Fromveur Strait Research Articles

Use of an environmental proxy to determine turbulence regime surrounding a full-scale tidal turbine deployed within the Fromveur Strait, Brittany, France

Establishing a relationship between tidal current conditions and tidal turbine performance and loads is a critically important consideration for turbine reliability. Nonetheless, obtaining in-situ information is often challenging, and as a result both environmental and load data may be more sparse than desired. This study presents a method to make use of limited data sets by establishing a relationship between measurements of hydrodynamic variability and turbine power or blade strain variability, even when these measurements are not taken simultaneously. The method is tested on data from the deployment of a full-scale pilot tidal turbine: in situ velocity measurements and turbulence characteristics taken at times when the turbine was not installed were associated with power and strain measurements during the turbine’s deployment via a Delft3D proxy. The data show that the variability of active power correlates well with larger turbulence kinetic energy (TKE) when comparing similar populations via the proxy. Examination of blade strain variance against TKE shows a weaker correlation, with fat-tailed distributions and extremely high strain values prominent across all flow speeds. Acceleration or deceleration of the flow influenced the power variability of the turbine, with larger standard deviations recorded across accelerating flows. No significant difference was found when comparing blade strain variance in accelerating and decelerating flows. We conclude that the proxy method studied can establish a population-level relationship between non-simultaneous environmental and load data, but that the accuracy and precision of this relationship depends on the amount of data available: this method is therefore only suitable where there is a sufficiently rich dataset.

On nodal modulations of tidal-stream energy resource in north-western Europe

Estimation of tidal-stream turbine loading and energy yield requires a thorough understanding of the hydrodynamic processes that influence tidal currents over a wide range of timescales. In this study, we focus on the long-term variability of the tidal-stream energy resource associated with the 18.6-year lunar cycle. Three sites in north-western Europe, with strong potential for tidal array development, are considered; the Alderney Race (English Channel), the Fromveur Strait (western Brittany) and the Ramsey Sound (Irish Sea). The investigation relies on harmonic analysis and associated predictions of depth-averaged tidal currents. Results show that the variability in predicted annual power densities is comparable at the three measurement locations. This variability reaches +/- 10% over the 18.6-year lunar cycle, and is mainly associated with M2 nodal modulations which dominate the tidal signal in north-western European shelf seas. Maximum and minimum power density occur in 2015 and 2024, respectively. 2015 is characterized by (i) reduced spring-neap variabilities and diurnal inequalities of tidal currents, and (ii) increased tidal asymmetries (between peak ebb and flood), whereas 2024 shows reverse effects. This cycle also influences the annual maximum/mean current magnitudes. The differences in maximum speeds between 2015 and 2024 are of 0.16–0.17 m/s at the three sites, which represents 4-5% of the peak speeds. The simple approach considered here may be applied in broader locations as a preliminary assessment of the long-term variability of the available tidal stream energy resource. The modulations predicted from harmonic analysis are finally expected to be on the high side of the forecasting range. Indeed, the results of an 18-year numerical simulation performed on the Alderney Race suggested a reduced effect of the 18.6-year lunar cycle on the resource.

The Tidal Stream Energy Resource of the Fromveur Strait—A Review

Refined assessments of the available tidal stream energy resource are required to optimize turbines design and guarantee successful implementations and operations of devices in the marine environment. Investigations primary focused on identifying areas with maximum current speeds. However, further information may be reached by exhibiting (i) resource temporal variability, (ii) superimposed effects of meteo-oceanographic conditions (including especially wind-generated surface-gravity waves), and (iii) potential environmental impacts of operating turbines at the regional (e.g., changes in sediment transport and surrounding seabed features, effects on marine water quality, etc.) and local (wake-wake interactions and energy output) scales. These aspects are here investigated by reviewing a series of research studies dedicated to the Fromveur Strait off western Brittany, a region with strong potential for tidal array development along the coast of France. Particular attention is dedicated to the exploitation of combined in-situ and remote-sensing observations and numerical simulations. Beyond a site specific characterization of the tidal stream energy resource, this review promotes a series of original approaches and analysis methods for turbines optimization, thus complementing technical specifications to secure the key steps of a tidal energy project and promote the growth of a reliable tidal stream energy exploitation.

