Wind power

An example of a wind turbine. This 3 bladed turbine is the most common design of modern wind turbines.

Wind power is the conversion of wind energy into more useful forms, such as electricity, using wind turbines. At the end of 2006, worldwide capacity of wind-powered generators was 73.9 gigawatts; although it currently produces just over 1% of world-wide electricity use,, it accounts for approximately 20% of electricity production in Denmark, 9% in Spain, and 7% in Germany. 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 (a much older technology), 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. The intermittency of wind seldom creates problems when using wind power at low to moderate penetration levels.

Wind energy

Further information: Wind

There is an estimated 50 to 100 times more wind energy than plant biomass energy available on Earth. 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.

Wind variability and turbine power

A 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:

$P={\begin{matrix}{\frac {1}{2}}\end{matrix}}\alpha \rho \pi r^{2}v^{3}$,

where P = power in watts, alpha = efficiency constant, rho = mass density of air in kilograms per cubic meter, r = radius of the wind turbine in meters, and v = velocity of the air in meters per second.

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 power 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.

Worldwide installed capacity and prediction 1997-2010, Source: WWEA

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; utilities that use wind power must provide backup generation or grid power reception capability for times that the wind is weak.

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 a typical capacity factors of 90% for nuclear plants (like wind farms, they have negligible fuel cost, and are therefore often run at maximum capacity with the load following relegated to other plants). The lower values of 70% for coal plants and 30% for oil plants reflect a throttling-back of plants with high cost fuel in times of low demand.

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 (PHEV or EV), making it very convenient. PHEV/EV's are likely to be a very important source of demand management, which would mostly charge at night when wind power is most likely to be surplus, and whose charging could be scheduled in an automated fashion for periods of greatest wind output.

Turbine placement

Map 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. 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. 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.

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 30m 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 or on water within ten kilometers of land. These areas tend to be windy and are good sites for turbine installation, because a primary source of wind is convection caused by the differential heating and cooling of land and sea over the cycle of day and night. Wind speeds in these zones share the characteristics of both onshore 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. Local residents in some potential sites have strongly opposed the installation of wind farms due to these concerns.

Offshore

Offshore wind turbines near Copenhagen

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 18% of total electricity production 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, 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 fresh water and one on the Pacific west coast.

In most cases offshore environment is more expensive than onshore but this depends on the unique attributes of the specific site. Offshore towers are generally taller than onshore towers once the submerged height is included, and offshore foundations may be more difficult to build and more expensive but again this will be determined by the specific site of the proposed development. 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. 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 more difficult or slower, and generally more costly, than on onshore turbines due to the location of the offshore site. These costs may vary greatly depending on the exact site of the offshore development. 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.

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

Main article: Airborne wind turbine

Wind turbines might also be flown in high speed winds at altitude, although no such systems currently exist in the marketplace. An Ontario (Canada) company, Magenn Power, Inc., is attempting to commercialize tethered aerial turbines suspended with helium

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. A number of other designs for vertical-axis turbines have been developed or proposed, including small scale commercial or pilot installations. However, vertical-axis turbines remain a commercially unproven technology.

Utilization

Large scale

Installed windpower capacity (MW)
Rank	Nation	2005	2006	Latest
1	Germany	18,415	20,622	21,283
2	Spain	10,028	11,615	12,801
3	United States	9,149	11,603	12,634
4	India	4,430	6,270	7,231
5	Denmark (& Færoe Islands)	3,136	3,140
6	China	1,260	2,604	2,956
7	Italy	1,718	2,123
8	United Kingdom	1,332	1,963	2,191
9	Portugal	1,022	1,716	1,874
10	Canada	683	1,459	1,670
11	France	757	1,567
12	Netherlands	1,219	1,560
13	Japan	1,061	1,394
14	Austria	819	965
15	Australia	708	817
16	Greece	573	746	795
17	Ireland	496	745	866
18	Sweden	510	572
19	Norway	267	314
20	Brazil	29	237
21	Egypt	145	230	580
22	Belgium	167	193
23	Taiwan	104	188
24	South Korea	98	173
25	New Zealand	169	171	322
26	Poland	83	153	216
27	Morocco	64	124
28	Mexico	3	88
29	Finland	82	86	107
30	Ukraine	77	86
31	Costa Rica	71	74
32	Hungary	18	61
33	Lithuania	6	55
34	Turkey	20	51
35	Czech Republic	28	50
36	Iran	23	48
	Rest of Europe	129	163
	Rest of Americas	109	109
	Rest of Asia	38	38
	Rest of Africa & Middle East	31	31
	Rest of Oceania	12	12
	World total (MW)	59,091	74,223	79,341

There are many thousands of wind turbines operating, with a total capacity of 73,904 MW of which Europe accounts for 65% (2006). The average output of one megawatt of wind power is equivalent to the average electricity consumption of about 250 American households. Wind power was the most rapidly-growing means of alternative electricity generation at the turn of the century and world wind generation capacity more than quadrupled between 2000 and 2006. 81% of wind power installations are in the US and Europe, but the share of the top five countries in terms of new installations fell from 71% in 2004 to 55% in 2005.

By 2010, the World Wind Energy Association expects 160GW of capacity to be installed worldwide, up from 73.9GW at the end of 2006, implying an anticipated net growth rate of more than 21% per year.

Germany, Spain, the United States, India, and Denmark have made the largest investments in wind generated electricity. Denmark is prominent in the manufacturing and use of wind turbines, with a commitment made in the 1970s to eventually produce half of the country's power by wind. Denmark generates over 20% of its electricity with wind turbines, the highest percentage of any country and is fifth in the world in total wind power generation (which can be compared with the fact that Denmark is 56th on the general electricity consumption list). Denmark and Germany are leading exporters of large (0.66 to 5 MW) turbines.

