No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Modelling and analysis of the horizontal configuration of tidal fences in barrages
ResearchGate has not been able to resolve any citations for this publication.
The tidal flow between bridge pillars and through open barriers is a promising source of ocean energy which can be exploited using tidal stream turbines, as proven recently by operational demonstration plants. The aim of this study is to clarify the consequences for the power output of tidal turbines when placing them in a hydraulic structure. To this end, experimental measurements of turbine power and wakes are performed, using a down-scaled turbine mounted at a submerged weir. The results are compared to an analytical model, validating its range of application for optimising turbine-weir geometries. The experimental data show that the power coefficient of the turbine can be increased by optimising the blockage of the channel and the distance between the turbine and the structure, which is related to the wake configuration. In this way, the power coefficient increased by 40 percent when the turbine was re-positioned from the upstream to the downstream end of the structure. The theoretical model could reproduce the measured power within 10 percent accuracy, proving its value as a rapid assessment tool. As such, this work advances the knowledge needed to meet targets on the transition towards renewable energy.
An economical way to harvest tidal energy is by integrating free stream turbines in coastal infrastructure. While numerous studies have investigated how turbines should be arranged in idealized geometries to optimize their performance, only a few have considered the influence of realistic bed features. This research investigates the influence of a hydraulic structure on the performance of a tidal turbine, using the combination of field monitoring of full scale turbines installed in a Dutch storm surge barrier - comprising a weir and pillars - and by developing a corresponding theoretical model.
The observed production by the turbines was large compared to situations with an unconstrained flow for two reasons. Firstly, the flow contraction by the weir increased the mass flux through the rotor plane. Secondly, the turbine suppressed energy losses in the recirculation zone downstream of a weir. The proposed model provides a quantitative estimate of these effects and is validated against field data. The model can be used as a design tool or parametrization of turbines in a large scale shallow water model, providing performance estimates covering a range of turbine-weir configurations. The work contributes to efficiently exploiting tidal energy with turbines in coastal bridges or flood defenses.
Scour in the vicinity of a hydraulic structure may compromise its geotechnical stability, or that of nearby structures. Much fundamental research has been dedicated to the understanding and prediction of these processes, in order to enable the efficient design of mitigation measures. Whilst most of these efforts consider laterally uniform flows, non‐uniformity is the rule rather than the exception. This study presents field observations near the storm surge barrier in the Eastern Scheldt estuary, The Netherlands, where significant scour holes have developed. The tidal flow through this semi‐open barrier exhibits characteristics of a shallow jet. A pronounced contraction of this jet towards the nearby scour hole during particular stages of the tidal cycle is revealed, attributed to potential vorticity conservation; the depth‐averaged vertical vorticity increases proportionally to the depth increase. Measurements show a vertically uniform velocity profile in the scour hole that is attributed to a reduction of the adverse pressure gradient due to the contraction, that suppresses boundary layer separation from the upstream slope of the scour hole. A positive feedback mechanism is thus revealed; lateral velocity gradients lead to relatively high near‐bed velocities in the scour hole which enhances erosion − causing an even stronger horizontal contraction − maintaining the scouring potential. If the upstream flow field is laterally uniform (observed around slack tide) horizontal contraction does not occur and the boundary layer separates from the upstream slope. The scour depth will then tend to an equilibrium. The results infer the importance of the non‐uniformity of the horizontal flow field.
A large-eddy simulation (LES) of a laboratory-scale horizontal axis tidal stream turbine operating over an irregular bathymetry in the form of dunes is performed. The Reynolds number based on the approach velocity and the chord length of the turbine blades is approximately 60,000. The simulated turbine is a 1:30 scale model of a full-scale prototype and both turbines operate at very similar tip-speed ratio of λ ≈ 3. The simulations provide quantitative evidence of the effect of seabed-induced turbulence on the instantaneous performance and structural loadings of the turbine revealing how large-scale, energetic turbulence structures affect turbine performance and bending moments of the rotor blades. The data analysis shows that wake recovery is notably enhanced in comparison to the same turbine operating above a flat-bed and this is due to the higher turbulence levels generated by the dune. The results demonstrate the need for studying in detail the flow and turbulence characteristics at potential tidal turbine deployment sites and to incorporate observed large-scale velocity and pressure fluctuations into the structural design of the turbines.
A methodology associated with the simulation of tidal range projects through a coastal hydrodynamic model is discussed regarding its capabilities and limitations. Particular focus is directed towards the formulations imposed for the representation of hydraulic structures and the corresponding model boundary conditions. Details of refinements are presented that would be applicable in representing the flow (and momentum flux) expected through tidal range turbines to inform the regional modelling of tidal lagoons and barrages. A conceptual tidal lagoon along the North Wales coast, a barrage across the Severn Estuary and the Swansea Bay Lagoon proposal are used to demonstrate the effect of the refinements for projects of a different scale. The hydrodynamic model results indicate that boundary refinements, particularly in the form of accurate momentum conservation, have a noticeable influence on near-field conditions and can be critical when assessing the environmental impact arising from the schemes. Finally, it is shown that these models can be used to guide and improve tidal impoundment proposals.
