Researchers

Brian Polagye

Adjunct Investigator

AIRS Department

APL-UW

Research Assistant Professor, Mechanical Engineering

Jim Thomson

Principal Oceanographer

AIRS Department

APL-UW

Associate Professor, Civil and Environmental Engineering

Tidal Power from the Seafloor

The surging water below is generated by the predictable and dependable ebb and flow of the tide. This begs the question: Can this natural energy be harnessed?

Plans call for two 33-foot tidal power turbines to generate electricity. But before the turbines are lowered into Admiralty Inlet, APL-UW researchers are compiling an intensive profile of the proposed turbine site.

We're trying to determine the current water quality in Admiralty Inlet so that when a tidal power device is installed we can have a baseline to compare whether there was any effect — changing the mixing of the water coming in through Admiralty Inlet.

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Recent Publications

Measurements of turbulence at two tidal energy sites in Puget Sound, WA

Thomson, J., B. Polagye, V. Durgesh, M.C. Richmond, "Measurements of turbulence at two tidal energy sites in Puget Sound, WA," IEEE J. Ocean. Eng., 37, 363-374, doi:10.1109/JOE.2012.2191656, 2012.

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15 May 2012

Field measurements of turbulence are presented from two sites in Puget Sound, WA, that are proposed for electrical power generation using tidal current turbines. Time series data from multiple acoustic Doppler instruments are analyzed to obtain statistical measures of fluctuations in both the magnitude and direction of the tidal currents. The resulting turbulence intensities (i.e., the turbulent velocity fluctuations normalized by the deterministic tidal currents) are typically 10% at the hub heights (i.e., the relevant depth) of the proposed turbines. Length and time scales of the turbulence are also analyzed. Large-scale, anisotropic eddies dominate the turbulent kinetic energy (TKE) spectra, which may be the result of proximity to headlands at each site. At small scales, an isotropic turbulent cascade is observed and used to estimate the dissipation rate of TKE, which is shown to balance with shear production. Data quality and sampling parameters are discussed, with an emphasis on the removal of Doppler noise from turbulence statistics. The results are relevant to estimating the performance and fatigue of tidal turbines.

Underwater noise measurements of a 1/7th scale wave energy converter

Bassett, C., J. Thomson, B. Polagye, and K. Rhinefrank, "Underwater noise measurements of a 1/7th scale wave energy converter," In Proceedings, MTS/IEEE OCEANS 2011, Waikoloa, 19-22 September, doi:110.1109/OCEANS.2010.5664380 (MTS/IEEE, 2011).

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22 Sep 2011

Field measurements of the underwater acoustic signature of Columbia Power Technologies (Columbia Power) SeaRay wave energy converter (WEC) prototype are presented. The device was deployed in the vicinity of West Point (Puget Sound, Washington State) at a depth of approximately 20 meters. The 1/7th scale SeaRay prototype is a heave and surge, point absorber secured to the seabed with a three-point mooring. Acoustic measurements were made in order to satisfy permit requirements and assure that marine life is not adversely affected. A series of one-minute hydrophone recordings were collected on March 30, 2011 for approximately 4 hours. During these recordings, significant wave height varied from 0.4 to 0.7 m, peak wave periods varied from 2.9 to 3.2 seconds, and southerly winds varied from 5 to 10 m s-1. These are approximately twice the amplitude of typical operating conditions for the SeaRay in Puget Sound. Shipping vessel and ferry traffic levels also were typical. Received sound pressure levels during the experiment vary from 116 to 132 dB re 1 µPa in the integrated bands from 20 Hz to 20 kHz. At times, ship traffic dominates the signal, as determined from spectral characteristics and vessel proximity. Received sound pressure levels attributed to the WEC cycle from 116 to 126 dB re 1 µPa in the integrated bands from 60 Hz to 20 kHz at distances from 10 to 1500 m from the SeaRay. The cycling is well correlated with the peak wave period, including peaks and harmonics in the pressure spectral densities. Masking by ship noise prevents rigorous extrapolation to estimate the WEC source level at the conventional 1 m reference.

Characterizing underwater noise at a proposed tidal energy site in Puget Sound

Bassett, C., J. Thomson, and B. Polagye, "Characterizing underwater noise at a proposed tidal energy site in Puget Sound," In Proceedings, MTS/IEEE Oceans 2010, 20-23 September, doi:10.1109/OCEANS.2010.5664380 (MTS/IEEE, 2010).

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20 Sep 2010

Ambient underwater acoustics data are presented for one year at a potential tidal energy site in Admiralty Inlet, WA (USA) with maximum currents exceeding 3 m/s. The site, at a depth of approximately 60 meters, is located near shipping lanes, a local ferry route, and a transit area for many cetacean species. A key finding is that the statistical distribution of total sound pressure levels are dependent on tidal currents at the site. Pseudosound, cobbles shifting on the sea bed, and vibrations induced by forces on the equipment are possible explanations. Non-propagating turbulent pressure fluctuations, termed pseudosound, can mask ambient noise, especially in highly energetic environments suitable for tidal energy development.

A statistical method identifies periods during which changes in the mean and standard deviation of the one-third octave band sound pressure levels are statistically significant and thus suggestive of pseudosound contamination. For each deployment, recordings with depth averaged tidal currents greater than 1 m/s are found to be contaminated, and only recordings with currents below this threshold are used in the subsequent ambient noise analysis. Mean total sound pressure levels (0.156 - 30 kHz) over all recordings are 117 dB re 1 micoPa. Total sound pressure levels exceed 100 dB re 1 microPa 99% of the time and exceed 135 dB re 1 microPa 4% of the time. Commercial shipping and ferry traffic are found to be the most significant contributors to ambient noise levels at the site, with secondary contributions from rain, wind, and marine mammal vocalizations. Post-processed data from an AIS (Automatic Identification System) receiver is used to determine the location of ships during each recording.

Shipboard acoustic Doppler current profiler surveys to assess tidal current resources

Epler, J. B. Polagye, and J. Thomson, "Shipboard acoustic Doppler current profiler surveys to assess tidal current resources," In Proceedings, MTS/IEEE Oceans 2010, Seattle, 20-23 September, doi:10.1109/OCEANS.2010.5664387 (MTS/IEEE, 2010).

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20 Sep 2010

A compelling aspect of power generation from tidal currents is the predictability of the resource, which is generated by the gravitational pull of the sun and moon on the earth's oceans. For technical feasibility studies, it is presupposed that once the currents at a site have been well characterized it is possible to make accurate predictions of the electricity that would be generated by an array of turbines. These data are generally collected by Acoustic Doppler Current Profilers (ADCP), which use active acoustics to measure currents throughout the water column.

Stationary ADCP deployments in Admiralty Inlet, WA, indicate operationally important variability over length scales less than 100 m and use of shipboard ADCP surveys aims to identify regions of peak currents without deploying a high resolution grid of stationary ADCPs. Shipboard surveys involve multiple laps during a tidal cycle around a short "racetrack". Data are aggregated within bins such that multiple laps produce time series at 100 m spatial resolution along the track. The time series is then fitted with a half sine wave assuming that each tidal cycle can be represented as such due to the periodic nature of tidal currents. The amplitude and timing of the peak currents along the survey track are estimated from the fit. Multiple ebb current surveys indicate that relative amplitude trends are consistent between cycles of differing strength and time of the year. Therefore, by overlapping surveys, greater spatial coverage can be achieved from multiple cycles without changing the survey characteristics.

Results obtained to date suggest a noise floor of approximately plus/minus 15 minutes for phase. This method could be applied to other tidal energy sites as a lower cost alternative to siting studies using arrays of stationary ADCPs.

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