Wind accounts for 1% of the total electricity production on a global scale (2005). Germany is the leading producer of wind power with 28% of the total world capacity in 2006 (7.3% of German electricity); the official target is that by 2010, renewable energy will meet 12.5% of German electricity needs — it can be expected that this target will be reached even earlier. Germany has 18,600 wind turbines, mostly in the north of the country — including three of the biggest in the world, constructed by the companies Enercon (6 MW), Multibrid (5 MW) and Repower (5 MW). Germany's Schleswig-Holstein province generates 36% of its power with wind turbines.

Spain and the United States are next in terms of installed capacity. In 2005, the government of Spain approved a new national goal for installed wind power capacity of 20,000 MW by 2012. According to trade journal Windpower Monthly; however, in 2006 they abruptly halted subsidies and price supports for wind power. According to the American Wind Energy Association, wind generated enough electricity to power 0.4% (1.6 million households) of total electricity in US, up from less than 0.1% in 1999. In 2005, both Germany and Spain have produced more electricity from wind power than from hydropower plants. US Department of Energy studies have concluded wind harvested in just three of the fifty U.S. states could provide enough electricity to power the entire nation, and that offshore wind farms could do the same job.[3]

In recent years, the United States has added more wind energy to its grid than any other single country, and capacity is expected to grow by 3 gigawatts (3,000 megawatts) in 2007. Texas has become the leader in Wind Energy production, far surpassing California. In 2007, the state expects to add 2 gigawatts to raise its existing capacity to approximately 4.5 gigawatts. Iowa and Minnesota are expected to reach the 1 gigawatt mark by the end of 2007. Wind power generation in the U.S. was up 31.8% in February, 2007 from February, 2006.

India ranks 4th in the world with a total wind power capacity of 6,270 MW in 2006. Wind power generates 3% of all electricity produced in India. The World Wind Energy Conference in New Delhi in November 2006 has given additional impetus to the Indian wind industry. The windfarm near Muppandal, India, provides an impoverished village with energy for work. India-based Suzlon Energy is one of the world's largest wind turbine manufacturers.

In December 2003, General Electric installed the world's largest offshore wind turbines in Ireland, and plans are being made for more such installations on the west coast, including the possible use of floating turbines.

On August 15, 2005, China announced it would build a 1000-megawatt wind farm in Hebei for completion in 2020. China reportedly has set a generating target of 20,000 MW by 2020 from renewable energy sources — it says indigenous wind power could generate up to 253,000 MW. Following the World Wind Energy Conference in November 2004, organised by the Chinese and the World Wind Energy Association, a Chinese renewable energy law was adopted. In late 2005, the Chinese government increased the official wind energy target for the year 2020 from 20 GW to 30 GW.

Mexico recently opened La Venta II wind power project as an important step in reducing Mexico's consumption of fossil fuels. The project (88MW) the first of its kind in Mexico, will provide 13 percent of the electricity needs of the state of Oaxaca and by 2012 will have a capacity of 3500 MW.

Another growing market is Brazil, with a wind potential of 143 GW. The federal government has created an incentive program, called Proinfa, to build production capacity of 3300 MW of renewable energy for 2008, of which 1422 MW through wind energy. The program seeks to produce 10% of Brazilian electricity through renewable sources. Brazil produced 320 TWh in 2004. France recently announced a very ambitious target of 12 500 MW installed by 2010.

View of wind farm near Muppandal, Tamilnadu in India

Over the 7 years from 2000-2006, Canada experienced rapid growth of wind capacity — moving from a total installed capacity of 137 MW to 1,451 MW, and showing a growth rate of 38% and rising. Particularly rapid growth has been seen in 2006, with total capacity growing to 1,451 MW by December, 2006, doubling the installed capacity from the 684 MW at end-2005. This growth was fed by provincial measures, including installation targets, economic incentives and political support. For example, the government of the Canadian province of Ontario announced on 21 March 2006 that it will introduce a feed-in tariff for wind power, referred to as 'Standard Offer Contracts', which may boost the wind industry across the province. In the Canadian province of Quebec, the state-owned hydroelectric utility plans beside current wind farm projects to purchase an additional 2000 MW by 2013.

Wind power in Europe

Wind Power in Europe 2006 (MW)
No	Country	Addition	Total
1	Germany	2 233	20 622
2	Spain	1 587	11 615
3	France	810	1 567
4	Portugal	694	1 716
5	UK	634	1 963
6	Italy	417	2 123
7	Netherlands	356	1 560
8	Ireland	250	745
9	Greece	173	746
10	Austria	146	965
11	Poland	69	152
12	Sweden	62	572
13	Lithuania	49	55
14	Hungary	43	61
15	Belgium	26	193
16	Czech Republic	22	50
17	Bulgaria	22	32
18	Denmark & F.I.	11	3 140
19	Finland	4	86
20	Romania	1	3
21	Luxembourg	0	35
22	Estonia	0	32
23	Latvia	0	27
24	Slovenia	0	5
25	Slovakia	0	0
26	Cyprus	0	0
27	Malta	0	0
EU27 (MW)		7 609	48 061
28	Norway	47	314
Europe (MW)		7 708	48 545
ref in discussion

Small scale

This rooftop-mounted urban wind turbine charges a 12 volt battery and runs various 12 volt appliances within the building on which it is installed.

Small Wind is defined as wind generation systems with capacities of 100 kW or less and are usually used to power homes, farms, and small businesses. Individuals purchase these systems to reduce or eliminate their electricity bills, to avoid the unpredictability of natural gas prices, or simply to generate their own clean power.

