Wind Energy Of all the forces of nature, I should think the wind contains the greatest amount of power. ~ Abraham Lincoln CHAPTER 3 In the last quarter of a century, wind energy technology has gone through revolutionary changes making wind the fastest growing source of electricity in the world. Worldwide, 120 gigawatts (120,000 megawatts), or about 1.5 percent of the total electricity, is generated by wind turbines, though numbers are growing rapidly. In the last decade, the average annual growth of the installed capacity of wind power plants has been increasing by 32%.1 Many European countries, in particular Denmark, Germany, and Spain, have been heavily investing in wind energy. Germany, with more than 18,400 MW, is the leader in wind generation capacity with Spain, the United States, India, and Denmark ranking next. In terms of the percentage of total electricity production, Denmark ranks first; it currently produces 20% of its electricity from wind and is planning to raise this figure to 40% by 2030. In the United States, only three states - North Dakota, Kansas, and Texas - have sufficient wind that, if harnessed, could satisfy national electricity needs. Though these states have the most wind, they are sparsely populated and there are not many transmission lines. Most wind turbines are located in three areas, all of them in California. Today, the US generates about 9,100 MW, or a little more than half a percent of its annual electrical generation capacity from wind, but is expected to increase its capacity to about 5% of its total electricity demand by 2025.2 Rotor Wind Gearing Overview Wind is air in motion. We can’t see it, but we can see its effect all around us. Just like moving water, the wind represents a tremendous source of natural energy. Like waterwheels, windmills were among the original prime movers that replaced human beings as a source of power. Wind power has been used as early as 5000 years ago by the Egyptians to sail ships across the Nile. The first windmills were invented by Persians around 200 BC to pump water from wells and to grind grain. They were constructed by fastening bundles of reeds onto wooden frames mounted on vertical shafts (hence called vertical windmills) housed in brick or clay walls. Wind entered through an opening at the side and was caught between the spokes radiating from the shaft (Figure 3-1). The technology was exported to China after Genghis Khan imprisoned Persian millers and forced them to build windmills to power irrigation systems in China. 1 2 Figure 3-1 Persians invented the first practical windmills for grinding wheat and pumping water. K enisarin, M. M., “Worldwide State of Wind Power Engineering,” A pplied Solar Energy, Vol. 38, No. 4, 2002, Allerton Press, Inc., New York. A merican Wind Energy Association Website, http://www.awea.org. It was not until the twelfth century that windmills found their way to Europe, where their use became increasingly widespread until the early 19th century. These mills used vanes that looked like huge paddles mounted on a horizontal pole (hence called horizontal windmills). The Dutch used windmills mainly for draining water from their low-lying land (hence Netherlands), which was quite prone to flooding (Figure 3-2). Figure 3-2 Dutch horizontal wind turbine Figure 3-3 Brush windmill in Cleveland Figure 3-4 Two-bladed wind turbines 0.35 25,000 20,000 15,000 10,000 5,000 0 1980 1984 1988 1992 1996 2000 Cost of electricity $/kWh 0.30 0.25 0.20 0.15 0.10 0.05 0 Production Cost Megawatts The first use of a large windmill to generate electricity was in 1888 by Charles Brush in Cleveland, Ohio.3 The Brush machine shown in Figure 3-3 had a rotor 17 meters in diameter with 144 blades and provided enough electricity to light 350 incandescent lamps. The device worked for 20 years until the advent of steam engines and the popularization of low-cost, seemingly inexhaustible fossil fuels made windmills less and less attractive. Later works by aerodynamicists showed that the most efficient number of blades is 2-4, a far smaller number than those found in earlier windmills (Figure 3-4). The world’s wind electric energy generation capacity has been climbing steadily since 1980, reaching 60,000 megawatts in 2005, and 120,000 megawatts in 2008; this is largely due to new technological innovations that have reduced production cost from $1.00 to around $0.04 for one kilowatt-hour (Figure 3-5). In 1980, the United States was the leading producer of electricity from wind and accounted for 80% of the world’s capacity. Its share, however, has been declining ever since and accounts for only 15% of the world’s capacity today.4 Figure 3-5 As the electricity output from wind energy has increased, cost of electrical generation has decreased dramatically. Global and Local Wind Patterns Winds are generated as a result of two factors, the non-uniform heating of the earth by the sun and the rotation of the earth. Equatorial regions receive the most radiation, whereas polar regions receive the least. The difference in ground temperatures between the equator and the poles induces a global circulation pattern where hotter (and lighter) air rises 3 4 “Mr. Brush’s Windmill Dynamo,” Scientific American, December 20, 1890. Global Wind Energy Council, “Global Wind Power Continues Expansion,” press release, 17 February 2006. 46 Chapter 3 - Wind Energy near the equator, and colder (and heavier) air sinks at the poles. As a result, overall wind flow direction is from the poles toward the equator close to the surface and from the equator toward the poles in the upper atmosphere. The detailed flow pattern, however, is much more complicated than this. The upper atmosphere wind (called geostrophic wind) is largely driven by the earth’s rotation and temperature (and thus pressure) differences. Close to the earth’s surface, other factors such as mountains, valleys, and shorelines are important in establishing the local wind patterns. Sea breezes occur during the daytime when landmasses are heated more quickly than the sea. Sand has a lower heat capacity than water and cannot hold solar heat as effectively, resulting in its temperature rising above that of the water nearby. As the air rises above the hotter land, air from the cooler sea moves to replace it, resulting in a sea breeze. At night, the land gives off heat more quickly and its temperature drops faster than the surrounding sea, resulting in land breezes. At dusk, there is often a period of tranquility when the temperatures of land and sea are equal (Figure 3-6a). Valley and mountain breezes are due to a combination of both differential heating and local topography. As the sun rises, it hits the mountain tops first and, as