Tidal Energy Site Research Articles

Merging velocity measurements and modeling to improve understanding of tidal stream resource in Alderney Race

Tidal circulation and tidal stream resource in Alderney Race (Raz Blanchard) were assessed by using a towed acoustic Doppler current profiler (ADCP) system and tidal modeling. Optimal Interpolation (OI) was applied to process the underway velocity measurements recorded at neap tide flood and ebb flow. The interpolation technique allows reconstructing space-time evolution of the velocity field within the domain during surveying periods. The method employs velocity covariances derived from numerical simulations by a 2D hydrodynamic model MARS. Model covariances are utilized by the OI algorithm to obtain the most likely evolution of the velocity field under the constraints provided by the ADCP observations and their error statistics. The resulting velocity fields were used for assessing the tidal stream resource at site. The largest overall difference between the kinetic power density derived from simulated and interpolated velocity fields was found for ebb tide. Model simulations constrained by velocity measurements demonstrated a significant (up to 30%) decrease of power available in the flow. A significant change in spatial pattern of power density distribution was also identified. It is demonstrated that by merging high resolution velocity measurements at tidal energy site with modeling the tidal stream potential estimation becomes more accurate.

On Tidal Current Velocity Vector Time Series Prediction: A Comparative Study for a French High Tidal Energy Potential Site

Estimating the energy potential of tidal stream site is a key feature for tidal energy system deployment. This paper aims to compare two methods of prediction of tidal current velocities. The first one is based on the use of a fully three-dimensional (3D) numerical approach. However, while being accurate, the numerical model is highly time-consuming. The second method is based on a linear approximation of the tidal current, which only requires preliminary knowledge of local current velocities time series during two typical tidal cycles. This second method allows a very quick evaluation of the tidal stream resource during a long time period. The proposed comparison is done in three different locations of a high potential tidal energy site in west of France. It is carried out in terms of current velocity and energy harnessing for several turbines technology options (with and without yaw). The achieved results show that the linear approximation gives satisfactory evaluation of the tidal stream potential and can be a very interesting tool for preliminary site evaluation and first technology options selection. However, the fully 3D numerical model can obviously be very useful in more advanced steps of a project.

Towards a turbulence characterization in tidal energy sites. First results of THYMOTE project

Tidal turbine will be installed in area with high current and high turbulence level. A characterisation of this last is required. The aim of the project THYMOTE is to characterize and understand the generation of eddies from smaller to several tens of meters. Three technics are used: Numerical modelling, Physical modelling, field measurements. Physical and numerical modelling show clearly the appearance of the eddies close to the bottom in presence of dunes or rocks and their motion towards the free surface.

Tidal stream resource characterisation in progressive versus standing wave systems

Characterisations of the tidal stream resource and its variability over various timescales are crucial for the development of the tidal stream energy industry. To date, no research has compared resource sensitivity in standing wave (when peak currents occur midway between high and low water) and progressive wave (where peak currents occur at high and low water) tidal systems. Here, we compare the flow regimes of standing wave versus progressive wave systems and the associated variations in tidal stream power with applications to device deployment options (floating-platform turbines versus bottom-mounted turbines). We use a validated 3D numerical model (ROMS) of a globally-significant tidal energy shelf sea region (Irish Sea), to test the hypotheses that the influence on potential extractable energy, and suitability for different devices, may be markedly different between these contrasting systems. Power density was also calculated and compared for floating versus bottom-mounted devices using in-situ current data (ADCPs) obtained from a standing wave site and a progressive wave site. We show that progressive wave systems are characterised by velocity-asymmetry over a tidal cycle (i.e. stronger peak flows at high water than at low water), leading to power-asymmetry. Such power asymmetry was shown to have more of an effect on floating device technology, where an assumed turbine depth tracks the sea surface, in contrast to bottom-mounted technology, where the hub height is fixed at a certain position above the sea bed. Shallow, high-flow regions where tidal range is large contained up to 2.5% more power density from bottom-mounted compared with floating turbines; however, there were areas where floating devices were exposed to higher mean currents over a tidal cycle. Standing wave systems, where flow asymmetry is minimised, did not particularly favour either technology. The results highlight the requirement for detailed resource assessments to consider the vertical plane, and are applicable to all potential tidal stream energy sites.

Assessment of the Malaysian tidal stream energy resource using an upper bound approach

In this paper, an upper bound approach is used to determine the maximum power available to tidal stream turbines placed at five sites along the west coast of Peninsular Malaysia. A depth-averaged hydrodynamic model of the Malacca Strait is built and validated against field measurements. Actuator disc theory is then used to introduce rows of tidal stream turbines as line sinks of momentum and to determine the maximum time-averaged power available to rows of both moderately sized and very large turbines, placed strategically at the locations of highest naturally occurring kinetic energy flux. Results suggest that although the Malaysian tidal stream energy resource is not large enough to make a significant contribution to the country’s energy mix, there may yet be opportunities to use low-speed tidal turbines in small-scale and off-grid electricity generation schemes. Methods are described in detail and links to source codes and results are provided to encourage the application of this simple, yet effective resource assessment methodology to other promising tidal energy sites.

