Wind resources

Wind is the movement of air in response to pressure differences within the atmosphere (Figure 11.1). These pressure differences exert a force that causes air masses to move from a region of high pressure to one of low pressure. That movement is wind. The pressure differences are caused primarily by differential heating effects of the sun on Earth’s surface, although Earth’s rotation will also play a part. Thus, wind energy can be considered to be primarily another form of solar energy.

The effects that lead to the generation of winds are complex and unpredictable, and as a consequence wind is a variable and unpredictable resource. This can make wind power hard to manage on a conventional grid. Advanced weather forecasting techniques are rendering short-term wind variations more predictable and this is helping to make the energy generated from wind easier to manage on a grid. Geographical averaging of wind capacity over a specific region can also lead to more reliable output levels, because when the wind does not blow in one part of the region it will often be blowing somewhere else. In this case the larger the region, the more predictable the resource becomes. Nevertheless, wind will always be an intermittent source of energy and this must be taken into account when installing wind capacity.

To assess whether a particular wind site is suitable for exploitation it is necessary to monitor the wind for a period of at least a year, preferably longer. Long-term average wind speeds are usually much more predictable than short-term values, so such long-term assessments provide a much more reliable means of establishing the size of the resource at a particular site.

Annually, over Earth’s land masses, around 1.7 million TWh of energy is generated in the form of wind. When oceans are included too, the figure is much higher. Even so, only a small fraction of the available wind energy can be harnessed to generate useful energy. Estimates of how large this might be are difficult; some recent estimates have suggested that global onshore wind could support generating capacities ranging from 100,000 GW to as much as 1,000,000 GW. Even the smaller of these figures is greater than the total global generating capacity from all sources of around 5000 GW. Offshore wind might provide an even greater potential than onshore wind, though exploitation of far-from-shore sites in deep waters is not possible today.

While the potential is clearly large, the exploitation of wind energy is often limited, particularly onshore, by additional restrictions that prevent construction. Urban areas are normally not considered suitable and neither are remote sites if they are close to habitations. Elsewhere, aesthetic considerations may prevent construction of wind farms. In addition, the onshore wind regime will depend critically on local geographic features that can reduce wind speeds in some areas while enhancing them in others. Another factor to take into account is the nature of the ground-cover vegetation, which will affect turbulence levels in the air layers close to Earth’s surface. As a consequence, onshore sites need to be chosen carefully and surveyed over an extended period before being developed to ensure they will provide a sufficiently rich wind regime. The best onshore wind sites are often in regions remote from urban centers. This can cause its own problems because these regions are also far from the backbone of the grid system and dedicated transmission capacity may be needed to bring the output to the users.

Offshore construction is usually less constrained than construction onshore, though it should generally avoid shipping lanes or fishing waters. The wind regime offshore is more predictable too, and the relatively smooth surface of the sea means that surface-generated turbulence is often lower than onshore while the extended open spaces allow high wind speeds to develop. Against this, turbines have to cope with regular storms at sea. Like onshore sites, offshore wind farms may also be distant from the grid system and require lengthy transmission connections. However, there are also good offshore sites relatively close to large coastal cities, such as along the eastern seaboard of the United States, and these sites are likely to be attractive for future wind development.

An important factor for consideration at any wind site is that the wind speed increases with height above Earth’s surface. In consequence, the taller the tower upon which a wind turbine is mounted, the better the wind regime available. Large wind turbines mounted on tall towers will therefore perform better than smaller turbines on proportionally smaller towers. This wind speed gradient means that the wind speed at the lowest point of the rotor will be smaller than at the highest point. This will create a bending force on the rotor that must be taken into account in wind turbine design.

Wind turbine technology

The modern history of the wind turbine for power generation began during the oil crises of the 1970s. During these early years of wind development many different types of wind turbines were tested. The majority was horizontal-axis wind turbines with a rotor at one end of a shaft and a generator at the other, the whole mounted on the top of a high tower (Figure 11.2). Machines of this type were fitted with rotors carrying one, two, three, and more blades. They could be upwind designs, with the rotor facing into the wind and the generator behind, or downwind designs that reversed this arrangement. All used gearboxes to match the rotor speed to generator speed and they often relied on the grid for frequency synchronization and control.