the day progresses, the mountain slopes, causing differential heating between the two. As warmer air rises off the slopes, cool valley air moves up to fill the vacuum (valley breeze). In the afternoon, as the sun sets, the opposite occurs and we have mountain breezes (Figure 3-6b). Wind Rose Selection of a proper site for installing wind farms requires detailed meteorological data at different times. Wind data are routinely collected by wind anemometers installed on top of towers in the direction of the prevailing wind. Because speeds and directions are continually changing, 10-15 minute average values are recorded. The data is plotted on a wind rose, which is a single, graphical representation of speed, direction, and frequency of occurrence. Data are often normalized to the total period of observation in order to indicate frequencies. One simple way to Warm air Cool land breeze Cool sea breeze Warm air Cool air Warm air Warm air Warm air Cool air Cool air Land warmer Sea cooler Land cooler Sea warmer (a) Figure 3-6 Wind formation along a) the coast lines, and b) mountain ranges. (b) 47 Digging Deeper ... Global Circulation Pattern I f the earth were not rotating, wind near the earth’s surface would flow directly from the poles towards the equator and, in the upper atmosphere, from the equator to the poles, completing the loop. Since the globe is spinning, observers at the equator have a higher tangential velocity than observers farther north. To observers in the northern hemisphere, air moving south is turning to the right, acquiring a westward component and appearing to lag behind the earth. This deflection can be viewed as if a force (called Coriolis force) were pushing the wind toward the west. The opposite is true for Polar cell Polar high the southern hemisphere. The result is that the wind flows horizontally, Polar front H more or less along the isobars (paths of constant pressure). These winds Ferrel cell 60¼ are known as geostrophic winds. L One factor that complicates the general circulation pattern is the radiative cooling of the upper atmosphere; this causes flow to become unstable and air to subside at 30o north and south latitudes, thus falling short of either pole. Air parcels sinking at 30o latitude will be pushed both north and south, towards the poles and the equator. These streams will collide at 60o latitude, producing new lows and polar fronts-- breaking up the large cells into two smaller Polar and Ferrell cells. The result is the prevailing winds and a global circulation pattern resembling that of the figure shown. H Hadley cell H L H L H 60¼ 30¼ L H L 0¼ Equatorial lows 30¼ The geostrophic wind pattern described above occurs at altitudes above 1,000 meters. At regions below, we have synoptic winds that are associated with large-scale movements of warm and cold fronts. Hurricanes, tornadoes, and typhoons are of this kind. At ground level, surface effects dominate. Surface obstacles such as tall buildings, hills, and valleys, as well as proximity to the sea and the heating by local sources (such as factories, power plants and freeways), will determine the local wind pattern. Figure 3-7 Wind rose for Fresno, California for the month of April. Concentric circles represent the frequency, and different colors indicate ranges of wind velocity. Source: Natural Resources Conservation Service, US department of Agriculture, http:// wcc.nrcs.usda.gov/climate/windrose.html) graphically indicate both duration and direction is to draw bars extending radially from the centre of the rose in the direction of the wind; magnitude is represented proportionally to the time the wind spent in that direction at a given speed. The direction of the wind is traditionally taken as the direction it is coming from. The thickness or the color of the bars can be used to indicate the range of speeds in a given direction. A typical wind rose is shown in Figure 3-7. The data were accumulated over a period of ten years in Fresno, California, for the month of April. From the figure, it can be deduced that over 50% of the time the wind blows to the northwest at speeds of 1.8-3.3 m/s (16% of the time), 3.3-5.4 m/s (20% of the time), 5.4-8.5 m/s 11% of the time, 8.5-11 m/s 2% of the time and greater than 11 m/s less than 1% of the time. Depending on the wind power density and speed, different geographical areas have been divided into seven classes. Class 1 regions are not suitable for wind energy development, and class 2 regions may become only marginally acceptable. Class 3 areas will be suitable in the future, as technology matures. Class 4 and higher are considered suitable for wind power with existing technology. Figure 3-8 shows the yearly electricity production in W/m2 of rotor area for different locations within the continental United States. As the data indicates, the strongest winds are found along coastlines, along ridges, 48 Chapter 3 - Wind Energy and on the Great Plains.5,6 Furthermore, it is estimated that up to 6% of US land is suitable for development of wind farms and has the potential to generate 1.5 times its electricity needs.7 Wind Turbines Wind mills are devices that convert the kinetic energy of wind into the mechanical energy of rotating blades. The more kinetic energy, the greater the potential for producing work and power. The total energy transferred depends on the wind speed, the size of the rotor, and the density of the air. In practical terms, there are limits to the wind velocities that wind turbines can utilize. If the wind velocity is too low (cut-in speed), the kinetic energy is not sufficient to overcome friction at the bearings. At velocities above about 50 mph (cut-out speed), wind power is strong enough to knock the blades off, rendering the turbine inoperable and creating a real danger to nearby buildings, traffic, and people. Question: W hat is the difference in operation of an electric fan and a wind turbine? Answer: A fan uses electricity to produce wind, whereas a wind turbine uses wind to make electricity. In ancient windmills and other early vertical axis windmills, the wind pushed and rotated the sail with a force called a drag. These windmills were inherently less efficient than later horizontal axis wind turbines that have been put into operation in the past four hundred years. Modern wind turbine operation is similar to that of the wings of an aircraft. Because of the shape and angle of the airfoil (blade) cross-section, the air changes direction as it approaches the airfoil. The change in airstream direction results in a change in its momentum. As we have learned from Newton’s second law, a change in momentum requires a force. The force of the airfoil on the air is counteracted with