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.
This paper assesses an upper bound for the tidal stream power resource of the Pentland Firth. A depth-averaged numerical mode of the tidal dynamics in the region is set-up and validated against field measurements. Actuator disc theory is used to mode the effect of turbines on the flow, and to estimate the power available for generation after accounting for losses owing t mixing downstream of the turbines. It is found that three rows of turbines extending across the entire width of the Pentlan Firth and blocking a large fraction of the channel can theoretically generate 1.9 GW, averaged over the spring–neap cycle.
However, generation of significantly more power than this is unlikely to be feasible as the available power per additiona swept area of turbine is too small to be viable. Our results differ from those obtained using simplified tidal channel model of the type used commonly in the literature. We also use our numerical model to investigate the available power from row of turbines placed across various subchannels within the Pentland Firth, together with practical considerations such as th variation in power over the spring–neap tidal cycle and the changes to natural tidal flows which result from power extraction.
The characteristics of flow past a partial cross-stream array of (idealized) tidal turbines are investigated both analytically and computationally to understand the mechanisms that determine the limiting performance of partial tidal fences. A two-scale analytical partial tidal fence model reported earlier is further extended by better accounting for the effect of array-scale flow expansion on device-scale dynamics, so that the new model is applicable to short fences (consisting of a small number of devices) as well as to long fences. The new model explains theoretically general trends of the limiting performance of partial tidal fences. The new model is then compared to three-dimensional Reynolds-averaged Navier–Stokes (RANS) computations of flow past an array of various numbers (up to 40) of actuator disks. On the whole, the analytical model agrees well with the RANS computations, suggesting that the two-scale dynamics described in the analytical model predominantly determines the fence performance in the RANS computations as well. The comparison also suggests that the limiting performance of short partial fences depends on how much of device far-wake mixing takes place within the array near-wake region. This factor, however, depends on the structures of the wake and therefore on the type/design of devices to be arrayed.
The Dutch Delta project, initiated in 1959 for the protection of the south-western Netherlands, reached its conclusion in 1987 with the completion of the Oosterschelde project: an open storm surge barrier and the Oesterdam and Philipsdam in the landward parts of the Oosterschelde basin. The Delta project has increasingly influenced the tidal characteristics of the various basins in the area. The Oosterschelde basin was showing a tendency towards erosion until ca. 1980, when the Oosterschelde project was launched. Completion of this project in 1987 caused enormous changes in hydraulic conditions and initiated a large-scale transformation in the geomorphology of the Oosterschelde tidal basin and ebb tidal delta. Since 1987, the tidal prism (ebb or flood volume) has decreased by 30% and the average tidal range by 12%. Tidal current velocities have generally declined by 20-40% in the western and central parts of the Oosterschelde, and by as much as 80-100% near to the closure dams in the landward end of th
Oceanic tides have the potential to yield a vast amount of renewable energy.
Tidal stream generators are one of the key technologies for extracting and
harnessing this potential. In order to extract an economically useful amount of
power, hundreds of tidal turbines must typically be deployed in an array. This
naturally leads to the question of how these turbines should be configured to
extract the maximum possible power: the positioning and the individual tuning
of the turbines could significantly influence the extracted power, and hence is
of major economic interest. However, manual optimisation is difficult due to
legal site constraints, nonlinear interactions of the turbine wakes, and the
cubic dependence of the power on the flow speed. The novel contribution of this
paper is the formulation of this problem as an optimisation problem constrained
by a physical model, which is then solved using an efficient gradient-based
optimisation algorithm. In each optimisation iteration, a two-dimensional
finite element shallow water model predicts the flow and the performance of the
current array configuration. The gradient of the power extracted with respect
to the turbine positions and their tuning parameters is then computed in a
fraction of the time taken for a flow solution by solving the associated
adjoint equations. These equations propagate causality backwards through the
computation, from the power extracted back to the turbine positions and the
tuning parameters. This yields the gradient at a cost almost independent of the
number of turbines, which is crucial for any practical application. The utility
of the approach is demonstrated by optimising turbine arrays in four idealised
scenarios and a more realistic case with up to 256 turbines in the Inner Sound
of the Pentland Firth, Scotland.
