Written by Stuard Baird, M.Eng.,M.A.
Edited by Douglas Hayhoe, M.Ed., Ph.D. (Title changed by Ryan Boyd)
Humans have harnessed the energy of the wind for over 2000 years. The first windmills were built in Persia and converted wind energy into mechanical power. Until the industrial revolution, windmills were used extensively to provide power for many purposes such as pumping water and grinding grain. Wind was second only to wood as a source of energy.
As recently as the early 1900s, thousands of multi-bladed windmills were used on farms throughout Ontario and North America for pumping water. Many of these are still standing today, although few are actively used. The extension of transmission grids into rural areas and cheap electricity prices resulted in the replacement of wind pumps with electrically powered water pumps.
Throughout this century, some remote areas not serviced by electricity grids have used small wind turbines to supply electricity, but the extent of these operations was very small. It was not until the “energy crisis” of the early 1970s, when oil prices skyrocketed, that anybody seriously considered the wind as an important source of energy for the future. Since the early 1970s new wind turbine designs have been developed which have transformed the wind into a clean, reliable and renewable source of electricity.
The Wind Resource
Winds are caused by differential heating of the earth’s surface by the sun. The wind is indirect form of solar energy, and is therefore “renewable”, that is, it is always being replenished by the sun. Every location on the planet experiences wind, but the absolute amount of wind in any one area is highly variable. In order to determine the magnitude of wind power available in a region, maps of average wind speeds must be developed.
The wind speed contour lines on the map show that the areas with the highest average wind speed in Ontario are around the shores of the Great Lakes, and near Sudbury. Wind speed is also very dependant upon elevation above ground level. This is why most wind turbines are situated on the top of tall towers.
The average wind speed of a site is a very important factor in determining the cost of electricity generated from wind turbines. This is because the amount of energy which can be captured by a wind turbine increases as the cube of wind velocity. For example, in Figure 1, the potential amount of energy which could be captured by a wind turbine in the Sudbury area where wind speeds average 20 km/h is about 2.5 times that in the Toronto region where the average wind speed is 15 km/h.
The more electricity a single wind turbine can generate in a year, the more economical it is to operate. This is because the costs of wind generated electricity are mainly the costs of manufacturing the wind turbines. Unlike other sources of electricity such as coal or oil, the “fuel” for wind turbines is free.
On a worldwide scale, the total kinetic energy contained in the wind is more than 80 times that of human energy consumption. Unfortunately in practice only a small fraction of this potential resource could be captured for use, due to the dispersed and variable nature of this resource. It is difficult to estimate the total size of the wind resource.
This total depends upon many factors such as local wind speeds, the availability of land for wind turbines, turbine efficiency, and the costs of manufacturing wind turbines.
It has been estimated that many countries, including the U.S., the United Kingdom, Denmark and the Netherlands could easily generate 20 – 40% of their electricity from wind power. In Ontario, although the theoretical size of the wind resource is very large, wind speeds on average are only moderate, resulting in higher costs for wind power. Use of wind power will be much higher if less cost effective sites with more moderate wind speeds or off-shore wind farms are developed.
Wind Turbine Technology
Wind turbines come in two basic types, horizontal and vertical axis, as shown in Figure 2. Despite their different appearances, the basic mechanics of the two systems are very similar. Wind passing over the blades is converted into mechanical power, which is fed through a transmission to an electrical generator. The transmission is used to keep the generator operating efficiently throughout a range of different wind speeds. The electricity generated can either be used directly, fed into a transmission grid or stored for later use.
Modern wind turbines come in a wide range of sizes, from small 100 watt units designed to provide power for single homes or cottages, to huge turbines with blade diameters over 50 m, generating over 1 MW (1 million watts) of electricity. The vast majority of wind turbines produced at the present time are horizontal axis turbines with three blades, 15 – 30 m in diameter, producing 50 – 350 kW of electricity (1kW = 1000 W).
These turbines are often grouped together to form “wind farms” which provide power to an electrical grid. In Canada, where vast areas of land with moderate wind speeds exist, wind turbine development is concentrating on vertical axis machines which may work more efficiently than horizontal axis machines at lower wind speeds.
The location of wind turbines is a very important factor which influences the performance of the machines. In general, wind speeds increase with elevation. This is why most wind turbines are placed at the top of a tower. Limitations in the strength of affordable materials has limited most towers to heights of approximately 30 m. On wind farms, turbines are most often spaced at intervals of 5 – 15 times the blade diameter. This is necessary to avoid turbulence from one turbine affecting the wind flow at others.
One of the benefits of wind generated electricity is that it avoids most of the traditional environmental impacts associated with electricity generation. Wind power has none of the greenhouse gas and acid gas emissions which result from the combustion of fossil fuels such as coal, oil and natural gas, the traditional sources of electrical power. Similarly, wind power obviously does not result in the risks of radioactive exposure associated with nuclear power plants. Although use of the wind for generating electricity would help reduce the problems of global warming and acid rain, no source of energy is totally without environmental impacts.
