Our growing use of wind power

Aside from powering sailboats, the earliest recorded use of wind power was in windmills for grinding grain, by the Persians, as far back as 500–900 BC.

The first electricity-generating wind turbines were invented in the United States and Europe in the late 1800s. In the early 1900s, as electricity became more widely available in towns and cities, many rural communities and homesteads turned to small-scale wind turbines for their electricity supply. Many were built on site, using old car generators and hand-carved rotor blades or old biplane propellers.

Since then, wind power has gathered momentum in a world that is increasingly looking for alternatives to fossil fuels for power generation.

According to statistics from the US Energy Information Administration, in 2012, global energy production stood at around 21,532 terawatt hours (1 terawatt = 10¹² watts). Wind power accounted for around 520 terawatt hours of this, a share of around 2.5 per cent. The Renewables 2015 Global Status Report states that this share increased to around 3.1 per cent by the end of 2014.

Although wind power's share of total energy production around the world is small, it's definitely growing. Figures from the Global Wind Energy Council show that since 2000, wind-power capacity around the world has grown by an average of 24 per cent per year, reaching a total capacity of around 370,000 megawatts of power in 2014. Nearly 60 per cent of this capacity is accounted for by China (31 per cent), the United States (17.8 per cent) and Germany (10.6 per cent). 

• The units of electricity

  You’re probably familiar with seeing the number of watts a light bulb uses as being something between 15 and 100. This number indicates the amount of power that the light bulb uses at any given moment (when turned on!). 

  The units you most likely see on your electricity bill are kilowatt hours GLOSSARY kilowatt hoursA unit of energy that is normally used to measure the consumption of domestic electricity. The joule (1 watt per second) could be used but the numbers become very large and it is common to use the kilowatt hour (1 kilowatt hour = 3,600,000 joules or 3.6 megajoules). , which is the amount of electricity your household used over the course of an hour.

  When we start looking at the amount of energy produced to power entire cities, or by entire power plants, or the capacity of entire countries, the numbers scale dramatically.

  • 1 kilowatt = 1,000 watts = 10³ watts
  • 1 megawatt = 1,000 kilowatts = 10⁶ watts
  • 1 gigawatt = 1,000 megawatts = 10⁹ watts
  • 1 terawatt = 1,000 gigawatts GLOSSARY gigawattsThe unit of energy is the joule (J) and the unit of power is the watt (W), which is the power involved in doing 1 joule of work (or using 1 joule of energy) each second. This is a very small amount of power and in most mechanical applications, we count power in kilowatts (1 kilowatt = 1000 watts). A kilowatt is about equal to the heat energy put out by a single bar radiator, and is also about equal to the power expended by a person running up stairs. A car engine typically produces 50 to 100 kilowatts. When we consider power generation we use larger units. The megawatt is a 1,000,000 watts or 1000 kilowatts. A typical coal-burning power station produces about 1000 megawatts of power and this is the same as 1 gigawatt. = 10¹² watts

In 2014, wind power accounted for nearly 40 per cent of Denmark’s energy production. On a particularly windy day in July 2015, Denmark produced sufficient wind power to supply 140 per cent of the country’s needs. The excess power was transferred to neighbouring countries, Germany and Sweden.

And here in Australia, according to the 2014 Clean Energy Australia Report, wind power accounts for more than 30 per cent of our country’s renewable energy, with around 4 per cent of our electricity derived from wind. South Australia leads the way, with the greatest number of wind farms of all the Australian states, and generating around 40 per cent of the state’s electricity from wind power.

The science behind the wind

In generating electricity from wind, strictly speaking, we start with the sun. The energy radiated by the sun heats up the air in Earth’s atmosphere. Air expands when it warms, making the warmer air less dense and ‘lighter’ than cooler air, creating a region of lower pressure. Cool air is usually more dense.

Earth is not heated evenly by the sun’s energy. Some regions heat up more than others: the poles, for example, are cool, while the equator is hot. Warm, low-density air over the equator rises and spreads outwards towards the poles and cool, dense air rushes in to replace it. It’s this moving air that we call wind. These temperature and pressure differences between air masses drive global air circulation patterns, and the wind.

• Air circulation

  Because of Earth's rotation, winds move in complicated patterns. In northern Australia the winds blow mostly from the east, while in southern Australia they blow mostly from the west. On top of this general circulation pattern we find other winds associated with tropical cyclones in northern Australia or with cold fronts in the south.

  Most winds blow in circles. The major circulation patterns of easterly winds or westerly winds blow right around Earth, carrying weather patterns with them. Local winds, as in cyclones or other low-pressure systems, rotate in circles clockwise (in the southern hemisphere, anticlockwise in the northern hemisphere) around the centre or eye of the storm, although they have a non-circular path.

