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For the tidal current power industry to progress, simple models for predicting the performance of tidal current turbines are needed. Currently the aerodynamic models developed for wind turbines are typically used to predict tidal turbine performance. There are a number of significant differences between wind and tidal turbines so these models will not provide a good representation of tidal turbine performance. The main aim of this project was to show the magnitude of the differences between wind and tidal turbine performance caused by dissimilarities in wind and tidal turbine operation. Two main areas were investigated: the effects of energy extraction on flow speeds and the effect of the proximity of the boundaries. An analytical model was developed which allowed the effect of the number of turbines at a site on the performance of each turbine to be investigated. The magnitude of the change in performance was found to depend on the design and geometry of the turbines and the drag coefficient of the site. The model results indicated that, typically, there will be no noticeable reduction in the power generated by single or small farms of turbines, but installing large farms of turbines will cause significant reduction in the power generated per turbine. An analytical model was developed which allowed the effects of blockage on performance to be investigated. Applying the model to a single turbine in the Sound of Islay indicated that the performance of a realistically sized turbine in an actual tidal channel will differ from that predicted by an aerodynamic model. The effect of the proximity of the turbine to channel boundaries was investigated and performance was found to decrease as the free surface was approached. In all of the areas investigated the performance tidal turbines has been found to differ significantly from the performance predicted by aerodynamic models. This implies that it is not appropriate to use aerodynamic models to predict tidal turbine performance.

This thesis outlines the use of a new design of Tidal Energy Conversion device which has application in near shore shallow water. The design is applicable for use by coastal communities, either to generate revenue through power sales or just a stand alone system to generate off grid electricity. Previous work conducted on large scale tidal installations have shown that they suffer from excessive costs and time lines, due to their up front design philosophy. This thesis discusses the reasons behind such cost/time overruns and concludes that several technologies and techniques can be incorporated from the subsea oil and gas industry. The early ethos in the offshore oil industry in the 1970’s and 1980’s was to build large offshore structures such as steel and concrete platforms. This has now been replaced by a field development philosophy that looks at simple lower cost subsea well infrastructure as the most cost effective route to exploit a reservoir. The emerging tidal industry has not learned this lesson, yet. A set of new Tidal Energy exploitation designs are proposed and Patented. The chief advantages of this new design are their modular nature, fabrication simplicity, lower build and installation cost. Prototype work is described and further work also highlighted.

The costs of installing tidal energy technology are high, requiring expensive vessels to drill sockets in the sea bed or to handle gravity based structures of substantial mass, and this impacts on the commercial viability of any proposed marine renewables development. This thesis offers a viable alternative to socketed or gravity based installations by proposing that the downwards lift force that can be developed from the flow over a hydrofoil can be used to resist the slip and overturning moments applied to a structure by the flow. The fundamental theory of axial and crossflow energy conversion devices is outlined and the current methods of fixing and supporting tidal stream devices are analysed. The origins of tidal stream flows are discussed and the effects of local topography, bathymetry and system resonance are used to explain the significant differences between real tidal behaviour and the ideal of Newton’s equilibrium theory. The idiosyncratic and localised nature of tidal streams is thereby made clear as well as the need for a solid understanding of the resource prior to device design and installation. The principles of classical hydrodynamics and conformal mapping are used in the context of relating theoretical lift and drag functions to low aspect ratio hydrofoils with endplates, and a numerical model of distributed surface pressures around a hydrofoil is demonstrated. Subsequently, the concept is evaluated using two 1/7th scale test devices, one is field tested in a large stream under real flow conditions, and the second in a tow tank under ideal laboratory conditions. The limitations and challenges of model scaling are shown and the semi-empirical Froude method of scaling using residual forces is applied to the towing model. Analysis of the experimental data shows a correlation with normal distribution and extrapolation of the experimental results shows that the Sea Snail can operate with an average lift coefficient of 0.7 and drag coefficient of 0.18. Application of the experimental data to the full scale device demonstrates that the Sea Snail principle is not only valid, but is a significant advance on existing installation methodologies.

