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Generalities - 1
06/2006
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Wind power is the conversion of wind energy into more useful forms, usually electricity using wind turbines. In 2005, worldwide capacity of wind-powered generators was 58,982 megawatts; although it currently produces less than 1% of world-wide electricity use, it accounts for approximately 23% of electricity use in Denmark, 9% in Spain, and 6% in Germany. Globally, wind power generation more than quadrupled between 1999 and 2005.
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 to rural residences or grid-isolated locations.
Wind energy is ample, renewable, widely distributed, clean, and mitigates the greenhouse effect if used to replace fossil-fuel-derived electricity.
Cost and growth
The cost of wind-generated electric power has dropped substantially. Since 2004, according to some sources, the price in the United States is now lower than the cost of fuel-generated electric power, even without taking externalities into account. 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. A British Wind Energy Association report gives an average generation cost of onshore wind power of around 3.2 pence per kilowatt hour. Wind power is growing quickly, at about 38% in 2003, 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.
Most major forms of electric generation are capital intensive, meaning that they require substantial investments at project inception, and low ongoing costs (generally for fuel and maintenance). This is particularly true for wind and hydropower, which have fuel costs close to zero and relatively low maintenance costs; in economic terms, wind power has an extremely low marginal cost and a high proportion of up-front costs. The "cost" of wind energy per unit of production is generally based on average cost per unit, which incorporates the cost of construction, borrowed funds, return to investors (including cost of risk), estimated annual production, and other components. Since these costs are averaged over the projected useful life of the equipment, which may be in excess of twenty years, cost estimates per unit of generation are highly dependent on these assumptions. Figures for cost of wind energy per unit of production cited in various studies can therefore differ substantially.
Estimates for cost of production use similar methodologies for other sources of electricity generation. Existing generation capacity represents sunk costs, and the decision to continue production will depend on marginal costs going forward, not estimated average costs at project inception. For example, the estimated cost of new wind power capacity may be lower than that for "new coal" (estimated average costs for new generation capacity) but higher than for "old coal" (marginal cost of production for existing capacity). Therefore, the choice to increase wind capacity by building new facilities will depend on more complex factors than cost estimates, including the profile of existing generation capacity.
Wind energy
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 throughout 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 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 : P = (1/2).rho.pi.R^2.v^3
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.
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 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%. This compares to typical capacity factors of 90% for nuclear plants, 70% for coal plants, and 30% for oil plants. 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. 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.
Turbine siting
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. 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 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.
Onshore
Onshore turbine installations in hilly or mountainous regions tend to be on ridgelines generally three kilometers or more inland from the nearest shoreline. This is done to exploit the so-called topographic acceleration. The hill or ridge causes the wind to accelerate as it is forced over it. The additional wind speeds gained in this way make large differences to the amount of energy that is produced. Great attention must be paid to the exact positions of the turbines (a process known as micro-siting) because a difference of 30 m can sometimes mean a doubling in output. Local winds are often monitored for a year or more with anemometers and detailed wind maps constructed before wind generators are installed.
For smaller installations where such data collection is too expensive or time consuming, the normal way of prospecting for wind-power sites is to directly look for trees or vegetation that are permanently "cast" or deformed by the prevailing winds. Another way is to use a wind-speed survey map, or historical data from a nearby meteorological station, although these methods are less reliable.
Wind farm siting can sometimes be highly controversial, particularly as the hilltop, often coastal sites preferred are often picturesque and environmentally sensitive (for instance, having substantial bird life). Local residents in a number of potential sites have strongly opposed the installation of wind farms, and political support has resulted in the blocking of construction of some installations.
Near-Shore
Near-Shore turbine installations are generally considered to be inside a zone that is on land within three kilometers of a shoreline and on water within ten kilometers of land. Wind speeds in these zones share wind speed characteristics of both onshore wind and offshore wind depending on the prevailing wind direction. Common issues that are shared within Near-shore wind development zones are aviary (including bird migration and nesting), aquatic habitat, transportation (including shipping and boating) and visual aesthetics amongst several others.
Sea shores also tend to be windy areas and good sites for turbine installation, because a primary source of wind is convection from the differential heating and cooling of land and sea over the course of day and night. Winds at sea level carry somewhat more energy than winds of the same speed in mountainous areas because the air at sea level is more dense.
Near-shore wind farm siting can sometimes be highly controversial as coastal sites are often picturesque and environmentally sensitive (for instance, having substantial bird life). Local residents in a number of potential sites have strongly opposed the installation of wind farms due to visual aesthetic concerns.
Offshore
Offshore wind development zones are generally considered to be ten kilometers or more from land. Offshore wind turbines are less obtrusive than turbines on land, as their apparent size and noise can be mitigated by distance. Because water has less surface roughness than land (especially deeper water), the average wind speed is usually considerably higher over open water. Capacity factors (utilisation rates) are considerably higher than for onshore and near-shore locations which allows offshore turbines to use shorter towers, making them less visible.
In stormy areas with extended shallow continental shelves (such as Denmark), turbines are practical to install — Denmark's wind generation provides about 25-30% of total electricity demand in the country, with many offshore windfarms. Denmark plans to increase wind energy's contribution to as much as half of its electrical supply.
Locations have begun to be developed in the Great Lakes - with one project by Trillium Power approximately 20 km from shore and over 700 MW in size. Ontario is aggressively pursuing wind power development and has many onshore wind farms and several proposed near-shore locations but presently only one offshore development.
In most cases offshore environment is more expensive than onshore. Offshore towers are generally taller than onshore towers once the submerged height is included, and offshore foundations are more difficult to build and more expensive. Power transmission from offshore turbines is generally through undersea cable, which is more expensive to install than cables on land, and may use high voltage direct current operation if significant distance is to be covered — which then requires yet more equipment. The offshore environment can also corrosive and abrasive in salt water locations but locations such as the Great Lakes are in fresh water and do not have many of the issues found in the ocean or sea. Repairs and maintenance are usually much more difficult, and generally more costly, than on onshore turbines. Offshore wind turbines are outfitted with extensive corrosion protection measures like coatings and cathodic protection however some of these measures may not be required in fresh water locations.
While there is a significant market for small land-based windmills, offshore wind turbines have recently been and will probably continue to be the largest wind turbines in operation, because larger turbines allow for the spread of the high fixed costs involved in offshore operation over a greater quantity of generation, reducing the average cost. For similar reasons, offshore wind farms tend to be quite large—often involving over 100 turbines—as opposed to onshore wind farms which can operate competitively even with much smaller installations.
Airborne
Wind turbines might also be flown in high speed winds at altitude, although no such systems currently exist in the marketplace. An Ontario company, Magenn Power, Inc., is attempting to commercialize tethered aerial turbines suspended with helium.
Vertical-axis turbines
The Italian project called "Kitegen" uses a prototype vertical-axis wind turbine. It is an innovative plan (still in the construction phase) that consists of one wind farm with a vertical spin axis, and employs kites to exploit high-altitude winds.
The Kite Wind Generator (KWG) or KiteGen is claimed to eliminate all the static and dynamic problems that prevent the increase of the power (in terms of dimensions) obtainable from the traditional horizontal-axis wind turbine generators. According to its developers, a one gigawatt installation will be 1/40 the cost of the corresponding nuclear powerplant.
The Kitegen project has received 15 million euros of finance from the Italian state. It will begin with a 1 MW prototype and then with the planning of a 20 MW Kitegen.
According to : Wikipedia