Billions of watts of energy continuously flow down rivers and streams, or wash up on U.S. shores, just to dissipate. As part of ocean waves and flowing currents, this renewable energy can be more reliable than solar and wind. According to the Electric Power Research Institute, untapped "technically recoverable" wave energy alone could provide about 25 percent of U.S. electricity demand in the future.
This potential has accelerated research in the field of hydrokinetics, the science of understanding how to convert the kinetic energy in moving water—surface waves, tidal currents, and rivers, streams, and ocean currents—into electricity. One big advantage of hydrokinetic devices is that they are placed directly in the flow path and generate energy only from the power of the moving water; no dam or diversion structure is required to increase hydraulic head to drive a turbine.
Many hydrokinetic devices are buoy-like and float on the ocean surface. Wave movement creates a piston-like action within the device that is converted via a sophisticated power take-off to drive an electrical generator. Other devices consist of cylinders connected by hydraulic joints that also float on the surface—wave motion drives internal hydraulic motors that produce electricity.
Ocean Renewable Power Company's first grid-connected power system. Image: ORPC
The most popular hydrokinetic device is the turbine. The basic technology is well-known and comes in a variety of sizes and shapes. They also work well in tidal environments and tend to be lower cost than wave energy recovery. As a result, tidal projects comprise over 90 percent of today's marine kinetic capacity totals—which is driving advanced research in tidal turbine design.
Tidal turbines are very similar to wind turbines in that they convert a directional flow of energy into rotational mechanical energy. They also work well in smaller-scale river environments. Because these turbines are installed underwater, which is much denser than air, hydrokinetic turbines provide much more power than wind turbines at relatively low water current speeds. They can also be modular in design and stacked in different arrays, depending on the characteristics of the site.
New Turbine Projects
Ocean Renewable Power Company, Portland, ME, installed the first grid-connected tidal energy project in the Americas when it placed the first of several planned turbine generator units in Cobscook Bay, ME, in late summer 2012. The unit began sending electricity to the grid in September. Built primarily with composite materials, the turbines resist corrosion, require no lubricants, and emit no discharge. When fully operational, the three-device power system will generate enough energy to power 75 to 100 homes.
In New York City's East River, another tidal deployment is under way—New York City-based Verdant Power's 1050 KW Roosevelt Island Tidal Energy project will generate electricity from turbine generator units mounted on the riverbed. This kinetic hydropower system will capture energy from both ebb and flood directions by yawing with the changing tide, using a passive system with a downstream rotor.
"We have a three-blade downstream axial flow rotor and use the nose cone to orientate the turbine to the water current," says Verdant Power's president Trey Taylor. "So with the East River tides, the turbine can turn around to get the ebb and flood tides."
The turbines are positioned in an offset arrangement. "We inadvertently came to that conclusion as we were looking at existing wind turbine modeling to determine how to space turbines," Taylor comments. "If you take a percentage of energy out you need to allow enough distance for the water current to restore itself before you take another percentage of energy out, otherwise it will take the path of least resistance and the water current will start to move around the turbines."
For both tidal and wave converters, machine speeds tend to be quite low, particularly compared to wind turbines. Consequently, device components experience very high torques near rated power, which will only increase as devices scale up.
"Developing reliable and highly efficient drivetrains is a significant engineering challenge for the marine renewable sector," says Brian Polagye, research assistant professor of mechanical engineering and co-director of the Northwest National Marine Renewable Energy Center at the University of Washington, Seattle. "Similarly, control algorithms to optimize power production will be constrained between the realities of structural loads and the benefits of capturing the power from turbulent 'gusts' in the tidal sector, and large waves in the wave sector. This represents a structural-fluids-control problem that has not yet been fully explored."
Hydrokinetic technology tends to get the most attention. However, important behind-the-scenes research is dealing with critical aspects of site evaluation, seabed mechanics and engineering, environmental impacts, and regulatory compliance.
For example, the Northwest National Marine Renewable Energy Center is using underwater noise data from tidal energy sites and models for the sound produced by tidal turbines to assess the effects these sounds may have on marine life.
"We determined that the probability of marine animals detecting sound from the project is quite low beyond one kilometer," states Polagye. "Marine mammals at specific distances to a turbine may also be exposed to very different received levels of sound, depending on how fast the tidal currents are running and if vessels are nearby. This type of interdisciplinary engineering and environmental analysis is at the forefront of identifying potential environmental impacts of hydrokinetics and mitigating them through engineering design."
Mark Crawford is an independent writer.
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