Uncle Bobs Tips: Generator Guide

Wind Turbine Generator



A wind turbine generator converts mechanical energy into electrical energy.

A wind turbine generator converts mechanical energy into electrical energy. Wind turbine generators are a bit unusual, compared to other generating units, as the generator has to work with a power source (the wind turbine rotor) that supplies very fluctuating mechanical power (torque).

HISTORY

Charles F. Brush (1849-1929) is one of the founders of the American electrical industry. During the winter of 1887-88 Brush built what is today believed to be the first automatically operating wind turbine for electricity generation. It was a giant - with a rotor diameter of 17 m (50 ft.) and 144 rotor blades made of cedar wood. The turbine ran for 20 years and charged the batteries in the cellar of his mansion. Despite the size of the turbine, the generator was only a 12 kW model! This was due to the fact that slowly rotating wind turbines of the American wind type do not have a high average efficiency. Paul la Cour later discovered that fast rotating wind turbines with few rotor blades are more efficient for electricity production than slow moving wind turbines. Paul la Cour (1846-1908) was the pioneer of modern electricity generating wind turbines and built his own wind tunnel for experiments. La Cour used the electricity from his wind turbines for electrolysis in order to produce hydrogen for the gaslight in his school.

During World War II the Danish engineering company F.L. Smidth (now a cement machinery maker) built a number of two- and three-bladed wind turbines. All of these machines (like their predecessors) generated DC (direct current). In 1951 the DC generator was replaced with a 35 kW asynchronous AC (alternating current) generator, thus becoming the second wind turbine to generate AC.

The engineer Johannes Juul, one of the first students of Paul La Cour in the 1950's, became a pioneer in developing the world's first alternating current (AC) wind turbine. The innovative 200 kW Cedser wind turbine (35K/PEC) was built in 1956-57 by J. Juul for the electricity company SEAS at Cedser coast in the Southern part of Denmark. The turbine was incredibly durable - it ran for 11 years without maintenance.

The Cedser wind turbine was refurbished in 1975 at the request of NASA who wanted measurement results from the turbine for the new U.S. wind energy program. The machine ran for a few years after which it was dismantled. The nacelle and rotor of the wind turbine are now on display the Electricity Museum at Bjerringbro, Denmark.

After the oil crisis in 1973, interest in wind energy was rekindled. However, the turbines built in the 70's shared a similar drawback: they were extremely expensive, and the high energy price subsequently became a key argument against wind energy.

A carpenter, Christian Riisager, however, built a small 22kW wind turbine in his own back yard using the Cedser Wind Turbine design as a point of departure. He used inexpensive standard components (e.g. an electric motor as generator, and car parts for gear and mechanical brakes) wherever possible. Riisager's success gave present day wind turbine manufacturers inspiration to start designing their own wind turbines from around 1980.The 55 kW wind turbines which were developed in 1980 - 1981 became the industrial and technological templates for modern wind turbines. With this generation of wind turbines, the cost per kilowatt hour (kWh) of electricity dropped by about 50 per cent and the wind industry became more professionalized.

MODERN WIND TURBINES
Growing concern about global warming and greenhouse gas production has lead governments worldwide to consider clean alternatives to coal and diesel for the production of electricity. Wind, in essence, is a proven technology, as some European countries have had wind turbines in operation for over 20 years. Wind energy production costs in Europe are now competing with coal fired power stations. Since 1990 the Wind Energy Industry has been the fastest growing sector of the power generation industry.

All wind systems consist of a wind turbine, a tower, wiring and the balance of system components: controllers, inverters and/or batteries. Wind turbines consist of blades on a rotor, a generator mounted on a frame and a tail. The spinning blades turn the rotor capturing the kinetic energy of the wind. This is converted to rotary motion to drive the generator. The best indication of how much energy a turbine will produce is the diameter of the rotor. This determines the amount of wind that will intercept the turbine.

Wind speeds increase with height in flat terrain, thus towers are used to mount the wind turbine. Generally speaking, the higher the tower the more power the wind system can produce. A general rule of thumb is to mount the wind turbine 30 feet above any obstacle that is within 300 feet of the tower.

Wind turbines are typically installed in small groups of 2 to 5 units directly connected to an existing utility grid, or in larger groups of 10 to 30 units with an indirect grid connection, where the current from the turbine passes through a series of electric devices that adjust the current to match that of the grid. When the wind speed increases above a certain speed, known as the cut in speed - typically about 3 to 4m/s (meters per second), the turbine begins to generate electricity and will continue to do so until the wind speed reaches the cut out speed, (about 25m/s). At this point the turbine will shut down, rotate out of the wind and wait for the wind speed to drop to a suitable speed to allow the turbine to start again. The turbine's optimum operating wind speed at which maximum output will be achieved is typically about 13 to 16m/s. During operation the generator ensures that the blades maintain a constant speed of about 20 revolutions per minute, which the gearbox transforms into 1500 revolutions per minute. Higher wind loads acting on the blades result in increased power production but not a higher number of revolutions per minute.

