Wind Mill

How a windmill works

Parts of the windmill

Figure E21.2.1 shows the important parts of a windmill. The wind is shown heading perpendicular toward the hub and blade assembly (this example uses three blades, but some use only two, and, as Fig. 21.7 shows, some use many blades).

Fig. E21.2.1 A view of a horizontal axis wind energy machine. (U.S. Department of Energy, Ref. 30)

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Wind machines come in two basic types—horizontal axis or vertical axis machines. The illustration Fig. E21.2.1 shows a horizontal axis machine, the type most often seen. The blades and hub rotate as air streams by. The shape of the blades “channels” the air, and creates regions of higher and lower pressure that result in a net force being applied to the blade, which causes it to turn. This rotational motion represents mechanical energy that will eventually drive the generator to produce electricity. However, usually, the rotational speed of the turbine is less than the 60 Hz AC, so there is an arrangement of gears to change the speed. This axle, the gearbox, and generator arrangement is designated the drivetrain in Fig. E21.2.1.

Fig. E21.2.2 The Sandia 34 m vertical axis windmill. (U.S. Department of Energy, Sandia National Laboratory, Ref. 33)

Figure E21.3.2 in extension 21.3 shows a photo of a vertical axis machine, of the Darrieus type, and Fig. E21.2.2 shows a drawing of the Sandia 34 m vertical axis

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test windmill. Another type of vertical axis machine is called a Savonius turbine, which is an S-shaped windmill that turns slowly; it is used primarily to grind grain.(34)

The blades attached to the hub take on a special shape. According to Ref. 35, “the major features recognized by modern designers as being crucial to the performance of modern wind turbine blades, ... [are] 1) camber along the leading edge, 2) placement of the blade spar at the quarter chord position (25% of the way back from the leading edge toward the trailing edge), 3) center of gravity at the same 1/4 chord position, and 4) nonlinear twist of the blade from root to tip.” The newest wind energy machines are designed to “give” somewhat rather than resist the force of the wind, and the shape as described is very important in assuring efficient transfer of energy.(30,35)

Fig. E21.2.3 A farm windmill showing the rear wing that keeps the mill presented to the wind. (U.S. Department of Energy, National Renewable Energy Laboratory)

The yaw motor can be used to change the orientation of the turbine blades to keep it perpendicular to the wind. In old farm windmills, there was a wing at the back that kept the windmill facing the wind. In Fig. 21.7, the farmer has locked the wing

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sidewise so he could work on the windmill. In Fig. E21.2.3, the wing assuring the windmill presents its rotors perpendicular to the wind is clearly visible

In very strong winds, the windmill can be subjected to large stresses. To prevent damage, windmills are usually stopped in high wind conditions to protect them from harm. The brake can stop the windmill’s motion.

The control system indicated in Fig. E21.2.1 may be mechanical, as it was in olden times. Or, in more modern windmills, it is likely to be run by a microprocessor or hooked into a computer control network.(36)

Wind speed and area—two important parameters

Local mean wind speed is the most important parameter to consider in siting a windmill, because the power transferred to the windmill grows as the cube of the wind speed. There are many regions in the world suitable as wind turbine sites. (See Fig. E21.3.1 in Extension 21.3, Power from the wind for such regions in the United States.) Generally, the wind at windmill height above the ground should be a few meters per second. When the wind speed is too low, the vanes do not turn at all; when it is too high, it could endanger the machine.

Efficient windmills can produce 175 W per square meter of area swept by the propeller. (37) This compares favorably to the total solar annual energy flux. Someone planning to erect a wind turbine would look for a region with steady, relatively fast winds, because the turbine’s delivered power grows as the cube of the speed.

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Windmills cannot extract all the power in the wind, because air must be able to move away from behind the blade and because conversion of mechanical energy involves losses. The maximum fraction of wind power extractable is only 27 ≈ 0.6.(37,38) This is known as the Betz limit. The limitation arises because, if the blades slow the wind too much, most wind will just flow around the blades, but if the blades do not slow the wind enough, the energy will be lost.(39) Sites with mean wind speeds in excess of 7 m/s at 25 m height are prime wind energy resources.
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Windmills produce power at a rate that depends on the effective area swept by the windmill blades and on the speed of the wind. The area is basically the circle cut by the windmill blades. Air entering this region pushes against the blades of the propeller. Since the kinetic energy of any air parcel having mass m is 2 mv 2, the rate of mass flow through the blade’s effective area, A, determines the power transferred to the blades.
1

