Wind energy

Editor's Notes

• #5: Power generation from wind has emerged as one of the most successful programmes in the renewable energy sector, and has started making meaningful contributions to the overall power requirements of some States. Wind turbines today are up to the task of producing serious amounts of electricity. Turbines vary in size from small 1 kW structures to large machines rated at 2 MW or more.      Energy is a major input for overall socio-economic development. Use of fossil fuels is expected to fuel the economic development process of a majority of the world population during the next two decades. However, at some time during the period 2020-2050, fossil fuels are likely to reach their maximum potential, and their price will become higher than other renewable energy options on account of increasingly constrained production and availability. Therefore, renewables are expected to play a key role in accelerating development and sustainable growth in the second half of the next century, accounting then to 50 to 60% of the total global energy supply.
• #11: For the operation of wind turbine to be commercially feasible, average wind speed should be in the range of 13-30 mi/hr. at 25-30 mi/hr, a turbine operates at full capacity, and at higher wind speeds the turbine should be shut down to avoid damage.
• #12: There are two primary physical principles by which energy can be extracted from the wind; these are through the creation of either drag or lift force (or through a combination of the two). The basic features that characterize lift and drag are: • drag is in the direction of airflow • lift is perpendicular to the direction of airflow • generation of lift always causes a certain amount of drag to be developed • with a good aerofoil, the lift produced can be more than thirty times greater than the drag • lift devices are generally more efficient than drag devices
• #13: The tip speed ratio is defined as the ratio of the speed of the extremities of a windmill rotor to the speed of the free wind. It is a measure of the 'gearing ratio' of the rotor. Drag devices always have tip speed ratios less than one and hence turn slowly, whereas lift devices can have high tip speed ratios and hence turn quickly relative to the wind. Tip speed ratio = blade tip speed/wind speed Solidity is usually defined as the percentage of the circumference of the rotor which contains material rather than air. High solidity machines carry a lot of material and have coarse blade angles. They generate much higher starting torque than low-solidity machines but are inherently less efficient than low-solidity machines. The extra materials also cost more money. However, low-solidity machines need to be made with more precision which leads to little difference in costs.
• #14: The proportion of the power in the wind that the rotor can extract is termed the coefficient of performance (or power coefficient or efficiency; symbol Cp) and its variation as a function of tip speed ratio is commonly used to characterize different types of rotor. It is physically impossible to extract all the energy from the wind, without bringing the air behind the rotor to a standstill.
• #16: Wind energy is freely available, widely distributed, renewable and also nature-friendly. Wind has huge potential. It is estimated that if all the available wind energy is harnessed, it can contribute about five times the total energy demands of the world at present
• #17: Wind energy is the fastest growing renewable energy source in the world. The world wide installed capacity is growing at a rapid pace of over 30% per year. Factors: declining cost (4-6 cents k/wh) technological advances revenue for landowners & tax jurisdictions consumer demand
• #18: World wide wind generating capacity is less than 5000 MW in 1995 and is 39000MW in 2003, an increase of nearly eight fold.
• #21: World wide wind generating capacity is less than 5000 MW in 1995 and is 39000MW in 2003, an increase of nearly eight fold. Wind energy is the fastest growing renewable energy source in the world. The world wide installed capacity is growing at a rapid pace of over 30% per year.
• #23: From the graph, we can see that the available potential for wind in India is 45000MW out of which at present we r using only ~3500 MW.
• #26: Installed capacity is rapidly increasing in India
• #29: Over 5,000 years ago, the ancient Egyptians used wind power to sail their ships on the Nile River. Later, people built windmills to grind their grain. In 1891, the first electrical output wind machine was developed incorporating the aerodynamic design principles.
• #30: However, a disadvantage of most VAWT configurations is the fact that they have either low or insignificant starting torque, so that in the case of Darrieus devices, for example, the rotor must be brought up to speed either by using the generator as a motor or by means of a small secondary rotor, such as a Savonius, mounted on the Darrieus main shaft.
• #31: Upwind turbines are used mostly. Because wind velocity increases at higher altitudes, the backward force and torque on a horizontal axis wind turbine (HAWT) blade peaks as it turns through the highest point in its circle. The tower hinders the airflow at the lowest point in the circle, which produces a local dip in force and torque. These two effects combine to produce a cyclic twist on the main bearings of a HAWT. The combined twist is worst in machines with an even number of blades, where one is straight up when another is straight down. To improve reliability, teetering hubs are used which allow the main shaft to rock through a few degrees, so that the main bearings do not have to resist the torque peaks.
