Blades are the leading edge of a wind turbine so their aerodynamic surfaces carry critical shapes in a fluid (air) that seems bent on removing the shape. Blade tips often hit 150 mph, so it’s no surprise that sand, rain, insects, and hail wear and damage their surfaces. Trends in blade maintenance show up in three areas: damage detection, repair, and improvement.
“To spot initial damage, we suggest visually examining about 10% of a fleet to get a statistical handle on the possible blade problems,” says Rope Partner director of R&D Chris Bley. “If that finds significant damage, then it’s a good time for technicians on ropes to inspect the blades more closely. After establishing a baseline and at least one follow on inspections, it’s possible to calculate trending, an indicator of how the blades are performing for particular locations.” One trend is to conduct such inspections before turbines exit warranty.
“Over the last decade, we’ve seen damage at sites across the globe due to leading-edge erosion when conducting inspections for wind turbine maintenance and repairs,” adds Bley. “At some sites, significant erosion occurred in a little as two years after installation.”
Should a closer inspection become necessary, technicians can use optical devices to evaluate the damage. Ultrasonic units have been useful in these tasks but require applying a jelly to the blade surface and examining small areas–a few inches square–at a time. Recently, a more promising method uses light for a more in-depth diagnosis, over a larger area without contacting the blade.
The optical inspection method, shearography, has had widespread use in the aerospace and marine industries. “It’s laser based, on holography, so it allows measuring small subsurface deformations,” says Dantec Dynamics’ Matt Crompton. The defects could result from manufacturing, or lightning, or from weathering. The unit works fast taking roughly 30 sec to sample an area about 1 ft2, and it’s highly repeatable. The device works well on glass or carbon-fiber constructions.
The equipment works by subjecting a composite surface to a slight temperature rise, a stressing technique of 1 or 2 Celsius degrees, and then measuring changes in laser light to indicate subsurface fiber movements. The unit quickly detects imperfections, flaws, or weaknesses just microns below the surface. There is no surface preparation required, and 150 ft2 can be scanned in an hour. It’s used in field and up tower for instant results,” he says. A structural engineer will have to decide how to treat the problem. The identified flaw could lead to cracking and ultimately blade failure.
Additional trends include using data to predict which other blades from a lot might have a particular flaw. The idea: Early detection allows better and less costly repairs.
Because the best defense is a good offense, Bley suggests preventing blade wear by placing a durable tape or coating over its leading edge. 3M developed has developed one such tape that is tough enough to pierce with a scissor point. Originally intended for the leading edge of helicopter rotors, it works well on wind turbine rotors.
More recently, other formulators have developed gel coatings that roll on like paint. One service company says it is easier to apply to a leading edge than the tape, especially when working from a suspended platform. Watch for the coating to be applied at the blade factory as asset owner pressure OEMs for improved reliability.
Studies in wind tunnels or CFD simulations of proposed blade attachments can better reveal blade aerodynamics. The tactic is to look at a blade in two configurations, one is smooth without flow interruptions and the other with a flow disruptor on the surface–something to induce a separation or turbulence of flow around the blade.
One concept for restoring performance to wearing blades comes from the aircraft industry. Vortex generators (VGs), used since the 1940s, are visible on modern passenger-aircraft wings. They appear as small rectangles placed along the wing’s crest. When used on a wind turbine blade, VGs generally provide greater output for the turbine.
Vortex generators have been applied to wind turbines several times over the last 20 years with varying degrees of success. One area where they’ve been helpful is in combatting flow separation—when air moving smoothly along the airfoil, separates at so it’s no longer attached and predictable. The VG sort of steers the low momentum air away from the blade and brings high momentum air back down to the surface which re-energizes the flow and reduces flow separation. This can gain back some rotor performance losses caused by flow separation. But it is important to look at the effects on turbine loads and performance before adding VGs. WPE
Filed Under: Blades