Tidal Stream Turbine Modelling

As tidal turbine farms grow they interact with the larger scale flow along a channel by increasing the channel's drag coefficient. This interaction limits a channel's potential to produce power. A 1D model for a tidal channel is combined with a theory for turbines in a channel to show that the tuning of the flow through the turbines and the density of turbines in a channel's cross-section also interact with the larger scale flow, via the drag coefficient, to determine the power available for production. To maximise turbine efficiency, i.e. the power available per turbine, farms must occupy the largest fraction of a channel's cross-section permitted by navigational and environmental constraints. Maximising of power available with these necessarily densely packed farms requires turbines to be tuned for a particular channel and turbine density. The optimal through-flow tuning fraction varies from near 1/3 for small farms occupying a small fraction of the cross-section, to near 1 for large farms occupying most of the cross-section. Consequently, tunings are higher than the optimal through-flow tuning of 1/3 for an isolated turbine from the classic turbine theory. Large optimally tuned farms can realise most of a channel's potential. Optimal tunings are dependent on the number of turbines per row, the number of rows, as well as the channel geometry, the background bottom friction coefficient and the tidal forcing.

Abstract
Details are given herein, of the development of an equation for predicting the longitudinal dispersion coefficient in riverine flows, based on 81 sets of measured data, and obtained from 30 rivers in the USA. This equation relates the dispersion coefficient to the hydraulic and geometric parameters of the flow and has been derived using dimensional and regression analysis, with a high correlation coefficient (i.e. R2=0.84). The formulation has been compared with many other existing empirical equations, frequently used to predict the longitudinal dispersion coefficient in riverine flows, with the comparisons based on four different statistical methods. These statistical comparisons have shown that the new equation appears to be more accurate than the other equations considered. The new dispersion equation was then linearly combined with a similar equation recently proposed by Seo and Cheong (J. Hydraul. Eng. ASCE 124 (1998) 25) and this combined equation was then also analysed using statistical methods. The existing empirical equations used to estimate the longitudinal dispersion coefficient and the new equations proposed in this study were included in the advective dispersion equation to predict the suspended sediment concentrations at three sites in the Humber Estuary sited along the northeast coast of England. The average percentage errors between the predicted- and measured-field data for the proposed new dispersion equations were less than those obtained using the previously documented equations.

Blade boundary layer resolved simulations of two different tidal turbine rotor designs are presented. The first rotor was designed to achieve its maximum power coefficient at tip speed ratio of 5 in unblocked conditions and the other was designed to achieve its maximum power coefficient at a tip speed ratio of 5 but at a higher local blockage ratio of 0.197. Both designs achieve their maximum power coefficient close to a tip-speed-ratio of 5 for which they were originally designed. In addition, the maximum power coefficients for both rotors operating in an unblocked domain compare favourably with full-scale deployed horizontal axis tidal turbines. With increasing blockage ratio, it is observed that greater power coefficients can be achieved, 0.62 to 0.63, but that this requires an increased thrust. The blockage designed rotor can provide this extra thrust naturally , primarily as it is designed with greater solidity. In contrast, the rotor designed for unblocked flow must be made to operate at a significantly higher tip-speed-ratio to achieve the required thrust.

The Betz limit sets a theoretical upper limit for the power production by turbines expressed as a maximum power coefficient of 16/27. While power production by wind turbines falls short of the Betz limit, tidal turbines in a channel can theoretically have a power coefficient several times larger than 16/27. However, power extraction by turbines in large tidal farms also reduces the flow along the channel, limiting their maximum output. Despite this flow reduction, turbines in tidal farms can produce enough power to meet a stricter definition of what it means to exceed the Betz limit, one where the maximum power output of a turbine at the reduced flow exceeds the maximum output from a single Betz turbine operating in the unreduced flow. While having a power coefficient > 16/27 is easily achieved by turbines in a channel, generating enough power to meet this stricter definition of exceedance is much more difficult. Whether turbines meet this stricter definition depends on their number, how they are arranged and tuned, and the dynamical balance of the channel. Arranging a tidal turbine farm so that the turbines within it exceed the stricter Betz limit would give tidal turbine farms an economic advantage over similarly sized wind farms. However, exceeding the stricter limit comes at a cost of both higher structural loads on the tidal turbines and the need to produce power from weaker flows. Farms in a channel loosely based on the Pentland Firth are used to discuss exceedance and structural loads.