• #33: Counter rotating turbines can be used to increase the rotation speed of the electrical generator. When the counter rotating turbines are on the same side of the tower, the blades on the one in front are angled forwards slightly so as to never hit the rear ones. They are either both geared to the same generator or, more often, one is connected to the rotor and the other to the field windings. Counter rotating turbines geared to the same generator have additional gearing losses. Counter rotating turbines connected to the rotor and stator are mechanically simpler; but, the field windings need slip rings which adds complexity, wastes some electricity and wastes some mechanical power. As of 2005, no large practical counter-rotating HAWTs are commercially sold. Counter rotating turbines can be on opposite sides of the tower. In this case it is best that the one at the back be smaller than the one at the front and set to stall at a higher wind speed. This way, at low wind speeds, both turn and the generator taps the maximum proportion of the wind's power. At intermediate speeds, the front turbine stalls; but, the rear one keeps turning, so the wind generator has a smaller wind resistance and the tower can still support the generator. At high wind speeds both turbines stall, the wind resistance is at a minimum and the tower can still support the generator. This allows the generator to function at a wider wind speed range than a single-turbine generator for a given tower. To reduce sympathetic vibrations, the two turbines should turn at speeds with few common factors, for example 7:3 speed ratio. Overall, this is a more complicated design than the single-turbine wind generator, but it taps more of the wind's energy at a wider range of wind speeds. Winds at a height of a few kilometers are quite constant and very fast (often over 40 m/s). Theoretically, flying wind turbines could tap into the energy in these winds. One design has four turbines linked together forming a kind of kite. Winds keep the construction in the air, while causing the blades on the turbines to rotate. The kite wire would carry the electrical energy to the ground. This approach would require no-fly-zones around the deployment site, and might require electrical energy to remain aloft if the winds would ever die down
• #36: One of the best construction materials available (in 2001) is graphite-fibre in epoxy. Graphite composites can be used to build turbines of sixty meters radius, enough to tap a few megawatts of power. Smaller household turbines can be made of lightweight fiberglass, aluminum, or sometimes laminated wood. Wood and canvas sails were originally used on early windmills. Unfortunately they require much maintenance over their service life. Also, they have a relatively high drag (low aerodynamic efficiency) for the force they capture. For these reasons they were superseded with solid airfoils. Wind power intercepted by the turbine is proportional to the square of its blade-length. The maximum blade-length of a turbine is limited by both the strength and stiffness of its material. The wind blows faster at higher altitudes because of the drag of the surface (sea or land) and the viscosity of the air. The variation in velocity with altitude, called wind shear is most dramatic near the surface. Typically, the variation follows the 1/7th power law , which predicts that wind speed rises proportionally to the seventh root of altitude. Doubling the altitude of a turbine, then, increases the expected wind speeds by 10% and the expected power by 34%. Doubling the tower height generally requires doubling the diameter as well, increasing the amount of material by a factor of eight. For HAWTs, tower heights approximately twice the blade length have been found to balance material costs of the tower against better utilisation of the more expensive active components.
• #37: Although turbines can be built with any number of blades, there are many constraints. There are a number of vibration modes that increase in peak intensity as the number of blades decreases. Some of these vibrations, besides causing wear on the machine, are also audible. Thus, noise and wear considerations point to larger numbers of blades, generally at least 3. Many small scale wind turbines, such as the Whisper 175, use 2 blades because such turbines are easy to construct as they avoid the need for using a hub with linkages to individual blades, and the blade(s) can be shipped easily in one long package. Three-bladed turbines, which are much more efficient, and more quiet, require more complicated onsite assembly.
• #38: The speed at which wind turbines rotate must be controlled for several reasons: Maintenance; because it is dangerous to have people working on a wind turbine while it is active, it is sometimes necessary to bring a turbine to a full stop. Noise reduction; As a rule of thumb, the noise from a wind turbine increases with the fifth power of the relative wind speed (as seen from the moving tip of the blades). In noise-sensitive environments (nearly all onshore installations), noise limits the tip speed to approximately 60 m/s. High efficiency turbines may have tip speed ratios of 5-6, which, for onshore turbines, limits high efficiency operation to winds of just 10 m/s. Centripetal force reduction; as the rotational speed increases, so does the centripetal force working on the central hub or axis. When it exceeds safe limits blades could snap off, and the turbine would fail dramatically. On a pitch controlled wind turbine the turbine's electronic controller checks the power output of the turbine several times per second. When the power output becomes too high, it sends an order to the blade pitch mechanism which immediately pitches (turns) the rotor blades slightly out of the wind. Conversely, the blades are turned back into the wind whenever the wind drops again. (Passive) stall controlled wind turbines have the rotor blades bolted onto the hub at a fixed angle. The geometry of the rotor blade profile, however has been aerodynamically designed to ensure that the moment the wind speed becomes too high, it creates turbulence on the side of the rotor blade which is not facing the wind as shown in the picture on the previous page. This stall prevents the lifting force of the rotor blade from acting on the rotor. An increasing number of larger wind turbines (1 MW and up) are being developed with an active stall power control mechanism. Technically the active stall machines resemble pitch controlled machines, since they have pitchable blades. In order to get a reasonably large torque (turning force) at low wind speeds, the machines will usually be programmed to pitch their blades much like a pitch controlled machine at low wind speeds. (Often they use only a few fixed steps depending upon the wind speed). When the machine reaches its rated power , however, you will notice an important difference from the pitch controlled machines: If the generator is about to be overloaded, the machine will pitch its blades in the opposite direction from what a pitch controlled machine does. In other words, it will increase the angle of attack of the rotor blades in order to make the blades go into a deeper stall, thus wasting the excess energy in the wind. One of the advantages of active stall is that one can control the power output more accurately than with passive stall, so as to avoid overshooting the rated power of the machine at the beginning of a gust of wind. Another advantage is that the machine can be run almost exactly at rated power at all high wind speeds. A normal passive stall controlled wind turbine will usually have a drop in the electrical power output for higher wind speeds, as the rotor blades go into deeper stall. The pitch mechanism is usually operated using hydraulics or electric stepper motors.