On large wind turbines (above 100-150 kW) the voltage generated by the turbine is usually 690V three-phase alternating current (AC). The current is subsequently sent through a transformer next to the wind turbine (or inside the tower) to raise the voltage to somewhere between 10,000 and 30,000 volts, depending on the standard in the local electrical grid. Large manufacturers will supply both 50 Hz wind turbine models (for the electrical grids in most of the world) and 60 Hz models (for the electrical grid in America).

Generators need cooling while they work. Most turbines use a large fan for air- cooling but a few manufacturers use water- cooled generators. Water-cooled generators may be built more compactly, which also gives some electrical efficiency advantages, but they require a radiator in the nacelle to get rid of the heat from the liquid cooling system. Wind turbines are big! A typical turbine is installed on a tower that is between 50 meters (150 feet) and 80 meters (240 feet) tall. The rotor diameter (blade span) will be between 50 meters (150 feet) and 80 meters (240 feet). Turbine towers are constructed from rolled steel plate and are normally about 4 to 5 meters (12 to 15 feet) diameter at the base and about 2 to 3 meters (6 to 9 feet) diameter at the top. Turbines are installed on concrete foundations that are buried well below ground level with a pedestal to mount the tower on so the landholder can work the land right up to the base of the tower.

Yet, wind turbines are quiet! A typical 1.5 megawatt (15,000,000 watt) turbine will produce 45dB(A) or less at 300 meters. The average noise level in a typical home is 50dB(A). Indeed, with the turbine running at its rated speed a normal conversation can be held at the base of the tower.

Large-scale wind turbines can be installed for about $2.00 per watt or about 2 million dollars per megawatt. A typical wind farm will use about 1% of the area where it is constructed, leaving the rest for normal farming or grazing. Wind turbines have a designed working life of 20 to 25 years, require very little maintenance and are safe (there are no recorded injuries to a member of the general public anywhere in the world).

Capacity factor, also called the load factor, is the amount of time an energy production source is able to produce electricity. A coal power station has a capacity factor of 65 to 85% - that is, it will be able to produce output for 65 to 85% of the time and it will be out of action the rest of the time (due to maintenance, breakdowns etc.) A typical wind turbine has a capacity factor of 25 to 40% depending on the available wind resource. However, a typical large wind turbine recovers the energy used to manufacture and construct it (embodied energy) in 4 to 5 months of operation in a reasonable wind regime. Note that a coal-fired power station never recovers the energy used to construct and operate it. Yet, since wind is neither constant nor consistent, wind turbines can at best supplement regular power stations and aid in the reduction of coal consumption and emissions.

Germany is one of the largest users of wind energy with 6113megawatts of wind turbines installed, thus assisting the German government to close its nuclear power stations. The Danish government has determined that wind energy will provide 50% of the country's energy requirements by 2030; meaning a 50% reduction in greenhouse emissions.

Pitch vs. Stall Controlled Wind Turbines

Strong winds can damage the wind turbine, thus all wind turbines are designed with some sort of power control. There are two different ways of doing this safely on modern wind turbines. 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.

Designing a pitch controlled wind turbine requires some clever engineering to make sure that the rotor blades pitch exactly the amount required. On a pitch controlled wind turbine, the computer will generally pitch the blades a few degrees every time the wind changes in order to keep the rotor blades at the optimum angle to maximize output for all wind speeds. The pitch mechanism is usually operated using hydraulics.

Stall (passive) controlled wind turbines have the rotor blades bolted onto the hub at a fixed angle. The geometry of the rotor blade is aerodynamically designed to ensure that the moment the wind speed becomes too high, it creates turbulence on the side of the rotor blade not facing the wind. This stall prevents the lifting force of the rotor blade from acting on the rotor. The rotor blade for a stall controlled wind turbine is twisted slightly in order to ensure that the rotor blade stalls gradually rather than abruptly when the wind speed reaches its critical value.

The basic advantage of stall control is that one avoids a complex control system and avoids moving parts in the rotor itself. On the other hand, stall control represents a very complex aerodynamic design problem. About two thirds of the wind turbines currently being installed in the world are stall controlled machines.

Finally, 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. When the machine reaches its rated power, an important difference from the pitch controlled machines occurs: 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, (and avoid overshooting the rated power of the machine at the beginning of a gust of wind). Further, the turbine 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.







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