• Introduction to Wind Power Generation

    Wind is a form of solar energy and is a result of the uneven heating of the atmosphere by the sun, the irregularities of the earth's surface, and rotation of the earth. Wind flow patterns are modified by the earth's geographical area, bodies of water, and vegetative cover. This wind flow, or motion energy, when "harvested" by modern wind turbines, can be used to generate electricity. This one is a great way of harnessing an unlimited resource generated by the natural processes of the planet's weather systems.

     

    When air hits the wind turbine, the blades spin, converting the wind’s kinetic energy into mechanical energy. This rotary motion then travels down the shaft and drives a generator where the electricity is produced. Typically most wind turbines are mounted in the horizontal plane, and therefore it is key the blades are facing directly into the wind. Yaw control mechanism aim to minimize the yaw angle (the difference in angle between the wind direction and the direction in which the rotors are facing) as much as possible. Thus yaw mechanism orients upwind turbines to keep them facing the wind when the direction changes. Downwind turbines don't require a yaw control mechanism because the wind manually blows the rotor away from it. Vertical  axis  machines  are  affected  by  the  wind  from  all  directions  and  thus  do not need yaw control. 

     

    Wind Turbine

    There are two types of wind turbines:

    1. Horizontal Axis Wind Turbines (HAWT):

    This is a propeller type rotor mounted on the horizontal axis. As mentioned previously, the blades need to be aligned with the wind and this is done by either a simple tail, or an active yaw. These are more efficient at producing electricity than VAWTs however they are impacted more by changes in wind direction. Horizontal Axis Wind Turbines: Upwind or Downwind Machines

    A. Upwind Turbines: Upwind turbines have the rotor facing the wind. The basic advantage of upwind designs is that one avoids the wind shade behind the tower. By far the vast majority of wind turbines have this design. On the other hand, there is also some wind shade in front of the tower, i.e. the wind starts bending away from the tower before it reaches the tower itself, even if the tower is round and smooth. Therefore, each time the rotor blade passes the tower, the power from the wind turbine drops slightly.

    The basic drawback of upwind designs is that the rotor needs to be made rather inflexible, and placed at some distance from the tower (as some manufacturers have found out to their cost). In addition an upwind machine needs a yaw mechanism to keep the rotor facing the wind.

    B. Downwind Turbines: Downwind turbines have the rotor placed on the lee side of the tower. They have the theoretical advantage that they may be built without a yaw mechanism, if the rotor and nacelle have a suitable design that makes the nacelle follow the wind passively. However for large wind turbines this is a somewhat doubtful advantage.

     

    A more important advantage is that the rotor blades may be made more flexible. This is an advantage both in regard to weight and the structural dynamics of the machine, i.e. the blades will bend at high wind speeds, thus taking part of the load off the tower. The basic advantage of the downwind machine is thus, that it may be built somewhat lighter than an upwind machine. The basic drawback is the fluctuation in the wind power due to the rotor passing through the wind shade of the tower. This may give more fatigue loads on the turbine than with an upwind design.

     

    2. Vertical Axis Wind Turbines (VAWT):

    These are aligned in the vertical axis (like the rotor blades on a helicopter). These are only really deployed within urban areas, where the flow of air is more uneven. Due to their alignment, wind direction has little impact on this type of turbine; however it is apparent that these are less efficient than their HAWT cousins.

    Most turbines tends to have two or three blades, two bladed turbines are cheaper but suffer from blade chatter which puts stress on the system, which can lead to increased maintenance.

     

    A wind turbine is made up of the following components:

    1. Tower: The tower construction doesn’t just carry the weight of the nacelle and the rotor blades, but must also absorb the huge static loads caused by the varying power of the wind. Generally, a tubular construction of concrete or steel is used.

     

    2. Rotor and rotor blades: The rotor is the component which, with the help of the rotor blades, converts the wind energy into rotary mechanical movement. Currently, the three-blade, horizontal axis rotor dominates. The rotor blades are mainly made of glass-fibre or carbon-fibre reinforced plastics (GRP, CFRP). The blade profile is similar to that of an airplane wing. They use the same principle of lift: on the lower side of the wing the passing air generates higher pressure, while the upper side generates a pull. These forces cause the rotor to move forwards, i.e. to rotate.

     

    3. Nacelle with drive vane: The nacelle holds all the turbine machinery. It contains the gear box, low- and high-speed shafts, generator, controller, and brake. Some nacelles are large enough for a helicopter to land on. Because it must be able to rotate to follow the wind direction, it is connected to the tower via bearings.

    A. Gearbox: It connects the low-speed shaft to the high-speed shaft and increases the rotational speeds from about 30-60 rpm, to about 1,000-1,800 rpm; this is the rotational speed required by most generators to produce electricity. Gearbox is a costly and heavy part of the wind turbine and engineers are exploring "direct-drive" generators that operate at lower rotational speeds and don't need gearboxes.

     

    B. Generator: For high power wind turbines, doubly-fed asynchronous generators (there are two three-phase windings, one stationary and one rotating, both separately connected to equipment outside the generator) are most frequently used. Here, the operating rotation speed can be varied somewhat, unlike when using conventional asynchronous generators. Another concept uses synchronous generators. A grid connection of synchronous generators is only possible via transformers, due to the fixed rotation behavior. The disadvantage of requiring complicated control systems is countered by the overall efficiency and better grid compatibility.

