[Report of] Specialist Committee V.4: ocean, wind and wave energy utilization

The committee's mandate was :Concern for structural design of ocean energy utilization devices, such as offshore wind turbines, support structures and fixed or floating wave and tidal energy converters. Attention shall be given to the interaction between the load and the structural response and shall include due consideration of the stochastic nature of the waves, current and wind

Methodology for tidal turbine representation in ocean circulation model

The present method proposes the use and adaptation of ocean circulation models as an assessment tool framework for tidal current turbine (TCT) array layout optimization. By adapting both momentum and turbulence transport equations of an existing model, the present TCT representation method is proposed to extend the actuator disc concept to 3-D large-scale ocean circulation models. Through the reproduction of experimental flume tests and grid dependency tests, this method has shown its numerical coherence as well as its ability to simulate accurately both momentum and turbulent turbine-induced perturbations in both near and far wakes in a relatively short period of computation time. Consequently the present TCT representation method is a very promising basis for the development of a TCT array layout optimization tool

Flow curvature effects on dynamic behaviour of a novel vertical axis tidal current turbine: numerical and experimental analysis

The paper deals with performances analysis of vertical axis turbine to exploit tidal marine currents. Flow curvature effects on performences of a novel vertical axis turbine have been investuigated. It has been shown that the flow curvature effect allows to design properly an accurate airfoil shape to increase turbine performances

State of the Art in the Optimisation of Wind Turbine Performance Using CFD

Wind energy has received increasing attention in recent years due to its sustainability and geographically wide availability. The efficiency of wind energy utilisation highly depends on the performance of wind turbines, which convert the kinetic energy in wind into electrical energy. In order to optimise wind turbine performance and reduce the cost of next-generation wind turbines, it is crucial to have a view of the state of the art in the key aspects on the performance optimisation of wind turbines using Computational Fluid Dynamics (CFD), which has attracted enormous interest in the development of next-generation wind turbines in recent years. This paper presents a comprehensive review of the state-of-the-art progress on optimisation of wind turbine performance using CFD, reviewing the objective functions to judge the performance of wind turbine, CFD approaches applied in the simulation of wind turbines and optimisation algorithms for wind turbine performance. This paper has been written for both researchers new to this research area by summarising underlying theory whilst presenting a comprehensive review on the up-to-date studies, and experts in the field of study by collecting a comprehensive list of related references where the details of computational methods that have been employed lately can be obtained

Hydrodynamic modelling of marine renewable energy devices : a state of the art review

This paper reviews key issues in the physical and numerical modelling of marine renewable energy systems, including wave energy devices, current turbines, and offshore wind turbines. The paper starts with an overview of the types of devices considered, and introduces some key studies in marine renewable energy modelling research. The development of new International Towing Tank Conference (ITTC) guidelines for model testing these devices is placed in the context of guidelines developed or under development by other international bodies as well as via research projects. Some particular challenges are introduced in the experimental and numerical modelling and testing of these devices, including the simulation of Power-Take-Off systems (PTOs) for physical models of all devices, approaches for numerical modelling of devices, and the correct modelling of wind load on offshore wind turbines. Finally, issues related to the uncertainty in performance prediction from model test results are discussed.The paper is based on the report of the International Towing Tank Conference specialist committee on Hydrodynamic Modelling of Marine Renewable Energy Devices to the 27th ITTC held in Copenhagen, Denmark in 2014 (ITTC Specialist Committee on Hydrodynamic Modelling of Marine Renewable Energy Devices, 2014a. Final Report and Recommendations to the 27th ITTC Proc. 27th International Towing Tank Conference, Copehagen, Denmark, vol. 2, pp. 680–725)

