Ken Youssefi / Hsu1

WindTurbine

Mostafa GhadamyariMostafa Ghadamyari

Ferdowsi universityFerdowsi universityof Mashhadof Mashhad

Winter 2012Winter 2012

Wind Turbine

Sun -> Different absorption -> dT -> density difference -> WIND

Wind energy is created when the atmosphere is heated unevenly by the Sun, some patches of air become warmer than others. These warm patches of air rise, other air rushes in to replace them – thus, wind blows :

A wind turbine extracts energy from moving air by slowing the wind down, and transferring this energy into a spinning shaft, which usually turns a generator to produce electricity. The power in the wind that’s available for harvest depends on both the wind speed and the area that’s swept by the turbine blades.

How wind is created ?

Two types of turbine design: Horizontal axis and Vertical axis.

Horizontal axis turbines: Can reach higher altitude wind but requires a substantial tower structure. Used in most modern wind turbine designs.

Vertical axis turbines: No need to turn into wind (yaw), easier construction and maintenance (generator and gear box are on the ground) level, lower efficiency.

Wind Turbine Design

Horizontal axis Turbine

Vertical axis Turbine

Drag or Lift Design

Ken Youssefi / HsuEngineering 10, SJSU4

Drag Design

The wind literally pushes the blades out of the way.

Slower rotational speeds and high torque capabilities. Useful for providing mechanical work (water pumping e.g.).

Wind turbines are designed based on either aerodynamic Drag or Lift force.

Lift Design

•Blade is essentially an airfoil (like wings of airplanes).

•When air flows past the blade, a wind speed and pressure differential is created between the upper and lower blade surfaces. The pressure at the lower surface is greater and thus acts to "lift" the blade.

•The lift force is translated into rotational motion.

•Lift design generally has higher efficiency and is used in most modern turbines.

Lift

We focus our discussion on the Lift Design Horizontal Axis Turbine.

Main components of a Horizontal Axis Wind Turbine

Blades and rotor: Converts the wind power to a rotational mechanical power.

Generator: Converts the rotational mechanical power to electrical power.

Gear box: Wind turbines rotate typically between 40 rpm and 400 rpm. Generators typically rotates at 1,200 to 1,800 rpm. Most wind turbines require a step-up gear-box for efficient generator operation (electricity production).

Power Generated by HWind Turbine

Power = ½ (ρ)(A)(V)3 (Cp)

A = swept area = (radius)2, m2

V = Wind Velocity, m/sec.

ρ = Density of air = 1.2 kg/m3 at sea level, 20 oC and dry air

Cp = Efficiency=0.35~0.45, typically

The power in the wind is Pwind = ½ (ρ)(A)(V)3. The amount of power that can be captured by a turbine is only 35% to 45% of that amount (i.e., Cp = 0.35 ~ 0.45).

The theoretical maximum for Cp is 0.593, i.e., the theoretical maximum efficiency of a turbine is 59.3%. This maximum efficiency is called Betz Limit.

A

What should be the angle of attack?

What should be the blade profile?

How many blades to use?

Blade Length Blade Number

Blade Pitch Blade Shape

Blade Materials Blade Weight

Rotor Blade Variables

Number of Blades – One• Rotor must move more rapidly to capture same amount of wind

– Gearbox ratio reduced– Added weight of counterbalance negates some benefits of lighter design– Higher speed means more noise, visual, and wildlife impacts

• Blades easier to install because entire rotor can be assembled on ground• Captures 10% less energy than two blade design• Ultimately provide no cost savings

Number of Blades - Two

• Advantages & disadvantages similar to one blade

• Need teetering hub and or shock absorbers because of gyroscopic imbalances

• Capture 5% less energy than three blade designs

Number of Blades - Three

• Balance of gyroscopic forces• Slower rotation

– increases gearbox & transmission costs

– More aesthetic, less noise, fewer bird strikes

Blade Angle

The angle between the chord line of the blade and the wind direction (called angle of attack) has a large effect on the lift force (see figure below). Typically, maximum lift force is achieved with 1.0 to 15.0 degrees angle of attack.

Wind Turbine – Blade Design

chord lineAngle of Attack

WindLift

Relative Wind direction

Wind direction relative to blade depends on wind speed and rotor speed.

Wind Turbine – Blade Design

Relative wind

direction relative wind due to blade speed

Angle of attack

Blade motion direction

wind direction

Wind Turbine – Blade Design (Shape)To see the wind moves relative to the rotor blades, red ribbons are attached to the tip of the rotor blades and yellow ribbons about 1/4 of distance from the hub.

If the tip of the rotor blade moves through the air with a tip speed = 64 m/s, the speed at the centre of the hub is zero. 1/4 out from the hub, the speed will then be ~16 m/s.

The yellow ribbons close to the hub will be blown more towards the back of the turbine than the red ribbons at the tips of the blades.

Wind Turbine – Blade Design (Shape)

Recall that to reach the maximum lift, the angle of attack must be at a specific value for a given blade design.

Angle of attack depends on the relative wind speed which varies along the blade (highest at the tip).

To achieve an optimal angle of attack throughout the length of the blade, the blade must be “twisted” along the blade length.

To maintain a uniform stress on the blade, the chord length (blade width) is narrower near the tip.

Wind Turbine – Blade Design

5-station design as seen from the tip

Blade size and shape Last profile next to the hub

First profile at the tip

0 ~ 5 m/s --- Wind speed is too low for generating power. Turbine is not operational. Rotor is locked.

5 ~ 15 m/s ---- 5 m/s is the minimum operational speed. It is called “Cut-in speed”. In 10 ~ 25 mph wind, generated power increases with the wind speed.

15 ~ 25 m/s ---- Typical wind turbines reach the rated power (maximum operating power) at wind speed of 15 m/s (called Rated wind speed). Further increase in wind speed will not result in substantially higher generated power by design. This is accomplished by, for example, pitching the blade angle to reduce the turbine efficiency.

> 25 m/s ---- Turbine is shut down when wind speed is higher than 50mph (called “Cut-out” speed) to prevent structure failure.

Typical Wind Turbine Operation

Worldwide Energy Sources

Engineering 10, SJSU

85%

1.24%

Questions?Questions?

AcknowledgementSpecial thanks to :-Ken Youssefi / Hsu (San Jose State University)-Jaime Carbonell (Carnegie Mellon University)-Joseph Rand (The Kidwind Project)For their powerpoints about wind power.

21