Note: This is the third article of a four-part series covering plastics in wind energy (1) trends, (2) material advances, (3) process technologies and (4) applications.

Wind energy designs increasingly depend on plastics processing and design technologies. Plastic parts and components are extensively used in all major sectors of wind energy generation to accomplish the following goals:

  • Reduce weight
  • Extend service life
  • Lower cost
  • Advance design and provide design flexibility
  • Maximize productivity and safety

Processing developments in blade manufacturing and service are advancing rapidly. For example, modular blade design and manufacturing is a good place to start.

The primary drivers in wind turbine development are power generation and cost. Assuming there are no changes in design, materials or construction methods, the amount of generated power increases with blade length in proportion to the square of the turbine rotor's diameter, with blade mass increasing in proportion to the cube of its diameter.

As a result, longer, exponentially heavier blades vastly increase transportation and installation difficulties and mass/cost of the turbine/tower system. The engineering challenge has been to develop a longer wind turbine blade that will produce more power output without increasing blade weight, the load on the turbine or the costs for transportation to and assembly at the wind farm site.

Gamesa addressed this challenge with a design solution creating a segmented blade assembled with a reliable bolted joint and a proprietary structural concept, using optimized composite materials, processes and automation for a 25 percent weight reduction.

Segmented wind turbine blade design.

Gamesa's G128 5 megawatt (MW) turbine is fitted with a commercial, segmented composite blade. Their blade segmentation enables customization of inboard and outboard section lengths and shapes for each wind farm site's unique wind conditions.

The segmented blade sections can be shipped on standard 90-foot flatbed trucks. Standard 100-meter rotors need blade-specific trucks, which are much more expensive. Rotors with 117-meter diameter are pushing the limits for transporting them to the wind farm.

The Gamesa blade weighs less than blades used on standard 100-meter rotors and is as easy to install as the much shorter blades used on 2 MW systems thanks to the use of carbon fiber. The G128 uses both glass and carbon fiber, balsa core and a proprietary new multipanel internal structural concept to optimize the weight allocation inside the blade.

A combination of load reduction and internal structure optimization has reduced the weight of blades for a rotor over 100 meters in diameter from an average 20 metric tonnes (mt) per blade to 15 mt, which translates into a 25 percent weight reduction.

When the blade is in operation, it has to look and act as a one-piece blade, with a continuous bend and no flat spot, in order to avoid disrupting the aerodynamics and loading. Segments are joined using insert and bolt connections within the joint. The inserts are metallic and are bonded into the blade laminate in such a way that they form a double lap-shear joint, considered by the aerospace industry to be one of the strongest options for adhesively bonded joints in composite structures.

After the bolts are secured, a metallic external fairing covers and protects the joint's metal components and provides a smooth transition across the joint. Gamesa claims its joint and assembly design achieves low cost by enabling transport via standard equipment used for 2 MW turbines. The joint adds approximately 10 percent to blade cost, but the increase is more than offset by transport savings.

Gamesa/Free Patents Online
Metallic fittings are bonded into the spar laminate of each blade segment and then bolted together to join the two segments.

Gamesa/4C Offshore Limited
Blade segments delivered to a wind farm await assembly.

Elsewhere, Enercon is also using segmented blades for its most powerful machines, and Blade Dynamics is also developing segmented blades. Enercon's 2.5MW E-115 turbine features new-generation, load-reducing segmented rotor blades comprising two glass-fiber-reinforced epoxy sections joined by cross-sectional and longitudinal bolts. Their larger E-126 turbines are fitted with segmented blades but with the inner sections in steel.

Continuing, wind blade factory restoration is coming into its own from a servicing standpoint. The science of repairing wind turbine blades in the field has seen significant advances during the last five years as a result of improvements in diagnostic tools, materials, equipment and technician training, but field repair still has limitations.

Blades are frequently damaged beyond the ability of field repair crews to economically or safely restore them to optimum working condition. A new service option for wind energy operators offered by MFG Energy Services extends the life of seriously damaged wind turbine blades.

Factory repair and restoration — a new alternative to field repair is successfully extending productive life and value to previously doomed blades. In the factory, it's possible to restore even seriously damaged blades to "like new" condition.

Factory repairs are executed in a clean, controlled environment at a regulated temperature, ensuring that plastic resins and materials cure properly. When it's possible for blades to be repaired in situ, factory service is generally not the optimum solution.

MFG Energy Services
Wind turbine catastrophic damage (left) and factory restoration (right).

Thorough cost analysis of transportation from site to factory against the cost of mobilizing a repair team, the risks/limitations of field work, and projected life extension of the blades often supports the case for factory repair. Wind blade suppliers in North America are looking to provide needed aftermarket products and services for all wind composites — from blades to nacelles to spinners.

Wind farm owners are diverting funds previously allocated to farm expansion to projects for improving and maintaining their aging fleet with strong demand expected for leading-edge repairs, erosion protection measures and updated or repaired lightning systems.

Refurbishments done in a controlled factory environment, where blades can be thoroughly inspected prior to the restoration, are often more consistent and predictable than field repairs. Plus, additional improvements, including blade protection, lengthening and functional lightning systems can be safely and cost-effectively added to the process.

Finally, on the simulation and monitoring technology front, software development to optimize wind turbine blade design is advancing rapidly. As they are getting increasingly longer, wind turbine blades need to be stiffer and lighter to avoid cracks from fatigue loading.

Software is being designed to address aerodynamics, structural modelling and optimization of emerging composite wind turbine blades. Advanced capabilities are needed to generate high shape quality, i.e. a powerful and complete set of modeling capabilities, including realistic and fast quality-analysis tools, and shape optimization.

AnalySwift, a provider of efficient, high‐fidelity modeling software for aerospace and energy composites and other advanced materials, has partnered with Altran in the release of Altran's upcoming optimization code for preliminary design of composite wind turbine blades. Altran's code, a generalistic tool, is based on aerodynamic and structural calculations and will include an optimization loop to modify structural predesign and allow for simulation of more complex scenarios to improve the design.

AnalySwift's VABS/PreVABS software will interface with and complement Altran's code. VABS (now in version 3.7) enables rigorous modeling of complex composite slender structures (commonly called beams), such as wind turbine blades.

VABS and PreVABS software architecture protocol.

Due to its versatility, VABS can model beams of any shape and a wide variety of materials. Real structures with complex microstructures such as composite rotor blades with hundreds of layers can be designed and analyzed. The analysis can easily be handled by a laptop computer with analysis time typically reduced from several hours to just seconds.

Cross-sectional composite wind turbine beam.