Wind power gathers speedWind power is set to become an important means of generating electricity worldwide.
Key text
Back to basics You will get more from this topic if you have mastered the basics of energy – this link will take you to an annotated list of sites with helpful background information. Key textThe world has never been short of wind. For thousands of years it has turned windmills, flown kites, cooled houses and filled sails (Box 1: What causes the wind?). Now, technological advances are breathing new life into our use of wind power as a clean, renewable, cost-effective means of generating electricity.Wind has its turn 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. Wind power is increasing in popularity as an energy source. In 2008, the installed capacity of wind turbines worldwide was about 120 gigawatts and was increasing at approximately 30 per cent every year. Much of the growth is in the USA and European countries such as Spain, the United Kingdom and Germany. Denmark obtains about 20 per cent of its electricity from wind turbines and aims to increase this to 35 per cent by 2015. Interest in wind power is also growing in countries such as India and China, and Australia is paying increasing attention to the concept. Why the recent interest? There are probably two main reasons for the increasing interest in wind power. First, most electricity generated today uses non-renewable fuels such as coal, oil and gas. These contribute vast quantities of carbon dioxide to the atmosphere, which many scientists think cause an enhanced greenhouse effect, leading to a warming of the Earth's atmosphere. The second reason is that advances in wind power science and technology are reducing the cost of wind power to a point at which it is becoming competitive with many other energy sources (at about 7 cents per kilowatt hour). The world has long been searching for a non-polluting, renewable source of energy that is as cheap as coal and oil (Box 2: The environmental credentials of wind power). The science of wind generation 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 steam. A high pressure jet of steam is used to turn a turbine (mechanical energy), which is then used to turn a generator to produce electrical energy. (See Box 3: Energy basics for a simple introduction to energy.) In generating electricity from wind, the chemical and heat energy steps are not needed: the kinetic energy of the wind turns the turbine (or blades), which then turns a generator to produce electricity. The potential power of wind turbines The power available from a wind turbine increases very rapidly with wind speed: a doubling of wind speed results in as much as an eight-fold 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 four-fold increase in power (Box 4: The power of the winds). Whipping up the wind A drawback to wind power is that the wind can be erratic, changing direction by the hour. There may be no wind at all one day and a howling gale the next. It may blow hard at times when electricity demand is low, and be a mere gentle breeze when demand is high. But many of the problems of wind power are now being solved. For example, locating wind turbines in areas where the wind blows regularly and at optimum speeds would be a good way to start. In Australia, important advances have been made in this regard: CSIRO researchers have used computer models of wind-flow over complex terrain, together with extensive wind measurements, to calculate potential wind energy yield at different locations. There are other relatively simple tricks to catching the wind. For example, the wind is slowed by friction with the land surface. Modern wind turbines are therefore mounted on towers 40-60 metres high to expose the blades to a higher wind speed. Rotor length Rotor blades need to be strong, light and durable. These qualities become more elusive as blade length increases. Recent advances in fibreglass and carbon-fibre technology have enabled the production of lightweight rotor blades (usually two or three per turbine) between 20 and 30 metres long. These blades are capable of performing for years in the rugged conditions of some of the world's windiest locations. Turbines with blades of this length can generate up to 1 megawatt of power. Plugging into the power supply The large-scale production of wind-powered electricity involves the use of windfarms. These are concentrations of wind turbines – from just a few to hundreds that 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 windfarms feeding into a common grid, for example, may help provide a steady supply: when one windfarm is becalmed, others elsewhere in the region and the continent may continue to operate. Alternative energy sources (such as solar, coal, hydroelectricity or gas) may also help smooth the load. And new technologies are available to store surplus energy generated during windy periods for use at a calmer time. The winds of change How far can wind power take us? The Australian government wants 20 per cent of the nation's electricity to be obtained from renewable sources such as wind power. Wind power technology has the potential to supply a significant proportion of the nation's electricity needs – just as long as the wind keeps blowing. Related Nova topics: Harnessing direct solar energy – a progress report Generating new ideas for meeting future energy needs Which way ahead for hydrogen cars?