Tidal Turbines’ Layout in a Stream with Asymmetry and Misalignment

A refined assessment of tidal currents variability is a prerequisite for successful turbine deployment in the marine environment. However, the numerical evaluation of the tidal kinetic energy resource relies, most of the time, on integrated parameters, such as the averaged or maximum stream powers. Predictions from a high resolution three-dimensional model are exploited here to characterize the asymmetry and misalignment between the flood and ebb tidal currents in the “Raz de Sein”, a strait off western Brittany (France) with strong potential for array development. A series of parameters is considered to assess resource variability and refine the cartography of local potential tidal stream energy sites. The strait is characterized by strong tidal flow divergence with currents’ asymmetry liable to vary output power by 60% over a tidal cycle. Pronounced misalignments over 20 ∘ are furthermore identified in a great part of energetic locations, and this may account for a deficit of the monthly averaged extractable energy by more than 12%. As sea space is limited for turbines, it is finally suggested to aggregate flood and ebb-dominant stream powers on both parts of the strait to output energy with reduced asymmetry.

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.

Modelling 3D hydrodynamics governing island-associated sandbanks in a proposed tidal stream energy site

A 3D numerical modelling study to investigate the existing hydrodynamic regime of the Pentland Firth Inner Sound Channel, Scotland, UK is presented. Hydrodynamics that govern some sensitive sedimentary deposits in the Inner Sound Channel are discussed. A 3D hydrodynamic model Delft3D is set up for Pentland Firth, located between Orkney Islands and mainland Scotland and a full sensitivity analysis of the numerical model is carried out. The current model set up sufficiently captures the existing hydrodynamics during a full spring-neap tidal cycle inside Pentland Firth. Using model results, the 3D structure of the dynamics of the tidal flows in the Inner Sound Channel is investigated. The temporal variability of tidal flows, the residual tidal flows in the channel and local flow interactions with the Island of Stroma are described. It is proved that the tidally dominant flows drive the sediment transport gradient model to explain the principle maintenance mechanisms of two island-associated sandbanks present in the Inner Sound. The present study provides detailed information on the physics of the tidal regime in the Inner Sound and explains the presence of sandbanks in an area of high tidal flows. Due to extremely high tidal flows, Inner Sound is considered as one of the most favourable sites for tidal energy extraction in the UK. The findings of this study will be very useful in assessing the significance of impacts of future tidal energy extraction on natural hydrodynamics and sediment dynamics of the area.

Multi-scale temporal patterns in fish presence in a high-velocity tidal channel.

The natural variation of fish presence in high-velocity tidal channels is not well understood. A better understanding of fish use of these areas would aid in predicting fish interactions with marine hydrokinetic (MHK) devices, the effects of which are uncertain but of high concern. To characterize the patterns in fish presence at a tidal energy site in Cobscook Bay, Maine, we examined two years of hydroacoustic data continuously collected at the proposed depth of an MHK turbine with a bottom-mounted, side-looking echosounder. The maximum number of fish counted per hour ranged from hundreds in the early spring to over 1,000 in the fall. Counts varied greatly with tidal and diel cycles in a seasonally changing relationship, likely linked to the seasonally changing fish community of the bay. In the winter and spring, higher hourly counts were generally confined to ebb tides and low slack tides near sunrise and sunset. In summer and fall of each year, the highest fish counts shifted to night and occurred during ebb, low slack, and flood tides. Fish counts were not linked to current speed, and did not decrease as current speed increased, contrary to observations at other tidal power sites. As fish counts may be proportional to the encounter rate of fish with an MHK turbine at the same depth, highly variable counts indicate that the risk to fish is similarly variable. The links between fish presence and environmental cycles at this site will likely be present at other locations with similar environmental forcing, making these observations useful in predicting potential fish interactions at tidal energy sites worldwide.

Tidal energy site characterization for marine turbine optimal installation: Case of the Ouessant Island in France

This paper deals with an energy characterization method of tidal sites. This method is illustrated by the study of a typical tidal site close to the Ouessant (Ushant) Island of the Bretagne region in France. The main objective of this study is to find the optimal location of a tidal turbine including the effect of using or not a yaw drive system.After presenting the different data used in this work, the marine current velocity is first evaluated at different specific locations and times of the studied zone. Afterwords, the tidal energy is evaluated for different systems (tidal turbine with a yaw and bidirectional tidal turbine with fixed axis) and presented in terms of energy distribution maps.