Figure 11.2

Alongside these horizontal-axis turbines a range of vertical-axis turbines were also developed (Figure 11.3). The most common of these was the Darrieus or eggbeater wind turbine, so-called because its blades were shaped like those of an eggbeater. Other blade designs tested included an H-shaped vertical-axis configuration. The primary advantages claimed for vertical-axis turbines was that they do not need to yaw to keep the rotor facing into the wind while their massive mechanical components—the gearbox and the generator—can be sited on the ground.

Figure 11.3

In spite of these advantages, vertical-axis machines have never prospered. As the technology has matured most of these designs have disappeared so that today virtually all wind turbines have a similar configuration: a three-blade rotor attached to the front of a horizontal-axis drive-train shaft in an upwind design in which the rotor is always facing the wind. A generator and gearbox (if used) make up the remainder of the drive train that is housed in a protective nacelle mounted on top of a tall steel tower. From an average turbine size of 30 kW in the early 1980s, today’s largest onshore machines are in the 2–3 MW range, while offshore machines of 5 MW are now common, and larger machines up to 15 MW are being planned.

Alongside the market for utility wind turbines there is also a parallel market for smaller wind turbines of less than 100 kW. These are often used to supply power to remote sites, for off-grid domestic supply or for a range of small distributed generation applications. One of the largest markets for such small turbines is the United States.

Wind turbine anatomy

The standard utility scale wind turbine for both onshore and offshore applications has, as noted already, a three-bladed rotor attached to a drive train and generator with the whole assembly mounted at the top of a tall tower inside a protective housing called a nacelle. The nacelle must be able to rotate, so it is attached to the tower through a yaw bearing that allows the complete structure to turn as the wind direction changes, with the rotor always facing into the wind and the nacelle behind it. In an upwind design, the rotation must be powered by a yaw motor although this is not necessary for a downwind rotor.

The drive train within the nacelle will often include a gearbox that increases the rotational speed of the drive shaft to be able to drive a generator that is synchronized to the local grid frequency, usually 50 Hz or 60 Hz. Other turbines use a variable-speed system with a power electronic converter to ensure the output is always at grid voltage even as the generator speed varies. There are, however, an increasing number of wind turbines that dispense with the gearbox—a component that has often proved unreliable in the past—and use a direct-drive system instead. Direct-drive generators tend to be more expensive but the benefits in terms of higher reliability can outweigh this.

The tower-top structure may have a helicopter pad and will usually be accessible via a ladder or lift within the tower. There may also be a crane fitted to the top for maintenance purposes. Power from the generator will be carried down the tower in cables to a transformer fitted at the bottom of the tower that converts the output to the local distribution grid voltage. If the unit is part of a large wind farm this power may then be carried to a wind farm substation where the voltage is raised further to be fed into the transmission system.

The reliability of modern wind turbines is much higher than it was in the 1980s and 1990s, and most turbine manufacturers aim for lifetimes of 20 years of more. However, anecdotal evidence suggests that maintenance costs rise as machines get older.

Drive Trains and Generator

The drive train of a wind turbine begins with the shaft to which the rotor is attached (Figure 11.4). This transmits the mechanical energy generated by the rotor in the form of a rotational force or torque. In most early wind turbines and in many modern units the shaft is connected to a gear box that increases speed of rotation from perhaps 20 rpm to 1000 rpm or 1500 rpm (50 Hz) or 1200 rpm or 1800 rpm (60 Hz), suitable to drive a synchronized generator. A drive shaft from the gearbox is then linked to the generator.

Figure 11.4

The drive train has to endure more than simply the rotational torque produced by the rotor. The force of the wind on the rotor blades can be extremely uneven and this will generate lateral or bending forces too, which are transmitted into the gearbox and generator. While shock-absorbing components can help reduce the effect of such lateral forces, the effect on the gearbox can often be severe and this can lead to early failure.

Various attempts have been made to improve gearbox reliability but perhaps the optimum, if most expensive, solution is to remove the gearbox all together and drive the generator directly from the rotor. Direct-drive generators are becoming increasingly popular in large wind turbines and work is being carried out to develop a superconducting direct-drive generator for large offshore wind turbines of 10 MW or more.

The generators used in early wind turbines were asynchronous generators (often motors operated in reverse) that relied on the grid to control their rotational frequency. This generally weakened the grid and large wind farms with this type of turbine usually required some form of reactive compensation to improve grid stability. Modern wind turbines are normally required to be able to maintain their synchronization with the grid independent of the grid itself, so a more sophisticated design is necessary. This has the added advantage of allowing them to help maintain grid stability rather than reducing it.