     

    C. Brake: Stops the rotor mechanically, electrically, or hydraulically, in emergencies.

     

    D. Electronic Equipment: The electronic equipments of a wind turbine are composed of various sensors. The sensors for measuring temperature, wind direction, wind speed and many other things can be found in and around the nacelle, and assist in turbine control and monitoring.

     

    Figure: Wind Turbine Components

     

    4. Inverter: Most wind turbines produce AC current, so this should be able to be directly fed into your home and the grid, however the voltage and frequency of the power produced is very erratic, so an inverter is used to convert the erratic AC to DC, then back to a smoother AC which can be synchronised with the grid, or for use directly into your home. Battery based wind turbines normally operate at 12 or 48 Volts, and therefore the inverter must also act to convert this relatively low voltage to high voltage (240 volts AC in India).

     

    5. Batteries: In wind turbine systems, the electricity produced may stored in deep-cycle lead acid batteries which look very similar to the ones found in most cars today (although structurally different). Batteries (for off-grid and backup systems) provide energy storage for periods of calm or during utility grid outages.

     

    6. Charge controllers: Charge controllers are used in wind turbine systems to prevent the batteries from being overcharged. A charge controller is necessary for any battery setup as it prevents damage to the battery by monitoring the flow of electricity in and out. If our system overcharges the battery it will damage it. The same is also true if we completely discharge all the charge held within the battery. Most charge controllers associated with wind turbines have dump load capability associated with them. This allows any additional charge to be diverted from the batteries when they are fully charged, potentially to a hot water heating system (so the electricity is not completely wasted). If wind turbine system is connected to the grid, this electricity would instead be sold there, providing an additional income stream.

     

    7. Anemometer and Wind vane: The anemometer and the wind vane are used to measure the speed and the direction of the wind. The electronic signals from the anemometer are used by the wind turbine's electronic controller to start the wind turbine when the wind speed reaches approximately 5 m/s. The computer stops the wind turbine automatically if the wind speed exceeds 25 m/s in order to protect the turbine and its surroundings. The wind vane signals are used by the wind turbine's electronic controller to turn the wind turbine against the wind, using the yaw mechanism.

     

    8. Safety equipment: Disconnects are simply switches that allow isolating the parts of the system so that troubleshoot or repair of faulty parts become possible without the risk of being electrocuted. In addition many wind turbine systems are grounded, so that if there is surge in current anywhere in the system it is safely dissipated rather than damaging the system or more importantly human.

     

    Advantages of Wind Power Generation

    1. Wind energy itself is both renewable and sustainable. The wind will never run out, unlike the earth’s fossil fuel reserves (such as coal, oil and gas), making it the ideal energy source for a sustainable power supply.

    2. Once the wind turbine is built the energy it produces does not cause green house gases or other pollutants.

    3. Generating electricity from wind energy reduces the need to burn fossil fuel alternatives such as coal, oil and gas.

    4. Wind turbines have a relatively small land footprint. Although they can tower high above the ground, the impact on the land at the base is minimal. The area around the base of a wind turbine can often be used for agricultural purposes.

    5. It can also be installed on a domestic scale, with many landowners opting to install smaller, less powerful wind turbines in order to provide part of a domestic electricity supply.

    6. Wind power generation can be used for remote areas that are not connected to the electricity power grid.

    7. As wind energy is free, running costs are considered to be low. The only ongoing cost associated with wind energy is for the maintenance of wind turbines, which are considered low maintenance in nature.

    8. By using wind energy to generate electricity, we are helping to reduce our dependency on coal, oil and gas. Thus increase its energy security.

     

    Disadvantages of Wind Power Generation

    1. The speed of the wind is not constant and it varies from zero to storm. This means that wind turbines do not produce the same amount of electricity all the time. There will be times when they produce no electricity at all.

    2. Although wind power plants have relatively little impact on the environment compared to conventional power plants, concern exists over the noise produced by the turbine blades. Each one can generate the same level of noise as a family car travelling at 70 mph.

    3. Many people see large wind turbines as unsightly structures and not pleasant or interesting to look at. They disfigure the countryside (a geographic area that is located outside towns and cities) and are generally ugly.

    4. Turbine blades could damage local wildlife. Birds have been killed by flying into spinning turbine blades.

    5. Large wind farms are needed to provide entire communities with enough electricity. For example, the largest single turbine available today can only provide enough electricity for 475 homes, when running at full capacity.

     

    How fast must the wind be blowing in order for the turbines to function?

    There is an anemometer on each turbine to measure the wind speed.  When wind speeds reach 8-16 mph, the turbine begins generating electricity.  For the 1.5MW turbine, a wind speed of 27 mph produces peak power.  At about 55 mph, the turbine shuts down and turns out of the wind to protect from overspeed failures.  A brake is equipped to help protect from overspeed as well.

     

    How does the wind direction affect performance?

    In addition to the anemometer, there is also a wind vane that communicates the direction of the wind to the computer. The computer then commands the yaw motor to turn the turbine into the wind.

     

    Doesn’t intermittency of wind affect performance?

    This question leads to discussion of the capacity factors of wind turbines.  A capacity factor is how much electricity is produced in relation to how much could be produced at maximum performance of the system.  According to the Department of Energy, capacity factors have increased from 22% before 1998 to about 35% in 2007.  In other words, the average turbine produces about 35% of the energy that it could produce given steady optimal wind conditions year round.  A 1.5 MW turbine would provide only about 0.5 MW on average. However, because no fossil fuels go towards electricity generation once the turbine is built and established; this capacity factor does not relate to low efficiencies or wasted fuels.