available is far larger than any practical means to develop it.
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Distribution of wind speed
The strength of wind varies, and an average value for a given location does not alone indicate the amount of energy a wind turbine could produce there. To asss the frequency of wind speeds at a particular location, a probability distribution function is often fit to the obrved data. Different locations will have different wind speed distributions. The Weibull model cloly mirrors the actual distribution of hourly wind speeds at many locations. The Weibull factor is often clo to 2 and therefore a Rayleigh distribution can be ud as a less accurate, but simpler model.
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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
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Becau so much power is generated by higher wind speed, much of the energy 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 conquence is that wind energy from a particular turbine or wind farm does not have as consistent an output as fuel-fired power plants.
Electricity generation
In a wind farm, individual turbines are interconnected with a medium voltage (often 34.5 kV), power collection system and communications network. At a substation, this medium-voltage electric current is incread in voltage with a transformer for connection to the high voltage electric power transmission system.争论作文600字初一
Typical components of a wind turbine (gearbox, rotor shaft and brake asmbly) being lifted into position
The surplus power produced by domestic microgenerators can, in some jurisdictions, be fed into the network and sold to the utility company, producing a retail credit for the microgenerators' owners to offt their energy costs.
Grid management
Induction generators, often ud for wind power, require reactive power for excitation so substations ud in wind-power collection systems include substantial capacitor banks for power factor correction. Different types of wind turbine generators behave differently during transmission grid disturbances, so extensive modelling of the dynamic electromechanical characteristics of a new wind farm is required by transmission system operators to ensure predictable stable behaviour during system faults (e: Low voltage ride through). In particular, induction generators cannot support the system voltage during faults, unlike steam or hydro turbine-driven synchronous generators. Doubly-fed machines generally have more desirable properties for grid interconnection. Transmission systems operators will supply a wind farm developer with a grid
code to specify the requirements for interconnection to the transmission grid. This will include power factor, constancy of frequency and dynamic behavior of the wind farm turbines during a system fault.孕妇能吃薏仁吗
Capacity factor
Since wind speed is not constant, a wind farm's annual energy production is never as much as the sum of the generator nameplate ratings multiplied by the total hours in a year. The ratio of actual pro
ductivity in a year to this theoretical maximum is called the capacity factor. Typical capacity factors are 20–40%, with values at the upper end of the range in particularly favourable sites. For example, a 1 MW turbine with a capacity factor of 35% will not produce
8,760 MW·h in a year (1 × 24 × 365), but only 1 × 0.35 × 24 ×
365 = 3,066 MW·h, averaging to 0.35 MW. Online data is available for some locations and the capacity factor can be calculated from the yearly output.
Unlike fueled generating plants, the capacity factor is limited by the inherent properties of wind. Capacity factors of other types of power plant are bad mostly on fuel cost, with a small amount of downtime for maintenance. Nuclear plants have low incremental fuel cost, and so are run at full output and achieve
a 90% capacity factor. Plants with higher fuel cost are throttled back to follow load. Gas turbine plants using natural gas as fuel may be very expensive to operate and may be run only to meet peak power demand. A gas turbine plant may have an annual capacity factor of 5–25% due to relatively high energy production cost.
In a 2008 study relead by the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy, the capacity factor achieved by the wind turbine fleet is shown to be increasing as the technology improves. The capacity factor achieved by new wind turbines in 2004 and 2005 reached 36%.杨孟衡
Penetration
Wind energy "penetration" refers to the fraction of energy produced by wind compared with the total available generation capacity. There is no generally accepted "maximum" level of wind penetration. The limit for a particular grid will depend on the existing generating plants, pricing mechanisms, capacity for storage or demand management, and other factors. An interconnected electricity grid will already include rerve generating and transmission capacity to allow for equipment failures; this rerve capacity can also rve to regulate for the varying power generation by wind plants. Studies have indicated that 20% of the total electrical energy consumption may be incorporated with minimal difficulty. The studies have been for locations with geographically disperd wind farms, some degree of dispatchable energy, or hydropower with storage capacity, demand management, and interconnection to a large grid area export of electricity when needed. Beyond this level, there are few technical limits, but the economic implications become more significant. Electrical utilities contin
ue to study the effects of large (20% or more) scale penetration of wind generation on system stability and economics.
