Tidal Farm Research Articles

A Numerical Methodology to Predict the Maximum Power Output of Tidal Stream Arrays

Due to its high level of consistency and predictability, tidal stream energy is a feasible and promising type of renewable energy for future development and investment. Numerical modeling of tidal farms is a challenging task. Many studies have shown the applicability of the Blade Element Momentum (BEM) method for modeling the interaction of turbines in tidal arrays. Apart from its well-known capabilities, there is a scarcity of research using BEM to model tidal stream energy farms. Therefore, the main aim of this numerical study is to simulate a full-scale array in a real geographical position. A fundamental linear relationship to estimate the power capture of full-scale turbines using available kinetic energy flux is being explored. For this purpose, a real site for developing a tidal farm on the southern coasts of Iran is selected. Then, a numerical methodology is validated and calibrated for the established farm by analyzing an array of turbines. A linear equation is proposed to calculate the tidal power of marine hydrokinetic turbines. The results indicate that the difference between the predicted value and the actual power does not exceed 6%.

Goal-Oriented Metric-Based Mesh Adaptive Tidal Farm Modelling

Goal-Oriented Metric-Based Mesh Adaptive Tidal Farm Modelling

3D Numerical Study of the Impact of Macro-Roughnesses on a Tidal Turbine, on Its Performance and Hydrodynamic Wake

Biofouling is an important factor to consider when calculating the energetic efficiency of tidal farms. Despite the fact that biofouling effects have been widely investigated in the past for naval applications, very few studies concern tidal turbines. This paper proposes a numerical approach to assess the impact of biofouling on tidal turbines, which is efficient for testing many configurations. Two turbulence models are tested (RANS k-ω SST and LES Smagorinsky) for the motionless blade case to validate them. Then we chose to use the Smagorinsky model for the case of a complete tidal turbine rotor with realistically fouled blades. The pressure coefficient is strongly affected by the barnacle in the motionless blade case and the power coefficient is slightly degraded in the complete rotor case. Motionless blade cases do not represent the real biofouling behaviour for two reasons. First, sessile species settle in the down flow part of the chord where their impact is less important. Then, the surrounding turbulence provoked by the blades rotation in the rotor case reduces the impact of biofouling. In the wake, biofouling generates small vortexes that propagate into the larger ones, causing them to spread their energy.

The influence of tidal turbine arrangement on the wake interaction in shallow water

The arrangement of tidal turbines in the tidal farm is known to be complicated due to the resistance to the tidal flow which causes the flow to be channeled around the individual devices. To successfully implement the tidal turbine, the wake interaction between the device and its implication needs to be fully understood. Typically, the wake interaction in the array depends on the arrangement and spacing between the device in the array. In this study, a numerical analysis is conducted through the use of Computational Fluid Dynamic (CFD) approach to investigate the influence of the array setup in shallow water application and to propose a suitable array layout for possible application in Malaysia. The numerical analysis is carried out with 2 combination sets of lateral and longitudinal spacing covering 15 turbines in staggered and squared array layout. Hypothetical ‘actuator’ disk and ‘actuator’ cylinder model are used to represent the Horizontal axis turbine (HATT) and Vertical axis turbine (VATT) respectively. The results showed that the VATT model has faster wake recovery and obeys the definition of the far wake. Staggered arrays with bigger spacing are preferable for application in shallow water due to the low probability of wake merging between the rows.

Tidal current resource assessment and study of turbine extraction effects in Banks Strait, Australia

Banks Strait, Australia has been identified as highly promising for tidal turbine deployments. To assess this site's feasibility, high-resolution unstructured mesh two-dimensional (2D) models were generated using the newly developed COMPAS hydrodynamic model. Model results were calibrated and validated against results from five Acoustic Doppler Current Profiler field datasets. Simulation results indicating that considerable tidal energy resources exist, with maximum tidal currents of almost 3 m/s found at depths suitable for both small and large-scale Tidal Energy Converters (TEC) array installations, making this region promising for tidal energy extraction for both on and off-grid energy demands. Two examples of tidal farms consisting of a 1.2 MW array of 3 m diameter TEC's in shallow water and a 60 MW array of twin-rotor 20 m diameter TEC's in deep water were then simulated. The study used two methods to determine farm power output using the 2D models: the first method estimated the output directly from current velocity predictions based on the developed resource model, whilst the second method simulated the TEC energy extraction using a bottom friction model. For both TEC arrays minimal changes in near and far-field mean current flow and surface elevations were found.

