Alderney Race Research Articles

Multi-criteria analysis to evaluate tidal energy potential in France

France benefits from an important tidal energy resource, with the second largest potential in Europe, and one of the biggest in the world. In addition, France has an important industrial and academic fabric, with several tidal developers, shipyards, and research centers, creating, theoretically, an attractive environment. However, though some tidal turbines already installed, such as Sabella demonstrator at Ouessant Island, or Guinard energy turbine in the Etel river, the sector still has a large development potential. One limitation of such a development is the lack of available data to assess the total potential. Previous studies mainly focused on the most energetic sites, such as the Alderney Race or the Fromveur Passage, while other sites are neglected. Yet, those sites, such has rivers or estuaries, offer softer conditions, lowering the risks. Moreover, although identifying a suitable area for tidal array implementation requires technical criteria (maximum current velocity, water depth, installable potential, etc.), it should also consider other aspects such as ecological indicators (migratory fishes, specific habitats, etc.), regulation specificities (Natura 2000, ZNIEFF, etc.) or human activities conflicts (fishery, UXO, etc.). On top of that, socioeconomic indicators, such as potential local content or partnerships with relevant academic stakeholders, are encouraged by authorities, and must be accounted for.
 Hence, this study proposes an extensive multi-criteria analysis methodology, to assess tidal energy potential of ten sites in France, including five nearshore locations (Alderney race, Fromveur passage, Raz-de-Sein, Raz-Barfleur, Paimpol-Bréhat), and five estuaries, river or similar (Gironde estuary, Gulf of Morbihan, Adour river, Etel river, Arcachon Bay). The analysis is limited to ten sites, selected based on a pre-screening analysis, which will be detailed in the study.
 The different criteria used will be presented, including their weight to the final score which reflect their importance during project development, and the 3-level grading system used to discriminate sites. The methodology developed to assess full-time equivalent employments created by a tidal array, using world input-output tables and other economic parameters will also be detailed. A specific focus will be proposed on methodology used to assess the installable capacity, relying on data from MARS 2D model for nearshore sites and public data for the others.
 First analysis shows that Raz-de-Sein and Paimpol-Bréhat are the two most suitable sites for tidal arrays implementation, while the Alderney Race is the most energetic one. Those results are driven by high scores obtained by these sites in categories such as ecological or human activities. Direct, indirect, and induced employments created during construction and operational phase by each project are also calculated, as well as potential growth value added. Hypothesis and sensitivity to them will be discussed.
 This research was funded by the H2020 projet ELEMENT, grant number 815180.

Estimation and characterisation of the wave-induced turbulent kinetic energy and turbulent dissipation from ADCP data

Turbulence in the water flow causes small-scale variations in the mechanical stress acting on submerged tidal turbines. As such it increases their fatigue loading and impacts greatly their lifetime. It is therefore essential for engineers to have an accurate knowledge and characterisation of turbulence at a given site as they design the structures to be deployed. The strength of the tidal currents is the main parameter influencing the intensity of turbulence through their friction with the sea bed. However, most potential tidal energy sites are located in a coastal environment with shallow enough water depths so that the direct impact of waves on turbulence can not be overlooked. Steepness-induced wave breaking is indeed observed to increase the turbulent mixing for such applications. In this context, we propose to estimate the contribution of surface processes to the total turbulence in Alderney Race, France, the most energetic tidal site in western Europe. The turbulent kinetic energy (TKE) specifically induced by waves and wind is characterised using measurements from a 5-beams ADCP deployed between 27/02/2018 and 06/07/2018.Analytical profiles are fitted to the data, the only fitting parameter of the model is an evaluation of the turbulence penetration depth, it determines how deep surface processes impact the water column. Its dependence towards mean wave and current parameters is studied. The results do not allow to conclude on the nature of turbulence observed in the mid water column.

