Sandia researchers are investigating the seasonal effects of a wave-energy converter (WEC) array on nearshore wave propagation using SNL-SWAN. WECs were simulated as obstacles with frequency-dependent transmission coefficients that were determined based on the incoming wave height and period. The research team created frequency distributions of seasonal wave parameters (significant wave height, peak wave period, and wave direction) from hourly time series of National Oceanic and Atmospheric Administration National Data Buoy Center (NOAA NDBC) wave data collected between 1987 and 2013 in Monterey Bay, California. Median and extreme seasonal wave conditions were determined from frequency distributions and used as inputs to SNL-SWAN. The model was run with and without an array of 50 WECS consisting of floating two-body heaving converters (F-2HB) or floating oscillating water column (F-OWC) device types centered on the 40 m depth contour.
Initial model results indicate that wave-height reductions were largest directly in the lee of the array and ranged from less than 1% to nearly 10%, depending on initial wave conditions (see the figure). The F-2HB device type resulted in slightly larger decreases in wave height compared to the F-OWC buoy due to the closer spacing of individual WECs in the array; F-OWC effects were more dispersed.
Wave-height reductions were largest when initial peak wave periods were between 8 and 12 seconds, which is the wave-period range where power extraction is maximized for both WEC types regardless of initial wave height. These optimal initial wave periods were generally observed during winter for southerly wave directions and during summer and spring for west-northwesterly initial wave directions. Wave periods were largely > 13.5 seconds during extreme wave conditions; therefore decreases in wave height were more pronounced for median wave conditions.
The results from wave seasonality studies such as this will be used to guide the selection of conditions with which to run full-ocean circulation models that consider waves, currents, and winds. The ocean circulation model, coupled with sediment transport simulations, will indicate potential coastal geomorphological variability due to the presence of WEC arrays.