Study of a Bi-Vertical Axis Turbines Farm Using the Actuator Cylinder Method

Vertical axis turbines, which extract kinetic energy from currents, can produce electricity independently from a current’s direction. Hence, this type of turbines raises interest for harvesting energy from tidal currents, where flow changes direction during flood and ebb tides, and where currents present large variation of direction during tide. Methods for representing vertical axis turbines in tidal farms should be implemented in order to predict correctly power production with an acceptable time cost. The Actuator Cylinder (AC) is one of them. Numerical results in terms of wakes, with the study of velocity profiles, and efforts are compared to experiences, as well as showed that the method is sufficiently accurate and for a reasonable computing time, which is of prime importance for tidal turbines farms studies. The Actuator Cylinder method is implemented in ANSYS Fluent in a 2D stationary resolution. The method is then applied to a double levels of two counter-rotating rotors marine turbine designed by Hydroquest. Wake and power production of a single turbine and several farm configurations are studied under different current conditions (magnitude and direction).

Study on characteristics of turbine wake and the effect of array arrangement

A prerequisite in undertaking large-scale developments and utilisation of tidal current energy is to comprehend the hydrodynamic characteristics of the wake field of tidal current turbines. The wake of an upstream turbine can strongly affect the power output of downstream turbines and their arrangement. The structure of turbine wakes was investigated. One experiment was conducted on a two-bladed horizontal turbine in a laboratory flume. Actuator disc momentum theory, combined with Reynolds-averaged Navier–Stokes equations, was used to parameterise the tidal turbine. The numerical model was validated using experimental data. The degree of wake mixing improved when the axial distance was eight times the turbine diameter (8D) or larger. The influence of the arrangement and spacing of turbines was investigated at two tidal farms, one with three aligned turbines and one with seven staggered turbines. The incoming flow velocity of the downstream turbine was reduced by 31% when turbines were linearly aligned. The extent of influence of the third column was larger than that of the second column, indicating an expanding wake. Different lateral spaces of staggered turbines were analysed. Different distances between rows strongly influenced both the incoming velocity of downstream turbines and the expansion of the whole wake. The incoming flow of the third column reduced by 31, 17 and 16% with increasing lateral space. The flow velocity increased as water passed over the turbine tip. For staggered turbines, a centreline spacing of up to 1.5D for adjacent rows is recommended.

An effective framework for wake predictions of tidal-current turbines

Deployment of tidal array farms is the next stage of tidal energy development which concentrates on the capture of the enormous energy. Characterization into the wake of tidal turbine aids in determining the tidal farm layout and energy yields. This paper develops a framework that employs the Computational Fluid Dynamics (CFD) simulation and machine learning to analyze turbine wake with high accuracy and good efficiency. Multilayer perceptron neural network (MLP-NN) was introduced to establish the interrelation between the incoming flow conditions and the wake profiles. The Reynolds-averaged Navier-Stokes equations (RANS) are associated with a k − ε turbulence model to offer a number of datasets of wake profile for training, testing, and validation of the MLP-NN models. It was found that the MLP–NN–based model has achieved a considerably high level of accuracy by comparing it with empirical and numerical models. The reliability of the MLP-NN based model coupled with the wake combination RSS model to predict the resultant wake profile and power output of multiple turbines are also assessed. The techniques significantly enhance the efficiency and accuracy of wake predictions.

Modelling Morphological Changes and Migration of Large Sand Waves in a Very Energetic Tidal Environment: Banks Strait, Australia

Banks Strait, Tasmania, Australia, has been identified as a potential site for the deployment of tidal turbines. In this study, the characterization of sediment transport and large sand waves for this site is performed. Observations of bed level change collected from surveys in 2018 showed a migration of large sand waves over a period of nine months. Migration rates in an excess of one hundred meters for nine months were found, which are large compared to the rate reported at other coastal sites, by several meters per year. A validated hydrodynamic model is coupled with a morphodynamic model to perform sensitivity tests and identify what parameters influence migration to better understand sediment dynamic in the Banks Strait. Numerical analysis showed a constant shift of the sand waves profile in an eastward direction, consistent with the observations. This migration was strongly linked with tidal asymmetry, with a residual current flowing towards the east. The principal parameters driving the migration of sand waves in the Banks Strait were found to be sediment sorting, bed friction and residual current. This study gives new insights for the seabed of Banks Strait and provides an assessment of the natural variability of sediment for futures tidal farms deployments.

