Principles of wind energy conversion

There are two primary physical principles by which energy can be extracted from the wind; these are through the creation of either drag or lift force (or through a combination of the two). Drag forces provide the most obvious means of propulsion, these being the forces felt by a person (or object) exposed to the wind. Lift forces are the most efficient means of propulsion but being more subtle than drag forces are not so well understood.

 

Figure – Illustration of drag and lift

The basic features that characterise lift and drag are:

• ·        drag is in the direction of airflow
• ·        lift is perpendicular to the direction of airflow
• ·        generation of lift always causes a certain amount of drag to be developed
• ·        with a good aerofoil, the lift produced can be more than thirty times greater than the drag
• ·        lift devices are generally more efficient than drag devices