Overall tidal farm optimal design–Application to the Alderney Race and the Fromveur Strait (France)

This article presents an overall optimization method of Tidal Energy Converters arrangement and their electrical connection topology. This methodology is implemented as a software tool and formulated as a bi-level programming where two sequential genetic algorithms are used. A semi-analytical method is adopted to evaluate the loss of production caused by the wake effect. Electrical connection costs as well as line losses are taken into account. This developed tool takes a large panel of variables into consideration and delivers an optimal number of turbines and their locations, optimal AC electrical architecture and optimal sizing of electrical equipment as its output. The optimization approach is applied to the Alderney Race and the Fromveur Strait which represent the first and the second greatest tidal potential in France, respectively. The results show that the energy production cost considerably differs from one site to another and it strongly depends on the size of the farm.

A comparative study for tidal current velocity prediction using simplified and fast algorithms

Interest in marine renewable energies has grown in recent years, especially in northern European shelf seas where several technological devices have been developed. In this context, numerous investigations focused on accurate predictions of tidal currents and their respective kinetic energy. An empirical estimation of the current velocity is explored in this study, where three different methods are presented. The first method is a linear approximation of the tidal vector velocity as a function of the tidal coefficient (ratio of the tidal range over its maximal value). The second and the third methods are a piece-wise linear and an exponential relationships between tidal vector velocity and the tidal coefficient, respectively. These three methods are compared to reference values obtained using a numerical simulation of tidal hydrodynamics. These methods are applied at two prominent tidal stream energy sites along the coast of France, the Fromveur Strait (western Brittany) and the Alderney Race (between the island of Alderney, Channel islands, and the Cap de la Hague, France). Results show that the piece-wise linear and the exponential models are more accurate than the linear approximation, where the mean error decreased by 12.7% and 14.9% respectively at the worst case scenario.

Spatio-temporal variability of tidal-stream energy in north-western Europe.

Initial selection of tidal stream energy sites is primarily based on identifying areas with the maximum current speeds. However, optimal design and deployment of turbines requires detailed investigations of the temporal variability of the available resource, focusing on areas with reduced variability, and hence the potential for more continuous energy supply. These aspects are investigated here for some of the most promising sites for tidal array development across the north-western European shelf seas: the Alderney Race, the Fromveur Strait, the Pentland Firth and the channels of Orkney. Particular attention was dedicated to asymmetry between the flood and ebb phases of the tidal cycle (due to the phase relationship between M2 and M4 constituents), and spring-neap variability of the available resource (due to M2 and S2 compound tides). A series of high-resolution models were exploited to (i) produce a detailed harmonic database of these three components, and (ii) characterize, using energy resource metrics, temporal variability of the available power density. There was a clear contrast between the Alderney Race, with reduced temporal variability over semi-diurnal and fortnightly time scales, and sites in western Brittany and North Scotland which, due to increased variability, appeared less attractive for optimal energy conversion. This article is part of the theme issue 'New insights on tidal dynamics and tidal energy harvesting in the Alderney Race'.

Optimization of the electrical connection topology of a tidal farm network

This article presents an approach to optimize the electrical connection topology of tidal energy converters in a tidal farm. The methodology is based on a genetic algorithm (GA). The main purpose is to present a technique of coding to find the best electrical connection topology of the tidal farm network. The optimization model takes into account the energy loss in the submarine cables. The model gives as its output the optimal number of turbine clusters connected to each offshore substation, the number of turbines in each cluster, the cross-section of MV and HV cables, the connection design for each cluster of turbines as well as the number and the locations of the offshore substations. A particle swarm optimization algorithm (PSO) is used to confirm the results obtained with the GA. The optimization approach is applied to the Fromveur Strait (France).

