Tidal Stream Energy

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.

Today, the world is heavily dependent on fossil fuels, as most of the energy requirements are being met through conventional methods of burning these fuels. The energy demand is increasing day with growing population. Consequently, fossil fuel reserves are depleting continuously and will soon run out in coming years. Therefore, renewable energy resources have gained enormous attention in recent years. The growing interest in exploring tidal current technologies has many compelling reasons such as its renewable nature, tidal energy is cleaner than fossil fuels, intermittent but predictable, security and diversity of supply, and limited social and environmental impacts. Tidal current technologies are still in development phase, yet need some time to mature to prove their full potential. Tidal current turbine is an important tidal current technology. The purpose of this paper is to present a comprehensive review of tidal current turbine, its potential and associated challenges. The pap...

This paper presents the results of a pilot experiment with an existing tidal energy converter (TEC), Evopod 1 kW floatable prototype, in a real test case scenario (Faro Channel, Ria Formosa, Portugal). Operational results related to the description of power generation capacity, energy capture area and proportion of energy flux are presented and discussed. The data is now available to the scientific community and to TEC industry developers, enhancing the operational knowledge of TEC technology concerning efficiency, environmental effects, and interactions (i.e. device/environment).

This paper presents a combined theoretical and CFD study on the fluid-mechanical limit of power extraction by a closely-spaced lateral array of wind turbines. The idea of this study originates in recent studies on the array optimisation of tidal/marine turbines, for which the power coefficient of each turbine is known to increase significantly if the lateral spacing between turbines, or the local blockage, is optimised. The present study, using 3D Reynolds-averaged Navier-Stokes (RANS) simulations of a boundary-layer flow over a closely-spaced lateral array of up to 9 actuator discs, suggests that a similar—albeit less significant—power increase due to the effect of local blockage can be achieved even for wind turbines. A possible theoretical approach to estimating this power increase is also discussed.

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 study of hydrodynamic performance of vertical axis marine current turbine in unsteady marine current environment is still an open-ended research question. The interaction between the turbine and marine current frequencies can lead to resonance condition. In this work, a numerical approach to investigate the performance of VAMCT under unsteady current velocity was proposed. A three blades turbine was examined in this study in order to investigate the relationship between the current fluctuation frequency and turbine frequency. Fluent is used to solve the 2-D model using the time-accurate incompressible Unsteady Reynolds-averaged Navier-Stokes (URANS) method with k-ω SST turbulence mode in addition to a transient rotor-stator model with a sliding mesh technique. The simulation results showed that there are significant increases in turbine power coefficient when resonance happens between turbine and marine current frequencies.

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..

Workshop  on ocean energy technologies structured around nine different emerging technology families. First generation tidal, already at the pre-commercial stage, is also included in the overview. Technology families were created based on the input received from the experts prior the workshop.
First generation tidal energy converters;
Rotor innovation for tidal energy turbines;
Floating tidal concepts;
Third generation  tidal energy converters;
Novel approaches tp first generation wave energy concepts
Novel wave energy concepts;
Innovative tidal and wave energy  power take off;
Control systems;
Moorings and station keeping systems;
Materials and components.

Sustainable energy generation through renewable sources is a rapidly expanding industry within the energy sector and as part of that industry tidal power creates the potential to tap into a previously untapped resource. Existing tidal barrage schemes are leading the way in exploiting tidal energy resources but are only producing a very small percentage of the world's generated electricity. Further development and expansion of this technology has been somewhat hindered by opposition from green political parties whom believe that the environmental impact of such a scheme is too great.
Tidal stream devices have therefore been developed as an alternative method of extracting the energy from the tides. This form of tidal power technology poses less threat to the environment and does not face the same limiting factors associated with tidal barrage schemes, therefore making it a more feasible method of electricity generation.
This paper describes work involved with modelling, using the CFD package SolidWorks Flow Simulation, a contra-rotating double row bladed tidal turbines. The first rotor has three blades rotating in a anti-clockwise direction while the second rotor, located directly behind the first, has four blades rotating in an clockwise direction. A contra-rotating marine current turbine has a number of attractive features: near zero reactive torque on the support structure, near-zero swirl in the wake, and high relative inter-rotor rotational speeds. The design of these turbines is to assess the potential increase in the power, torque and axial thrust generated over a conventional single row propeller.
The results from the CFD models show that there is a negligible increase in the power generated but an increase in the axial load on the turbine. The nett torque acting on the device is, however, considerably reduced, and potentially negated, so potentially helping the turbine to align to the tidal flow. Flow visualization of the wake verified the lack of swirl behind the turbine.