Designing an effective generator system for a wind turbine can be difficult because of the variable wind speed conditions. A conventional synchronous generator can only rotate at one speed if it is to supply power at the grid frequency so that the wind turbine must be maintained at a single rotational speed. To overcome this some wind turbine designs include two generators, one for low-speed operation and one for higher-speed operation.

While this is the cheapest variable-speed solution, maintaining fixed rotational speeds creates additional stresses on the rotor and drive train, something that manufacturers are seeking to avoid to improve reliability and lifetimes. The best way to avoid many of the problems associated with variable wind speed is to use a variable-speed generator. The disadvantage of this is that a generator operating at varying speeds will produce an output of variable frequency. Variable-speed operation can, therefore, only be achieved by using some form of power electronic frequency conversion system to maintain grid frequency independent of the frequency from the generator. These electronic systems convert the output from the generator to direct current and then back to alternating current at the grid frequency.

Two types of variable-speed generators have been used in recent years. The first is called a partial conversion system and uses a doubly fed generator to provide limited speed variation. The second is a full conversion generator that is more expensive but also more flexible.

Variable-speed operation reduces the stress on the rotor because the wind turbine can always operate at the optimum speed for the wind conditions. In addition, it means that energy can be harvested over a wider range of wind conditions than is possible with fixed-speed generators. A further advantage is that variable-speed generators with full AC–DC–AC converters can provide grid frequency support facilities, as noted before. This can make them easier to integrate into modern grids.

Offshore wind turbine technology

Offshore wind power started to accelerate toward the end of the first decade of the 21st century and has become an important part of global wind power expansion. By the end of 2012 global capacity had exceeded 5400 MW. Most of this capacity is in Europe but development in both Asia and the United States are anticipated. China has started to build offshore wind farms, as has South Korea, and Japan is exploring its use as a potential replacement for nuclear capacity.

Wind turbines for use offshore are similar to those used onshore, and offshore wind turbines lean heavily on the technology developed for onshore units. However, offshore turbines have to be more rugged than onshore turbines because of the harsher environment. Additionally, offshore wind turbines can be significantly larger than those used onshore. This is important because using larger units can partially outweigh the additional cost of creating a wind turbine foundation offshore. It is the latter that is primarily responsible for making offshore construction more expensive than onshore.

Offshore construction has several advantages over onshore construction. One of the most important is that there are fewer environmental restrictions so that it is often easier to gain permission for development offshore than for construction onshore. The wind regime is generally better offshore too and this means that similarly sized wind farms will generate more power, more reliably. As a result, offshore wind development, where it is feasible, is likely to become increasingly important over the next 10–20 years.

Against these advantages, offshore development is more expensive than development onshore. As already noted, the most important additional expense is the construction of the foundation for an offshore wind turbine (Figure 11.5). Then, once the turbines have been installed, they are subject to much more severe conditions than onshore so reliability is a key issue. On top of that, offshore maintenance is much more difficult to carry out and therefore also more costly than for a similar unit onshore. All these factors affect the economics of offshore construction.

There are a variety of structures that can be used to establish an offshore wind turbine foundation. The most common of these is a simple monopole tower, similar to the tower used onshore. The foundation for this type of structure is created by using a pile driver to drive a steel monopole into the seabed. This base structure normally terminates with a flange at just above sea level and the tower to support the nacelle and rotor is bolted to this flange. Monopoles can be used in depths up to around 30 m, provided the seabed is suitable for pile driving, but beyond that depth the driving the base becomes more difficult. For long monopoles, resonance effects may also start to come into play and this will limit their applicability.

For shallow waters the main alternative to the monopole is the gravity base structure. This is a massive foundation, usually fabricated onshore from concrete sections then floated to the wind turbine site and then sunk to the seabed by loading it with ballast. The wind turbine is then attached to the concrete base that relies on its mass to remain stable. Gravity structures can be used for a variety of seabed conditions but site preparation is necessary to ensure the base sits horizontally. Otherwise, the result is a wind turbine that is not perpendicular.

For deeper waters a range of tripod and space frame foundations can be used. These will have three or more legs, each anchored to the seabed to provide a wide, stable base. The wind turbine is again bolted to the top of the foundation structure. Such structures are the most economical in water depths of 40–60 m.

