Many nations have set challenging targets for renewable energy production and it was in this setting that AMI held the latest global conference on Wind Turbine Blade Manufacture in December 2011 in Düsseldorf, Germany.
The UK Minister for the Environment had announced potential plans for another 29,000 wind turbines mainly offshore, and Siemens Energy had unveiled its latest 6 MW turbine with rotor diameters of 120 m and 154 m. In the same month the American Wind Energy Association (AWEA) announced that more than 1200 MW were installed in the US in the third quarter of 2011, which is better than any quarter since 2008: the US wind energy market has been boosted by the federal Production Tax Credit.
Advances in blade manufacture
The advances in the wind industry were reviewed by Frank Virenfeldt Neilsen, Chief Technology Officer of LM Wind Power, from the earliest windmills dating back to 4000 BC. The first windmill to produce electricity was built by Poul La Cour in 1891. The global wind energy market took off in the late 1970s as oil prices rose and many governments commissioned research; for example NASA and Tvind both had a 2 MW project.
LM Wind Power has been involved in marine composites for 55 years and in wind blade manufacture for several decades. The company is developing longer offshore blades of 73.5 m in length, weighing in at 26 tons, using experience from current installations such as Alpha Ventus in Germany, Vattenfall in the UK, Thornton Bank in Belgium and Vindeby in Denmark. The latter installation was set up in 1991 and used 33 blades: inspection over the next 20 years showed only minor leading edge erosion, which was restored.
In 2000, 60 more blades were installed at Middelgrunden, Copenhagen, and inspection 10 years later revealed only limited surface damage to gel-coats, nine blades with laminate cracks and two with lightning damage.
Routine inspection and maintenance of blades can keep operating costs to a minimum and LM has extended its services beyond manufacture to support wind farm operators. The limiting factor in blade scale-up has been weight, and the solutions lie in new technology coming from studies of aerodynamics, load control and integrated design.
Managing loads on blades can maximise the windswept area and increase energy production. A prototype glass fibre polyester blade takes around 9 months to develop and testing takes a further 6-12 months. LM has set up blade production in many areas of the world, with around 36% in China and 32% in Europe, and relies on suppliers to provide consistent, cost-effective supplies.
Vestas has 30 years of experience in wind turbine development and has 44 000 installed in 66 countries on six continents. The company’s V112 3 MW turbine is established for operating in low and medium onshore winds. The company manufactures blades at eight sites worldwide. Vestas blades were initially manufactured as polyester shells and spars. This changed in 1991 to an epoxy spar, and in 2002 the company used a carbon fibre-reinforced spar for load bearing. This allowed it to add 5 m to the length of the blade without increasing the weight.
Frank Weise, Vice President of Vestas Blades Deutschland, described the slender V112 blades including the wider root diameter to reduce blade bearing wear, integrated lightning receptors and grounding cable, and profile design for low noise level. The planned lifespan is 20 years.
Weise’s manufacturing background began in Japan and he is studying the whole production process and introducing standard operating methods with controls at each step to ensure consistent, high blade quality. This is a key precursor to instigating automation, which is still proving difficult in the wind composites industry.
At the end of 2011 Suzlon completed its purchase of REpower Systems. Offshore is an expanding market for the company and the 5 MW/6W manufacturing facilities are located at Bremerhaven. In 2011, 30 5 MW turbines were installed at Ormonde, and in 2012-13 a further 290 MW are scheduled for installation at Nordsee Ost and Thornton Bank.
Design of the new blades is complex involving over 800 load cases, and the effects of the environment like rain, ice and salt and temperature cycling. The scale up of blade size leads to far greater demands in manufacturing, for example the 40 m blade weighs 7 tons and has a surface area of 180 m2 and the 61.5 m blade weighs 22 tons and has a surface area of 470 m2.
Automation is an obvious solution to improve efficiency and quality, and due to the large size of the new blades, manual labour is almost impossible to use for some tasks. Aerospace has implemented some solutions, but these would be too slow for blade making. The biggest manual job is hand laydown of glass fibre and the difficulties of automating this are that the fabric is limp and difficult to handle, the automated process is currently too slow, the complexity of the task and the huge size of the task, not to mention the high investment cost.
IDP Sistemas y Aplicaciones has robotic systems for finishing wind blades, which the company claims can cut the time take for a 40 m blade from 100 hours to 10 hours.
The cost breakdown of an onshore wind turbine shows that the drivetrain, hub and nacelle absorbs 25%; the tower, electrical systems and civil work each take up about 13%; and the rotor blades make up 11% of the total expense.
Blade development can affect the load and cost of other components and this is being studied extensively by Christian Bak at Riso Danish Technical University (DTU), who is looking at drivers such as high aerodynamic efficiency (high power), blade-tower clearance, loads, transportation and moulding.
The virtual 5 MW turbine of the EC Upwind project was used as the model in this investigation, which took into account the structural dynamics of the whole turbine. In the new slender blade design, the blade chord length was reduced by 16.7%, and the slender blades reduced the extreme loads on components by up to 24% and fatigue loads by up to16%.
Design is the key issue in blade scale-up. Dassault Systemes Deutschland supplies composite simulation software to the wind industry covering aerodynamics, structural analysis, design and manufacturing planning. Quality control can offer big opportunities for cost savings. In one aerospace project with EADS Composites Atlantic the scrap rate was reduced from 13% to zero.
Certification and testing
One of the certification bodies for wind blade manufacturing sites is Germanischer Lloyd (GL) and some wind farm authorities and insurers expect this standard from suppliers. The standard covers facilities, materials, processes, and personnel qualifications.
For example, the workshop is expected to keep temperature and humidity within limits and the laminating resin is specified as having good wetting and impregnation, resistance to ageing, cure between 16°C and 30°C with a maximum of 12% filler by weight.
