Wind power

Wind power is the conversion of wind energy into useful form, such as electricity, using wind turbines. In windmills, wind energy is directly used to crush grain or to pump water. At the end of 2006, worldwide capacity of wind-powered generators was 73.9 gigawatts. Although wind currently produces just over 1% of world-wide electricity use,^[1] it accounts for approximately 20% of electricity production in Denmark, 9% in Spain, and 7% in Germany.^[2] Globally, wind power generation more than quadrupled between 2000 and 2006.^[3] Wind power is produced in large scale wind farms connected to electrical grids, as well as in individual turbines for providing electricity to isolated locations. Wind energy is plentiful, renewable, widely distributed, clean, and reduces greenhouse gas emissions when it displaces fossil-fuel-derived electricity. The intermittency of wind seldom creates insurmountable problems when using wind power to supply up to roughly 10% of total electrical demand (low to moderate penetration), but it presents challenges that are not yet fully solved when wind is to be used for a larger fraction of demand.^[4]

Wind energy

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 drives a global atmospheric convection system reaching from the Earth's surface to the stratosphere which acts as a virtual ceiling. Most of the energy stored in these wind movements 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. There is an estimated 72 TW of wind energy on the Earth that can potentially be converted to electricity and that is commercially viable ^[5].

Potential turbine power

Wind Turbine Power Coefficent

The power in the wind can be extracted by allowing it to blow past moving blades 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 usable power <math>P</math> available in the wind is given by:

<math>P = \begin{matrix}\frac{1}{2}\end{matrix}\alpha\rho\pi r^2 v^3</math>,

where P = power in watts, α = an efficiency factor determined by the design of the turbine, ρ = 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. ^[6] As the wind turbine extracts energy from the air flow, the air is slowed down, which causes it to spread out. 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, that is α can never be higher than 0.59 in the above equation. The Betz limit applies regardless of the design of the turbine. This equation incorporates two effects:

• 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 (28.8 km/h or 18 mi/h) 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 total power of the example breeze above through a 100 meter diameter rotor would be about 2.5 megawatts. The maximum power that could be extracted according to Betz' Law would be about 1.5 megawatts.

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.

Distribution of wind speed

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 frequency 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 Rayleigh model 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 energy comes in short bursts. The 2002 Lee Ranch sample is telling; half of the energy available arrived in just 15% of the operating time. The 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 for times that the wind is weak.

Grid management

Induction generators typically used for wind power projects require reactive power for excitation, so typically substations used in wind-power collection systems include substantial capacitor banks for power factor correction. Groups of induction generators behave differently during transmission grid disturbances, so extensive modelling of the dynamic electromechanical characteristics of a new wind farm is required by transmission grid operators to ensure predictable stable behaviour during system faults. In particular, induction generators cannot support the system voltage during faults, unlike steam or hydro turbine-driven synchronous generators.

Capacity factor

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%. Capacity factors of other types of power are based mostly on fuel cost, with a small amount of downtime for maintenance. Nuclear plants have low fuel cost, and so are run at full output and achieve a 90% capacity factor.^[7] Plants with higher fuel cost are throttled back to follow load. According to a 2007 Stanford University study published in the Journal of Applied Meteorology and Climatology, interconnecting 10 or more well-sited wind farms over a dispersed geographic area allows roughly 1/3 of the total energy produced to be relied on for baseline loads.[4]

