Technical concept

Wind energy refers to the conversion of the kinetic (movement) energy in air into a usable form.  This could be potential energy in the pumping of water or indeed rotational kinetic energy for the grinding of wheat to make flour, as with traditional windmills.  However wind energy nowadays generally refers to the generation of electricity from the wind.  The wind passing through the rotor of a wind turbine transfers it energy to the rotation of the blades. For electricity generation this rotation results in the rotation of a magnetic field within an alternator, the output of which is electric power.  In order to convert this electric power into a form that can be distributed and used to operate appliances and electric plant, power electronics are typically used to alter the waveform and then a transformer is used to adjust to a voltage suitable for distribution.  Power electronics may not be needed for fixed speed wind turbines with a gear box, but even for this type of wind turbine they are generally incorporated nowadays.

Figure 1 – Components of a wind turbine [source: World Wind Energy Association, www.wwindea.org]

Figure 1 – Components of a wind turbine [source: World Wind Energy Association, www.wwindea.org]

The blades (1) of a horizontal axis turbine as shown in Figure 1 have a similar profile to an aircraft wing. They are generally made of glass fibre or carbon fibre reinforced plastics.  Because the blades have a different profile either side, as with an aircraft wing, the air moves at different speeds across each side of the blade, causing a pressure difference across the each blade and therefore rotational movement of the rotor (2). The pitch of the blade (3) can be varied to optimise power output and also to stall the turbine should the wind speed be too high.  The turbine may then be stopped and held motionless through the use of the brake (4).  The control of the turbine in accordance with wind speed, and shutdown if necessary, is enabled through the measurement of wind speed by the anemometer (9) on top of the turbine.  In order for the turbine to be aligned with the wind, it needs to be made to rotate around the axis of the tower (15) or ‘yaw’.  The wind direction is detected by the wind vane (10) and via the controller (8) the yaw motor (14) is called to operate and turn the yaw drive (13) and thereby the top of the turbine.  The gearbox (6) facilitates an increase in rotational speed between the low speed shaft (5) and the high speed shaft (12) so that the rotational speed is more suited to electricity generation.  Gearboxes are sometimes avoided through the use of power electronics, which can ensure the right frequency of electrical supply.  The rotational kinetic energy of the shaft is converted into electricity in the generator (7) through rotating electromagnetic fields.  The rotating machinery is housed in the nacelle (11) of the turbine and the electricity generated is transmitted to the base of the turbine and through a transformer to convert to distribution voltage.

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Different types of wind turbine

Lift versus drag wind turbines

There are different types of wind turbine available on the market.  The most effective blades for extraction of energy from the wind are those with an aerofoil cross-section operating on the Bernoulli principle.  The shape of the aerofoil is such that the air has to travel different distances over each side of the blade resulting in different velocities across the surface of the blade thereby leading to differential pressures forcing the blades round, also known as ‘lift’.  Other types of wind turbine operate predominantly on the ‘drag’ principle rather than the ‘lift’ principle.  For this type of turbine, the blade offers resistance to the wind and the blades are pushed round directly by the wind rather than the differential pressure set up with a ‘lift’ turbine.  Drag turbines are less efficient because there is significant energy loss through greater friction.

Horizontal versus vertical axis wind turbines

In the initial development of wind turbines, much attention was paid both to the design of both vertical axis and horizontal axis wind turbines.  Generally the efficiency of horizontal axis turbines is greater than that of vertical axis turbines, and their starting torque is lower.  Furthermore vertical axis turbines can present difficulties of access for maintenance, as to reach the bearings the whole blade assembly generally has to be removed.  It is sometimes claimed by suppliers of vertical axis wind turbines that they are better able to deal with wind with varying direction, such as that found in a more urban environment.  There are also designs of vertical axis wind turbines that lend themselves to modular scale-up more easily, through extension of blades with constant cross-section, unlike the horizontal wind turbines.  This could be an advantage for mass-produced turbines tailored to specific needs.  For the time being the horizontal axis turbine dominates the market.

Number of blades on a wind turbine

In the initial development of wind turbines, much attention was paid both to the design of both vertical axis and horizontal axis wind turbines.  Generally the efficiency of horizontal axis turbines is greater than that of vertical axis turbines, and their starting torque is lower.  Furthermore vertical axis turbines can present difficulties of access for maintenance, as to reach the bearings the whole blade assembly generally has to be removed.  It is sometimes claimed by suppliers of vertical axis wind turbines that they are better able to deal with wind with varying direction, such as that found in a more urban environment.  There are also designs of vertical axis wind turbines that lend themselves to modular scale-up more easily, through extension of blades with constant cross-section, unlike the horizontal wind turbines.  This could be an advantage for mass-produced turbines tailored to specific needs.  For the time being the horizontal axis turbine dominates the market.

Generally multiple bladed machines with their lower starting torque are more suited to the pumping of water and wind turbines with one, two or three blades, with their higher rotational velocity, are more suited to electricity generation.  However there are also considerations of aesthetics and balancing to minimise stress of the machinery.  One of the reasons that 3-bladed wind turbines have prevailed is because they are regarded as more aesthetically appealing than wind turbines with one or two blades.