The effects of free-surface proximity on the flow field around tidal stream turbines are modelled using actuator disc theory. Theoretical results are presented for a blocked configuration of tidal stream turbines such as a linear array that account for the proximity of the free surface and the seabed. The theoretical results are compared to open channel flow experimental results in which the flow field has been simulated using a porous disc and strip. These results are complemented by more detailed measurements of the performance of a model horizontal-axis turbine carried out in a water flume and a wind tunnel. The two sets of experiments represent highly blocked and effectively unblocked cases, respectively. The theoretical model of the effects of free-surface proximity provides a blockage correction for axial induction that can be incorporated in blade element momentum codes. The performance predictions obtained with such a code are in good agreement with the experimental results for CP and CT at low tip-speed ratios. The agreement weakens with increasing tip-speed ratio, as the wake of turbine enters a reversed flow state. A correction following the philosophy of Maskell is applied to CT in this region, which provides a better agreement.
Underway profiles of current velocity were combined with stationary profiles of temperature and salinity around a vertically mixed scour pit of the Seto Inland Sea throughout a semidiurnal tidal cycle. This was done with the purpose of determining (a) whether flood flow is asymmetric relative to ebb over a pit with weakly stratified conditions, and (b) whether there is a dynamic transition from frictionally dominated tidal flow to advectively dominated tidal flow over the pit. These questions arose from previous studies elsewhere under stratified water columns, in contrast to the unstratified conditions at the study site. Observations showed an acceleration of the flood tidal flow over the pit and a deceleration during ebb. The flow acceleration over the pit during flood and deceleration during ebb was attributed to asymmetric patterns of flow convergence/divergence. In turn, these divergence patterns were influenced by the direction and strength of the baroclinic pressure gradient force, which was 10 to 30% of the advective term, despite the relatively weak horizontal gradients of order 10 5 kg/m 4 . The non-negligible influence of the baroclinic pressure gradient was possible from the relatively large depths that exceeded 100 m at the deepest part of the pit, compared to the surrounding depths of 30 m. From depth-averaged dynamical terms derived for flood and ebb phases of the tidal cycle, it was found that the advective terms became more important than frictional terms over the deep parts of the pit. Advection became more prominent than friction where the bottom slope exceeded the value of the bottom drag coefficient (0.003). Otherwise, frictional effects dominated outside the pit.
An extension of actuator disc theory is used to describe the properties of a tidal energy device, or row of tidal energy devices, within a depth-averaged numerical model. This approach allows a direct link to be made between an actual tidal device and its equivalent momentum sink in a depth-averaged domain. Extended actuator disc theory also leads to a measure of efficiency for an energy device in a tidal stream of finite Froude number, where efficiency is defined as the ratio of power extracted by one or more tidal devices to the total power removed from the tidal stream. To demonstrate the use of actuator disc theory in a depth-averaged model, tidal flow in a simple channel is approximated using the shallow water equations and the results are compared with the published analytical solutions.
A hybrid method for the incompressible Navier--Stokes equations is presented.
The method inherits the attractive stabilizing mechanism of upwinded
discontinuous Galerkin methods when momentum advection becomes significant,
equal-order interpolations can be used for the velocity and pressure fields,
and mass can be conserved locally. Using continuous Lagrange multiplier spaces
to enforce flux continuity across cell facets, the number of global degrees of
freedom is the same as for a continuous Galerkin method on the same mesh.
Different from our earlier investigations on the approach for the
Navier--Stokes equations, the pressure field in this work is discontinuous
across cell boundaries. It is shown that this leads to very good local mass
conservation and, for an appropriate choice of finite element spaces, momentum
conservation. Also, a new form of the momentum transport terms for the method
is constructed such that global energy stability is guaranteed, even in the
absence of a point-wise solenoidal velocity field. Mass conservation, momentum
conservation and global energy stability are proved for the time-continuous
case, and for a fully discrete scheme. The presented analysis results are
supported by a range of numerical simulations.
Located in between the Flores and Adonara islands, Larantuka strait has one of the strongest tidal currents in Indonesia. An 810 m length bridge is planned to be constructed across the strait to connect the islands. Commencing in 2019, the Palmerah Bridge Project is planned to be the first combined tidal energy/bridge project worldwide. By combining the installation of turbines with bridge construction, the project is intended to achieve certain economies. However, the financial viability would depend on the power production at this
site. This study is an independent assessment regarding the tidal energy resources, estimating the average power available at possible array locations in the area of the Palmerah Tidal Bridge. This paper examines alternative locations for turbine deployment along the strait as well. Moreover, this paper also discusses the flow field change and its effects on the Indonesian Throughflow (ITF) in the region. Assessments with realistic turbine deployments as well as idealised cases are conducted. The project promises to deliver power capacity of 18 to 23 MW at the first stage and may be followed by an extension to a capacity of 90 to 115 MW. It is unclear whether these figures are the average or maximum power. This paper finds that the available power from this site is lower than the output that has been predicted for this project. However, if the prediction is the maximum (as opposed to average) power available for the subsequent stages, this study finds that numbers are achievable.
