Physics and Engineering Questions of Importance in the Wind Energy Field  How much power is in the wind?  How much of this power can we extract?  How do we design devices to extract this energy?  What designs are already available, and what are their strengths and weaknesses?  What additional equipment do we need?  Where should we put these devices?  Can they ever pay for themselves? .....?

Total Wind Power  This power is measured in Watts = N-m/sec = Voltage-Amps  The power in the wind increases with the cube of the velocity!  It also increases linearly with the capture area, A.  Air Density, , doesn’t vary much in a fixed location, but it may vary several percent with altitude. –The air density in Flagstaff is only about 80% of that at sea level. –See next chart ρ = Air Density (Kg/m 3 ) A = Cross Section Area (m 2 ) V = Wind velocity (m/sec)

Air Density in kg/m 3 Z Z Z Z Z Elevation in meters Temp in ºC

Efficiency of your Wind Energy Device  Efficiency is the Output Power/Input Power

How much of the Total Wind Power can actually be converted to Mechanical Power?  We applied the 1 st law of thermodynamics – conservation of energy to find Wind Power.  We also must apply conservation of mass and conservation of momentum (Newton’s 2 nd law) to the system to determine how much of this power it is possible to retrieve.  After we do this we find….

Maximum Useable Power to the Power Contained in the Wind  Define a power Coefficient as:  This corresponds to 59.3%, or about 60%, and is an upper limit on wind turbine efficiency, usually called the Betz limit.