To best understand how wind turbines and wind energy works, let’s first have an understanding of how wind is produced.
How is Wind Produced?
Wind is essentially a form of solar energy. The Principle of Conservation of Energy stipulates that energy cannot be created or destroyed. Rather it can only be converted from one form to another.
The Sun is the source of most other forms of energy on Earth. Most of the energy in the rays of the Sun, that penetrate the Earth’s atmosphere, is converted into heat. Uneven heating of the Earth’s surface as a result of the Earth’s natural irregularities, inclination and rotation results in temperature differences around the world. The areas around the equator naturally receive more direct sunshine than those closer to the Earth’s poles. As the Earth's surface is warmed differentially, the air above these surfaces absorbs different amounts of heat. The heated areas warm up first and fast. At a certain temperature, the warm air begins to rise quickly upwards. Rising air creates a low-pressure zone at the ground level and a higher-pressure zone above the ground. Cooler air from surrounding areas rushes in to occupy the low-pressure space. This creates surface wind. It is estimated that about 1-2% of the energy from the Sun is converted into wind energy.
As wind blows, its flow patterns are enhanced and transformed by land formation, elevation, topography, water bodies and other barriers. As the air moves, patterns are created, gentle breezes are experienced and even tornados witnessed. The resultant phenomenon is what we know as local and global wind circuits and patterns. Winds flow patterns across latitudes, take heat from equatorial regions to Polar Regions resulting in an equalizing process of major importance in determining the environments for life on Earth.
Wind Energy Applications
Wind has been identified as a clean, inexpensive source of energy. Inventions have been made that can harness the energy of the wind and put it to practical use.
» Windmills have been used over centuries to turn millstones for grinding grains such as wheat, barley, and corn to make flour or meal.
» In transportation, sails have been used over centuries.
» Generation of electricity is probably the most important use wind can be put to.
How Wind Turbines Work
At a microscopic level wind is moving air-molecules. We can therefore best refer to wind as the kinetic energy of the air molecules.
Wind turbines are a human invention utilized to capture the kinetic energy in surface winds and convert it into mechanical and eventually, electrical energy in the form of electricity.
An electric fan consumes electricity to produce a drought (wind). A wind turbine works the opposite of an electric fan by using wind to make electricity.
The energy in the wind turns the turbine blades, which spin a shaft, which connects to a generator that generates electricity. Wind turbines comprise of four basic parts:
» The blades
» The shaft
» The generator
» The tower
The wind moves over the turbine blades, generating a lift. The lift makes the blades rotate and in turn rotate the shaft. The turning shaft moves a magnetic field in the generator, which in turn creates electricity. This therefore implies that the faster the wind, the more energy can be produced. Larger wind turbines are more efficient and cost effective.
Wind Turbine Components
The nacelle sits atop the tower and contains a gearbox, low- and high-speed shafts, a generator, a controller, and a mechanical brake. The nacelle is properly covered to protect the components inside. Some nacelles are large enough for a technician to stand inside while working.
The blades of a wind turbine together with the hub on which they are attached are simply referred to as the rotor. Most rotor hubs are made from cast iron and are fitted to the main shaft with a large flange.
The bigger the surface swept by a rotor the more energy can be harvested from the wind. The energy that is available to the wind turbine is proportional to the swept area of the rotor.
Main (Low-speed) shaft
The rotor is coupled to the low-speed shaft. This shaft turns according to the speed of the rotor. The main shaft is mainly forged in alloy steel.
The low speed shaft is connected to the high-speed shaft through a gearing mechanism. The gears alter the speed from the low speed-shaft to enable the high speed-shaft attain high speeds for driving the generator.
Pitching is the twisting or turning of blades at the base so that they do not turn in winds that are either too low or too high.
Typically, a generator in a wind turbine needs a rotational speed of 25 rotations per second. For this reason, a gear is utilized to enhance the rotor speed to enable the generator attain electricity generating speeds.
Between the gearbox and the generator a coupling is utilized, mainly with two flexible elements, to aid in damping and shock-absorbing properties in order to tolerate slight misalignments.
A normal induction generator is sufficient. The generator rotor design and the stator windings are specially designed for high efficiency at partial loads.
A mechanical disc-brake is fitted to the gearbox high speed shaft. This brake can be applied mechanically, electrically, or hydraulically to stop the rotor in emergencies and normal shutdowns.
Upwind turbines face into the wind whereas downwind turbines are the opposite. For upwind turbines, the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. A yaw motor is used to power the yaw system. Downwind turbines don't require a yaw drive, because the rotor is naturally blown downwind by the wind.
The wind vane measures wind direction and transmits this information to the yaw drive, which orientates the turbine properly with respect to the wind.
The controller is an industrial microprocessor, complete with switchgear, protection devices and instrumentation. The controller starts up the machine at wind speeds of about 4 metres per second, and shuts off the machine at about 25 metres per second. At speeds exceeding 25 metres per second, wind turbine generators could overheat. The turbine controller is placed at the bottom of the tower.
This is a very important instrument for measuring the wind speed and transmitting this information to the controller.
Wind turbine towers play a crucial role in the harvesting of wind energy. Wind speed increases with height and it is for this reason that taller towers enable turbines to capture more energy and generate more electricity.
Most towers are made from tubular steel or steel lattice. The tower must be strong enough to support the nacelle and rotor as well as withstand powerful loads from the wind.
The tower has internal ascent with proper lighting to allow direct access to the yaw system and nacelle. Towers are usually bolted onto concrete foundations, but another erection method is where part of the bottom section of the tower is cast into the concrete foundation and the lowest section of the tower is subsequently welded together directly on the site.
Operation and Generation of Electricity
When the wind speed increases from calm depending on design, the turbine will self-start at about 4-6 m/s.
The turbine rotor will continue to accelerate until a synchronous speed is attained. At this time, the generator, through the small windings and using thyristors to avoid flickering, is connected to the grid, Once connected to the grid, the thyristors, due to their high energy losses, are by-passed by a main contactor. The turbine will continue to supply the grid through the small generator windings up to approximately 7 m/s wind speed, but at higher wind speeds the generator switches to the main generator windings.
The output from the wind turbine increases in a roughly linear manner with increasing wind speed. At wind speeds of about 14-15 m/s the power output is limited by stalling of the rotor. If the wind speed exceeds the set maximum limit, say 25 m/s, the turbine is shut down. But if the wind speed drops below a re-starting limit the safety systems are automatically reset to enable the turbine to start up again.
In case of emergencies, the turbine is shut down through the application of the mechanical brake. All these are enabled by the automatic monitoring system of the turbine controller.
The design figure from the manufacturers for Masinga machines ESCHER WYSS of Germany is 20.6 MW which is quite close to the one from the formula.