Effects of the Current Direction on the Energy Production of a Tidal Farm: The Case of Raz Blanchard (France)

Abstract

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.The hydrodynamics of tidal energy sites is intricate and often shows an asymmetry in tidal current direction. As an example, the current of the Raz Blanchard, which is located between the Alderney Island and La Hague Cape (Normandy, France), is quasi bidirectional in the Southern part of the Race but is characterized by a high directional spreading in the Northern part. At some locations, the current incidence with respect to the predominant direction reaches up to 30 • [4]. Thus, the multi-directionality of the flow should be considered when optimizing the turbines' arrangement.Numerous numerical modelling studies investigated the effect of the current asymmetry (amplitude and direction) on the production of tidal farms. However, previous studies were essentially performed with regional models where the turbines' thrust was spatially averaged over the area occupied by the tidal farm. This (spatially averaged) turbine representation implies that the interactions between the turbines' wakes are negated [5][6][7][8][9][10][11]. Such regional approach has been used at sites located in West Brittany such as the Sein Race [7] and the Fromveur strait [8], at Scottish sites such as the Orkney archipelago [9], at the Ramsey Sound strait [10], and at the Normand site of the Raz Blanchard [1].Recent works [11][12][13][14] deal with the implementation of methods developed (based on numerical results or experimental mapping) to optimize the identification of suitable sites for ene...

Numerical modelling of hydrodynamics and tidal energy extraction in the Alderney Race: a review

“…With a semi-empirical wake model and a particle swarm optimisation model, Lo Brutto et al [77] adjusted the density and positions of turbines in order to maximise the profit of a potential array in the AR. Using a more reliable approach to simulate the flow within the array, Nguyen et al [82] and Thiébot et al [20] compared different turbine arrangements chosen a priori. Their wake-field studies, relying on a 3D solver in which the turbines are represented as Actuator Disks, confirmed the advantage of staggering the turbines, in comparison to an aligned layout.…”

“…The interactions between horizontal-axis turbines deployed in array was investigated by Nguyen et al [82] and Thiébot et al [20] with 3D RANS models in which the turbines were represented by Actuator Disk theory, hereafter AD. This technique applies a momentum sink across the swept area of the turbine rotors that is equivalent to the turbine thrust.…”

“…However, those processes may dissipate rapidly behind the turbine and numerous AD models validated against experimental data on scaled turbines have demonstrated that AD can give reliable estimates of the flow in the mid-to far-wakes (at distance from the turbines greater than five turbine diameters) provided that the unresolved processes are properly simulated by the turbulence model [28,84]. Although the turbine representation (based on the AD concept) is comparable between [82] and [20], those two wake-field studies differed as shown in Table 3, which summarises the main characteristics of these investigations. Nguyen et al [82] showed that the staggered layout was more efficient than the aligned layout when the current was rectilinear and that, when there was variation in the current direction, there was less benefit in using a staggered layout (in comparison to the aligned layout).…”

“…Although the turbine representation (based on the AD concept) is comparable between [82] and [20], those two wake-field studies differed as shown in Table 3, which summarises the main characteristics of these investigations. Nguyen et al [82] showed that the staggered layout was more efficient than the aligned layout when the current was rectilinear and that, when there was variation in the current direction, there was less benefit in using a staggered layout (in comparison to the aligned layout). In line with this finding, Thiébot et al [20] also demonstrated that the staggered layout gave a greater AEP than the aligned layout, especially when the lateral distance exceeded 5D.…”

“…Reference [82] [20] 1 spring tide. Realistic tidal conditions (3D model forced validated with ADCP data).…”