Stabilizing an offshore wind turbine in water depths greater than 60 m becomes increasingly expensive, and at this depth the only real solution is some form of floating support. No major wind farms have yet been deployed with floating supports but a number of designs are being tested. These include fully buoyant platforms that are anchored in deep water and can support multiple wind turbines; single turbine supports including spar buoys that are partially submerged and anchored to the seabed; and tension leg platforms with several legs that are partially buoyant but are held in position under water by steel guys under tension.

Wind turbines for offshore use have to be more rugged than onshore units so that they can resist the corrosive effects of seawater. Maintenance is much more difficult to carry out offshore too because of the access problems. To improve offshore reliability wind turbine manufacturers are developing remote monitoring and control systems that can both identify and manage faults as they develop so that units can be kept in service and until maintenance can be carried out.

Another issue with offshore development is the means of bringing power ashore. Most of the early offshore wind farms in European waters use simple AC transmission lines to bring power from an offshore substation to a substation on shore. However, as the distance from shore increases, AC transmission becomes less effective because of capacitive losses and it is necessary to switch to high-voltage DC (HVDC) transmission. The crossover between the two systems, economically, is generally considered to be around 100 km. HVDC transmission lines are beginning to be introduced for offshore wind farms in European waters.

For agglomerations of far-from-shore wind farms there are also arguments for setting up dedicated offshore grids that can be linked to onshore grids at more than one point. Typical of such proposals is the North Sea Supergrid, which would link several littoral countries both to multiple offshore wind farms and to one another.

Wind farms

A wind farm is a collection of wind turbines that operate as a single power station. Depending on its size, a wind farm will normally have a dedicated substation into which power from all the wind turbines is fed and from which it is carried to the nearest access point to the grid system (Figure 11.6).

Power can be fed either into a local distribution system, or, for the largest wind farms, directly into the transmission grid. As many wind farms are located in regions remote from existing transmission system backbones, wind power development will often involve additional transmission lines. This can add to the expense of a wind project.

When wind turbines are built close together they will often affect one another because, depending on the wind direction, some turbines will be downwind of others and will therefore experience turbulence from the upwind turbines. Correct turbine placement is vital if a wind farm is to extract the maximum energy from the resource.

Onshore wind farms are generally limited in size and this, together with the nature of the terrain, will often make turbine placement less critical. For offshore construction, however, where individual turbines are larger and the number of turbines can be larger too, placement can be critical and can in some cases affect the economics of a project. Energy losses due to poor placement of up to 10% have been recorded. Modeling airflow over the wind farm site is the best way to determine the optimum placement, but computer modeling techniques have yet to reach the level of sophistication to accurately model a large offshore wind farm.

Maintaining a good distance between wind turbines means that wind farms can end up occupying a very large area of land or sea. This is not necessarily a handicap because the turbines generally occupy less than 1% of the actual area. The remainder is usually accessible for farming use onshore or for fishing or the passage of ships offshore.

Environmental effects of wind power

The environmental impacts of wind power, with the exception of the aesthetic impact of a series of wind turbines being added to the landscape, are generally limited. Most of the problems that arise when attempting to gain permission to erect a wind turbine or build a wind farm are related to the siting of wind turbines due to the visual impact. This has proved a major issue, particularly in densely populated countries such as the United Kingdom where wind farms are often subject to lengthy permitting procedures. It is partly for this reason that offshore construction is accelerating around European shores.

Of the other potential environmental effects, noise has been considered an issue in the past, but most modern wind turbines with their large, low-speed rotors have limited noise impact provided they are reasonably distant from habitations. Other areas of concern, such as the danger to birds from rotating turbine blades or impacts on marine life from construction offshore, have not generally proved serious and studies have suggested the impact is small, although anecdotal evidence sometimes contradicts this.

Wind intermittency and grid issues

Electricity generated from wind turbines suffers from two problems that make it difficult to accommodate on a conventional grid. First the wind is an intermittent source of energy so a single wind turbine cannot ever provide electric power continuously. Second, the wind is unpredictable so that it is impossible to know in advance when a wind turbine will supply power and when it will not. Together these make the management of wind energy more difficult than most other sources of power.

