Development of the epoxy composite complex permittivity and its application in wind turbine blades

Abstract

Offshore wind farm structures may have the potential to affect marine navigation and communication systems by reflecting radar signals. With ever increasing size of wind turbines it is necessary to better understand the influence of radar signals on wind turbine blades in order to minimise the radar reflecting potential. One possible way of reducing radar reflection is to use radar absorbing materials. In this thesis, epoxy composite materials reinforced with five different types of nano-size additives: carbon nanotubes (CNTs), carbon blacks (CBs), silver, tungsten carbide and titanium oxide are manufactured and tested to investigated their potential as wind turbine blade material that absorb radar signals. Nanoadditives/epoxy composites with additives content ranging from 0.05-1 wt. % were fabricated by a simple cast moulding process. The nanoadditives were dispersed in the epoxy resin by sonication method. The degree of nanoadditives dispersion was observed by examining the surface of the composite materials using scanning electron microscope (SEM). Complex permittivity of the nanoadditives/epoxy composites was studied using a free wave transmittance only method at a frequency range of 6.5-10.5 GHz. The effect of the percolation threshold of the direct current conductivity on the composite permittivity was analysed and discussion. In order to get a better insight in the importance of the results they were compared to existing models (Maxwell- Garnett, Bruggeman, Bottcher, Lichtenecker and Lichtenecker-Rother). A new model based rule of mixtures is developed to predict the complex permittivity of the composite. A model of wind turbine rotor blade made of the nanoadditives/epoxy composite was developed using Comsol-multiphysics software. The data obtained from the experimental work was inputted in to the model to generate result of backscattered energy verses composite permittivity as a function of nanoadditives content. A decrease in backscattered energy was noticed with increasing nanoadditives content. The results demonstrate that radar reflecting signals will be significantly reduced by incorporating nanoadditives in the composite materials.