Wind speed and Wind Energy

Understanding wind speed is critical to working with modern residential wind turbines. Let's begin by looking at some of the common terms associated with wind speed.

Wind Terminology

• Start-up Speed - This is the speed at which the rotor and blade assembly begins to rotate.
• Cut-in Speed - Cut-in speed is the minimum wind speed at which the wind turbine will generate usable power. This wind speed is typically between 7 and 10 mph for most turbines.
• Rated Speed - The rated speed is the minimum wind speed at which the wind turbine will generate its designated rated power. For example, a "10 kilowatt" wind turbine may not generate 10 kilowatts until wind speeds reach 25 mph. Rated speed for most machines is in the range of 25 to 35 mph. At wind speeds between cut-in and rated, the power output from a wind turbine increases as the wind increases. The output of most machines levels off above the rated speed. Most manufacturers provide graphs, called "power curves," showing how their wind turbine output varies with wind speed.
• Cut-out Speed - At very high wind speeds, typically between 45 and 80 mph, most wind turbines cease power generation and shut down. The wind speed at which shut down occurs is called the cut-out speed, or sometimes the furling speed. Having a cut-out speed is a safety feature which protects the wind turbine from damage. Shut down may occur in one of several ways. In some machines an automatic brake is activated by a wind speed sensor. Some machines twist or "pitch" the blades to spill the wind. Still others use "spoilers," drag flaps mounted on the blades or the hub which are automatically activated by high rotor rpm's, or mechanically activated by a spring loaded device which turns the machine sideways to the wind stream. Normal wind turbine operation usually resumes when the wind drops back to a safe level.

Wind Speed Facts

One of the key things to know about wind speed is that the amount of energy which wind can generate is not a one to one function.  Rather energy increases by the cube of the wind speed.  If you double the wind speed, you get eight times the energy. That is one reason that looking at wind maps is so useful.  Even a small difference in wind speed within a given area can have a big impact on the amount of energy a wind turbine can generate. It is also one of the reasons why a taller wind tower can make so much of a difference.  If the wind speed increases even a few miles an hour by going with a taller tower the energy generation potential goes way up (see sidebar chart).

One way to get a sense of the amount of energy a wind turbine will produce at different speeds is look at a power curve graph.  Most wind turbine manufacturers will show the power curve for their particular turbines.  This type of chart shows the power output (usually in watts) on one axis and the wind speed on the other. The chart below shows power curve for an Air-X wind turbine. It should be noted that the sudden drop off at above 30 mph is caused by a safety cut off.

Betz Limit

It is the flow of air over the blades and through the rotor area that makes a wind turbine function. The wind turbine extracts energy by slowing the wind down. The theoretical maximum amount of energy in the wind that can be collected by a wind turbine's rotor is approximately 59%. This value is known as the Betz limit. If the blades were 100% efficient, a wind turbine would not work because the air, having given up all its energy, would entirely stop. In practice, the collection efficiency of a rotor is not as high as 59%. A more typical efficiency is 35% to 45%. A complete wind energy system, including rotor, transmission, generator, storage and other devices, which all have less than perfect efficiencies, will (depending on the model) deliver between 10% and 30% of the original energy available in the wind.