Intermittency on its own is not necessarily a problem. Tidal power is intermittent but the output can be predicted with great certainty so that the dispatching of tidal power is relatively easy. Solar power is intermittent too and has a degree of unpredictability, but during daylight hours there will be generally some solar power available so the problem here is less severe. In addition, in regions where the sun shines often, power demand, particularly from air conditioning, will often follow solar output so that solar power can provide a reliable source of peak power that will coincide with rises in this type of demand. Wind power, on the other hand, cannot be relied on to coincide with anything. (However, the wind often blows more strongly in winter than in summer, so wind and solar can be complementary over a seasonal timescale.)

There are various ways of managing the intermittent and unpredictable output from wind power plants. The simplest solution, and the one often applied where wind is used for remote or small domestic supplies, is to store the wind energy—most frequently with a small battery storage system. So long as the wind power–generating capacity is sufficiently large enough compared to the demand, the wind power plant will be able to supply enough stored energy to provide a continuous supply of electricity. Such systems usually include some backup source of electricity too, for those rare periods when there is a lengthy calm period.

Energy storage is, in principle, the most robust solution for wind energy management on grid systems too. Unfortunately, energy storage capacity is generally expensive and very few grids have storage sufficient to manage the large amounts of wind power being introduced into grid systems across the world today. As the proportion of renewable energy grows, it may become economically expedient to expand storage capacity. Meanwhile, novel solutions to this problem for wind have been proposed, including the integration of wind generation with hydrogen production as a means of energy storage. It has yet to be established that this is an economically viable solution.

Where storage is not available, wind output variability is usually managed by maintaining a sufficiently large standby capacity to step in when wind output fails. The cheapest way of achieving this, but one that is not available everywhere, is to use hydropower capacity as backup. Provided this capacity is based on dam and reservoir power plants, hydropower can be brought online or taken offline rapidly and at will. In a sense this is much like pumped storage hydropower but without the full flexibility. Nevertheless, where hydropower capacity of this sort is available, the cost of grid integration of wind energy has been shown to be cheaper than where it is not available. When such hydropower capacity is not available the standby capacity will probably be based on natural gas–fired combined cycle power plants, which are more expensive to operate and therefore increase overall wind integration costs. Coal plant manufacturers are also trying to adapt their technologies to be able to provide the flexibility to provide grid support of this type too.

There are also other means of helping to integrate wind power. While wind unpredictability and intermittency cannot be avoided, there are ways of ameliorating the problems associated with them. One is to use sophisticated weather forecasting techniques. If wind output can be predicted with reasonable accuracy several hours or even a day ahead, dispatching of the power on the grid becomes much easier. Integrating weather forecasting into automated dispatching systems is already being used with advanced dispatching systems and the accuracy and applicability of such techniques can be expected to improve in the future.

A further factor that can help make wind output more predictable is geographical averaging. An individual wind turbine will always have a varying and intermittent output. However, if the output of one turbine is combined with that of a second at a different location, the combined output will generally vary less because the wind level at one location will not be exactly correlated with that in another. This idea can be expanded so that the output from wind farms over a wide geographical area can be considered collectively as one source of power with much less variability than any one wind farm individually can provide. Modern computerized grid management systems can treat groups of power plants such as wind farms as single, virtual power plants, creating more reliable and therefore more valuable wind energy power sources.

Wind capacity limits

It is clear from figures already quoted that there is, in principle, enough wind energy to supply global electricity demand 20 times over, or more. There would appear to be no limit, therefore, to the amount of electricity that might be generated from wind turbines except under exceptional regional circumstances.

What will limit wind power is the amount of wind energy that can be satisfactorily managed on grid systems without endangering grid security and reliability. This limit will depend on the availability of the means of providing energy when the wind does not blow, including energy storage and alternative power sources. Various estimates have been made in the past for the amount of wind energy that can be absorbed, practically, on modern grids. However, the best way of assessing what is practical is to look at the proportion of wind capacity on existing grids where wind penetration is high.

The highest wind penetration in the world in on the Danish grid where 29% of power was provided by wind energy in 2012. Denmark has links with neighboring countries and so can export excess wind power allowing it to use most of the wind energy it generates. Elsewhere, countries have had problems absorbing all the wind energy they generate. Germany, with 11% of its electricity supplied by wind power in 2012, has often had to curtail wind power during periods of high output because its transmission system does not allow large amounts of power to be transmitted from the north, where most wind generation is situated, to demand centers in the south.

