Vestas Wind Systems A/S (Randers SV, DK) in U.S. Patent Application 20100134946 details how carbon nanotubes can be used to create an improved lightning strike protection system for wind turbines.
Inventors Srikanth Narasimalu, Yoshiki Haraguchi and Erwin Merijn Wouterson, all based in Singapore, developed a system, method and software for protecting wind turbines from lightning strikes. They also developed a wind turbine comprising the system for lightning protection in which at least one lightning strike protection element of the lightning strike protection (LSP) system is ionized prior to a lightning strike, thus reducing the chance of damaging unprotected parts of the blades of the wind turbine.
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A study published in 2002 by the National Renewable Energy Laboratory (B. McNiff, Wind Turbine Lightning Protection Project 1999-2001, National Renewable Energy Laboratory (NREL), U.S. Dept. of Energy, Golden, Colo., USA) states that about 8 out of every 100 wind turbines could be expected to receive one direct lightning strike every year.
According to the statistics quoted in IEC TR 61400-24, 7-10% of damages caused by lightning was to the rotor blades (IEC TR 61400-24, Wind Turbine Generator Systems, Part 24: Lightning Protection, International Electrotechnical Commission, First edition 2002-07). The blades of a wind turbine are most likely to be damaged when struck by lightning due to the presence of fiber glass which exhibits a highly resistive behavior. Even carbon fiber reinforced epoxies are not capable of conducting away the large amounts of current and heat from a single bolt of lightning. The wind turbine blades account for 15-20% of the cost of a wind turbine and hence damage thereto is very costly.
Wind turbines are prone to being struck by lightning because they are often erected in areas which are often vast, flat terrain coinciding with areas of thunderstorm activity.
When a lightning bolt strikes an unprotected structure, a current up to about 200 kA seeks the path of least resistance. Direct effects typically include vaporization of resin in the immediate strike area, with possible burn-through of the laminate. Indirect effects occur when magnetic fields and electrical potential differences in the structure induce transient voltages, which can damage and even destroy onboard electronics that have not been EMF (electromagnetic field) shielded or lightning protected. The need for protection of composite structures has prompted the development of a number of specialized LSP (lightning strike protection) materials.
Effective protection against lightning is crucial for ensuring the structural integrity of the blades, lifespan, safety and efficiency of the wind turbine. Common lightning strike protection (LSP) technology requires the use of a number of receptors and an inner down conductor inside the blade to conduct the charge away to the tower. This method is effective for blades with a length between about 20-30 meters. When lightning hits a blade of above about 40 m in length, the arc might either penetrate the surface and proceed towards the inner lightning conductor, or move along the surface to the nearest receptor. The penetration is critical and usually implies a delamination, while the arc movement along the surface may leave physical erosion referred to as tracking.
Independently and separately, it has been reported by Lightning Eliminators & Consultants, Inc. that ionization of air in their lightning strike collectors comprising ion plasma generators, improves the attraction of lightning strikes (http://lecglobal.com/solutions/lightning/protection/ion-plasma-generator- /).
As wind turbines continue to increase in size, there is a corresponding increase in the risks of being struck by lightning, which requires the development of an improved lightning protection system for wind turbines.
Hence, an improved and more efficient system, method and software for protecting wind turbines from lightning strikes, as well as an improved wind turbine incorporating the improved system for lightning protection, would be advantageous.
Wind turbines are prone to being struck by lightning because they are often erected in areas which are often vast, flat terrain coinciding with areas of thunderstorm activity.
When a lightning bolt strikes an unprotected structure, a current up to about 200 kA seeks the path of least resistance. Direct effects typically include vaporization of resin in the immediate strike area, with possible burn-through of the laminate. Indirect effects occur when magnetic fields and electrical potential differences in the structure induce transient voltages, which can damage and even destroy onboard electronics that have not been EMF (electromagnetic field) shielded or lightning protected. The need for protection of composite structures has prompted the development of a number of specialized LSP (lightning strike protection) materials.
Effective protection against lightning is crucial for ensuring the structural integrity of the blades, lifespan, safety and efficiency of the wind turbine. Common lightning strike protection (LSP) technology requires the use of a number of receptors and an inner down conductor inside the blade to conduct the charge away to the tower. This method is effective for blades with a length between about 20-30 meters. When lightning hits a blade of above about 40 m in length, the arc might either penetrate the surface and proceed towards the inner lightning conductor, or move along the surface to the nearest receptor. The penetration is critical and usually implies a delamination, while the arc movement along the surface may leave physical erosion referred to as tracking.
Independently and separately, it has been reported by Lightning Eliminators & Consultants, Inc. that ionization of air in their lightning strike collectors comprising ion plasma generators, improves the attraction of lightning strikes (http://lecglobal.com/solutions/lightning/protection/ion-plasma-generator- /).
As wind turbines continue to increase in size, there is a corresponding increase in the risks of being struck by lightning, which requires the development of an improved lightning protection system for wind turbines.
Hence, an improved and more efficient system, method and software for protecting wind turbines from lightning strikes, as well as an improved wind turbine incorporating the improved system for lightning protection, would be advantageous.
According to the inventors, in an improved lightning protection or LSP system for wind turbines, the metallic materials or particles in the lightning strike protection elements or diverter strips are replaced by materials comprising a higher conductivity compared to existing LSP materials. Candidate materials include single wall and/or multi-wall carbon nanotubes or buckypaper. The advantages are as follows:
Materials with a higher conductivity such as carbon nantoubes than the existing LSP materials are expected to improve attraction and guidance of lightning strike charges.
The thickness of a strip made of a material exhibiting higher conductivity is expected to be thinner compared to existing metallic diverter strips, minimizing the disruption of the aerodynamic flow around the blade profile. The thickness of the strip will probably be in the order of about 100 microns. However, these strips are not limited to these dimensions.
No change in the current production process or blade design is required. The diverter strips can be applied onto ready-made wind turbine blades.
New patterns and shapes of carbon nanotube diverter strips can be created using conventional methods for growing carbon nanotubes.
Materials with a higher conductivity such as carbon nantoubes than the existing LSP materials are expected to improve attraction and guidance of lightning strike charges.
The thickness of a strip made of a material exhibiting higher conductivity is expected to be thinner compared to existing metallic diverter strips, minimizing the disruption of the aerodynamic flow around the blade profile. The thickness of the strip will probably be in the order of about 100 microns. However, these strips are not limited to these dimensions.
No change in the current production process or blade design is required. The diverter strips can be applied onto ready-made wind turbine blades.
New patterns and shapes of carbon nanotube diverter strips can be created using conventional methods for growing carbon nanotubes.
The improved conductivity of the carbon nanotube LSP system is expected to increase the attraction of the lightning strike, thus preventing and/or reducing the chance of unprotected parts of the blade from being hit by lightning. Additional advantage of the materials like carbon nanotubes is that the density is far less compared to that of copper, which will result in weight savings compared to the existing LSP systems.
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