Conclusions

The main conclusions of this thesis are:

 The effect of turbulence on power production of the two FVAWTs resulted in an increased power generation and higher power variation.

 The statistical analysis of the dynamic responses of both FVAWTs under turbulent and steady wind condition exhibited similar trends. The comparisons between the statistical parameters (the mean values, standard deviations and maximum values) of selected response parameters (BM and tension, global motions in six DOFs,) under steady wind and turbulent wind conditions revealed the effect of turbulence on the global motions and the structural responses. For the 5 MW Optimized FVAWT global motions, it was observed that the amplitude of motions are clearly higher under turbulent wind condition than under steady wind condition under both design load cases (DLC3 and DLC6) for all 6 degree of freedom motion.

 For the tower base bending moments, the statistical results revealed that as the wind speed increases, the effect of turbulence on the mean values remained relatively low but became significant at wind speeds above 18m/s with a larger bending moment under turbulent wind condition. The turbulent effect resulted is prominent in the load variation

than in the mean values. The effect of turbulence had its largest effects at 25m/s wind speed.

 For the blade bending moments, the blade bending moment distribution for both FVAWT rotors revealed the positions of high bending were at the blade extremes. Furthermore, the 5 MW Optimized FVAWT experienced high bending at the blade center, while this is considered negligible when compared with the bending moment at the blade extremes for the 5 MW Baseline FVAWT. The effect of turbulence is insignificant for the mean values especially for the 5 MW Baseline FVAWT. This effect for the 5 MW Optimized FVAWT was almost zero from cut-in wind speed up to 18m/s wind speed. However, above 18 m/s wind speed, the turbulent effects on the load variability is obvious within this region of wind speeds. However, the turbulent effects had its greatest effect on variation of the blade bending moment from the mean values rather than mean values.

 Through the spectral analysis of the global motions, the tower base and the blade BMs, and the mooring lines tension under two wind conditions (steady wind and turbulent wind), the effect of 2P frequency on the structural responses and global motions were identified. The 2P effect resulting from the VAWT rotation leads to large fluctuations of the aerodynamic loads on the rotor, the internal loads in the rotor, the internal loads in the mooring line tension and the tower base, and the motion amplitude even in steady wind conditions. Furthermore, the 2P frequency experienced a shift from the natural 2P frequency (for 2-bladed VAWTs, 2P frequency is twice the rated rotational speed) at higher wind speeds. Furthermore, the spectral analysis of both FVAWTs under two load cases (DLC3 and DLC6), demonstrated that the 2P effect increases as the wind speed increases, this demonstrate the importance of the 2P effect at high wind speeds in the operational conditions.

 The power spectra of the tower base and the blade BMs, the mooring lines tension, and the global motions in six DOFs under turbulent condition and under steady wind condition were compared with further evaluate the effect of turbulence on these parameters. From the power spectra plots, the contributions to the differences in the load variation under the respective wind conditions were identified at different frequencies. This further revealed that low-frequency responses were tremendously excited by the turbulent wind except for the heave motion which exhibited the opposite response. Furthermore,

responses at the 2P frequency is prominent in the roll and the pitch motions. However, the 2P effect is reduced by the turbulent wind.

 The structural integrity and fatigue damage in the VAWTs is significant. The comparison of the Short-Term fatigue Damage Equivalent Loads (STDEL) due to the tower base bending moments, the blade root (bottom) bending moment and the mooring line 1 tension under steady wind and under turbulent wind conditions for the 5MW Baseline FVAWT revealed the effect of turbulence leads to higher STDELs. The effect of turbulence was observed for all the selected points for fatigue analysis. The difference between the STDEL under turbulent wind and steady wind conditions widens as wind speed increases for both SS and FA bending moments but experience a decrease at 25m/s for the SS bending moment. This showed that the SS bending moment at the blade under turbulent wind condition could have a similar damaging effect as the bending moment under steady wind condition at wind speeds above the 25m/s wind speed. The statistical analysis of STDELs for the 5MW Optimized FVAWT showed rapid drop in STDEL values at wind speeds above the FVAWT rated wind speed, this further indicates that the induced loads on the FVAWT under turbulent wind condition could have a similar damaging as under steady wind condition effect at wind speeds above the rated wind speed. Also, the statistical results demonstrated that at wind speeds farther from the rated wind speed (14 m/s), the internal loads could have lower damaging effect than at wind speeds closer to the rated wind speed for this FVAWT model. In terms of STDELs, the 5 MW Baseline FVAWT experienced larger short-term fatigue damage than the 5 MW Optimized FVAWT as the wind speed increases with twice as large as the STDELs for the 5 MW Optimized FVAWT at the cut-out wind speed of 25 m/s for the tower base SS bending moment.

 A comparison between the 5MW Optimized FVAWT and the 5MW Baseline FVAWT in terms of power production, structural responses, global motion and STDELs. Below the rated wind speed, the 5 MW Optimized FVAWT produced higher power than the 5 MW Baseline FVAWT. A value of about 3 MW and 2.5 MW were reached for the 5 MW Optimized FVAWT and the 5 MW Baseline FVAWT respectively at 10 m/s wind speed.

However, the variation in the power produced continued to increase for the 5 MW Baseline FVAWT but decreases for the 5 MW Optimized FVAWT as the wind speed increases above the rated wind speed. This implies if an improved controller for the 5 MW

optimized FVAWT is applied, the generated power will more than that generated by the 5 MW base line FVAWT, especially below the rated wind speed. That means cost of energy (COE) will be reduced for the 5 MW optimized FVAWT.

 Comparisons between the results of spectral analysis of the global motions showed the 5 MW Optimized FVAWT experienced a lower excitation at the wave excitation frequency than the 5 MW Baseline FVAWT. Furthermore, the 5 MW Optimized FVAWT experienced a lower excitation at the 2P frequency than the 5 MW Baseline FVAWT. This implies a reduced 2P effects for the 5 MW Optimized FVAWT.