an equal and opposite force on the airfoil (remember action and reaction), which is called lift. In airplanes, lift causes the plane to become airborne, whereas in wind turbines, lift forces the blades’ rotation about the hub, which in turn drives a generator. Perpendicular to the lift force, a drag force impedes rotor rotation. Unlike helicopter blades and aircraft wings, which are designed for the greatest lift, turbine blades are designed to reduce drag. High lift is essential in aircrafts to prevent stall. Stall refers to a condition at which the angle of attack is so steep that no lift is produced. Even a small dent in the blade or airfoil can trigger a stall. In wind turbine applications, some stalls are welcome in high wind conditions to slow the turbine down and prevent W ind Energy Resources of United States, U.S. Department of Energy (DOE), can be found at the National Wind Technology Center web site at http://www.nrel.gov/wind/. DOE “Wind Powering America,” web site at http://www.eren.doe.gov/windpoweringamerica/. 7 US Department of Energy (http://www.doe.gov) 5 6 Figure 3-8 United States Wind Resource Map. Source: National Wind Technology Center, http://www.nrel.gov/wind/wind_map.html. 49 damage. In new designs, the blade angles can be adjusted to change the lift-to-drag ratio and to optimize the turbine’s energy output for different wind speeds and directions. Question: Most rotor blades have twisted surfaces. Why? Answer: To optimize the angle of attack and prevent stall as wind flows from the tip (with a very high rotational speed) toward the hub (with a low rotational speed). The major components of a wind power generator are the hub (on which rotor blades are attached), a gearbox, an electrical generator, and a controller with associated cooling units. Except for the hub, the entire assembly is housed inside an enclosure called a nacelle that is mounted on top of a tower (Figure 3-9). In addition, modern wind generators have anemometers for measuring wind speed, vanes for determining direction, and a yaw mechanism that can tilt the rotor in the direction of the wind. The vane assures that the turbine continuously faces into the wind. The tower carries the weight and raises the turbine above trees, buildings and other nearby obstructions. Wind turbines are more efficient when installed on tall towers because they face faster winds. In addition, most turbines are equipped with automatic governors that protect rotors from spinning out of control in gusts and during high wind speeds. Generator win d Blades Gear-Box Transmission Brake Hub Main Shaft Housing Tower High-Speed Shaft Figure 3-9 Principle of operation of a wind turbine. Basic operation of wind power generation is as follows. Wind drives the blades mounted on a shaft rotating at about 10 to 50 revolutions per minute. The shaft is connected to a transmission or gearbox and changes the speed to the 3,000 rpm needed to generate utility grade electricity. An electrical generator converts the rotational speed of the shaft to electricity, at which point it is transmitted through an underground cable to a field transformer and, eventually, to a utility substation. Here, electricity is added to the grid and combines with electricity generated by a variety of other sources such as coal, nuclear, and hydroelectric power plants. Not all kinetic energy can be converted to useful work. As incoming wind collides with the blades, some reflects off of the rotor surface, and some is lost as a result of friction at the generator, limiting the wind turbine’s efficiency to about 40% at best.8 Wind turbines are available in a variety of sizes. Machines vary in size from 0.6 m in diameter (rated at about 500 W) to 60 m in diameter (rated at about 3 MW). R ated power is the power that a wind turbine produces when running at its optimum, i.e. at the highest wind speed. Wind Power Not all the kinetic energy present in winds can be extracted by a wind 8 It can be shown that the theoretical maximum efficiency of wind turbines is 59%, known as the Betz limit. 50 Chapter 3 - Wind Energy turbine. In addition to various losses, such as slippage and drag, some of this energy must also be carried downstream of the turbine in order to maintain the air flow. The power generated by a wind turbine is proportional to the kinetic energy of the mass of air swept through its rotor. The mass itself increases with the air density, the size of the rotor, and the speed of the wind. As a result, total power generated by a wind turbine increases with the air density, the cube of the wind velocity, and rotor area, that is, the square of the rotor diameter (See Box “Power and Torque”). Wind speed increases with height above the ground, so the height of the tower on which the rotor is mounted indirectly affects power production. Figure 3-10 shows the increase in power with the height of the tower. For example, a wind turbine mounted on a tower 120’ (36 m) tall will be exposed to wind speeds twice that of a tower 30’ (9 m) tall. This alone results in the production of eight times more power. Air density (and power output) decreases with altitude at the rate of roughly 2.5% for every 1000’ above sea level. Temperature affects the power derived from wind energy in two ways. First, they affect wind patterns and intensity. Second, they affect the air density and thus the mass of air drawn through the turbine. In desert and open areas, where most wind turbines are installed, temperature differences as high as 60oC are possible between summer and winter seasons. As a result, as much as 20% more power can be extracted in winter when the air is denser. Design Considerations There are two types of wind turbines, horizontal and vertical. In horizontal turbines, the axis of the rotor is parallel to the ground, whereas in vertical turbines, the axis of the rotor is perpendicular to the ground. Horizontal turbines are usually of a propeller type, while vertical turbines have C-shaped blades and look like eggbeaters. These turbines (called Darrieus machines) are made with 2-3 blades and are designed to capture wind from any direction; they have the advantage that the heavy components (the gearbox, generator, and controllers) can be placed on the ground where wind is weaker. Because these turbines are mounted close to the ground, wind speeds are relatively low so they are generally less efficient than horizontal-axis machines. As a result, most modern wind turbines are of the horizontal type. Turbines can also be designed such that either the front side (upwind) or rear side (downwind) of the rotor faces the wind. Most commercial turbines use an upwind design. The major advantage of upwind turbines is that