"This thesis details the development, verification and validation of an unsteady unstructured high order (≥ 3) h/p Discontinuous Galerkin - Fourier solver for the incompressible Navier-Stokes equations on static and rotating meshes in two and three dimensions. This general purpose solver is used to provide insight into cross-flow (wind or tidal) turbine physical phenomena.
Simulation of this type of turbine for renewable energy generation needs to account for the rotational motion of the blades with respect to the fixed environment. This rotational motion implies azimuthal changes in blade aero/hydro-dynamics that result in complex flow phenomena such as stalled flows, vortex shedding and blade-vortex interactions. Simulation of these flow features necessitates the use of a high order code exhibiting low numerical errors. This thesis presents the development of such a high order solver, which has been conceived and implemented from scratch by the author during his doctoral work.

To account for the relative mesh motion, the incompressible Navier-Stokes equations are written in arbitrary Lagrangian-Eulerian form and a non-conformal Discontinuous Galerkin (DG) formulation (i.e. Symmetric Interior Penalty Galerkin) is used for spatial discretisation. The DG method, together with a novel sliding mesh technique, allows direct linking of rotating and static meshes through the numerical fluxes. This technique shows spectral accuracy and no degradation of temporal convergence rates if rotational motion is applied to a region of the mesh. In addition, analytical mappings are introduced to account for curved external boundaries representing circular shapes and NACA foils.

To simulate 3D flows, the 2D DG solver is parallelised and extended using Fourier series. This extension allows for laminar and turbulent regimes to be simulated through Direct Numerical Simulation and Large Eddy Simulation (LES) type approaches. Two LES methodologies are proposed.

Various 2D and 3D cases are presented for laminar and turbulent regimes. Among others, solutions for: Stokes flows, the Taylor vortex problem, flows around square and circular cylinders, flows around static and rotating NACA foils and flows through rotating cross-flow turbines, are presented."

— This work presents a numerical simulation of a Vertical Axis Marine Current Turbines (VAMCT) in both steady and unsteady current velocities. Three different turbines were examined in this study in order to investigate the relationship between the current fluctuation frequency, the number of turbine blades and turbine frequency. Three, four and five blades turbines were numerically simulated. Fluent is used to solve the 2-D model using the time-accurate incompressible Unsteady Reynolds-averaged Navier-Stokes (URANS) equation with k-ω SST turbulence mode. The simulation results showed that there existed a significant relationship between the number of turbine blades, the turbine Tip Speed Ratio and the current fluctuation frequency. In an unsteady current the increase in the number of blades led to a reduction in the turbine's instability; however it might not increase the power performance especially at high TSR..

The extent of vertical velocity shear expressed in tidal flows is highlighted with extracts of field data taken from the Falls of Warness in the Pentland Firth. Resulting velocity profiles reveal distinct divergences in shape for the same mean speed during flood and ebb tides with the ebb profile observed to be much fuller than the low shear 1/7 th power law used by the wind industry. A method of applying a Log-law fit to the field data profiles is presented resulting in the calculation of ebb and flood bed roughness heights of 0.015 and 0.127 m. The impact of operating turbines within the high shear tidal environment is assessed computationally at approximately 1/30th geometric scale.

This work investigates the predicted effects of realistic tidal conditions on a 1MW tidal stream turbine (TST) using EDF's open-source CFD solver, Code_Saturne. The turbine is to be installed at the European Marine Energy Centre (EMEC) in Orkney. A strongly sheared profile is employed to represent flood tide and a roughly parabolic profile, with maximum just below hub height, to represent ebb tide. The effect of these different flow conditions are assessed through instantaneous and mean quantities of power and thrust coefficients.

