Composites in Wind Energy (USA)
The wind energy market is experiencing rapid growth worldwide and has doubled in size every 3 years for the past 30 years. It is expected to have generated 331 TWh in 2010 alone, amounting to 1.6% of global energy consumption. This expansion requires new manufacturing plant including facilities for large composite blades. Applied Market Information Ltd. (AMI) organised a global networking conference on Wind Turbine Blade Manufacture in Germany in December 2010 to bring together energy companies, wind industry players and the composites supply chain, with top blade manufacturers and researchers reviewing the latest innovations and the issues.
The process of blade manufacture is very labour-intensive involving hand lay-up: automation is being developed and quality control from production stages to installation is key to wind farm success. The cost of energy production is lower with larger turbines due to lower installation costs, which is driving up the size of blades. It is even more of a factor offshore where turbines and blades are being designed to need less servicing, because of the difficulties of access. The blades comprise a root area providing the connection to the hub, and an aerodynamic area. The structure is supported by the load-bearing spars. The sequence in blade manufacturing is material preparation, tool preparation, component moulding, assembly and finishing. Manufacturing process factors that can affect blade quality include fibre misalignment and waviness, dry spots (no resin), and voids in bonding.
LM Wind Power is a global leader in blade manufacturing: the company has produced 140,000 blades since 1978 and has 13 production facilities. It is operating in Asia, Europe and the Americas and estimates that more than 1 in 3 wind turbines use its blades. In line with other parts of this industry, it offers to service blades and turbines through its logistics division. In-house design and testing facilities allow LM Wind Power to adjust its blades for different operating conditions. Blade service life is determined by factors such as fracture mechanics and wind loading, because composites degrade when loaded. Surface damage occurs due to erosion and lightning strikes: lightning is a problem in composite structures that needs to be managed.
Vestas Wind Systems has installed more than 41,000 wind turbines in 65 countries. It has its own blade production facilities and research. The company is prioritising quality control aiming for Six Sigma systems, which means 3.4 defects per million opportunities – the process is gradual and the target for 2010 was 5 Sigma. One case study on blades examined areas that might cause problems, like damage to the core foam from an extended cure cycle due to trapping of heat inside the laminate: in this instance the designer can check the exotherm before starting production. In a two-mould factory this work can increase productivity by 9%, equivalent to 66 more blades per year. Quality control is the focus of the company Vesper, because current measures tend to be manual and irregular. The company is using a non-contact laser-based scanner to inspect blades, taking around 12 hours, and detecting shape variation.
In India WinWinD has set up a series of blade manufacturing plants. Location is selected for easy access to transport (highway, port and airport) and size of premises. The major problem for Indian composites manufacturing has been the climate, with tropical conditions. Capital costs have been coming down as production technology has been optimized. WinWinD uses vacuum assisted resin transfer moulding (VARIM); prepreg is the growing alternative. Production stages include material preparation like glass cutting, resin mixing and foam preparation; prefabrication of the structural supports like the spar cap and root insert; moulding; wet and dry finishing; then assembly with metal parts and lightning connections. The company has in house testing facilities and works to GL, DNV and CWET (India) standards.
Structural design optimisation of blades is being studied by ACENTISS, which is working with Airworks of Italy and IABG to form A2Wind to focus on rotor blade development using aerospace technology. The structural constraints of blades include a minimum stiffness to avoid collisions with the turbine tower, and a minimum buckling factor. There are manufacturing constraints too and cost is a big factor. Studies showed that carbon fibre will be needed for structural and cost optimisation as blades get longer.
In blade design there are limits due to aerodynamic stress, rotational stress, mass-dependent stress and frequency. The SGL Group both manufactures wind turbine blades and supplies carbon fibre: it has studied the use of carbon fibre in the spar cap load-carrying structure of rotor blades, compared to glass fibre. The carbon fibre blades were 20% lighter and had about 25% lower mass moment. In addition, the use of carbon fibre had a big effect on lower blade root fatigue loads, which means that the root and pitch system can be smaller, with a longer blade.
The trend towards carbon fibres requires new expertise as they are finer than glass and more difficult to infuse with resin. BASF supplies epoxy to the wind blade market and Dr Gregor Daun has conducted research to improve infusion for big blades. With geometric scale up of structures, most forces are squared and torques are cubed, so laminate thickness needs to increase. Over long flow paths infusion fronts slow down and become more viscous, so BASF has a new latent system with slower gelation. This can assist with the thicker parts too as it improves homogeneity of curing.
Euros Entwicklungsgesellschaft was founded in 1996 in Berlin to focus on blade design and has since started two plants in Poland at Ustron and Zory-Warszowice for blade and master plug/mould manufacturing. It supplies around 1% of the European market. The Materials Department is carrying out research on foam cores for spar webs and the shell, looking at resin absorption into the slits in the sandwich and the cellular structure, which depends on the cell diameter and wall thickness. The shear stiffness after resin absorption increases by a factor of approximately 3.2, which can be taken into account in the design process and used to reduce blade weight.
