To boost energy yield, rotor blades are becoming larger and heavier. The longest production blades can span about 61-m and weigh 15 metric tons. Future designs call for lengths of 80 to 100 m and beyond. They are primarily polymer composite structures (e.g. glass-reinforced plastic) expected to last at least 20 years in a challenging service environment. When rotor blade tips slice through the air at speeds up to 300 km per hour, it’s not hard to imagine the extreme stresses they must withstand.

The trend toward larger, stronger, heavier blades is expected to continue because such designs will deliver more energy. In addition, the area available to capture wind is greater, making the turbine more efficient at lower wind speeds.

But as blade length increases, stresses on the blade are magnified. Larger blades experience more deflection across the length of the blade, increasing the dynamic load and stress on the adhesive bond line. Adhesives that offer superior long-term dynamic fatigue strength will provide better performance over the blade’s life and lower the risk of damage that requires repair or replacement.

Along with the need to improve performance, wind-turbine manufacturers must also lower their total production cost. Wind-equipment manufacturers work diligently to shorten production cycles and lower overall time and turbine production costs. Also, manufacturers are evaluating options to make reductions in lengthy production cycle times. Shorter cycle times translate into capital, labor, utility, and overhead savings, while also allowing greater flexibility in meeting unplanned demands.

Henkel Macroplast UK 1340, represented in blue, bonds the spar in place and the leading and trailing edges of the blade assembly.

Traditional blade assembly
There are two fundamentally different approaches to rotor-blade construction. Non-self supporting structures involve a box spar with an aerodynamic profile. Self-supporting structures consist of two blade halves with bonded spars that transmit all forces. The structural bonding of self-supporting blades requires adhesives with exceptional mechanical properties.

Regardless of size or construction, the quality of a rotor blade depends upon the reliability of its adhesive bonds. Glass-reinforced plastic rotor blades are fabricated in a simple sandwich design. Two blade halves are created in a composite mold. After demolding, the two halves are mated and bonded together to form the rotor blade.

The bonded blade construction is exposed to long-term direct loading. If the bond fails under stress, the blade may be damaged or even ruptured. For this reason, Germanischer Lloyd (GL) approval for blade bonding adhesives is only awarded after an adhesive passes numerous physical parameter checks.

A technician applies Macroplast UK 1340, a polyurethane adhesive that speeds rotor blade production and reduces costs by 15 to 30%. Offering breakthrough Tg better than typical polyurethanes, it is the only adhesive in its class to satisfy GL’s requirements.

Until recently, self-supporting structures such as rotor blades have been typically bonded with GL-certified two-component reactive epoxy resins. Widely accepted in the aerospace industry for structural bonding applications, epoxies offer excellent adhesion, tensile strength, and chemical and heat resistance. Limitations include extended cure times, high exotherm, and higher process costs.

Two-part epoxy adhesives are thermoset materials that cure using heat, or heat with pressure. Used for high-load assemblies and in severe service conditions, cured thermoset adhesives may soften when heated, but do not melt or flow.

Epoxies begin curing once the two parts are mixed, but require substantial time to cure completely. The total cure involves several heat-cure cycles and can last in excess of 24 hours. Longer cycle times limit a manufacturer’s throughput and result in elevated work-in-progress inventories, decreased mold use, and increased energy costs associated with cure ovens.

Epoxies also generate extreme heat, greater than 120 to 150ºC, when curing. This naturally-occurring heat can cause the bond line to swell during cure, and then shrink as it cools, which leads to stress building in the blade assembly. This stress can translate into a concentration that causes cracks under dynamic loading. Furthermore, epoxies can be inherently brittle, which limits the blades’ resistance to crack propagation. Also, epoxies can become more brittle over time, so the likelihood of cracking, especially under dynamic loads, may increase.

These limitations increase the risk for warranty claims, loss of production, and finally increased maintenance cost and blade failure. With the new manufacturing requirements for larger blades, epoxyies are reaching their limits for bonding blades.

An advanced polyurethane
A recent polyurethane (PUR) adhesive satisfies specific mechanical requirements for use in the wind industry while improving the long-term reliability of rotor blades and making rotor-blade production faster and less expensive. The material, Macroplast UK 1340 from Henkel Corp., contributes to the assembly of highly efficient wind turbines.

