Wind Turbines

Carolyn Lauer EE '03

Modern wind turbines are becoming a popular alternative source of energy in all parts of the world, due to the increasing abundance and relative low cost of wind turbine technology. Although wind turbines have been utilized for many years, they have only recently been considered a viable option for large-scale energy production, particularly as the concerns surrounding dwindling fossil fuel resources have intensified.

HISTORY

In the mid-1800s wind was first considered as an energy source in the modern sense by Poal la Cour. La Cour was a trained meteorologist, but soon became the pioneer in wind turbine technology. He was most concerned about the storage of energy, and used the electricity he generated from his wind turbine to produce hydrogen by electrolysis. La Cour subsequently used this hydrogen to fuel the gaslights at his school in Askov, Denmark. La Cour founded the Society of Wind Electricians in 1905, and published the Journal of Wind Electricity, the first such journal of its kind. The popularity of wind energy surged, and in 1918 more than 120 utilities in Denmark were powered, at least partially, by wind turbines. These turbines produced a total of 3 megawatts of power, about 3% of Danish electricity consumption at the time. Interest in wind energy plateaued until the energy crisis of the Second World War, when Danish wind turbines were looked to as an alternate energy source.


This prototype of the Nordex in Grevenbroich, Germany has a rotor diameter of 80 meters and is the largest commercial wind turbine in the world.

It was during WWII that F.L. Smidth and several other companies began successfully producing two and three-blade wind turbines. These turbines subsequently became important research models for the Danish energy program in the 1950s. The turbines were important because of their energy-producing capabilities, and also because a portion of these turbines were equipped with the first AC generators, rather than the conventional DC generators. The interest in wind turbines dropped again when the energy crisis of WWII had passed, but enjoyed a feverish renewal a few years later at the onset of the oil crisis in 1973.

The interest in wind turbines was not localized in Denmark during this crisis as it had been historically, as Germany, Sweden, The United Kingdom, and the United States all became interested in wind technology. In response to this renewed interest in wind energy these countries began to build large, 630 kW wind turbines to create electricity. Unfortunately, even with this heightened global interest, wind turbines fell out of favor by the 1970s because of their astronomical costs.

A new generation of wind turbines emerged in the early 1980s that dropped the cost per kilowatt of electricity by about 50 %. This drop was due to the development of the European Wind Atlas Method, software used to predict wind climate and power production, and a more professional climate in the wind industry. The drastic decrease in price fueled a major wind program in California in the early 1980s, and thousands of wind turbines were delivered and installed for alternate energy. Several impressive wind farms were established including one in Palm Springs, CA that included more than 1000 wind turbines. This interest and investment in wind energy disappeared when funding was lost in 1985. Very few new wind turbines have been installed since then, although the market is performing increasingly better as of late. Currently, Germany is the largest market in the world for wind turbines, and they also have the largest amount of wind power installed.

PHYSICS

Wind turbines in use today are continually being updated with modern technology, but the principles are unchanged from those employed in the 1800s. All turbines are governed by several basic physical principles, the most important of which is the idea of lift. Lift is created as wind hits the rotor blades and causes a force perpendicular to the direction of the wind as given by the Bernoulli principle. This force is used to drive the wind turbine, but also to limit the turbine in high winds. Limits can be imposed on the wind turbines by the specific design of its rotors.

Most turbine rotors operate on the stall principle. Stall, or turbulence, is what prevents the lifting force of the rotor blade from acting on the rotor when the winds are too high. This phenomenon occurs because as the angle of the wind and the rotor increases the lift also increases until the wind can no longer travel smoothly along the upper surface of the blade, and it begins to tumble over the top of the rotor. The turbulence destroys the low pressure created by smoothly flowing air, and lift disappears. Another way that the turbine rotors control lift is by drag. Drag is created or eliminated by changing the pitch, or angle, of the rotors in accordance to the wind speed in order to decrease efficiency in high winds, thereby protecting the turbine's structural integrity. Pitch adjustments are also used to increase efficiency in normal winds to deliver as much power as possible.

HOW WIND TURBINES WORK

Both stall and pitch wind turbines have several basic features: rotors, transmission system, generator, braking system, yaw system, control and monitoring system, and the tower. For design and service purposes, these major components are usually inside or connected to the nacelle, the capsule at the tip of the tower where the rotor attaches. In case of malfunction, or for general servicing, service personnel can enter the nacelle from the tower of the turbine. Wind energy is captured by the rotation of the wind turbine's rotor blades. These rotors travel from 50 to 70 m/s at the tip, and turbines usually have two or three of them, although the turbines with only two rotors suffer a 2-3% efficiency loss. Rotors have historically been made of wood, but because of its sensitivity to moisture and processing costs modern materials such as glass reinforced plastic (GRP), carbon fiber reinforced plastic, steel and aluminum are replacing the traditional wooden units. The most widely used material for wind turbines manufactured today is GRP. GRP is used because of its stiffness and relative lost cost of production. Steel was once thought to be an optimum choice for blade fabrication, but was soon abandoned because of its high weight and low fatigue level. Aluminum was only implemented in testing situations because it was found to have a lower fatigue level than steel.