At the Wind Turbine Blade Manufacture 2011 conference questions were asked about the compatibility with IEC guidelines and the rationale for the GL requirements.
The Fraunhofer IWES conducts rotor blade testing, which can take a year for fatigue tests. It is developing facilities for up to 80 m blades. At the end of 2010 biaxial testing was implemented, for example with vertical loading in blade eigenfrequency plus horizontal quasi-static loading. Movement of the blade was monitored at different radii and the tip at different load levels.
The Materials Engineering Research Laboratory (MERL) has looked at the causes and modes of delamination in composites in order to predict fatigue life. This can then be factored into the design process.
Euros Entwicklungsgesellschaft Fuer Windkraftanlagen designs blades in Germany and has two manufacturing sites in Poland. The company carries out its own testing programme – on coupons to determine material properties, on samples to see the performance of complex structures and on full-scale blades to exclude size effects on smaller samples. Mass content measurements can be misleading if the stitching yarn and the sizing covering the glass fibre are not taken into account as these will decompose during calcination of a specimen and this may affect the calculation of void content.
There are a variety of techniques in use for non-destructive testing (NDT) of wind turbine blades. The University of Stuttgart has used an ultrasonic-echo method to examine the bond between the shell and the spar by taking measurements of a pulse sent through the outer shell and reflected as the material changes. It was also possible to look at bonding on the trailing edge despite the sound damping effect of the foam component, using a through transmission method. Local resonance spectroscopy on the blade shell detected delamination and air inclusions.
Force Technology has applied the pulse echo ultrasound technique to whole blades in an automated inspection procedure. This can be used to verify prototype blades before commencing production. The AMS-46 cart scanner crawls along the blade surface and is used to detect laminar defects, dry areas in a girder spar, to indicate waviness and wrinkles, and to check adhesive bond quality.
During use wind blades can be continuously monitored using a system such as that developed by Igus ITS, now Bosch Rexroth Monitoring Systems. Accelerometers are placed in the blade to measure vibrations, which are then transmitted, converted to a frequency spectrum and analysed. As one example, in hull damage with multiple trailing edge crackings, the spectrum is affected between 150 and 300 Hz. In the case of severe lightning damage, the turbine can be switched off in a few seconds and limit the effects. In the case of ice formation on blades, the system is said to detect 5-10 kg on a 2 MW turbine blade, which may not always be visible from the ground, but formed as a surface layer in icy rain.
End-of-life blade disposal
Research in Bremerhaven has indicated that around 2020 there will be about 26 000 tons of waste blades in Germany. Disposal of end-of-life blades has been reviewed by Holcim (Deutschland), looking at processes like pyrolysis, incineration and landfill. The option that Holcim chose to implement involved cutting the blades in the field to ease transportation, size reduction in a closed system and separation of materials like metals for recycling, then homogenisation of the shredded blade to provide fuel and raw material in a cement plant. The blade ash forms part of the clinker matrix.
There are a variety of coating systems for wind blades. Re-Turn AS focuses on development and has a background in marine coatings including the use of carbon nanotubes. The claim is that the new Advanced Marine Coatings (AMC) gel-coats lower friction and thus allow boats to travel faster, and Re-Turn is looking to take this technology to the wind industry. Coatings can assist in many areas such as UV stability, fouling and erosion.
Zoltek has been examining when it is cost-effective to use carbon composites in wind blades. The industry uses heavy-tow carbon, most commonly as pre-impregnated unidirectional tapes. These materials are used in spar caps and sometimes in the trailing edge girder, and allow blades to be made longer without increasing weight. The cost-effectiveness increases with blade size, and in one example of a 57 m blade, use of carbon led to a 27% weight reduction alongside a 14% cost increase compared to glass fibre only designs. At 90 m the cost comparisons are predicted to be about the same.
Other aspects such as labour and load on other turbine components can also contribute to the cost-effectiveness of carbon fibre usage.
New amine cure systems are being developed by Huntsman Performance Products after requests from the wind composites industry for longer pot life and faster cure for big blades.
The National Technical University of Athens is developing process monitoring during resin infusion using sensors to detect factors such as resin ageing, viscosity changes and the end of cure. Intelligent automation in composites production is the subject of the iREMO project.
Epoxy adhesives have played a critical role in blade assembly and bonding for several decades and 3M has produced a new high performance type. It has been compared with standard epoxy for a range of properties including the cure exotherm where it releases 240J/g compared to 290J/g for standard epoxy, thus limiting the bond line temperature, and linear shrinkage during cure is 0.33%. There was no reduction in bond strength after 1000 hours of immersion in a variety of solvents, high humidity and high temperature. The cure speed is significantly faster and there is a longer pot life.
Core materials are used to reinforce the shell structure of blades as they are lightweight with high bending stiffness. CTC Engineering BV based in the Netherlands has reviewed the different core materials in the wind market. The most common are balsa and PVC, but due to demand the price has increased. The newer alternatives are cork and foams from other polymers like polyethylene terephthalate (PET), styrene acrylonitrile (SAN), polystyrene (XPS), polymethacrylimide (PMI) and glass reinforced polyurethane (PUR/PIR).
CTC Engineering looked at a set of properties for each material namely, density, compression modulus, shear modulus, peel strength, temperature stability and resin consumption. The comparisons are not always simple, for example PET foam is 10-15% cheaper than PVC by volume, but has higher density and resin consumption. The SAN foam has better properties than PVC and is comparable in price; however there is currently only one supplier, which could cause issues in manufacturing.
Wind Turbine Blade Manufacture 2012
Wind Turbine Blade Manufacture 2012 will take place on 27-29 November 2012 at the Maritim Hotel, Düsseldorf, Germany.