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Wind blade composites market grows as offshore wind energy advances

IDP Sistemas y Aplicaciones has robotic systems for finishing wind blades, which the company claims can cut the time take for a 40m blade from 100h to 10h. 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) gelcoats 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 57m blade, use of carbon led to a 27% weight reduction alongside a 14% cost increase compared to glass fibre only designs: at 90m 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 1,000 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.