Tidal energy resource complexity in a large strait: The Karori Rip, Cook Strait

Successful extraction of tidal stream energy will require a good understanding of flow at a range of scales, including those relating to average energy, variability in energy supply and fatigue. Current and turbulence measurements from the Karori Rip area of Cook Strait, the prime focal region of open-water tidal stream electricity generation in New Zealand, are described. A key issue is that a significant portion of the energy is contained in waters deeper than normally considered for energy extraction. Here we compare shallow and deep sites. Velocity data were derived from acoustic Doppler current profiler moorings, as well as spatial surveys and show flow magnitudes reaching 3.4 m s(-1) in the shallow regions. The maximum speeds in both shallow and deep sites were typically located in the upper part of the measured water column although moored acoustic Doppler current profiler (ADCP) observations showed some complexity in this regard. Benthic boundary-layers were resolved in the bottom similar to 20 m of the water column. Measured turbulent kinetic energy dissipation rates epsilon exceeded 10(-5) m(2) s(-3) and estimated maximum epsilon is a factor of 10 greater. This was not distributed evenly through the water column, with stratification and velocity shear clearly persisting, especially around the turn of the tide. The implications for tidal stream energy are that (i) there is sufficient energy resource in the region for a moderate sized array of turbines, (ii) the vertical variability in the flow speed suggests turbines that can operate near the surface would be more effective at accessing the resource, (iii) stratification may persist and influence the scales of turbulence and (iv) wave-current interaction effects will influence any near-surface structure as well as vessel operations. (C) 2011 Elsevier Ltd. All rights reserved.