Wind is a natural resource that we can use to harness energy. For thousands of years we have used wind to provide us with motive power as in the case of ships. Through time and the industrial revolution we began to use it as a way of channelling the force of the wind by way of turning blades. The revolving blades were used to turn a shaft which was then used to turn a grindstone. This provided us with a fast way to mill grain and the name given to this machine was a ‘windmill’. During the last century we replaced the shaft turning a grindstone, to the shaft now turning a gear box which feeds into a generator (See Diagram 1). This now provides us with electrical energy that can now be used to power other types of machinery or appliances. The name given to this machine is ‘wind-turbine’.

Diagram 1: The Internal Structure of a Wind Turbine¹

How does a turbine work?

Wind turbines are built out of three main body parts. These are the tower (including base), hub and blades. The tower is what gives the turbine access to ‘clean’ wind, wind that is less affected by turbulence than that nearer the ground. The tower is clamped to an underground concrete base. The hub (the box behind the blades) contains all of the workings of the turbine; it pivots around on the tower point towards the direction of the oncoming wind. A rotor is attached to the hub, and the blades are then fixed to this. The blades are manufactured rather like an aeroplane’s wing and can be pitched to change their angle. The blades are pitched so that they can extract the maximum amount of energy out of the wind. They can also be pitched to minimise the amount of energy they extract in the event of excessive wind speeds (over 20m/s), in order to protect the turbine. The turbine can automatically be shut down if the wind speed is too high. This is known as the’ cut out’ speed. The turbine will also only start up when there is enough wind. This is called the ‘cut-in’ speed and is typically 3 to 4m/s depending on the design of the blades. The wind speed at which the maximum electrical output is achieved (typically 11m/s) is called the ‘rated wind speed’. When the wind speed is greater than this the blades are controlled so that the output is maintained at the rated electrical output.

The hub is connected to the tower with a ‘yaw drive’ controlling the direction of the hub. The yaw drive is used to maximise the energy extracted from the wind by making sure the rotor is facing the direction of the wind. If there is a misalignment between the two then the turbine will be receiving a lower share of the winds energy and therefore not generating the maximum amount of electrical energy.

The use of a ‘gear box’ within the hub is rather like the use of a gearbox in a car. It is designed so that the ‘generator’ can run at optimum speed whatever the speed at which that the rotor turns. With early gear boxes, the rotor was allowed to run as fast or slow as it likes whatever the speed of the wind. This resulted in poor power quality and fluctuations on the local electricity distribution grid. Early gearboxes were the main cause of the loud noises associated with turbines. Modern day turbines use more efficient generators, or in some cases no gearbox is used at all, reducing the noise generation to only aerodynamic noise (‘swooshing’ sounds) – however the stigma of turbines producing lots of noise is still associated with modern turbines today.

Why is it installed on this site?

There is a huge amount of planning that is involved with installing a wind farm regardless of whether this is for a single, or multiple turbines. The owners of the wind farm will try to maximise the amount of available space as much as possible. Considerations for the maximum amount of turbines utilised within a space are given below;

Turbulence and Feasibility

• Grid Suitability: Within the UK there are a plentiful amount of ‘windy’ sites. However, whether a site is suitable to build a wind turbine on depends on feasibility. One of the main issues arising here is that many of these sites, as well as being windy, are also isolated and therefore quite a long distance away from the electricity grid transmission lines. Building new connections to a grid is often costly. A new connection may need to be replaced by one able to carry a higher voltage so that the generated electricity can be safely transported. Sometimes it may be more feasible to build two wind turbines as opposed to building several, as the cost in connection far exceeds the money potentially recuperated in installing several (however the number is more than likely affected by other issues).
• Wind Speed: This will always be an important factor; as if the average wind speed for the year is too low then installing a wind farm will not be feasible. Prior to planning and installation an average wind speed for a year will be taken. This will involve either putting up an anemometer or using two sets of data from anemometer/met masts that are already recording close to the proximity of the site. By correlating these pieces of data, software analysis can generate an accurate and valid estimate of the wind speed for the year. Tell tale signs observed when a site is first visited will also tell you if the site is windy – just look for numerous trees and shrubs that have grown, bent over in the same direction as a result of the prevailing wind. This indicates that the wind has been forceful and frequent enough for the vegetation to grow in this way. The minimum economic wind speed is 5.5m/s at 10m above ground level for a wind farm to be viable. If it is much higher than this, say 9 or 10m/s, then the turbine will produce more profit!
• Positioning: If a turbine is subjected to unsteady turbulent flows then it cannot produce power efficiently as it cannot slow down or speed up its rotation quick enough. A wind turbine creates turbulence behind itself called the ‘wake’. For this reason, it is important to position turbines in such a way that air can regain almost normal flow (turbulence diminishes) so that the following turbines can work at maximum efficiency.

The rule of thumb for a grid of turbines is 4x blade diameter width, and 6x blade diameter depth.

