Proceedings of the 2nd International Conference on Environmental Interactions of Marine Renewable Energy Technologies (EIMR2014), 28 April – 02 May 2014, Stornoway, Isle of Lewis, Outer Hebrides, Scotland. www.eimr.org -1- EIMR2014-713 THE ROLE OF TIDAL ASYMMETRY IN CHARACTERIZING THE TIDAL ENERGY RESOURCE OF ORKNEY Simon P. Neill 1 School of Ocean Sciences Bangor University Menai Bridge, UK M. Reza Hashemi School of Ocean Sciences Bangor University Menai Bridge, UK Matthew J. Lewis School of Ocean Sciences Bangor University Menai Bridge, UK ABSTRACT When selecting sites for marine renewable energy projects, there are a wide range of economical and practical constraints to be considered, from the magnitude of the resource through to proximity of grid connections. One factor that is not routinely considered in tidal energy site selection, yet which has an important role in quantifying the resource, is tidal asymmetry, i.e. variations between the flood and ebb phases of the tidal cycle. Here, we present theory and develop a high-resolution three-dimensional ROMS tidal model of Orkney to examine net power output for a range of sites along an energetic channel with varying degrees of tidal asymmetry. Since power output is related to velocity cubed, even small asymmetries in velocity lead to substantial asymmetries in power output. We also use the 3D model to assess how tidal asymmetry changes with height above the bed, i.e. representing different device hub heights, how asymmetry affects turbulence properties, and how asymmetry is influenced by wind-driven currents. Finally, although there is minimal potential for tidal phasing over our study site, we demonstrate that regions of opposing flood- versus ebb-dominant asymmetry occurring over short spatial scales can be aggregated to provide balanced power generation over the tidal cycle. INTRODUCTION From a resource and device perspective, it is clearly beneficial to select tidal energy sites where the tidal currents have an equal magnitude between the flood and ebb phases of the tide (tidal symmetry), and less desirable to exploit sites which have either strong flood- or ebb-dominance (tidal asymmetry). Tidal asymmetry not only affects the primary variables of the flow field such as velocity and water elevation – it is also expected to cause asymmetry in turbulence properties such as Reynolds stresses and turbulent kinetic energy, important variables in site selection [1]. Tidal waves are progressively distorted and dampened as they propagate in shallow-water coastal regions [2]. Although tidal waves in such regions still satisfy the criteria of long waves (i.e. wavelength is much greater than water depth), in shallow water the amplitudes of the waves become a significant fraction of the total water depth [3]. As a result of these non-linear shallow-water processes, tidal waves in such regions are often more complex than their linear wave counterparts, with the occurrence of double high or low water, and asymmetries observed in velocity time series due to the presence of overtides. Focussing on the principal semi-diurnal lunar constituent (M 2 ) and its first overtide (M 4 ), we can estimate tidal asymmetry from the phase relationship [4]     METHODS We apply the ROMS model to simulate the 3D barotropic currents of the northeast region of Orkney at high resolution (   ~ 75 m), extending from  to , and from  to , covering the Westray Firth and Stronsay Firth, which connect via the Fall of Warness (the EMEC tidal test site) (Fig. 1). The model was run with 10 vertical (sigma) levels, used the Generic Length Scale (GLS) turbulence scheme, with the coefficients tuned to represent the  model, and we used a drag coefficient    . Since this is primarily a study of tidal asymmetry, and is not intended as a detailed resource study, we considered only the principal semi-diurnal lunar (M 2 ) and solar (S 2 ) constituents. We ran the model for a period of 2 weeks, and validated the M 2 and S 2 components of the vertical tide against data from 6 tide gauges. To validate the horizontal tide, we used ADCP data from the EMEC tidal test site at the Fall of Warness. RESULTS We restrict our analysis only to sites where water depth is in the range 25-50 m, and where the peak spring (M 2 and S 2 ) currents exceed 2 m/s, i.e. locations which are suitable for the majority of first generation tidal stream devices (Fig. 2). These sites are primarily located in Westray Firth and Stronsay Firth. The total area where these depth and velocity criteria are satisfied within the model domain is around 70 km 2 – a substantial region for tidal energy arrays. We selected 21 sites evenly distributed along a 30 km longitudinal transect through Westray Firth and Stronsay Firth, representing a large variability in tidal asymmetry with which to examine its influence on the tidal energy resource. 1 Corresponding author: s.p.neill@bangor.ac.uk