Wind blades in the global sea

There are waves of changes in funding worldwide for alternative energy, matching the move to more wind offshore - the whole market is at sea. There are economic and structural pressures on wind turbines especially the blades. This article reviews wind composite technology and markets.

The wind energy industry is continuing to expand worldwide with the industry ebbing and flowing as global governments decide policy on supporting renewable power and then reconsider the economics, causing delays in financing major projects. The industry is caught in the financial tide but is growing with offshore and new countries coming into the marketplace. The US wind energy industry will be helped by the renewal of the Production Tax Credit in 2013. The manufacturers of wind turbines are constantly looking at new regions and types of turbine to keep their market share. In terms of the wind blades, the industry not only supplies blades for new turbines, but also to upgrade and provide replacements for established generators. This means setting up composite construction facilities worldwide to ease the transport logistics of large blades so ports are the most popular locations. Thus in the UK the ports are competing to win new blade business to supply the planned offshore wind farms – the UK is expected to be the global leader in wind farms at sea.

Some turbine makers build all of their own components, whereas others commission construction from other firms. REPower does a combination of the two and has taken on Dr Jan Kranich, a leading economist, as Global Commodity Manager for Composites to determine the best place to manufacture blades and the best way to source them for international business, considering factors such as location of the industry, transportation and labour costs.

Blade design
LM Wind Power has retained its place as top independent blade manufacturer in the world although it has not been immune to the economic situation. The latest concept from the company is to offer a flexible blade platform (GloBlade) where a set of variations can be built in to tailor a blade design for each customer, with different blade lengths for different powers. The company has found that its clients want distinct designs and the aim is to be able to provide the blades at each of the LM Wind Power global manufacturing sites. Dr Lars Fuglsang spoke on this topic at the AMI annual conference on Wind Turbine Blade Manufacture 2012 at the end of the year in Dusseldorf, Germany.

One of the growing turbine manufacturers is Siemens Wind Power, which has its own distinct method of manufacturing blades. The company studies all aspects of design in detail including aerodynamics, which have become more important with the rise in size – a 3.5 MW turbine rotor is now bigger than a Boeing 747. The driver for size is to maximise annual energy production and a 10% increase in rotor area approximates to 12% more energy. However, other factors come into play like the potential for more noise and the rise in weight. Design improvements have been tremendous over the 30 years of wind blades: the original Bonus 5m blade, for example, was more shaped like a rowing oar, whereas the latest Siemens blade is tapered, with reduced solidity (from around 10% to 5%) and trailing edge add-ons for noise reduction. The technology of passive twist bend is showing great promise.

The leading research group for wind blades in the United States is Sandia National Laboratories. One of the latest projects involved scaling up a theoretical blade design, SNL100-00, to 100 metres, the longest model in the world, and are now starting to look at some of the challenges like weight and the need to meet the certification standards such as GL and IEC. The initial design was carried out with all-glass fibre and no carbon with two shear webs, however it was necessary to add a third shear web at around 14.5 meters. The software indicated a long fatigue life but there could be a problem with flutter as very large blades can develop aerodynamic instability. The studies continue.

Weathering the weather
One of the big issues for wind energy in cold climates is the level of ice that can build up on blades and prevent the rotor from turning. Various solutions have been proposed and there is a large market for energy production if the problem can be resolved. Nordex Energy has employed Dr Astrid Lowe, a scientist with research experience in Antarctica to work on this for Northern Sweden. The Nordex system is pro-active with continuous monitoring of icing conditions, and using minimal energy from the turbine while it is operational to heat the aerodynamically relevant blade surfaces. In tests the anti-icing turbine generated considerably more energy during the winter months than a reference turbine.

Climate has been known to cause erosion since the world began and this process can also occur on wind blades particularly on the leading edge. 3M has looked at this problem, which is observed irrespective of hub height, location, blade length or manufacturer. Key factors are tip speed, the quality of blade finishing and the environmental conditions. The erosion usually starts in the area of the blade tip with damage to paint or coating and can affect energy output by altering the aerodynamics (up to 20%). 3M has several protective products for blades including a film-based tape based on 40 years of experience in aerospace applications like helicopter blades, and a new protection coating W4600, which is VOC free, fast curing and re-coatable. This new PU-based material has been studied for liquid/rain erosion using current methods like pulsating jet erosion testing – this is an area of Standards that is under review.