Intermittency and penetration limits

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. 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 energy demand management, load shedding, or storage solutions. At low levels of wind penetration, fluctuations in load and allowance for failure of large generating units requires reserve capacity that can also regulate for variability of wind generation. Storage, such as with pumped hydroelectric storage or other forms of grid energy storage, can be used to "shape" wind power (by assuring constant delivery reliability), adds a cost of about 25% to yield viable commercial performance.^[8] Storage of electrical energy 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 and operating cost of storage facilities. Electricity consumption can be adapted to production variability by offering variable market pricing over the course of the day. Wind speeds are generally much lower during periods of the highest peak-load demand (the months of June, July and August) in North America. [5][6] 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. There is no generally accepted "maximum" level of wind penetration; the limit for a particular grid will depend on the existing generating plants, pricing mechanisms, capacity for storage or demand management, and other factors. Studies have indicated that 20% of the total electrical energy consumption may be incorporated with minimal difficulty^[9]. These studies have been for locations with geographically dispersed wind farms, some degree of dispatchable energy, or hydropower with storage capacity, demand management, and interconnection to a large grid area export of electricity when needed. Beyond this level, there are few technical limits, but the economic implications become more significant. At present, few grid systems have penetration of wind energy above 5%. Germany, Spain, and Portugal all have penetration levels below 10%. Denmark's penetration is over 20%, but the Danish grid is heavily interconnected to the European electrical 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.^[10]. Intermittency is a major problem that may well limit the penetration of wind power generation.[7] The 2006 Energy in Scotland Inquiry report [8] expresses concern about some aspects of wind power.

"The inherent intermittency of wind power means that it cannot be relied on to deliver firm output at any given time. However, its input when available has to be accepted into the grid. A diversity of supply is essential to achieve maximum security and flexibility in the supply of electricity."

A study commissioned by the state of Minnesota^[11] considered penetration of up to 25%, and concluded that integration issues would be manageable and have incremental costs of less than one-half cent ($0.0045) per kWh. A similar report from Denmark noted that their wind power network was without power for 54 days during 2002.[9]

Predictability

Main article: Wind Power Forecasting

Related to variability is the short-term (hourly or daily) predictability of wind plant output. Like other electricity sources, wind energy must be "scheduled". The nature of this energy source makes it inherently variable. Wind power forecasting methods are used, but predictability of wind plant output remains low. Storage, such as with pumped hydroelectric storage or other forms of grid energy storage, can be used to "shape" wind power (by assuring constant delivery reliability), adds a cost of about 25% to yield viable commercial performance.^[8] Storage of electrical energy 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.

Turbine placement

Measurements

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. An 'ideal' location would have a near constant flow of non-turbulent wind throughout the year with a minimum likelihood of sudden powerful bursts of wind. A vitally important factor of turbine siting is also access to local demand or transmission capacity. Usually sites are pre-selected on basis of a wind atlas, and validated with wind measurements. meteorological wind data alone is usually not sufficient for accurate siting of a large wind power project. Collection of site specific data for wind speed and direction is crucial to determining site potential. ^[12] To collect wind data a meteorological tower is installed 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 are guyed steel-pipe structures which are left to collect data for one to two years and then disassembled. Data is collected by a data logging device which stores and transmits data for analysis. 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 double energy production.

Altitude

The wind blows faster at higher altitudes because of the reduced influence of drag of the surface and lower air viscosity. 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 wind profile power law, 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%.

Wind park effect

Wind farms have many turbines and each extracts some of the energy of the wind. 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.

Low temperature

Utility-scale wind turbine generators have minimum temperature operating limits which apply in areas that 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. If the low-temperature interval is combined with a low-wind condition, the wind turbine will require an external supply of power, equivalent to a few percent of its rated power, for internal heating. 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 as the wind accelerates over a ridge. 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 when sites are picturesque or environmentally sensitive (for instance, having substantial bird life).

Near-Shore

Near-Shore turbine installations are on land within three kilometers of a shoreline or on water within ten kilometers of land. These areas are good sites for turbine installation, because of wind produced by convection due to differential heating of land and sea each day. 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 bird migration and nesting, aquatic habitat, transportation (including shipping and boating) and visual aesthetics. Residents near some 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 20% of total electricity production in the country, with many offshore windfarms.^[13] Denmark plans to increase wind energy's contribution to as much as half of its electrical supply. The United Kingdom plans to use offshore wind turbines to generate enough power to light every home in the U.K. by 2020.^[14] 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 pursuing 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 installation is more expensive than onshore but this depends on the attributes of the site. Offshore towers are generally taller than onshore towers once the submerged height is included. Offshore foundations may be more expensive to build. Power transmission from offshore turbines is through undersea cable. Offshore installations may use high voltage direct current operation if significant distance is to be covered. 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. 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. Offshore wind turbines will probably continue to be the largest turbines in operation, since the high fixed costs of the installation are spread over more energy production, reducing the average cost. Offshore wind farms tend to be quite large&mdash, often involving over 100 turbines.