Abstract:
Blade Element Momentum Theory (BEMT) performance models for wind turbines lead to a robust BEMT model of marine current or tidal stream turbines. It is shown that numerical convergence methods for models reported in the literature are problematic when away from the normal operating range and this paper reports a robust numerical scheme using combined Monte Carlo and sequential quadratic optimisation. The model is extended by Prandtl corrections for losses at the blade tip and hub. Results are validated against an industrial code, Garrad Hassan's Tidal Bladed Software (GH Tidal Bladed) evaluation version, and a lifting line theory model.

The extent of vertical velocity shear expressed in tidal flows is highlighted with extracts of field data taken from the Falls of Warness in the Pentland Firth. Resulting velocity profiles reveal distinct divergences in shape for the same mean speed during flood and ebb tides with the ebb profile observed to be much fuller than the low shear 1/7 th power law used by the wind industry. A method of applying a Log-law fit to the field data profiles is presented resulting in the calculation of ebb and flood bed roughness heights of 0.015 and 0.127 m. The impact of operating turbines within the high shear tidal environment is assessed computationally at approximately 1/30th geometric scale.

A new theoretical model is proposed to explore the efficiency of a long array of tidal turbines partially blocking a wide channel cross section. An idea of scale separation is introduced between the flow around each device (or turbine) and that around the entire array to assume that all device-scale flow events, including "far-wake" mixing behind each device, take place much faster than the horizontal expansion of the flow around the entire array. This assumption makes it possible to model the flow as a combination of two quasi-inviscid problems of different scales, in both of which the conservation of mass, momentum and energy is considered. The new model suggests the following: When turbines block only a small portion of the span of a shallow channel cross section, there is an optimal intra-turbine spacing to maximise the efficiency (limit of power extraction) for a given channel height and width. The efficiency increases as the spacing is reduced to the optimal value due to the effect of local blockage, but then decreases as the spacing is further reduced due to the effect of array-scale choking, i.e., reduced flow through the entire array. Also, when the channel is infinitely wide, the efficiency depends solely on the local area blockage rather than on the combination of the intra-turbine spacing and the channel height. As the local blockage is increased, the efficiency increases from the Lanchester-Betz limit of 0.593 to another limiting value of 0.798, but then decreases as the local blockage is further increased.

Three-dimensional incompressible Reynolds-averaged Navier-Stokes (RANS) computations are performed for water flow past an actuator disk model (representing a tidal turbine) placed in a rectangular channel of various blockages and aspect ratios. The study focuses on the effects of turbulent mixing behind the disk, as well as on the effects of channel blockage and aspect ratio on the prediction of the hydrodynamic limit of power extraction. To qualitatively account for the effect of turbulence generated by the turbine (rather than by the shear flow behind the turbine), we propose a new approach, called a blade-induced turbulence model, which does not use any additional model coefficients other than those used in the original RANS turbulence model. Results demonstrate that the power removed from the mean flow by the disk increases as the strength of turbulent mixing behind the disk increases, being consistent with the turbulent shear stress on the interface between the bypass and core flow passages acting in such a way as to decelerate the bypass flow and accelerate the core flow. The channel aspect ratio also affects the flow downstream of the disk but has less influence upstream of the disk; hence its effect on the limit of power extraction is relatively minor compared to that of the channel blockage, which is shown to be significant but satisfactorily estimated using one-dimensional inviscid theory previously reported in the literature.

