Why is the swept area of the blades an important factor in choosing a turbine?

Swept area is a term used to describe the imaginary circular area within the blade tips of a turbine as they rotate. It is probably the most important specification to reference when considering a turbine. If the blades are efficient it can be a clearer indicator of the likely output of a turbine than the manufacturer's rating.

Swept area (A) = π x (R)2 where R is the rotor radius and π = 3.14.(I)

For example a turbine with a rotor diameter of 6m (i.e. radius of 3m) has a swept area of 28.26m2 because:

A = π x (3)2 = π x 9 = 28.26m2 (II)

A larger swept area has more air passing through the plane of the turbine and so greater is the energy that can be harvested. The ability of a turbine to avail of the energy in an air stream is approximately proportional to the diameter of the blades.

The maximum available power in the swept area of a turbine rotor is found using the following formula:

Power = ½ ρx swept area x V3 (III)

Where ρis the air density, V is the wind speed.

If air density and wind speed are made constant it can be seen that the theoretical power available in an air stream is proportional to the swept area. If this area doubles the power available doubles.

It is worth noting that the power captured by the turbine can not be calculated by using the above formula (III). The formula shows how much power is available but the actual power captured is governed by Betz's law, the efficiency of the blades and the efficiency of the generator and electrical conversions. The above formula is shown to illustrate the relationship between power, swept area and wind speed. From the formula it can also be seen that if the wind speed doubles the power output is multiplied by 8! Demonstrating clearly why access to a high average wind speed is so important.