Development of nanoengineered coatings for leading edge protection of wind turbine blades

This research is focused on developing rain erosion resistant coatings for leading edge of the wind turbine blades. One of the critical problems of wind turbine blades is erosion of its leading edge. Leading edge erosion (LEE) will degrade the aerodynamic performance of the wind turbines by increasing the drag force and decreasing the lift force. Past studies showed that the annual energy production (AEP) of the wind turbine can be reduced by up to 25% due to LEE. Hence, applying an erosion resistant coating to the wind turbine blades is necessary. Elastomeric polyurethane (PU) has been used for LEE protection. The approach of this research is to use PU and enhance its the mechanical properties by introducing carbon nanoparticles (CNPs) multi-walled carbon nanotubes (MWCNT) and graphene nanoplatelets (GNP) and also increase the hydrophobicity of the PU by introducing silica-based sol-gel (SG). Initially the effect of the environmental temperature on mechanical properties of the pure PU was studied by performing tensile tests at different temperatures and strain rates. It was found that increasing the temperature decreases the tensile properties of the pure PU and increasing the strain rate will increase these properties. For optimising mixing parameters of CNPs in PU, PU was modified by CNPs at three different mixing speeds and three different mixing durations. Tensile tests were performed on these nanocomposites, and the optimum mixing duration (18 minutes) and speed (8000 rpm) where nanocomposite materials showed the highest mechanical performance were established. The optimum weight percentage of nanoparticles loading was also required. The PU was modified at different CNPs loading and the tensile tests were performed on pure and modified PUs. The results of the tensile tests showed that PU with 0.5wt% of MWCNTs and PU with 0.5wt% of GNP-COOH loading resulted in the highest amount of Young's modulus, UTS, elongation at break and modulus of toughness. Other CNPs such as GNP-NH3 and combined GNP-COOH/CNT and GNP-NH3/CNT were also investigated. The results showed that modifying PU with GNP-COOH at 0.5wt% loading gives the best tensile properties. Finally, the hydrophobicity of the coating was improved by adding silica-based sol-gel to the GNP modified PU. The water contact angle (CA) experiments showed that modifying PU with GNP and SG increased the CA of neat PU from 56 degree to 110 degree for PU+GNP+SG while the free surface energy reduced from 114.6 mJ/m2 to 50 mJ/m2 . The cyclic compression tests were carried out and the results revealed that the maximum stress at maximum strain of 0.5 for PU is 107.9 MPa, for PU + GNP is 77.4 MPa and for PU + GNP viii + SG is 71.5 MPa. This indicates PU + GNP + SG experiences the least stresses during cyclic compressive loading. Tearing test results showed that the PU + GNP nanocomposite has the highest tearing strength and PU + GNP + SG has the highest elongation at break. The PU + GNP + SG nanocomposite has much higher value for Young's modulus (95%), tensile strength (115%), modulus of toughness (124%) and elongation at break (102%) relative to the neat PU at room temperature. In addition, the tearing energy for both modified PU nanocomposites was higher than the neat PU (137% increase for PU + GNP and 148% increase for PU + GNP + SG). In addition to the mechanical tests, water absorption test was carried out for a period of six months to analyse the amount of the water that can be absorbed by developed materials and the effect of absorbed water on the tensile properties of the coating materials were identified. Experimental results showed that after six months, the weight of the pure PU, PU+GNP and PU+GNP+SG increased by 4%, 3.7% and 3.6%, respectively. The results showed that absorbing water by PU decreases the tensile properties of the material. Microstructural analysis of the developed PU coatings by FTIR, field emission scanning electron microscope (FESEM) and energy-dispersive X-ray spectroscopy (EDX) were carried out and the detailed results are presented in this thesis. Finally the developed coatings were tested for anti-erosion performance using the single point impact fatigue testing (SPIFT) technique. It is demonstrated that graphene / silica reinforced PU coating can provide better erosion protection with substantial longer time before material loss than non-reinforced PU.