Proceedings of the 11th European Wave and Tidal Energy Conference 6-11th Sept 2015, Nantes, France Mapping tidal energy resources by High Frequency Radar: Application to resource characterization around the Ushant Island (W. Brittany coast) Alexei Sentchev∗, Maxime Thiébaut∗ ∗Laboratoire d’Océanologie et de Géosciences (CNRS UMR 8187), Université du Littoral - Côte d’Opale, 62930 Wimereux, France. E-mail: [email protected]; [email protected] Abstract—A methodology of assessing the hydrokinetic re- powerful tool and allowed a considerable gain of the quality sources and tidal flow potential at a near-shore site in the Iroise of velocity and available power estimation. Nowadays, 3D Sea is presented. More than a year-long surface current velocity models are routinely applied for tidal energy site assessment, time series recorded by high frequency radars (HFR) were used for site screening. We focused on the analysis of tidal current resource quantification, and studies of the impact of energy pattern around the Ushant Island which is a promising site of devices on the local circulation and environment. For example, tidal energy generation. Our results revealed current velocities numerical simulations by Regional Ocean Modeling System of 4 m/s northwest of the island and in the Fromveur Strait, (ROMS) allowed examining the tidal asymmetry in a promis- % % with 1 m/s value exceeded 60 and 70 of time respectively. ing site of Orkney Islands [3], or evaluating the wave influence The monitoring of surface currents allowed to discover an exceptionally high asymmetry between the flood and ebb flow on tidal stream energy resources [4]. varying in a wide range, from 0.5 to 2.5, around the Ushant Underway velocity measurements by towed or vessel Island. The strongest variation of asymmetry was found in the mounted ADCP is another efficient tool for tidal flow char- Fromveur Strait. Spatial distribution of asymmetry coefficient acterization and energy resource quantification. In the recent showed persistent pattern and fine scale structures. Asymmetry studies [5], [6], velocity profiles were recorded while the in current direction was also quantified. Analysis of the bottom mounted ADCP data in the radar coverage zone allowed the vessel steamed around a circuit with sufficient frequency quantification of the variation of current velocity with depth, allowing to resolve the vertical structure of the tidal current which increased confidence in the HFR results. The power density and its spatial irregularities. Gooch et al. [7] used spatial inter- of tidal flow was estimated in two particular locations where polation of underway ADCP data to reconstruct a general tidal the strongest currents are observed and the highest potential is flow pattern neglecting the tidal phase difference during the expected. A method is presented to find the optimal location for energy conversion devices and to optimize the power generation surveying period. Goddijn-Murphy et al. [5] showed that the by moving devices in space and to a different altitude. It was accuracy in reconstruction of the full 4-dimensional tidal flow demonstrated that in the region of opposing flood- versus ebb- can be significantly increased by merging observed velocities dominated asymmetry occurring over a limited spatial scale (e.g. with the dynamical constraints provided by numerical models. Fromveur passage), it is possible to provide a balanced power [5], [6]) generation. Index Terms—Tidal flow - Hydrokinetic energy - Resource In the absence of spatial coverage with observations, point characterization - HF radar - Iroise Sea measurements of velocity by Acoustic Doppler Current Pro- filer (ADCP) and Acoustic Doppler Velocimeter (ADV) rep- resent a unique opportunity of resource quantification from in I. INTRODUCTION situ data [8]. At the same time, the data are used to validate Resource characterization is the first step for project devel- numerical models which provide spatial extension of the study opment activity. In order to optimize the energy conversion, it area [7]. is required to properly characterize the environment conditions In our study, we present a new and very promising technique at the sites and thus facilitate the process of selecting tidal of site assessment based on the analysis of velocity data power devices. The European Marine Energy Centre (EMEC) recorded by High Frequency Radar (HFR). We applied this has proposed a number of metrics, which helps to evaluate technology for site screening and resource evaluation in a the potential of tidal flow in the most efficient way. This is region of high potential in the Iroise Sea. As the HF radars usually done by using different approaches. measure only the surface current velocity, the knowledge of In early studies, tidal current velocity data from Admiralty velocity profile is an important issue for site assessment. In Charts have been used to estimate tidal current energy re- this work, we completed the surface velocity data set with