Because of a large spectrum of possible polymer architectures, polyurethane adhesives have been employed as a bonding agent in many different industrial sectors for more than 30 years. Construction, automotive, transportation, and shipping vessels have all benefited from the use of polyurethanes.

The recent polyurethane adhesive provides superior dynamic fatigue strength and increased resistance to crack propagation, while making the production of rotor blades more efficient than with epoxy technology. For instance, the PUR adhesive requires fewer curing steps than epoxies, resulting in reduced production costs and production cycles that are 15 to 30% shorter.

In addition to blade bonding, the two-component polyurethane adhesive is also used in other structural bonding applications on turbines including bonding components to the rotor blade, performing field repairs of blades, and securing various components inside the tower assembly.

GL’s requirements for the adhesive primarily relate to its tensile shear strength, long-term durability, creep behavior, and glass transition. The adhesive’s physical properties are temperature-dependent. Within a temperature range known as glass transition (Tg), the change in the adhesive’s mechanical properties is considerable. The glass-transition temperature separates the lower, brittle, or glass range from the upper, flexible, or rubbery-elastic range.

Extensive tests demonstrate a tensile shear strength for Macroplast UK 1340 exceeding 20 MPa in the -40 to +80°C temperature range and a Tg of 65°C and higher.

For turbine rotor blades, a Tg of at least 65°C is required to prevent bond creep or relative movement of the substrates, and achieve a certain degree of rigidity at higher ambient temperatures. However, for typical polyurethane adhesives this is usually within the range of only -30 to 45°C, depending on the required elasticity.

Tests have demonstrated a tensile shear strength exceeding 20 MPa in the -40 to +80°C temperature range for the material and a Tg of 65°C and higher. This improved tensile fatigue strength lets wind blades handle the deflection across the length of the blade and the dynamic load and stress on the adhesive bond line better than epoxies, thereby reducing the risk of damage that may require repair or replacement.The two-component polyurethane adhesive consists of a resin and a hardener. After mixing, pot life ranges from 60 to 80 minutes at the optimal ambient temperature of 20°C. The reaction speed of polyurethane-based adhesives can be altered, for considerably decreased time spent to produce rotor blades.

The adhesive’s pot life can adapt as required for a specific manufacturer’s production without the disadvantage of partially overheating the adhesive joint. Manufacturers can maximize throughput without the risk of elevated stress in the part or the potential cracking associated with high exotherm.

The recent PUR adhesive cures at a much lower reaction temperature of up to 75°C maximum when compared to epoxies that cure at 120 to150ºC. Reducing the exothermic reaction benefits the process in two ways. First, when bonding composite materials, stress cracking caused by excessive thermal loading may weaken the rotor blade. This risk is significantly reduced when the reaction temperature is lower.

Second, polymerization that takes place at high temperatures is always accompanied by changes in volume. Low-heat emission and lower thermal loading during chemical crosslinking reduces heat-related shrinkage of the adhesive. Shrinkage in the bond line can increase internal stresses, hence, a controlled shrinkage results in a more durable bond over time. The heat generated from an exothermic reaction will vary depending on the quantity of adhesive undergoing cure. In areas where larger amounts of adhesive are applied, temperatures get much hotter than in areas with less adhesive volume.

On a single rotor blade, there are considerable differences in the quantity of adhesive applied and, therefore, considerable differences in stress. At interfaces between such stress fields, mechanical flaws will occur unless the stress is relieved through elaborate tempering. This is where polyurethane systems with low-temperature exothermic reactions have big advantages, particularly when applied in thick films.

Adhesives that address the higher dynamic load and stress associated with larger blades will provide better performance over the life of the blade and lower the risk of damage that may require repair or replacement.

Adhesives that cure at room temperature and those which cure rapidly at elevated temperatures can shorten production cycles, and reduce the overall time and cost of turbine production. Room temperature curing adhesives eliminate the need for ovens, while faster curing adhesives improve throughput during the blade bonding assembly process.

Compared to conventional epoxy resin systems, the latest polyurethane delivers a number of improvements for wind-turbine manufacturers. The adhesive’s shorter cycle times boost productivity while reducing energy costs. It provides superior tensile and fatigue strength, and resists crack propagation, meeting high standards of reliability and quality.

