Wind Power - Tech

 

Tip speed ratio = (2pi R N)/V

where: R=radius(ft) N=Rpm V=wind velocity (ft/sec.)

The tip speed ratio depends on the blade efficiency, blade area to swept area ratio, and operating conditions.

Power Equation:

P = 1/2 * A * q * V^3 * Cp

Where; P = Power A = Area q = Density of air V : Wind velocity Cp = Power coefficient

Assuming sea level air density, this can be rewritten as: P = K * A * V^3 * Cp

The value of K depends on the different units of measurement in use:

The power coefficient (Cp) is a non-dimensional number that reflects the amount of energy a given wind generator can extract from the wind.The theoretical maximum is 0.593 (with a 100% efficient generator etc!).

The power coeficient, or efficiency of any design is dependant on a number of factors, but some approximate figures for some common designs are as follows:

High Speed Propeller: 0.45 2 or 3 bladed, commonly used for electricity generation,
Savonius Rotor: 0.15 This is a vertical axis machine. Often made from large oil drums
Darrius Rotor: 0..35 A vertical axis machine, often referred to as the 'egg beater',
Dutch Four Arm: 0.17 A horizontal axis mill traditionally used for grinding grain and pumping water.
Sail Windmill: 0.3 Originally developed in the Eastern Mediterranean
American Multi-bladed Fan: 0.3 Used extensively in the West and Middle West.

A typical wind generator blade design (after 'LeJay' and Hugh Piggot)