• #39: That part of the rotor which is closest to the source direction of the wind, however, will be subject to a larger force (bending torque) than the rest of the rotor. On the one hand, this means that the rotor will have a tendency to yaw against the wind automatically, regardless of whether we are dealing with an upwind or a downwind turbine. On the other hand, it means that the blades will be bending back and forth in a flapwise direction for each turn of the rotor. Wind turbines which are running with a yaw error are therefore subject to larger fatigue loads than wind turbines which are yawed in a perpendicular direction against the wind. Sensor:It simply consists of a ball resting on a ring. The ball is connected to a switch through a chain. If the turbine starts shaking, the ball will fall off the ring and switch the turbine off. There are many other sensors in the nacelle, e.g. electronic thermometers which check the oil temperature in the gearbox and the temperature of the generator. Overspeed Protection It is essential that wind turbines stop automatically in case of malfunction of a critical component Aerodynamic Braking System: Tip Brakes The primary braking system for most modern wind turbines is the aerodynamic braking system, which essentially consists in turning the rotor blades about 90 degrees along their longitudinal axis (in the case of a pitch controlled turbine or an active stall controlled turbine ), or in turning the rotor blade tips 90 degrees (in the case of a stall controlled turbine ). Aerodynamic braking systems are extremely safe. Mechanical Braking System The mechanical brake is used as a backup system for the aerodynamic braking system
• #41: WARP system amplifies the ambient wind speed, through its multi-tasking aerodynamic modules or wind frames, to simple, standardized commodity horizontal axis (propeller-type) wind turbines. Each modular wind frame provides highly amplified wind flow fields from over 50% to 80% over free air wind speed to each conventional, small diameter wind turbine of no more than 1 meter to 3 meters in diameter. Each module also serves as a support for the wind turbines, a yaw assembly and protective housing for the core support tower and other internal subsystems.
• #42: Rotatable shutters mounted on a circular disk automatically open when directed into the wind, irregardless of the wind's direction.  Pairs of upper and lower shutters are geared together. The lower shutter acts as a counterweight to the upper shutter. The bottom shutter opens in the downward direction and its weight helps to lift the upper shutter in the upward direction, as the wind applies an opening force against both shutters. When the shutters reach the vertical position, stops prevent them from opening further and the force of the wind is transferred from the open shutters to the circular disk.  And the circular disk is attached to the vertical axis for power output. The circular disk, shutters, and outer vertical axis rotate together. The outer vertical axis is mounted via bearings over an inner vertical axis that is stationery. The shutters are blown closed by the wind (no stops in the opposite direction) as they reverse direction during their rotation and move into the wind on the opposite side of the wind turbine.  When the wind is not blowing, the shutters open by gravity because the lower shutter is weighted to be slightly heavier than the upper shutter and it therefore can cause the upper shutter to open via the force of gravity as the two shutters are geared together.  Wind blows against the open shutters and the open shutters with stops apply a force against the disk, but the open shutters with no stops (opposite side going into the wind) merely close due to the force of the wind (not applying a force against the disk) and the wind turbine begins spinning no matter what direction the wind comes from. Operation of the turbine is remarkably quiet as compared to the appearance of the video due to biasing members that absorb the shock of the opening and closing and provide useful energy output. Conventional turbines must be very tall in order to create leverage by having very long blades to sweep a very large area.  Hunt's vertical axis creates leverage by increasing its width instead of height.  This allows the vertical turbine to be used in many applications, in which horizontal axis turbines cannot be used, such as flat building rooftops or just above the rooftop of a house or portable office building, as a sailboat wind turbine over a cabin area, attached to cellular telephone towers, on top of advertisement billboards, on the top of water towers, at the top of power line towers, etc.