Comparative study of power production using a generic empirical model in a tidal farm

This study presents a generic model for estimating the velocity deficit and turbulence intensity ina tidal turbine farm. The proposed model considers a range of ambient turbulence intensity, the rotor diameter-to-depth ratio, and the rotor thrust coefficient in realistic applications. We evaluate the power generation of a large-scale tidal farm composed of 16 turbines in an in-line and staggered configuration in an ideal channel similar to the Alderney Race in the English Channel. The added turbulence effect is taken into account when assessing the velocity deficit in the farm. As supported by previous studies, the results show that the staggered array produces more power than the rectilinear array. The staggered arrangement benefits from flow acceleration and wide turbine spacing, which improves wake recovery. According to the results, the farm can be resized by decreasing the lateral spacing in the rectilinear array and decreasing the longitudinal spacing in the staggered array without affecting the farm’s efficiency. The reduction in farm size will reduce cable costs and provide an opportunity for future expansion. For the tidal turbines in shallow water regions, the ratio of rotor diameter to depth is shown to affect the power generated by the turbines. The power produced in the farm decreases with an increase in the rotor diameter-to-depth ratio due to the limited wake expansion along the vertical plane. This low-computational model can be useful in studying the wake interaction of tidal turbine parks in different configurations.

Model and method to predict the turbulent kinetic energy induced by tidal currents, application to the wave-induced turbulence

A prediction model for the turbulent kinetic energy (TKE) induced by tidal-currents is proposed as a function of the barotropic velocity only, along with a robust method evaluating the different parameters involved using Acoustic Doppler Current Profiler (ADCP) measurements from Alderney Race. We find that the model is able to reproduce correctly the TKE profiles with coefficients of correlation on average higher than 0.90 and normalised root-mean-square errors (NRMSE) less than 14%. Different profiles are also tested for the mean velocity, no satisfactory prediction model is found but we are able to have decent estimates of the velocity shear and friction velocity. Two applications are then carried out. First the turbulent budget terms are estimated and discussed. We identify the turbulent production and dissipation of TKE as the most important mechanisms, then we discuss the validity of several theoretical results derived for isotropic turbulence for this application. A strong departure for the estimation of the turbulent dissipation is notably found and explained by the turbulent anisotropy. At last the prediction model for the TKE is used to infer the wave-induced TKE. We show the importance of removing the tidal component, waves can have a strong influence down to mid-depth.

An empirical model accounting for added turbulence in the wake of a full-scale turbine in realistic tidal stream conditions

The increased turbulence behind the turbine induces a fatigue load on the downstream turbine blades. An accurate estimate of turbulent intensity in turbine wake is paramount to optimizing the placement of turbines in tidal turbine parks. A simple empirical model is developed using data fitting from numerical simulation of a non-rotational actuator disc model to estimate the added turbulence in realistic tidal stream conditions similar to the Alderney Race, with a rotor diameter to depth ratio of 40%. The study shows a self-similar Gaussian shape streamwise turbulent intensity in a lateral direction similar to the velocity deficit profile. The turbulent wake radius expands according to a power-law depending on the ambient turbulent intensity. The added turbulence model justifies that the major turbulence source in the near wake is attributed to the rotor as it is weakly dependent on the ambient turbulence in the flow. As no known existing empirical model for tidal turbine wake added turbulence, the model is compared with existing models of unbounded wind turbines. The proposed model estimates the average turbulence in the far wake can assist turbine placement in a tidal farm.

Blockage Corrections for Tidal Turbines—Application to an Array of Turbines in the Alderney Race

Tidal turbines are located in shallow water depths in comparison to their dimensions (15 m-diameter turbines in 40 m depths, typically). Constrained vertically by the water depth and laterally by neighbouring turbines, the flow within a tidal farm is subjected to blockage effects that influence the performance of individual devices. The Betz limit (which is the maximum power extractable from an unconstrained flow) can, therefore, be exceeded as demonstrated by Garrett and Cummins. Thus, beyond a significant blockage ratio, blockage effects should be considered when assessing the energy production of a tidal farm. The actuator disk method is particularly suited to simulate the flow field within an array of turbines under realistic tidal flow conditions. However, the implementation of actuator disks in coastal numerical models relies on relationships that neglect the blockage effects on the thrust and power of devices. We propose here an actuator disk formulation corrected to integrate these effects. This modified formulation, based on the model of Whelan et al., is integrated into a regional implementation of a three-dimensional model Telemac3D targeted towards the Alderney Race (English Channel). The method is applied to two hypothetical tidal farms with aligned and staggered arrangements, respectively. Blockage corrections of the thrust and power coefficients are found to have counterbalanced effects on the array production. Thrust correction results in a noticeable flow reduction within the array. However, the associated decrease of the array production is counterbalanced by the increase of the turbine power coefficient. Blockage corrections were, therefore, found to result in a slight increase, by 3%, of the array production over a mean spring tidal cycle.