Wind turbines have been used for household electricity generation in conjunction with battery storage over many decades in remote areas, but increasingly, U.S. consumers are choosing to purchase grid-connected turbines in the 1 to 10 kilowatt range to power their whole homes. Household generator units of more than 1 kW are now functioning in several countries, and in every state in the U.S.

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 sometimes, but not always, have direct drive generators, direct current output, aeroelastic blades, lifetime bearings and use a vane to point into the wind; while 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 predictable or 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.

While installing a small wind turbine on a roof (rather than a tall tower elsewhere on a property) can be done successfully, there are a few inherent issues that this type of installation faces: Whether the roof can support the turbine's weight, how the building tolerates the vibrations from the spinning rotor, and the turbulence caused by the roof ledge and the resulting unpredictability in wind patterns.

Small-scale wind power in rural Indiana.

Small scale turbines for residential-scale use are available that are approximately 7 feet (2 m) to 25 feet (8 m) in diameter and produce electricity at a rate of 900 watts to 10,000 watts at their tested wind speed. Some units are designed to be very lightweight, e.g. 16 kilograms (35 lb), allowing rapid response to wind gusts typical of urban settings and easy mounting much like a television antenna. It is claimed that they are inaudible even a few feet under the turbine. Dynamic braking regulates the speed by dumping excess energy, so that the turbine continues to produce electricity even in high winds. The dynamic braking resistor may be installed inside the building to provide heat (during high winds when more heat is lost by the building, while more heat is also produced by the braking resistor). The proximal location makes low voltage (12 volt, or the like) energy distribution practical. An additional benefit is that owners become more aware of electricity consumption, possibly reducing their consumption down to the average level that the turbine can produce.

The American Wind Energy Association has released several studies on the small wind turbine market in the U.S. and abroad, showing that the U.S. continues to dominate the Small Wind industry.[4] According to another organization, the World Wind Energy Association, it is difficult to assess the total number or capacity of small-scaled wind turbines, but in China alone, there are roughly 300,000 small-scale wind turbines generating electricity.

The dominant model on the market, especially in the United States, is the propeller-shaped "Horizontal Axis" type, which resembles the large, utility-scale turbines used in wind "farms." An alternative model is known as "Vertical Axis," and rotates like a top and can come in many different designs.

There have been a number of recent developments of mini-windmills which could be adapted to home use, including:

• The AeroTecture vertical-axis turbine
• The AeroVironment Architectural Wind Project
• The piezoelectric windmill project
• The Swift home wind turbine. The Swift project peaked in 2004 and has had some implementation difficulties while promising to be a low-noise/safe roof-mount/low-cost alternative
• The Motorwave micro-wind turbine

Consumer guides are available to help potential customers learn about residential-scale wind systems, three of which are:

• "Small Wind Electric Systems: A U.S. Consumer's Guide" by the Dept. of Energy's Wind Powering America program [5]

• "Wind Turbine Buyer's Guide" From Home Power Magazine[6]

• "Apples & Oranges 2002: Choosing a Home-Sized Wind Generator" [7]

Much more information is also available at the American Wind Energy Association's web site at:

• www.awea.org/smallwind [8]
• FAQ: http://www.awea.org/smallwind/faq.html [9]
• www.awea.org/smallwind/toolbox2/index.html[10]

Wind power: key issues

Wind power can be a controversial issue, and several main areas of dispute are debated between supporters and opponents.

Erection of an Enercon E70-4 in Germany

Growth and cost trends

Global Wind Energy Council (GWEC) figures show that 2006 recorded an increase of installed capacity of 15,197 megawatts (MW), taking the total installed wind energy capacity to 74,223 MW, up from 59,091 MW in 2005. Despite constraints facing supply chains for wind turbines, the annual market for wind continued to increase at an estimated rate of 32% following the 2005 record year, in which the market grew by 41%. In terms of economic value, the wind energy sector has become one of the important players in the energy markets, with the total value of new generating equipment installed in 2006 reaching €18 billion, or US$23 billion.

The countries with the highest total installed capacity are Germany (20,621 MW), Spain (11,615 MW), the USA (11,603 MW), India (6,270 MW) and Denmark (3,136). Thirteen countries around the world can now be counted among those with over 1,000 MW of wind capacity. In terms of new installed capacity in 2006, the US leads with 2,454 MW, followed by Germany (2,233 MW), India (1,840 MW), Spain (1,587 MW), China (1,347 MW) and France (810 MW).

In 2004, wind energy cost one-fifth 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 US$1,600 per kilowatt, compared to $1200/kW just a few years before. A British Wind Energy Association report gives an average generation cost of onshore wind power of around 3.2 pence per kilowatt hour (2005). 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. Other sources in various studies have estimated wind to be more expensive than other sources (see Economics of new nuclear power plants, Clean coal, and Carbon capture and storage).

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 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. 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.

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.

Research from a wide variety of sources in various countries shows that support for wind power is consistently between 70 and 80 per cent amongst the general public.

Scalability

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. See, for example, the annual report of the Independent Electricity System Operator in Ontario, Canada [11]. 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; see the section below on Economics and Feasibility. 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 (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.

A significant part of the debate about the potential for wind energy to substitute for other electric production sources is the level of penetration. With the exception of Denmark, no countries or electrical systems produce more than 10% from wind energy, and most are below 2% (of course, this is in large part because wind power is a relatively new technology, with the vast majority of installations having taken place within the last 10 years). While the feasibility of integrating much higher levels (beyond 25%) is debated, significantly more wind energy could be produced worldwide before these issues become significant. In Denmark, wind power now accounts for close to 20% of electricity production and a recent poll of Danes show that 90% want more wind power installed.