Three-dimensional modelling of turbine wake interactions at a tidal stream energy site

One of the biggest uncertainties in tidal stream energy resource assessment is how tidal energy conversion, particularly at large scale, will interact with the resource. As few arrays are currently operational, data collected from these developments tends to be commercially sensitive. Therefore, array interaction with the resource is generally assessed using numerical models. A fully three-dimensional numerical approach based on Actuator Disk theory was implemented into the Regional Ocean Modelling System (ROMS) to simulate the energy extraction by tidal stream turbines. Emphasis was placed on wake interactions and cumulative effects of individual devices on energy extraction at array scale. This model was applied at the tidal stream energy site of the Fromveur Strait (western Brittany, France) considering an array of horizontal-axis turbines of 10-m diameter, matching the device technology currently operating in the Strait. Two tidal energy metrics were considered to describe asymmetries in tidal current magnitude and direction. The area with reduced asymmetry in current magnitude was selected to implement the turbine array. A nested grid technique was adopted to cascade processes from the regional scale to the high-resolution local farm domain. The computation was conducted over the inner-nested array domain covering the tidal farm with horizontal and vertical resolutions of 1 m, matching the 1/10th turbine diameter (D) recommended to resolve velocity and turbulence intensity along device wakes. The array layout initially followed recommended staggered configurations with longitudinal and lateral spacings of 10D and 5D, respectively. However, during mean spring tidal conditions, the misalignment of peak flood currents induced significant wake interactions that reduced the array output by about 15% in comparison to peak ebb. These interactions were investigated to adapt array layouts, minimise wake interactions, and optimise the energy conversion. By reducing the lateral spacing between devices to 3D (measured centre to centre rather than tip to tip), the flood ebb asymmetry in energy extraction was lowered from 15% to 2%.

Turbines’ effects on water renewal within a marine tidal stream energy site

As tidal stream energy sites contribute to a significant part of water transport in coastal shelf seas, turbines’ implementation may lead to system-wide changes of the hydrodynamic circulation with potential effects on marine water quality. These aspects were investigated in north-western coastal waters of Brittany (France, western Europe) by simulating the decay of a tracer concentration within a control domain surrounding the tidal stream energy site of the Fromveur Strait. Simplified simulations were adopted to analyse the sensitivity of tracer distribution to stream energy extraction. While this marine area showed important renewal capacity with a 50% loss of tracer concentration in less than three days, residual values were still present after two months. This result exhibited that the concentration decay could not be calibrated by simple exponential functions. The renewal of marine waters appeared furthermore dependent on tracer release time during a tidal cycle. A full energy-extraction scenario was finally simulated suggesting weak effects of turbines on the renewal capacity with changes in residence times below 5%. The spatio-temporal Eulerian evolution of the concentration revealed, however, local trapping areas in close correlation with the Lagrangian residual circulation that vanished after two months.

Assessing the impact of tidal stream energy extraction on the Lagrangian circulation

Whereas numerous numerical modelling have been conducted to gain further insights about the hydrodynamic characteristics of tidal stream energy sites, the long-term transport of water particles has been routinely ignored. However, a refined evaluation of circulation pathways is fundamental to assess water-quality changes induced by tidal energy converters. The present study investigates the tide-induced Lagrangian circulation in north-western coastal waters of Brittany (France, western Europe), including numerous islands, shoals and straits with strong potential for turbine farm implementation. Particular attention is dedicated to the hydrodynamic impact of tidal stream power extraction from a series of horizontal-axis turbines. An original approach, based on the coupling of the Lagrangian barycentric method with predictions from a model at high spatial resolution, is considered to assess circulation pathways in a single synoptic view. Spatial results reveal a complex Lagrangian circulation highly controlled by bathymetry and islands geometry. The residual flow in the Fromveur Strait, separating Ushant island from Molène archipelago, displays thus a strong asymmetry, between a prominent north-eastern pathway with residual currents up to 0.45ms-1 and a southward circulation. The positions of cyclonic and anti-cyclonic recirculations, both upstream and downstream the Strait, are furthermore precisely defined. The north eddy is found closely located to the sand bank of the Four. The extraction of tidal stream power impacts mainly the residual Lagrangian circulation along the current stream emerging from the Strait, with a tendency for surrounding eddies to get closer to the tidal stream energy site. The displacements of the south and north recirculations may thus exceed 1.5km with possible implications on the evolution of surrounding seabed features.