Airborne

Main article: Airborne wind turbine

Wind turbines might also be flown in high speed winds at altitude, although no such systems are in commercial operation.

Utilization of wind power

Main article: :Category:Wind power by country

There are many thousands of wind turbines operating, with a total capacity of 73,904 MW of which wind power in 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 20th century. World wind generation capacity more than quadrupled between 2000 and 2006. In some countries (Spain and Denmark) wind supplies 10% or more of the nation's electricity. 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^[1], 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 world's total electricity production (2005). Germany was the leading producer of wind power with 28% of the total world capacity in 2006 (7.3% of German electricity); the official target is for renewable energy to meet 12.5% of German electricity needs by 2010 — this target may 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 gross 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.[10] 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 its existing capacity of approximately 4.5 gigawatts. Iowa and Minnesota are expected to each produce 1 gigawatt by late-2007.^[17] Wind power generation in the U.S. was up 31.8% in February, 2007 from February, 2006.^[18] India ranks 4th in the world with a total wind power capacity of 6,270 MW in 2006, or 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.^[1] The windfarm near Muppandal, Tamil Nadu, India, provides an impoverished village with energy for work.^[19][20] India-based Suzlon Energy is one of the world's largest wind turbine manufacturers.^[21] 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.^[22] 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.^[23] The federal government has created an incentive program, called Proinfa,^[24] 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.^[25] 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.^[26] 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.^[27] 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.^[28]

Small scale wind power

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. Isolated communities that otherwise rely on diesel generators may use wind turbines to displace diesel fuel consumption. 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 batteries, photovoltaic or diesel systems to supplement the wind turbine. 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.

Economics and feasibility

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.^[15] 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.^[29] 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,^[30] 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).^[31] 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.^[32] 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). Wind and hydro power have negligable fuel costs and relatively low maintenance costs; in economic terms, wind power has a low marginal cost and a high proportion of capital cost. The estimated average cost per unit incorporates the cost of construction of the turbine and transmission facilities, borrowed funds, return to investors (including cost of risk), estimated annual production, and other components, averaged over the projected useful life of the equipment, which may be in excess of twenty years. Energy cost estimates are highly dependent on these assumptions so published cost figures can differ substantially. Similar methods apply to other electrical energy sources. 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 will depend on factors 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.^[33]

Theoretical potential

Wind power theoretical potential is much greater than current world energy consumption. The most comprehensive study to date^[34] 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. 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.^[35][36] This number could also increase with higher altitude or airborne wind turbines.^[37]

Direct costs

Many potential sites for wind farms are far from demand centers, requiring substantially more money to construct new transmission lines and substations. 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 usually less than 1 cent per kilowatt-hour ^[38] 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. 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.

External costs

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. 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. 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.

Incentives

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.^[39] 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. 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.

Environmental effects

CO₂ emissions and pollution

Wind power consumes no fuel for continuing operation, and has no emissions directly related to electricity production. Operation does not produce carbon dioxide, sulfur dioxide, mercury, particulates, or any other type of air pollution, as do fossil fuel power sources. Wind power plants consume resources in manufacturing and construction. 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 initial carbon dioxide emissions "pay back" within about 9 months of operation for off shore turbines.^[40] Wind power may affect emissions at fossil-fuel plants used for reserve and regulation:

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.^[41]

A study by the Irish national grid stated 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 0.33 to 0.59 tonnes of CO2 per MWh.^[42]

Net energy gain

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. Since energy produced is several times energy consumed in construction, there is a net energy gain. The energy used for construction is produced by the wind turbine within a few months of operation.