Lightning strikes tall objects and wind turbines fall into that category, so protection is essential. For an average turbine tip at 160m, even in low lightning risk areas like the North Sea there will be 1.4 flashes per year. The current blade lightning protection technology incorporates a set of receptors which attract lightning in a storm. The University of Manchester School of Electrical and Electronics provides testing to the IEC 61400-24 standard: the blade is suspended over a ground plane and high voltage is applied to simulate an electric field. The result is positive if all the leaders/streamers come only from the lightning receptor. There are potential new problem areas from innovative blade design where conductive materials are added to blades like carbon fibre or de-icing systems.

The world's longest manufactured prototype blade was sent for testing by SSP Technology at the end of 2012. The 83.5m structure was made for Samsung, which has expertise in carbon fibre in helicopter blades and expects high quality standards. The use of carbon fibre in the spar cap was discussed with lightning protection experts and the leading edge was tested for rain erosion and protected with a combination of tape and paint as the tip speed is increased.

Manufacturing improvements
The US Department of Energy has funded an advanced manufacturing innovation initiative for wind turbine blades and TPI Composites is part of this research project, which has an overall objective to create sustainable local production by improving labour productivity and cutting cycle time by 35% each. TPI is an independent US blade manufacturer supplying companies such as GE Energy. For this study typical big blade production times were estimated as: mould prep 4.5h, lay-up 95h, infusion 60h, pre-bond 60.5h, bonding/assembly 10.8h, and fabrication of spar cap 53h, of shear web 26h and root 52h. These are complex multi-axis processes and very labour intensive. Automation is the aim, but the return on capital expenditure is not justifiable with current systems used in applications like aerospace as the blade value is more like $5-10$ per pound not the $200-700 per pound that would make it affordable. There are many parts to this project including developing rotating carts for blade handling to eliminate crane availability constraints, create an ergonomic working height for finishing operations and to speed up throughput. TPI has been using ultrasonic non-destructive testing for over a decade to look at bond lines and more recently for laminate inspection, with new portable systems that can be used up-tower.

Dr Frank Peters at Iowa State University is part of the same project with ongoing work on fabric placement in spar cap assembly, which is very challenging due to the geometry. Students were asked to paint regular stripes on the fabric so that laser scanner detectors could determine whether all waves had been removed after placement. Aerospace techniques have not proved suitable so the university is developing its own systems including software. ISU is also looking at ultrasonic evaluation of wind blades to improve reliability and productivity and is reviewing non-contact systems. President Obama has visited the TPI and Iowa projects in 2012 and did some lay up work to demonstrate his support for wind energy. It is not regarded as cost effective to use the President to make wind blades!

In Germany the Fraunhofer IWES research institute is also looking at automation and cost reduction in blade manufacturing. There are a range of investments before production even starts such as design (aerodynamics, structure, load calculation, material testing, component testing and design for manufacture) and preparation (making the premaster, mould manufacture, lay up planning, prototype manufacture, certification and testing). The time taken in each step has been calculated, for example in the main mould preparation takes 12 minutes per square metre, 1 minute per kg for fibre placement , 5 minutes per kg for core material kit placement and around 5 hours for curing. Analysis shows that automated fibre placement in shells will be difficult to achieve due to the high investment costs, whereas it is more viable for the spar cap.

The Gamesa turbine company is recognised for leading the field in blade automation, one of the dream goals of wind composites manufacturing to increase production and minimise variation. In 2005 the company produced the INNOBLADE measuring 62.5m and the longest on the market, using glass and carbon fibre and divided into two modules. The latest Innoblade turbine is the G10X-Platform: the G128 prototype at 4.5MW has been tested and the first wind farm is being erected. The blade mass is round 40% lower than the current market at 15 tonnes with a 128m rotor diameter. As it is made in two parts it is simpler to transport and easier to inspect and repair, although more work is needed to join the blade on site.

Beijing Goldwind Windpower Equipment is the second largest turbine manufacturer in the world and has installed over 12GW. The company mainly outsources blade manufacture to businesses like LM Windpower, but also conducts its own research. Goldwind has three research sites based in Germany and China, with cooperation with Universities across the globe. The latest products in development are 6.0MW and 10.0MW turbines. Two new blade types were studied for a 2.5MW turbine, the GW-59 and GV-52. Using a preformed root increased productivity, embedding bolt systems saves resin and means no drilling.