there are no wake losses. The primary advantage of downwind machines is that the blades can be made from cheaper and more flexible materials because there is no danger of them hitting the tower. The main 51 150 120 Tower height, feet 90 60 30 0 0 41 100 124 Percent increase in wind power Figure 3-10 Effect of tower height on wind power Power and Torque Mathematical Interlude ... A s the air rushes through the rotor, it follows an imaginary stream tube, slowing down to convert its kinetic energy into the rotational energy of the blades. Since the total volume of the air must remain nearly the same, it must expand as it crosses the rotor. The volume swept is proportional to the cross-sectional area of the blades and the velocity of the wind perpendicular to the rotor: Q/t = A.V = πd2 V 4 (i) where: Q is the air volume(m3) V is the wind velocity(m/s) t is the time(s) A is area of the disk rotor(m2) d is the rotor diameter(m) The kinetic energy of this parcel of air is given as 1 1 π E = mV2 = ρQV2 = d2V3t 2 2 8 (ii) (iii) Power is energy per unit time, or π P = ρd2V3 8 This equation shows that wind velocity is the most important factor affecting power output. As the wind speed doubles, kinetic energy increases 4-fold, while twice as much air sweeps past the blades per unit time, resulting in a power increase of 2x2x2 = 8 times. Equation (iii) assumes that all the energy contained in the approaching wind is converted to useful work. In practice, there are a number of factors that limit the energy utilization and rate of power production. Some of the wind will spill over and be deflected off of the rotor blades. Also, the velocity of the air after passing through the rotor is not negligible. Furthermore, there are losses at the generator, gearbox, converter, and other power conditioning devices. Overall efficiencies of 20-50% are reasonable. Including losses, equation (iii) is modified to give electrical power output as: Pe = π ρd2V3η 8 Where h is the turbine efficiency. Torque is calculated by dividing power by angular velocity of the shaft. P ω where w is the angular velocity of the rotor. τ= V2 V1 Stream tube. (iv) (v) Example: Calculate the maximum power output and torque delivered by a 12-m diameter wind turbine when wind blows at 10 m/s. Assume rotor spins at 200 rpm and air density of 1.16 kg/m3. Solution: The maximum power is delivered when h = 1 P= 2 3 π (1.16)(12)(10) = 65,596 W = 65.6 kW and, 8 t= 65.6 (1.16)(12) = 3.1 kJ 2 π (200/60) 52 Chapter 3 - Wind Energy disadvantage of downwind turbines is that wind speed suddenly drops behind the tower, so there is additional wind shade where the blade passes behind the tower which causes vibration of the rotor and results in more fatigue. Rotor Efficiency (percent) 40 Three Blade (H) How many blades are optimal? Theoretically a wind turbine needs only one blade, but most have two and a few have three or more. The greater the number of blades, the more stable the turbines are, but also the heavier and more expensive. Multi-bladed rotors used in older designs are less efficient, but provide more torque and have therefore been used traditionally for pumping water even at low wind speeds. Two- and threebladed rotors are cheaper, lighter, and run at faster speeds; however the increased tip speed can also make them noisier. They also have moderate starting torque which makes their operation difficult at very low wind speeds. One especially important factor in two-bladed wind turbines is wind shear. Because air has a greater velocity further from the ground, wind puts greater force on the top blade than the bottom blade. To correct this, wind turbines are equipped with a teetering mechanism, which equalizes the forces by allowing the blades to tilt slightly around their central pin. In some other designs, blades have a fixed pitch so instead of pivoting at the hub, these turbines flex as wind speed picks up. To guard against overspeeding, each blade has a small tip brake that at high speeds tips into the wind, stalling the turbine. Figure 3-11 compares performance of various rotor designs at different wind speeds. Question: Windmills traditionally used for pumping water have small solid rotors with many blades. What is the main advantage of these designs? Answer: The many blades assure operation at very slow wind speeds, allowing continuous operation all year. These windmills are, however, very inefficient at high speeds and must be shut down to prevent damage. Size 30 Two Blade (H) G Blade (V) 20 10 Multi-Blade (H) Savonius (H) 0 0 10 20 30 40 Darrieus (V) 50 Wind Velocity (MPH) Figure 3-11 Effect of rotor design on performance (Adapted from Wind Energy Systems, by G. L. Johnson, Prentice Hall, 1985.) The size of wind turbines varies greatly and, depending on application, they produce power from a few watts to many megawatts. Micro turbines produce power in the range of 20-500 watts and are used in such applications as battery chargers and recreational vehicles. Turbines generating less than 50 kW are considered small, those producing between 50 kW and 1 MW are medium-sized, and turbines producing above 1 MW are large (Figure 3-12). Small turbines are used in applications such as pumping water, whereas medium and large wind turbines are used for producing electricity. For larger metropolitan areas, either a large number of small turbines or a few large turbines must be installed. There are advantages and disadvantages to each option. The choice of using small or large rotors Figure 3-12 The world’s largest wind turbine is now the Enercon E-126. This turbine has a rotor diameter of 126 meters (413 feet). The turbine being installed in Emden, Germany and will produce 7 megawatts (or 20 million kilowatt hours per year). That’s enough to power about 5,000 households of four in Europe. (Image courtesy of Enercon Corporation). 