A turbine will need to be positioned at a height where it can access ‘clean air’.  As wind travels, any contact it makes with a surface will cause it slow down. The resulting affect causes turbulence.  In simple terms, as a layer of wind travels along a surface, it encounters friction between that surface. This slower moving layer of wind then creates some friction with the layer above causing that layer to slow down (but not as much as the 1^st layer. Eventually it will even out to a less frictionless turbulent flow and this is the height that will need to be accessed.

Buffered zones

• Shadow Flicker: Shadow flicker occurs when something passes in front of the sun. This could be anything from driving past trees or wrought iron fencing to (in this case) a wind turbine. As the blades of a turbine rotate it causes flickering, the sun is temporarily blocked from view on a repeated basis. This often only occurs at a certain time of the day and can be influenced by the season. It is worth mentioning that this not only causes discomfort for a short period of time during the day but also creates a health risk, particularly with individuals suffering from epilepsy, as this could trigger a seizure; however the British Epilepsy Association (2007) has stated that there was no evidence that shadow flicker triggers seizures. Using specialist software (such as ‘WindFarmer’² or ‘Windpro’³), shadow flicker predictions amongst other things can be calculated to see if, and for how long flicker may affect a dwelling. If flicker is a problem, measures can be taken in order to reduce its impact. This could be fixing blinds to windows or planting vegetation to act as a screen to flicker. Turbines could also be programmed to shut down at the time of the day where it might be a problem, and started up once the sun has moved from behind it. Modern turbines tend not to revolve as frequently as they used to do. The frequency of flickers per second are greatly reduced and from a health point of view, the affect of flickering is not as great. Modern wind turbines have a rotation that is often less than 1Hz (1 Rotation per second).
• Dwellings: This is an important factor to consider in planning a development. The proximity to houses should be such as to reduce the chance of ‘spoilt’ views, shadow flicker and noise. However, there is no statutory limit on how close a wind turbine may be situated to a dwelling, although turbines will always be built to a distance that exceeds noise, visual impact and fall over distance (height of turbine to tip of blade + 10%). Turbines are typically situated far enough away from residential dwellings so that the noise does not exceed 40-45dB.
• Aspect, Slope and Terrain: These three factors are important in the productivity of a wind farm. Aspect relates to the direction of the prevailing wind speed. I.e. In Cornwall, this is south westerly. Slope denotes the topography of the area in which a wind turbine could be installed. If an area is too steep, then this poses engineering difficulties. So, in identifying potential sites, slope data will be used to find only areas that have slopes less than a 10% gradient. Terrain relates to the turbulence/frictional effect when wind travels along uneven ground. This is known as wind shear. Wind shear is far greater when there are trees or buildings in the way. The lowest wind shear can be found out at sea, as the sea is relatively flat. Also, it is better to build a turbine on top of a hill rather than on the bottom to gain maximal wind speeds.
• Electromagnetic Interference: There is a chance that wind farms may interfere and reflect microwave links. If a wind farm is built in the way of the Ministry of Defence’s microwave links, then the wind farm will almost certainly not receive planning permission.
• Supply of Machinery to Site: The blades and the tower of a wind turbine are not small in size. Blades will be extremely long, and are made up of a single piece. They range in length from anywhere between 20 and 45m plus and therefore need an extremely long truck to transport them. The tower is typically made up of smaller sections. This poses a difficult scenario (especially for blades) in transporting them to a site as often the winds farms are being built in the middle of nowhere, far away from main straight roads. Bridges and wider roads may need to be built to allow for their transportation. Potential routes to the wind farm site should be scouted to see if there are any sharp bends and so on, noted and budgeted for in the planning stages.
• Noise: Noise is always an issue. However to quickly put the level of noise into context please see the table below:

Source	Noise Level (decibels)
Threshold of Pain	140
Jet Aircraft at 250m	105
Pneumatic Drill at 7m		95
Truck at 30mph at 100m		65
Busy General Office		60
Car at 40mph at 100m		55
Wind farm at 350m		35-45
Quiet Bedroom		20
Rural Night time Background		20-40
Threshold of hearing		0 (decibels)

ODPM, Planning for Renewable Energy: a Companion Guide to PPS22, 2004

• Conservational Issues⁵: Throughout the UK many sites are protected due to their aesthetic or conservation importance. Special Areas of Conservation (SACs) and National Nature Reserves (NNRs) contain no buildings or roads and the building of a wind turbine within one is not permitted. This is the same as for Sites of Special Scientific Interest (SSSIs) where even planning applications nearby can be rejected. Areas Of Outstanding Natural Beauty (AONBs) are often owned between local communities and local authorities and National Parks by large organisations such as the National Trust. Request for planning permission needs to be approved by these organisations before planning can go ahead although it is unlikely it will be approved.
• When assessing the suitability of a site it is important to check first whether there are any rare species living on the site that could be affected by the building of wind turbines. This is because the construction of wind turbines has been thought to disrupt the migratory routes of some bird species and more recently, to cause the death of different bat species.

What happens to the electrical energy generated by a wind turbine?

Usually a wind turbine is connected to the local distribution grid, supplying electricity virtually straight to the consumer. However, large wind farms may be connected to the transmission network.

Intermittency

Intermittency is defined as something being sporadically interrupted. In the case of wind, a turbine may be subjected to fluctuating wind speeds; a generation issue.

