The impact of seabed rock roughness on tidal stream power extraction
Introduction
The exploitation of tidal stream power is currently considered as a promising solution for rising the proportion of marine renewable energy to the worldwide energy production [1], [2]. Major well-known advantages are the highly predictable and regular characteristics of the resource, the substantial degree of modularity in extracting the energy [3], the high load fluid factor and the reduced visual impact of tidal stream devices in comparison with wind or waves systems. Tidal kinetic power has thus attracted significant interest from leading developers of tidal energy converters (TEC) with numerous full-scale devices tested in the real environment [4]. Considering the increasing technical development of devices, the question of the hydro-environmental impact of tidal stream farms is of major interest to guarantee successful deployment in the marine environment. Indeed, besides a local modification of available power distribution, potential far-field effects are expected with modifications of hydrodynamics components and associated transport of particles at several kilometres from the proposed array [5], [6], [7].
As observations in the real environment with tidal stream farms are not available today, investigations of far-field impacts rely nowadays on regional numerical modelling. Hydrodynamic effects of tidal stream devices, which generally take the form of horizontal axis turbines [8], are most of the time approached with an equivalent drag force term redistributing the sum of turbines' thrust and structural drag forces over the area covered by a proposed array [9], [10], [11], [12]. Whereas uncertainties exist about parameterisations of drag coefficients, upstream velocities [13], [14] or wake-wake interactions within the array [15], these formulations provide a global assessment of tidal stream power extraction particularly suited for the early stages of a tidal farm project while restricting the computational costs of simulations. Numerical sensitivity studies to the array-drag coefficient have thus helped potential developers to optimize projects of stream farms in terms of size, thrust and structural drag coefficients and number of turbines [7], [16], [17].
Nevertheless, in spite of local sensitivity studies to array-drag coefficients, no additional investigation has been devoted to the influence of surrounding seabed friction on TEC-induced modifications. Indeed, numerical sensitivity studies devoted to the hydrodynamic impact of seabed roughness have primarily focused on the regional influence of bottom friction on tides exhibiting, in continental shelf environments, its effects on currents amplitude and asymmetry, and transport of particles [18], [19], [20]. These aspects have however been set aside in numerical assessments of available tidal stream power and TEC effects assuming uniform bottom friction coefficients, based (1) on Manning-Strickler [7], [10] or Chezy [21] formulations in depth-averaged models or (2) on uniform quadratic friction law in three-dimensional approaches [22], [23], [24]. Further investigations about the influence of bottom friction, integrating the spatial heterogeneity of sediment bottom types, are thus necessary as (1) induced modifications of tidal stream array depend on initial estimates of current fields and (2) more significant effects are expected on predicted energy outputs and sediment transport rates which vary with a power of tidal current amplitude.
The present study investigates the numerical sensitivity of tidal stream power extraction to the parameterisation of bottom roughness. Bottom friction depends however on different settings and forcings including bed-sediment composition and morphology of the sea bottom [25], and influence of hydrodynamic processes such as interactions of wave and current bottom boundary layers [26]. Taking into account the difficulty to separate each contribution, the influence of time-variable bottom roughness is disregarded here focusing on effects of intrinsic seabed properties. The attention is devoted to sensitivity of numerical predictions to roughness parameter of rock outcrops, a typical seabed of tidal stream sites whose roughness is characterised by a relative uncertainty in relation to the shape of seabed features [25]. This sensitivity study will furthermore give further insights about the calibration of seabed roughness in numerical assessments of TEC effects at the early stages of a farm project.
After a description of the study site (Section 2.1), the emphasis is put on the theoretical formulation and implementation of hydrodynamic model (Section 2.2). Predictions are first assessed against available in-situ observations of current amplitude and direction (Section 3.1) exhibiting the local numerical sensitivity to roughness parameterisation of rock outcrops (Section 3.2). Synoptic effects on tidal current amplitude and associated kinetic energy are then evaluated in spring tidal conditions. Modifications induced by the presence of a tidal stream farm are finally investigated focusing on key parameters of maximum currents amplitude and available stream power 10 m above the seabed (the assumed technology hub height for the region considered), and bottom shear stress (Sections 3.3 Effects of tidal stream power extraction, 3.4 Sensitivity of tidal stream energy extraction to rock bottom roughness).
Section snippets
Study region
The site of application is the Fromveur Strait, off western Brittany, separating the isle of Ushant from a group of islets and rock belonging to the Molène archipelago (Fig. 1). With annual peak velocities exceeding 4 ms−1 [27] and mean water depths of 50 m, this location is one of the largest tidal stream resource along the coasts of France. A restricted area of interest of 4 km2 has thus been identified by the French government for the development of tidal farm projects. The exploitation of
Evaluation of model predictions
The evaluation of model predictions is performed against available in-situ observations of current amplitude and direction at points 1010 and 1011 (Fig. 1 and Table 1). These observations have been acquired by a 600 kHz RDI ADCP (Acoustic Doppler Current Profiler) deployed by the French Navy SHOM (“Service Hydrographique et Océanographique de la Marine”). Measurements period corresponds to different neap-spring tidal conditions: (1) from 19 March to 2 April 1993 at point 1010 and (2) from 14
Conclusions
The depth-averaged tidal circulation model TELEMAC 2D has been implemented in a region off western Brittany, that has been identified with strong potential for tidal array development, in order to investigate and evaluate the impact of bottom roughness parameterisation of rock outcrops on effects of tidal kinetic energy extraction. Numerical predictions are assessed against available in-situ observations of tidal currents amplitude and direction at two locations in the Fromveur Strait and
Acknowledgements
The authors are grateful to André Simon (Cerema) for the process of sedimentological data. Current measurements were supplied by the Service Hydrographique et Océanographique de la Marine (SHOM). Simulations were performed on computer facilities CAPARMOR (“CAlcul PARallèle Mutualisé pour l’Océanographie et la Recherche”). The present paper is a contribution to the research programs DIADEMS and FLUSED of the Laboratory of Coastal Engineering and Environment (Cerema, http://www.cerema.fr, //memphys-lgce.fr.ht
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