Assessment of tidal current energy in Bohai Sea and Yellow Sea, China

In china, Bohai Sea and Yellow Sea are major shallow water areas. There are many regions with high current velocities which are potential sites for tidal energy development. In this paper, a 3D numerical model, verified by the measured data, was built to quantify the extractable current energy along the coast of the Bohai Sea and Yellow Sea. By using tidal harmonic analysis method, 4 major constituents’ harmonic constants and ellipse parameters, including M2, S2, K1, O1, were calculated. The tidal currents’ characteristics in Bohai Sea and Yellow Sea were analyzed and their maximum probable velocity could be obtained. The FLUX method was applied to calculate the kinetic energy flux through the front cross-sectional area of the flow channel. Based on numerical simulation results, the tidal current energy in three regions, including Shandong Peninsula, Bohai Strait and Liaodong Bay, was evaluated. The results showed that the maximum tidal current energy density, 6.2 kW/m2 and 5.5 kW/m2, might occur at mid-ebb in the east of Shandong Peninsula and in the northeast of Bohai Strait, respectively. In addition, Tidal Stream Exploitability (TSE) method was used to find optimal areas of tidal farm in Chengshan Cape. The detailed results about power density distribution in Bohai Sea and Yellow Sea would be given by further analysis.

Tidal Energy Potential in the Center Zone of the Colombian Pacific Coast

Introduction: Tidal energy is evidencing interest worldwide because it can be accurately estimated due to the cyclical behavior of the tides. This energy can be extracted from differences in sea level height through the use of gates and turbines of a reservoir (tidal power plant), or extracted from tidal currents through a grid of reversible horizontal axis microturbines as proposed in this study. Thus, the Colombian Pacific region has the opportunity to implement microturbines in strategic areas with the challenge of optimizing them to extract energy with local tidal heights up to 4 m and current velocities up to 1.5 m/s.
 Objective: Calculate the energy potentials from tidal currents in the central zone of the Colombian Pacific coast obtained through hydrodynamic modeling (Delft3D) validated it with in situ information.
 Methodology: The present study starts from level 1 of technological development (Technology Readiness Levels, TRL) till reaching the TRL 2. Then, the research performed a documentary review of national and international experiences related to tidal energy, evidencing the great opportunity that Colombia has to take advantage of this type of energy and join to the growing international interest in the development of clean and renewable energies. In addition, this study characterized the tidal heights and did calculations of energy potentials from tidal currents in the Colombian Pacific derived from hydrodynamic modeling (Delft3D) validated with measured in situ data. The results of this study encourage the application of these technologies in Colombia and represents a contribution to the fulfillment of the UN sustainable development objectives (Affordable and non-polluting energy, SDG 7).
 Results: From the 4 analysis points, the point B (Buenaventura) registered the highest accumulated potential for electricity generation per month (31,546.56 Wh / month). The maximum tidal heights at points A, B, C and D during 2018 were 1.88 m. Furthermore, the mean velocity range at points A, B, C and D was 0.28 m/s to 0.54 m/s, and the maximum velocity for these points were 0.54 m/s, 0.49 m/s, 0.31 m/s and 0.28 m/s respectively. Then, by means of 5 tidal microturbines with a reversible horizontal axis of 1 m sweep area for the point of highest potential (B), it is possible to generate electrical energy to meet the electrical consumption of a house, and with 544 microturbines the consumption of 99 houses. The tidal farm (544 microturbines) would require an area of 2079.36 m2 (e.g. 45.6 m x 45.6 m) and an average depth of 5 m; these requirements are feasible to be met because of the study area characteristics.
 Conclusions: This research determined the tidal energy potential in the central zone of the Colombian Pacific for 4 points of interest, (A, B, C and D) and the highest potentials were found in points A and B located in Bahía Málaga and Buenaventura respectively. In addition, the minimum velocities, the duration of these and the diurnal tidal regime (12 h) were analyzed, and it was found that points A and B showed the lowest times (1 h and 2 h) of minimum velocity, evidencing a capacity of daily electricity generation of 22 h and 20 h respectively. In consideration of the obtained results, this research shows the opportunity to perform studies for promoting the development of reversible horizontal-axis microturbines for the exploitation of tidal energy. The generated energy could be supplied to communities with access difficulties to the electricity distribution network, what would reduce poverty and the emission of Greenhouse Gases (GHG) due to the burning of fossil and plant-based fuels (firewood).
 