Theoretical potential

Wind's long-term theoretical potential is much greater than current world energy consumption. The most comprehensive study to date found the potential of wind power on land and near-shore to be 72 TW (~171,000 Mtoe), or over fifteen times the world's current energy use and 40 times the current electricity use. The potential takes into account only locations with Class 3 (mean annual wind speeds ≥ 6.9 m/s at 80 m) or better wind regimes, which includes the locations suitable for low-cost (0.03–0.04 $/kWh) wind power generation and is in that sense conservative. It assumes 6 turbines per square km for 77 m diameter, 1.5 MW-turbines on roughly 13% of the total global land area (though that land would also be available for other compatible uses such as farming). However, the authors are quick to point out that many practical barriers would need to be overcome to reach this theoretical capacity. The calculations of potential assumes a capacity factor of 48% and does not take into account the practicality of reaching the windy sites, of transmission (including 'choke' points), of competing land uses, of transporting power over large distances, or of switching to wind power.

To determine the more realistic technical potential, it is essential to estimate how large a fraction of this land could be made available to wind power. In the 2001 IPCC report, it is assumed that a use of 4% – 10% of that land area would be practical.

Although the theoretical potential is vast, the amount of production that could be economically viable depends on a number of exogenous and endogenous factors, including the cost of other sources of electricity and the future cost of wind energy farms.

Offshore resources experience mean wind speeds about 90% greater than those on land, so offshore resources could contribute about seven times more energy than land. This number could also increase with higher altitude or airborne wind turbines.

Economics and feasibility

Some of the over 6,000 wind turbines at Altamont Pass, in California. Developed during a period of tax incentives in the 1980s, this wind farm has more turbines than any other in the United States, producing about 125 MW. Considered largely obsolete, these turbines produce only a few tens of kilowatts each.

Wind energy in many jurisdictions receives some financial or other support to encourage its development. A key issue is the comparison to other forms of energy production, and their total cost. Two main points of discussion arise: direct subsidies and externalities for various sources of electricity, including wind. Wind energy benefits from subsidies of various kinds in many jurisdictions, either to increase its attractiveness, or to compensate for subsidies received by other forms of production or which have significant negative externalities. Without the handsome tax incentives (also know as subsidies) in fact, almost no wind power installation is economically feasible at present.

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 argue that the level of required subsidies, the small amount of energy needs met, and the uncertain financial returns to wind projects — that is, the all-in cost of wind energy compared to other technologies - make it inferior to other energy sources. Intermittency and other characteristics of wind energy also have costs that may rise with higher levels of penetration, and may change the cost-benefit ratio.

• Conventional and nuclear power plants receive substantial direct and indirect governmental subsidies. If a comparison is made on total production costs (including subsidies), wind energy may or may not be competitive compared to other energy sources. If the full costs (environmental, health, etc.) are taken into account, wind energy may be competitive in more cases. Wind energy costs have generally decreased due to technology development and scale enlargement. However, the cost of other capital intensive generation technologies, such as nuclear and fossil fueled plants, is also subject to cost reductions due to economies of scale and technological improvements.
• Nuclear power plants generally receive special immunity from the disasters they may cause, which prevents victims from recovering the cost of their continued health care from those responsible, even in the case of criminal malfeasance. In many cases, nuclear plants are owned directly by governments or substantially supported by them. This is a form of indirect subsidy, although the size of this subsidy is difficult to ascertain precisely.
• To compete with traditional sources of energy, wind power often receives financial incentives. In the United States, wind power receives a tax credit for each kilowatt-hour produced; at 1.9 cents per kilowatt-hour in 2006, the credit has a yearly inflationary adjustment. Another tax benefit is accelerated depreciation. Many American states also provide incentives, such as exemption from property tax, mandated purchases, and additional markets for "green credits." Countries such as Canada and Germany also provide incentives for wind turbine construction, such as tax credits or minimum purchase prices for wind generation, with assured grid access (sometimes referred to as feed-in tariffs). These feed-in tariffs are typically set well above average electricity prices.
• Many potential sites for wind farms are far from demand centers, requiring substantially more money to construct new transmission lines and substations.
• Intermittency and the non-dispatchable nature of wind energy production can raise costs for regulation, incremental operating reserve, and (at high penetration levels) could require demand-side management or storage solutions. However, it is highly unlikely that any of these storage systems could replace the energy deficit produced, say, on windless days. See 'Grid Energy Storage' section below.
• Since the primary cost of producing wind energy is construction and there are no fuel costs, the average cost of wind energy per unit of production is dependent on a few key assumptions, such as the cost of capital and years of assumed service. The marginal cost of wind energy once a plant is constructed is close to zero.
• The cost of wind energy production has fallen rapidly since the early 1980s, primarily due to technological improvements, although the cost of construction materials (particularly metals) and the increased demand for turbine components caused price increases in 2005-06. Many expect further reductions in the cost of wind energy through improved technology, better forecasting, and increased scale. Since the cost of capital plays a large part in projected cost, risk (as perceived by investors) will affect projected costs per unit of electricity.
• Apart from regulatory issues and externalities, decisions to invest in wind energy will also depend on the cost of alternative sources of energy. Natural gas, oil and coal prices, the main production technologies with significant fuel costs, will therefore also be a determinant in the choice of the level of wind energy.
• The commercial viability of wind power also depends on the pricing regime for power producers. Electricity prices are highly regulated worldwide, and in many locations may not reflect the full cost of production, let alone indirect subsidies or negative externalities. Certain jurisdictions or customers may enter into long-term pricing contracts for wind to reduce the risk of future pricing changes, thereby ensuring more stable returns for projects at the development stage. These may take the form of standard offer contracts, whereby the system operator undertakes to purchase power from wind at a fixed price for a certain period (perhaps up to a limit); these prices may be different than purchase prices from other sources, and even incorporate an implicit subsidy.
• In jurisdictions where the price paid to producers for electricity is based on market mechanisms, revenue for all producers per unit is higher when their production coincides with periods of higher prices. The profitability of wind farms will therefore be higher if their production schedule coincides with these periods (generally, high demand / low supply situations). If wind represents a significant portion of supply, average revenue per unit of production may be lower as more expensive and less-efficient forms of generation, which typically set revenue levels, are displaced from economic dispatch. This may be of particular concern if the output of many wind plants in a market have strong temporal correlation. In economic terms, the marginal revenue of the wind sector as penetration increases may diminish.