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What affects the power output of a wind turbine?

Maximum efficiency

The efficiency of all types of wind turbine is limited by the linear momentum theory to the value of 0.59, known as the Betz criterion.  There efficiency of a wind turbine may be expressed in terms of the ‘power coefficient’ or Cp.  The power coefficient for commercial wind turbines has reached around 0.45.

Capacity factor

The efficiency of a wind turbine is not to be confused with the ‘capacity factor’.  Whereas the efficiency of a wind turbine is a measure of the inherent performance of the machine, the capacity factor is a measure of the performance of a wind turbine in a given situation.  The capacity factor may be defined as the ration between the average annual output of a wind turbine and the maximum, or rated, output of the wind turbine.  For example if a 1MW (1000kW) wind turbine in a given location were found to have a capacity factor of 35%, then over the course of the year the turbine would have generated on average an output of 350kW.  This does not mean that the turbine blades are only turning for 35% of the time – the output of a wind turbine varies with wind speed, so for much of the time the wind turbine will be generating at less than its rated output.  The way in which the output of a wind turbine varies with wind speed is described below.

Power curve

The relationship between wind speed and the power output of a wind turbine may be expressed in terms of a power curve.  An example of a power curve for a wind turbine is given in the figure below.

 

Powercurve1-300x238.png

Figure 2 – Example Power Curve

Wind turbines do not operate until the wind reaches a certain speed, known as the ‘cut-in’ speed.  The cut-in speed varies between turbine types – the starting torque depends on the blade design and also the inertia of the machinery within the turbine.  The energy available in the wind is proportional to the cube of the wind speed, so as the wind speed exceeds the cut-in speed the power output of the turbine varies with an approximately cubic relationship with the wind speed.  In practice there may be some variation in the power coefficient (measure of efficiency) with wind speed, so there may be some variation from this cubic relationship.  The ‘furling’ speed is the wind speed at which the output of the wind turbine reaches its rated value.  At this point the output of the wind turbine is curbed.  This is typically done through power electronics, but certain wind turbines are designed to stall based on the form of the blades.  An aerofoil only works effectively if the incident wind is within a certain range of angles with respect to the blade.  As the blade rotational velocity increases the relative angle varies, and so beyond a certain point the angle becomes unsuitable and the turbine will stall.  For a stall machine there is a curved drop-off from the rated value, but if power electronics are used to moderate the output, then the output remains constant beyond the furling speed, as with the power curve above in Figure 2.  If the wind speed is too high there can be damage to the wind turbine, and therefore the turbine shuts down at a certain wind speed, known as the cut-out speed, to prevent this happening.  This is also shown in the example power curve above.

Impact of rotor blade diameter on power output

The power output of a wind turbine is proportional to the swept area (capture area) of the rotor blades.  This means that for turbines of the same power rating there may be a difference in annual generation due to different blade diameters.  Some models of wind turbines are optimised for lower wind speeds by having larger rotor diameters, but with maximum output limited to that of smaller rotor diameter machines through the use of power electronics.

Impact of tower height on power output

Wind speed is lower closer at a lower height due to the frictional effects of the earth’s surface.  Therefore wind turbines with taller towers will generally generated more electricity than those with shorter towers.  For many models of wind turbine the purchaser has choice of different power heights.  It is not always the case that the tallest will be chosen.  Sometimes the height of the tower will be constrained for reasons of visual impact or interference with aviation.  Furthermore certain tower heights are only suitable for certain classes of wind speeds, and for a more turbulent, gusty wind regime a shorter tower may be required.

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Wind Energy Environmental Considerations

Visual

Wind turbine projects often meet resistance from local residents on account of their visual impact.  This is particularly the case where the turbines are to be located in an area of attractive landscape.  In order to take advantage of the best wind resource, wind turbines are often installed in upland locations that are visible from a long way away, and with their large size some people regard their presence as having a negative impact on the view.  There are people however to whom the sight of a wind farm is appealing and the motion of the blades graceful. 

It is important that the need for renewable energy is balanced by the needs of the local people in terms of their quality of life, and therefore visual impact has to be reviewed carefully.  Consideration has been given in the design of turbines through the type of paint or coating used, which is generally non-reflective and white.  The suitability of location of turbines however has to be considered on a case-by-case basis in consultation with local people and the local planning authority.  There are also guidelines in planning policy that should be referred to covering land designations such as National Parks and Areas of Outstanding Natural Beauty (AONBs).

Noise

Whereas with old wind turbines much of the noise emitted came from the gearbox almost all the audible noise from modern turbines comes from the movement of the blades – a ‘swooshing’ sound.  This noise can be irritating to people and therefore there are guidelines to ensure that the noise level from wind turbines should not be above the level of that acceptable at sensitive receptors (dwellings and other places of extended occupation).  In order to determine whether a planned wind turbine installation is likely to have an excessive noise impact a desktop noise assessment may be carried out.  The critical condition under noise regulations is where the noise level from the wind turbine is a certain amount above background noise level.  Therefore where the background noise level is high then a wind turbine can be sited closer to a building.  At night time there is assumed to be some reduction of the noise from the wind turbine by virtue of the fact that people will generally be in their bedrooms, and even an open window reduces the sound level.  With the initial desktop assessment, if there is no data available on background sound level then an estimate can be made by reference to sound levels from similar environments.  It should also be taken into account that background noise level varies with wind speed such that at a certain wind speed the background noise starts to be dominant.