But the atmosphere is warmed unevenly by the sun: the poles, for example, are cold while the equator is hot. Warm air over the equator rises and then spreads outwards towards the poles. In the meantime, the cooler air from nearer the poles rushes in to replace the warm air that has risen over the equator. In this way, air is circulated around the globe – the moving air is what we call wind. Patterns of air circulation Because of the rotation of the Earth, 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. All winds blow in circles. The major circulation patterns of easterly winds or westerly winds blow right around the Earth, carrying weather patterns with them. Local winds, as in cyclones or other low-pressure systems, blow in circles clockwise around the centre or eye of the storm. 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. Scientists use computer models and extensive wind measurements to assist in this. Related sites
The towers themselves cover only a small land area, and farming activities can continue virtually up to the base, even under the revolving blades. However, the roads needed to service the turbines take up space, as do the towers supporting connecting powerlines: the total land requirement for a wind turbine is estimated to be about 10 square metres per kilowatt of its potential power. A 600 kilowatt turbine would therefore require about 6000 square metres, or just over half a hectare. Wind power has some critics One problem with wind turbines is that that they produce a low-frequency drone, which increases with increasing wind speed. However, the remoteness of most windfarms means that noise isn't a major problem at most locations. Critics also say that the modern windfarm 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 (eg, turbines mounted on tubular towers are generally less of an eyesore 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 windfarm attractive if they have a financial share in it, while windfarms erected without the participation of the community are less likely to enjoy local acceptance. Future windfarms may be located at sea There may be other ways of avoiding disputes over the loss of scenic quality caused by windfarms. 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. In the future, windfarms may be located 20-30 kilometres offshore, where their visual impact would be minimal. Their outputs would be brought to where it is needed by power cables. Related sites
Potential energy Potential energy can have many forms. The wound-up spring of a toy has potential energy that can be converted into energy of motion when the toy is set running. Water in a high dam similarly has potential energy that can be converted into energy of motion when we open the sluice gates of the dam and the water streams down. One of the important things about energy is that it can be used to do things. In formal terms we call this 'useful work', but it might not be useful and it might be play rather than work! Simply making something move might be useful work, but so could be stirring liquids, lifting loads or heating foods. Energy comes in so many forms that it is worthwhile, first of all, to take stock of these. Energy can be stored in many ways. One way is energy of position. We have already mentioned the energy of water stored in a high dam, and there is a similar energy of position stored in any heavy object that has been lifted up. The heavier the object and the higher it is lifted, the more energy of position it has, and the more useful work it could do if it was allowed to fall. Another very important way in which energy can be stored is as chemical energy. Petrol clearly has stored energy because it can be used to run a car, and electric batteries similarly have stored energy because they can be used to run many things. Energy of motion Heat is a special sort of energy which is actually a form of energy of motion, but we can't see any movement because it is the random motion of atoms. When atoms receive energy they move more rapidly – moving around in different directions in a gas or a liquid, or simply vibrating in one position in a solid. Although we can't see anything happening, we have nerve cell endings that detect this heat energy and tell us whether something is hot or cold. Light energy is rather different from heat energy, though it is given out by things that are very hot. Boiling water is 100°C, a red-hot electric radiator is about 1000°C, a light bulb filament is about 3500°C and the surface of the sun is about 6000°C. (Some things however, such as the phosphors in fluorescent lights or in the screens of television sets, give out light by electrical processes at room temperature.) Conversion of energy One form of energy can be readily converted to another. The elastic energy of a wound spring can be converted to energy of motion of a toy. The chemical energy of petrol can be converted to the energy of motion of a car, and also to heat energy in the engine. The chemical energy in an electric battery can be converted into electrical energy and then into light (in a torch) or into motion (in a toy). It can even be converted into sound energy (in a radio or CD player). Conservation of energy One of the most important things recognised by scientists about 100 years ago was that energy is never actually created or destroyed. It simply changes from one form to another or moves from one place to another. But all along this chain of conversion there is 'waste' energy that appears as heat. It is important to realise that heat energy is different from temperature. In fact the amount of heat energy in something depends on its mass as well as its temperature. There is more total heat energy in the lukewarm water of a large bath than in the small amount of boiling water inside a kettle.