Germany’s problem can be solved with grid modification. Meanwhile, the International Energy Agency has estimated that a grid such as that in Denmark would be capable of managing around 60% wind penetration—that is, 60% of all electricity could be provided by wind capacity—although others countries might have lower capabilities. This suggests that with appropriate adaptation and grid integration, perhaps as much as 50% of the power on a typical modern grid might feasibly be provided by wind power. Whether that is desirable is probably more a matter of politics than technology.

Repowering

As wind turbine technology has evolved, wind turbines have become both larger and more efficient at capturing energy from the wind. As a consequence early wind farms, based on relatively large numbers of small wind turbines, are starting to appear significantly less economical than modern wind farms with smaller numbers of larger wind turbines. Aesthetically the modern wind farms are often more attractive too.

This change is creating a market for the repowering of existing wind farms with new wind turbines. Repowering can be economically viable and it has the attraction of allowing a new, potentially more financially attractive, wind farm to be built at a site where a wind facility already exists, avoiding the need to acquire the various permits that might be needed at a new site.

The earliest repowering took place in California when large numbers of small wind turbines were scrapped during the late 1980s and early 1990s. Repowering has also taken place in Denmark where, before 2002, around 1800 wind turbines were taken down. Meanwhile, Germany began to encourage repowering in 2004 with financial incentives after most of the best onshore sites had already been used.

Typically, repowering of a wind farm with new turbines aims to double the output of the farm while reducing the number of wind turbines by 50%. Repowering also creates a market for secondhand wind turbines that can be refurbished and then sold on. This has led to older turbines from western European countries being re-erected in countries of the Balkans and eastern Europe. Older turbines can also be exported to developing countries for reuse, potentially cutting the cost of introducing the technology to these nations.

Cost of wind power

As with many renewable technologies, wind power is a capital-intensive form of power generation in which most of the investment over the life of the plant is required for its construction, while operational costs are relatively low and there are no fuel costs. The cost of wind turbines fell sharply during the 1990s and at the beginning of the 21st century as technology development and improved manufacturing techniques allowed economies to be made. During the middle of the first decade of this century costs started to level out as the economies made by development were counterbalanced by some steep increases in commodity prices. This led to the prices for the installation of onshore wind generation starting to rise in the latter part of the decade. However, the financial crisis at the end of the decade and the rapidly increasing competition in the wind turbine market led to costs starting to fall again as the second decade began.

The cost of building offshore wind farms is higher than the cost onshore. For U.K. waters the cost is probably close to 50% higher than for building onshore. Added to this, as offshore wind farms have moved into deeper waters farther from shore the cost of installation has risen, offsetting any gains from improvements in technology and installation techniques. Against this, offshore wind technology is at an earlier point in its development cycle than onshore wind technology, and there are likely to be further economic gains to be made in the cost of offshore technology, particularly for foundation construction. Floating wind turbine platforms and supports, in particular, could cut costs significantly if they can be perfected.

The U.K. Department of Energy and Climate (DECC) has estimated that the installed cost of onshore wind turbines in the United Kingdom in 2011 was £1452/kW. For offshore wind turbines, the installed cost in 2011 was estimated to be £2722/kW, as shown in Table 11.2. The U.K. DECC expects the cost of onshore wind turbines to fall only slightly over the next two decades. This view may be modified by the growing global competition between wind turbine manufacturers that could lead to capital costs falling more than had been expected. For offshore wind the U.K. DECC expected a significant fall in costs over the same period so that by 2030 the installed cost was predicted to be around 30% lower than in 2011. Competition for offshore wind turbines is not yet as fierce as for onshore machines but this could change, leading to steeper falls in price than this suggests.

The actual cost of electricity from a wind farm, as measured by the levelized cost, is also shown for U.K. wind farms in 2011. For large offshore wind farms (greater than 5 MW in capacity) the levelized cost was £91/MWh. Offshore wind, at £169/MWh, was significantly more expensive. In the United States, meanwhile, the U.S. Department of Energy found average wholesale wind energy costs for onshore wind capacity in 2012 to be $40/MWh based on new contracts for electricity from wind farms.

Based on these figures, electricity from wind power plants remains generally more expensive than power from the best conventional sources such as natural gas–fired combined cycle power plants, existing coal-fired power plants, and established hydropower plants. It is widely expected that this situation could change during this decade and that sometime toward the end of the decade wind power will achieve parity with these other sources. Whether this will happen will depend on a number of unpredictable factors including whether competition within the turbine market continues to lead to falling prices and the impact of shale gas in the United States and elsewhere.