The prototype od the NEG Micon 2 MW turbine was commissioned in August 1999 and is located in Hagelsholm, Denmark. It has a 72m rotor diameter and is mounted on a 68m tower. In the background are the foundations for two sister wind turbines.

Material selection is not the only aspect of rotor design, but also the physical principles that are important for efficient energy production. For example, on stall regulated turbines, the angle of the blades is constant and eliminates the need for a control system which increases the aerodynamic forces on the rotor as the wind speed increases therefore limiting the power capabilities of the wind turbine. However, pitch regulated turbine rotor blades can be rotated radially as the velocity of the wind varies making it possible to have optimum blade angle at all wind velocities maximizing the power. This pitch-regulated system also decreases the efficiency of the turbine in high winds so that the power of rotor is limited to that of the capability of the generator. Even with the modifications of modern rotor blades, it is said that the most ideal blades are only 10% more efficient than a plank of wood. This is not entirely true, as modern rotors are able to contend with a broader range of wind directions and speeds, but it shows the effective power generation of even the earliest wind turbine blades.

Mechanical power generated by the rotor blades is transferred to the transmission system that consists of a gearbox and a braking system. The rotor is connected to the gearbox in the transmission system via the rotor hub. This connection has an input of 20-50 rpm from the low-speed shaft attached to the rotor and an output of 1000-1500 rpm for the high-speed shaft that will drive the generator. The step up is needed because the wind turbine generator cannot be driven directly by the low revolutions per minute created by the wind turbine. Also included in the transmission system is an emergency mechanical brake. The emergency brake is attached to the high-speed shaft and is used only when the aerodynamic brake fails, or when the wind turbine is being serviced.


A wind farm placed on the coast in order to benefit from the undisturbed air coming across the water.

Once the transmission system increases the rpm to that of the high-speed shaft, the shaft is able to drive the induction, or asynchronous, generator. The current is induced by the relative motion of the rotor against the rotation field produced the stator winding, a non-rotating three phase winding on an iron core. An induction generator always operates at a higher speed than that of the rotating stator, since no current could be induced in the absence of relative motion. The difference between the speed of the generator and that of the rotor is about 1% under normal conditions. For modern wind turbines, the maximum electric power that can be generated is between 500 and 1,500 kilowatts. Finally, the power produced by the generator is sent through a soft-start mechanism, which limits the inrush of current when the system is started, a power factor correction, and a transformer, so that it will be compatible with the electrical network.

Other systems in the wind turbine are not used directly in the power generation, but are used to maximize the power generation, keep the wind turbine itself from being ruined in high wind, and to add structural support for the whole system. The power in the wind causes considerably high forces when the wind speeds are high. Therefore, the braking system is essential to safe operation of a wind turbine. In a pitch controlled wind turbine, the rotating blades are turned to a zero or negative angle to slow the rotors and in a stall regulated wind turbine; the tips of the rotors have special brake devices built in. In order to stop the rotors completely the mechanical brakes mentioned earlier in the transmission system are implemented.

During safe operating conditions, the yaw system is used to maximize the amount of power the wind turbine can generate. This system turns the nacelle in reference to the actual wind direction given by the wind vane so that the span of the rotors is perpendicular with the direction of the wind. When the wind vane senses a change in the wind direction it causes the wind turbine controller to activate the yaw system until the rotors are in their optimum position.

The wind control and monitoring system directs many other turbine functions than just the yaw system. This system is responsible for starting the wind turbine when winds reach 5 m/s, and stopping the turbine if the wind velocity exceeds 25 m/s. The wind control and monitoring system is responsible for system startup and shutdown, monitoring of alarms, control of the pitch mechanism on pitch controlled turbines, and communication with the wind farm controller or remote computer.

The tower is the physical support for the entire power generating system. The most common types of towers are tubular or lattice, usually constructed from steel or concrete. Smaller and less expensive towers can be constructed using guy wires, but they are less desirable because they are the most susceptible to destruction by the elements or vandals. Most towers, however, are tubular because they are safer for service personnel and more aesthetically pleasing. These towers are manufactured in 20-30 sections and brought to the construction site. Tubular towers are conical in shape to minimize the amount of material needed and to increase their strength. Lattice towers are much cheaper to build because they require much less material than tubular towers, and are of essentially the same stiffness, but for aesthetic reasons tubular towers predominate. Guyed pole towers are still found on wind turbines, but they are used only on small systems. Such setups are not sensible for larger towers, since the wires make it difficult to access areas around the turbine.

Wind turbines have been revolutionized since their introduction in the 1800s. They are now highly reliable and very efficient in producing power. Not only is this method of power production environmentally conscious, but it is increasingly becoming economically advantageous. Countries around the world are recognizing these benefits of wind energy and its implementation is becoming more widespread, particularly as other energy sources are being depleted. In the future, there will undoubtedly be a more widespread use and acceptance of wind turbines as large-scale energy producers.







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