WIND ENERGY COLLECTORS

• Wind turbine power production depends on the interaction between the rotor and the wind.

• The wind may be considered to be a combination of the mean wind and turbulent fluctuations about that mean flow.

• Experience has shown that the major aspects of wind turbine performance (mean power output and mean loads) are determined by the aerodynamic forces generated by the mean wind.

• Periodic aerodynamic forces caused by wind shear, off-axis winds and rotor rotation and randomly fluctuating forces induced by turbulence and dynamic effects are the source of fatigue loads and are a factor in the peak loads experienced by a wind turbine.

Practical horizontal axis wind turbine designs use airfoils to transform the kinetic energy in the wind into useful energy.

• A quantitative measure of the wind energy available at any location is called the Wind Power Density (WPD)

• It is a calculation of the mean annual power available per square meter of swept area of a turbine, and is tabulated for different heights above ground.

• Calculation of wind power density includes the effect of wind velocity and air density.

Theoretical power captured by a wind turbine

Total wind power could be captured only if the wind velocity is reduced to zero. In a realistic wind turbine this is impossible, as the captured air must also leave the turbine.

Practical wind turbine power

Further insufficiencies, such as rotor blade friction and drag, gearbox losses, generator and converter losses, reduce the power delivered by a wind turbine.

The basic relation that the turbine power is proportional to the third power of velocity.

HORIZONTAL AXIS WIND TURBINE

• Wind turbines can rotate about either a horizontal or a vertical axis

• Horizontal-axis wind turbines (HAWT) have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind.

• Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor.

• Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive an electrical generator

• Since a tower produces turbulence behind it, the turbine is usually positioned upwind of its supporting tower. Turbine blades are made stiff to prevent the blades from being pushed into the tower by high winds. Additionally, the blades are placed a considerable distance in front of the tower and are sometimes tilted forward into the wind a small amount.

Advantages

• Blades are to the side of the turbine’s center of gravity, helping stability.

• Allowing the angle of attack to be remotely adjusted gives greater control, so the turbine collects the maximum amount of wind energy.

• The ability to pitch the rotor blades in a storm so that damage is minimized.

• Tall tower allows access to stronger wind in sites with wind shear and placement on uneven land or in offshore locations.

• Most of them are self-starting.

• Can be cheaper because of higher production volume.

Disadvantages

• Has difficulties operating near the ground and with turbulent winds because the yaw and blade bearing need smoother, more laminar wind flows.

• The tall towers and long blades are difficult to transport and need a special installation procedure.

• When placed offshore, they can cause navigation problem.

VERTICAL AXIS WIND TURBINE

• Vertical-axis wind turbines (or VAWTs) have the main rotor shaft arranged vertically.

• Key advantages of this arrangement are that the turbine does not need to be pointed into the wind to be effective. This is an advantage on sites where the wind direction is highly variable, for example when integrated into buildings.

• With a vertical axis, the generator and gearbox can be placed near the ground, using a direct drive from the rotor assembly to the ground-based gearbox, hence improving accessibility for maintenance.

Advantages

• VAWTs are not affected by the direction of the wind, which is useful in areas where the wind changes direction frequently and quickly. Unlike traditional horizontal axis wind turbines, a yaw mechanism is not needed to turn the wind turbine towards the wind.

• Because of this, VAWTs outperform horizontal axis turbines in areas where a tall tower isn't feasible

• VAWTs are better able to harvest turbulent air flow found around buildings and other structures

• VAWTs are ideal for both rural and urban applications, including roof top installations. Depending on the shape of the roof, the wind flow over the roof can be concentrated, leading to an increased energy output

• Simple to install and maintain

• Quiet operation

• Pleasant appearance

Disadvantages

• They are less efficient than horizontal axis wind turbines. Most of them are only half as efficient as the horizontal ones because of the additional drag that they have as their blades rotate into the wind.

• Air flow near the ground and other objects can create turbulent flow, which can introduce issues of vibration. This can include noise and bearing wear which may increase the maintenance or shorten the service life.

• The machine may need guy wires to hold it up. Guy wires are impractical in heavily farmed areas.

• Largest capacity: The Enercon E-126 has a rated capacity of 7.58 MW, has an overall height of 198 m (650 ft), a diameter of 126 m (413 ft), and is the world's largest-capacity wind turbine since its introduction in 2007.