Tidal currents provide a significant energy source in many locations worldwide,
particularly at coastal areas where bathymetric conditions intensify their magnitudes.
Potential sites for tidal-stream energy resource harvesting require realistic assessments
and reliable simulations of effects of turbine arrays on tidal dynamics. Accuracy of
the analysis has direct implications on economic and technical aspects of tidal energy
project developments.
An alternative approach for simulating turbine array energy capture, momentum
sink-TOC, was developed to improve conventional methodologies for assessing tidalstream energy resource. The method uses a non-constant thrust force coefficient calculated based on turbines operating-conditions, and relates turbine near-field changes
produced by power extraction to turbine thrust forces. Sink-TOC combines linear momentum actuator disc in open channel flows theory with the momentum sink method.
Momentum sink-TOC was implemented in two depth-average complex hydrodynamic models to simulate different marine turbine configurations and to perform tidalstream energy resource assessments. The first model solves smooth and slow flows
(SSF) with an alternating direction implicit scheme. The second model solves rapidly
varying flows (RVF) combining MacCormack and total variation diminishing schemes.
The RVF solver incorporates a computational less expensive approach for simulating
sharp gradients produced by power extraction than existing techniques. Benchmarking
of numerical results against analytical solutions indicates that both models correctly  compute momentum extracted by turbines.
Calculation of turbine velocity coefficients and head drops across turbine arrays
enabled the calculation of turbine efficiency, total power extracted, power dissipated
by turbine wake-mixing, and power available for electricity generation. These metrics
represent an advantage over traditional methodologies used to assess resources. As
sessment of bounded flow scenarios through a full fence configuration performed better
using the SSF solver, because head drop was more accurately simulated. However,
this scheme underestimates velocity reductions due to power extraction. Evaluation
of un-bounded flow scenarios through a partial-fence was better performed by the
RVF solver as the head drop was more correctly approximated by this scheme. The
free-surface flow simulations led to identification of non-uniform upstream conditions
for the partial-fence configuration. Computational performance comparisons indicated
that the RVF solver requires higher computational cost independently of domain-size,
and whether energy extraction procedure is incorporated or not.
Tidal-stream energy resource evaluations with fence and partial-fence configurations indicate that a computationally economical pre-assessment can be adequately
performed using an SSF solver. However, more accurate evaluation requires solution of
the discontinuities produced in the tidal-stream by power extraction. The methodol
ogy and numerical models obtained in this research could be use to determine realistic
upper limits of available power with turbine arrays in farm format in real-world coastal
environments.

Blockage effects are currently not accounted for in cavitation analyses of tidal turbine rotors. At higher blockage ratios, rotors are more heavily loaded and have potentially stronger suction peaks, so cavitation inception is more likely. In this paper, blade resolved computations are used to carry out a cavitation analysis over a range of blockage ratios and tip-speed-ratios. Our analysis suggests that increasing the blockage ratio from 0.01 to 0.197 reduces the minimum static pressure head in the fluid by approximately 0.5 m. To mitigate this reduction, either the submersion depth of the rotor can be increased or the maximum permissible tip speed ratio reduced. However, reducing the maximum permissible tip speed ratio is shown to severely restrict the rotor thrust and power. Spanwise flow effects are shown to reduce the strength of the suction peak on the outboard blade sections, reducing the likelihood of cavitation inception. Blade element based methods are shown to inadequately account for spanwise flow effects and thus are overly-conservative. Hence, rotors designed with these methods could potentially be operated at higher tip-speed-ratios or reduced submersion depths.

The challenge in the hydrodynamic modelling of tidal and marine turbine farms is to take into account the interaction of flow events across a wide range of scales, such as as the blade scale, turbine scale, array scale and regional scale. Whilst the interaction of the blade and turbine scales can be studied using the classical Blade- Element-Momentum (BEM) theory, no basic theory was available until recently to describe the interaction of the turbine and larger scales. The two-scale actuator disc theory (ADT), first proposed in 2012 by Nishino and Willden, explain the interaction of the turbine and array scales at a fundamental level; however, its validity or applicability to real problems has only partially been confirmed. Hence in this study we perform 3D RANS simulations of single and double rows of porous discs (8 discs for each row) in the middle of a shallow open channel with a vertically sheared flow. The simulation results are shown to agree qualitatively with the two-scale with the two-scale ADT and importantly, the optimal intra-disc spacing predicted by the simulations (to maximise the total power) agrees well with the theory, for both single-row and double-row cases

This paper presents the predicted effect of realistic velocity profiles on a 1MW tidal stream turbine (TST) using computational fluid dynamics (CFD). A baseline study using a uniform onset velocity and low turbulence intensity is first performed. The effects of realistic flow conditions typical of Flood and Ebb tides are then modelled using velocity profiles based on field measurements recorded at the EMEC test site. The effect of these different flow conditions are assessed through instantaneous and mean quantities of power and thrust coefficients. Both coefficients are related to average velocity across the turbine's swept area, which is 2% and 3% greater than the uniform flow for the flood and ebb profiles respectively. An effect of the velocity profiles is that the power coefficient is increased, by 1% and 2.7% for the flood and ebb velocity profiles respectively. To assess the influence of the mast and depth-varying velocity on individual blades the instantaneous loading is investigated. For the baseline uniform flow study the mast causes some variation in the instantaneous blade loads over a range that is 4% of the mean. Increased levels of turbulence are shown to have little effect on the mean power and thrust coefficients whilst causing the immediate wake to recover much more rapidly.