Ecological footprint

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. Several incidents have been reported of oil or hydraulic fluid being leaked into the surrounding environment, in some cases contaminating protected drinking water areas. The liquid can run down the blades during motion and be dispersed over a wide area.^[43]

A wind turbine at Greenpark, Reading, England

Land use

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 200 MW wind farm, which might produce as much energy each year as a 100 MW baseload power plant, might have turbines spread out over an area of approximately 20 square kilometres. Clearing of wooded areas is often unnecessary. Farmers commonly lease land to companies building wind farms. In the U.S., farmers may receive annual lease payments of two thousand to five thousand dollars per turbine.^[44] 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.^[45] Turbines can be sited on unused land in techniques such as center pivot irrigation. The clearing of trees around tower bases may be necessary for installation sites on mountain ridges, such as in the northeastern U.S.^[46] Turbines are not generally installed in urban areas. Buildings may interfere with wind, and the value of land is high. Despite these issues, Toronto's demonstration project demonstrates that such installations are possible. 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

Birds

Some wind turbines kill birds, especially birds of prey. ^[47] More recent siting generally takes into account known bird flight patterns. 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.^[48] In the United States, turbines kill 70,000 birds per year, compared to 57 million killed by cars and 97.5 million killed by collisions with plate glass.^[49] An article in Nature stated that each wind turbine kills on average 0.03 birds per year, or one kill per thirty turbines.^[50] 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."^[51] 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 paths of bird migration, particularly for birds that fly by night, are unknown. Another study suggests that migrating birds adapt to obstacles; those birds which continue to fly through a wind farm avoid the large turbines,^[52] at least in the low-wind non-twilight conditions studied. A Danish 2005 (Biology Letters 2005:336) study showed that radio tagged migrating birds traveled around offshore wind farms, with 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^[53] 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 wind farm was canceled because of the possibility that a single endangered bird of prey was nesting in the area. A 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.

Bats

The numbers of bats killed by existing onshore and near-shore facilities has troubled even industry personnel.^[54] 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.^[55] 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, red bat, and the silver-haired bat 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.

Fish

In Ireland, construction of a wind farm caused pollution feared to be responsible for wiping out vegetation and fish stocks in the Lough Lee.^[56] A separate landslide is thought to have been caused by wind farm construction, and has killed thousands of fish by polluting the local rivers with sediment.^[57]

Offshore 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), which could impact baleen whales communication and stress levels, and possibly prey distribution.^[58]

Safety

A turbine on fire at Nissan Motor Manufacturing (UK) Ltd after an oil leak

Operation of any utility-scale energy conversion system presents safety hazards. Wind turbines do not consume fuel or produce pollution during normal operation, but still have hazards associated with their construction and operation. Although organizations like the British Wind Energy Association claim that wind power is "one of the safest energy technologies" and that "no member of the public has ever been harmed by operating wind turbines",^[59][60] there have been at least 35 fatalities directly related to wind turbine accidents, including both workers and members of the public, and other injuries and deaths attributed to the wind power life cycle. Most worker deaths involve falls or becoming caught in machinery while performing maintenance inside turbine housings. Blade failures and falling ice have also accounted for a number of deaths and injuries.^[61] Deaths to members of the public include a parachutist colliding with a turbine and small aircraft crashing into support structures. Other public fatalities have been blamed on collisions with transport vehicles and distracted motorists seeing wind turbines along highways.^[62][63] When a turbine's brake fails, the turbine can spin freely until it disintegrates or catches fire. Turbine blades may fail spontaneously due to manufacturing flaws. Lightning strikes are a common problem, also causing rotor blade damage and fires.^{[63][64][65][66]} When ejected, pieces of broken blade and ice can be thrown hundreds of meters away. Although no member of the public has been killed by a malfunctioning turbine, there have been close calls, including injury by falling ice. Large pieces of debris, up to several tons, have dropped in populated areas, residential properties, and roads, damaging cars and homes.^[63] Turbine fires often cannot be extinguished because of the height, and are left to burn themselves out. In the process, they generate toxic fumes and can scatter flaming debris over a wide area, starting secondary fires below. Several turbine-ignited fires have burned hundreds of acres of vegetation each, and one burned 80,000 hectares (200,000 acres) of Australian National Park.^{[63][67][68][69]} Certainly the number of deaths and injuries caused by wind power are lower than most other power sources, but the total amount of energy produced is also low. When considered together, wind power is not an especially safe form of energy generation, per kilowatt-hour produced. Wind power proponent and author Paul Gipe estimated in Wind Energy Comes of Age that the mortality rate for wind power from 1980–1994 was 0.23 deaths per terawatt-hour, which he later corrected to 0.4.^[70][71] Even assuming the more conservative estimate, this is equivalent to 2000 deaths per terawatt-year. By comparison, hydroelectric power was found to kill 883 members of the public for every TW·yr generated from 1969–1996,^[72] which includes the Banqiao Dam collapse in 1975 that killed thousands. Although the wind power death rate is much higher than other power sources, the numbers are necessarily based on a small sample size, and it is difficult to predict whether the safety rate would improve if wind power were widely adopted.