The rate of mass flow past the blades is given by Av, where

is the air density,

since the mass flowing through area A in a small time ∆t is the density times the volume A(v∆t) of air having that mass. As a result, the wind’s power is the rate of energy flow, ∆E/∆t, which is 2 (the rate of mass flow)v 2, P = 2 ( Av3). The power per unit area (“power density”) is P 1 = ( v 3). A 2 Because of the wind’s variability, we must average the power density over time. That average, or mean, is given by P 1 = (average of the factor v 3 over time). A 2 We call the average of the factor v 3 over time , and it is found by calculation to be
1 1

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6 = π ()3 ≈ 2 ()3. Therefore, the mean power density is almost twice as great as the power density at the average speed.(40)

To increase the power collected by the wind turbine, then, we must have a location having winds with mean speeds great enough, and then we need a large collecting area. So we would like to make the blades as large as possible.

Wind shadows, or wakes, can be an important consideration as well. If machines are spaced too close to one another, the change in the flow pattern caused by the windmill in front or to the side can interfere with neighboring windmills. In the mountain passes in California, the machines are spaced 2.5 rotor diameters apart in a row perpendicular to the prevailing winds, and the rows are spaced 8 rotor diameters apart, which reduces wake problems.(25)

The rotor blades are very important to the windmill. They must be big in order to capture as much wind as possible, but not so big that they lose their structural integrity when subjected to large forces. As noted in Extension 21.3, the smaller machines represent reasonably well-known technology, but if we want to operate more cost-effectively, we would want to make the machines bigger. As of 1997, the average size of an installed new windmill was 600 to 750 kW.(30,41) By 2000, machines rated from 1 to 2 MW are being installed.(30) Research is proceeding to scale the blades to even bigger sizes.

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Why is research on “old technology” important?

The first windmill used to generate electricity was built only a little over a century ago. Charles Brush built that wind machine in Cleveland, Ohio in 1888.(35,42) It generated only 12 kW. Developments came slowly, because the real work of windmills was for smallscale pumping of water on farms, not for electricity for the grid.

Fig. E21.2.4 Large-scale windmills were developed in the early response to the 1973 and 1979 energy crises. This is a two-bladed research turbine. (U.S. Department of Energy)

Windmills seem such old technology, having been around for at least a millennium, it might seem that there is nothing more to learn about the behavior of windmills. This is far from the truth. It is true that small windmills, such as the one in Fig. 21.7, are better understood. Because of the immense amount of energy that could be produced by tapping the winds, even at 35% efficiency, the 1970s and 1980s saw many companies enter the

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wind turbine business.(43) Immediately after the 1973 energy crisis, large wind units were designed and studied (Fig. E21.2.4). They were found to be uneconomic at that time, and smaller units of size 50 to 70 kWe were preferred. The technology of small wind units is now relatively mature, and reliable units are available up to 0.5 MWe. The 1990s saw tremendous growth in the size and output of wind generators.(18,30,35,41,42)

Engineering research is often thought of as “cleaning up around the edges,” but it has made real contributions to the efficiency and profitability of wind turbines. Problems were identified in research in the 1970s that were rectified in the wind turbines of the 1980s, but even those machines are not the most efficient.

Nevertheless, the new technology was pushed and many investments made, and direct government research money dried up. Renewable energy research and development, which languished during the Reagan-Bush years, made a comeback after the election of President Clinton. So much progress was made in this newer round of research that the technology will probably withstand any decision of the George W. Bush administration to deemphasize renewable energy.

One important change is that new wind units are being certified, which should prevent “hidden” problems from coming to light only after the purchase of the unit. DOE’s National Wind Technology Center is the only certification unit in the United States accredited by the American Association of Laboratory Accreditation.(30)

Much 1990s research focused on mid-size turbines. Three sites were chosen by the Department of Energy in 1993 for significant new wind energy feasibility tests: Texas, Vermont, and Maine. There are 150 windmills at the Maine site, producing 30 to 50 MW. The plan calls for 20 wind turbines at the Vermont site. The windmills were built by

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Kenetech-U.S. Windpower; they are a new design that can be used even when winds are gusty. (44) The rotors are about 30 m in diameter; each mill, rotating 50 to 100 times per minute, will generate 300 kW. Texas is also interested in wind energy. Ft. Davis, Texas, will have at least 20 windmills (producing about 6 MW). The Department of Energy is putting up $6 million of a total of $40 million for tests, with the remainder coming from local utilities. A different sort of wind energy system has been tested at Rensselaer Polytechnic Institute in Troy, New York. It will give variable-frequency power, first rectified, then transformed to 60 Hz.