The main environmental concerns surrounding the use of wind energy are impacts on land use, noise, effects on wildlife and disruption of radio transmissions. Since the available wind resource is so spread out, vast areas of land are required to provide significant amounts of electricity. Wind turbines can however be placed on areas used for grazing of animals, or land of marginal value. The aesthetics of wind farms has also been questioned, but the public seems to accept the thousands of electricity transmission towers which dot our landscape.
The noise of wind turbines has also been cited as a negative impact of wind turbines. It is true that wind turbines create increased noise levels, but the decibel level of wind turbines drops quickly with distance from the turbine. At a distance of 300 m from the average wind turbine, the ambient noise level is similar to that in a library. Turbines are generally located at least this far away from human activity for safety reasons.
Similarly wind turbines can disrupt radio signals and therefore can’t be located near airports or other sources of important radio transmissions.
Naturalists have always been concerned that wind farms would have a negative effect on bird populations. These fears may not be justified however, as recent studies by the Royal Society for the Protection for Birds have shown wind turbines to have very little effect on bird populations. The damage to wildlife habitat caused by traditional fossil fuel electricity generation has a much greater impact on wildlife than wind turbines.
The Present Situation
Spurred on by increased fossil fuel prices in the 1970s, and favourable tax incentives in California, worldwide wind generated electricity has grown from virtually nothing to over 1800 MW of capacity at the present time. Most of this capacity is located in California and Denmark, both of which obtain about 1% of their electricity needs from the wind. In Canada, wind power is still predominantly used in remote locations, but grid connected wind farms are currently being developed in southern Alberta where average wind speeds are quite high.
In the last decade, the costs of wind generated electricity have decreased considerably, and the reliability of wind turbines has increased dramatically. During the 1980s the cost of wind generated electricity dropped from about 15 – 20 cents per kWh to the current costs of 7 – 9 cents per kWh. This is similar to the costs of generating electricity from fossil fuels and is cheaper than the cost of electricity from most recent nuclear power plants. The costs came down largely because of improvements in the reliability of wind turbines, the best of which are now available to operate 95 – 98% of the time.
Wind speeds in any area are variable and therefore the wind is an intermittent source of electricity. During periods of calm winds, wind turbines cannot generate any electricity. As long as wind remains as a small fraction of total electricity generation, there is no problem integrating wind energy into an electrical grid. Demand for electricity varies with time of day, and day of week, therefore excess capacity is built into every electrical grid. When the wind is blowing, electricity from wind turbines can be used to displace fossil fuel power plants, and when there is no wind, other sources of electricity can make up the difference.
If wind or other intermittent sources of electricity such as solar energy comprise a large fraction (>25%) of an electrical system’s generating capacity, there must be some method of “storing” electricity, to ensure an adequate supply at all times. An example of this is a stand alone wind turbine in a remote location which is connected to batteries. On a larger scale, electricity can be stored with pumped hydropower reservoirs or, excess electricity can be used to break water down into hydrogen and oxygen. The hydrogen could then be burned at a later time to produce electricity. Other storage schemes under development include compressed air storage and large batteries. Storage capacity is very expensive, and therefore if the use of wind power increases above the 25% level, costs will rise dramatically.
The Future of Wind Power
Wind turbines are a mature technology capable of providing large amounts of power at prices competitive with most other sources of electricity. The proliferation of wind turbines as a source of electricity in the future will probably depend more upon the costs of other electricity generating options and public reaction to their environmental impacts than on the improvement of wind turbine technology. Despite this there are a few technological advancements that would improve the potential of wind power.
New lightweight composite materials for use in turbine blades could result in larger turbines (over 1 MW) becoming more reliable and thus more cost effective. Similarly advancements in control systems could allow larger turbine blades to be used without experiencing damage during storms.
Breakthroughs in electricity storage would also make wind power more attractive as an electricity supply option. Since their are large areas around the world which only have moderately strong winds, any improvements which result in economic generation of electricity from areas with slightly poorer wind resources would have a big impact on the future of wind power. The development of “offshore” wind farms in coastal areas would also improve the prospects for wind power.
Wind power is a vast resource of clean, reliable cost effective electricity. Electricity generated from the wind does not contribute to global warming and acid rain. As part of an electrical system which has a diversity of sources or significant storage capacity, wind turbines can be a valuable source of low cost electricity.
The amount of electricity supplied in the future by wind turbines will depend upon the costs of wind power compared to other sources of electricity and the value that society places on reducing atmospheric pollution, rather than the ultimate size of the wind resource. All methods of generating electricity, including wind turbines have some negative environmental impacts and therefore it is important that electricity is used efficiently regardless of source.
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