Wind Power

Wind Power: Renewable Energy for Home, Farm, and Business
by Paul Gipe

Main Points:

• Wind power ultimately depends upon three key things: wind velocity, rotor length, and air density. Due to the lapse rate, temperature drops by 6.5 degrees Celsius for every kilometer of elevation thus making the pressure decrease, and hence why the air is less dense at higher elevations. The wind velocities mainly an exponential function of height, where the exponent is the shear factor and is 0.14 (1/7) for standard applications, higher for rougher terrain, and lower for smoother terrain. Normally if height increases by 5x, then the power increases by 2x- this is the case behind the company Makani Power (see below). Rotor length is the key consideration as this controls the swept area of the wind turbine. Power is proportional to the square of the turbine blade length, hence why 0.5m turbine blades produce 100 Watts, while 20 meter long blades can produce 3 megawatts.
• The two most common types of wind turbines are horizontal axis wind turbines (HAWT) and vertical axis wind turbines (VAWT). Important consideration is the lift to drag ratio. Any airfoil has a lift component and a drag component, and by maximizing the lift/drag ratio, you receive optimum performance. Turbines that depend on lift can get 50x more power than those dependent on drag. Vertical axis wind turbines benefit from the fact that they receive wind energy from all directions, however centrifugal forces through the blade cause poor reliability and deformation. Horizontal axis wind turbines can change in the direction of the wind if connected to tail vanes, but are the generally accepted type of wind turbine.
• Denmark leads the world in wind energy development through 3 interesting investment vehicles. (1) Mutual ownership (fellesmolle) is a where a wind coop formed as an LLC pool money from several individuals to own a wind turbine. This only works if there is a fixed price for electricity or they receive payments for CO2 tax. (2) Cooperative net billing is basically net metering- where energy generated is subtracted from the retail rate of electricity in one's monthly bill. (3) Buying clubs- pools of money from individuals to buy small wind turbines and negotiate pricing with utilities when they sell generated electricity back to them. 

For an exciting lecture on how wind turbines work, why they are designed the way they are, and the physics behind wind power listen to F. John Hay from the University of Nebraska! 

Listen to Saul Griffith, founder of Makani Power, talk about the history of kites and using a tethered kite wind power project that can produce more energy than conventional turbine!

For an awesome derivation of Betz's Law and the maximum theoretical wind turbine efficiency of 59.3% , watch this video!