Wind power is the conversion of wind energy into more useful forms, usually electricity, using windmills and wind turbines. At the end of 2006, worldwide capacity of wind-powered generators was 74,223 megawatts, or about 1% of world-wide electricity use. Globally, wind power generation more than quadrupled between 2000 and 2006. 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 wind energy is used to turn mechanical machinery to do physical work, such as 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 plentiful, renewable, widely distributed, clean, and reduces toxic atmospheric and greenhouse gas emissions, if used to replace fossil-fuel-derived electricity. Wiki n.p.
Windmills are practical when a regular, strong supply of wind is available. The vanes are adjusted and rotated to maximize wind force, but in a calm, the mill must curtail or stop operations. However, with the current interest on substitutes for polluting fossil-fuel plants, modern windmills are now showing a resurgence where wind conditions make them practical. Unlike the old windmills that converted potential water power into mechanical power through shafts and pulleys, modern windmills convert water power to battery power or electrical generator power.
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. 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. Wiki n.p.
Winds 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 increases the expected wind speeds by 10% and the expected power by 34%. Wiki n.p.
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. Wiki n.p.
Onshore turbine installations in hilly or mountainous regions tend to be on ridgelines generally 3 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 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, i.e., 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. Wiki n.p.
Near-shore turbine installations are generally considered to be inside a zone that is on land within 3 kilometers of a shoreline and on water within 10 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. Near-shore wind farm siting can sometimes be highly controversial as coastal sites are often picturesque and environmentally sensitive. Local residents in a number of potential sites have strongly opposed the installation of wind farms due to visual aesthetic concerns. Wiki n.p.
Offshore wind turbines are generally considered to be 10 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, 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, turbines are practical to install — Denmark's wind generation provides about 18% of total electricity demand in the country, with many offshore windfarms. Locations have begun to be developed in the Great Lakes, with one project approximately 20 km from shore and over 700 MW in size. Ontario, Canada, 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 requires more equipment. Offshore saltwater environments can also raise maintenance costs by corroding the towers, but fresh-water locations such as the Great Lakes do not. Repairs and maintenance are usually much more difficult, and generally more costly, than on onshore turbines. Offshore saltwater wind turbines are outfitted with extensive corrosion protection measures like coatings and cathodic protection, which may not be required in fresh water locations. Wiki n.p.
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. Wiki n.p.
Wind turbines have been used for household electricity generation in conjunction with battery storage over many decades in remote areas. Household generator units of more than 1 kW are now functioning in several countries. To compensate for the varying power output, grid-connected wind turbines may utilise some sort of grid energy storage. Off-grid systems either adapt to intermittent power or use photovoltaic or diesel systems to supplement the wind turbine. Wind turbines range from small four hundred watt generators for residential use to several megawatt machines for wind farms and offshore. The small ones have direct drive generators, direct current output, aeroelastic blades, lifetime bearings and use a vane to point into the wind. The larger ones generally have geared power trains, alternating current output, flaps and are actively pointed into the wind. Direct drive generators and aeroelastic blades for large wind turbines are being researched and direct current generators are sometimes used. In urban locations where it is difficult to obtain large amounts of wind energy, smaller systems may still be used to run low power equipment. Distributed power from rooftop mounted wind turbines can also alleviate power distribution problems as well as provide resilience to power failures. Equipment such as parking meters or wireless internet gateways may be powered by a wind turbine that charges a small battery, replacing the need for a connection to the power grid and/or maintaining service despite possible power grid failures. Wiki n.p.
In 2004 wind energy cost 1/5-th of what it did in the 1980s, and some expected that downward trend to continue as larger multi-megawatt turbines are mass-produced. However, installation costs have increased significantly in 2005 and 2006, and according to the major U.S. wind industry trade group, now average over $1600 U.S. dollars per kilowatt, compared to $1200/kW just a few years before. Cost per unit of energy produced was estimated in 2006 to be comparable to the cost of new generating capacity in the United States for coal and natural gas: wind cost was estimated at $55.80 per MWh, coal at $53.10/MWh and natural gas at $52.50. Most major forms of electricity 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 hydro power, 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 capital costs. The estimated 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 20 years, cost estimates per unit of generation are highly dependent on these assumptions. The cost of wind energy per unit of production cited in various studies can therefore differ substantially. The cost of wind power also depends on several other factors, such as installation of power lines from the wind farm to the national grid and the frequency of wind at the site in question. Wiki n.p.
A key issue debated about wind power is its ability to scale to meet a substantial portion of the world's energy demand. There are significant economic, technical, and ecological issues about the large-scale use of wind power that may limit its ability to replace other forms of energy production. Most forms of electricity production also involve such trade-offs, and many are also not capable of replacing all other types of production for various reasons. A key issue in the application of wind energy to replace substantial amounts of other electrical production is intermittency. At present it is unclear whether wind energy will eventually be sufficient to replace other forms of electricity production, but this does not mean wind energy cannot be a significant source of clean electrical production on a scale comparable to or greater than other technologies, such as hydropower. Most electrical grids use a mix of different generation types for baseload generating capacity and peaking capacity to match demand cycles by attempting to match the variable nature of demand to the most economic form of production. With the exception of hydropower, most types of production capacity are not used for all production; hydropower usage is limited by the presence of appropriate geographical sites. For example, nuclear power is effective as a baseload technology, but cannot be easily varied in short timeframes, and gas turbine plants are most economically used as peaking capacity. Coal generation is primarily considered appropriate for baseload generation with some capacity to cycle to meet demand. Wiki n.p.
Most forms of energy production create some form of negative externality: costs that are not paid by the producer or consumer of the good. For electric production, the most significant externality is pollution, which imposes costs on society in the form of increased health expenses, reduced agricultural productivity, and other problems. In addition, carbon dioxide, a greenhouse gas produced when fossil fuels are burned for electricity production, may impose even greater costs on society in the form of global warming. Few mechanisms currently exist to impose (or internalise) these external costs in a consistent way between various industries or technologies, and the total cost is highly uncertain. Other significant externalities can include national security expenditures to ensure access to fossil fuels, remediation of polluted sites, destruction of wild habitat, loss of scenery/tourism, etc. Wind energy supporters argue that, once external costs and subsidies to other forms of electrical production are accounted for, wind energy is amongst the most cost-effective forms of electrical production. Critics may debate the level of subsidies required or existing, the cost of pollution externalities, and the uncertain financial returns to wind projects. Wiki n.p.
One potential means of increasing the amount of usable wind energy in a given electrical system (penetration rates) is to make use of wind energy storage systems. Surplus wind energy would be used to store electricity in usable form, e.g., pumped storage hydroelectricity. Storage of electricity would effectively arbitrage between the cost of electricity at periods of high supply and low demand, and the higher cost at periods of high demand and low supply. The potential revenue from this arbitrage must be balanced against the installation cost of storage facilities and efficiency losses. Many different technologies exist to store usable electric energy, including air ballast, battery technologies, even flywheel energy storage, etc. For large energy grids, pumped storage hydroelectric has been implemented at large scale, but capital requirement include accessing the acreage of the potential area sites as suitable for such facilities. Most storage technologies are currently unproven commercially at large scale. and often dependent on government induced environmental credits, and renewable energy subsidies. One solution currently being piloted on wind farms is the use of rechargeable flow batteries as a rapid-response storage medium. An alternate solution is to use flywheel energy storage. This type of solution has been implemented by EDA  in the Azores on the islands of Graciosa and Flores. This system uses a 18MWs flywheel to improve power quality and thus allow increased renewable energy usage. Wiki n.p.