• #44: Some of the factors determining the economics of the utility scale wind energy are: Costs depend very much on the wind speed at that site since the power varies as cube of the wind speed. Turbine design and construction: more than 60% of total costs are contributed by the turbine costs Rated capacity of the turbine: larger wind farms are known to be more economical than small wind farms. Exact location and orientation of the turbine greatly affects the economics of the wind energy. Improvements of turbine design: for example, use of light weight material Wind energy is a capital intensive source of energy.
• #52: Advantages: No delay in construction: wind turbines are easy to construct and does not require long gestation periods. Low maintenance costs: maintenance costs are very small compared to installation costs. Reliable and durable equipment: except for wind speeds greater than 30 mi/hr, once installed, wind equipment last for more than 25 years. Farmers and ranchers earn additional income by leasing their land for wind turbine. Wind industry produces more jobs per unit energy produced than other forms of energy. No hidden costs, which greatly reduces the environmental impacts Greater fuel diversity. Wind energy is big business turning over A$13 billion globally and employing 100,000 people in 2003. By 2020, the industry is expected to employ 1.8 million people and be worth $A120 billion a year.
• #62: Aesthetic concerns may be addressed through the use of modern turbines -- tubular towers and sleek, minimalist features contribute to a more attractive appearance. Further, some developers try to arrange a wind plant's turbines in an orderly fashion, giving a more purposeful and efficient appearance. Following the contours of a ridge, for example, helps turbines blend into the surroundings. Avoiding construction of conspicuous roads and clearings, burying transmission lines, and hiding buildings and structures behind ridges or vegetation are also prudent steps. Finally, educating nearby communities prior to construction about wind energy and its benefits can reduce opposition to visual effects. For wind plants currently experiencing bird conflicts, the immediate task is to find practical measures to reduce bird deaths and injuries. Mitigation proposals include changing the color of wind turbine blades, using tubular towers with diagonal stringers, eliminating places for birds to perch on the towers (especially perches near uninsulated electricity transmission lines) and using radar to alert wind project operators to the passage of large flocks of birds. Federal and state agencies and environmental organizations are collaborating on a research program to address the bird issue.
• #63: Most rotors are upwind: A wind turbine can be either "upwind" (that is, where the rotor faces into the wind) or "downwind" (where the rotor faces away from the wind). A downwind design offers some engineering advantages, but when a rotor blade passes the "wind shadow" of the tower as the rotor revolves, it tends to produce an "impulsive" or thumping sound that can be annoying. Today, almost all of the commercial wind machines on the market are upwind designs, and the few that are downwind have incorporated design features aimed at reducing impulsive noise (for example, positioning the rotor so that it is further away from the tower). Towers and nacelles are streamlined: Streamlining (rounding or giving an aerodynamic shape to any protruding features and to the nacelle itself) reduces any noise that is created by the wind passing the turbine. Turbines also incorporate design features to reduce vibration and any associated noise. Soundproofing in nacelles has been increased: The generator, gears, and other moving parts located in the turbine nacelle produce mechanical noise. Soundproofing and mounting equipment on sound-dampening buffer pads helps to deal with this issue. Gearboxes are specially-designed for quiet operation: Wind turbines use special gearboxes, in which the gear wheels are designed to flex slightly and reduce mechanical noise. In addition, special sound-dampening buffer pads separate the gearboxes from the nacelle frame to minimize the possibility that any vibrations could become sound.
• #64: Somnath Baidya Roy of Princeton University headed up a related project that studied the impact of simulated, extensive wind farms on local weather and found they could cause a drying and warming effect in the morning when somewhat inefficient turbines end up pushing warm air across moist and cool overnight soil. Local wind speed would also increase slightly, the experiment showed. The culprit in local climate impacts is the turbulent air left in the wake of each turbine’s rotors. This artificially energized air stirs up horizontal layers of air near the surface more than normal, leading to more vertical mixing of the atmosphere. Engineers could reduce the turbulence by designing rotors and farms differently so they produce less wake, possibly by turning rotors up to the sky like helicopter blades or a ceiling fan, Baidya Roy says. To reduce the global climate effects, engineers could install wind farms in ways that their effects counteract one another globally, Keith says. Or designers could take advantage of hills and dales to tailor the interaction of turbines with the atmosphere. How serious? Unaddressed, the severity of the local weather impact induced by large wind farms would fall somewhere between the environmental costs of deforestation and global warming, Baidya Roy said. But people should take these findings in context. In the past, older turbines with metal blades caused television interference in areas near the turbine. Interference from modern turbines is unlikely because many components formerly made of metal are now made from composites.