The earliest record of man using the wind to his advantage is of the Egyptian sailors from around 3500 BC.  Since the 7th century AD, man has harnessed the wind to grind grain, pump water, and more recently to generate electricity.  But how does the energy in the wind get captured then converted to electrical energy?  The process is both straightforward and complicated.    

The energy that is contained in the wind is dependent on three factors:  the density of the air, the cross-sectional area the wind blows through, and most importantly the speed (or velocity) of the wind.  As each one of these factors increases, so does the energy stored in the wind.  The reason that the wind velocity is the most important factor is because the energy increases as a cube of the wind velocity, meaning that if the wind speed doubles then the energy increases by a factor of eight.

In order to capture this energy and convert it to electrical energy, one needs to have a device that is capable of ‘touching’ the wind.  This device, or turbine, is usually composed of three major parts:  the ‘blades’, the drivetrain, and the generator.  The blades are the part of the turbine that touches the wind and rotates about an axis.  The drivetrain connects the rotating blades to the generator, and the generator converts the mechanical energy into electrical energy.  The blades can be based on a lift principle (similar to aircraft wings) or a drag principle (similar to a boat sail).  Traditional 3-bladed propeller-type wind turbines are lift machines, for instance.  The drivetrain exists so that the mechanical energy created by the blades can be transferred to the generator.  Sometimes the drivetrain can be used to increase the speed of rotation, dependent on the generator that is used.  The generator then passes wire through an electromagnetic field, which creates current flow.  This current is then converted to a form that is usable by everyday electrical devices.

The design of the turbine determines how much of the energy from the wind can be extracted.  Albert Betz, a German physicist, determined that the most energy that any device can extract is 59% of the total energy contained in the wind.  He derived an equation called the Betz Law that tells how much of the energy from the wind that can be extracted.  Based on this equation, wind turbine designers can only manipulate two factors:  the efficiency of the turbine blades (called the Coefficient of Performance, or Cp) and the wind-swept area (or cross-sectional area).  The higher these values are, the more energy that can be extracted from the wind.  Of course, there will be mechanical and electrical losses due to friction and heat, and the designer should work to minimize those values.