1. Wind Power and Wind Turbines

2. Outline Wind – causes? Wind Power – example Wind Turbine Design
Aerodynamics

3. Wind – what causes it? Atmospheric pressure differences
Where does the pressure come from?
Weight of air in atmosphere
~31 km
(99% of mass)
Earth’s surface
~12,800 km
Consider the relative thickness of the atmosphere to the diameter of the earth: 31 km/12800 km =  0.24% A dimensionally accurate analogy would be to take a hair from your head and wrap it over the tip of your thumb. The thickness of your hair is about inch. The tip of your thumb to the first joint on your thumb is about 1 inch. The thickness of the hair to this distance is the same as that of the atmosphere to the diameter of the earth. Pretty thin, huh?
Avg. pressure at sea level
Pa (Pascal)
mb (millibar)
29.92 in. Hg (inches of mercury
1 atm (atmosphere)
14.7 psi (pound per square inch)

4. Pressure Check the formula by checking the units! Okay! (Definition)
Space (zero pressure)
(Definition)
Earth’s surface
Edge of atmosphere
Check the formula by checking the units!
Units multiply and divide like numbers!
Okay!

5. Wind – what causes it?, cont.
Pressure differences cause the flow of fluids (gases and liquids)
pressure is always measured relative to some reference pressure
Sometimes relative to vacuum  absolute
Sometimes relative to atmospheric pressure
Fluid density
Acceleration due to gravity
Fluid height h PB PA
The higher pressure at B will cause fluid to flow out of the tank.
So, what causes pressure variations in the atmosphere?

6. Prevailing Winds Heating and cooling of the air

7. Wind – what causes it?, cont.
Pressure maps
Contours of constant pressure (usually 4 mb between contours
Close spacing means stronger winds
In N.H., winds circulate around a low pressure region in CCW direction
Wind barbs show direction (from circle out toward barb tip). Speed by number and size of barbs, where whole barbs are 10 knots.

8. Weather Processor Symbols
WXP Legend

9. Review Question 1 What causes wind? Air pressure
Weight of the atmosphere
Pressure difference in atmosphere
Low pressure
High pressure

10. Review Question 2 What are the units of pressure? Force/Area
Pascals (Pa)
Pounds per square inch (psi)
Millirads
B and C

11. Wind Energy and Power
Atmospheric pressure differences accelerate and impart kinetic energy into the air
Wind energy conversion machines (WEC) convert wind energy into electrical or mechanical forms
How much power can we extract?

12. Wind Power - Example Example: V = 10 m/s A = (2 m)2 = 4 m2 = 1.2 kg/m3
10 m/s = mi/hr

13. Wind Power – Example, cont.
Theoretical Maximum
Betz Limit: 59.3% of the theoretical is the maximum amount extractable by a wind energy conversion device (WEC)
Practical Maximum

14. Wind Turbine Size-Power Comparison

15. Wind Turbine Configurations
HAWT
VAWT
Boyle, G., Renewable Energy, 2nd ed., Oxford University Press, 2004

16. Configuration Tradeoffs
Factors
Efficiency
Power produced per unit cost
Directionality
Support configuration
Speed of rotation
Reliability
Cost
Maintainability
Which type is best, HAWT or VAWT?

17. Common HAWT Construction
Rotor
Blades are connected to a hub, which is connected to a shaft
Rotational speed will depend on blade geometry, number of blades, and wind speed (40 to 400 revolutions per minute typical speed range)
Gear box needed to increase speed to RPM for generator

18. Aerodynamics of Wind Turbine Blades
Forces are transmitted from a moving fluid to an object in the flow stream
Lift = the force component perpendicular to the original flow direction
Drag = the force component in line with the original flow direction
Lift
Drag

19. Two Types of Turbine Designs
Drag Designs
Savonius
Lift Designs
VAWT Darrieus
Most HAWT designs

20. Aerodynamics of HAWT Blade
r[L(sinf) - D(cosf)] = Torque
Torque x rotational speed= Power
Boyle, G., Renewable Energy, 2nd ed., Oxford University Press, 2004

21. Aerodynamics of HAWT Blade, cont.
Angle of attack, a (blade angle between chord and relative wind direction)
Has a large effect on the lift and drag
Typical values between 1 and 15 degrees (what is optimum?)

22. Design of HAWT Turbine Blade
Blade size and shape
5-station design as seen from the tip
The blade twists to keep angle of attack constant

23. Design of HAWT Turbine Blade, cont.
Number of blades
Increasing the number of blades tends to increase the aerodynamic efficiency
Increasing the number of blades increases the cost (material and manufacturing
Turbines with fewer blades tend to run most efficiently at lower tip speed ratios (ratio of tip speed to wind speed)