Aesthetics

Historical experience of noisy and visually intrusive wind turbines may create resistance to the establishment of land-based wind farms. Residents near turbines may complain of "shadow flicker" caused by rotating turbine blades. Wind towers require aircraft warning lights, which create light pollution, which bothers humans. Complaints about these lights have caused the FAA to consider allowing fewer lights per turbine in certain areas.^[73]

Windmills at La Mancha, Spain, made famous by the 1605 novel Don Quixote, are a national treasure.

These effects may be countered by changes in wind farm design. Modern large turbines have low sound levels at ground level. For example, in December 2006, a Texas jury denied a noise pollution suit against FPL Energy, after the company demonstrated that noise readings were not excessive. The highest reading was 44 decibels, which was characterized as about the same level as a 10 mile/hour (16 km/hr) wind.^[74]

Wind turbines at Magrath, Alberta, Canada.

Newer wind farms have larger, more widely spaced turbines, and so look less cluttered than old installations. 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). 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.

Examples of opposition to wind power

• After a wind farm was proposed several miles off the coast of Cape Cod, environmentalists raised objections. Ted Kennedy, typically a supporter of wind power, owns a summer home in the area and objected to the proposal.^[75]
• On October 16, 2003 in Galway, Ireland, construction of the foundation of a wind farm caused almost half a square kilometer of bog to slide 2.5 kilometers down a hillside. The slide destroyed an unoccupied farmhouse and blocked two roads. Nearby residents expressed concern over these environmental impacts.^[76]
• On December 4, 2007, environmentalists filed lawsuits to block two proposed wind farms in southern Texas. The lawsuits expressed concerns over wetlands, habitat, endangered species and migratory birds.^[77]
• On January 12, 2004, it was reported that the Center for Biological Diversity filed a lawsuit against wind farm owners for killing tens of thousands of birds at the Altamont Pass Wind Resource Area near San Francisco, California.^[78]
• On December 7, 2007, it was reported that environmentalists opposed a plan to build a wind farm in western Maryland. Ajax Eastman, whom the article described as "a conservationist from Baltimore whose opposition has helped stall construction of other wind farms in western Maryland," was quoted as saying, "The idea of destroying the Appalachian ridge tops for such a little bit of energy capacity doesn't make any sense to me." Paulette Hammond, president of the Maryland Conservation Council, was quoted as saying, "This would denude some very valuable forest tree canopy ... and wouldn't provide nearly the amount of energy we'll need." The article also said, "Dan Boone, a former state wildlife biologist who has been fighting wind farms in western Maryland, said that the Savage River and Potomac state forests contain rare old-growth trees and that threatened species that shouldn't be disturbed." ^[79]

See also

Energy Portal

Sustainable development Portal

• Energy development
• List of wind turbine manufacturers
• Solar updraft tower
• Wind-Diesel
• The Windbelt, a non-turbine approach to tapping wind power
• World energy resources and consumption
• Distributed Energy Resources
• Kite Wind Generator
• Merchant Wind Power
• Green energy
• Green tax shift
• Grid energy storage
• Renewable energy
• Category:Wind power by country
• List of large wind farms

References

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
• Wind power in the United States
• Renewable energy in Scotland
• Database of offshore wind projects in North America