Comparison of ADCP observations and 3D model simulations of turbulence at a tidal energy site

Field measurement of turbulence in strong tidal currents is difficult and expensive, but the tidal energy industry needs to accurately quantify turbulence for adequate resource characterisation and device design. Models that can predict such turbulence could reduce measurement costs. We compare a Regional Ocean Modelling System (ROMS) simulation with acoustic Doppler current profiler (ADCP) measurements from a highly-energetic tidal site. This comparison shows the extent to which turbulence can be quantified by ROMS, using the conventional k−ε turbulence closure model. Both model and observations covered the same time period, encompassing two spring-neap cycles. Turbulent kinetic energy (TKE) density was calculated from measurements using the variance method; turbulent dissipation, ε, was calculated using the structure function method. Measurements show that wave action dominates turbulent fluctuations in the upper half of the water column; comparing results for deeper water, however, shows very strong agreement. A best fit between ROMS and ADCP results for mean velocity yields R2=0.98; for TKE, R2 is 0.84 when strongly wave-dominated times are excluded. Dissipation agrees less well: although time series of ε are well-correlated at similar depths, ROMS estimates a greater magnitude of dissipation than is measured, by a factor of up to 4.8.

Parametric analysis of three dimensional flow models applied to tidal energy sites in Scotland

This paper presents a detailed parametric analysis on various input parameters of two different numerical models, namely Telemac3D and Delft3D, used for the simulation of tidal current flow at potential tidal energy sites in the Pentland Firth in Scotland. The motivation behind this work is to investigate the influence of the input parameters on the above 3D models, as the majority of past research has mainly focused on using the 2D depth-averaged flow models for this region. An extended description of the models setup, along with the utilised parameters is provided. The International Hydrographic Organisation (IHO) tidal gauges and Acoustic Doppler and Current Profiler (ADCP) measurements are used in calibrating model output to ensure the robustness of the models. Extensive parametric study on the impact of varying drag coefficients, roughness formulae and turbulence models has been investigated and reported. The results indicate that both Telemac3D and Delft3D models are able to produce excellent comparison against measured data; however, with Delft3D, the model parameters which provided higher correlation with the measured data, are found to be different from those reported in the previous literature, which could be attributed to the choice of boundary conditions and the mesh size.

Effects of hydrokinetic turbine sound on the behavior of four species of fish within an experimental mesocosm

The development of hydrokinetic energy technologies (e.g., tidal turbines) has raised concern over the potential impacts of underwater sound produced by hydrokinetic turbines on fish species likely to encounter these turbines. To assess the potential for behavioral impacts, we exposed four species of fish to varying intensities of recorded hydrokinetic turbine sound in a semi-natural environment. Although we tested freshwater species (redhorse suckers [Moxostoma spp], freshwater drum [Aplondinotus grunniens], largemouth bass [Micropterus salmoides], and rainbow trout [Oncorhynchus mykiss]), these species are also representative of the hearing physiology and sensitivity of estuarine species that would be affected at tidal energy sites. We evaluated changes in fish position relative to different intensities of turbine sound as well as trends in location over time with linear mixed-effects and generalized additive mixed models. We also evaluated changes in the proportion of near-source detections relative to sound intensity and exposure time with generalized linear mixed models and generalized additive models. Models indicated that redhorse suckers may respond to sustained turbine sound by increasing distance from the sound source. Freshwater drum models suggested a mixed response to turbine sound, and largemouth bass and rainbow trout models did not indicate any likely responses to turbine sound. Findings highlight the importance for future research to utilize accurate localization systems, different species, validated sound transmission distances, and to consider different types of behavioral responses to different turbine designs and to the cumulative sound of arrays of multiple turbines.

Multisensor Acoustic Tracking of Fish and Seabird Behavior Around Tidal Turbine Structures in Scotland

Despite rapid development of marine renewable energy, relatively little is known of the immediate and future impacts on the surrounding ecosystems. Quantifying the behavior and distribution of animals around marine renewable energy devices is crucial for understanding, predicting, and potentially mitigating any threats posed by these installations. The Flow and Benthic Ecology 4D (FLOWBEC) autonomous seabed platform integrated an Imagenex multibeam echosounder and a Simrad EK60 multifrequency echosounder to monitor marine life in a 120 $^{\circ}$ sector over ranges up to 50 m, seven to eight times per second. Established target detection algorithms fail within MRE sites, due to high levels of backscatter generated by the turbulent physical dynamics, limiting and biasing analysis to only periods of low current speed. This study presents novel algorithms to extract diving seabirds, fish, and fish schools from the intense backscatter caused by turbulent dynamics in flows of 4 m s $^{{-1}}$ . Filtering, detection, and tracking using a modified nearest neighbor algorithm provide robust tracking of animal behavior using the multibeam echosounder. Independent multifrequency target detection is demonstrated using the EK60 with optimally calculated thresholds, scale-sensitive filters, morphological exclusion, and frequency-response characteristics. This provides sensitive and reliable detection throughout the entire water column and at all flow speeds. Dive profiles, depth preferences, predator–prey interactions, and fish schooling behavior can be analyzed, in conjunction with the hydrodynamic impacts of marine renewable energy devices. Coregistration of targets between the acoustic instruments increases the information available, providing quantitative measures including frequency response from the EK60, and target morphology and behavioral interactions from the multibeam echosounder. The analyses draw on deployments at a tidal energy site in Scotland to compare the presence and absence of renewable energy structures across a range of physical and trophic levels over complete spring-neap tidal cycles. These results can be used to inform how animals forage in these sites and whether individuals face collision risks. This quantitative information can de-risk the licensing process and, with a greater mechanistic understanding at demonstration scales, its predictive power could reduce the monitoring required at future arrays.

