Fossil fuels dominate energy sources – oil, coal, gas
Wind TurbineMostafa GhadamyariFerdowsi university 1 of Mashhad Ken Youssefi / Hsu Winter 2012
Wind TurbineHow wind is created ?Sun -> Different absorption -> dT -> density difference -> WINDWind energy is created when the atmosphere is heated unevenly bythe Sun, some patches of air become warmer than others. Thesewarm 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 winddown, and transferring this energy into a spinning shaft, whichusually turns a generator to produce electricity. The power in thewind that’s available for harvest depends on both the wind speedand the area that’s swept by the turbine blades.
Wind Turbine DesignTwo types of turbine design: Horizontal axis and Vertical axis.Horizontal axis turbines: Can reach higher altitude wind but requiresa substantial tower structure. Used in most modern wind turbinedesigns.Vertical axis turbines: No need to turn into wind (yaw), easierconstruction and maintenance (generator and gear box are on theground) level, lower efficiency. Vertical Horizontal axis axis Turbine Turbine
Drag or Lift DesignWind turbines are designed based on either aerodynamicDrag or Lift force. 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.).4 Engineering 10, SJSU Ken Youssefi / Hsu
Lift Design•Blade is essentially an airfoil (like wings ofairplanes).•When air flows past the blade, a wind speedand pressure differential is created between Liftthe upper and lower blade surfaces. Thepressure at the lower surface is greater andthus acts to "lift" the blade.•The lift force is translated into rotationalmotion.•Lift design generally has higher efficiencyand is used in most modern turbines. We focus our discussion on the Lift Design Horizontal Axis Turbine.
Main components of a Horizontal AxisWind TurbineBlades 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 requirea step-up gear-box for efficient generator operation (electricity production).
Power Generated by HWind Turbine Power = ½ (ρ)(A)(V)3 (Cp) ρ = Density of air = 1.2 kg/m3 at sea level, 20 oC and dry air A = swept area = π(radius)2, m2 V = Wind Velocity, m/sec. A 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.
Rotor Blade Variables Blade Length Blade Number Blade Pitch Blade Shape Blade Materials Blade WeightWhat should be the blade profile?What should be the angle of attack? How many blades to use?
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
Wind Turbine – Blade DesignBlade AngleThe angle between the chord line of the blade and the winddirection (called angle of attack) has a large effect on the liftforce (see figure below). Typically, maximum lift force isachieved with 1.0 to 15.0 degrees angle of attack. ch or d Angle of lin e Attack Lift Wind
Wind Turbine – Blade DesignRelative Wind directionWind direction relative to blade depends on wind speedand rotor speed. Angle of attack Blade wind e motion tiv n relative wind direction e l a c ti o R e due to blade d ir speed 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 ofattack must be at a specific value for a given bladedesign.Angle of attack depends on the relative wind speedwhich varies along the blade (highest at the tip).To achieve an optimal angle of attack throughout thelength of the blade, the blade must be “twisted” alongthe blade length.To maintain a uniform stress on the blade, the chordlength (blade width) is narrower near the tip.
Wind Turbine – Blade DesignBlade size and shape Last profile next 5-station design as seen from the tip to the hub First profile at the tip
Typical Wind Turbine Operation0 ~ 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.
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Acknowledgement Special 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