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.

A winter storm in Alderney Race: Impacts of 3D wave–current interactions on the hydrodynamic and tidal stream energy

Due to the climate change, it is necessary to modify the energy modes of production. The mix energetic, based on renewable energies as tidal currents, is one of the solutions to decrease the energy production carbon footprint. This article focuses on hydrodynamic interactions in Alderney Race (France), which is the most energetic tidal site in Western Europe with a maximum potential of 5.1GW according to Coles et al. (2017). The impact of a winter storm occurring during spring tide is assessed thanks to numerical modeling with a 3D fully-coupled wave–current model and in-situ data. This study starts to analyze the impacts of the storm on the wave field and the current effects on waves. Then, the modifications of the current and tidal stream energy caused by waves are discussed. After a successful validation step with excellent PBIAS and R2 scores, the main finding are: (i) although the current intensity is strong (around 3–4m s−1), the wave action significantly changes the vertical profile of the current, with a reduction of the PBIAS by a factor of 1.78 between simulations with and without wave effects, (ii) ocean waves affect the tidal asymmetry, with a flood current whose intensity is 13% higher than for the ebb current, inducing a decrease of 30% in the tidal stream energy, (iii) the flow is very sensitive to the angle between the directions of propagation of waves and current, with an acceleration or a reduction of the velocity, as observed in the presence of a 3D turbulent structure, (iv) current effects on waves cause a wavenumber shift, changes in significant wave height (modulated by tide), wave direction due to refraction and an increase of the energy transfer from waves to ocean ascribed to the wave breaking. By a feedback mechanism, the modifications of the wave field by current and water level significantly alter the flow with a decrease of its velocity when waves propagate against current. This study shows that the 3D wave–current interactions need to be considered during a storm even during a spring tide event where currents are the strongest.

Open-Source Code-Based Tidal Modeling of Tropical and Temperate Waters

Tidal energy is the most reliable and predictable form of renewable energy capable of ensuring energy security in coastal regions of the world. Many developing countries are prone to energy self-sufficiency due to a lack of tidal data, expensive commercial tidal modeling tools, and program codes. In the present study, an open-source finite element code along with available open-source data was used to predict the tidal resource potential of sites in both temperate and coastal waters. This paper also investigates the suitability of open-source code towards accurate tidal resource prediction and provides a comparative study on tidal resource prediction of sites in both temperate and coastal waters. Based on knowledge gained from tidal experts all around the globe, the straits of the Alderney race were selected as a temperate water site because of their high tidal flow conditions and high tidal energy resource potential. Singapore was selected as a tropical water site because of its low tidal flow conditions and lack of open-source tidal resource data in the tropical belt. From the results, temperate waters such as Alderney Race experience high tidal velocity in the range of 3.5–4.5 m/s with an average power density of about 15 kW/m2 in comparison with tropical waters such as Singapore that experiences tidal velocity in the range of 1–1.5 m/s with an average power density of about 1.5 kW/m2. The thrusting force behind the coastal dynamics is mainly due to tides, their interactions, and changes in seabed topography. The seabed roughness profile creates a drag force on the flow on the velocity field. Lack of understanding of the effects of seabed friction on tidal modeling might reduce the accuracy of the model prediction. Thus, the present study also focuses on the effects of seabed roughness on tidal prediction of Alderney race straits using the open-source finite element-based 2-dimensional depth average ocean model. It can be found that an increase in seabed friction reduces the flow velocity and thus the average power density of the location due to its major energy dissipating phenomenon for the energetic ocean flow.