Tidal stream energy has the potential to supply power with low carbon dioxide emissions, but the technology is still being developed. Horizontal axis tidal stream turbines have seen the greatest development, and to generate energy at a commercial scale they will be installed in multi-device arrays. In the array, performance of downstream devices will be affected by the wake of those upstream. The actuator disc-RANS model can be used to optimise array configuration. A review of previous studies which used actuator disc-RANS showed that the model has been implemented with a wide range of parameters (boundary conditions, turbulence model, turbine model and dimensions), and predictions of velocity magnitudes in the wake of a turbine varied between similar published studies. The accuracy of the model for tidal turbines has not been demonstrated, and appropriate parameters for applying the model are not apparent from previous work. Without validation the model may not be applied to arrays with confidence. The purpose of this thesis is to identify appropriate parameters, and to demonstrate their accuracy through verification and validation studies. The first aspect of the work is to identify parameters which achieve accurate predictions using the uniform turbine model. If the k - f turbulence model, turbulence sources at the disc, and a free-slip wall to model the free surface are used, the model produces accurate results. Comparison to centreline velocities measured experimentally in the wake of a porous disc (acting as a turbine simulator) gives a coefficient of determination of 0.61-0.97 depending on the turbine thrust coefficient, with a maximum discretization error of 1.5%. Comparison to centreline velocities measured in the wake of a scaled tidal stream turbine gives a coefficient of determination of 0.92 at a thrust coefficient of 1, and a maximum discretization error of 9%. When the blade-element actuator disc turbine model is used comparison to measured cen- treline wake velocities gives a coefficient of determination of 0.94 at a thrust coefficient of 1, and a maximum discretization error of 9%. Comparison of turbine power coefficient shows that the model under predicts experimental data by between 2% at low tip speed ratio, and 9% at high tip speed ratio. The second aspect examines the capability of the turbine models to predict the power output from an array. A comparison between the uniform, and blade element actuator disc models finds that the blade element model predicts 13% higher power output due to compounded differences between the models over the array. Modelling the support structure increases predicted power output by 0.2%. Results also show that if ambient turbulence intensity is increased from 15% to 40%, overall power output from the array increases by 14%. The results provide array modellers with appropriate parameters to achieve accurate predic- tions using actuator disc-RANS. They demonstrate that the model may be used with confidence provided these parameters are used, discretization errors are minimised, and Froude number and blockage ratio are similar to the cases tested. The results also demonstrate the difference be- tween turbine models when estimating the power from an array, and a requirement to accurately characterise turbulence intensity at a tidal site and represent this data in the model.

Similar to wind turbines, it is planned that tidal energy converters (TECs) will be deployed in arrays. However, before industry can progress to this stage, more information is required on the optimum spacing between tidal turbines. Experimental studies were carried out to understand how a single TEC interacts with its surrounding environment, with a view to informing device developers on the spacing requirements. The availability of two turbine models also permitted tests to be conducted assessing the performance of a tidal turbine in the wake of another. A new large-scale towing test facility was established at Montgomery Lough. As a simplification of the marine environment, the lake provided in principle the steady, uniform flow conditions required to quantify and understand the wake produced by a tidal device. A 16m long x 6m wide twin-hull catamaran was constructed for the test programme. This doubled as a towing rig and instrument measurement platform, providing a fixed frame of reference for measurements in the wake of the turbine. Tests carried out documented the performance of the single TEC models tested and mapped the downstream wake generated. Tests were also completed to investigate the influence of the wake of a TEC, on the performance of a second device, at varying separation distances. The large-scale experiments also provided a test bed to compare the ability of different velocity measurement instruments to measures the wake of a tidal turbine. Three different acoustic instruments were used, two varieties of acoustic Doppler current profilers and an acoustic Doppler velocimeter

Wave energy converters, by their nature, extract large amounts of energy from incident waves. If the industry is to progress such that wave energy becomes a significant provider of power in the future, large wave farms will be required. Presently, consenting for these sites is a long and problematic process, mainly due to a lack of knowledge of the potential environmental impacts. Accurate numerical modelling of the effect of wave energy extraction on the wave field and subsequent evaluation of changes to coastal processes is therefore required. Modelling the wave field impact is also necessary to allow optimum wave farm configurations to be determined. This thesis addresses the need for more accurate representation of wave energy converters in numerical models so that the effect on the wave field, and subsequently the coastal processes, may be evaluated. Using a hybrid of physical and numerical modelling (MIKE21 BW and SW models) the effect of energy extraction and operation of a WEC array on the local wave climate has been determined. The main outcomes of the thesis are: an improved wave basin facility, in terms of wave climate homogeneity, reducing the standard deviation of wave amplitude by up to 50%; experimental measurement of the wave field around WEC arrays, showing that radiated waves account for a significant proportion of the wave disturbance; a new representation method of WECs for use with standard numerical modelling tools, validated against experimental results. The methodology and procedures developed here allow subsequent evaluation of changes to coastal processes and sediment transport due to WEC arrays.

The drive to develop marine renewable energy is pushing new technology and has highlighted a gap in the current knowledge. The flow round a cylinder has been extensively tested for the offshore industry and is reasonably well understood. Deploying marine renewable energy devices means installing cylindrical structure in areas with current-wave ratios that have not been tested in the offshore industry. The force acting on the cylindrical structure is very small when compared to the forces acting on a tidal turbine. There has been very limited work done on the inline wave-current forces acting on tidal turbines. This work uses Computational Fluid Dynamics to investigate the inline forces acting on a vertical axis turbine and a model experiment to calculate the inline forces acting on a horizontal turbine. Various CFD models were run and compared to published data as a means of validating the models. Experiments were carried out in the Newcastle Towing Tank on a towed oscillating tidal turbine. The in-line force on it was measured, and a Morison equation regression analysis was carried out on the experimental data. In the process of computer modelling the Savonius turbine was selected for the inline force simulations. This selection was based on the accuracy of CFD results for a give amount of processing time. The inline force acting on the Savonius was broken down into component parts representing a cylinder, the rotating turbine and the interaction of the cylinder and turbine. A relationship was found for these interactions and their relative importance in contributing to the in-line force was evaluated. The turbine displayed significantly different hydrodynamic characteristics from those of a simple equivalent cylinder.