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Horizontal Axis Wind Turbine (HAWT)
Horizontal axis wind turbines have the main rotor shaft and electrical generator at the top of a tower, and they must be pointed into the wind. Small turbines are pointed by a simple wind vane placed square with the rotor (blades), while large turbines generally use a wind sensor coupled with a servo motor. Most large wind turbines have a gearbox, which turns the slow rotation of the rotor into a faster rotation that is more suitable to drive an electrical generator. Since a tower produces turbulence behind it, the turbine is usually pointed upwind of the tower. Wind turbine blades are made stiff to prevent the blades from being pushed into the tower by high winds. Additionally, the blades are placed a considerable distance in front of the tower and are sometimes tilted up a small amount. Downwind machines have been built, despite the problem of turbulence, because they don't need an additional mechanism for keeping them in line with the wind, and because in high winds, the blades can be allowed to bend which reduces their swept area and thus their wind resistance. Since turbulence leads to fatigue failures, and reliability is so important, most HAWTs are upwind machines. Types of HAWT wind farm installations are:
- On-Shore: Mountains and hilly areas have been the original choice to setup these farms. Individual wind turbines at these farms contribute towards power generation of 100 MW or more. The land occupied by the wind parks are often used for agriculture or animal grazing. Denmark, Spain and Portugal are some of the leading countries in the onshore wind farm electricity production.
- Offshore wind farms are the results of revolutionary technology that has encouraged man to set up wind energy harvesting farms on the water surface. Apart from oceans, lakes also act as sites for the installation of wind parks. An advantage of offshore wind farm is that it makes use of powerful winds blowing over the water surface. Moreover, it is easy to transport huge parts of a wind turbine to the offshore sites using big ships and vessels. Some of the other advantages of these farms include mitigation of noise due to distance from land and higher capacity factors. The United Kingdom is the nation that leads in electricity generation using offshore wind parks. Denmark, Sweden and Netherlands are other countries that follow.
Off-shore Wind in India
- A near-shore wind farm is the third type of farms used for harvesting wind power. As suggested by its name, a near-shore wind farm is installed near the shore, thus making use of land and see breezes to turn the turbines. In future, you might come across air-borne wind farms, with wind turbines requiring no towers for installation.
- The tall tower base allows access to stronger wind in sites with wind shear. In some wind shear sites, every ten meters up the wind speed can increase by 20% and the power output by 34%.
- High efficiency, since the blades always move perpendicularly to the wind, receiving power through the whole rotation. In contrast, all vertical axis wind turbines, and most proposed airborne wind turbine designs, involve various types of reciprocating actions, requiring airfoil surfaces to backtrack against the wind for part of the cycle. Backtracking against the wind leads to inherently lower efficiency.
- Massive tower construction is required to support the heavy blades, gearbox, generator and an additional yaw control mechanism to turn the blades toward the wind.
- Downwind variants suffer from fatigue and structural failure caused by turbulence when a blade passes through the tower's wind shadow (for this reason, the majority of HAWTs use an upwind design, with the rotor facing the wind in front of the tower).
- HAWTs generally require a braking or yawing device in high winds to stop the turbine from spinning and destroying or damaging itself.
Vertical Axis Wind Turbine (VAWT)
VAWTs, have the main rotor shaft arranged vertically. The main advantage of this arrangement is that the wind turbine does not need to be pointed into the wind. This is an advantage on sites where the wind direction is highly variable or has turbulent winds. With a vertical axis, the generator and other primary components can be placed near the ground, so the tower does not need to support it, also makes maintenance easier. The main drawback of a VAWT is that it generally creates drag when rotating into the wind. It is difficult to mount vertical-axis turbines on towers, meaning they are often installed nearer to the base on which they rest, such as the ground or a building rooftop. Hence these models are not frequently used for off-shore installations which if used might require water proof casings that adds to extra costs. Also, offshore installations require very high height towers. The wind speed is slower at a lower altitude, so less wind energy is available for a given size turbine. Air flow near the ground and other objects can create turbulent flow, which can introduce issues of vibration, including noise and bearing wear which may increase the maintenance or shorten its service life. However, when a turbine is mounted on a rooftop, the building generally redirects wind over the roof and these can double the wind speed at the turbine. If the height of the rooftop mounted turbine tower is approximately 50% of the building height, this is near the optimum for maximum wind energy and minimum wind turbulence. Various types of VAWT are:
- Darrieus wind turbine: "Eggbeater" turbines, or Darrieus turbines, were named after the French inventor, Georges Darrieus. They have good efficiency, but produce large torque ripple and cyclical stress on the tower, which contributes to poor reliability. They also generally require some external power source, or an additional Savonius rotor to start turning, because the starting torque is very low. The torque ripple is reduced by using three or more blades which results in greater solidity of the rotor. Solidity is measured by blade area divided by the rotor area. Newer Darrieus type turbines are not held up by guy-wires but have an external superstructure connected to the top bearing.
- Giromill: It is a subtype of Darrieus turbine with straight, as opposed to curved, blades. The cycloturbine variety has variable pitch to reduce the torque pulsation and is self-starting. The advantages of variable pitch are: high starting torque; a wide, relatively flat torque curve; a lower blade speed ratio; a higher coefficient of performance; more efficient operation in turbulent winds; and a lower blade speed ratio which lowers blade bending stresses. Straight, V, or curved blades may be used.
- Savonius wind turbine: These are drag-type devices with two (or more) scoops that are used in anemometers, Flettner vents (commonly seen on bus and van roofs), and in some high-reliability low-efficiency power turbines. They are always self-starting if there are at least three scoops.
- Twisted Savonius: Twisted Savonius is a modified savonius, with long helical scoops to give a smooth torque, this is mostly used as roof windturbine or on some boats (like the Hornblower Hybrid)
- No yaw mechanism is needed because they have lower wind startup speeds than the typical the HAWTs.
- A VAWT can be located nearer the ground, making it easier to maintain the moving parts.
- VAWTs situated close to the ground can take advantage of locations where rooftops, mesas, hilltops, ridgelines, and passes funnel the wind and increase wind velocity.
- Most VAWTs have an average decreased efficiency from a common HAWT, mainly because of the additional drag that they have as their blades rotate into the wind.
- Having rotors located close to the ground where wind speeds are lower due and do not take advantage of higher wind speeds above.
Wind Turbine Technology Based on Applications
Wind power can be divided into three size ranges, which are used for different applications. The size is chosen differently depending on the turbine’s purpose. The height/span of wind turbines can be anticipated by comparing with some other large objects and installations. Typical sizes in the three ranges available are:
Residential: below 30 kW
- Choose a size based on electrical load
- Diameter: 1 - 13 m (4 - 43 ft)
- Height: 18 - 37 m (60 - 120 ft)
- Example: 20,000 kWh/year
Medium: 30 - 500 kW
- May be sized to a load. Typically used when there is a large electrical load.
- Diameter: 13 - 30 m (43 - 100 ft)
- Height: 35 - 50 m (115 - 164 ft)
- Example: 600,000 kWh/year
Commercial scale: 500 kW - 2 MW
- Usually fed into the grid, not sized to a single load
- Diameter: 47 - 90 m (155 - 300 ft)
- Height: 50 - 80 m (164 - 262 ft)
- Example: 4,000,000 kWh/year