A theoretical model is proposed for a row of sub-arrays of tidal turbines aligned in a cross-stream fashion across part of a wide channel. This model builds on previous work investigating the behaviour of a single partial row array that split the problem into two flow scales; device and channel. In the present work, three flow scales are proposed: device, sub-array and channel flow, allowing the mass, energy and momentum conservation balance to be assessed separately at each scale. The power potential of a row of sub-arrays with varying blockage ratios at each flow scale is investigated, and it is found that increasing device local blockage has the greatest potential to increase power yield. It is also found that, for such a single row tidal farm with a sufficient number of devices in a very wide channel, splitting the long fence array into multiple smaller co-linear sub-fences can increase the overall energy extraction potential. A new maximum power coefficient is found in infinitely wide flow, increasing from the Lanchester-Betz limit of 0.593 for turbines in unblocked flow, past the partial row array limit of 0.798, to a new limiting value of 0.865 for a row of multiple sub-arrays.
Energy production based on fossil fuel reserves is largely responsible for carbon emissions, and hence global warming. The planet needs concerted action to reduce fossil fuel usage and to implement carbon mitigation measures. Ocean energy has huge potential, but there are major interdisciplinary problems to be overcome regarding technology, cost reduction, investment, environmental impact, governance, and so forth. This article briefly reviews ocean energy production from offshore wind, tidal stream, ocean current, tidal range, wave, thermal, salinity gradients, and biomass sources. Future areas of research and development are outlined that could make exploitation of the marine renewable energy (MRE) seascape a viable proposition; these areas include energy storage, advanced materials, robotics, and informatics. The article concludes with a sustainability perspective on the MRE seascape encompassing ethics, legislation, the regulatory environment, governance and consenting, economic, social, and environmental constraints. A new generation of engineers is needed with the ingenuity and spirit of adventure to meet the global challenge posed by MRE.
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.
A recently developed stabilised finite element method which draws upon features of both continuous and discontinuous finite element methods is generalised for the incompressible Navier–Stokes equations on moving domains and for free surface problems. The approach allows the natural incorporation of upwinding of the advective flux, and the formulation is stable when using equal-order Lagrange basis functions for the velocity and pressure fields. Particular attention is paid to the formulation of a fractional step algorithm for moving domain problems. Numerous applications to environmental free surface flows are presented, ranging from simple test cases to simulations of sophisticated laboratory experiments. It is shown that the formulation, which contains no flow-dependent stabilisation parameters, is able to model a wide range of flow conditions stably without user intervention, and it is shown that negligible numerical dissipation is introduced. The physics of the considered numerical examples involves sensitive free surface wave behaviour, which has been accurately captured due to the stability of the model together with negligible introduced numerical dissipation.
The conversion of the kinetic energy presented by ocean or marine currents offers an exciting proposition as it can provide regular and predictable energy resource. The majority of the proposed designs for converting this type of kinetic energy are based on the concept of the horizontal axis turbines, which has common characteristics to those being used in wind energy. Although a lot can be learnt and transferred from wind turbine technology, there are significant differences. These include the effects of the free surface and the occurrence of cavitation. Consequently, any developed numerical methods need to be verified. This study reports on the development and verification of simulation tools based on blade element momentum theory—a commercial code (GH-Tidal Bladed) and an academic in-house code (SERG-Tidal). Validation is derived from experimental measurements conducted on a model 800mm diameter turbine in a cavitation tunnel and a towing tank. The experimental data includes measurements of shaft power and thrust generated by the turbine for a series of blade pitch settings and speeds. The results derived from the two codes are compared. These indicate that the two developed codes demonstrate similar trends in the results and provide a satisfactory representation of the experimental turbine performance. Such results give the necessary confidence in the developed codes resulting in appropriate tools that can to be utilised by developers of marine current turbines.
There is an upper bound to the amount of power that can be generated by turbines in tidal channels as too many turbines merely block the flow. One condition for achievement of the upper bound is that the turbines are deployed uniformly across the channel, with all the flow through them, but this may interfere with other uses of the channel. An isolated turbine is more effective in a channel than in an unbounded flow, but the current downstream is non-uniform between the wake of the turbines and the free stream. Hence some energy is lost when these streams merge, as may occur in a long channel. We show here, for ideal turbine models, that the fractional power loss increases from 1/3 to 2/3 as the fraction of the channel cross-section spanned by the turbines increases from 0 to close to 1. In another scenario, possibly appropriate for a short channel, the speed of the free stream outside the turbine wake is controlled by separation at the channel exit. In this case, the maximum power obtainable is slightly less than proportional to the fraction of the channel cross-section occupied by turbines.
Rapid assessment of (small) morphological impacts in intertidal systems: methodology and application to tidal energy
- Gatto
Validatie van het oosterscheldemodel, sophiastrand
- van Leeuwen