  Predicting windy sites

  Other factors also have a profound influence on the nature of atmospheric circulation and wind patterns. These include the location of the continents, the season, and topographic features such as mountain ranges, valleys and ridgelines. This means that predicting the best areas for electricity generation from wind power can be quite complex. Engineers use computer models and extensive wind measurements to assist in this.

Harnessing the wind

All forms of electricity production rely on the conversion of other forms of energy into electricity. In a coal-fired power station, chemical energy stored in coal is converted first to heat energy by burning and then into kinetic energy (energy of motion) by heating water to produce high pressure steam. The steam is used to turn a turbine GLOSSARY turbineA device in which a stream of water or gas turns a bladed wheel, converting the kinetic energy of the fluid flow into mechanical energy available from the turbine shaft. The earliest turbines were water wheels. Now, steam turbines are driven by jets of high-temperature steam; gas turbines are driven by burning fuel vapour; and wind turbines use the power of moving air. (also kinetic energy), which is then used to power a generator GLOSSARY generatorA machine that converts mechanical energy into electrical energy. In a normal generator, a shaft spins a magnetic rotor. The moving magnet produces an alternating current. (It is the reverse of an electric motor.) Generators are extremely efficient in converting mechanical energy to electrical energy. to produce electrical energy.

With a wind turbine, we firstly need the heat energy of the sun to warm the air in the atmosphere to create wind. Then, the blades of the wind turbine (usually two or three per turbine) catch the wind, which causes them to rotate. We can then skip the chemical and heat energy steps used in the coal-fired power plant—the kinetic energy of the moving blades of the wind turbine are used to turn a generator that produces electricity.

As the blades of a modern large turbine spin, they cause a low-speed shaft to rotate at a speed of 10–20 times per minute. In most turbines, this low-speed shaft is connected to a gearbox; the gearbox provides a connection between the low-speed shaft and a generator, which rotates 1,000–1,800 times per minute. The generator uses these rapid rotations to produce electricity. The electricity then passes through complex power electronics which help to control the turbine operation, and a transformer to be converted into the correct voltage for the electricity grid. Recently, very large wind turbines for offshore application have used direct drive generators with no gearbox, which is one of the costly maintenance items of onshore turbines.

The largest wind turbine at present is the 8 megawatt Vestas V164 with blades 80 metres long. This is longer than the wingspan of the Airbus A380, the largest commercial aircraft in the world.

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The turbine

The blades of a wind turbine need to be strong, light and durable. They are generally made from fibreglass or carbon-fibre and have a diameter between 60 and 80 metres.

The low-speed shaft, the high-speed shaft, the gearbox and the generator are all housed inside a compartment called the nacelle.

In order to best catch the wind, the turbines need to be up high, and they often sit atop towers that are anywhere from 40 to 100 metres tall. The generators and gearboxes that perch on these towers can weigh more than 10 tonnes. These factors add expense and logistical difficulty to the construction of a wind farm. 

The power produced by a wind turbine

The power produced by a wind turbine increases very rapidly with wind speed: a doubling of wind speed results in as much as an eightfold increase in power. Therefore it is important to site wind generators in a place where the wind speed is high, as well as reasonably constant. The length of the rotor blades is also important—doubling the diameter of the circle made by the blades produces a fourfold increase in power.

• The maths behind a wind turbine's power production

  The potential power offered by wind turbines can be demonstrated by a series of equations. The kinetic energy (\(KE\)) of a mass is expressed by the equation:

  $$KE = \frac{1}{2}mv^{2}$$

  where \(m\) is the mass of the air (or other object) and \(v\) is its velocity or speed.

  For a unit volume (1 cubic metre) of material:

  $$KE = \frac{1}{2}dv^{2}$$

  where \(d\) is the density, or mass per unit volume. (For air, \(d\) is about 1 kilogram per cubic metre.)

  The mass of the air that actually supplies energy to a wind turbine is related to the area (\(A\)) covered by the sweep of the blades. For the standard wind turbine with blades that revolve on a horizontal axis (that is, the blades themselves are perpendicular to the ground), this area is expressed in the following equation:

  $$A = \frac{ \pi D^{2}}{4}$$

  where \(\pi=1.31416\) and \(D\) is the diameter swept by the rotor blades.

  The kinetic energy available to the wind turbine therefore becomes:

  $$KE = \frac{1}{2}d (\frac{\pi D^{2}}{4})v^{2}$$

  The power available to the wind turbine in a given unit of time is a factor of kinetic energy and the distance that the wind travels in that time, which is determined by the wind velocity. Thus:

  $$power = (KE)v = \frac{1}{2}d(\frac{\pi D^{2}}{4} v^{2})v$$

  Simplifying the equation:

  $$power = \frac{1}{2}d(\frac{\pi D^{2}}{4})v^{3} = \frac{\pi}{8} × dD^{2}v^{3}$$

  This last equation is the one of most interest. Since the density (\(d\)) of air varies only slowly with temperature or height above Earth's surface, the only variables that influence the power available for electricity generation are the diameter (\(D\)) of the turbine blades and the velocity (\(v\)) of the wind. (If \(d\) and \(D\) are in metres and \(v\) is in metres per second then the calculated power is in watts.)