Lagrangian Vortex Computations of a Four Tidal Turbine Array: An Example Based on the NEPTHYD Layout in the Alderney Race

This study investigates the wake interaction of four full-scale three-bladed tidal turbines with different ambient turbulence conditions, in straight and yawed flows. A three-dimensional unsteady Lagrangian Vortex Blob software is used for the numerical simulations of the turbines’ wakes. In order to model the ambient turbulence in the Lagrangian Vortex Method formalism, a Synthetic Eddy Method is used. With this method, turbulent structures are added in the computational domain to generate a velocity field which statistically reproduces any ambient turbulence intensity and integral length scale. The influence of the size of the structures and their density (within the study volume) on the wake of a single turbine is studied. Good agreement is obtained between numerical and experimental results for a high turbulence intensity but too many structures can increase the numerical dissipation and reduce the wake extension. Numerical simulations of the four turbine array with the layout initially proposed for the NEPTHYD pilot farm are then presented. Two ambient turbulence intensities encountered in the Alderney Race and two integral length scales are tested with a straight flow. Finally, the wakes obtained for yawed flows with different angles are presented, highlighting turbine interactions.

Quantifying the Effects of Wave—Current Interactions on Tidal Energy Resource at Sites in the English Channel Using Coupled Numerical Simulations

Numerical modeling of currents and waves is used throughout the marine energy industry for resource assessment. This study compared the output of numerical flow simulations run both as a standalone model and as a two-way coupled wave–current simulation. A regional coupled flow-wave model was established covering the English Channel using the Delft D-Flow 2D model coupled with a SWAN spectral wave model. Outputs were analyzed at three tidal energy sites: Alderney Race, Big Roussel (Guernsey), and PTEC (Isle of Wight). The difference in the power in the tidal flow between coupled and standalone model runs was strongly correlated to the relative direction of the waves and currents. The net difference between the coupled and standalone runs was less than 2.5%. However, when wave and current directions were aligned, the mean flow power was increased by up to 7%, whereas, when the directions were opposed, the mean flow power was reduced by as much as 9.6%. The D-Flow Flexible Mesh model incorporates the effects of waves into the flow calculations in three areas: Stokes drift, forcing by radiation stress gradients, and enhancement of the bed shear stress. Each of these mechanisms is discussed. Forcing from radiation stress gradients is shown to be the dominant mechanism affecting the flow conditions at the sites considered, primarily caused by dissipation of wave energy due to white-capping. Wave action is an important consideration at tidal energy sites. Although the net impact on the flow power was found to be small for the present sites, the effect is site specific and may be significant at sites with large wave exposure or strong asymmetry in the flow conditions and should thus be considered for detailed resource and engineering assessments.

Multi-Disciplinary and Multi-Scale Assessment of Marine Renewable Energy Structure in a Tidal System

The French coast of the Atlantic and English Channel (EC) is promising for the development of Marine Renewable Energy (MRE), including wind, wave, and tidal stream, due to the high velocity of currents in some parts of the area. This paper, focusing on wind and tidal energy, discusses how the implementation of MRE converters influences biodiversity, and vice versa, through biofouling and reef effects. The understanding of these interactions requires the knowledge of the hydro-sedimentary conditions and the macrofauna. The research on these topics, performed at the Continental and Coastal Morphodynamic laboratory (M2C) (UNICAEN, France), is presented through a multi-disciplinary approach by i) studying the hydrodynamic conditions and the macrofauna in Alderney Race, ii) studying the biofouling effects on tidal turbines and their influence on the turbulent wake, iii) assessing the hydro-sedimentary impacts induced by the offshore wind farm, like scouring, and iv) taking an ecosystem approach on MRE, such as the reef effect. From an ecological perspective, the reef effect can be responsible for changes in the structure and function of the ecosystem. Although several studies have analyzed this effect at the species-or community-scale, the propagation of the reef effect at the ecosystem-scale remains unclear. Thus, understanding these ecosystem-scale effects is urgent for future research. From an engineering perspective, biofouling changes the structural characteristics (i.e., supplementary mass) of the converters and thus, affects their performance.

