Assessing the impact of tidal stream energy extraction on the Lagrangian circulation

Abstract

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 with 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 0.45 m s −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 1.5 km with possible implications on the evolution of surrounding seabed features.

Tidal Turbines’ Layout in a Stream with Asymmetry and Misalignment

“…This asymmetry is associated with a prominent flood-dominated region in the north of the strait with residual currents up to 0.8 m s −1 and an ebb-dominated area in the south. Such current asymmetry is typical of tidal strait dynamics as confirmed by numerical investigations conducted in the potential stream energy sites of Orkney [8] or the Fromveur Strait [13,20]. Moreover, the divergence area, with reduced current asymmetry in the center of the strait, presents, most of the time, the strongest tidal currents.…”

“…However, except local studies such as simulations conducted by Neill et al [8] in Orkney archipelago, Lewis et al [7] in the Irish sea or Guillou and Chapalain [13] in the Fromveur Strait (western Brittany, France), few numerical modeling studies have investigated the asymmetry and misalignment of tidal currents and its effects on the generated technical resource, at the scale of potential stream energy sites.…”

“…As tidal stream energy sites occupy, in a high proportion, restricted surfaces in the vicinity of headlands and straits characterised by complex coastline geometry and significant water-depths variations [12], numerical modelling at high-spatial resolution, below 100 m, are required to assess tidal stream power variability. However, except local studies such as simulations conducted by Neill et al [8] in Orkney archipelago, Lewis et al [7] in the Irish sea or Guillou and Chapalain [13] in the Fromveur Strait (western Brittany, France), few numerical modelling have investigated the ¼ asymmetry and misalignment of tidal currents, and its effects on the generated technical resource, at ½ the scale of potential stream energy sites.…”

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

“…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 energy extraction.…”

Techno-economic assessment of energy production potential from tidal streams in Nigeria

“…Thus, in this work, we perform resource characterization of Nigeria's tidal energy resources, for selected sites, together with an LCOE analysis of the respective sites. Several studies have been reported on geographical sites in various parts of the world, to the assessment of the potential for tidal current energy utilization [4][5][6][7][8][9]. Tidal current technology continues to progress rapidly particularly in the United Kingdom.…”