U.S. Windpower (Kenetech) developed a variable-pitched blade turbine that is expected to produce electricity at a total cost of $0.05/kWh where the wind speed at hub height is at least 7.2 m/s (a power density of 450 W/m ).

Fig. E21.2.5 Technicians at Sandia National Laboratory examine turbine blades. (U.S. Department of Energy, National Renewable Energy Laboratory, Ref. 30)

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Compared to a decade ago, turbine blades are as much as 30% more efficient.(30) According to the Department of Energy, 40 m blade lengths as at present are very close to the size limit imposed by the strength of the materials.(30) As discussed above, the larger the size of the blade, the greater the area swept out. The larger the area swept out, the greater the energy that can be coaxed from the wind on a piece of real estate. Research at the National Renewable Energy Laboratory (NREL) shows that size only helps so much—that is, economy of scale is not a total panacea.(30) However, the size of wind turbines has increased steadily, from the 50 kW machines installed in California wind farms in the early 1980s to the 1.5 MW turbines now being mounted.(35)

One of the many new things found recently is that intermittent energy output decreases in wind turbines arises because of the piling up of insect bodies on the turbines.(45) The insects deposit on slow-moving blades and cause the long-observed apparently inexplicable decline in generator output. The output rose after every rain or manual cleaning. Clearly, this has implications for increasing efficiency of all sorts of wind turbines.

Variable speed turbines are more able to harvest wind energy than fixed speed machines. The ability to absorb energy from gusts can add significantly to the capacity factor of windmills, which has sometimes been very low.(30,42) Electronic inverter circuits convert the energy into the 60 Hz used in the electricity grid. Variable speed turbines are more expensive to produce than fixed-speed versions. American companies generally are committed to variable speed turbines, while European companies have remained with fixed speed machines.

Another area of research is rotor design. Should there be two or three rotors? It seems to depend. According to NREL,(30) “Configuration options include the number of blades; the

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orientation upwind or downwind of the tower; the amount of movement or flexing allowed in the blades and the hub; and the way blades are used for speed and power control.” For example, it had been found in German research in the 1960s that putting a bearing in the rotor hub allowed the rotor to move in response to wind gusts and stabilized the windmill, but this result was not widely known.(35) The teetering hubs turned out to be crucial for reducing loads and are a feature of every advanced machine. The idea had to be rediscovered through laborious research. Another product of research was the nodding nacelle—a hinge was inserted between nacelle and tower to allow better dynamic response to turbulence. (35)

Seven different drivetrain combinations are also being studied. Surprisingly, except for the foundations, the cost of running a windfarm was found to be independent of the size of the turbine installed.(30) Drivetrains are important for another reason: to change the much slower revolution frequency of the wind-caused rotation to 60 Hz, they have many moving parts and must be serviced regularly. This is why a new Swedish design that works without a gearbox may be important.(46)

In another research project, it was found by examining data from Germany that having wind farm turbines that are widely separated, rather than bunched, tends to even out the electricity production and make a better match to the load. This implies that tying windfarms in separated geographic areas together may be advantageous.(47)

Lightning strikes were found to be a problem at several windfarms due to a Department of Energy program for sharing experience among utilities. This led to installation of better grounding for the turbines.(30)

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Finally, two Croatian researchers from Rijeka University found that by changing the cowling on the generator, they could increase the output by a factor of 3.5.(48) Their device works by shunting off air in high winds, stabilizing output in almost all wind conditions. This approach holds promise, but it does cost a substantial amount more (perhaps an additional 75%) than conventional windmills. This configuration even gives electricity when the windspeed is below 5 m/s, the usual limit for wind machines—it provides electricity at windspeeds as low as 2 m/s.(48)

The International Energy Agency predicts that renewables (excluding hydroelectricity) will be only about 3% by 2020.(49) Renewables are expected to continue to grow at over 1% per year, higher than any other form of energy. Wind is expected to be a large part in this surge. Figure E21.2.6 shows the expectation for 2020.

Fig. E21.2.6 Onshore wind energy projected in 2020. (International Energy Agency, Ref. 49, Fig. 3)

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Germany has installed windmills having a capacity of 4.5 MW. The larger the mill, the more difficult repairs are and the more reliable the machine must be in order to be put into service. This development is a vote of confidence in these giant windmills (the machine stands 120 meters tall, and its wind vanes are 52 meters long).(50) As of this writing, the Germans are working on the development of 5 MW windmills for offshore placement. The mills are based on a design already in use in Finland (at 1 MW, however).(51)

Whatever the final word, it is research that will help give us the final best configurations for future wind energy machines. Progress made since the early 1980s is startling. There is sure to be more to come.