Abstract
The renewable energy route map for Wales outlines ambitious targets for 50% renewables by 2025. The Welsh coast, subject to tidal ranges of the order of 13m and tidal flows in excess of 3 m/s, is thus in an ideal position to
significantly contribute to these targets. Tidal stream energy is an emerging energy sector and a relatively small number of devices are at various stages of development in Wales. However, before such demonstration devices or
arrays can be applied at a larger scale, a number of consents and permissions must be obtained to ensure safe and environmentally responsible deployment. This paper describes the multidisciplinary work that has been undertaken on an area of ocean that could be used for the
deployment of a tidal stream turbine. The paper aims to put the scientific work undertaken into the context of device deployment in a complex marine environment and to provide an overview of the surveying and modelling
required for the deployment of a single demonstration device off the Welsh coast. The Bristol Channel was chosen as a case study because of its high tidal flows and proximity to national grid connections and support
infrastructure. This paper provides an overview of the research carried out during the project; the details of each discipline will be provided in individual papers by the respective subject authors

Computational fluid dynamics is used to study the impact of the support structure of a tidal turbine on performance and the downstream wake characteristics. A high-fidelity computational model of a dual rotor, contra-rotating tidal turbine in a large channel domain is presented, with turbulence modelled using large eddy simulation. Actuator lines represent the turbine blades, permitting the analysis of transient flow features and turbine diagnostics. The following four cases are considered: the flow in an unexploited, empty channel; flow in a channel containing the rotors; flow in a channel containing the support structure; and flow in a channel with both rotors and support structure. The results indicate that the support structure contributes significantly to the behaviour of the turbine and to turbulence levels downstream, even when the rotors are upstream. This implies that inclusion of the turbine structure, or some parametrisation thereof, is a prerequisite for the realistic prediction of turbine performance and reliability, particularly for array layouts where wake effects become significant.

Blade element momentum theory (BEMT) allows much more rapid modelling of turbines than full Navier-Stokes simulations. Classical BEMT greatly simplifies flow conditions to make this rapidity possible; we can, however, modify BEMT to incorporate additional flow phenomena without sacrificing much in the way of calculation speed. Using an already-existing extended BEMT model, this paper explores how turbulent flows can be synthesised in order to predict their effects on turbine performance.

Tuning wind and tidal turbines is critical to maximizing their power output. Adopting a wind turbine tuning strategy of maximizing the output at any given time is shown to be an extremely poor strategy for large arrays of tidal turbines in channels. This impatient-tuning strategy results in far lower power output, much higher structural loads and greater environmental impacts due to ow reduction, than an existing patient-tuning strategy which maximizes the power output averaged over the tidal cycle. This paper presents a smart patient tuning strategy, which can increase array output by up to 35% over the existing strategy. This smart strategy forgoes some power generation early in the half tidal cycle in order to allow stronger flows to develop later in the cycle. It extracts enough power from these stronger flows to produce more power from the cycle as a whole than the existing strategy. Surprisingly, the smart strategy can often extract more power without increasing maximum structural loads on the turbines, while also maintaining stronger flows along the channel. This paper also shows, that counter-intuitively, for some tuning strategies imposing a cap on turbine power output to limit loads can increase in a turbine's average power output.

The effect of wind turbine blade tip geometry is numerically analysed using Computational Fluid Dynamics (CFD). Three different rotating blade tips are compared for attached flow conditions and the flow physics around the geometries are analysed. To this end, the pressure coefficient (Cp) is defined based on the stagnation pressure rather than on the inflow dynamic pressure. The tip geometry locally modifies the angles of attack (AOA) and the inflow dynamic pressure at each of the studied sections. However not all 3D effects could be reduced to a change of these two variables. An increase in loadings (particularly the normal force) towards the tip seem to be associated to a spanwise flow component present for the swept-back analysed tip. Integrated loads are ranked to asses wind turbine tip overall performance.It results from the comparison that a better tip shape that produced better torque to thrust ratios in both forces and moments is a geometry that has the end tip at the pitch axis. The work here presented shows that CFD may prove to be useful to complement 2D based methods on the design of new wind turbine blade tips.

