Designing composite wind turbine blades
is about balancing aerodynamic performance and structural integrity. Blades
must extract as much energy from the airflow as possible while resisting huge
forces and deformations over a lifespan of 20 years with minimum lifecycle
costs.
While blades need to be as narrow and
thin as possible to achieve maximum energy extraction, sufficient strength and
stiffness can only be provided by larger cross sections and/or the use of
higher performance materials, leading to a design that is a compromise between
efficiency, endurance and cost.
In all, hundreds of composite plies and numerous pieces of core materials are required to make a wind turbine blade. Blades must
be able to stand up under the duress of tens of millions of rotations and
fatigue cycles for at least 20 years. This is no easy engineering task,
especially when you consider that the blade can weigh as much as 20 tons and
the speed at the tip of the blade can reach up to 200 mph. Blades must also
withstand harsh sun, heavy rain, snow, ice, hail, gusty winds
and lightning strikes.
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Finding the Right Approach
Blade manufacturers are currently struggling with four major
issues. First, they are not able to fully optimize the design of the blade
because the analysis model and simulation lack some key elements of the
composite definition. Second, a number of manual steps are required
to produce the manufacturing engineering documentation for blade production,
which leads to errors and a lack of repeatability. Third, a lot of touch labor is involved in
the manufacturing process, which raises costs and slows the process. Finally,
there is no formal design change management process so there can be a major
disconnect between tooling and part design.
Therea euro 'are other
complications. As blades increase in length, weight reduction becomes a
critical concern because weight increases faster with blade length than energy
throughput. Today, large blades are almost all made of glass-fiber-reinforced
polymer (GFRP) because it currently represents the best way to strike the
balance between performance and structural integrity. The good thing about GFRP is it is relatively
inexpensive and provides sufficient strength and stiffness. However, as blade
size increases, carbon-fiber-reinforced polymer (CFRP) is becoming more popular
for developing some parts of the blades, such as spar caps and some of the root
areas.
The root laminate of a blade is typically very thick -
including several inches of GFRP plies - in order to withstand the enormous
bending and torsion forces acting at the base of the blade. It is usually
pre-cured separately to avoid overheating issues during curing and then joined
to the blade main laminate.
The spar cap or the box beam provides stiffness against
flap-wise bending due to the wind pressure. The upper and lower shells -
pressure side and suction side - joined at the leading and trailing edges,
provide the appropriate aerodynamic outer shape. Together, they also act as a
torsion box to counter blade twisting and provide stiffness against edge wise
bending that is due to drag forces and gravity.
The blade shells are typically built
using balsa or foam material over some areas in order to increase bending
stiffness and reduce the risks of buckling. The leading and trailing edges are
typically reinforced with unidirectional material for both local reinforcement
and also to increase the edge-wise bending stiffness.
So what is the most appropriate process
for designing and manufacturing such a complex composite assembly that needs to
satisfy stringent structural and environmental requirements? We can look to the
aerospace industry for some of the answers. The aerospace and defense
industries were early adopters of high-performance composites so it is no
surprise that the bulk of the expertise is owned by people who have worked in
those industries. Some of that expertise is transferable to other applications
such as wind turbines.
For instance, some of the design
methodologies and manufacturing engineering processes used to develop aircraft
wings and fairings are similar to the process for developing blades. However,
the wind industry presents some major differences in terms of part size,
material types, layup processes and design tolerances. For example, a large
variety of biax/triax/quadrax and multilayered matte/woven/uni materials are
used on wind blades. Some ply draping and covering techniques are more
pertinent to composite blade design, such as the extensive use of 2D-to-3D
mapping of rolls of material, as opposed to aerospace where most plies, which
are much smaller, are defined using 3D-to-2D flattening and trimming.
Employing End-to-End Solutions
In order to support their new advanced composite engineering
and manufacturing processes, wind blade manufacturers must look beyond
acquiring point solutions. What companies are really looking for is to create
an end-to-end engineering environment with the best-in-class assets that can maximize efficiency and effectiveness.
Implementing an integrated composite
design, analysis and manufacturing environment is a must if you want to develop
a better and faster engineering process.
This environment must be open and flexible so engineers can easily and
rapidly adapt the tools to the needs of the wind turbine industry as well as
specific customer requirements. It must also allow the company to select the
best software components, be it the CAD platform for 3-D design, the CAE
solution for structural analysis, CAM software for manufacturing simulation, or
a PDM system for data management.
As the linchpin of the new engineering
environment, the composite design software must support and be integrated with
a diversified CAD and CAE base. It must also account for easy and reliable data
transfer across the supply chain and different engineering sites that may use
different CAD, CAE, CAM and PDM platforms.
Relying on a Master Model
It is vital that the digital composite
model of a blade contains all the information required for properly
manufacturing the part, including definition of all laminates and plies,
associated flat patterns, manufacturing sequences and steps, accurate
definition of the cored panels and interface definition for all mating parts.
This enables seamless collaboration between engineering and manufacturing.
Such a master model must also enable
so-called producibility simulations, or simulations of the manufacturing
process. Producibility simulations enable the design or manufacturing engineer
to predict manufacturing issues such as composite fabric wrinkling or bridging,
that may appear due to material deformation when laid up in the blade molds. By
accurately predicting such issues, simulation software enables early resolution
of the manufacturing issues without the need for making many costly prototypes
that lengthen the development process.
As the market increasingly demands
larger, lighter and better performing wind turbine blades, the need to automate
the development process will only become greater. Companies that are able to
master automation will thrive because they will enjoy dramatic reductions over
manual methods in labor and manufacturing costs as well as cycle times. Those
cost reductions will be essential to making wind energy a sustainable and
profitable energy source.
Olivier Guillermin is director of product and market strategy for Vistagy
Inc.