The top university in the world for wind energy studies is the Technical University of Denmark where the wind industry started. Professor Povl Bronsted has 35 years of experience in wind composites. One of the most important parts in composites is the interface between fibre and resin particularly the sizing on the fibre and its thickness. The sizing is an organic material that is coated onto individual glass fibres for processability and protection. It is essential for the fibre manufacturers to liaise with the resin companies to ensure compatibility. The ideal matrix resin is low cost, ductile, tough, with viscosity less than 500 cps, processable (gel time more than 20 minutes), has low moisture absorption and is compatible with fibres, sizing and adhesives. New glass fibres are coming into the market with improved stiffness (80-90GPa) compared to traditional E-glass fibres (72-74GPa).

The automotive industry uses tough structural adhesives which are being offered to the wind energy market by Sika Technology. As one example, the properties of Silkadur WTG-1184 are: 45 MPa tensile strength, 1.5% elongation at break, E-Modulus of 4800MPa, 95C Tg and lap shear strength of 15 MPa.

The BASF company strategy in wind energy is growth to 300 million Euros in sales in 2020, including matrix and adhesive epoxy, foam core and polyurethane coatings. The epoxy division is reviewing material compatibility in rotor blades, particularly at interfaces like the trailing and leading edges, the spar cap/web and the shell/spar cap. In one chemical compatibility study the epoxy resin coloured PVC foam core material probably due to the formation of conjugated double bonds, but had no effect on balsa, PET or SAN. In tests with epoxy and a new 3B sizing on glass fibre, the static strength was improved by 14-19%. Temperature is critical to material performance, for example good adhesion is achieved between in-mould PU gel coat and epoxy at 45C and less at 55C.

The root joint usually connects the blade to the rotor using T-bolt connectors and as blades grow in size this component needs to increase in aerodynamic performance and fatigue load. Owens Corning has developed Ultrablade Fabrics to assist this development. Twelve variations in ply thickness and ply angle were analysed for 3 materials. The moment on the bolt and joint region can be very large, for example when the wind changes suddenly from 3.5 to 18.5 m/sec in 10 seconds and there is delayed pitch adjustment.

At the composite specialist Gurit, studies are going on into failure mechanisms related to core materials. Large areas of a wind blade only require thin face laminate for strength and foam core is used in the rest of the blade to create a sandwich laminate to massively improve the performance under compressive load. In these reinforced areas failure modes include panel buckling due to insufficient bending stiffness, shear crimping where the core has insufficient shear stiffness to stabilise the faces, and skin wrinkling where the core provides inadequate support to the laminate faces. In a recent study resin infused cores were subjected to a series of tests to determine safety factors.

There are many demands on core materials such as lightweight, stiff, fatigue resistant, cost effective, processability including resin compatibility, long-term stability, sustainability and global availability. 3A Composites is a world supplier of core and has looked at the options not just for balsa, PVC and PET, but also some hybrid combinations of balsa/PVC and balsa/PET. Balsa has a high resin uptake, with the new Airex sealed core the resin uptake is around 20% of that amount in tests, giving very significant cost savings on resin and lighter weight.

In one of the world centres for wind energy, Bremen, Professor Henning Albers is studying the end of life options for glass fibre composites such as wind turbine blades. The possibilities are: reconditioning and re-use for intact blades and energy recovery with residual waste in an incinerator for example in a cement plant. Current household waste plants are not geared to take large amounts of GRP material. There have been attempts at material recovery over the years by ERCOM, Seawolf Design and ReFiber ApS. In the latter case the process involves pre-crushing to 25cm pieces, pyrolysis at 600C and separation into glass fibre and filling material. The recovered glass showed a 50% loss of strength and was used in insulation. The drivers to find a solution are the expected growth in the need to dispose of old blades and the waste management regulations.

As the wind energy industry continues to adapt to the varying weather conditions worldwide and to build larger and larger turbines, so the rotor is being upgraded too. The next advanced forum will take place from 3-5 December 2013 at the Maritim Hotel in Dusseldorf, Germany at AMI's Wind Turbine Blade Manufacture 2013.

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