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.

Numerical model simulation of island-headland induced eddies in a site for tidal current energy extraction

As an important marine renewable energy resource, tidal stream energy has increasingly attracted public attention. However, the detailed hydrodynamic properties are often poorly understood at potential tidal stream energy sites, where local topographical features usually induce complicated current structures, such as eddies. In the present study a three-dimensional hydrodynamic model is employed to reproduce the flow field at a promising coastal site for tidal current energy extraction, where there is an island located near a headland. Comparisons between the model predictions and the survey data show good agreement. Two large eddies are found near the island-headland system in each tidal cycle. The eddies form during the flood and ebb tides, and the rotation does not cease until slack water. When the eddies move across the island-headland system, fluctuations are detected both in current speed and direction. The eddies induced velocity fluctuations cause a significant change in the energy density, with its magnitude being about half the maximum, whilst the water elevation change is limited. Therefore, the impact of eddies should be taken into account seriously in practical tidal stream energy explorations.

Tidal stream resource assessment in the Dover Strait (eastern English Channel)

A methodology of tidal flow resource assessment in the Dover Strait is presented. The resource assessment is performed using surface velocity time series recorded by Very High Frequency Radars (VHFR) and ADCP velocity measurements. Following the EMEC guideline, the major parameters of tidal flow conventionally used for tidal energy site screening are estimated and mapped. The combination of two sources of data allowed to characterize the current velocity variation in three spatial dimensions and in time, which increased confidence in hydrokinetic resource assessment from the radar data. Current velocities provided by the radars show strong spatial variation and fortnightly modulation. The most energetic area was found west of the Cape Gris Nez with the peak velocity of 2.5m/s, mean velocity of 1m/s, and spring tide average velocity of 1.4m/s. Velocities exceeding 1m/s are observed more than 50% of time there. Averaged velocity profiles derived from ADCP data were obtained for different stages of the tidal cycle and then approximated by a power law function. Using velocity time series provided by the radars and the power law velocity profiles, the power density time series in the surface and bottom layers were generated. The analysis of these data show that west of the Cape Gris Nez, the mean power density attains its maximum value 0.9kW/m2 in the surface layer and a peak value 5kW/m2. In the rest of the domain, the mean power density varies from 0.1 to 0.6kW/m2. The power density is found three times lower in the bottom layer. A three dimensional hydrodynamic model MARS-3D is used for comparison with experimental data. The model results are in good agreement with observations thus allowing the use of the model for assessing tidal stream resource in extended area.

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.

Comparing nekton distributions at two tidal energy sites suggests potential for generic environmental monitoring

Tidal energy is a renewable resource that can contribute towards meeting growing energy demands, but uncertainties remain about environmental impacts of device installation and operation. Environmental monitoring programs are used to detect and evaluate impacts caused by anthropogenic disturbances and are a mandatory requirement of project operating licenses in the United States. In the United Kingdom, consent conditions require monitoring of any adverse impacts on species of concern. While tidal turbine sites share similar physical characteristics (e.g. strong tidal flows), similarities in their biological characteristics have not been examined. To characterize the generality of biological attributes at tidal energy sites, metrics derived from acoustic backscatter describing temporal and spatial distributions of fish and macrozooplankton at Admiralty Inlet, Washington State and the Fall of Warness, Scotland were compared using t-tests, F-tests, linear regressions, spectral analysis, and extreme value analysis (EVA). EVA was used to characterize metric values that are rare but potentially associated with biological impacts, defined as relevant change as a consequence of human activity. Pelagic nekton densities were similar at both sites, as evidenced by no statistically significant difference in densities, and similar daily density patterns of pelagic nekton between sites. Biological characteristics were similar, suggesting that generic biological monitoring programs could be implemented at these two sites, which would streamline permitting, facilitate site comparison, and enable environmental impact detection associated with tidal energy deployment.