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venerdì 6 giugno 2008
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Sustainable Energy Options
Sustainable Energy Options
Wind Power (From Wikipedia, the free encyclopedia)
Wind power is the conversion of wind energy into useful form, such as electricity, using wind turbines. Wind energy is plentiful, renewable, widely distributed, clean, and reduces greenhouse gas emissions when it displaces fossil-fuel-derived electricity.
Wind Turbine: TYPES
Wind turbines can be separated into two types based on the axis about which the turbine rotates. Turbines that rotate around a horizontal axis are more common. Vertical-axis turbines are less frequently used.
Horizontal axis
Horizontal-axis wind turbines(HAWT) have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable for generating electricity.
Since a tower produces turbulence behind it, the turbine is usually pointed upwind of the tower. 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.
HAWT Subtypes:
There are several types of HAWT:
Windmills
These four- (or more) bladed squat structures, usually with wooden shutters or fabric sails, were developed in Europe. These windmills were pointed into the wind manually or via a tail-fan and were typically used to grind grain. In the Netherlands they were also used to pump water from low-lying land, and were instrumental in keeping its polders dry. Windmills were also located throughout the USA, especially in the Northeastern region.
Modern Rural Winsmills:
The Eclipse windmill factory was set up around 1866 in Beloit, Wisconsin and soon became a huge success building mills for farm waterpumping and railroad tank filling. Other firms like Star, Dempster, and Aeromotor also entered the market.
These windmills, invented in 1876 [1] by Griffiths Bros and Co (Australia [2]), were used by Australian and later American farmers to pump water and to generate electricity. They typically had many blades, operated at tip speed ratios (defined below) not better than one, and had good starting torque. Some had small direct-current generators used to charge storage batteries, to provide a few lights, or to operate a radio receiver. The American rural electrification connected many farms to centrally-generated power and replaced individual windmills as a primary source of farm power by the 1950's. Such devices are still used in locations where it is too costly to bring in commercial power.
Common modern wind turbines
Usually three-bladed, sometimes two-bladed or even one-bladed (and counterbalanced), and pointed into the wind by computer-controlled motors. The rugged three-bladed turbine type has been championed by Danish turbine manufacturers. These have high tip speeds of up to 6x wind speed, high efficiency, and low torque ripple which contributes to good reliability. This is the type of turbine that is used commercially to produce electricity. The blades are usually colored light gray to blend in with the clouds and range in length from 20 to 40 metres (65 to 130 ft) or more. The posts range from about 200 to 295 feet high. Contemporary models rotate at 16.6 rpm with a planetary gearbox which steps up the speed of generator components to 2,200 rpm. All are equipped with high wind shut down features to avoid over speed damage.
HAWT advantages
Blades are to the side of the turbine's center of gravity, helping stability.
Ability to wing warp, which gives the turbine blades the best angle of attack. Allowing the angle of attack to be remotely adjusted gives greater control, so the turbine collects the maximum amount of wind energy for the time of day and season.
Ability to pitch the rotor blades in a storm, to minimize damage.
Tall tower 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%.
Tall tower allows placement on uneven land or in offshore locations.
Can be sited in forests above the tree line.
Most are self-starting.
Can be cheaper because of higher production volume, larger sizes and, in general higher capacity factors and efficiencies.
HAWT disadvantages
HAWTs have difficulty operating in near ground, turbulent winds because their yaw and blade bearing need smoother, more laminar wind flows.
The tall towers and long blades (up to 180 feet (55 m) long) are difficult to transport on the sea and on land. Transportation can now cost 20% of equipment costs.
Tall HAWTs are difficult to install, needing very tall and expensive cranes and skilled operators.
Supply of HAWTs is less than demand and between 2004 and 2006, turbine prices increased up to 60%. At the end of 2006, all major manufacturers were booked up with orders through 2008.
The FAA has raised concerns about tall HAWTs effects on radar in proximity to air force bases.
Their height can create local opposition based on impacts to viewsheds.
Offshore towers can be a navigation problem and must be installed in shallow seas.
Downwind variants suffer from fatigue and structural failure caused by turbulence.
Cyclic stresses and vibration
Cyclic stresses fatigue the blade, axle and bearing material failures were a major cause of turbine failure for many years. Because wind velocity often 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 effects 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 have been 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.
When the turbine turns to face the wind, the rotating blades act like a gyroscope. As it pivots, gyroscopic precession tries to twist the turbine into a forward or backward somersault. For each blade on a wind generator's turbine, precessive force is at a minimum when the blade is horizontal and at a maximum when the blade is vertical. This cyclic twisting can quickly fatigue and crack the blade roots, hub and axle of the turbines.
