Brian Polagye

Measurements of Turbulence at Two Tidal Energy Sites in Puget Sound, WA

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.

Tidal energy resource characterization: methodology and field study in Admiralty Inlet, Puget Sound, WA (USA):

Tidal energy resource characteristics are presented from a multi-year field study in northern Admiralty Inlet, Puget Sound, WA (USA). Measurements were conducted as part of a broader effort to characterize the physical and biological environment at this location ahead of a proposed tidal energy project. The resource is conceptually partitioned into deterministic, meteorological, and turbulent components. Metrics with implications for device performance are used to describe spatial variations in the tidal resource. The performance differences between passive and fixed yaw turbines are evaluated at these locations. Results show operationally significant variations in the tidal resource over length scales less than 100 m, likely driven by large eddies shed from a nearby headland. Finite-record length observations of tidal currents are shown to be acceptable for estimating device performance, but unsuitable for direct investigation of design loads.

In-stream tidal energy potential of Puget Sound, Washington

Abstract The far-field, barotropic effects of in-stream tidal energy extraction from Puget Sound are quantified using a one-dimensional channel model. In-stream turbines are modelled in regions of energetic flow in northern Admiralty Inlet and Tacoma Narrows. The far-field extraction effects include changes to the tide (amplitude and phase), transport, power dissipation, and kinetic power density. These effects are observed throughout Puget Sound and are dependent on the magnitude and location of extraction. The model indicates that a 5 per cent reduction in transport in the South Sound would correspond to either 260 MW of dissipation by in-stream turbines in Admiralty Inlet, 120 MW in Tacoma Narrows, or an intermediate level of dissipation in both locations. The environmental and economic limits on future developments are discussed. For pilot-scale development, this modelling indicates that the barotropic, far-field extraction effects on Puget Sound will be immeasurably small.

Quantifying turbulence for tidal power applications

Using newly collected data from a tidal power site in Puget Sound, WA, metrics for turbulence quantification are assessed and discussed. Of particular interest is the robustness of the “turbulent intensity,” defined as the ratio of velocity standard deviation to velocity mean. Simultaneously, the quality of raw ping Acoustic Doppler Current Profiler (ADCP) data for turbulence studies is evaluated against Acoustic Doppler Velocimeter (ADV) data at a point. Removal of Doppler noise from the raw ping data is shown to be a crucial step in turbulence quantification. Excluding periods of slack tide, the corrected turbulent intensity estimates at a height of 4.6 m above the seabed are 10% and 11% from the ADCP and ADV, respectively. Estimates of the turbulent dissipation rate are more variable, from 10-3 to 10-1 W/m3. An example analysis of coherent Turbulent Kinetic Energy (TKE) is presented.

A vessel noise budget for Admiralty Inlet, Puget Sound, Washington (USA)

One calendar year of Automatic Identification System (AIS) ship-traffic data was paired with hydrophone recordings to assess ambient noise in northern Admiralty Inlet, Puget Sound, WA (USA) and to quantify the contribution of vessel traffic. The study region included inland waters of the Salish Sea within a 20 km radius of the hydrophone deployment site. Spectra and hourly, daily, and monthly ambient noise statistics for unweighted broadband (0.02-30 kHz) and marine mammal, or M-weighted, sound pressure levels showed variability driven largely by vessel traffic. Over the calendar year, 1363 unique AIS transmitting vessels were recorded, with at least one AIS transmitting vessel present in the study area 90% of the time. A vessel noise budget was calculated for all vessels equipped with AIS transponders. Cargo ships were the largest contributor to the vessel noise budget, followed by tugs and passenger vessels. A simple model to predict received levels at the site based on an incoherent summation of noise from different vessels resulted in a cumulative probability density function of broadband sound pressure levels that shows good agreement with 85% of the temporal data.

Limits to the predictability of tidal current energy

The predictability of tidal currents in the context of hydrokinetic power generation are assessed using current data from a series of surveys in Admiralty Inlet, Puget Sound, Washington, USA. Both current speed and kinetic power density are shown to be well-described by harmonic analysis. Three challenges to predictability are identified. First, non-sinusoidal fluctuations over time scales on the order of hours are observed but cannot be replicated by conventional harmonic analysis. Second, turbulent fluctuations over time scales on the order of seconds are relatively large and inherently unpredictable. Third, for this site, predictions may not be extrapolated more than 100 m from the location of measurement. While none of these issues are insurmountable, they contribute to a degree of unpredictability for tidal hydrokinetic power.

Effect of large-scale kinetic power extraction on time-dependent estuaries:

Abstract An open and important question in tidal in-stream energy conversion is the level of kinetic power extraction that is possible without unacceptable environmental degradation. In general, the effects of large-scale kinetic power extraction on estuary-scale fluid mechanics are not well understood. In this paper, these effects are quantified for an idealized estuary using a one-dimensional, time-dependent numerical model. The numerical domain consists of a long, wide inlet and basin connected by a constricted channel. Kinetic power densities within this constriction are suitable for in-stream energy conversion. Modelling shows that the extraction of kinetic power has a number of effects, including: (a) reduction of the volume of water exchanged through the estuary over the tidal cycle; (b) reduction of the tidal range landward of the array; and (c) reduction of the kinetic power density in the tidal channel. These impacts are strongly dependent on the magnitude of kinetic power extraction, estuary geometry, tidal regime, and non-linear turbine dynamics. It is shown that it may be misleading to relate these impacts to the fraction of kinetic energy extracted from the system. Results highlight the importance of time-dependent modelling and the incorporation of non-linear turbine dynamics.

Resource Mapping at Tidal Energy Sites

Station keeping, a vessel-based spatial surveying method for resolving details of the hydrokinetic resource, is presented in the context of the general methodology and also for the specific case of a survey conducted in northern Admiralty Inlet, Puget Sound, WA, in June 2011. The acoustic Doppler current profiler (ADCP) measurements collected during the June 2011 survey were part of a broader effort to characterize the resource at this location before tidal turbine installation. Autonomous bottom-lander (bottom-mounted) ADCP measurements are used to evaluate the accuracy with which data collected from this vessel-based survey reflect stationary measurements and also to analyze the potential for cycle-to-cycle variations in the conclusions drawn. Results indicate good agreement between shipboard and bottom-mounted observations in capturing spatial resource differences. Repeated surveys over several tidal cycles are required to obtain results consistent with long-term observations. Station-keeping surveys help to optimize bottom-mounted ADCP deployments that are then used to estimate turbine power generation potential and make final siting decisions.

Characteristics of underwater ambient noise at a proposed tidal energy site in puget sound

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 1μPa. Total sound pressure levels exceed 100 dB re 1μPa 99% of the time and exceed 135 dB re 1μPa 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. Referencing 368 individual recordings with the distance between the ferry and the site obtained from AIS data, the source level of the ferry is estimated to be 179 ± 4 dB re 1μPa at 1m with a logarithmic spreading loss coefficient of 18.

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

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.