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“When the wind blows, a pocket of low pressure air forms, pulling the blade toward it, causing the rotor to turn. This is called lift. The “When the wind blows, a pocket of low pressure air forms, pulling the blade toward it, causing the rotor to turn. This is called lift. 

 The turbines that use a new, low wind speed technology. The sweep twist adaptive blade automatically twists to reduce stress on the turbine from the mechanical load produced during high wind. 

Today, a standard HAWT (Horizontal Axis Wind Turbine) captures the wind’s energy with three propeller-like blades mounted on a rotor. The turbine sits more than 200 feet atop a tower to take advantage of stronger and less-turbulent wind. Elevated so high above the surface makes maintenance and repair costly; bearings and shafts that transfer wind power from the turbine blades must be to withstand exceptional rigors in various weather conditions.

 Newer HAWTs can make use of wind at lower velocities; such advanced design has meant the addition of utility scale wind turbines in less windy (Class Four*) areas.

 *Note: Wind power classes designate a range of mean wind power density of approximate wind speed at specified heights above the ground. Areas designated as “Excellent” have wind power Class Five or greater, and are suitable for utility-scale wind energy applications. These areas represent regions where the wind power density exceeds 500 watts/m2 at 50m above the ground. Areas designated as “Good” have wind power at four or better, and are also suitable for utility scale wind energy applications. 

Nevertheless, these advanced designs can have increased stress at higher velocities requiring the turbine to be stopped or the drive train disengaged. The innovative wind blade, which Knight & Carver’s Wind Blade Division has developed, can operate over a wider range of wind speed thereby increasing the amount of energy that a low wind speed turbine can produce.

 Sized at 27.2 (85 ft) meters x 2.4 meters (7 ft),

the Adaptive Sweep Twist Blade is designed both for maximum efficiency at lower-speed wind conditions and to automatically adjust to higher wind gusts when necessary.

 In partnership with the U.S. Department of Energy, the University of California at Davis and Sandia National Laboratory Knight & Carver designed, fabricated and field tested this innovative component as part of the Low Wind Speed Technology Project. The goal of the project is to reduce wind-powered electricity generation costs at low-speed sites, and to open new areas of wind production by utilizing next-generation configurations, designs and concepts. The DoE wind technology program supports public-private partnerships for multiple large wind systems (turbines over 100 kilowatts). The program has a goal of achieving costs of 3 cents/kWh in class 4 wind regions by 2012. 

 

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Betz Limits

Albert Betz was a german physicist who in 1919 concluded that no wind turbine can convert more than 16/27 (59.3%) of the kinetic energy of the wind into mechanical energy turning a rotor. To this day this is known as the Betz Limit or Betz' Law. This limit has nothing to do with inefficiencies in the generator, but in the very nature of wind turbines themselves.

Wind turbines extract energy by slowing down the wind. For a wind turbine to be 100% efficient it would need to stop 100% of the wind - but then the rotor would have to be a solid disk and it would not turn and no kinetic energy would be converted. On the other extreme, if you had a wind turbine with just one rotor blade, most of the wind passing through the area swept by the turbine blade would miss the blade completely and so the kinetic energy would be kept by the wind.

Real World Wind Turbine Power Efficiencies

The theoretical maximum power efficiency of any design of wind turbine is 0.59 (i.e. no more than 59% of the energy carried by the wind can be extracted by a wind turbine). Once you also factor in the engineering requirements of a wind turbine - strength and durability in particular - the real world limit is well below the Betz Limit with values of 0.35-0.45 common even in the best designed wind turbines. By the time you take into account other inefficiencies in a complete wind turbine system - e.g. the generator, bearings, power transmission and so on - only 10-30% of the power of the wind is ever actually converted into usable electricity. (see the graphic above from the Iowa Energy Center, USA.)

Horizontal axis wind turbines (HAWT) theoretically have higher power efficiencies than vertical axis wind turbines (VAWT) however wind direction is not important for a VAWT and so no time (and power) is wasted chasing the wind. In turbulent conditions with rapid changes in wind direction more electricity will be generated by a VAWT despite 

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copyright 2007 Centre for Application of Renewable Energy 
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