Much of the global tidal current energy resource lies in the accelerated flows along narrow channels. These channels have the potential to produce 10s -1000s of MW of electricity. However, realizing 100 MW of a channel's potential is much more complex than installing 100 one MW turbines because large scale power extraction reduces tidal currents throughout the channel, changing the resource. This synthesis and review gives an overview of the issues and compromises in designing the layout of the large tidal turbine arrays required to realize this potential. The paper focuses on macro- and micro-design of arrays. Macro-design relates to the total number of turbines and their gross arrangement into rows, while micro-design adjusts the relative positions of the turbines within a grid and the spacing between rows. Interdependent macro-design compromises balance the total number of turbines, array power output, the power output of each turbine, the loads turbines experience, turbine construction costs, maintaining navigability along the channel and any environmental impacts due to flow reduction. A strong emphasis is placed on providing physical insights about how “channel-scale dynamics” and the “duct-effect” impact on the compromises in array design. This work is relevant to the design of any “large” array which blocks more than 2%-5% of a channel's cross-section, be it 2 turbines in a small channel or 100 turbines in a large channel.

Due to the low current speeds associated with Malaysian tidal currents, Savonius turbine was
chosen as a device in extracting this energy. However, this turbine suffers from poor efficiency. The present paper describes some works carried out to improve the turbine. The effect of speeds on performance was found to be minimal while the use of deflectors improved flow into the rotor and contributes significantly to the coefficient of performance improvement. The use of ducts was also studied, indicating improved flow characteristics.

This paper presents analytical bounds for blade–wake interaction phenomenona occurring in rotating cross-flow turbines for wind and tidal energy generation (e.g. H-rotors, Darrieus or vertical axis). Limiting cases are derived for one bladed turbines and extended to the more common three bladed configuration. Additionally, we present a classification of the blade–wake type of interactions in terms of limiting tip speed ratios.

These bounds are validated using a high order h/ph/p Discontinuous Galerkin solver with sliding meshes. This computational method enables highly accurate flow solutions and shows that the analytical bounds correspond to limiting blade–wake interactions in fully resolved flow simulations.

Interest in the marine renewable energy devices, and particularly tidal stream turbines, has increased significantly over the past decade and several devices such as vertical and horizontal axis turbines and reciprocating hydrofoils are now being designed around the world to harness tidal stream energy. While tidal stream turbines are being developed at a high rate and getting closer to commercialisation, it is important to acquire the right tools to assist planners and environmentalists, not only in finding a right location for the turbines, but also in identifying their potential impacts on the surrounding marine and coastal environment.

Thick walled components such as high pressure (HP) steam turbine casings operating under high parameter conditions are subjected to a complex stress state. As a result of that stress state, some parts of HP turbine casing undergo to the creep fatigue caused by the combination of thermal fatigue resulted from repeated start/stop operation and the creep which occurs during long-term operation at high temperature and high-pressure. It is well known that domestic thermal power plants have been in use over 100000 h which means that significant cost is required not only for maintenance, but often for renewal of equipment. Based on comprehensive investigation, the results of residual life assessment of one high pressure steam turbine casing, which belongs to the older turbine generation, taking into account simultaneous action of thermal fatigue and creep, are presented in this paper. Also, the critical flaw crack size of HP turbine casing is determined because this parameter has a strong influence on casing integrity and residual life. The results of residual life assessment provide not only a basis for further maintenance, but also estimated time for reparation or renewal.