Fiberline Composites makes subcomponents of blades and has been involved in studies of carbon fibre composites: carbon fibre is very sensitive to wrinkles. There are several methods for measuring fibre misalignment including Multiple Field Image Analysis (MFIA), Confocal Laser Scanning Microscopy (CSLM), X-ray tomography and Fourier Transform Misalignment Analysis (FTMA). Shear properties can be monitored using methods like the Iosipescu shear test combined with Digital Image Correlation (DIC). Fiberline has developed an injection pultrusion processing method that gives highly aligned fibres.
Owens Corning OCV Technical Fabrics is a supplier of glass-based fabrics for the global composites industry including wind blades. The Ultrablade fabric helps to improve modulus and strength in the vacuum infusion process (VARTM) supporting the longer blades, while reducing spar weight by around 17% in a study using the IEC 61400-1 class II blade model.
Headquartered in California, Clipper Windpower is a relatively young company producing 2.5 MW Liberty wind turbines and developing 10 MW Britannia models for the offshore market. It is now affiliated to United Technologies Corp. (UTC) and can develop wind projects. According the American Wind Energy Association it had 66% market share of the turbine market of 2.5 MW and larger in the United States in 2009. It has a new blade to enter the market in the 3rd quarter of 2011, and is focussing on innovative production processes like mechanisation and automation. Laser projection systems can be use for ply placement, for example. The design process takes into account the manufacturing methods. There are non-destructive test methods for inspecting blades such as DIC, ultrasonics, back scatter X-ray and thermography.
Riso-DTU is one of the top universities in the world for wind composites research and has reviewed problems in blades. Various defects may be introduced in manufacturing such as matrix cracks caused by shrinkage, fibre wrinkles and breaks, poor bonding and voids in adhesive layers. Interface defects are a common cause of failure, for example due to lack of adhesive at bond lines. Some faults are not important, but others should be addressed quickly to prevent blade failure: Riso-DTU is examining crack propagation, buckling-driven delamination, cohesive laws and traction-separation laws to try to determine which issues are critical. There are various types of repairs including plug and patch, or a scarf repair.
On the Isle of Wight, Solent Composite Systems is looking at ways to reduce manufacturing costs including automation and low power curing systems. It has tested automated tape laying (ATL) on blade shells, which entailed adjusting the machine to make sure that the leading edge was covered, along with light resin transfer moulding (LRTM). The mould can be heated with insulated water pipes, air or electrical methods. The SmartCure system applies heat in the mould shell combined with zone temperature monitoring to deliver power only where it is most needed. As the exotherm of the cure develops the power is cut to those areas.
In Ireland and Germany, EireComposites Teo is looking at future blade technologies such as thermoplastics; alternative reinforcements like basalt and natural fibres; and VOC (volatile organic compound)-free thermosets like the polyester and epoxy powders currently in use by the paint and coating industries. One of the issues with both thermoplastics and VOC-free resins is that they need to be processed at temperatures above 200C, which in turn requires tooling that can withstand 300C for fast heating. The company has experimented with ceramic tooling and embedded electrical heating with moulds up to 12.6 meters long.
The UK is preparing for massive growth in its offshore wind industry, and Narec has built new test facilities for static and fatigue testing for 5 MW blades of 60 meters and above. One of the challenges is handling, for example a 70 meter blade is predicted to weigh 30 tonnes. Blade deflections during test could be up to 25 meters at the tip for an 80 meter blade. There are several certification bodies including GL, DNV and DEWI and one of the main blade testing specifications is IEC-61400-23. Currently there are test facilities at Riso-DTU in Denmark, WMC in the Netherlands, Fraunhofer IWES in Germany, CENER in Spain, NREL in the USA, at Baroda in India, being built in China, and at a blade manufacturer in Brazil.
Wind blade service engineers such as CP. Max Rotortechnik have built up years of experience of blade maintenance. The company has seen problems from superficial pinholes and flaking on the surface, to internal defects such as bonding failure at the leading edge and the trailing edge, and loose and cracked web visible as stress whitening. Some damage is caused during blade transport. Lightning strikes most commonly affect the blade tip and are a problem if the lightning protection system is faulty.
Wind blades have to be lightning tested to International Electrotechnical Commission standard IEC 61400-24 (updated June 2010) at facilities like Testinglab Denmark: Kim Bertelsen comments, “It is not a matter of if the turbine will be hit by lightning, but how many times per year”. A strike begins with a high electric field surrounding the blade tip when a lightning leader from a cloud comes downward. Tests are carried out to see where on the blade the lightning attachment points occur and whether the design can handle the impact from the lightning current. Lightning protection systems like receptors are built into blades.
Some wind farms in cold regions stop operating for much of the winter according to WindREN, because of the ice that builds up on the blades adding tonnes of weight. Various anti-icing systems have been tested including black blades (not enough sun at the right times), a carbon fibre layer under the coating (WinWind/Skelleftea Kraft), hot air fan systems (Enercon/Svevind), hydrophobic coatings (Nordex/LM/Dong Energy), and foil systems (EcoTEMP/O2VK/Kelly/Goodrich). The geographical areas affected by icing need to be mapped so that wind turbines can be specified appropriately: there is vast potential for wind energy in areas such as Canada, Sweden and Russia that could benefit from turbines that operate under severe icing conditions. The International Energy Agency (IEA) is running projects on this area of research, for example measuring icing on wind turbines at different locations.
Source: amiplastics, February 28, 2011