sources [1]. However these estimates suffered from lack of velocity time series recorded by ADCP in radar coverage zone. precision. The use of old data survey, not originally intended We used a power law to best fit the ADCP data and, on this for resource assessment, also generated big prediction errors basis, we characterized the horizontal velocity variation with [2]. Numerical modeling of coastal circulation appeared as a depth. A combination of remotely sensed surface velocity and ISSN 2309-1983 Copyright © European Wave and Tidal Energy Conference 2015 09A5-4-1 ADCP velocity profiles allowed performing the analysis of tidal current variability in three dimensions and thus assessing energy resources in the most efficient way. In recent years, ocean radar systems, installed on the seashore in many countries, have had stunning success, and their ability to map surface circulation has allowed significant advances in our understanding of circulation and oceano- graphic conditions in many coastal ocean regions. Today, HF radar networks form the backbone of many ocean observing systems, and the data are assimilated into ocean circulation models (e.g., see [9] for review of HF radar applications). However, this technology is completely underexploited in tidal energy projects. Our study represents the first attempt of using HFR for resource assessing at a very promising tidal energy site in the Iroise Sea. We showed the skill and ability of the system for resource characterization, mapping, and searching the best locations for in-stream tidal devices. Our work aims to popularize its application at sites of high potential, such as Orkney or Bristol Channel, where similar radar systems have Fig. 1. Study area in the Iroise Sea with HF radar coverage zone (grey been already installed. shading), radar sites (grey circles), ADCP location (red triangles). Contour interval of the bathymetry is 50 m (grey solid lines). Geographic names used II. DATA AND METHODS in the text are also shown. A. HFR data A system of two high-frequency Wellen Radars (WERA) HFR was estimated using empirical expression derived by operating at 12.4 MHz is deployed on the western Brittany Sentchev et al. [12] from numerical studies of wind-current coast since July 2006. Individual radar sites are located at relationship [13], and then subtracted from radar velocity Cape Garchine (site G), a seashore flat-ground area, and Cape records. The requested wave quantities were provided by the Brezellec (site B), 50 km southward (Fig. 1). The radars numerical model WAVEWATCH III. Analysis of the wave measure the current velocity in the surface layer (0.7 m thick). induced current component during selected periods in 2007 The recorded radial velocities are the projections of the current showed that the highest magnitude of Stokes current did not velocity vector onto radar beam directions. In the present exceed 0.18 m/s and that a peak of occurrence was close 0.05 study, the radar network was configured to provide velocity m/s whereas it was of the order of 0.40 m/s for radar derived estimates at high spatial resolution: 1.5 km along beam and 2o velocities. A detailed description of the experimental settings, azimuthal spacing, and time resolution of 20 minutes. The ra- the methods of data processing of the radar network in the dial velocities measured by the two radars were interpolated on Iroise Sea and the sea state removing can be found in Sentchev regular grid of 1 km spacing and combined to provide current et al. [12]. vector maps. A powerful variational interpolation technique 2dVar [10] was used, providing continuous velocity data on a B. ADCP data regular grid within the area shown in Fig. 1 by grey shading. The knowledge of velocity profiles is extremely important The size and shape of the area was chosen so as to ensure a for resource assessing at marine energy sites. We comple- high degree of coverage by the data (velocities were recorded mented HFR derived surface velocities by ADCP measure- more than 60% of time) and also high accuracy of the retrieved ments in two particular locations within the radar coverage velocity values. The accuracy of the radar-derived velocities zone: in the Fromveur Strait and in the offshore sector – has been estimated by SHOM (Oceanographic Division of the on the southern periphery of the study site (Fig. 1). In both French Navy) through a comparison with surface drifters and locations, the velocity profiles were recorded by bottom- ADCP current measurements for a period of 7 months. In the mounted broadband RDI ADCPs. majority of situations, the discrepancy in velocity measured by The offshore ADCP was deployed at 120 m depth from different instruments did not exceed 0.15 m/s [11]. It was also June 6 to September 13, 2007. Velocities were recorded every noticed that a large fraction of this discrepancy might not be 5 m starting from 8 m above the bottom and averaged within due to instrumental errors, but to the definition of the “surface 5 min.