Concern over global climate change has led policy makers to accept the importance of reducing greenhouse gas emissions. This in turn has led to a large growth in clean renewable generation for electricity production. Much emphasis has been on wind generation as it is among the most advanced forms of renewable generation, however, its variable and relatively unpredictable nature result in increased challenges for electricity system operators. Tidal generation on the other hand is almost perfectly forecastable and as such may be a viable alternative to wind generation. This thesis paper shows the important of Tidal power over other sources and it’s generation using DFIG. This thesis is analyzed based on the following configuration; A 6 MW tidal power site having four, 1.5 MW tidal turbines connected to a 25 kV distribution system which ensures power to a 120 kV grid through a 30 km, 25 kV feeder line. A 2300V, 2 MVA plant with a motor load (1.68 MW induction motor at 0.93 pf) and of a 200 kW resistive load which is also linked into the same feeder. Both tidal turbine and motor is attached to a protection system which was used to monitor voltage, current and machine speed. DFIG DC link voltage is also monitored. Tidal turbines used an induction generator having a wound rotor and an AC/DC/AC IGBT based PWM converter. The stator winding is also linked directly to the 50 Hz grid and the rotor is fed at variable frequency through the AC/DC/AC converter. DFIG technology was also used for extraction of maximum energy from the tides for low tidal speeds through optimization of turbine speed, while lowering turbine mechanical stresses during heavy tides. The turbine’s optimum speed used to produce maximum mechanical energy for a given tidal speed which is directly proportional to tidal speed. The rotor runs at sub synchronous speed for tidal speeds will also be lowered at 10 m/s and at super synchronous speeds for higher tidal speeds. The simulation of 6MW tidal site was carried out for normal condition and at different types of faults introduced on the 25kV line. They are carried out as follows; a) For normal conditions, at a tidal speed of 13m/s. b) For a phase A to ground fault in the 25 kV line for t=5 to 5.1 seconds. c) For a phase A and B fault in the 25 kV line t=5 to 5.1 seconds. d) For a phase A and B to ground fault in the 25 kV line t=5 to 5.1 seconds. e) For a symmetric fault in the 25kV line for t=5 to 5.1 seconds

Computations of the blade loading and the local flow field around the Model Rotor Experiments In Controlled Conditions (MEXICO) rotor are presented using an actuator line method, implemented within the open source code OpenFOAM. The nacelle and near wake mesh refinement are shown to have little influence on the computed blade loads but a significant impact on the near wake flow field. In addition, the blade loads and near wake flow field calculated with 3 different distributions of the Gaussian smearing parameter ǫ are compared with experimental measurements. Local chord and lift coefficient scaled smearing distributions are shown to yield a significant improvement in the representation of the computed tip vortices and also a small improvement in the blade loading prediction, when compared with a spanwise constant smearing distribution. Despite these improvements in performance prediction, the performance of the rotor is shown to be more strongly influenced by the tip correction fac...

http://dx.doi.org/10.1016/j.apm.2013.06.004 Studies of tidal stream turbine performance and of wake development are often conducted in tow-tanks or in regulated flumes with uniform flows across the turbine. Whilst such studies can be very useful, it is questionable as to what extent the results would differ if the flows were more complex in nature, for instance if the flows were unsteady or non-uniform or even both. This study aims to explore whether the results would be affected once we move away from the uniform flow scenario. A numerical modelling study is presented in which tidal stream turbine performance and wake development in non-uniform flow conditions are assessed. The model implements the Blade Element Momentum method for characterising turbine rotor source terms which are used within a Computational Fluid Dynamics model for predicting the interaction between the turbines and the surrounding flow. The model is applied to a rectangular domain and a range of slopes are implemented for the water surface to instigate an increase in flow velocity along the domain. Within an accelerated flow domain wake recovery occurred more rapidly although rotor performance was not affected.

A modelling approach based on Blade Element Momentum Theory is developed for the prediction of tidal stream turbine performance in the ocean environment. Through the coupling of the Blade Element Momentum method with Computational Fluid Dynamics, the influence of upstream hydrodynamics on rotor performance is accounted for. Incoming flow onto the rotor can vary in speed and direction compared to free-stream conditions due to the presence of obstructions to the flow in the upstream, due to other devices for example, or due to the complexity of natural bathymetries. The relative simplicity of the model leads to short run times and a lower demand on computational resources making it a useful tool for considering more complex engineering problems consisting of multiple tidal stream turbines. Results from the model compare well against both measured data from flume experiments and results obtained using the Classical Blade Element Momentum model. A discussion considering the advantages and disadvantages of these different approaches is included.