The Efficiency of a Fence of Tidal Turbines in the Alderney Race: Comparison between Analytical and Numerical Models

Assessing the efficiency of a tidal turbine array is necessary for adequate device positioning and the reliable evaluation of annual energy production. Array efficiency depends on hydrodynamic characteristics, operating conditions, and blockage effects, and is commonly evaluated by relying on analytical models or more complex numerical simulations. By applying the conservations of mass, momentum, and energy in an idealized flow field, analytical models derive formulations of turbines’ thrust and power as a function of the induction factor (change in the current velocity induced by turbines). This simplified approach also gives a preliminary characterization of the influence of blockage on array efficiency. Numerical models with turbines represented as actuator disks also enable the assessment of the efficiency of a tidal array. We compare here these two approaches, considering the numerical model as a reference as it includes more physics than the analytical models. The actuator disk approach is applied to the three-dimensional model Telemac3D in realistic flow conditions and for different operating scenarios. Reference results are compared to those obtained from three analytical models that permit the investigation of the flow within tidal farm integrating or excluding processes such as the deformation of the free surface or the effects of global blockage. The comparison is applied to the deployment of a fence of turbines in the Alderney Race (macro-tidal conditions of the English Channel, northwest European shelf). Efficiency estimates are found to vary significantly from one model to another. The main result is that analytical models predict lower efficiency as they fail to approach realistically the flow structure in the vicinity of turbines, especially because they neglect the three-dimensional effects and turbulent mixing. This finding implies that the tidal energy yield potential could be larger than previously estimated (with analytical models).

LANS-α turbulence modeling for coastal sea: An application to Alderney Race

The Lagrangian-Averaged Navier-Stokes-α (LANS-α) turbulence model was implemented for the first time in a coastal hydrodynamic model. We present in this paper the details of the implementation, as well as the difficulties encountered. To overcome the difficulties, a convolution filter was used instead of the Helmoltz operator, and incompressibility was imposed in both rough and smooth velocities. The results of the second numerical implementation were tested against the results of simulations without LANS-α for a realistic application showing the tidal dynamic in Alderney Race, Normandy, France, which has the strongest currents in western Europe. The behavior of LANS-α is consistent with the conclusions of former studies, which supports our results. The findings are: i) LANS-α re-energizes the flow recovering higher-resolution turbulence statistics in lower-resolution simulations, leading to 30% savings in computing time, ii) LANS-α produces the two kinds of inertial range for barotropic turbulence, with turbulent energy decays in k−5/3 and in k−3 (k being the wave number) and iii) LANS-α strongly characterizes the turbulence induced by deformation. In the future, these results need to be compared to measurements and to other turbulence modeling approaches, including dissipative LES or DNS, to evaluate their relevance for the community.

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.

One year of measurements in Alderney Race: preliminary results from database analysis.

The Alderney Race is assumed to have the largest tidal-stream energy potential in the north-western European coastal seas. Interaction of the powerful tidal stream with strong wind, high waves and irregular bathymetry creates hydrodynamic conditions of extreme complexity, with high levels of turbulence. A comprehensive dataset has been created to improve the understanding of physical processes, turbulence, tidal stream and resource variability at the site. The database contains a large amount of oceanographic and meteorological measurements acquired in Alderney Race in 2017-2018. This exceptionally long period of observations (nearly one year) became possible due to modern tools and strategies of data acquisition. The paper presents some significant results from the database analysis. Among many results, we would like to underline the following: (i) a wide range of variability of mean flow and sea state parameters was documented; (ii) exceptionally large values of current velocity (7 m s-1) and significant wave height (8 m) were measured during extreme meteorological conditions; (iii) high-frequency variability of current speed during storm events was also found to be very large, with the standard deviation of velocity reaching 0.3 m s-1 in the bottom boundary layer, and 0.6 m s-1 in the surface layer; and (iv) predominant wind and wave direction relative to the flow impacts the wave height and significantly increases the turbulence kinetic energy of the flow. To our knowledge, this is the largest multi-variable database available on potential tidal energy sites. The results of database analysis can represent a significant advance in environmental conditions and resource characterization and provide advanced information to turbine developers. This article is part of the theme issue 'New insights on tidal dynamics and tidal energy harvesting in the Alderney Race'.