  The equation shows us that power is directly proportional to the velocity cubed. This means that a small increase in wind speed can produce a large increase in power. For example, if wind speed is 10 kilometres per hour, the value of \(v^{3}\) in the equation would be 1,000. If wind speed doubled to 20 kilometres per hour, the value of \(v^{3}\) would be 8,000. A doubling of wind speed therefore leads to an eightfold increase in power.

  Similarly, since power is proportional to the square of turbine diameter, rotor length is also an important determinant of the amount of electricity produced. For example, if the turbine diameter is 10 metres, then the value of \(D^{2}\) in the equation would be 100. If \(D = 20\), then \(D^{2} = 400\). Thus, a doubling of diameter produces a fourfold increase in power.

  These two factors have important implications for the design and location of wind turbines. First, the stronger the wind, the more effective the turbine—up to a point. If the wind is too strong, the turbine will be damaged. All turbines will shut down when winds reach around 25 metres per second(~90 km/h). Second, the longer the rotor blades, the better (again, up to a point—if they are too long they become unwieldy and more susceptible to damage).

  Unfortunately not all the power available in the wind can be converted to electricity. The famous Betz limit (which was also discovered independently by the Russian Joukowsky) states that the actual power cannot exceed 16/27 of that given by the equation. 16/27 = 0.593 so the maximum possible conversion efficiency is close to 60 per cent. Modern wind turbines (which have inefficiencies in the drivetrain, generator and power electronics) can achieve up to 50 per cent overall efficiency.

Catching the wind

Obviously, we want to locate our wind turbines in windy places. Generally, a wind farm will be built in an area with consistent winds that regularly reach at least 25 km/h. However, a big factor is that there may be no wind at all one day and a howling gale the next. This fundamental variability in the wind means it’s unlikely that wind power will become the sole source of electricity generation in the future, unless energy storage becomes cheaper and more efficient.

That said, while wind can be variable, it’s also predictable over long time periods. The Australian Energy Market Operator has developed the Australian Wind Energy Forecasting System, a tool that can forecast the wind energy output of wind farms on the east coast of Australia several days into the future, with high accuracy.

Wind can be erratic, changing direction by the second. The turbines are fitted with a ‘yaw’ motor, which allows them to swivel around so they can always be facing into the wind.

Plugging into the power supply

The large-scale production of wind-powered electricity involves the use of wind farms, where multiple wind turbines, from a few up to hundreds, feed electricity directly into the supply network.

Electrical engineers know that the wind doesn't blow all the time and have devised a number of strategies to ensure that electricity supply meets electricity demand. A network of wind farms feeding into a common grid, for example, may help provide a steady supply: when one wind farm is becalmed, others elsewhere in the region and the continent may continue to operate. Other energy sources (such as the sun, coal, water or gas) are often used to supplement wind-generated power. Hydroelectricity and gas are the best technologies for this purpose as they can be turned off and on very quickly. In the future, battery systems may be used to store excess wind power and dispatch it during periods of low wind.

Credentials and criticism

Recent analyses show that wind power is considerably more environmentally friendly than, for example, electricity produced by coal-fired power stations. It uses fewer non-renewable resources, causes less local or regional air pollution and makes virtually no contribution to greenhouse gas emissions.

The towers themselves cover only a small land area, and farming activities can continue virtually up to the base, even under the revolving blades.

With regard to the materials and resources that go into building wind turbines, lifecycle analysis studies have found that turbines pay for themselves very quickly—a 2 megawatt turbine with an expected working life of 20 years will provide a net benefit within five to eight months.

Criticism

The movement of the wind turbine blades produces low-frequency noise, which increases with wind speed, and may include ‘infrasound’ at frequencies too low for a human to hear. The blades can also make a swishing noise. However, the remoteness of the majority of wind farms means that noise isn't a major problem at most locations. There are planning regulations to ensure people living in rural areas are not subjected to annoying or stressful levels of noise.

Wind Turbine Syndrome

However, in parts of Australia, Canada, the UK and the USA, there has been some opposition to wind farms from proponents of a phenomenon known as Wind Turbine Syndrome. This ‘syndrome’ is not recognised by any health agencies or authorities in Australia; the term was coined after the publication in 2009 of a self-published book of the same title. The content of the book has not been through the standard scientific peer review process.

Globally, small proportions of people living close to a small number of wind farms have reported a variety of symptoms that they have attributed to the low-frequency noise and infrasound that wind turbines produce. A huge array of ailments have been attributed to this—including headaches, sleeplessness, irritability, heart palpitations and even a deterioration in a person’s diabetes symptoms.

