• Energy Research

A system of two coupled four-beam acoustic Doppler current profilers was used to collect turbulence measurements over a 36-h period at a highly energetic tidal energy site in Alderney Race. This system enables the evaluation of the six components of the Reynolds stress tensor throughout a large proportion of the water column. The present study provides mean vertical profiles of the velocity, the turbulence intensity and the integral lengthscale along the streamwise, spanwise and vertical direction of the tidal current. Based on our results and considering a tidal-stream energy convertor (TEC) aligned with the current main direction, the main elements of turbulence prone to affect the structure (material fatigue) and to alter power generation would likely be: (i) the streamwise turbulence intensity ( I x ), (ii) the shear stress, v ′ w ′ ¯ , (iii) the normal stress, u ′ 2 ¯ and (iv) the vertical integral lengthscale ( L z ). The streamwise turbulence intensity, ( I x ), was found to be higher than that estimated at other tidal energy sites across the world for similar height above bottom. Along the vertical direction, the length ( L z ) of the large-scale turbulence eddies was found to be equivalent to the rotor diameter of the TEC Sabella D10. It is considered that the turbulence metrics presented in this paper will be valuable for TECs designers, helping them optimize their designs as well as improve loading prediction through the lifetime of the machines. This article is part of the theme issue ‘New insights on tidal dynamics and tidal energy harvesting in the Alderney Race’.

Vertical axis tidal turbines are devices that extract the kinetic energy from tidal currents. Tidal currents can be highly turbulent. Since ambient turbulence affects the turbine hydrodynamic, it is critical to understand its influence in order to optimize tidal farms. Actuator Line Model (ALM) combined with Large Eddy Simulation (LES) is a promising way to comprehend this phenomenon. In this article, an ALM was implemented into a Lattice Boltzmann Method (LBM) LES solver. This implementation gives good results for predicting the wake of a vertical axis tidal turbine placed into a turbulent boundary layer. The validated numerical configuration was then used to compute the wake of a real size ducted vertical axis tidal turbine. Several upstream turbulence rates were simulated. It was found that the shape of the wake is strongly influenced by the ambient turbulence. The cost-to-precision ratio of ALM-LBM-LES compared to fully resolved LBM-LES makes it a promising way of modeling tidal farms.

A regional 2DH hydrodynamic model is used to estimate the tidal stream resource of a site located in a macrotidal environment with extreme tidal velocities. The study site is the Alderney Race (Raz Blanchard in French) which is a straight located in the English Channel between the Alderney Island and La Hague cape (France). The estimation of the resource is used to build two realistic tidal energy extraction scenarios consisting in placing a 290 MW tidal turbine array in two different areas. Then, we analyze the impact of turbines on the hydrodynamics and the sediment transport. The hydrodynamic perturbation is restricted to the vicinity of the array where the mean current velocity reduction reaches 0.3 m/s locally (corresponding to 15% of the baseline velocity). Focusing on the variable driving the bedload (the critical erosion threshold exceedance), we confirm that tidal energy extraction tends to reduce the bedload rate and to deflect the sediment fluxes. Our simulations with a simple model for the suspended sediment transport suggest that tidal energy extraction has a significant effect on the area of deposition of the particles transiting through the tidal farm. For the baseline, the sediment particles transiting through the tidal farm deposit mostly in the eastern part of the English Channel. Depending on the location of the tidal farm, the sediment mass balance between the eastern and the western part of the English Channel changes drastically.

Abstract The deployment of tidal turbines requires a precise hydrodynamic characterisation of the production site. Acoustic Doppler Current Profilers (ADCP), usually employed for measuring the time-mean characteristics of environmental flows, could also be used for assessing the main features of turbulence. ADCP measurements are sensitive to many sources of uncertainties associated mainly with the spreading of the beams or the assumptions made on flow homogeneity. The ability of ADCPs to accurately measure the hydrodynamic parameters of a given flow can be tested on a synthetic dataset. However, it is difficult to generate a dataset representative of a real environmental flow. In this work, large-eddy simulation of a high Reynolds flow over a rough seabed is performed and used to assess the accuracy of two, coupled, 4-beam ADCP systems forming an 8-beam arrangement. The study confirms the relevance and efficiency of the tested 8-beam configuration for the characterisation of turbulence. The results near the seabed are of a lower quality, with up to 50 % error on the Reynolds stresses for elevations under twice the roughness height, which questions the interpretation of ADCP measurements in the lower part of the water column. Also, the spatial averaging over ADCP cells leads to an underestimation of the turbulence intensity of 10 % to 20 %.

This study seeks to establish a comprehensive model for estimating both the velocity deficit and turbulence intensity within a tidal turbine farm across various layout configurations. The model incorporates a spectrum of ambient turbulence intensity ranging from 5% to 20%, a rotor diameter-to-depth ratio between 20% and 60%, and a rotor thrust coefficient that varies from 0.64 to 0.98. The influence of added turbulence is factored into the evaluation of the velocity deficit within the farm. Consistent with findings from prior research, the results indicate that in a tidal farm consisting of 16 turbines, a staggered array configuration yields 21% more power compared to a rectilinear array. This staggered setup benefits from enhanced flow acceleration and greater spacing between turbines, which facilitates improved wake recovery. The findings suggest that the farm’s dimensions can be optimized by reducing lateral spacing in the rectilinear array and longitudinal spacing in the staggered array without compromising efficiency. Such reductions in farm size can lead to decreased cable expenses and create opportunities 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. The efficiency of a tidal farm can be increased by high ambient turbulent intensity, sufficient turbine spacing, and low rotor diameter-to-depth ratio. These factors improve the wake recovery to allow more energy to be extracted by a downstream turbine. This low-computational model can be useful in studying the wake interaction of tidal turbine parks in different configurations.

Sites suitable for the deployment of tidal turbines generally show a combination of complex seabed morphologies and extreme current magnitudes. Such configurations favour the formation of vortices, which can be very powerful. Anticipating the vortex effect on the turbine performance and/or lifespan requires refined description of the turbulence. Thanks to increased calculation resources, large-eddy simulation (LES) can now be applied to natural flow. An LES approach developed within the TELEMAC-3D open-source software is presented here. After validating the model with in-situ measurements, the model is applied to characterize the flow statistics of the Alderney Race. This article is part of the theme issue ‘New insights on tidal dynamics and tidal energy harvesting in the Alderney Race’.

This study aims to investigate the influence of the current direction on the energy production of a tidal turbines array. It is based on a three-dimensional (3D) numerical simulation of the flow where the turbines are represented with actuator disks. The case study consists of modelling the energy extraction of a small array of turbines (staggered and aligned layouts) placed in the Raz Blanchard (Alderney Race, France). The simulations are performed with hydrodynamic data (current magnitude and direction) representative of a mean tide, with several resistance forces and ambient turbulence intensities. The influence of the current direction on the energy production is highlighted by comparing the simulations forced with the real current direction with those in which the angle of incidence between the incoming flow and the turbine’s axis is “switched off” (bi-directional flow). When the flow is aligned with the turbines’ axis (misalignment “switched off”), the staggered layout produces more than the aligned arrangement. Comparison of the two types of simulations (misalignment switched off or not) shows that the misalignment of the flow around a predominant direction reduces the energy produced by the staggered layout and increases the production of the aligned layout. Furthermore, it suggests that the mean energy produced per machine is almost the same for both layouts. Higher turbulence intensity reduces the positive effect of the directional spreading on the aligned layout production and limits the negative effect on the staggered layout production.