53 S FYI ... Angle of Pitch and Angle of Attack ome mistake the pitch and the angle of attack as being the same. The pitch is the angle the airfoil chord makes with the rotor plane of rotation. The angle of attack is the angle the chord line makes with the direction of flight or the relative wind direction. They are only the same in the absence of induced flow; that is, when the aircraft is in horizontal flight or, in the case of a wind turbine, when wind blows parallel to the rotational axis of the rotor. Generally, the two angles are different. α θ Top and side views of a wind turbine rotor and propeller blades. V1 is the wind velocity, V0 is the component of wind in the axial direction (or in the case of the aircraft, its forward speed), q is the blade pitch angle, and a is the angle of attack. X α θ Pitch and angle of attack of an airplane in flight. and generators depends on the application and on the distribution of wind energy throughout the year. For the same total capacity, larger turbines occupy less space and are cheaper to install. For example, a 400-kW turbine costs considerably less than four 100-kW turbines. The cost of delivering electricity is also lower for larger turbines than smaller ones. In addition, larger turbines can utilize the energy contained in highspeed winds and have greater efficiency. The drawback is that they cannot produce power at low speeds. The main advantage of small turbines is that they can produce continuous power for most of the year, as they require only slow to moderate wind speeds to operate. However, much of the energy in high-speed wind is wasted. Taller towers are generally better than shorter towers, as wind velocity rapidly decreases near the ground. As shown earlier, the total energy in any wind stream is proportional to the cube of the wind speed, so turbines installed on taller towers could, in principle, be much more efficient. The cost of tower construction, however, could be much higher and may not justify the extra gain in efficiency. As a rule of thumb, tower heights are roughly equal to rotor diameter and cost one-fifth of the total price of the turbine. Stand-alone turbines are usually more economical, less complicated, and require less maintenance. They are used mainly in remote areas and small cities where local electrical grids are not able to support the power generation from larger wind turbines. Power Control To prevent failure, the system must have safety features that control 54 Chapter 3 - Wind Energy sudden increases in power. Modern wind turbines are equipped with mechanisms that automatically adjust the rotor speed as wind speed and direction change. This can be done by controlling the pitch, controlling the angle of yaw, or initiating a stall. The best way to control the power is by changing the blades’ pitch. Pitch is the angle that the airfoil chord makes with the rotor plane of rotation (also called the angle of incidence). With variable pitch designs, bearings are inserted between the blades and rotor hub. When the wind speed becomes too great, rotor blades are turned along their longitudinal axis and slightly out of the wind to allow some wind to pass by without increasing lift. At low wind speeds, blades are turned into the wind. The advantage of a pitch-controlled scheme is that relatively constant rotor speeds can be maintained within a large range of wind speeds. Another method of controlling rotor speed is to turn the rotor out of the wind by adjusting its yaw angle and tilting it either into or away from the direction of the wind. To optimize efficiency, turbine blades must face the wind at all times (they must be in a plane perpendicular to the direction of the wind). A large wheel called a yaw bearing turns the nacelle with the rotor into the wind. At very high speeds, the rotor is intentionally turned away from the direction of the trailing wind, reducing the total volume of air passing through the rotor and reducing the power. Since yawing bends the rotor, turbines running with yaw- or tilt-control experience large fatigue loads. In stall-controlled wind turbines, blades are designed to ensure stall when wind velocity reaches a set value. The blades are shaped such that excess wind speed creates sufficient turbulence on the backside and the rotor stalls. The basic advantage of stall control is that the rotors remain fixed to the hub, which removes many of the complex control-mechanisms necessary in pitch and yaw control methods. Most large wind turbines use this approach. Generator In order to generate electricity, the wind turbine must be coupled with a generator. Most generators are directly connected to the grid and produce power at a nearly constant frequency; they are therefore constrained to operate within a narrow range of wind speeds. Sudden gusts of wind can speed up the rotation, so the drive train and tower must absorb the extra torque. Normally, generators produce 690 V, three phase alternating currents at frequencies of 60 (for the US) or 50 (for most other places in the world) hertz. The current is amplified through transformers in order to increase the voltage to the 10-30 kV used in most local transmission lines. In indirect grid connections, wind turbines operate independently of the 55 generator and the system operates in a variable speed mode determined instantaneously by wind speed. This setup has the advantage of being able to run a generator at all wind speeds. The cost is, however, considerably higher. In this case, voltage from the generator must be modified to match that of the neighboring electrical power grid. This must be done in several steps. First, the variable frequency alternating current must be converted into a constant frequency alternating current. Next, the alternating current must be converted to direct current using a rectifier. Finally, using a DC-AC inverter, the direct current must be converted to an alternating current at a frequency matching the grid line. Some filtering may also be required to eliminate unwanted frequencies, thus producing clean electric power with the desired frequency. Power Train The power generated by the wind turbine must be transferred to a generator. If the generator and turbine rotor are connected directly, then the rotor must turn with the same rotational speed as the generator. For a 50 Hz generator, this translates to 3,000 rpm. For a large diameter rotor, the tip speed will be extremely high, reaching several times the speed of sound. This causes an extreme torsional load and an excessive amount of noise which is not acceptable in populated areas. Rotational speed of the generator can be lowered in several ways. The first option is to use a slow-moving AC generator with a large number of poles. For example, if the number of magnets in the stator of a synchronized generator is doubled, the magnetic field must rotate only half a revolution before it changes direction and the generator will run at 25 Hz (or 1,500 rpm). In theory, we can keep increasing the number of poles until the generator revolves at the same speed as the wind turbine rotor. The more practical approach is to use a gearbox. A gearbox is a device that converts slow-speed, high-torque power from the turbine into high-speed, low-torque power required to run the generator. An alternative approach is to use direct-drive generators that can operate at the rotational speed of the rotor, thus eliminating the need for a gearbox. Hybrid Systems According to World Bank estimates, as many as two billion people, 40% of the world’s population, live in villages