風能
维基百科，自由的百科全书
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風力發電被建议合并到本条目或者章节。（讨论） |
風能資源(Wind Energy Resources)因風力做功而提供給人類的一種可利用的能量。風具有的動能稱風能。風速越高，動能越大。人們可以用風車把風的動能轉化為旋轉的動作去推動發電機，以產生電力。方法是透過傳動軸，將轉子(由以空氣動力推動的扇葉組成)的旋轉動力傳送至發電機。到2005年為止，全世界以風力產生的電力約有 58,982 百萬瓦，供應的電力還不到全世界用量的百分之一。風能雖然對大多數國家而言還不是主要的能源，但在1999年到2005年之間已經成長了四倍以上。
Most modern wind power is generated in the form of electricity by converting the rotation of turbine blades into electrical current by means of an electrical generator. In windmills (a much older technology) wind energy is used to turn mechanical machinery to do physical work, like crushing grain or pumping water.
Wind power is used in large scale wind farms for national electrical grids as well as in small individual turbines for providing electricity in isolated locations.
Wind energy is abundant, inexhaustible, widely distributed, clean, and mitigates the greenhouse effect.
我們把地球表面一定範圍內。經過長期測量，調查與統計得出的平均風能密度的概況稱該範圍內能利用的依據，通常以能密度線標示在地圖上。
人類利用風能的歷史可以追溯到西元前，但數千年來，風能技術發展緩慢，沒有引起人們足夠的重視。但自1973年 世界石油危機以來，在常規能源告急和全球生態環境惡化的雙重壓力下，風能作為新能源的一部分才重新有了長足的發展。風能作為一種無污染和可再生的新能源有著巨大的發展潛力，特別是對沿海島嶼，交通不便的邊遠山區，地廣人稀的草原牧場，以及遠離電網和近期內電網還難以達到的農村、邊疆，作為解決生產和生活能源的一種可靠途徑，有著十分重要的意義。即使在發達國家，風能作為一種高效清潔的新能源也日益受到重視，比如：美國能源部就曾經調查過，單是德克薩斯州和南達科他州兩州的風能密度就足以供應全美國的用電量。
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[编辑] 經濟性
In recent years, the cost of wind-generated electric power has dropped substantially, and is now lower than the cost of fuel-generated 電動 power, even without taking externalities into account.[1] Since 2004, wind power has been the least expensive form of new power generation.[2][3] Wind power is growing quickly, at about 38%,[4] up from 25% growth in 2002. In the United States, as of 2003, wind power was the fastest growing form of electricity generation on a percentage basis.[5] In 2005, wind energy cost one-fifth as much as it did in the late 1990s, and that downward trend is expected to continue as larger multi-megawatt turbines are mass-produced.[6]
[编辑] 風的能量
- 主條目：風力
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. 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 all through the earth's surface and atmosphere.
The origin of wind is simple. 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.
The change of seasons, change of day and night, the Coriolis effect, the irregular albedo (reflectivity) of land and water, humidity, and the friction of wind over different terrain are some of the factors which complicate the flow of wind over the surface.
[编辑] 風力變化與轉子產能
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 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 about 1.22 kilograms per cubic metre (it gets less dense with higher humidity). An 8 m/s breeze blowing through a 100 meter diameter rotor would move about 76,000 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. A German physicist, Albert Betz, determined in 1919 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. More recent work by Gorlov shows a theoretical limit of about 30% for propeller-type turbines.[7] Actual efficiencies range from 10% to 20% for propeller-type turbines, and are as high as 35% for three-dimensional vertical-axis turbines like Darrieus or Gorlov turbines.
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 plotted to the right 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 of this is that wind energy is not dispatchable as for fuel-fired power plants; additional output cannot be supplied in response to load demand.
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 as much as 35%. 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 350 kW of fuel-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.[1]
[编辑] 風力密度的分級
Wind maps in the United States and Europe classify areas into seven classes of wind power density, which give an indication of the quality of wind power resource in the area.
Each class is a range of power densities, so that an area rated as class 4, for example, would have an average power density from 200 to 250 W/m2 at 10 m above ground. Generally, economic development of wind power for electricity generation takes place in areas rated Class 3 or higher.