A correction to the enhanced bottom drag parameterisation of tidal turbines

Hydrodynamic modelling is an important tool for the development of tidal stream energy projects. Many hydrodynamic models incorporate the effect of tidal turbines through an enhanced bottom drag. In this paper we show that although for coarse grid resolutions (kilometre scale) the resulting force exerted on the flow agrees well with the theoretical value, the force starts decreasing with decreasing grid sizes when these become smaller than the length scale of the wake recovery. This is because the assumption that the upstream velocity can be approximated by the local model velocity, is no longer valid. Using linear momentum actuator disc theory however, we derive a relationship between these two velocities and formulate a correction to the enhanced bottom drag formulation that consistently applies a force that remains closed to the theoretical value, for all grid sizes down to the turbine scale. In addition, a better understanding of the relation between the model, upstream, and actual turbine velocity, as predicted by actuator disc theory, leads to an improved estimate of the usefully extractable energy. We show how the corrections can be applied (demonstrated here for the models MIKE 21 and Fluidity) by a simple modification of the drag coefficient

Optimisation of ocean-powered turbines for seawater desalination

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

Numerical simulation for the hydrodynamic performance of hydropower turbine near free surface

The performance of hydropower turbine in shallow water can be affected by the presence of free surface. Therefore, it is of great interest to investigate the influence of free surface on hydropower turbine performance through computational simulations. For a better understanding of flow field around hydropower turbine operating in shallow water, it is important to analyze the flow over a single hydrofoil beneath free surface first. Therefore, as the first part of this thesis, the Computational Fluid Dynamics (CFD) methodology was used for numerical simulation of 2D unsteady incompressible viscous flow over a hydrofoil under the free surface. The computation was based on finite volume discretization incorporated with the interface capturing volume of fluid method (VOF) to solve the flow field. The SST − turbulence model was used to capture the turbulent flow in the field. A comparison of the present numerical results with experimental data and previous numerical results was presented to show how accurate to use turbulence model to simulate the result. A comprehensive simulation of quantities like wave profiles and forces was performed for various angles of attack ranging from -15 to 15 degree, and ℎ⁄ from 0.2 to 0.9 resulting in low Froude numbers ranging from 0.1 to 0.9. It was found that the presence of the free surface reduced the lift coefficient by 33.24% in the case of Froude number of 0.3 and increased the drag coefficient by 79.01%. As the second part of this thesis, the numerical simulations of flow over a 3-blade vertical axis hydropower turbine were performed. A good agreement between the current simulations and previous works was observed through validation process. Then, a comprehensive simulation was performed for submerged depths ranging from h/R = 1.2 to 2, and tip speed ratio from λ = 1 to 3 in the case of fix-pitch blades. Variation in submerged depth brought substantial changes in the flow and vortex pattern. The results revealed that the presence of the free surface decreased the power coefficient by 19.05% for the closest submerged depth of h/R = 1.2 at optimal tip speed ratio of λ = 2.5. The wave breaking also occurred when the submerged depth was smaller than h/R = 2. In order to understand the speeds limit for hydropower turbine, free-to-spin cases were investigated by six DOFs method. The top speed had a 4.24% drop by comparing the largest and the smallest submerged depths. Variable pitch improved the power coefficient by 28.10% when the free surface was far from the hydropower turbine. However, the power coefficient improvement became significantly small when the hydropower turbine was placed close to the free surface

Non-linear dynamic analysis of the response of moored floating structures

© 2016 Elsevier Ltd. The complexity of the dynamic response of offshore marine structures requires advanced simulations tools for the accurate assessment of the seakeeping behaviour of these devices. The aim of this work is to present a new time-domain model for solving the dynamics of moored floating marine devices, specifically offshore wind turbines, subjected to non-linear environmental loads. The paper first introduces the formulation of the second-order wave radiation-diffraction solver, designed for calculating the wave-floater interaction. Then, the solver of the mooring dynamics, based on a non-linear Finite Element Method (FEM) approach, is presented. Next, the procedure developed for coupling the floater dynamics model with the mooring model is described. Some validation examples of the developed models, and comparisons among different mooring approaches, are presented. Finally, a study of the OC3 floating wind turbine concept is performed to analyze the influence of the mooring model in the dynamics of the platform and the tension in the mooring lines. The work comes to the conclusion that the coupling of a dynamic mooring model along with a second-order wave radiation-diffraction solver can offer realistic predictions of the floating wind turbine performance.Postprint (published version