If it is found that for the proposed position of wind turbine the noise level is too high at nearby buildings then the options are generally: a) to install the turbine in a different location b) to use another model of turbine with different noise characteristics c) to switch off the wind turbine at times when the noise is deemed to be too intrusive.

Shadow flicker

Shadow flicker is the effect of the sun’s rays passing through the turning blades of a wind turbine.  This can cause irritation if people are exposed to it for long periods.  In order to determine whether shadow flicker is a problem or not for a proposed location, an analysis may be carried out taking into account the wind turbine dimensions, the path of the sun and the relative position of sensitive receptors (e.g. dwellings).  The output from this analysis may include a shadow map indicating periods of exposure over the year for different areas.  European guidelines can refer to a maximum acceptable number of hours’ exposure to shadow flicker over the year and a maximum number of minutes in the day.

If shadow flicker of found to be a problem for a proposed turbine location, then options may include locating the turbine somewhere else or introducing a control system to turn off the turbine during extended periods of shadow flicker.

Telecommunications

Wind turbines can interfere with radio communications such as microwave links for mobile phones.  When investigating a potential location for a wind turbine, the positions of mobile links and their radius of effect (defined in relation to the Fresnel radius) generally need to be reviewed.  If it is identified that there is likely to be interference then steps needed to avoid this may include moving the proposed location of the wind turbine or making modifications to the signal transmission system, for example by moving the transmitter or receiver, or through installing a booster station.

Aviation

Wind turbines may interfere with aviation through physical obstruction of air traffic movement (usually during take-off and landing) or through interference with the technical systems that support aviation, such as radar.  For an initial review of aviation interference when considering a wind turbine installation an inspection may be undertaken into proximity to nearby airports and military or air force sites.  Consultation generally needs to take place with the relevant bodies when the potential for aviation interference has been identified.  Generally it is the case that turbines have to be located elsewhere if there is deemed to be interference with aviation; however there is often room for negotiation when a wind turbine developer is faced with what might be an initially precautionary response from an aviation body.  There may be the possibility to upgrade radar systems to process wind turbines without confusing them with aircraft, but this is expensive and unless it is on the cards anyway would generally be too much for a single developer to accommodate.  There have been a number of recent technological developments to limit the technical interference impact of wind turbines including the development of ‘stealth’ rotor blades that are not picked up by radar.

Birds

Birds generally fly round turbines if they can identify a clear path.  Occasionally birds do die from flying into turbines, but the rate of this happening is far less than birds being killed by people’s pet cats, cars or indeed closed windows.  Nevertheless migration paths in particular should be taken into account and avoided.

Ice throw

In certain locations ice may collect on turbine blades and can pose a danger when the turbine blades rotate.  The risk of ice being flung of a wind turbine and causing damage can be mitigated through implementation of an exclusion zone, through halting operation during certain conditions or through blade heating systems.

Construction and decommissioning

Attention has to be paid to construction of wind turbine installations or wind farms in terms of access to the site and also the impact on the local environment.  Decommissioning has to be taking into account in the cost estimate to allow for the land to be returned to its previous use at the end of the turbine life.

CO2 displacement

Wind turbines result in significantly less carbon dioxide being emitted than fossil fuel power by virtue of the fact that they are converting energy from the wind and not a hydrocarbon.  Currently the intermittency of wind energy is countered by fossil power stations and in order to respond quickly to dips in demand fossil fuel plants have to be ‘warm’ and running on low load – this is known as spinning reserve.  For this reason some CO2 emissions may be attributed to wind energy, although when other methods of countering intermittency, such as energy storage and load management are introduced these emissions drop off.

Embodied energy

Energy is required to manufacture wind turbines and also during transportation and construction.  This energy burden prior to wind turbines generating electricity is known as embodied energy.  The energy payback is the period of time before a wind turbine has generated the energy equivalent to the embodied energy.  This varies with location and associated wind resource as well as turbine design.  In the UK for a reasonably sized turbine, the energy payback may be in the order of 6 months.

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Wind Energy Commercial Arrangements

The commercial case for a wind turbine is determined through balancing the initial capital outlay with a combination of the effective revenue from generation and the operating costs.  The effective revenue from generation comprises the value of displacing purchased electricity and additional support tariff that may be available (ROC or FiT in the UK).  The operating costs comprise mainly routine maintenance.  A single turbine installation may be owned and operated by the landowner, or can be put in the hands of a third party energy service company.  Large wind farms are generally owned and operated by utilities, but landowners are likely to receive lease payments for the use of their land in this case.  The type of commercial delivery vehicle opted for may be determined on the basis of appetite for risk, available capital and extent of alignment of core activities.

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