Wind Power (From Wikipedia, the free encyclopedia)
Wind power is the conversion of wind energy into useful form, such as electricity, using wind turbines. Wind energy is plentiful, renewable, widely distributed, clean, and reduces greenhouse gas emissions when it displaces fossil-fuel-derived electricity.
Wind Turbine: TYPES
Wind turbines can be separated into two types based on the axis about which the turbine rotates. Turbines that rotate around a horizontal axis are more common. Vertical-axis turbines are less frequently used.
Horizontal axis
Horizontal-axis wind turbines(HAWT) have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable for generating electricity.
Since a tower produces turbulence behind it, the turbine is usually pointed upwind of the tower. 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.
HAWT Subtypes:
There are several types of HAWT:
Windmills
These four- (or more) bladed squat structures, usually with wooden shutters or fabric sails, were developed in Europe. These windmills were pointed into the wind manually or via a tail-fan and were typically used to grind grain. In the Netherlands they were also used to pump water from low-lying land, and were instrumental in keeping its polders dry. Windmills were also located throughout the USA, especially in the Northeastern region.
Modern Rural Winsmills:
The Eclipse windmill factory was set up around 1866 in Beloit, Wisconsin and soon became a huge success building mills for farm waterpumping and railroad tank filling. Other firms like Star, Dempster, and Aeromotor also entered the market.
These windmills, invented in 1876 [1] by Griffiths Bros and Co (Australia [2]), were used by Australian and later American farmers to pump water and to generate electricity. They typically had many blades, operated at tip speed ratios (defined below) not better than one, and had good starting torque. Some had small direct-current generators used to charge storage batteries, to provide a few lights, or to operate a radio receiver. The American rural electrification connected many farms to centrally-generated power and replaced individual windmills as a primary source of farm power by the 1950's. Such devices are still used in locations where it is too costly to bring in commercial power.
Common modern wind turbines
Usually three-bladed, sometimes two-bladed or even one-bladed (and counterbalanced), and pointed into the wind by computer-controlled motors. The rugged three-bladed turbine type has been championed by Danish turbine manufacturers. These have high tip speeds of up to 6x wind speed, high efficiency, and low torque ripple which contributes to good reliability. This is the type of turbine that is used commercially to produce electricity. The blades are usually colored light gray to blend in with the clouds and range in length from 20 to 40 metres (65 to 130 ft) or more. The posts range from about 200 to 295 feet high. Contemporary models rotate at 16.6 rpm with a planetary gearbox which steps up the speed of generator components to 2,200 rpm. All are equipped with high wind shut down features to avoid over speed damage.
HAWT advantages
Blades are to the side of the turbine's center of gravity, helping stability.
Ability to wing warp, which gives the turbine blades the best angle of attack. Allowing the angle of attack to be remotely adjusted gives greater control, so the turbine collects the maximum amount of wind energy for the time of day and season.
Ability to pitch the rotor blades in a storm, to minimize damage.
Tall tower 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%.
Tall tower allows placement on uneven land or in offshore locations.
Can be sited in forests above the tree line.
Most are self-starting.
Can be cheaper because of higher production volume, larger sizes and, in general higher capacity factors and efficiencies.
HAWT disadvantages
HAWTs have difficulty operating in near ground, turbulent winds because their yaw and blade bearing need smoother, more laminar wind flows.
The tall towers and long blades (up to 180 feet (55 m) long) are difficult to transport on the sea and on land. Transportation can now cost 20% of equipment costs.
Tall HAWTs are difficult to install, needing very tall and expensive cranes and skilled operators.
Supply of HAWTs is less than demand and between 2004 and 2006, turbine prices increased up to 60%. At the end of 2006, all major manufacturers were booked up with orders through 2008.
The FAA has raised concerns about tall HAWTs effects on radar in proximity to air force bases.
Their height can create local opposition based on impacts to viewsheds.
Offshore towers can be a navigation problem and must be installed in shallow seas.
Downwind variants suffer from fatigue and structural failure caused by turbulence.
Cyclic stresses and vibration
Cyclic stresses fatigue the blade, axle and bearing material failures were a major cause of turbine failure for many years. Because wind velocity often 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 effects 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 have been 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.
When the turbine turns to face the wind, the rotating blades act like a gyroscope. As it pivots, gyroscopic precession tries to twist the turbine into a forward or backward somersault. For each blade on a wind generator's turbine, precessive force is at a minimum when the blade is horizontal and at a maximum when the blade is vertical. This cyclic twisting can quickly fatigue and crack the blade roots, hub and axle of the turbines.
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