• Largest swept area: The turbine with the largest swept area is a prototype installed by Gamesa at Jaulín, Zaragoza, Spain in 2009. The G10X – 4.5 MW has a rotor diameter of 128m

• Tallest: The tallest wind turbine is Fuhrländer Wind Turbine Laasow. Its axis is 160 meters above ground and its rotor tips can reach a height of 205 meters. It is the only wind turbine in the world taller than 200 meters

• Highest-situated: The world's highest-situated wind turbine is made by DeWind installed by the Seawind Group and located in the Andes, Argentina around 4,100 metres (13,500 ft) above sea level.

BETZ'S LAW

• Betz's law calculates the maximum power that can be extracted from the wind, independent of the design of a wind turbine in open flow.

• The law is derived from the principles of conservation of mass and momentum of the air stream flowing through an idealized "actuator disk" that extracts energy from the wind stream.

• The Betz law means that wind turbines can never be better than 59.3% efficient.

• The law can be simply explained by considering that if all of the energy coming from wind movement into the turbine were converted into useful energy then the wind speed afterwards would be zero.

• But, if the wind stopped moving at the exit of the turbine, then no more fresh wind could get in - it would be blocked.

• In order to keep the wind moving through the turbine, to keep getting energy, there has to be some wind movement on the outside with energy left in it. There must be a 'sweet spot' somewhere - and there is, the Betz limit at 59.3%

Assumptions

1. The rotor does not possess a hub, this is an ideal rotor, with an infinite number of blades which have no drag. Any resulting drag would only lower this idealized value.

2. The flow into and out of the rotor is axial. This is a control volume analysis, and to construct a solution the control volume must contain all flow going in and out, failure to account for that flow would violate the conservation equations.

3. This is incompressible flow. The density remains constant, and there is no heat transfer from the rotor to the flow or vice versa.

4. The rotor is also mass less. No account is taken of angular momentum imparted to either the rotor or the air flow behind the rotor, i.e., no account is taken of any wake effect.

COEFFICIENT OF WIND POWER

• The coefficient of power at a given wind speed, is given by the electricity produced divided by the total energy available in the wind at that speed

• Wind turbines extract energy by slowing down the wind. For a wind turbine to be 100% efficient it would need to stop 100% of the wind - but then the rotor would have to be a solid disk and it would not turn and no kinetic energy would be converted.

• On the other extreme, a wind turbine with just one rotor blade, most of the wind passing through the area swept by the turbine blade would miss the blade completely and so the kinetic energy would be kept by the wind.

• In the diagram shown above, the wind turbine converts 70% of the Betz Limit into electricity.

• Therefore, the Cp of this wind turbine would be 0.7 x 0.59 = 0.41. So this wind turbine converts 41% of the available wind energy into electricity.

• This is actually a pretty good coefficient of power. Good wind turbines generally fall in the 35-45% range.

WIND ENERGY AND POWER

• The kinetic energy (KE) of an object (or collection of objects) with total mass M and velocity V is given by the expression:

KE = ½ * M * V^2

• To find the kinetic energy of moving air molecules (wind), consider a large air parcel with the shape of a huge hockey puck: that is, it has the geometry of a collection of air molecules passing though the plane of a wind turbines blades (which sweep out a cross-sectional area A), with thickness (D) passing through the plane over a given time.

The volume (Vol) of this parcel is determined by the parcel's area multiplied by its thickness: Vol= A * D

Let ρ represent the density of the air in this parcel. Note that density is mass per volume and is expressed as: ρ = M / Vol and M = ρ * Vol

Now let's consider how the velocity (V) of our air parcel can be expressed. If a time T is required for this parcel (of thickness D) to move through the plane of the wind turbine blades, then the parcel's velocity can be expressed as V = D / T, and D = V * T.

KE = ½ *M * V^2 Substitute for M = ρ * Vol to obtain

KE = ½ * (ρ * Vol) * V^2 and Vol can be replaced by A * D to give

KE = ½ * (ρ * A * D) *V^2 and D can be replaced by V * T to give

KE = ½ * (ρ * A * V * T) * V^2 leaving with: KE = ½ *ρ*V^3*A*T

• Now, power is just energy divided by time, so the power available from our air parcel can be expressed as:

Pwr = KE / T = (½ *ρ * V^3 * A * T) / T = ½ * ρ * V^3 * A

• And if we divide Pwr by the cross-sectional area (A) of the parcel, then we are left with the expression:

Pwr / A = ½ * ρ * V^3

• Note two important things about this expression: one is that the power is proportional to the cube of the wind speed. The other is that by dividing power by the area, we have an expression on the right that is independent of the size of a wind turbine's rotor.

• In other words, Pwr / A only depends on (1) the density of the air and (2) the wind speed. In fact, there is no dependence on size, efficiency or other characteristics of wind turbines when determining Pwr / A.

• The term for the quotient Pwr / A is called the "Wind Power Density" (WPD)