WPE

The wind industry recognizes the importance of proper bolt tension. The acknowledgement will challenge some of the industry’s torque-tightening practices as it matures and adopts more sophisticated technology to improve reliability and reduce maintenance costs.

To start, let’s review current practices. For instance, bolts on turbines are often tightened in one of two ways: either by torque or by hydraulic tension tightening.

Torque is a rotational force applied to a lever and multiplied by its distance from the centre of the tightening end (fulcrum). Torque, measured in Nm or lbs-ft, rotates a bolt head or nut to tighten and stretch the bolt so it clamps two surfaces together. Hydraulic torque wrenches allow the operator to control the torque applied and are extensively used to tighten wind-turbine bolting.

One design for a tell-tale bolt comes from UK-based James Walker’s RotaBolt. A simple mechanism inside the bolt provides an indicator of the real load or tension. RotaBolt fasteners are guaranteed to consistently indicate within ±5% of the specified load regardless of environmental influences.

Although simple to use, the technique suffers from inaccuracy due to inconsistent friction. Even on lubricated threads and nuts, the level of friction is uncontrollable. This means there is a critical lack of tension control that can lead to bolt failure, and ultimately compromise joint integrity.

Hydraulic tension tightening is also used to tighten large diameter bolts on wind-turbine structures. A hydraulic ‘jack’ pulls the bolt axially. When the bolt is stretched by a specified amount, the nut inside the tensioner rises off the flange creating a gap between flange faces. The gap is closed by tightening the nut. Releasing the hydraulic pressure transfers tension to the bolt and flange.

If you can turn the indicator cap, the bolt is not tensioned.

The drawback here is that tension loss occurs when releasing the hydraulic pressure. It’s an effect called “load transfer relaxation.” To compensate for the loss, the hydraulic pressure originally applied is increased a bit, a “hydraulic overload value”. However, the amount of load-transfer relaxation is not accurately predicted and so varies from one bolt to another. Consequently, hydraulic tensioners are unable to control or measure the tension achieved for each bolt. The device is also unable to provide an in-service reliability check.

One way around the shortcomings of both tightening methods is with a bolt that indicates when its specified load or tension has been reached during tightening. The RotaBolt can be used in either torque or tension tightening. When this bolt is tightened, the cap locks at exactly the right amount of preload in the bolt. This allows for more control during tightening while ensuring that an OEM’s design criteria is met independent of the tightening system. It also allows checking the bolt’s status in service (if a cap on the bolt does not turn, it’s still tight) something traditional bolt retightening checks are unable to do.

Accuracy is not affected by friction, as in torque control, nor by load transfer errors, as in hydraulic tensioners. This bolt-tension solution can ensure joint integrity on wind turbines. Joint integrity, in turn, allows for greater reliability and the more efficient production of renewable energy.

WPE

Nick Harper
Applications Manager
Blade Sensing Systems
Moog Inc.
www.Moog.com

Cold weather presents special problems for wind turbines. Inside the nacelle, low-viscosity lubricants keep the gearbox turning and enclosure seals to keep moisture and ice off electronic components. But outside the nacelle, things are different. Ice easily forms on turbine blades possibly adding hundreds of kilograms, which degrades performance and shortens working life. About 65% of wind turbines in North America are in areas where icing is possible and likely. Also because wind farms are often in remote locations, shutdowns are occasionally necessary when icing conditions are present. Then, turbines require a visual inspection before a restart.

The problem with ice on a working turbine is that it can be thrown, and when not, it causes additional drive-train loads often in excess of design loads. For this reason, many turbines are shut down when ice buildup threatens and restarts come only after an inspection confirms the ice is gone. This is a difficult practice in remote locations or at night.

Ice on turbine blades is fairly obvious when the unit is clearly visible. But at night or in remote locations, the question of ice on blades becomes a matter of conjecture. Weather conditions right for ice is one indicator, but there are better ways to detect it and protect turbines.

Traditional ice detection uses meteorological equipment but this does not detect ice on blades. It simply measures conditions for icing, so it does not give operators enough warning or time to take action – such as shutting down the turbine to prevent damage.

Detecting ice
A recent solution to the problem of blade icing includes ice-detection sensors and controls. The unit is part of a rotor monitoring concept, called RMS, for wind turbines. It lets operators monitor wind-turbine performance to detect ice on blades and avoid damage. RMS uses optical strain sensors mounted inside at the root of each blade. They work optically so the sensors are immune to lightning and electromagnetic induction effects.

