State of the art:
Due to an on-going increase in rotor diameter today’s wind blade shell structures are designed in sandwich composites architectures to resist high loads. In particular these composites have to maintain the blade shape and resist the wind and gravitational forces in a wind blade. In a general view most blade shells are set up by two rigid top layers which are combined with a light core material to manufacture a stiff and light composite sandwich. Each blade manufacturer has defined an individual concept for the architecture of the composites which make different combinations available. In these sandwich schemes core materials are used to increase the moment of inertia to provide additional buckling capacity as well as providing a more lightweight design with similar buckling capacity. These cores have to withstand high shear loads whereas composite skins are exposed to bending stresses.
Currently two core material types are widely used in wind energy application according to the best suitability on the requirements stated above:
1) End-grain balsa wood
2) Extruded polyvinylchloride (PVC) foam
End grain balsa wood is currently used in a wide range of sandwich composite blades due to its good mechanical properties (high compressive and tensile strength) combined with a low density and the potential delivery from renewable resources. The structure of balsa is very stiff especially the compressive strength in grain direction. Nowadays 90% of the global balsa wood demand is delivered from artificial cultivated plants of balsa trees in Ecuador on the South American continent.
On the other hand PVC foam as a non-natural product is very popular for sandwich constructions. Generally speaking polymeric foams made of PVC for sandwich applications are lighter than balsa wood core materials. These foams have good mechanical properties and are widely available and formable to different shape geometries but are more expensive than balsa wood. PVC foam is highly consistent but carries fewer loads which make it twice as thick as balsa. Due to this fact, PVC foam is used in lower stressed areas of the blade from the mid-point of the tip.
Beside these two core types other new materials like styrene acrylonitrile (SAN) and polyethylene terephthalate (PET) foams are entering the market for sandwich cores. First approaches to use PET foams are available, that show high potential for this material in wind blade applications. At present several developments take place to investigate the behavior of the material with loads in sandwich
Previous core materials that are currently used in wind blade applications have several limitations regarding material properties and processing behaviour especially for the usage in next generation wind blades in off-shore areas.
Balsa wood in general has, beside the stated advantages several disadvantages. According to the prospected market growth there will be a global lack of availability in this field due to the “natural” production of the balsa tree. Balsa trees need special conditions to grow which are limited to small regions of the world. The artificial plant production cannot be increased in the same way as the worldwide demand is rising. PVC foams are widely used in wind blade applications. Currently there is no sustainable solution for recycling of PVC which is commercially implemented. Today PVC foam materials are sent to landfills after the blade lifetime. Due to the high chlorine content of the material these foams cannot be fed into recycling processes or used for the energy production (thermal recycling arises hydrochloric acid).
WALiD has developed new ultra-light and stiff foam materials which can withstand the requirements for sandwich cores in large off-shore wind turbines. Sandwich materials made from thermoplastic foams and fibre-reinforced plastics have been used for the outer shell of the blade as well as for segments of the inner supporting structure.
The incorporation of nanoparticles and fibres were investigated to provide a stronger material. As reported in the literature, nanoscale reinforcements are a new class of reinforcing materials which can be used for foams, as their dimensions are small enough not to disturb the foaming behaviour of the thermoplastic material.
The new thermoplastic foams have a high recyclability as shown in the life cycle analysis. The foams developed avoid resin uptake by modified surfaces and near-net shapeability. They also have closed celled surfaces avoiding infiltration of resin into the material during the resin infusion process.