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Stereo-Optical Imaging For Monitoring Hydrokinetic Turbines
Characterizing Environmental Impacts of Marine Renewable Energy


James Joslin

Brian Polagye

The energy in fast-moving tidal currents is a potential source of renewable, predictable electricity. Tidal turbines harness tidal currents in a manner analogous to wind turbines, and single-turbine demonstration projects have been successfully deployed with rated capacities exceeding 1 megawatt. However, before large-scale tidal energy projects can proceed, the approach must be proven to be not just technically feasible, but economically viable, environmentally compatible and socially acceptable.

Potential environmental impacts include those resulting from direct interactions between energy converters and marine animals such as invertebrates, fish, marine mammals and diving seabirds. Possible interactions include collision/strike with the moving rotor, attraction due to artificial reef effects and avoidance due to pressure fluctuations or sound. Resource agencies have expressed particular interest in understanding the type and frequency of close-range interactions between tidal turbines and marine animals, and, to date, there have been several attempts to collect this information using active acoustics (e.g., echosounders). These have provided valuable information about the behavior of fish in the vicinity of turbines, but have been less successful in identifying species or characterizing the nature of interactions between fish and the turbine rotor.

Public Utility District No. 1 of Snohomish County plans to deploy two Open-Centre turbines, manufactured by OpenHydro (Dublin, Ireland), in northern Admiralty Inlet, Puget Sound, Washington. The turbines are shrouded, horizontal-axis variants 6 meters in diameter and would operate for a five-year period as a demonstration project to evaluate environmental changes and turbine reliability. If the demonstration project is successful, Admiralty Inlet has significant potential for large-scale tidal energy generation. The water depth in the project area is approximately 55 meters and the turbine hub height is 10 meters above the seabed. During strong tidal exchanges, currents exceed 3 meters per second. Environmental studies for this project will include characterizing direct interactions between marine animals and the turbine rotors. The imaging system described here has been developed for those studies.

Imaging System
Optical monitoring of the near-field environment (e.g., up to a 10-meter range) of tidal turbines is intended to provide information capable of classifying marine animals to the lowest taxonomic level possible (ideally, species). The relative velocities between the camera and marine animals is up to several meters per second during strong currents. There is minimal ambient light at turbine hub depth and low turbidity (less than 1 nephelometric turbidity unit), but biological flocculent can limit the effective range of lighted video. The imaging system developed to meet the monitoring objectives is a shore-connected hybrid optical-acoustical system, incorporating stereographic optical cameras and a high-resolution, multibeam sonar (acoustical camera).

The principal components of the imaging system are a pair of Allied Vision Technologies GmbH (Stadtroda, Germany) Manta G-201 2-megapixel optical machine vision cameras, four strobe illuminators, a Teledyne BlueView (Bothell, Washington) P2250-900 acoustical camera, and the power/communications architecture to integrate them and communicate with shore via a fiber-optic link. Calibrated stereo cameras can provide information about the absolute position, size and speed of targets, which is crucial for classifying targets and characterizing their interaction with turbines. The strobe illumination provides crisp, synchronized images despite the high target velocities and low ambient light levels. While the use of full-spectrum, artificial light may cause behavioral changes in marine animals, the acoustic camera can be used to characterize these effects and potentially trigger the optical cameras when targets move into the system's field of view.

Functional Range in Field
One of the key uncertainties of integrating the imaging system with a tidal turbine is the functional range for detection (i.e., distinguishing a target from background), discrimination (i.e., distinguishing between marine animals and debris) and classification (i.e., taxonomic grouping) of marine animals by the stereographic cameras. This functional range establishes where the imaging system should be deployed relative to the turbine rotor. Another question is the relative effectiveness of the acoustical and optical camera systems. The main variables that could affect imaging system effectiveness are the target range, relative velocity of the target, attenuation of artificial lighting by flocculent, the cameras' digital gain and behavioral effects of the artificial lighting. To continue this article please click here.

James Joslin is a Ph.D. student working with the Northwest National Marine Renewable Energy Center at the University of Washington in Seattle. His research focuses on the development of instrumentation to increase monitoring capabilities of tidal and wave energy converters. He completed his bachelor's and master's degrees at Dartmouth College and was introduced to marine engineering while working with Sea Education Association out of Woods Hole, Massachusetts.

Dr. Brian Polagye is the co-director of the Northwest National Marine Renewable Energy Center and a research assistant professor in the Department of Mechanical Engineering at the University of Washington. His research focuses on marine renewable energy conversion. Polagye holds a bachelor's degree from Princeton University and a doctoral degree from the University of Washington.

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