Intermittency and variability

Main article: Intermittent Power Sources

Electricity generated from wind power can be highly variable at several different timescales: from hour to hour, daily, and seasonally. Annual variation also exists, but is not as significant. This variability can present substantial challenges to incorporating large amounts of wind power into a grid system, since to maintain grid stability, energy supply and demand must remain in balance.

While the negative effects of intermittency have to be considered in the economics of power generation, wind is unlikely to suffer momentary failure of large amounts of generation, which may be a concern with some traditional power plants. In this sense, it may be more reliable (albeit variable) due to the distributed nature of generation. That said, winds often stagnate during periods of peak demand, such as during heat waves. [12][13]

Wind speeds are generally much lower during periods of the highest peak-load demand (the months of June, July and August) in North America. There is an inverse relationship with wind speed and peak demand of electricity. Many grid planners do not even adjust their calculations to account for wind power installations because of that inverse (albeit happenstance) relationship.

Storage as pressurized gas

Cavern air storage is a method of storing wind energy in a large underground cavern for later use and can be used to change the time of use of wind power. It is also used as hybrid energy source, making natural gas burners more efficient with the stored wind energy. Two plants exist with this design and one new one is under consideration in Iowa. [14]

Grid management

Grid operators routinely control the supply of electricity by cycling generating plants on or off at different timescales. Most grids also have some degree of control over demand, through either demand management or load shedding. Management of either supply or demand has economic implications for suppliers, consumers and grid operators but is already widespread.

Variability of wind output creates a challenge to integrating high levels of wind into energy grids based on existing operating procedures. Critics of wind energy argue that methods to manage variability increase the total cost of wind energy production substantially at high levels of penetration, while supporters note that tools to manage variable energy sources already exist and are economical, given the other advantages of wind energy. Supporters note that the variability of the grid due to the failure of power stations themselves, or the sudden change of loads, exceeds the likely rate of change of even very large wind power penetrations.

There is no generally accepted "maximum" level of wind penetration, and practical limitations will depend on the configuration of existing generating plants, pricing mechanisms, capacity for storage or demand management, and other factors.

A number of studies for various locations have indicated that at least 20% of the total electrical energy consumption may be incorporated with minimal difficulty. These studies have generally been for locations with reasonable geographic diversity of wind; suitable generation profile (such as some degree of dispatchable energy and particularly hydropower with storage capacity); existing or contemplated demand management; and interconnection/links into a larger grid area allowing for import and export of electricity when needed. Beyond this level, there are few technical reasons why more wind power could not be incorporated, but the economic implications become more significant and other solutions may be preferred.

At present, very few locations have penetration of wind energy above 5%. Germany, Spain, and Portugal all have penetration levels above 20%, however, and Denmark's penetration is over 40%, demonstrating that the technical issues are manageable at relatively high levels. The penetration of intermittent powersources in Denmark is even higher since 20% of Denmarks electricity is produced by decentral combined heat-powerplants that only produce electricity when there is a demand for heat. However, it should also be noted that the Danish grid is heavily interconnected to the German and broader European electrical grid and can both supply and demand electricity from a broader area than just the Danish grid. In practice Denmark has solved its grid management problems by exporting almost half of its windpower to Norway. The correlation between electricity export and wind power production is very strong..

Grid energy storage

See article: Grid energy storage.

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'. Effectively, "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.

Unfortunately, it is unlikely that a pumped-water storage system could ever plug the gaps in renewable power supplies. Firstly, these power storage systems are very expensive to build. Secondly, many countries have very few geographical locations that can house such systems, and those they do have are normally situated in environmentally protected regions. This is why the Dinorwig pumped storage system in the UK proved so costly to build. Thirdly, renewable power and electrical generation systems can go off-line for days at a time, and there is no power storage system in the world that can cope with that amount of energy storage. Dinorwig can provide 5% of UK power generation for up to 5 hours before it runs out of water, and so a renewable outage lasting just two days would require 200 storage stations with the same generating capacity as Dinorwig to maintain normal power supplies. It is a meterorlogical fact that a large anticyclone sitting over a country can reduce wind, wave and solar power to absolute minimum levels, while tidal power is naturally cyclical anyway, switching off four times a day.

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 potential area sites as suitable for such facilities. Most storage technologies are currently unproven commercially at large scale - 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 [15]. Vanadium redox flow batteries are currently installed at Huxley Hill wind farm (Australia), Tomari Wind Hills at Hokkaidō (Japan), as well as in other non-wind farm applications. A further 12 MWh flow battery is to be installed at the Sorne Hill wind farm (Ireland) [16]. The supplier concerned is commissioning a production line to meet other anticipated orders.