Asymmetry of tidal currents off the W.Brittany coast and assessment of tidal energy resource around the Ushant Island

A method of tidal stream energy resource assessment around the Ushant Island in the Iroise Sea, using surface velocity time series from High Frequency radars (HFR) and ADCP measurements, is presented. Remotely sensed velocities provided by the radars allow to augment the industry standard approach of 3D numerical modeling and in-situ ADCP surveying to make a large-scale quantification of tidal stream resource. They capture the real ocean dynamics and thus provide context on the complex spatial variability of tidal currents that are so often feature at potential tidal energy sites. The observations show current velocities of 4 m/s northwest of the Island and in the Fromveur Strait, with 1 m/s value exceeded 60% and 70% of time respectively. Emphasis is given to the peculiarities in tidal flow asymmetry and to the study of the variation of vertical velocity profiles during different tidal stages. Radar derived velocities reveal a pronounced asymmetry between the flood and ebb flow around the Ushant Island, quantified by dimensionless number a - velocity asymmetry. The largest range of asymmetry variation, from 0.5 to 2.5, is observed in the Fromveur Strait. Harmonic analysis demonstrated that a joint variation of phase of the principal semi-diurnal (M2) and quarter-diurnal (M4) tidal velocity component accounts for flow asymmetry variation in the strait. Asymmetry in current direction is also quantified. ADCP measurements show that the vertical velocity profiles follow a 1/α power law with a power law exponent mostly depending on geographic location than on tidal stage. It was demonstrated that, in the Fromveur Strait, the 1/7 power law is appropriate to characterize the velocity profile. The combination of two sources of data enables characterization of the velocity variations in three spatial dimensions and in time thus increasing accuracy of the hydrokinetic resource assessment from HF radar observations. The estimation shows that the mean technical resource is 50% smaller in the lower half than in the upper half of the water column. The theoretical resource on average is three times higher than the technical resource and appears to be more sensitive to variations in the shape of the velocity profile in the lower layer.

Modelling effects of tidal currents on waves at a tidal stream energy site

Whereas tidal stream energy sites may be characterised by significant wave and current interactions, numerical resource assessments routinely ignored these processes neglecting, in particular, current-induced modifications of waves. Here, these effects are investigated in locations exposed to ocean waves, evaluating performances of a phase-averaged numerical wave model to approach observed semi-diurnal modulations of wave heights, periods and directions, off and within the Fromveur Strait (western Brittany), a region with strong potential for tidal array development. The introduction of tidal forcings in the wave model is found to improve predictions by approaching the large-scale influence of current-induced refraction leading to variations of the significant wave height by nearly 30% at the deep-water wave buoy. The model reproduces furthermore, with the inclusion of an additional dissipation term, the localised steepening and waves breaking on negative current gradients, limiting the overestimation of the significant wave height at the shallow-water wave buoy within the Fromveur Strait. Tidal currents may finally lead to a strong increase in wave power, over 100% in northern entrances of straits, contributing to the variability of the wave energy flux.

The influence of waves on the tidal kinetic energy resource at a tidal stream energy site

Successful deployment of tidal energy converters relies on access to accurate and high resolution numerical assessments of available tidal stream power. However, since suitable tidal stream sites are located in relatively shallow waters of the continental shelf where tidal currents are enhanced, tidal energy converters may experience effects of wind-generated surface-gravity waves. Waves may thus influence tidal currents, and associated kinetic energy, through two non-linear processes: the interaction of wave and current bottom boundary layers, and the generation of wave-induced currents. Here, we develop a three-dimensional tidal circulation model coupled with a phase-averaged wave model to quantify the impact of the waves on the tidal kinetic energy resource of the Fromveur Strait (western Brittany) - a region that has been identified with strong potential for tidal array development. Numerical results are compared with in situ observations of wave parameters (significant wave height, peak period and mean wave direction) and current amplitude and direction 10m above the seabed (the assumed technology hub height for this region). The introduction of waves is found to improve predictions of tidal stream power at 10m above the seabed at the measurement site in the Strait, reducing kinetic energy by up to 9% during storm conditions. Synoptic effects of wave radiation stresses and enhanced bottom friction are more specifically identified at the scale of the Strait. Waves contribute to a slight increase in the spatial gradient of available mean tidal stream potential between the north-western area and the south-eastern part of the Strait. At the scale of the region within the Strait that has been identified for tidal stream array development, the available mean spring tidal stream potential is furthermore reduced by 12% during extreme waves conditions. Isolated effects of wave radiation stresses and enhanced bottom friction lead to a reduction in spring tidal potential of 7.8% and 5.3%, respectively. It is therefore suggested that models used for tidal resource assessment consider the effect of waves in appropriately wave-exposed regions.

