The influence of waves on the tidal kinetic energy resource at a tidal stream energy site

Abstract

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 windgenerated 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 10 m 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 10 m 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 northwestern area and the southeastern 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.

Tidal Turbines’ Layout in a Stream with Asymmetry and Misalignment

“…The horizontal eddy viscosity is parametrized following Smagorinsky [28] while the vertical viscosity is computed with the mixing length model proposed by Quetin [29]. Whereas Brittany may be subjected to strong stormy waves [30][31][32] liable to interact with tidal currents in stream energy sites [21,33], the influence of wind-generated surface-gravity waves is disregarded in the present investigation assuming that the tide is the dominant hydrodynamic forcing. As thermal fronts develop at the west of the isle of Sein and in the entrance of bays with tidally-mixed waters in the "Raz de Sein" [34,35], the influence of density gradients on currents is also disregarded in the present study.…”

“…Furthermore, significant tidal recirculations with a diameter of around 2 km appear during flood and ebb peak periods, on both sides of the flow emerging from the strait. As local assessments of tidal stream power around France have primarily focused on the two prominent sites of the Alderney Race ("Raz Blanchard") in the western English Channel [18,19] and the Fromveur Strait [20,21] in northwestern Brittany, a reduced number of investigations was conducted in the "Raz de Sein". However, the values of the tidal velocities (>2.5 m s −1 ) and the water depths (>25 m) match the basic conditions required for the implementation of first-generation turbine technologies, covering the great part of existing prototype devices in pre-commercial demonstration stages [22].…”

“…The period between 17 June and 29 June 1993, characterized by spring tidal conditions, is retained for the assessment of numerical results as fair weather conditions prevail with significant wave heights below 1.5 m and wind speed velocities restricted to 8 m s −1 at the measurement point. This comparison is performed 10 m above the bed as this corresponds to the operating height of most horizontal-axis turbines in western Brittany [21].…”

“…The flow is considered to be turbulent over a rough bottom. Following Guillou and Thiébot [20,26] and Guillou et al [21], the associated roughness parameter z 0 is determined by matching sediment bottom types from the map established by Hamdi et al [16] with roughness observations compiled by Soulsby [27].…”

“…The 3D model setup is mainly based on the implementation performed by Guillou et al [21] for the assessment of tidal stream power in the Fromveur Strait. However, the main features of the model's configuration are briefly described hereafter as modifications have been made to databases considered in this simulation.…”