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
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:
where p = 3.1416 and D is the diameter swept by the rotor blades. The kinetic energy available to the wind turbine therefore becomes:
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:
Simplifying the equation:
and
This last equation is the one of most interest. Since the density (d) of air varies only slowly with temperature or height above the Earth's surface, the only variables that can 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 cube of velocity: this means that a small increase in wind speed can produce a large increase in power. For example, if windspeed is 10 kilometres per hour, the value of v3 in the equation would be 1000. If windspeed doubled to 20 kilometres per hour, the value of v3 would be 8000. A doubling of windspeed therefore leads to an eight-fold 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 D2 in the equation would be 100. If D = 20, then D2 = 400. Thus, a doubling of diameter produces a four-fold 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). 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). There are other considerations affecting the potential power of wind turbines, such as the efficiency at which they convert the kinetic energy of the wind into electricity. This can be expressed as the power coefficient, which is the power produced by the turbine as a percentage of the power of the undisturbed wind passing through an area equal to that swept by the rotor. For windmills built before 1900, the power coefficient was usually less than 5 per cent. Technological innovation has led to considerable improvements in modern wind turbines which can now achieve power coefficients of about 35 per cent. This means that they can convert more than a third of the wind's power into electricity. The theoretical maximum in most situations is 59 per cent. Related site
ATSE Focus October 2008, pages 20-21 Wind power can do it now (by Mark Diesendorf) Describes the status of wind power in Australia
March/April 2004 Renewables to the rescue? Wind and solar energy in Australia (by Ian Grey) Looks at the capacity of Australia to meet its energy requirements using wind and solar power.
Australasian Science August 2005, pages 25-26 Wind energy: A waking giant (by Dominique La Fontaine) Describes the potential for wind energy in Australia.
Australian Antarctic Magazine Autumn 2004 Mawson: Antarctica’s first wind-powered station Reports on the opening of Antarctica’s first wind farm at Mawson.
Economist 7 June 2007 Getting wind farms off the ground Explores the potential of flying generators.
June-July 2009, page 149
Nature 1 July 2004, pages 12-13 Breezing into town (by Jonathan Knight) Discusses the option of placing wind turbines on buildings in urban environments.
New Scientist A collection of articles on energy and fuels is available.
8 October 2008, pages 32-35 Renewable energy: Anywhere the wind blows (by Rob Edwards) Describes technological developments in harnessing wind energy
17 May 2008, pages 38-41 To make the most of wind power, go fly a kite (by Michael Brooks) Discusses the use of kites as an alternative for providing wind power.
1 October 2007, pages 44-47 Rocks could be novel store for wind energy (by Daniel Pendick) Investigates a novel approach to storing wind energy.
12 May 2007, page 4 Bats take a battering at wind farms Looks at the threat of wind farms to bats.
12 January 2007, pages 39-41 A bank for wind power (by Tim Thwaites) Looks at developments in the storage of power generated by the wind.
19 December 2006, page 7 An ill wind blows for UK turbines Reports that the UK's wind farms are not delivering the power they were predicted to generate.
8 July 2006, pages 36-39 Gone with the wind (by Ed Douglas) Looks at some potential environmental consequences of wind farms.
21 January 2006, pages 36-39 Every home should have one (by Mick Hamer) Looks at the increasing number of people generating their own energy using roof-top solar panels or wind turbines.
12 November 2005, page 18 Wind power takes a battering (by Rowan Hooper) Looks at the difficulties faced by wind power generation in the UK.
16 April 2005, page 20 The beauty of wind farms (by David Suzuki) Presents arguments in favour of wind farms.
6 November 2004, page 19 Weather hots up under the turbines (by Kate Ravilious) Reports that wind turbines can change air and temperature conditions.
31 July 2004 Power tower (by Rachel Nowak) Looks at the project to build the world’s tallest wind tower.
Our Planet Renewable Energy, 2006, pages 22-23 Blowing in the wind (by Kalpana Sharma) Describes how India has become the world’s fourth largest generator of wind power.
RTD Info February 2006 The wild card of distributed production Looks at the role of renewable energy sources in the move away from centralised production of power in Europe.
Scientific American September 2006, pages 60-69 The rise of renewable energy (by Daniel Kammen) Looks at the growth of wind and solar energy technologies, the development of biofuels and the need for research and development.
Briefly explains how wind is formed and how turbines convert wind into power.
Wind (Clean Energy Council, Australia) Provides information about wind energy in Australia including how it works and wind energy in Australia and globally.
The power of the wind: Cube of wind speed (Danish Wind Industry Association)
Provides detailed information about wind and wind energy. Includes diagrams.
Wind (New South Wales Department of Energy, Utilities and Sustainability, Australia)
Provides a diagram of a wind turbine, how much energy wind can produce and specific examples of wind farms in New South Wales.
Renewable energy and electricity (Uranium Information Centre)
Presents an overview of different types of renewable energy sources and relates the use of renewable sources to electricity demands.
Map of operating renewable energy generators in Australia (Department of the Environment, Water, Heritage and the Arts) Provides maps of proposed and operational renewable energy generators across Australia.
Australian Broadcasting Corporation
generator. A 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. gigawatt. The 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. kilowatt hour. A 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). turbine. A 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.
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