Several studies have been conducted in Australia and other countries to try and ascertain how, and indeed, if, wind turbines can affect the health of people living near them. There have been 25 reviews of these studies published since 2003, in which medical experts have assessed all the available evidence and each time come to the conclusion that there is no evidence that wind turbines have direct effects on people’s health. 

There is no direct evidence that exposure to wind farm noise affects physical or mental health. NHMRC Statement: Evidence on Wind Farms and Human Health

The most recent report from the Australian National Health and Medical Research Council reviewed the available quality evidence on wind farm noise and its impact upon physical and mental health, mood, sleep and quality of life. It considered both infrasound and low-frequency noise and perceptions of noise, shadow flicker and electromagnetic radiation. The findings are briefly summarised below.

• NHMRC Report: summary of results

  Product	Findings
  Noise (including infrasound)	No direct evidence for any impacts on physical and mental health Consistent but poor quality evidence that wind farm noise causes annoyance Poor, less consistent evidence that wind farm noise causes sleep disturbance Poor, less consistent evidence that wind farm noise reduces quality of life No direct evidence that infrasound and low-frequency noise impacts upon health. Data only available from laboratory studies that examine much higher levels than those caused by wind farms Unlikely to cause disturbance at distances greater than 1,500 metres away.
  Shadow flicker	Insufficient evidence to draw any conclusions regarding health impacts of shadow flicker; risk of inducing seizures in people with (the rare) photosensitive epilepsy extremely low
  Electromagnetic radiation	No direct evidence for any health impacts

Furthermore, a report released in 2014 by Health Canada also found that while people's feelings of annoyance could be related to wind turbine noise, no link was found between wind farm noise and ill health in people.

Another interesting feature of the Wind Turbine Syndrome debate is that it is basically non-existent in Europe where, in several countries, people have been living with wind turbines for years. This is not to say that there is no opposition to wind turbines in these countries, but it is primarily on environmental grounds, where opponents are concerned about the aesthetics of the turbines or their location in ecologically sensitive areas. There is little to no mention of ill effects on health. 

So it raises the question—what is going on here? Other researchers, such as Simon Chapman, a professor of Public Health at the University of Sydney, have suggested that the presence of anti-wind campaigning itself could be at the root of some of the health problems. The ‘nocebo’ (from the Latin term ‘I shall harm’, as opposed to ‘placebo’: ‘I shall please’) effect is when knowledge of and anxiety about a purported symptom induces some people to experience it—the stress or anxiety caused by being told they will suffer from ill effects causes them to experience the symptoms (and attribute them to the wind turbines). Stress levels can also be higher in communities where there was poor public consultation regarding the construction of a wind farm.

Chapman's research has shown that some wind farms in Australia have been operating for many years without any complaints, and that of the 129 people who made health or noise complaints during the period 1993–2012, 73 per cent lived near just six (out of a total of 51) wind farms, all of which had been targeted by anti-wind campaigners. More than two-thirds of the wind farms operating across Australia have never received any complaints from nearby residents. 

Interestingly, people receiving compensation or an income from having wind turbines located on their properties also rarely report any ill effects on health.

Whatever people’s motivations, it’s clear that there is currently no scientific evidence beyond anecdotes and non-rigorous case studies that supports any claims of Wind Turbine Syndrome or any harmful impacts of wind turbines upon human health. 

Aesthetics

Critics also say that the modern wind farm is a blight on the landscape. Social research has suggested that the perception of wind turbines as visual pollution depends on a number of factors. These may include the technology used (e.g. turbines mounted on tubular towers are generally regarded to be more aesthetically acceptable than those mounted on steel trusses) and the density and layout of the turbines. Ownership may also be a factor: locals are more likely to find a wind farm attractive if they have a financial share in it.

Developments in the technology, such as increasing the diameter of the rotor blades and the height of the towers, have allowed some turbines to operate effectively at lower wind speeds. This improves the capacity of the turbines, and means they can be located in areas that may otherwise have been deemed insufficiently windy. This can allow them to be located closer to the areas of consumption, or away from environmentally sensitive regions.

Wind power is often best at sea—there are fewer obstructions to air flow and less turbulence than on land and the wind is more constant. The wind industry in the UK and Europe is currently moving offshore in a big way. Although Australia does have good offshore prospects, it’s much more expensive to maintain wind farms at sea, and we have plenty of space on land that can be used.

Whether on land or offshore, wind power and its future are certainly not ‘all at sea’. Around the world, the uptake of wind power is growing fast.

Electricity has become a part of modern society that we all take for granted, and life without it would be nearly unimaginable. To meet our needs without the continued burning of fossil fuels, we will rely on wind power to be a key player in a mix of renewable energy sources that will provide a clean, low-carbon future.

 

See our infographic on sources of electricity.