‘The argument that wind turbines are not suitable because the wind is intermittent is often a flawed argument. The maximum electrical output of a turbine occurs above a fixed wind speed, yet a turbine still has the ability to produce usable electricity from wind speeds as low as 4m/s. Modern turbines use power electronics to control the power flow from the turbine to the distribution grid so that power generated adheres to the UK Grid Code. Just because a turbine is producing only a fraction of its total output, the voltage and frequency of the power is maintained at the correct level so not to cause a problem.’⁷

What is a ROC?

The Renewables Obligation is a system regulated by OFGEM (Office of Gas and Electricity Markets) that provides a financial incentive for companies to invest in renewable energy technologies. The RO scheme replaced the previous system for subsidising renewable energy called the Non Fossil Fuel Obligation, as the system benefited nuclear generators more than renewable energy technologies!

The RO came into effect in April 2002. In 2003 the specified proportion of renewable electricity generation was 3%. This rises every year and this year, 2010, 10.4% renewable generation is required from all suppliers. This will continue until 2027.

A ROC is a Renewables Obligation Certificate and is rewarded for each individual MWh produced from a renewable energy source. These certificates can then be sold to an energy supplier so that they can meet the percentage of renewable generation target. If a supplier has not produced enough ROCS then they can buy from other companies who have produced more ROCs then they have needed.

At the end of each financial year the companies that have not produced enough ROCs are penalised. This is by a monetary fine known as the ‘buy-out price’. The current buy-out price (2010) is £36.99 per MWh.

Those who have produced enough receive a financial reward by receiving a share of money generated from fines in proportion to the total number of ROCs produced throughout the UK that year.

Misconceptions and issues

They make too much noise

This is untrue, (how much is too much?), a turbine 350m away is far quieter than a car travelling at 40mph from a distance of 100m away. Where this misconception stems from is early wind turbine development. It can also be heard with small domestic turbines (which are simply built without any cut off devices). The noise sounds rather like a spitfire. The tips are travelling so fast that they have broken the sound barrier. Hence the deafening noise. The tips on modern turbines are not allowed to travel as fast, thus resulting in a much quieter noise.

Turbines cause Bird and Bat Deaths

Far more birds are killed by the domestic cat as opposed to wind turbines. The misconception occurred in the USA in early wind farm development where they had built the farm in a migratory bird path. Wind farms of today are not built on migratory routes, nor do the blades revolve quickly enough for birds to get knocked out.

Bats deaths however are a far greater concern. The air flow through a turbine changes pressure. Bats have delicate lungs that do not compensate quickly enough for the sudden drops in air pressure. This causes their lungs to haemorrhage as the capillaries burst. Again, these measurements were taken where there were bat migratory routes. ⁸

When they are turning really slowly, they aren’t producing energy

Another misconception explained in the Intermittency section of this article. Due to power electronics within the hub of a turbine, high and steady generation is possible.

NIMBY

Nimby –Not in my back yard! People here are given this term if they are for a development of something but do not wish to see it where they live. In this modern day with the problems of climate change, ask yourself this. As a country, we could power ourselves with Coal, Nuclear, Oil, Gas or Renewables. We can easily do this, we know how to. The real question is, if you want to be able to switch on your light bulb, it is not a case of how the electricity supplied to it is generated, it is how ethical is it to have electricity supplied  to the light bulb. Climate Change is an issue, and we will all be affected by it. Turbines are not permanent structures. They take a day to put up, and will take a day for them to be taken down. All that is left is the concrete base (which still could be broken up and taken away). Nobody is certain how long a nuclear plant will take to decommission, as it hasn’t ever been done. Estimates at this time are around 80-100years, mainly because of radiation issues. Where does the waste go?

For more information on wind sites within the UK, the BWEA have produced an interactive map, useful in identifying wind farms that are operational, in planning or under construction.

If you are interested in a small scale wind project to power your home then take a look at our informative pages.

http://www.bwea.com/ukwed/map-operational.html

 

[1] Diagram 1 showing makeup of a wind turbine Available [Online] at: http://upload.wikimedia.org/wikipedia/commons/5/52/EERE_illust_large_turbine.gif

[2] WindFarmer, GL-Garrad Hassan, information on WindFarmer computer package. Available [Online] at: http://www.gl-garradhassan.com/en/GHWindFarmer.php

[3] Windpro, EMD, information on Windpro computer package. Available [Online] at: http://www.emd.dk/WindPRO/Frontpage

[4] ODPM, Planning for Renewable Energy: a Companion Guide to PPS22, 2004 , Available [Online] at: http://www.communities.gov.uk/documents/planningandbuilding/pdf/147447.pdf

[5] Conservational Issues. Ive, S.E, 2010. Discussion on building regulations on protected sites in the UK. [Phone Call] (Personal Communication, 27 July 2010).

[6] Ofgem, http://www.ofgem.gov.uk/

[7] Intermittency Information. Haverson D. 2010. Verbal Communications 27 July 2010 with Haverson D.  MEng

[8] Bat Deaths. http://www.sciencedaily.com/releases/2008/08/080825132107.htm