Week 2 & 3 - Wind Power Flashcards
Other than physical size, capacity and application there are several main ways of classifying wind turbines. What are these?
Vertical axis - VAWT
Horisontal axis - HAWT
Concentrators
Also:
Lift or drag based
Solidity (describes the fration of swept area that is solid, high solidity - large no. of blades, low solidity - small no of narrow blades and large void)
Fixed or variable speed
Give some key points about VAWTs.
- Characterised by a vertical rotational axis.
- Exists in both lift and drag form.
- The Savonius Rotor and the simple cup anemometer are the best known vertical axis, drag type devices.
- The Darrieus Rotor is the best known vertical axis system.
- Can handle wind from all directions. heavy gearbox and generator on ground but fatigue problems and not self starting.
- VAHTs less popular than HAWT.
Give some key points about HAWTs.
- Standard horisontal axis wind devices operate with their rotational axis in line with the wind direction. (referred to axial flow devices).
- Almost universally they use lift.
- The rotation will be maintained in line with the wind direction by a “yaw” mechanism, which continually realigns the wind turbine rotor with the incoming wind direction.
What is “Solidity” defined?
“Solidity” is defined as the proportion of swept area occupied by blades.
High solidity rotors start easily with a high initial torque but reach their maximum power output at a low rotational speed. They are appropriate for direct mechanical drive applications such as water pumping. High solidity implies higher torque (higher gearbox costs), higher thrust (higher tower costs) and higher rotor material costs.
Low solidity rotors may require mechanical starting but reach their maximum power at high rotational speeds. They are appropriate for electricity generation.
Compare one blade, two blade and three blade HAWTs to each other.
One Blade:
- Speed > Two blade
- Noisier
- High drag losses
- Counterweight negates material savings
Two Blade:
- Speed > Three blade
- Slightly noisier
- Slightly higher drag
- Less sensitive to speeds
- Lighter structure
Three Blade:
- Balanced performance
(- Visually appealing)
Compare Upwind vs Downwind turbines.
Downwind (rotor at rear):
- Lighter, more flexible blades (cheaper)
- Can extend blades further from tower - loading
- Self orienting
- Tower shadow and fatigue
- Noisier
Upwind:
- Stiffer, heavier blades - fatigue down, cost up
- Extended nacelle to prevent tower strike
- Less tower shadow
- Reduced dynamic loading
- Yaw drive required
What does a typical wind turbine consist of?
- Tower: Mostly made of steel tubes painted light grey, 25 to 75 meters tall.
- Rotor blades: Made of fiberglass-reinforced polyester and between 30 to 80 meters long; blades can be rotated to change the pitch angle and modify power output.
- Yaw Mechanism: turns the turbine to face the wind.
- Wind speed/ Direction monitor uses anemometer to control power automatically as wind speed varies and wind vane to ensure rotors face into wind.
- Gear Box: Steps up slow 10-30 RPM motor speed to speed suitable for generators; increasing numbers of turbines are fitted with direct drive systems that use power electronics to decouple rotor speed from grid requirements.
How is the power available from the wind calculated?
Mass flow rate:
dm / dt = roh * A * U (Air density * Area * Wind speed)
Kinetic Energy:
KE = 1/2 * m * U^2 (1/2 * Mass * (Wind Speed)^2)
Wind power is the rate of kinetic flow:
P = dKE / dt = 1/2 * U^2 (1/2 * (Wind Speed)^2)
This wind power (using above equations) is:
P = 1/2 * roh * A * U^3
The wind power density (WPD) is:
WPD = P / A = 1/2 * roh * U^2
What is the Power coefficient (C_p) and how is it calculated?
The efficiency in wind power extraction is quantified by the Power coefficient, which is the ratio of power extracted by the turbine to the total power of the wind resource.
C_p = P_T / P_wind
How is turbine power capture calculated?
P_T = 1/2 * roh * A * U^3 * C_p
What is the Betz Limit?
The theoretical upper limit for C_p.
C_p = 16/27, i.e 59% efficiency.
What is Capacity Factor?
Capacity Factor (CF) is another metric for efficiency. It is the ratio of the actual generated energy to the energy which could be potentially be generated under ideal environments. CF = E_actual / E_ideal Typical value is about 30%. In regions of good resource it may increase to 50%.
Why is the power coefficient always smaller than the Betz limit?
Typically the power coefficient is between 0.4-0.45 for a modern turbine. This is due to:
- Rotation of wake behind rotor.
- Finite number of blades.
- Tip losses.
- Aerodynamic drag is non-zero.
What is an airfoil?
An airfoil is a streamlined body design to produce lift with minimum drag. Pressure and friction forces are developed on the surface. The resultant of all forces and moment act at a distance of c/4 from the leading edge.
The lift force will be perpendicular to the direction of the oncoming air flow.
The drag force will be parallel to the direction of air flow.
What affects the lift and drag coefficients?
- Shape of aerofoil
- Angle of attack
- Reynolds number (a little)
Why doesn’t modern turbines use drag for power generation?
C_Pmax = 22% for drag turbines, which is lower than that of lift.
What is the “stall angle”?
It’s a certain angle of attack where the lift coefficient falls drastically beyond that point. This us used by turbine designers to regulate turbine operation, i.e. stall control.
What is the ‘optimal’ solution to blade design?
- Blade tapers from root to tip
- Blade has a twist such that the pitch decreases from root to tip of blade.
However, blades are never really optimal. - Cost/fabrication limits
- Performance variation with wind a rotational speed, i.e. tip speed.
