Offshore Industry News

Calming the Waters: The Impact of Turbulence on Tidal Energy Systems

New research is helping the emerging tidal energy industry learn from their counterparts in the wind industry. By considering the effects of atmospheric turbulence when developing turbine designs, the wind industry has lowered the cost of energy to record lows and deployed turbines broadly across much of the nation.

Likewise, as the tidal energy industry continues to advance, developing a thorough understanding of turbulence is a critical step to designing durable devices and accelerating the pace of technology development. While the wind industry uses meteorological towers to measure turbulence, it would be expensive and unrealistic to place and maintain similar equipment in the ocean. Researchers funded by the U.S. Department of Energy (DOE) at the National Renewable Energy Laboratory (NREL) and the University of Washington (UW) are addressing this problem by developing a system to measure turbulence using instrumentation secured to mooring lines.

Beginning in October 2013, NREL closely collaborated with the Applied Physics Lab at UW to develop a mooring system that measures characteristics of the ocean turbulence expected to cause wear and tear on turbine components, which reduces lifetimes for devices in the water. The mooring suspends a collection of velocity sensors, known as velocimeters, along a rope between a buoy in the water and an anchor on the sea floor. These sensors measure the velocity at tidal energy sites across a wide-range of scales: from millimeter-diameter turbulent eddies, to the tidal ebb and flood that occupies the entire channel. These measurements help tidal energy device designers understand to what degree turbulence will reduce the lifetime of a tidal turbine. The two-year project, which concluded in September 2015, successfully developed a low-cost methodology for measuring turbulence at tidal energy sites.

Success was not easy; several iterations were needed to improve the design of the mooring and the accuracy of the measurement system. The project partners first deployed the Tidal Turbulence Mooring system in a channel in Puget Sound—the well-known network of waterways adjacent to Seattle, Washington—positioning turbulence sensors in the middle of the water-column at depths where tidal energy devices would be deployed. The sensors were mounted on a simple, weather-vane-like fin that held the sensor array upstream from the mooring. However, the scientists found that with this design, the sensors fluttered and swayed in the channel flow, distorting their measurements.

To correct the problem, NREL and UW researchers worked with Nortek AS, a scientific instrumentation company that makes turbulence sensors, to solve this problem. Nortek’s engineers integrated a motion sensor—similar to the sensor in smartphones that detects shaking gestures and screen orientation—into their high-precision velocity and turbulence instrument, known as an acoustic Doppler velocimeter.

Meanwhile, NREL developed open-source signal processing software that removed the mooring motion from the turbulence signals. As the project progressed, the team refined the hardware, signal processing techniques, and mooring configuration to improve the accuracy of the measurement system in the channel flow.

Now well calibrated, the measurements made using the Tidal Turbulence Mooring system provide information on inflow conditions that will help the industry design more efficient turbines, improving device lifetimes at lower cost. Current and future tidal projects can benefit from this research because it allows developers to understand and mitigate turbulence, which is critical to building long-lasting tidal energy systems, and helps them avoid the high costs of alternative measurement approaches.

Furthering this effort, NREL is also working with DOE, industry leaders, and international partners to identify tidal energy projects where turbulence monitoring is needed. The methodology used in this research has been published as an NREL technical report, accessible through the Water Power OpenEI page. The report was presented at the Ocean Sciences Meeting in New Orleans, Louisiana, in February 2016.

By: Gregory Wagner, Communications Specialist, U.S. Dept. of Energy, Wind and Water Power Technologies Office

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