An alternate solution is to use flywheel energy storage. This type of solution has been implemented by EDA [17] 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.

The conversion of excess wind energy into stored Hydrogen is also being developed. The Hydrogen is created using electrolysis of water and then stored for later use with hydrogen based generating equipment when there is not sufficient energy from the wind. In July 2007 the government of Newfoundland and Labrador announced a five year pilot Wind-Hydrogen Hybrid Power Systems program for this technology on the island of Ramea, which will replace the existing Wind-Diesel generating system.

V2G (Vehicle to Grid) offers another potential solution. In 2006, several companies (Altairnano, A123 Systems, Electrovaya) announced lithium batteries which could power future EVs (Electric Vehicles) and PHEVs (Plug-in Hybrid Electric Vehicles). A feature of these batteries is a high number of charge/discharge cycles per battery lifetime (Altairnano claim 15,000 cycles). By plugging thousands of cars to the grid when they are not in use (95% of the day on average), the electric car becomes an asset to the grid, rather than a drain only. Each participating vehicle would require upload as well as download capability.

Predictability

Main article: Wind Power Forecasting

Related to, but essentially different from variability, is the short-term (hourly or daily) predictability of wind plant output. Like other electricity sources, wind energy must be "scheduled" - this presents a challenge because the nature of this energy source makes it inherently variable over time. To overcome this problem, wind power forecasting methods are employed by utilities or system operators. Despite the use of forecasting, the predictability of wind plant output remains low for a variety of reasons.

Ecology and pollution

CO₂ emissions and pollution

It is sometimes said that wind energy, for example, does not reduce carbon dioxide emissions because the

intermittent nature of its output means it needs to be backed up by fossil fuel plants. Wind turbines do not displace fossil generating capacity on a one-for-one basis. But it is unambiguously the case that wind

energy can displace fossil fuel-based generation, reducing both fuel use and carbon dioxide emissions.

Wind power consumes no fuel for continuing operation, and has no emissions directly related to electricity production. Wind power stations, however, consume resources in manufacturing and construction, as do most other power production facilities. Wind power may also have an indirect effect on pollution at other production facilities, due to the need for reserve and regulation, and may affect the efficiency profile of plants used to balance demand and supply, particularly if those facilities use fossil fuel sources. Compared to other power sources, however, wind energy's direct emissions are low, and the materials used in construction (concrete, steel, fiberglass, generation components) and transportation are straightforward. Wind power's ability to reduce pollution and greenhouse gas emissions will depend on the amount of wind energy produced, and hence scalability, as well as the profile of other generating capacity.

• A study by the Irish national grid stated clearly that "Producing electricity from wind reduces the consumption of fossil fuels and therefore leads to emissions savings", and found reductions in CO2 emissions ranging from 0.59 tonnes of CO2 per MWh to 0.33 tonnes per MWh.
• Wind power is a renewable resource, which means using it will not deplete the earth's supply of fossil fuels. It also is a clean energy source, and operation does not produce carbon dioxide, sulfur dioxide, mercury, particulates, or any other type of air pollution, as do conventional fossil fuel power sources.
• Electric power production is only part (about 39% in the USA) of a country's energy use, so wind power's ability to mitigate the negative effects of energy use — as with any other clean source of electricity — is limited (except with a potential transition to electric or hydrogen vehicles). Wind power contributed less than 1% of the UK's national electricity supply in 2004 and hence had negligible effects on CO₂ emissions, which continued to rise in 2002 and 2003 (Department of Trade and Industry); the growth of installed wind capacity in the UK has been impressive (installed wind capacity doubled from 2002 to 2004, and again from end-2004 to mid-2006), but from low levels. Until wind energy achieves substantially greater scale worldwide, its ability to contribute will be limited.
• Groups such as the UN's Intergovernmental Panel on Climate Change cite wind power as a key mitigation technology available today to reduce carbon emissions in the energy supply . Intergovernmental Panel on Climate Change's 2007 Assessment Report
• During manufacture of the wind turbine, steel, concrete, aluminum and other materials will have to be made and transported using energy-intensive processes, generally using fossil energy sources.
• The energy return on investment (EROI) for wind energy is equal to the cumulative electricity generated divided by the cumulative primary energy required to build and maintain a turbine. The EROI for wind ranges from 5 to 35, with an average of around 18. This places wind energy in a favorable position relative to conventional power generation technologies in terms of EROI. Baseload coal-fired power generation has an EROI between 5 and 10:1. Nuclear power is probably no greater than 5:1, although there is considerable debate regarding how to calculate its EROI. The EROI for hydropower probably exceeds 10, but in most places in the world the most favorable sites have been developed.
• Net energy gain for wind turbines has been estimated in one report to be between 17 and 39 (i.e. over its life-time a wind turbine produces 17-39 times as much energy as is needed for its manufacture, construction, operation and decommissioning). A similar Danish study determined the payback ratio to be 80, which means that a wind turbine system pays back the energy invested within approximately 3 months. This is to be compared with payback ratios of 11 for coal power plants and 16 for nuclear power plants, though such figures do not take into account the energy content of the fuel itself, which would lead to a negative energy gain.
• The ecological and environmental costs of wind plants are paid by those using the power produced, with no long-term effects on climate or local environment left for future generations.

Ecology

• Because it uses energy already present in the atmosphere, and can displace fossil-fuel generated electricity (with its accompanying carbon dioxide emissions), wind power mitigates global warming. While wind turbines might kill some bird and bat species, conventionally fueled power plants also have the potential to affect other species through climate changes, acid rain, and pollution.
• Unlike fossil fuel and nuclear power stations, which circulate or evaporate large amounts of water for cooling, wind turbines do not need water to generate electricity.