Identifying economically viable tidal sites within the Alderney Race through optimization of levelized cost of energy.

Costs of tidal stream energy generation are anticipated to fall considerably with array expansion and time. This is due to both economies of volume, where arrays comprising of large numbers of turbines can split fixed costs over a greater number of devices, and learning rates, where the industry matures and so arrays of the same size become cheaper due to lessons learned from previous installations. This paper investigates how tidal energy arrays can be designed to minimize the levelized cost of energy (LCOE), by optimizing not only the location but also the number of devices, to find a suitable balance between decreased costs due to economies of volume and diminishing returns due to global blockage effects. It focuses on the Alderney Race as a case study site due to the high velocities found there, making it a highly suitable site for large-scale arrays. It is demonstrated that between 1 and 2 GW could be feasibly extracted as costs in the tidal industry fall, with the LCOE depending greatly on the assumed costs. A Monte-Carlo analysis is undertaken to account for variability in capital and operational cost data used as inputs to the array optimization. Once optimized, the estimated P50 LCOE of an 80 MW array is £110/MWh. This estimate aligns closely with the level of subsidy considered for tidal stream projects in the Alderney Race in the past. This article is part of the theme issue 'New insights on tidal dynamics and tidal energy harvesting in the Alderney Race'.

Underway velocity measurements in the Alderney Race: towards a three-dimensional representation of tidal motions.

The Alderney Race, located northwest of the Cotentin Peninsula (France), is a site with high tidal-stream energy potential. Circulation through the Alderney Race is complex, with current speed exceeding 3 m s-1 at neap tide. Towed acoustic Doppler current profiler (ADCP) measurements and static point velocity measurements were performed in July 2018 focusing on assessment of circulation and vertical structure of tidal currents. Transect surveys revealed peculiar features of local dynamics such as change in location of the tidal jet on ebb and flood flow. The spatial expanse of the tidal jet was quantified and regions with largely sheared or nearly homogeneous velocity distributions were identified on the cross-sections. Velocity profiles acquired along the cross-sections were accurately characterized using a power law. The spatial variability of the power-law exponent α was found to be large and correlated with the tidal conditions. The largest variation in profile shape was observed in the northern sector and assumed to be generated by the current interaction with a bathymetric constriction. The velocity profiles were found to vary from highly sheared on flood flow to nearly homogeneous on ebb flow, with corresponding range of power-law exponent α variation from 6 to 14. In the southern sector, over a relatively smooth bathymetry, the velocity profile shape was accurately approximated using the 1/7 power law with a range of variation of α from 6.5 to 8, with respect to the tidal conditions. To our knowledge, this is the largest field survey done using towed ADCP and the results could represent a significant advance in tidal site characterization and provide advanced information to turbine developers. This article is part of the theme issue 'New insights on tidal dynamics and tidal energy harvesting in the Alderney Race'.

The energy yield potential of a large tidal stream turbine array in the Alderney Race

This research provides an updated energy yield assessment for a large tidal stream turbine array in the Alderney Race. The original array energy yield estimate was presented in 2004. Enhancements to this original work are made through the use of a validated two-dimensional hydrodynamic model, enabling the resolution of flow modelling to be improved and the impacts of array blockage to be quantified. Results show that a range of turbine designs (i.e. rotor diameter and power capacity) are needed for large-scale development, given the spatial variation in bathymetry and flow across the Alderney Race. Array blockage causes a reduction in flow speeds in the array of up to 2.5 m s-1, increased flow speeds around the array of up to 1 m s-1 and a reduction in the mean volume flux through the Alderney Race of 8%. The annual energy yield estimate of the array is 3.18 TWh, equivalent to the electricity demand of around 1 million homes. The capacity factor of the array is 18%, implying sub-optimal array design. This result demonstrates the need for turbine rated speed to be selected based on the altered flow regime, not the ambient flow. Further enhancement to array performance is explored through increases to rotor diameter and changes to device micro-siting, demonstrating the significant potential for array performance improvement. This article is part of the theme issue 'New insights on tidal dynamics and tidal energy harvesting in the Alderney Race'.