Harnessing the power of ocean tides has long been imagined, but countries are only now putting it into practice. A demonstration project planned for Puget Sound, for example, will be the first tidal energy project on the west coast of the United States, and the first array of large-scale turbines to feed power from ocean tides into an electrical grid.
With the commercial roll-out of tidal turbines now being more than just a possibility, researchers around the world are investigation how to design and monitor tidal turbines to ensure they have a minimal impact on the local environment. At the University of Washington, Professor of Mechanical Engineering, Brian Polagye, and his colleagues are involved in environmental monitoring before and during a planned deployment of two 30-foot-wide turbines in Admiralty Inlet, the main entrance to Washington state's Puget Sound.
Polagye will present recent findings of this work in an invited talk at the American Geophysical Union's annual meeting in San Francisco this week. "There really isn't that much information, anywhere, about the environmental effects of tidal turbines," Polagye explains. "The results of this pilot project will help decide if this is an industry that has potential for going forward at the commercial scale, or if it stops at the pilot stage".
Although European countries have more experience with tidal energy devices, they are not as far ahead on environmental monitoring, Polagye says. He believes the Pacific Northwest installation will have the most comprehensive environmental monitoring of any tidal project so far.
The Snohomish County Public Utility District, just north of Seattle, received a $10 million grant from the Energy Department for the tidal project now in the final phase of obtaining permits. The turbines would generate an average of 100 kilowatts of electricity, enough to power 50-100 Washington homes during the pilot phase.
"We want to monitor the effects of this particular project, but also understand the processes so we can apply the findings to other potential tidal energy sites," Polagye says. To do this, the University of Washington team must assess a new technology that operates in a little-explored environment.
"There's surprisingly little known about the oceanography of these very fast waters," reveals collaborator Jim Thomson, a University of Washington assistant professor of civil and environmental engineering and an oceanographer in the University of Washington’s Applied Physics Laboratory. "These kinds of tidal channels where water is going very fast only happen in a few areas, and have not been well studied. The currents are so fast that it's hard to operate vehicles and maintain equipment. And it's too deep for conventional scuba diving."
Noise a concern
The pilot site lies roughly 200 feet below the surface of Admiralty Inlet, where the UW team has measured currents of up to 8 knots, or 9 miles per hour. One area of concern is how underwater noise generated by the turbines could affect marine mammals that use auditory cues to navigate and communicate with each other. Strong currents complicated the task of measuring how sound travels in the channel.
"When currents were more than about 2 knots the instruments are hearing considerable self-noise," Polagye comments. "It's similar to when you're bicycling downhill and the air rushes past your ears."
Chris Bassett, a University of Washington doctoral student in mechanical engineering, is testing approaches that would allow underwater microphones to work in fast-moving water. University of Washington researchers used sound from a Washington state ferry to learn how turbine noise would spread from the project site. The data suggest that Admiralty Inlet tends to lessen sound. This reduces the effect on animals' hearing, which is good, but it also means less noise for marine mammals to detect turbines and avoid them.
The University of Washington team has been measuring currents continuously at the proposed site for almost two years, using a monitoring tripod the size of a small refrigerator. With added ballast for stability, the device weighs 850 pounds in water. Even so, it can barely stay put on the ocean floor. The monitoring tripod holds instruments to track water quality, ambient noise, currents, temperature and salinity, and to record marine mammal calls and electronic tags on passing fish. This observational data will help determine precisely where to put the tidal turbines, and establish potential environmental effects once they are in the water.
So far, researchers say, the data support the notion that the Admiralty Inlet is well suited for a tidal energy installation from an engineering perspective. Once the turbines are in the water, likely in 2013, researchers will monitor environmental effects.
The Admiralty Inlet characterization is being conducted by the Northwest National Marine Renewable Energy Center, in which the University of Washington leads research on tidal energy. Polagye and Thomson lead research on characterizing the physical attributes, such as currents and sound propagation. University of Washington fisheries scientists recently received funding to test instruments for monitoring fish at the site, University of Washington mechanical engineers are using computer models to see how pressure changes caused by tidal turbines could affect sediments and fish, and University of Washington oceanographers are calculating when turbines would begin to affect the Sound's tides and currents.
The Washington state deployment is among three US tidal energy pilot projects now in the works (the others are in Maine and Alaska). An array of smaller turbines was operated during a pilot project in New York City's East River.
For additional information:
Northwest National Marine Renewable Energy Center
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