The whirling blades are probably the most critical components on wind turbines, and engineers are developing ever larger ones as they pull more power from the prevailing winds. For example, studies show that a 10% increase in rotor area equates to a 12% increase in electrical generation. But there are challenges to building ever-bigger blades, and these problems are being tackled by companies spread across the globe. Here are some of the most recent developments.
Custom blades: At LM Wind Power, a Danish company that is one of the world’s leading blade manufacturers, customers are demanding blades designed for specific installations and power requirements. To make it easier to satisfy these demands, the company developed a flexible blade design, the GloBlade. This lets LM evaluate customer requests and then simply adjust the GloBlade’s parameters to meet them. Because all blades are based on the common design, it was relatively simple to ensure it can manufacture the blades at any of its plants located across the globe.
Going big: To stay ahead of blade makers, researchers at Sandia National Laboratory are scaling up their computer model of a blade, the SNL 100-00, to 100 m. This makes it the longest blade model in the world. Researchers quickly put the model to work in helping them explore problems of blade weight and what it would take to get such a large blade certified to standards such as the GL (Germanischer Lloyd Industrial Services GmbH )and IEC. The researchers first designed the blade as being made of all-glass fibers with no carbon and two0 shear webs for strength, They soon discovered they had to add a third web. So far, studies indicate the blade would have a long fatigue life but it could have a flutter problem or aerodynamic instability, a common issue with long blades.
Withstanding the weather: Icing on blades is a major issue for turbines in northern climes. Too much ice on the blades will prevent the rotor from even turning. And there have been many cases where substantial chunks of ice built up on blades has been thrown at least 800 ft when the blades were turning, a safety hazard.
At Nordex in Sweden, scientists are studying a monitoring system that detects and tracks ice build up. When the systems determines that a set level of ice has formed, a minimal amount of energy is tapped from the wind turbine’s production to heat only those portions of the blades with ice on them. In a test, an anti-icing turbine generated more energy than one without the new technology.
Another climate problem for blades is the constant wind, rain, snow, and blowing dust, and the erosion they cause, especially on the thin, leading edges. According to 3M, this is problem for wind turbines regardless of hub height, location, blade length, or manufacturer. 3M also found that erosion usually starts at the tip with damage to the paint or coating, and this damage alters the blade’s aerodynamics which can increase drag by up to 20%.
3M has a couple of solutions for this problems, tapes and coatings. Their film-based tapes, for example, have been used for over 40 years in aerospace applications including helicopter blades. And the company’s new PU-based protective coating, W4600, is VOC-free, fast curing, and recoatable. The company is using pulsating jets of water to test the coating, a test method being considered by several organizations writing standards for turbine blades.
Lightning is also a growing problem as turbine heights continue to climb. Even in low-lightning areas such as the North Sea, an average blade tip that reaches 525-ft high gets struck 1.4 times per year, according to researchers at The University of Manchester. The current state-of-the-art at mitigating lightning-strike damage is to embed metal receptors that are suitably grounded. Wind experts agree that lightning could become a larger problem as more conductive materials are added to blades such as carbon fibers and anti-icing gear.
Lowering manufacturing costs: TPI Composites, with funding from the Energy Dept., is looking at ways to bring down the cost of making turbine blades. Their initial study totaled up the time it takes to go through each of the major long-blade manufacturing processes: mold prep 4.5 hr; lay-up 95 hr; infusion 60 hr; pre-bond 60.5 hr; bonding/assembly 10.8 hr; fabricating spar cap 53 hr; fabricating shear web 26 hr; fabricating root 52 hr. These are all multi-axis, labor-intensive processes.
TPI wants to automate as much of the manufacturing as possible. Unfortunately, techniques used in aerospace are too expensive. Aerospace suppliers industries can afford to use the techniques because they get $200 to $700 per pound for components. Turbine blades are currently valued at around $5 to $10 per pound. So TPI is taking smaller steps to cut costs. For example, it is developing rotating carts that will hold and maneuver long blades. These should eliminate the need for expensive cranes currently used to move and position blades. The carts would also keep the blades at an ergonomic working height. The company is also making ultrasonic equipment used for nondestructive testing more portable so it can be taken up into wind-turbine nacelles.
At Gamesa, a Spanish wind company, efforts to automate blade manufacturing has led to a two-part blade for a 4.5-MW turbine. It weighs about 15 tons, 40% less than comparable blades. Making it in two parts simplifies inspection, repair, and transport, but increases installation time.
Networking wind farms: Getting large turbines at wind farms to work together is one of the goals at GE Wind Power. The first step is optimizing individual turbines, and their largest, most recent turbine being designed and tested, the 5.2-120 (5.2 MW and 120-m rotor diameter), presents several challenges. It has blades 189-ft long. So when one blade tip crests the top of the arc at 650-ft high, the bottom blade is sweeping along 25-stories below it. The difference in altitude leads to a meaningful difference in wind speeds. To compensate, each blade’s pitch changes continually to extract the most power from the available wind.
The spinning turbine acts as the sensor to control blade pitch, and sensor data can be collected and exploited on a networked wind farm. In fact, the data can be shared with surrounding wind farms as well. For example, an upwind farm may relay reliable forecast what strength and duration the winds will be for downwind turbines. These updated farms may then inform the local utility that they will be able to deliver 60 MW for the next 15 minutes, and be 99% accurate in their forecasts. This is the type of data utilities need to make wing power a reliable part of the grid.
Another step toward making the output more consistent is the use of batteries to store excess wind-generated electricity during productive periods than meter it out to smooth out the dips when the wind comes up a little short. According to GE, a small amount of battery storage would let a wind farm send hundreds of megawatts of wind power into the grid seamlessly.
A wind farm populated with GE’s new networked turbines can operate at 45% of their full-capacity in areas with just 15-mph winds. This means wind farms can be sited in more areas that are removed from where people live. Overall, GE’s 5.2-120 turbines increase efficiency by 25% and output by 15% compared to the company’s current model.