Wind Energy Flashcards
HAWT vs VAWT
HAWT- Horizontal Axis Wind Turbine
Vertical Axis Wind Turbine- Vertical Axis Wind Turbine
VAWT components
Nacelle; This is the housing that contains all the main components
Blades; The molecules of the gases in the air strike the blades and make them turn
Hub; The part of the turbine the blades are attached to
Rotor; The blades and hub collectively
Low speed shaft; Shaft after the gearbox that now rotates at high RPM
Gearbox; The gearbox links the shaft attached to the generator. The generator increases the low RPM of the low speed shaft to the high speed shaft needed by the number
High speed shaft; Shaft after the gearbox that now rotates at high RPM required by the generator
Generator; Inside the generator electromagnetic induction takes place producing electricity
Wind vane; The wind vane is used to establish wind direction
Anemometer; A wind meter determines wind speed
Tower; This is the large pole that supports the nacelle
Betz Limit
Wind coming from upstream through a turbine possesses kinetic energy. When it passes through the turbine, it causes the blades of a turbine to turn. There must be a certain amount of swept area in order for rotation to take place, the Betz limit is the minimum and maximum of that area
Mechanical friction
Any moving parts rub against each other, causing some of the kinetic energy to change to thermal energy (heat) which increases the temperature of the material. These thermal losses can be minimised by the use of bearings and lubrication on the low speed shaft, high-speed shaft and gearbox. However, these thermal losses cannot be eliminated
Electrical resistance
Any current moving through a cable will encounter resistance. This causes the wire to heat up and so some electrical energy is converted to thermal energy. The rated output energy is much less due to heat losses through friction in the shafts and gearbox. Heat (thermal) losses also exist in electrical wires of the wind turbine
Relationship between swept area and power output
Power output = 1/2 x Area velocity cubed x COP
Relationship between power output and temperature + altitude
At high temperatures, such as in summer time, means that the molecules of the gases in air above have more kinetic energy and so move with a greater speed between collision. This makes the gas expand and so the particles move further apart
At higher altitudes there is lower air density meaning less energy can be extracted
Factors that effect maximum energy production in wind turbines
Wind speeds
Air density
COP
Obstructions
Hub height
Altitude
Temperature
Wind resource assessment
Wind speed; Knowing the average wind speed of an area is vital when making a wind turbine. It is possible to undertake a desktop study on the internet to get an estimate of the average wind speed at the site location. The benefit of this is that the desktop cost is low
Terrain; The topography of the ground around the turbine may be beneficial or a constraint for the turbine project. Obstructions may come in the form of nearby hedges, trees, forests, houses etc
Wind turbine; It is obvious and logical that the larger the wind turbine, the larger a tower will be. The larger the wind turbine’s power rating the larger the hub height will have to be
Visual impact; Some planners may object to larger hub heights on the the basis that wind turbines have an adverse visual impact. There is a lack of specific regulations or guidance for investors or planners to adhere to and this results in little more than opinion which can differ among planners
Wind survival speed
The maximum wind speed that a turbine is designed to withstand before sustaining damage
When a rotor spins too quickly, the forces and stresses experienced by the blades and tower may become too large and they may fail. This can lead to debris from the turbine being propelled hundreds of meters into the air and to nearby areas. If such debris struck a home or person, the injuries sustained could be fatal. This will also lead to very negative publicity in relation to wind turbines and perhaps increase opposition to their use. It is therefore critical that manufacturers are aware of when the failure takes place and do something to ensure that the turbine never reaches this point
Pitching
The method of power control used in turbines is called ‘pitching’. In this process the blades of the turbine change orientation relative to the wind direction, to control the rate of power production. For maximum power production the blades have a maximum area facing the incoming wind. A maximum number of collisions per second between the air particles and blade ensures the maximum force is transmitted and so the rotor will turn with maximum revolutions per minute (RPM). This position would be utilised from the cut-in speed to the speed at the rated power
When the wind speeds become dangerously large the cut-out speed is reached. At this point the turbine needs to cease rotation given the possibility of catastrophic failure. To shut the device down the blades orientate themselves such that they’re parallel to the incoming wind direction. The wind no longer causes the rotor to turn; hence no power is produced. This process is called feathering
Passive pitching
On a small scale wind turbines keeping production costs to a minimum are of critical concern. Complex engineering solutions that can facilitate precise are expensive and are not viable on small turbines. These smaller turbines utilise more simplistic solutions to keep down costs but have some limitations. The blades are designed to naturally adapt at higher wind speeds through various mechanisms. This might be the slight twisting of the blade when under higher wind loads or indeed the use of springs. The primary benefit of passive pitch solutions is cost which is reduced to a minimum while allowing some elements of power control
Active pitching
On medium to large scale projects achieving maximum energy production for all higher wind speeds is essential. To facilitate this, hydraulic ram systems or an electric motor with a gearwheel is utilised. Their costs are covered with the additional revenue the project can now produce per year. A signal from the anemometer at the back of the nacelle is sent to the pitching system which will precisely adapt the angle of pitch at higher wind speeds
