Despite more than 30 years of technology advancement, wind-turbine rotor-blade manufacture has remained a largely manual, multi-step process, with labour representing 25-30% of total blade costs.
The Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) coordinates the ambitious BladeMaker research project, which aims to cut manufacturing costs by up to 10% through the use of advanced industrialisation processes, innovative materials and carefully implemented automation steps.
A project milestone was the BladeMaker Demonstration Centre in Bremerhaven, northern Germany, inaugurated earlier this year. It is set up as an open "learning" environment for the wind industry to jointly develop, test and validate rotor-blade innovations.
The spacious 25-metre long blade manufacturing and mould-making area is located in the rear section of a large hall.
To industrialise factory production, Fraunhofer IWES and its industrial partners chose two lightweight, computer-controlled full-gantry systems that can be moved separately along a rail-track, offering a highly flexible industrial environment for advanced blade and mould making.
The innovative lattice-type gantry systems are strong, stiff structures made from carbon-reinforced polymers, making them ideally suited for precision movement tasks and high accelerations while providing an open working space and high payload capacity.
During the inauguration, various machining tools for direct mould-making were demonstrated, such as a rotating milling head and a belt grinder, together with various custom-developed "pick and place" tools.
These interchangeable devices can all be individually mounted on an externally powered universal tool holder, which is attached to the cross-sectional part of the gantry systems and can be moved in vertical and horizontal cross-wise directions.
"The same tool holders are used for various dedicated tasks — computer-controlled tasks during automated blade manufacture, such as a surface grinder; and a circular 'pick and place' device," explained Christian Dorsch, structural components project manager at Fraunhofer IWES.
"Typical tasks involve precision dry lay-up or wet lay-up of fabrics (prepreg), spar-caps tape laying inside blade shells, and the trimming of leading and trailing edges with a grinder. Automated grinding with integrated dust removal reduces unergonomic tasks and enables faster processes and improved product quality."
Time to market
Direct moulding in another example eliminates the manufacture of a master plug, allowing faster time to market and the cost-effective manufacture of smaller series.
The five-year BladeMaker research project, funded by the German economic affairs and energy ministry, runs until 30 September 2017 and has a budget of €13 million.
Fraunhofer IWES and its 16 industrial partners - including Siemens, adhesive-technologies firm Henkel, Fibretech Composites, PD Fibre Glass, mechanical-engineering specialist EEW Protec and chemical firms BASF and Hexion — aim to address main future industrialisation challenges.
Volume blades of up to 40-50-metre lengths being series manufactured in increasingly large numbers and 100-metre-plus offshore blades to be fitted at next-generation 10-12MW offshore turbines are at the top of the list.
Wind-turbine rotor blades have to meet a highly diverse and conflicting set of design and operational requirements.
These include favourable mass, high structural strength, accurate reproducible size and shape, and at least 20-year design life with minimal upkeep cost and effort while continuously facing severe fatigue loading.
Also, state-of-the-art blade material typically represents about 60% of total blade costs, according to Dorsch, who previously worked at BMW and was responsible for setting up the serial manufacturing of carbon-fibre reinforced body panels and structures for the German car maker's latest electric vehicles.
"One main challenge was that the number of pieces that can be produced in a single composite-parts mould per day is only a fraction of a steel-panel mould's daily capacity.
This represents an even bigger challenge for wind-turbine rotor blades, because numbers are again much smaller, while average component size and complexity go up substantially," he said.
Blade manufacture is labour-intensive because most commonly applied pre-cut dry glass-fibre textiles have low-stiffness and are handled manually.
Many current blade models are characterised by large integral sub parts and assemblies and high structural costs, while precision dry lay-up of multiple fibre layers in individual moulds is time consuming.
Labour intensive
The pick-and-place handling of dry textiles is a major bottleneck for introducing industrial automation, and a big contributing factor to why labour costs amounts to around 30% of blade costs.
