CALCOLO DELLA POTENZA ELETTRICA IN FUNZIONE DELL'ENERGIA DEL VENTO DISPONIBILE / CALCULATE THE AMOUNT OF POWER AVAILABLE AT A GIVEN WIND SPEED How Can I Calculate the Amount of Power Available at a Given Wind Speed? Contributed By Eric Eggleston, 5 February 1998 Because air has mass and it moves to form wind, it has kinetic energy. You may remember from science class that: kinetic energy (joules) = 0.5 x m x V2 where: m = mass (kg) (1 kg = 2.2 pounds) V = velocity (meters/second) (meter = 3.281 feet = 39.37 inches) Usually, we're more interested in power (which changes moment to moment) than energy. Since energy = power x time and density is a more convenient way to express the mass of flowing air, the kinetic energy equation can be converted into a flow equation: Power in the area swept by the wind turbine rotor: P = 0.5 x rho x A x V3 where: P = power in watts (746 watts = 1 hp) (1,000 watts = 1 kilowatt) rho = air density (about 1.225 kg/m3 at sea level, less higher up) A = rotor swept area, exposed to the wind (m2) V = wind speed in meters/sec (20 mph = 9 m/s) (mph/2.24 = m/s) This yields the power in a free flowing stream of wind. Of course, it is impossible to extract all the power from the wind because some flow must be maintained through the rotor (otherwise a brick wall would be a 100% efficient wind power extractor). So, we need to include some additional terms to get a practical equation for a wind turbine. Wind Turbine Power: P = 0.5 x rho x A x Cp x V3 x Ng x Nb where: P = power in watts (746 watts = 1 hp) (1,000 watts = 1 kilowatt) rho = air density (about 1.225 kg/m3 at sea level, less higher up) A = rotor swept area, exposed to the wind (m2) Cp = Coefficient of performance (.59 {Betz limit} is the maximum thoretically possible, .35 for a good design) V = wind speed in meters/sec (20 mph = 9 m/s) Ng = generator efficiency (50% for car alternator, 80% or possibly more for a permanent magnet generator or grid-connected induction generator) Nb = gearbox/bearings efficiency (depends, could be as high as 95% if good) If there is any single equation that the beginning wind enthusiast should memorize, this is it. IGET SS 524 N 21 66030 MOZZAGROGNA  CHIETI  ITAIA +39 0872 57123  e-mail iget@igetcol.it | Back |