Calvert K and Simandan D (2010) Energy, space, and society: a reassessment of the changing landscape of energy production, distribution, and use Journal of Economics and Business Research XVI(1), pp. 13-37.
ABSTRACT: While geography has always mattered for the energy sector, the relative effects of location and distance on the economics of energy regimes are increasing as we begin to deploy more renewable energy technologies. This reintroduction of the friction of distance is leading to an energy landscape that is far different from fossil-based regimes. The new energy paradigm, based as it is upon the physics and the economics of renewable energy, is being reflected in the landscape as distributed, decentralized, and diversified patterns of energy generation. Because the increased use of renewable energy technologies is beginning to change the spatial patterns of political and socioeconomic activities, a thorough understanding of these patterns is crucial to increasing the socio-political acceptability of new technologies and to avoiding the socially costly unintended consequences of policy and investment decisions.  This paper proposes a theoretical foundation upon which economists and economic geographers could scaffold their analyses of the spatial characteristics of the economics of energy use.  To this end, we bring together two complementary conceptualizations of economic geography: firstly, as the study of the effects of location and distance on energy economics, and secondly, as the study of the ways in which political, economic, and technological energy-related practices give rise to particular spatial patterns of socio-economic welfare. We end the paper by developing the concept of energy rationality and showing how it relates to discussions of metarationality, common sense, and wisdom.

A survey of Early Christian sites, artefacts and monuments in County Down in English and Polish, tracing in particular the sites associated with St Patrick and his followers. This project has been funded by the European Union's PEACE III Programme, managed by the Special EU Programmes Body and delivered by the North Down, Ards and Down Councils' Cluster.

Tidal turbine blades are smaller than wind turbine blades yet carry similar bending moments. This provides a structural challenge and limits how thin the blades can be made, which subsequently affects the hydrodynamic performance of the rotor. Thinner blades generally offer higher performance but require thicker laminates hence are more expensive to build. Opinions vary amongst tidal turbine developers as to the best compromise for overall cost of energy.
The parametric study presented in this paper includes a full structural and hydrodynamic design of a megawatt-scale axial flow tidal turbine rotor. Typical tidal turbine site parameters are assumed in terms of flow speed, turbulence and wave loading based on Gurit's experience of several rotors of this size. The blades are designed using epoxy composites in accordance with industry best practice. The blade thickness, chord and twist distribution are varied to find the effect on annual energy production and blade manufacturing cost in series production. Account is taken of the step changes in manufacturing method necessary as the blades are made thinner.
Conclusions are drawn on the optimum blade thickness distribution for the assumed flow conditions and the effects of deviating from it.

The world is fast becoming a global village due to the increasing daily requirement of energy by all population across the world while the earth in its form cannot change. The need for energy and its related services to satisfy human social and economic development, welfare and health is increasing. Returning to renewables to help mitigate climate change is an excellent approach which needs to be sustainable in order to meet energy demand of future generations. The study reviewed the opportunities associated with renewable energy sources as which includes; Energy Security, Energy Access, Social and Economic development, Climate Change Mitigation, and reduction of environmental and health impacts. Despite these opportunities, there are challenges that hinders the sustainability of renewable energy sources towards climate change mitigation. These challenges include Market failures, lack of information, access to raw materials for future renewable resource deployment, and our daily carbon footprint. The study suggested some measures and policy recommendations which when considered would help achieve the goal of renewable energy thus; to reduce emissions, mitigate climate change and provide a clean environment as well clean energy for all and future generations.

With the diminishing supply of fossil-based energy, new renewableenergy sources including marine tidal current are being explored.In vertical axis marine current turbine applications, Savonius-type rotorhas been shown as suitable for low current speeds normally associatedwith Malaysia seas. However, the efficiency of the rotor is low and assuch efforts are being made to optimise the rotor in variousmanners. This paper describes a validation procedure preparation for aparametric study to obtain an optimised Savonius turbine rotor. Theresearch work results have been validated by comparison with theexisting published experiment data. The study was based on conductingtwo and three dimensional analyses using Computational FluidDynamics (CFD) RANSE code. Some parametric analysis has beensuggested to obtain an optimal configuration for the turbine

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.

On the outboard sections of horizontal axis rotor blades (that are not enclosed within a duct or shroud), vorticity is shed into the wake of the rotor. The shed vorticity induces a downwash at the rotor plane and span- wise flow accelerations along the blade, which causes the blade loading to drop off as the tip is approached. These tip flow effects are currently not adequately accounted for by reduced order rotor models (such as the blade element momentum and actuator line methods) which are frequently used to represent wind and tidal turbine rotors in large simulations. Hence, the rotor thrust and torque may be considerably over-predicted by these models if they are not corrected appropriately for tip flow effects. In this thesis, the tip loss mechanism experienced by both wind and tidal turbine rotor blades is examined directly in a series of simulations, us- ing computational fluid dynamics. Two different correction methods that can account for tip flow effects are then presented and critically evaluated. Both methods are shown to lead to a significant improvement in the accuracy of the computed blade loading, which is principally achieved by allowing the sectional force vector to reduce in magnitude and rotate towards the streamwise direction as the tip of the blade is approached.

Tip flow effects also reduce the strength of the suction peak developed on the suction surface of the blade. This reduction has considerable implications for the operation of tidal turbine rotors, since tidal turbine operation may be limited by cavitation inception and cavitation inception is most likely to initiate on the outboard sections of the rotor blade (where the static pressure reaches a minimum). In this thesis, a cavitation analysis of two different tidal turbine rotors is carried out. When tip flow effects are properly accounted for, cavitation inception is shown to be less likely at a given operating condition (tip-speed-ratio and submersion depth). Hence, industry standard cavitation analyses that are based on the blade element momentum method (and do not adequately account for tip flow effects) are shown to be currently overly-conservative.