The main challenges for blade manufacturing in the future include growing dimensions, meeting stringent requirements for "low" structural costs, plus increased quality and reliability; and reproducibility, especially for smaller, high-volume blades. All this is closely linked to a drive to achieving better margins on finished blades.
"In contrast to a pure automated approach, BladeMaker focuses on industrialising the entire process. New materials and semi-finished products must be improved for optimising production logistics, enabling higher constant product quality and better process robustness," said Dorsch.
"These factors together must result in lower blade costs and reduced cycle time. Additional demands for 80-metre-plus offshore turbine blades are even further enhanced reliability because of marine-related O&M constraints, coping with size-related reachability and overall manufacturability issues, and acceptable mould occupation time," he said.
The centre's 25-metre overall length is sufficient to produce the inner 18-metre root section of a 40-metre long IWES design reference blade for a 1.5MW turbine, with all relevant design and other date available.
BladeMaker concentrates on the materials and labour-intensive inner blade section, as the outer blade part is comparatively easy to build, according to Dorsch, implying that blade segmentation is an option seriously being considered.
He said the multi-functional industrial automation system is applicable to all manufacturing processes except vacuum infusion:
"The combination of dry textiles lay-up vacuum infusion is now semi-standard. The use of prepreg, pre-manufactured glass or carbon-fibre layers, impregnated with epoxy resin plays a rather minor role in rotor-blade manufacture, but it is widely used in aerospace.
"A return to prepreg could be an enabler for enhanced efficiency in industrialised blade manufacture because the impregnation of fibres in a special offline process is robust and precise compared to the infusion of large composite parts.
"The logistics are easier as the resin and the required process consumables need not transported to the mould."
Halfway
Dorsch said his five-person team has already gained a lot of knowledge on how to build further upon the automation status.
He estimates that they are perhaps halfway in bringing these ideas to a next level through dedicated product and process development.
"Our first blade design was based upon an existing product and had to accept the limits this brings. Our challenge is how to improve on this existing design, by looking at materials, structural aspects and production methods.
"Two goals remained certain: the new in-house blade must be cheaper and we have to reduce failures and overall manufacturing and lifetime risk," he said.
Further BladeMaker goals are zero failures, stable repeatable processes and minimal waste through precision cutting. The finishing milestone will be a full-scale process demonstration using a blade design developed by Fraunhofer IWES.
RESEARCH FOCUS — CLEARLY DEFINED AREAS FOR INVESTIGATION
The BladeMaker project approach is built around three main themes: design for manufacturing, new materials, and innovative processes.
It has four distinct research focus areas:
- Design and analysis involves developing process key numbers, a cost model, blade optimising, a reference blade model, and programming;
- Materials focuses at new resin and adhesive systems, optimised fibre materials, new core materials and materials delivery for tests;
- Processes encompasses fabric placement of spare caps (internal reinforcements), and at the shells and root section, blade surface machining, mould production and heating, and blade handling;
- BladeMaker Demonstration Centre finally develops industrial hardware and software concepts, enables demonstration and validation, and facilitates test specimens and structures.
INDUSTRY VIEWS — FUTURE BLADE PRODUCTION OBSTACLES TO OVERCOME
The inauguration saw blade design and production experts give their views on the goals for advancing blade manufacture. Here are some key points:
- Enhancing process and quality control, optimising efficiency, minimising waste, and the modularity of solutions are essential. The equipment employed for production tasks is in general suitable for relatively simple repetitive tasks, but the production processes themselves are rather immature, with high variability. (Blade supplier)
- Automation is not applicable for all blade sections. "Medium opportunities" with specific benefits occur when using prepreg for the material and labour-intensive root-end with uncomplicated circular geometry. It is crucial that once placed individual prepreg layers stay in that position, even on sloping surfaces, until actual curing starts. To prevent sagging, prepreg layers need an adaptable surface tackiness, depending on application demands. (Composite materials supplier)
- Magic ideas are needed for future large blades. Blade stability and resistance to buckling are the main drivers for these blades, and new concepts are needed to design them differently. The cost of defects increases exponentially with blade length. (Blade design consultant)
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