PNNL

Abstract

The Aleutian Islands of Alaska are home to many island passes with fast (~ 4 m s-1) flowing tidal currents, making the region favorable for renewable tidal stream energy extraction. To date, a comprehensive spatial and temporally varying evaluation of the undisturbed tidal stream energy resource in the archipelago has been lacking. Here, we provide that assessment via a high resolution, barotropic tidal numerical model of the region run for a full year. We validate the model extensively with tidal water levels at 7 tide gauges and currents at 17 acoustic doppler current profiler (ADCP) deployments. Eight island passes are identified as tidal stream energy “hotspots”. The annual mean kinetic energy fluxes, KEF, calculated at each pass vary from 1,000 to 11,000 MW, while the annual available energy, AAE, varies from 5 to 42 MWh m-2. Notable seasonal modulation to monthly power density averages and ranges are noted at some passes and not others. Seasonal adjustment is linked to the semi-annual solar declination cycle which enhances (dampens) diurnal (D1) tidal amplitudes in summer/winter (spring/fall) as well as the time-varying phase lag between D1 and semidiurnal (D2) fortnightly tidal cycles. We quantitatively show, via normalized power density parameters, that annual variability in monthly mean power density scales with the tidal current form factor, F_u, with the largest seasonal change occurring for F_u>1 (D1 dominated tide). The spread in power density over a month is on average smaller for passes with mixed tides (F_u=1) than for those with D1 or D2 dominance, as changes to fortnightly phase lag become influential to net power density ranges when tides are mixed (corroborated via Pearson correlations). This study outlines overlooked, but relevant, long-term modulation to tidal streams which should be considered for all tidal energy assessments in regions with mixed tides.