I've been dinking with the various formula for calculating the amount of power extractable from the wind. I've seen various folks post tables where they have a column for efficiency and was wondering what formula is used.
Windstuff Ed sometimes uses 0.00508 * Sa * V where 0.00508 is constant, Sa appears to be swept area (in square feet), and V is windspeed (in mph) cubed. The formula I've been using from the AWEA site is :
P = 0.5 * rho * A * Cp * V^3 * Ng * Nb
P = power in watts
rho = air density (1.225 kg/m^3 at sea level, about 1.0583 where I am in Colorado)
A = rotor swept area in square meters
Cp = Coefficient of blade performance (.59 {Betz limit}, .35 is good)
V = wind speed in m/s (mph/2.24 = m/s)
Ng = generator efficiency (50% for a car alternator, 80% or more for a good pmg)
Nb = gearbox/bearing efficiency (varies, 95% is good)
So it seems that the theoretical peak would be something like:
P = 0.5 * rho * A * .59 * V^3 * .80 * .95
So for a 1 meter machine in a 30 mph wind the limit would be on the order of
910 watts using the above. Using Ed's formula, it comes out around 970 watts so my guess is Ed's constant is assuming a different value for rho.
Is this what people use for calculating efficiency? Thanks