In the previous post on the blade design project I discussed the commonly used composite materials in wind turbine design. In this post I compare natural fibers to the highly popular glass fibers and discuss why natural fibers truly are environmentally superior.
Comparison to glass fibers
Despite the fact that the use of natural fibres as reinforcement has tripled to nearly 50,000 tonnes in the first decade of this century , plant fibre composites still have no more than a 2% share in the EU market . They have mainly be used as replacement for glass fibre reinforced automotive parts. Also in sports and leisure, furniture and consumer goods they are used as light and cheap reinforcements. In all this applications they play little or no structural role. This tendency can be understood partially by making a comparative study of the mechanical properties between glass fibers and the most common natural fibers, as has been done by researchers at the Catholic University of Leuven .
Glass fibre’s popularity is a result of a low cost in comparison to aramid and carbon fibers and its very good mechanical properties. Yet is has a few serious disadvantages compared to natural fibres. Most surprising results are the low density and cost of the natural fibres. Densities of glass fiber lie around 2.6 g/cm^3 and have a cost of around $2/kg, while natural fibers typically have a density of around 1.5 g/cm^3 and a cost between 0.22 and 1.10$/kg .
The mechanical tests showed that mechanical properties such as tensile strength, flexural strengths and impact resistance compared favorably for the five tested natural fiber reinforced composites tested (kenaf, coir, sisal, hemp and jute). The specific properties were in some cases even better than those of the glass fiber reinforced composite. The research concludes natural fibers have a potential to replace glass in wide range of application as long as the loads are not too high. As discussed in an earlier post, the loads on wind turbine blades are high and the question arises if natural fibers are an option for this application. I hope to give an answer to this question with my own tests.
Are natural fibers really environmentally superior?
One can question whether natural fibers really offer an environmental advantage over typical fiber materials such as glass and carbon. A comparison of three in-depth life cycle analyses makes it possible to outline four reasons why natural fibers are generally environmentally superior to glass fiber .
Energy use, CO2 emissions and CO emissions are significantly lower. The inherent production energy of natural fibers is much lower because it uses energy of the sun to grow. However, emissions of nitrates and phosphates to water as well as NOx to air are higher due to the use of fertilizer during the cultivation of some of the natural fibers. Still, substitution of glass fibers with natural fibers of equal weight improves the net environmental performance.
Because of the better mechanical properties, less glass fiber is needed in comparison to natural fiber composites. To have the same mechanical properties, a higher volume fraction of the latter is used. The reduced volume and weight fraction of the resin also results in a decrease of energy use and emissions, since the production of the base polymer is mostly more demanding.
The final natural reinforced component mostly weighs less because of the higher volume fraction of the less dense natural fibers. This lowers the energy consumption during the use phase of the life cycle. This is why the automotive industry was one of the first to introduce natural fiber reinforced composites.
The end of life incineration of natural fibers is theoretically carbon neutral. The CO2 released when the fibers are burned, were stored by the plant during its lifetime.
Overall, one can conclude natural fibers do offer significant environmental benefits over glass fibers. The use in high load structural design situations remains a question..
Sources
- Can flax replace E-glass instructural composites? A small wind turbine blade case study, J. Schubel et al.
- Are natural fiber composites environmentally superior to glass fiber reinforced composites?, S.V. Joshi et al.
- Natural Fibres: can they replace glass in fibre reinforced plastics?, P. Wambua et al.