The impact of seabed rock roughness on tidal stream power extraction

Numerical assessments of environmental disturbances induced by a tidal farm project rely usually on local modifications of the friction coefficient over the area covered by a proposed array. Nevertheless, no study has investigated the sensitivity of predictions to surrounding seabed friction. The present investigation focuses on impacts of roughness parameterisation of rock outcrops, a typical seabed of tidal stream sites. A high-resolution depth-averaged circulation model is implemented in the Fromveur Strait off western Brittany, a region with strong potential for array development, integrating the heterogeneity of sediment bottom types. Rock roughness strongly influences initial predictions of tidal current and kinetic energy in the Strait with variations of available power up to 30 %. Tidal energy extraction induces noticeable reductions of tidal currents and bottom shear stresses up to 15 km from the array considered till surrounding sandbanks. Rock roughness impacts farm-induced modifications of tidal currents, bottom shear stresses and stream powers till north-eastern and southward edges of the Strait with major absolute differences identified in its central part. Surrounding sandbanks are finally suggested to variations of shear stresses from 9 to 17 % over the Bank of the Four with possible implications on local sediment deposition.

Estimation of Tidal Stream Potential in the Iroise Sea from Velocity Observations by High Frequency Radars

A year-long surface current velocity time series recorded by High Frequency radars (HFR) are used for assessing the hydrokinetic resources at a near-shore site in the Iroise sea. A combination of remotely sensed surface velocities and ADCP velocity profiles recorded in radar coverage zone allowed assessing the tidal current variability and power density variations in three dimensions. The analysis revealed two areas with high energy potential: the Fromveur Strait and the area NW of the Ushant Island where the kinetic power density in the surface layer attains 1.5kW/m2. The power available in the bottom layer is 3 times lower.

Surface circulation in the Iroise Sea (W. Brittany) from high resolution HF radar mapping

The data from two high-frequency radars (HFR) operating in the Iroise Sea are re-processed by applying an improved version of the direction finding algorithm, removing wave-induced surface currents and the variational interpolation on a regular grid. Combining these processing techniques allowed reconstruction of the surface currents at a level of details that was not previously available. Refined resolution enabled to identify fine-scale structures of surface circulation, to quantify the variability of tidal currents and the residual (time averaged) velocity field, and to explain spatial intermittence in polarization of the tidal current ellipses. The analyzed data span two month-long periods in spring and late summer of 2007. The major findings include (a) a dipole structure in the vorticity field characterized by two oppositely rotating eddies, generated on the leeward side of the Ushant Island at flood (negative polarity) and at ebb (positive polarity); (b) an extremely strong fortnightly variability of tidal currents northwest of the Ushant Island with the highest velocity magnitude of 3.9m/s caused by the interference of the major semi-diurnal tidal constituents; (c) a significant contribution of the higher order nonlinear tidal harmonics to the surface currents in the Fromveur strait, which maintains strong tidal currents and affects the shape of their fortnightly modulation. The residual circulation is characterized by two distinct zones approximately separated by the 100m isobath: in the offshore zone the residual currents have a significant contribution of the wind-driven component, whereas the nearshore zone is characterized by extremely strong (up to 0.4m/s) time-independent residual circulation featuring two permanent anticyclonic eddies: north of the western extremity of the Sein archipelago, and north the Ushant Island. The acquired data and the presented results could be useful for regional model validation and studies of the local eddy dynamics, tidal fronts, and passive tracer transport in the region.