Computations of the blade loading and local flow field around the Model Rotor Experiments In Controlled Conditions (MEXICO) rotor are presented using an actuator line method, implemented within the open source code OpenFOAM. The nacelle and near wake mesh refinement are shown to have little influence on the computed blade loads but a significant impact on the near wake flow field. In addition, the blade loads and near wake flow field calculated with 3 different distributions of the Gaussian smearing parameter are compared with experimental measurements. Local chord and lift coefficient based smearing distributions are shown to yield a significant improvement in the representation of the computed tip vortices and also a small improvement in the blade loading prediction, when compared with a spanwise constant smearing distribution. Despite these improvements in performance prediction, the performance of the rotor is shown to be more strongly influenced by the tip correction factor, where considerable improvement is still required before actuator line methods can represent real rotors with sufficient accuracy.

Abstract
Details are given herein of the current main proposals for tidal energy provision from the Severn Estuary, in the UK, with particular emphasis being focused on the Severn Barrage project, as originally promoted by the Severn Tidal Power Group.  In particular, emphasis has focused on assessing the potential hydro-environmental impacts and power outputs of a barrage across the estuary, with an unstructured grid, high resolution, model being developed and applied to the estuary to assess the implications of each of five shortlisted proposed schemes on the hydrodynamic, geomorphologic, flood risk and faecal indicator organism changes within the estuary.  An outline is given of recent research on power refinements to the model to assess the options for power generation.  The results show that the Severn Barrage has the potential to reduce the tidal currents in a highly dynamic estuary.  This leads to reduced suspended sediment loads (particularly upstream of the barrage), increased light penetration within the water column and, potentially, an increase in the benthic bio-diversity and the level of aquatic life in the estuary.  The results also show that the Severn Barrage will reduce the risk of flooding markedly upstream of the barrage and to a lesser extent downstream of the structure. In contrast the alternative options have far less impact on flood risk changes.  In addition to the Severn Barrage some results are shown herein for a typical lagoon option, namely the Fleming Lagoon.

An effective way of using CFD to simulate flow about a rotating device -for example, a wind or marine turbine -is to embed a rotating region of cells inside a larger, stationary domain, with a sliding interface between. This paper describes a simple but effective method for implementing this as an internal Dirichlet boundary condition, with interfacial values obtained by interpolation from halo nodes. The method is tested in two finite-volume codes: one using block-structured, and the other unstructured, meshes.

Tidal currents are one of the most promising sources of power within the renewable energy sector, especially for countries with suitable marine conditions. The United Kingdom has the edge over the rest of countries in this type of renewable energy and leads the innovation race in order to keep the UK economy competitive in future years.

Following these reasons, several types of marine turbines are currently being developed and tested. Nevertheless, one of the key issues, in the future development of marine hydrokinetic power generation systems, is to properly understand their prospective performance, not only for each power generation device alone but also for an array of devices as a whole. Recent theoretical studies have suggested that a dense cross-stream array of turbines (so-called turbine fence) is a promising way to extract power from marine currents; however, its optimal intra-turbine spacing depends on several physical factors, some of which are still uncertain. One of those uncertain factors is the effect of seabed friction causing vertical shear of the flow, which is difficult to study theoretically and requires 3-D CFD simulations. Also, little is known about the performance of multiple rows of marine turbines.

Consequently, throughout this thesis, numerous arrangements of marine turbines (modelled as actuator disks) have been tested using ANSYS Fluent®, with the idea of assessing the effects of various parameters such as: intra-device spacing, number of rows and turbine resistance coefficient. These CFD simulations have been compared with existing theoretical models (two-scale actuator disk models) for the power extracted by the turbines with the aim of validating these theoretical models. Also, the results of CFD simulations have been analysed in detail to better understand the characteristics of flow past these turbine arrays.


Keywords: Tidal Turbines, Optimal Spacing, Actuator Disk, RANS Simulations, Open Channel Flow, Marine Energy.