Wind Turbines - Wind Energy: Renewable Energy and the Environment - page 80

Lift

Drag

Blade path

Attack angle

Chord

Wind

figure 5.11 Forces on blade, lift and drag, due to airflow of relative wind.

The torque-rpm relationship also explains why drag devices are not used to produce electricity.
Drag devices have larger torque; however, the small rpm means the amount of power is low. Too
many inventors of drag devices equate torque with power.
From conservation of energy and momentum, the maximum theoretical efficiency for the cap-
ture of wind power and wind energy is 59%. Highest experimental efficiencies for wind energy
conversion systems are around 50%, from wind to electricity.
Lift and drag forces are measured experimentally in a wind tunnel for airfoils as a function
of the attack angle , the angle of the relative wind to the chord of the airfoil (Figure 5.11). Lift is
perpendicular and drag is parallel to the relative wind. The horizontal component of the lift on the
blades, which depends on the angle of attack, makes the rotor turn about the axis (Figure 5.12).

Drag

Lift

Drag

Lift
Windspeed
due to motion
of blade

Relative
wind

Ground
wind

Nacelle

Ta il

Blade

Ground
wind

Tower

figure 5.12 Wind produces forces on the blade. Relative wind (wind the blade sees) is the vector sum of the
blade speed plus the ground wind. Rotor is perpendicular to ground wind.