The ice detection system provides information for the operator to shut down the turbine when ice loads exceed the specs or present a danger of throwing ice. The system also detects when ice has been shed.

The ice-detection system works two ways. The left of Signals from ice shows the system working under normal operation conditions. The monitoring system measures the bending moment of the blade as it rotates, generating a sinusoidal wave pattern. As ice builds on the blade, the amplitude changes. When the trace goes above a predetermined threshold, the monitoring system shuts the turbine down.

The right side of Signals from ice shows a static signal trace. At this time, the turbine is closed down but not stopped. The rotor is still turning slowly while the system is working in the frequency domain. It is relying on the low wind speed, about 3 m/s, to excite the natural frequency of the blade. This natural frequency changes as ice builds on the blade and changes its mass.

The photos and traces show how ice looks to a technician and the detection system.

The buildup of ice correlates with stored data. The right of Reading the graph tracks a period from October to December. At the test site, there was no ice buildup for the first 70 days. At about day 72, moderate ice was verified. On day 78, a significant amount of ice was noted. After that, there is a reduction in mass to the point where it was safe to restart the turbine.

Now consider two turbines, A and B, on the same wind farm. The red and blue traces show strong correlation. Notice the apparent noise signal level indicating about a 100 to 150 kg addition to one blade. This is significantly reduced by providing blade pitch information.

The ellipses identify two severe ice events detected by the system and confirmed by inspection. Without ice, mass values should be zero. In this case, both turbines have inherent noise of 100 to 150 kg because pitch angle feeds were not provided. Overall system accuracy is maximized by supplying a pitch and angle feed from the turbine controls, along with blade mass and center of gravity.

Experience shows that to increase the overall accuracy of the ice detection system, blade pitch position from the turbine’s control system is combined with the optical data.

So if we can take blade-pitch information on the turbine, we can reduce the noise level. Even without it, we can clearly see that both turbines correlate well with each other – they are both icing at about the same rate. At 40 days, turbine A had ice for about half the time of turbine B. This tells that the operator was able to start Turbine A before B by two days. This, of course, improves the revenue for that particular machine.

When the RMS is functioning on a working turbine, it can generate the blue traces. When signal top a predefined threshold, the controls halt the turbine, but the RMS continues to track ice build up through the frequency domain, the red plots.

Data in Time domain and frequency analysis presents a comparison between a turbine rotating (blue trace) and one stopped (red data). Notice that the turbine on Day 2 is working without ice, but slowing over a period of 1 to 1.5 days. The blue curve is going up indicating ice build up on the blade.

At some point, the selected threshold is exceeded and the turbines will be stopped. The red information indicates that Turbine A is halted, but the system still provides useful data. The mass still increases on the blade, up for two more days, but then the data becomes flat again. After about day six, it reverts to zero. So the system works when the turbine is operating, but it is just as useful when it is not.

A closer look
The rotor monitor includes several components. First, there are four independent sensors at the root of each blade. These are installed at four positions corresponding to the leading edge, trailing edge, pressure surface, and suction surface. The sensors that connect to the interrogator unit, are rugged, well protected, and will last the life of the blade. The interrogator unit (OEM1030), mounts in the hub of the turbine. It can also mount in the root of the blade, but anywhere in the rotating part of the turbine would do.

There are several ways to transmit data out of the hub. One is a GRPS modem, which lets operators send data to the U.K. for analysis. Another way is to send the data by slip ring into the nacelle where it could be connected to the SCADA system for direct control of the turbine. So as the ice builds to a certain level, the SCADA can call for a shutdown and restart when the ice clears.

The RMS from Moog includes four optical strain gages in each blade, an interrogator unit in the hub, and at least one of two ways for transmitting signals across the turning hub.

The other main feature of the system is part of the RMS. Using the same system and additional software supplied by Moog, the system can interrogate the data and deduce information associated with blade damage and rotor imbalance. This can reduce the unit’s overall payback period.

Ice detection can be implemented on any wind turbine. It has been globally installed in many and we have not yet received word of any sensor failure. From experience, the system annually provides about $15,000 in additional revenue. In September 2009, GL certified the system.

WPE