Abstract
In this research, a novel conceptual desalination system was introduced which can be powered by Horizontal Axis Tidal (HAT) and Vertical Axis Tidal (VAT) turbines. Since in the proposed design, the most important part is the tidal turbine, the focus has been placed on optimisation of the turbines. The energy required for desalinating 1 m3/h was determined. Accordingly, a VAT turbine and a HAT turbine were separately designed to fulfil this amount of energy. The greatest weakness of these turbines is the high price of design, development, and manufacturing. Traditionally, optimisation of turbine geometry can be achieved by running several numerical models of the turbine which can become computationally expensive. In this work, a combination of the Taguchi method and CFD modelling was used as a straightforward solution for optimisation of geometry of tidal turbines. Although improving the hydrodynamic performance is a key objective in the design of ocean-powered turbines, some factors affect the efficiency of the device during its operation. In this study, the impacts of a wide range of surface roughness, as a tribological parameter, on stream flow around a hydro turbine and its power loss were studied. A comprehensive program of 3D Computational Fluid Dynamics (CFD) modelling, as well as an extensive range of experiments were carried out on a tidal turbine in order to measure reduction in hydrodynamic performance due to surface roughness. The results showed that surface roughness of turbine blades plays an important role in the hydrodynamics of the flow around the turbine. The surface roughness increases turbulence and decreases the active fluid energy that is required for rotating the turbine, thereby reducing the performance of the turbine. The geometry of the HAT turbine was optimised with combination of only 16 CFD simulations using the Taguchi method. The effects of blade size, number of blades, hub radius, and hub shape were studied and optimised. The results revealed that the most important parameters influencing the power output of HAT turbine are the number of blades, size of blade, hub radius, and hub shape. Moreover, the superposition model showed that the minimum signal-to-noise (S/N) ratio was 5% less than the amount achieved in the Taguchi approach. The power coefficient (Cp) of the optimised HAT turbine was 0.44 according to the results of CFD simulations, which was 10% higher than that of the baseline model (0.40) at tip speed ratio (TSR) of 5. The weight of the optimised model was less than the baseline model by 17%. Moreover, a number of CFD simulations were carried out using the mixed-level modified Taguchi technique to determine the optimal hydrodynamic performance of a VAT turbine. The effects of four parameters: twist angle, camber position, maximum camber, and chord/radius ratio were studied. The interaction of these parameters was investigated using the Variance of Analysis (ANOVA) approach. The Taguchi analysis showed that the most significant parameter affecting hydrodynamic performance of the turbine is the twist angle and the least effective parameter is chord/radius ratio. The ANOVA interaction analysis showed that the twist angle, camber position and maximum camber have significant interaction with each other. Moreover, the results showed that the power coefficient (Cp) for the optimised VAT turbine was improved by 26% compared to the baseline design. In addition, the flow separation in the optimised model was greatly reduced in comparison with the baseline model, signifying that the twisted and cambered blade could be effective in normalising the spraying vortices over blades due to suppressing dynamic-stall. The findings of this thesis can provide guidelines for optimisation of tidal turbines.