At prent, a few grid systems have penetration of wind energy above 5%: Denmark (values over 19%), Spain and Portugal (values over 11%), Germany and the Republic of Ireland (values over 6%). But even with a modest level of penetration, there can be times where wind power provides a substantial percentage of the power on a grid. For example, in the morning hours of 8 November 2009, wind energy produced covered more than half the electricity demand in Spain, tting a new record. This was an instance where demand was very low but wind power generation was very high.
Variability and intermittency
Electricity generated from wind power can be highly variable at veral different timescales: from hour to hour, daily, and asonally. Annual variation also exists, but is not as significant. Related to variability is the short-term (hourly or daily) predictability of wind plant output. Like other electricity sources, wind energy must be "scheduled". Wind power forecasting methods are ud, but predictability of wind plant output remains low for short-term operation.
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Becau instantaneous electrical generation and consumption must remain in balance to maintain gr
id stability, this variability can prent substantial challenges to incorporating large amounts of wind power into a grid system. Intermittency and the non-dispatchable nature of wind energy production can rai costs for regulation, incremental operating rerve, and (at high
penetration levels) could require an increa in the already existing energy demand management, load shedding, or storage solutions or system interconnection with HVDC cables. At low levels of wind penetration, fluctuations in load and allowance for failure of large generating units requires rerve capacity that can also regulate for variability of wind generation. Wind power can be replaced by other power stations during low wind periods. Transmission networks must already cope with outages of generation plant and daily changes in electrical demand. Systems with large wind capacity components may need more spinning rerve (plants operating at less than full load).
Wildorado Wind Ranch in Oldham County in the Texas Panhandle, as photographed from U.S. Route 385
Pumped-storage hydroelectricity or other forms of grid energy storage can store energy developed by high-wind periods and relea it when needed. Stored energy increas the economic value of wi
nd energy since it can be shifted to displace higher cost generation during peak demand periods. The potential revenue from this arbitrage can offt the cost and loss of storage; the cost of storage may add 25% to the cost of any wind energy stored, but it is not envisaged that this would apply to a large proportion of wind energy generated. The 2 GW Dinorwig pumped storage plant in Wales evens out electrical demand peaks, and allows ba-load suppliers to run their plant more efficiently. Although pumped storage power systems are only about 75% efficient, and have high installation costs, their low running costs and ability to reduce the required electrical ba-load can save both fuel and total electrical generation costs.
In particular geographic regions, peak wind speeds may not coincide with peak demand for electrical power. In the US states of California and Texas, for example, hot days in summer may have low wind speed and high electrical demand due to air conditioning. Some utilities subsidize the purcha of geothermal heat pumps by their customers, to reduce electricity demand during the summer months by making air conditioning up to 70% more efficient; widespread adoption of this technology would better match electricity demand to wind availability in areas with hot summers and low summer winds. Another option is to interconnect widely disperd geographic areas with an HVDC
"Super grid". In the USA it is estimated that to upgrade the transmission system to take in planned or
potential renewables would cost at least $60 billion.
In the UK, demand for electricity is higher in winter than in summer, and so are wind speeds. Solar power tends to be complementary to wind. On daily to weekly timescales, high pressure areas tend to bring clear skies and low surface winds, whereas low pressure areas tend to be windier and cloudier. On asonal timescales, solar energy typically peaks in summer, whereas in many areas wind energy is lower in summer and higher in winter. Thus the intermittencies of wind and solar power tend to cancel each other somewhat. The Institute for Solar Energy Supply Technology of the University of Kasl pilot-tested a combined power plant linking solar, wind, biogas and hydrostorage to provide load-following power around the clock, entirely from renewable sources.
A report on Denmark's wind power noted that their wind power network provided less than 1% of average demand 54 days during the year 2002. Wind power advocates argue that the periods of low wind can be dealt with by simply restarting existing power stations that have been held in readiness or interlinking with HVDC. Electrical grids with slow-responding thermal power plants and without ties to networks with hydroelectric generation may have to limit the u of wind power.[42]
Three reports on the wind variability in the UK issued in 2009, generally agree that variability of wind
needs to be taken into account, but it does not make the grid unmanageable; and the additional costs, which are modest, can be quantified. A 2006 International Energy Agency forum prented costs for managing intermittency as a function of wind-energy's share of total capacity for veral countries, as shown: Increa in system operation costs, Euros per MW·h, for 10% and 20% wind share
10% 20%
Germany 2.5 3.2
Denmark 0.4 0.8
Finland 0.3 1.5
Norway 0.1 0.3
Sweden 0.3 0.7