 Conclusions: It was determined that the tidal energy potential in the central zone of the Colombian Pacific for 4 points of interest, (A, B, C and D) and the highest potentials were found in points A and B located in Bahía Málaga and Buenaventura respectively. In consideration of the results obtained, this research shows the opportunity performe studies for promoting the development of microturbines for electricity generation from tidal energy. The generated energy would supply to communities with difficulties in accessing the networks of electricity distribution, thus reducing poverty and the emission of Greenhouse Gases (GHG) due to the burning of fossil fuels and of vegetable origin (firewood).

Variability of sediment processes around a tidal farm in a theoretical channel

Sediment transport plays a crucial role in coastal ecosystems and is one of the least known parameters in high energy sites. The recent development of tidal energy projects challenges the scientific community to better understand this natural phenomenon and the interactions with tidal turbines. Using MIKE21/3 software from DHI, a benchmark for sediment transport model was developed with simulations with and without a tidal farm in idealised two and three-dimensional tidal channels. Results reveal that a 2D approach is sufficient for regional scale morphological assessments, however 3D models allow for a closer examination of influences around the tidal farm. Differences in calculating sediment transport rates based on approaches after Engelund & Fredsøe and Van Rijn formulae illustrate the degree of uncertainty in modelling sediment transport rates. Results for the Engelund & Fredsøe models also show that they are sensitive to mesh resolution in equilibrium conditions and are more stable in non-equilibrium conditions for the bed level change rates. Tests have also demonstrated that models using Engelund & Fredsøe formulae are more sensitive to physical characteristics than models based on Van Rijn, accentuating the necessity to evaluate sediment transport formulae with field data before making a choice of model.

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).

Investigations on the hydrodynamic interference of the multi-rotor vertical axis tidal current turbine

In the development of tidal energy, in order to extract more energy in the limited tidal farm, large-scale deployment of tidal current turbine is needed. In this paper, based on the PimpleDyMFoam solver in Open Source Fluid Dynamics Software (OpenFOAM), an effective numerical method is proposed for the two-dimensional VATCT (Vertical Axis Tidal Current Turbine). Secondly, the symmetrical simplified model is introduced to simulate the multi-rotor VATCT array. The interaction and wake characteristics of each turbine in different array layout schemes are studied in order to enhance the understanding of the interaction between multiple turbines. It is well known that the interaction between multiple turbines improves the overall power conversion efficiency, so it is of great significance to conduct in-depth research. The CFD numerical simulation results of the multi-rotor turbine array are compared in three aspects: blade force, wake flow field and power conversion efficiency. The results show that the power conversion efficiency of multi-rotor turbine array is higher than that of the stand-alone turbine. The smaller the spacing of each turbine in the multi-rotor turbine, the larger the overall output power. The single-row multi-rotor array in the form of opposite-rotation is more advantageous for energy acquisition. The analysis of the multi-rotor array from the aspect of the wake flow field can provide some insights into the reasons for the increased power conversion in the array. In this paper, force characteristics, wake characteristics and the energy efficiency of multi-rotor VATCT under different array layouts are considered comprehensively. The array optimization scheme proposed from the point of view of hydrodynamics will provide technical insights for the industrialization of large-scale VATCT array farm.