that are not tied to a utility grid. For these villages, a hybrid of energy sources including wind, solar, and diesel-powered generators would be most suitable. In the United States, a typical stand-alone hybrid system involving photovoltaic cells and wind is advantageous over either system by itself because it takes advantage of longer and brighter sunlight in summers when wind speeds are lower. In winters, the opposite is true; winds are strong when there is less sunlight. During periods of peak demand or when neither wind nor sunlight is sufficiently available, an auxiliary diesel can produce the additional energy 56 Chapter 3 - Wind Energy required. In either case, to assure power is available at nights or during periods of low winds, a battery storage system is needed (Figure 3-13a). W hen grid connection is available, it is possible to sell extra power to the utility companies and buy back electricity when demand exceeds the capacity of the system. In this case, there is no need to store the electricity in batteries or have an auxiliary diesel power generating plant (See Figure 3-13b). Safety PV module Generator Wind turbine Power conditioner AC or DC Load Battery bank To make wind energy economically viable, wind turbines must endure a wide range of hazardous conditions and have a long lifetime. Modern designs deploy various sensors that protect turbines, gearboxes, and generators against excess vibration, overheating, and high speed wind gusts. Depending on wind speed and direction, the wind turbine produces fluctuating torque and varying forces on the blades, causing the rotor and tower to swing back and forth. The blades are made to be flexible and are able to vibrate. The frequency of oscillation depends on the height of the tower and on the material and weight of the rotor and nacelle. If the rotor spins at a synchronized speed with other vibrational frequencies, the oscillation can amplify and the tower could sway out of control. Wind turbines are often equipped with a controller system which starts the turbine when wind reaches a certain velocity (around 3-6 m/s) and shuts the machine off when wind velocity exceeds 20-30 m/s. Most wind turbines are designed to have a maximum output at wind speeds around 15 m/s. Higher wind speeds are rare, and lower wind speeds cannot produce sufficient power. Noise can also be a problem. There are two sources of noise: mechanical and aerodynamic. Major sources of mechanical noise are the gearbox, drive shaft, and turbine blades. The primary source of aerodynamic noise is the air flow from the trailing edges of the blades. One of the interesting features of electric motors and generators is that they require some electrical load to operate. If they overheat or a load is removed, the rotors will accelerate out of control and the devices will fail. To avoid overheating, most generators are equipped with either air or water cooling systems. To protect them from overspeeding and accidental disconnection from electrical grids, the common practice is aerodynamic braking, in which rotor blades are rotated 90 degrees along their longitudinal axis. Once the danger is over, a built-in hydraulic system returns the blades to their original orientation. In addition to the control strategies described above, large turbines are (a) PV module Motor Wind turbine Inverter AC (b) Load Figure 3-13 Hybrid systems involving a variety of energy sources can be used to power remote villages and generate income for their owners. (a) Stand-alone, (b) Grid-connected. Adapted from “Small Wind Electric Systems: A U.S. Customer’s Guide”, Office of Energy Efficiency, U.S. Department of Energy, DOE/ GO-102001-1293, [October 2002.] 57 equipped with complex, computer-operated control systems. These monitor environmental parameters (such as wind speed, direction, atmospheric temperature, and pressure), operating parameters (rotational speed of the rotor, torque, and the yaw angle), and a large number of devices such as pumps, actuators, and valves. Siting Figure 3-14 The Kappel wind farm in Denmark is a good example of how the technology can be integrated with the environment in an aesthetically pleasant manner. One of the important considerations in the design of any wind power generation station is its location. In general, wind generators must be installed in areas with an open view and follow the altitude contours of the prevailing winds. Accessibility to the site is also important. The site must be chosen so that the wind is mostly clear of obstacles, such as trees and tall buildings. Pattern and distances between wind turbines must be chosen so that the wake of one turbine does not interfere with the operation of adjacent turbines. The practical guideline is that turbines facing the prevailing wind direction should be spaced between 5 and 9 rotor diameters apart; turbines facing perpendicularly to prevailing winds should be from 3 to 5 rotor diameters apart.9 Hilltops have the added advantage that they can pick up drafts, and the wind speeds are generally higher. Valleys are also suitable because tunnel effects result in higher wind speeds in valleys than those found in open spaces. When wind energy is chosen to service a small community or a local region, a site must be selected near a 10-30 kV power line; otherwise, the additional cost of extending the power grid can be prohibitive. Because of their appearance, noise, and their potential adverse effects on the price of surrounding properties, wind farms can be objectionable to nearby communities. Modern planning procedures and sensitive site selection can help to minimize visual impact. Turbines can be obscured from view by planting trees or constructing other, similar screens. Some consider large wind turbines more aesthetically pleasing than smaller ones because they generally have lower rotational velocities. The selection of certain turbine colors, structures, and layouts can also help to minimize intrusiveness. Urban architects and city planners need to study these effects and design sites to match the local landscape or provide interesting additions to the nearby structures (Figures 3-14 and 3-15).10 Offshore Wind Farms Figure 3-15 Conceptual architectural design of a twin-tower building with three integrated 35-m diameter, 250 kW horizontal axis wind turbines. This arrangement is expected to provide 20% of the building’s electricity. Source: Compbell, N. et al., Wind Energy for the Built Environment Project (Project WEB), PF4.11, EWEC 2001, Copenhagen, 2-6 July2001. Figure 3-16 The world’s first offshore wind farm north of the island of Lolland in Denmark, consists of 11 turbines each producing 450 kW of energy. 