Ecological footprint

Large-scale onshore and near-shore wind energy facilities (wind farms) can be controversial due to aesthetic reasons and impact on the local environment. Large-scale offshore wind farms are not visible from land and according to a comprehensive 8-year Danish Offshore Wind study on "Key Environmental Issues" have no discernible effect on aquatic species and no effect on migratory bird patterns or mortality rates. Modern wind farms make use of large towers with impressive blade spans, occupy large areas and may be considered unsightly at onshore and near-shore locations. They usually do not, however, interfere significantly with other uses, such as farming. The impact of onshore and near-shore wind farms on wildlife—particularly migratory birds and bats—is hotly debated, and studies with contradictory conclusions have been published. Two preliminary conclusions for onshore and near-shore wind developments seem to be supported: first, the impact on wildlife is likely low compared to other forms of human and industrial activity; second, negative impacts on certain populations of sensitive species are possible, and efforts to mitigate these effects should be considered in the planning phase. According to recent estimates published in Nature, each wind turbine kills on average 0.03 birds per year, or one kill per thirty turbines . However, the birds that are killed may on average be larger, so their populations affected more strongly by individual deaths. Aesthetic issues are important for onshore and near-shore locations in that the "visible footprint" may be extremely large compared to other sources of industrial power (which may be sited in industrially developed areas), and wind farms may be close to scenic or otherwise undeveloped areas. Offshore wind development locations remove the visual aesthetic issue by being at least 10 km from shore and in many cases much further away.

A wind turbine at Greenpark, Reading, England

Land use

• Clearing of wooded areas is often unnecessary, as the practice of farmers leasing their land out to companies building wind farms is common. In the U.S., farmers may receive annual lease payments of two thousand to five thousand dollars per turbine. The land can still be used for farming and cattle grazing. Less than 1% of the land would be used for foundations and access roads, the other 99% could still be used for farming. Turbines can be sited on unused land in techniques such as center pivot irrigation.
• The clearing of trees around onshore and near-shore tower bases may be necessary to enable installation. This is an issue for potential sites on mountain ridges, such as in the northeastern U.S.
• Wind turbines should ideally be placed about ten times their diameter apart in the direction of prevailing winds and five times their diameter apart in the perpendicular direction for minimal losses due to wind park effects. As a result, wind turbines require roughly 0.1 square kilometres of unobstructed land per megawatt of nameplate capacity. A 2 GW wind farm, which might produce as much energy each year as a 1 GW baseload power plant, might have turbines spread out over an area of approximately 200 square kilometres.
• Areas under onshore and near-shore windfarms can be used for farming, and are protected from further development.
• Although there have been installations of wind turbines in urban areas (such as Toronto's exhibition place), these are generally not used. Buildings may interfere with wind, and the value of land is likely too high if it would interfere with other uses to make urban installations viable. Installations near major cities on unused land, particularly offshore for cities near large bodies of water, may be of more interest. Despite these issues, Toronto's demonstration project demonstrates that there are no major issues that would prevent such installations where practical, although non-urban locations are expected to predominate.
• Offshore locations, such as that being developed on a large underwater plateau in eastern Lake Ontario by Trillium Power use no land per se and avoid known shipping channels. Some offshore locations are uniquely located close to ample transmission and high load centres however that is not the norm for most offshore locations. Most offshore locations are at considerable distances from load centres and may face transmission and line loss challenges.
• Wind turbines located in agricultural areas may create concerns by operators of cropdusting aircraft. Operating rules may prohibit approach of aircraft within a stated distance of the turbine towers; turbine operators may agree to curtail operations of turbines during cropdusting operations.

Impact on wildlife

• Onshore and near-shore studies show that the number of birds killed by wind turbines is negligible compared to the number that die as a result of other human activities such as traffic, hunting, power lines and high-rise buildings and especially the environmental impacts of using non-clean power sources. For example, in the UK, where there are several hundred turbines, about one bird is killed per turbine per year; 10 million per year are killed by cars alone. In the United States, onshore and near-shore turbines kill 70,000 birds per year, compared to 57 million killed by cars and 97.5 million killed by collisions with plate glass. Another study suggests that migrating birds adapt to obstacles; those birds which don't modify their route and continue to fly through a wind farm are capable of avoiding the large offshore windmills, at least in the low-wind non-twilight conditions studied. In the UK, the Royal Society for the Protection of Birds (RSPB) concluded that "The available evidence suggests that appropriately positioned wind farms do not pose a significant hazard for birds." It notes that climate change poses a much more significant threat to wildlife, and therefore supports wind farms and other forms of renewable energy.
• Some onshore and near-shore windmills kill birds, especially birds of prey. More recent siting generally takes into account known bird flight patterns, but some paths of bird migration, particularly for birds that fly by night, are unknown although a 2006 Danish Offshore Wind study showed that radio tagged migrating birds traveled around offshore wind farms. A Danish survey in 2005 (Biology Letters 2005:336) showed that less than 1% of migrating birds passing an offshore wind farm in Rønde, Denmark, got close to collision, though the site was studied only during low-wind non-twilight conditions. A survey at Altamont Pass, California, conducted by a California Energy Commission in 2004 showed that onshore turbines killed between 1,766 and 4,721 birds annually (881 to 1,300 of which were birds of prey). Radar studies of proposed onshore and near-shore sites in the eastern U.S. have shown that migrating songbirds fly well within the reach of large modern turbine blades. In Australia, a proposed onshore/near-shore wind farm was canceled before production because of the possibility that a single endangered bird of prey was nesting in the area.
• An onshore/near-shore wind farm in Norway's Smøla islands is reported to have destroyed a colony of sea eagles, according to the British Royal Society for the Protection of Birds. The society said turbine blades killed nine of the birds in a 10 month period, including all three of the chicks that fledged that year. Norway is regarded as the most important place for white-tailed eagles.
• The numbers of bats killed by existing onshore and near-shore facilities has troubled even industry personnel. A study in 2004 estimated that over 2200 bats were killed by 63 onshore turbines in just six weeks at two sites in the eastern U.S. This study suggests some onshore and near-shore sites may be particularly hazardous to local bat populations and more research is urgently needed. Migratory bat species appear to be particularly at risk, especially during key movement periods (spring and more importantly in fall). Lasiurines such as the hoary bat (Lasiurus cinereus), red bat (Lasiurus borealis), and the semi-migratory silver-haired bats (Lasionycteris noctivagans) appear to be most vulnerable at North American sites. Almost nothing is known about current populations of these species and the impact on bat numbers as a result of mortality at windpower locations. Offshore wind sites 10 km or more from shore do not interact with bat populations.

