Tidal Stream Resource Potential
Wind and waves are not the only alternative energy sources the ocean possesses. Tidal streams are high velocity sea currents created by periodic movement of the ocean caused by the gravitational pull of the moon. High and low tides exist even far up tidal creeks, just like in the ocean. Tidal stream energy is created from the kinetic energy of the moving flow of water. This energy is one of the alternative energy sources that is renewable and clean. Because of their persistent and predictable nature, tidal streams have become one of the more promising energy sources. So why should an ocean energy planner pay attention to the tidal stream resource-potential data sets?
- Tidal energy is a viable option. As stated above, tidal stream energy is renewable, clean, continuous, and predictable, making for the ideal energy situation. Tidal energy is one of the fastest-growing technologies in the renewable energy sector. Up until now, there has been no national database of tidal resource potential, a key piece of information needed to plan for tidal energy. With the creation of this data set, planners can begin delving into the full potential of tidal energy. Current tidal energy sites in the U.S. include Cook Inlet (Alaska), East River (New York) and Eastport (Maine).
- Tidal siting involves many factors. Tidal stream energy is dependent on factors such as current velocity and flow rate—the direction, speed, and volume of water. Other factors, such as bathymetry, geology of the seabed, and environmental impacts, will determine the deployment method needed and the cost of installation. Planners need to pay attention to all these factors when deciding on a tidal energy facility site, and this data set puts that information together in an easy-to-use format. Most tidal stream power converters require a minimum flow speed of 0.5 to 1.0 meters per second with an ideal speed between 1.5 and 3.5 meters per second. The factors taken into consideration include the water depth at low tide, minimizing erosion, clearance for sea traffic, and the optimal turbine hub height to maximize the efficiency of capturing tidal currents.
- Tidal stream mean power density depends on the mean current. Within the MarineCadastre.gov National Viewer and Data Registry, there are two data sets concerning tidal stream resource potential: mean current and mean power density. The mean current data set represents the average speed for the year of the depth-averaged current at that location. To create the mean current data set, experts used a numerical model to simulate the tidal flows along the coast of the United States. After validating this model, experts converted the mean current data into mean power density. The mean power density data set depicts the amount of tidal stream power per cross-section perpendicular to the flow direction at that point. Depending on site conditions and environmental constraints, the power harvested could be more or less. This study is based only on the existing tidal currents under undisturbed flow conditions.
Quick Caveats. It should be noted that tidal currents and associated power per unit area can have significant spatial variability. Therefore, currents at one location are generally a poor indicator of conditions at another location, even nearby. As with any resource potential data set, the actual power density harvested at any location will depend on the technology. To standardize the types of resource assessments, the International Electrotechnical Commission (IEC) has defined a conceptual framework for assessing tidal stream resource potential. Planners should keep in mind that numerical model results should be confirmed before finalizing site location. For more information, please consult the report.
Kevin Haas, PhD, Georgia Institute of Technology, School of Civil and Environmental Engineering
Zafer Defne, PhD, U.S. Geological Survey, Coastal and Marine Geology Program
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