9 Wind turbines do not always need to be placed on land and, if they are installed offshore, may actually benefit from generally cooler and smoother lake and sea surfaces. Furthermore, because noise is not as much of a concern as with onshore facilities, turbines can be designed to operate at higher rotational speeds. As a result, a 20% greater efficiency is achieved at offshore wind farms (Figure 3-16). The main disadvantages of offshore wind farms are potential interferences with shipping routes and 10 Danish Wind Industry Association, (http://www.windpower.org). Palmer, A., “Towers of Power,” Popular Science, Vol 259, Iss 6, Dec 2001. 58 Chapter 3 - Wind Energy Is wind energy practical for you? A F YI ... small wind electric system may work for you if: • Your house is thermally well-insulated. Making your house more energy efficient saves energy and money, thus requiring a smaller wind generator • There is enough wind where you live • You live in a house or ranch over one acre in size • Tall towers are allowed in your neighborhood • You have enough space • You are comfortable with long term investments • Your electricity cost is over $150 a month Reference: “Small Wind Electric Systems: A US Customer’s Guide”, Office of Energy Efficiency, U.S. Department of Energy, DOE/ GO-102001-1293, October 2002. the additional cost of undersea cabling needed to connect the generator to the main electrical grid. Summary Wind energy is a clean, renewable source of energy. Only 1% of the total wind energy available is enough to fulfill all global energy needs, meaning wind is a potential source of an enormous amount of energy. The main drawback to wind energy is that it is noisy, intermittent, and to some unsightly. Other disadvantages cited are potential danger to birds, which may result in changes in migration patterns of some bird species, and shadow flicker – when moving blades cast shadow on nearby residences. However, wind turbine technology is undergoing rapid development: new composites have allowed substantial weight reduction, turbine noise is continually reduced, and new manufacturing technologies have allowed larger and larger blades-- making construction of multi megawatt turbines possible. At present, the average cost of production of electricity from wind energy is slightly higher than that from fossil fuel, but as demand for energy increases and fossil fuel sources deplete, the cost is rapidly becoming competitive with or even cheaper than those of nonrenewable sources of energy. Wind energy is expected to be a major energy source for most developing and island nations, as well as many developed European countries in the 21st century. Additional Information Books 1. Gipe P., Wind Energy Basics, —A comprehensive guide to modern small wind technology. AWEA (http://www.awea.org). 2. Elliott, D. et al., Wind Energy Resource Atlas of the United States, by American Wind Energy Association (http://rredc.nrel.gov/wind/ pubs/atlas). 3. Khennas, S., Small wind systems for rural energy services, London: ITDG Pub., 2003. 59 Periodicals 1. Solar Energy, Direct Science Elsevier Publishing Company, the official journal of the International Solar Energy Society ®, is devoted to the science and technology of solar energy applications, and includes the indirect uses such as wind energy and biomass. 2. Home Power Magazine—bimonthly magazine for farm and home wind turbines (http://www.homepower.com). Government Agencies and Websites 1. Energy Efficiency and Renewable Energy Clearinghouse (http:// www.eren.doe.gov). 2. National Wind Technology Center, National Renewable Energy Laboratory (http://www.nrel.gov/wind). 3. Energy Efficiency and Renewable Energy: Wind Energy Technologies, US DOE (http://www.eere.energy.gov/windandhydro/ wind_technologies.html). Non-Government Organizations and Websites 1. American Wind Energy Association (http://www.awea.org). 60 Chapter 3 - Wind Energy Exercises I. Essay Questions: 1. Airports are rarely located near big mountains. W hat concerns prevent building an airport near mountains? 2. Discuss the principles of operation of wind 2. Which country utilizes the most wind energy today? a. US b. Germany c. Denmark d. India e. Iran 3. 4. 5. 6. 7. 8. II. 1. turbines. What are the major components of a wind 3. Since 1980, the US percentage share of wind generator? How do the wind turbine blades differ capacity has been from those used in helicopters? a. Declining b. Increasing Define the pitch and angle of attack. Under what c. Remaining about the same conditions are they the same? 4. What percentage of the US’s electrical capacity Name the components of a modern wind power comes from wind? plant and describe their functions. a. Less than 1% b. About 5% How does the height of a tower on which a wind c. About 15% turbine is installed affect its operation? What are d. About 50% the advantages and disadvantages of taller versus e. About 99% shorter towers? 5. How much does the energy content of the What is the difference between a horizontal and a wind change if wind speed suddenly doubles? vertical wind turbine? How do they vary in shape? a. About one-half W hat is the advantage and disadvantage of an b. About the same upwind design over a downwind design? c. About twice as much d. About four times as much What are the conditions under which wind power e. About eight times as much generation can be economical for an average family? Discuss tax and other economical ramifications of 6. How much does the energy content of wind change switching from fossil fuel power plants to wind in if the rotor size suddenly doubles? your neighborhood. a. About one-half b. About the same Which one generates more power, a wind turbine c. About twice as much 10-m in diameter at a wind speed of 10 m/s, or a d. About four times as much wind turbine 20-m in diameter at a wind speed of 5 e. About eight times as much. m/s? 7. How much does the energy content of wind change Multiple Choice Questions: if the rotor size suddenly doubles, while at the same time the wind speed reduces by one half? People from which country invented the first a. About one-half practical wind turbine? b. About the same a. US c. About twice as much b. Netherlands d. About four times as much. c. Egypt e. About eight times as much d. Persia e. China 61 8. The rated power of a wind turbine is a. The maximum power a turbine can produce b. The average between maximum and minimum power c. The average between powers produced at the highest and lowest possible speeds under which wind turbine can operate d. The average power a turbine produces over a period of one day e. The average power a turbine produces over a period of one year 9. Two wind mills 10 feet and 30 feet in diameters are standing