Aerofoils, the cross-section of wind turbine blades, form the basis of turbine blade design. Generating lift and drag during movement through the air, aerofoils play a key role in improving the aerodynamics and structural durability of turbine blades.
Aerofoils have undergone significant evolution since the early days of the wind power industry. In the 1970s, designers chose blade shapes from a library of standard pre-World War II aerofoils designed for aircraft wings by the National Advisory Committee for Aeronautics, the predecessor to NASA.
Initially, wind turbine blade designers focused on key features such as twist and taper to optimize aerodynamics, increasing speed and efficiency while reducing drag. However, in the 1980s and 1990s, engineers discovered that this was not enough, and deficiencies in operational efficiency could be linked to the performance of aerofoils.
For example, engineers noticed that the surface of the blade’s leading edge (the front edge that first comes into contact with the air) became rougher over time. This was due to contamination, resulting from the accumulation of dirt and insects (much like a car windshield without windshield wipers), as well as minor damage from general wear and tear. In the 1980s, at early wind farms in California (Altamont Pass and Palm Springs), contaminants caused energy production to drop by as much as 30%, until technicians washed the blades, a significant workload during windy and therefore more profitable seasons.
Wind gusts exerted additional pressure on the shovels, leading to loads that exceeded predicted values. This caused the blades to stop or slow down their rotation, which in turn reduced the efficiency and energy production of the wind turbine.
It became clear to researchers at the National Renewable Energy Laboratory (NREL, formerly known as the Solar Energy Research Institute) that achieving better and more reliable performance required the development of new aerodynamic profiles tailored specifically for wind turbines. In 1984, with funding from the U.S. Department of Energy (DOE), NREL researchers partnered with Airfoils Inc. a small company specializing in aerofoil design, to help solve these problems.
History of airfoils.
1975 Shared designs.
1970s: Designers selected wind turbine blade shapes from a library of standard pre-World War II aerofoils designed for aircraft wings.
1982: A new vision for aerofoils.
1980s: Researchers conclude that achieving better and more reliable wind turbine performance requires the development of new aerofoils tailored specifically for these applications.
1984: NREL research begins.
NREL researchers team up with Airfoils Inc. to reduce the effects of leading edge contaminants on the performance of aerofoils and improve their aerodynamic properties.
1991 Award-winning research.
NREL receives an R&D 100 award for its work on aerofoils for wind turbines.
1994 – 1995 NREL patents.
NREL obtains patents in the United States and Europe for its aerofoils, leading to 12 commercial licenses between industry companies and NREL.
1998 Fundamental publication.
NREL publishes “Advanced Airfoils for Wind Turbines,” showcasing the families of aerofoils developed.
2023 Continued progress.
NREL’s aerofoil families continue to be used commercially. Their basic designs have contributed to many other aerofoil families.
Changing the shape of blades has improved the efficiency of wind turbines.
DOE-supported design efforts were aimed at:
- Reduce the impact of leading edge debris on aerofoil performance.
- Improve the aerodynamic performance of aerofoils.
NREL and Airfoils Inc. used both experimental and computational methods in the design process. The experimental method made it possible to verify the new airfoil designs in clean and contaminated versions. Tests in a low-turbulence wind tunnel at the University of Delphi in the Netherlands showed which models needed tweaking. Digitally, the researchers created two-dimensional and three-dimensional aerofoil designs to simulate potential functional improvements.
Experimental measurements and design work indicated to the researchers that:
- The leading edges of aerofoils can be adjusted to be less sensitive to debris.
- The parts of the blades closer to the ends generate most of the power. At these locations, aerofoils should be as thin as possible to increase aerodynamic efficiency and fouling resistance.
- Improving the design near the blade tip improves turbine performance in the face of fouling and roughness.
These findings have unequivocally shown that new, customized aerofoils are needed for each section along the wind turbine blade.
All in the family of aerofoils.
The result of this research was the first “aerofoil family” for wind turbines. These families prioritized aspects of aerofoil design to meet aerodynamic and structural requirements.
With DOE funding, a total of seven aerofoil families were created, which were revised and published in a series of reports and patents in the mid-1990s. Together, they set the standard for efficiency in wind turbine blade design.
A family of thin aerofoils for medium-sized blades.
A family of thick aerofoils for large blades.
Two examples of “families” of aerodynamic profiles developed by NREL, tailored to the aerodynamics and needs of wind turbines. Source: NREL. 1998. Revised 2000 Advanced Airfoils for Wind Turbines.
Since their introduction in the early 1990s, these DOE-developed aerofoils have set the global standard for efficiency in turbine blade design.
The standardized designs have brought awards.
The broader wind power industry quickly recognized the benefits of the families of aerofoils developed by NREL. Contamination no longer affected aerodynamic performance, and load requirements were better matched. Annual energy production increased by 10%-35% compared to standard aerofoils from the 1970s.
These achievements earned the project the prestigious R&D World R&D 100 award in 1991.
NREL also obtained five patents in the United States for these aerofoils, as well as numerous corresponding European patents. These patents have led to 12 commercial licenses between companies in the industry and NREL.
Aerofoil families still in use.
Although NREL aerofoil families designed in the early 1990s are still in commercial use with active industrial licenses, advances in aerodynamic modeling, optimization methods and other computer design techniques have led to the development of new aerofoil families.
These newer aerofoils have benefited from knowledge gained through the DOE-funded NREL research program. Overall, DOE’s investment in aerofoils tailored to wind turbines has significantly improved their performance and reduced wind energy costs across the industry.
Sources:
https://www.energy.gov/eere/wind/articles/airfoils-where-turbine-meets-windÂ
https://pl.wikipedia.org/wiki/Turbina_wiatrowa
https://pl.wikipedia.org/wiki/Elektrownia_wiatrowa
https://pl.wikipedia.org/wiki/Energia_wiatru