Offshore and Ocean Noise

As the number of offshore wind farms increase and move further into deeper water, the question arises if the ocean noise that is generated due to mechanical motion of the turbines and other vibrations which can be transmitted via the tower structure to the sea, will become significant enough to harm sea mammals. Tests carried out in Denmark for shallow installations showed the levels were only significant up to a few hundred metres. However, sound injected into deeper water will travel much further and will be more likely to impact bigger creatures like whales which tend to use lower frequencies than porpoises and seals. A recent study found that wind farms add 80-110 dB to the existing low-frequency ambient noise (under 400 Hz) and this could impact baleen whales communication and stress levels, and possibly prey distribution. [18]

Safety and aesthetics

On the issue of safety, the British Wind Energy Association has said:

"...wind energy is one of the safest energy technologies, and enjoys an outstanding health & safety record. In over 20 years of operating experience and with more than 50,000 machines installed around the world, no member of the public has ever been harmed by operating wind turbines. High standards exist for the design and operation of wind energy projects as well as close industry co-operation with the certification and regulatory bodies in those countries where wind energy is deployed."

There have been a number of fatalities from accidents involving wind turbines. Most involve falls or workers becoming caught in machinery while performing maintenance inside turbine housings while blade failures and falling ice have also accounted for a number of deaths. Notable public fatalities have resulted from distracted motorists seeing wind turbines along highways.

Notable negative aesthetic effects of wind turbines include:

• Recorded experience that onshore and near-shore wind turbines are noisy and visually intrusive creates resistance to the establishment of land-based wind farms in many places. Moving the turbines far offshore (10 km or more) mitigates the problem, but offshore wind farms may be more expensive and transmission to on-shore locations may present challenges in many but not all cases.
• Some residents near onshore and near-shore windmills complain of "shadow flicker", which is the alternating pattern of sun and shade caused by a rotating windmill casting a shadow over residences. Efforts are made when siting onshore and near-shore turbines to avoid this problem.
• Large onshore and near-shore wind towers require aircraft warning lights, which create light pollution at night, which bothers humans and can disrupt the local ecosystem. Complaints about these lights have caused the FAA to consider allowing a less than 1:1 ratio of lights per turbine in certain areas.[19]

Windmills at La Mancha, Spain.

These effects may be countered by changed in wind farm design:

• Improvements in blade design and gearing have quietened modern turbines to the point where a normal conversation can be held underneath one. In December 2006, a jury in Texas denied a suit for private nuisance against FPL Energy for noise pollution after the company demonstrated that noise readings were not excessive, with the highest reading reaching 44 decibels, which was characterized as approximately the same noise level as a wind of 10 miles per hour.[20] The suit was initially for visual intrusion,[21] but that was disallowed, so it concentrated on noise, which with the large spreads involved, was bound to fail). Texas civil case law requires proof of personal injury in a suit against a neighbor's activities (Klein v. Gehrung, 25 Tex. Supp. 232), so even if the plaintiffs had presented data showing more substantial noise, they would not have prevailed unless they could prove injury.

Wind turbines at Magrath, Alberta, Canada.

• Newer wind farms have more widely spaced turbines due to the greater power of the individual wind turbines, and to look less cluttered.
• The aesthetics of onshore and near-shore wind turbines have been compared favorably to those of pylons from conventional power stations.
• Offshore sites have on average a considerably higher energy yield than onshore sites, and generally cannot be seen from the shore even on the clearest of days.

Hurricanes

Main article: Hurricane

The theoretical wind energy from a hurricane is about one half the total world electrical generating capacity.

See also

Power generation

• Energy development
• Fossil fuel power and petro-free
• Hydropower
• Wind-Diesel
• Nuclear Power
• Solar power
• Solar updraft tower
• Steam engine
• List of wind turbine manufacturers
• World energy resources and consumption
• The Windbelt, a non-turbine approach to tapping wind power

Green energy

• Green energy
• Green tax shift
• Grid energy storage
• Renewable energy
• Wind
• Windmill
• Wind farm
• Wind turbine
• Merchant Wind Power

By country

• Category:Wind power by country
• List of large wind farms

Wind power projects

• Database of projects throughout the United States
• Database of projects throughout the whole World
• Altamont Pass
• Cape Wind (Massachusetts)
• Gharo Wind Power Plant in Pakistan
• Wind power in Denmark
• Wind power in Spain
• Wind power in Germany
• Wind power in Australia
• Wind power in the United Kingdom
• Renewable energy in Scotland
• Database of offshore wind projects in North America