side-by-side against wind blowing at 5 knots. The smaller wind turbine produces 10 kW of electric power. The larger turbine is expected to produce a. 10 kWe b. 30 kWe c. 60 kWe d. 90 kWe e. 270 kWe 10. A windmill system which produces 5 kilo watts of electrical power when the wind has a velocity of 3 m/s will produce _____ kilo watts when the wind velocity is 9 m/s. a. 1 2/3 b. 15 c. 45 d. 135 e. None of the above 11. The angle between the blade’s chord and the direction of the incoming relative wind is called a. Yaw b. Pitch c. Roll d. Angle of attack e. None of the above 12. The angle between the blade’s chord and the axis of rotation is called a. Yaw b. Pitch c. Roll d. Angle of attack 62 e. None of the above 13. The main function of the gearbox is a. To increase the turning force (torque) of the wind turbine rotor and use a smaller generator b. To reduce the turning force (torque) of the wind turbine rotor and use a smaller generator c. To increase the turning force (torque) of the wind turbine rotor and use a larger generator d. To increase both the speed and the torque of the wind turbine e. To reduce both the speed and the torque of the wind turbine 14. Which of the following statements is not correct? a. Theoretically, wind turbines need only one blade. b. Generally, multi-bladed rotors are less efficient than rotors with only 2-3 blades. c. Two and three-bladed rotors have moderate starting torques which make them unsuitable at low wind speeds. d. Two-bladed rotors are lighter and run at faster speeds, which make their operation quieter and with considerably less vibration. e. The greater the number of blades, the more stable the turbines are, but also the heavier and more expensive. 15. Which of the following statements is correct? a. The power generated by a wind turbine varies linearly with the kinetic energy of the mass of air passing through its rotor. b. For the same wind velocity and rotor size, more power is generated in summers than in winters. c. Total power generated by a wind turbine increases with the square of the rotor area. d. Total power generated by a wind turbine increases with the square of the wind speed. e. All of the above statements are correct. III. True or False? 1. Today, wind energy furnishes less than 1% of the Chapter 3 - Wind Energy total energy needs in the world. 2. The greater the number of blades, the higher the efficiency of the wind turbines. 3. Land breezes occur during the daytime when land warms up much faster than the surrounding water. 4. Valley and mountain breezes are affected strongly by local topography. 5. The wind rose is a mechanical device for measuring the wind velocities. 6. Lift is the force perpendicular to the surfaces of the wind turbine blades. 7. Rated power is the power that a wind turbine produces at its optimum operating conditions. 8. Modern wind turbines vary in size and can produce power ranging from a few watts to several megawatts. 9. Wind velocity decreases with height above the ground. 10. The best location for installing a wind farm is in wide open fields, away from the populated areas but close enough to the national power grid. IV. Fill-in the Blanks 1. Winds are generated as a result of two factors, the non-uniform heating of the earth by the sun and the _____________________. 2. Winds flowing on a path along the lines of constant pressure are called _________ wind. 3. The power a turbine produces when it is run at its optimum operating condition is called _________ power. 4. The total power generated by a wind turbine is not constant and varies with wind speed and direction, rotor diameter, and air __________. 5. For best efficiency, most wind turbines are designed with ____ to ____ blades. 6. The power output of a wind turbine ________ with altitude at the rate of 2.5% for every ______ feet above sea level. 7. The direction of the lifting force is roughly ________ to the local flow field. 8. To modify torque from a wind turbine to run a generator, wind turbines are usually equipped with ___________ . 9. The best way to control the power is by changing the blades’ _________ angle. 10. Only _______ percent of the total available wind energy is enough to satisfy all of the world’s energy needs. V. Project – Designing a Wind Farm A small village is planning to install a wind farm to meet its energy demand. In this project you are asked to evaluate various wind turbine technologies to meet the electricity needs of the community. The design options considered are the G-mill, two-blade, threeblade, multi-blade, Darrieus (egg-beater), and Savonius turbines. For comparison, assume that all wind designs must provide 60 kW of power when wind speed is around 33 mph. 1. Calculate the average wind speed and wind power factor for each speed range. Wind power factor is the ratio of power produced at a given wind speed to that at nominal speed (in this case 33 mph). 2. Use data given in Figure 3-11 to estimate the rotor efficiency. Calculate the power (kW) produced by each turbine when operated at a given wind velocity. 3. Find the total wind energy for each wind turbine by multiplying power times the number of hours of operation. Sum up capacities to find the annual cumulative value (kWh/year). 4. Assuming 50% conversion efficiency from mechanical to electrical power, determine electrical power for the most efficient turbine. 63 5. Assuming that the average cost of electricity generation is $0.12/kWh, determine the total annual cost. 6. Redo the problem if turbines are rated at points of their maximum efficiency. Which turbine do you Wind Velocity Hours per Year 0-5 1200 6-10 1600 11-15 1950 16-20 1700 choose? What is its power rating if we are to deliver 100 MWh of electricity annually? Comment! The following wind data are available from local meteorological offices: 21-25 1480 26-30 960 31-35 280 36-40 50 >40 10 Project Work Sheet Wind velocity (mph) Hours per year Average wind velocity Wind factor G-Mill h (%) kW kWh Two-Blade h (%) kW kWh Three-Blade h (%) kW kWh Darrieus h (%) kW kWh Multi-Blade h (%) kW kWh Savonius h (%) kW kWh The Most efficient turbine: _________ Total mechanical energy: _____________ kWh Total electricity generation capacity by one turbine: _____________ kWh Total annual operating cost of one turbine: US $ __________ * Wind factor = (V/Vnominal)3 = (18/33)3 = 0.162 ** Power calculated by scaling the nominal power (60 kW), corrected for wind velocity and efficiency, P (18 mph) = 60 kW x 0.162 x (0.27/0.18) = 14.58 kW 60 --------60 --------60 27% 14.58** 24,786 --------18% 60 --------60 --------60 0-5 1200 6-10 1600 11-15 1950 16-20 1700 18 0.16* 21-25 1480 26-30 960 31-35 280 33 1 36-40 50 >40 10 kWh/year --------------------- 64