Optimal configuration of a tidal current turbine farm in a shallow channel

This study investigates the macroscopic features of tidal farm layouts optimized to maximize power production in an idealized shallow channel with steady unidirectional flow. By varying the number of turbines and optimization constraints, numerical experiments were conducted using OpenTidalFarm, an open-source solver for tidal farm optimization using PDE-constraint gradient-based optimization. To alleviate the computational complexity for identifying a global optimum, a concept of quasi-global optimum was introduced, a local optimum that serves as a global optimum in a crude sense. From extensive numerical results, notable patterns in the shape of the quasi-global optimal layout were observed and explained by a nondimensional parameter, E, the ratio of the shortest hypothetical linear fence to the length of the lateral farm site constraint. The quasi-global optimal layout had a linear fence shape when E ≤ 1. The layout evolved into a downstream-concave parabola and subsequently into a V-shape as E increased beyond 1. Moreover, as more turbines were added to an array, the quasi-global optimal layout was no longer a single fence, and some turbines were separated from the main body. From the quantitative perspective, it is shown that the power production could be increased by up to 50% by tuning the optimization constraints.

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.

Towards a Tidal Farm in Banks Strait, Tasmania: Influence of Tidal Array on Hydrodynamics

The development of tidal energy in Australia is still a challenge with few studies performed on the characterisation of the resource, due to the difficulty to acquire data and uncertainties about the influence of this anthropogenic activity on the marine environment. Changes in flow could lead to alterations in sediment transport and have further influence on the marine habitat. A case study in a promising area, Banks Strait (Australia), was created using high resolution 2D and 3D models validated against in situ data to investigate changes to hydrodynamic conditions with two scenarios of tidal farms (100 and 300 turbines). Comparison between 2D and 3D is performed to find the best compromise between model accuracy and computational time for preliminary assessment. Changes to current speed and bed shear stress over a 35 day period were found to be localised around the tidal farms and did not extent more than 7 km from the farm (300 turbines) for both 2D and 3D. The results showed that for near field and far field, 2D models are sufficient to give a first approximation of the hydrodynamic influence of tidal farm deployment on its environment.

Modelling the wake of a tidal turbine with upstream turbulence

Tidal turbines are entering an industrial phase and farms will soon be installed. In order to optimize the power output of tidal farms, a good understanding of the interactions between the ambient turbulence and a single turbine is crucial. Computational Fluid Dynamics, and more precisely Large Eddy Simulation, is one way of acquiring such knowledge. This study proposed a comparison between a Lattice Boltzmann Method LES approach and a Navier-Stokes LES approach to model the wake of a tidal turbine. Numerical results are compared with experimental results and a relatively good concordance is observed. Differences inherent to the approaches are then pointed out.

Wake characterization of an undulating membrane tidal energy converter

Most of the research done on tidal energy focuses on marine current turbines. Therefore, tidal turbine’s wake is well-documented. The tidal energy converter studied here is based on the fluid-structure interactions that occur between a flexible membrane and an axial flow, resulting in an undulating motion that can be used to harvest energy. This device’s performance had been studied but it is the first time its wake is experimentally characterized from two-dimensional Particle Image Velocimetry (PIV) measurements. PIV is synchronized with a motion tracking system that gives information on trajectory and power conversion. Wake measurement gives access to velocity deficit, turbulence intensity and vorticity. Three configurations are tested in order to identify the influence of the main adjustment parameters. Pre-stress increases the membrane vertical speed, leading to a more important vertical expansion of the wake. The power extraction slows down the membrane’s motion, thus limiting the wake’s length and intensity. The flume tank measurements suggest that the best location for a downstream device in a tidal farm would be in the same horizontal position, at 5 membrane’s length. In the open sea, the membrane interaction effects should not predominant.

Preliminary assessment of a tidal test site on the Minho estuary

Abstract A characterization of tidal parameters at estuaries and the potential of these to be used for tidal energy extraction is performed. The tidal parameters analysed are crossed with estuaries around the world to obtain a selection of potentially feasible locations. A case study is addressed regarding the preliminary evaluation of location and overall characteristics of a tidal test centre and tidal farm in the Minho River estuary at the border between Spain and Portugal. Technical and environmental requirements are analysed to evaluate the potential of the location. Infrastructure characteristics, electrical connections and other equipment specification are determined considering the potential capacity available in the estuary; alternative configurations are studied and compared. Based on the study, the maximum capacity of the tidal technology centre is taken into account to produce an initial estimate of costs to deploy a device. The feasibility study shows that the economic evaluation of the investment is favourable, and the development of this tidal test site is feasible and economically viable. Even, a private organism could develop this project without any type of direct public investment.