The French aerospace lab, ONERA, has released information regarding their ongoing studies on the blades of wind turbines, with the objective being to make more effective and bigger turbines possible. Their expertise with helicopter rotors and blades position them ideally to undertake a thorough analysis of air flow running over the turbines.
With the monsterous size of many of the new turbines going up - as much as 6 MW - the wind industry is meeting some big challenges. A typical 5 MW unit can weigh between 400-500 tons, while each blade alone is about 20 tons. Being that the blades are generally built from fiberglass mixed with polyester resins or epoxy, they are fairly stiff and the stresses on them can be enormous, fatiguing and degrading them more quickly than we would like. ONERA believes that making the blades more supple, bendable, would relieve some of these stresses and even make the turbine more efficient.
Through specially designed helicopter modeling software, which they modified to deal with wind turbine specs, they calculated the characteristics of the blade composites to measure the response in elasticity. It also permitted them to see the wake of the blades. The main difficulty was to model the limiting layer, a very thin layer of air which makes contact with the blades and does what they call "unhooking", failing to transfer a large part of its energy to the blade.
"There are constant unhookings of the limiting layer in wind turbines. When the blades start, it's because of the sections closest to the rotor. Once the wind picks up, these sections don't work any more because the air is unhooking, and it's the ends of the blades that are doing the work. Thus, only 30-40% of the blade is working to transfer energy during normal operation," says one of the researchers.
Pitched blades, which represent 95% of the current market, help increase the efficiency over fixed blades, but there's still a lot of room for improvement.
Perfecting more supple blades that can bend will do some of the mechanical work themselves while unworking the deformations caused by the wind. Tests continue to find the optimum levels, but hopefully in a few years we'll see turbines that are 30% more efficient.