In a separate study, tidal power extraction is examined in a computational domain where the sea bed slopes in the streamwise direction. When the sea bed slopes downwards in the streamwise direction, the velocity shear across the swept area of the rotor increases, increasing the power available for extraction. However, the increased velocity shear also increases the strength of the suction peak developed on the blade at top dead centre, so the device is more likely to cavitate at a given tip-speed-ratio. Conversely, when the sea bed slopes upwards in the streamwise direction, the incident velocity profile is more uniform, so the device is less likely to cavitate at a given tip-speed-ratio. Power extraction is also found to be more efficient on the upwards facing slope, as the flow through the swept area of the rotor is accelerated by the downstream flow passage constriction. At higher blockage ratios, the strength of the suction peak is further increased by the acceleration of the flow through the swept area of the rotor, so the device is even more likely to cavitate at a given tip-speed-ratio.

We present a comprehensive set of two-dimensional (2D) unsteady Reynolds-averaged Navier-Stokes (URANS) simulations of flow around a pair of counter-rotating vertical-axis wind turbines (VAWTs). The simulations are performed for two possible configurations of the counter-rotating VAWT pair, with various gaps between the two turbines, tip-speed-ratios and wind directions, in order to identify key flow mechanisms contributing to the enhanced performance of a pair of turbines compared to an isolated turbine. One of the key mechanisms identified, for the case of two turbines arrayed side-by-side with respect to the incoming wind, is the change of lateral velocity in the upwind path of each turbine due to the presence of the neighbouring turbine, making the direction of local flow approaching the turbine blade more favourable to generate lift and torque. The results also show that the total power of a staggered pair of turbines cannot surpass that of a side-by-side pair of turbines. Some implications of the present results for the prediction of the performance of single and multiple rows (or a farm) of VAWTs are also discussed. The local flow mechanisms identified in the present study are expected to be of great importance when the size of the farm is relatively small.

Renewable energy can be used to decrease global dependence on natural resources, and tidal power can be the primary form of renewable power utilized. Built upon steam turbine knowledge, tidal turbines draw on innovative technology and design to operate on both the inflow and outflow of water through them. Tidal power utilizes twice the daily variation in sea level caused primarily by the gravitational effect of the Moon and, to a lesser extent by the Sun on the world's oceans. The Earth's rotation is also a factor in the production of tides. Control & Operation system for tidal power plant should be simple, reliable, cheap and with minimum interference of operating personal. Control system should be such that remote operation can be performed easily. The main functions of the controller for automation is to execute starting and the shut down sequences under normal and emergency conditions. In this paper an attempt has been made to elaborate the use of Programmable Logic Controller (PLC) for control and automation of tidal power plant, its advantages and cost effectiveness. Programmable logic controllers (PLC) can be used for control & automation of tidal power plant. The main reason for this is cost effectiveness. Various functions and controls can be achieved by programming the PLC. They can be used for full plant automation including governing of auto-operation includes speed control, load control, excitation control, and level control automatic start/stop sequencing, barrage gate control, start/stop of auxiliary systems, and protection requirement etc. Functions other than control like continuous monitoring, data recording, instrumentation and protections can also be performed. For remote operation, communication with PLC can be performed. For continuous monitoring purpose, a personal computer can be interfaced with PLC and continuous data can be recorded regularly .

Hydrokinetic energy is an unrecognized, low-cost renewable technology that can be deployed in Pakistan through a robust national energy strategy and international investment schemes to tackle the country’s acute energy crisis. This Article will be the first to show how national and local laws can be amended to favor progress in the sustainable energy sector and achieve hydrokinetic energy production in Pakistan, which if actualized, would be nothing short of a game changer – strategically and, even more so, environmentally. Despite current legal regimes that disfavor small scale hydroelectric power production, Pakistan and other less developed countries can adapt and deploy hydrokinetic technology through revamped investment laws, regulatory rules, and renewable energy tax reform.

The cost of utilizing kinetic energy of river stream, tidal and ocean current is considered to be higher than that of wind power generation because of difficulties in construction and maintenance of devices installed in seawater. As a solution to the problem, the authors propose a new concept of water stream turbine. The main idea is in the manner of supporting turbine. Although it is similar to a vertical axis turbine, the direction of turbine axis is not firmly fixed and its tilt angle is passively adjustable to the stream velocity. Since it does not have to keep the turbine axis in upright position, required structural strength and weight of the device will be reduced significantly. This paper describes the application ranging from the small hydro power in river streams to large application of tidal and ocean current turbine. In the large capacity plant for tidal stream and ocean current, the main mechanism of turbine axis support is the same as that of the wind turbine authors proposed in the previous paper. It leads to the further opportunity of cost reduction. The sample design of a multi-megawatt ocean current turbine shows the possibility of high economic performance of the concept. The results show that the cost of energy in the concept can be comparable to a land based wind turbine.

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