Harvesting Power from the Ocean
Researchers from SRI International, based in Menlo Park, CA, recently completed the first ocean tests of a system that uses a so-called artificial muscle to generate power from the motion of a buoy riding up and down on the waves. Although the prototype produces very little electricity, the researchers say that wave farms based on the technology could eventually rival wind turbines in power output, providing a significant source of clean energy.
Technology for harnessing the ocean’s energy already exists, but it has not been widely adopted, largely because it has trouble withstanding the pounding of the waves. The new system could prove both cheaper and more reliable, the researchers say.
Earlier systems used more-conventional electromagnetic devices, such as dynamos with complex transmissions, hydraulic pistons, and turbines. The gears of a transmission, in particular, are vulnerable to wear and tear from the erratic surging of ocean waves.
In contrast, the SRI system is not much more than a sheet of rubber attached to a weight. It has “the mechanical complexity of a rubber band,” says SRI senior researcher Roy Kornbluh. As a consequence, it is better able to absorb the shock of waves, says Yoseph Bar-Cohen, a senior research scientist at NASA’s Jet Propulsion Laboratory, in Pasadena, CA. What’s more, Bar-Cohen adds, the materials that the system is made from are cheap, which could help it compete in price with other sources of electricity.
Video: Power Buoy
The polymer-based system at the heart of the new generator is a variation on an artificial muscle–a device developed as an alternative to electric motors in applications such as robots. An artificial muscle will expand or contract when a voltage is applied to it, but the same process can work in reverse: if the muscle is stretched by an outside force, it can generate electricity. A few years ago, SRI developed a small device that, embedded in the heel of a shoe, enabled the wearer to charge a cell phone simply by walking. The wave-harvesting system is basically a larger version of the same technology.
The SRI researchers built their generator by sandwiching a commercially available rubbery material between two electrodes, which are themselves made of a greasy polymer containing conductive materials. The rubber sheet and electrodes are then rolled up, like a scroll, to form a hollow tube. When the tube is pulled by an outside force, the rubber layer is stretched thin, narrowing the gap between the electrodes. Initially, a small battery applies a voltage across the electrodes; when the rubber springs back into its original shape, it forces the electrodes apart, increasing the voltage between them. This excess energy can be siphoned off to generate a current. Part of that current feeds back into the system, so the battery is used only for the first cycle.
The researchers recently tested the system off the coast of Florida. A couple of square meters of rubber rolled into the shape of a hollow tube were attached to a weight and mounted at the center of a buoy. As the buoy bobs in the water, it causes the weight to rise and fall, repeatedly stretching the rubber and allowing it to rebound, generating electricity.
So far, this prototype produces only about five watts of power–enough for a small light bulb. But because the rubber is thin–about 0.1 millimeters thick–it’s possible to roll up much more of it and still fit it into the same buoy. A bundle of rubber about a meter long and half a meter thick, with optimized electronics and an improved buoy design, could generate a kilowatt of electricity, Kornbluh says. A string of buoys or larger floating structures could then generate appreciable amounts of electricity. (A thousand buoys could power about 750 houses.) An alternative design could involve submerged sheets of rubber that generate power as the force of currents or tides makes them flap back and forth. Such a system might prove more resilient than the turbines recently used in the East River in New York: their mechanical parts proved unable to withstand tidal forces.
The SRI system produces high voltages, in the range of a kilovolt. That was a problem for the shoe generator, which required a transformer to decrease the voltage enough that it wouldn’t fry cell phones and other devices but still had to fit in a shoe. But for the buoy application, high voltage is an advantage, since it makes it more efficient to transmit electricity back to shore along underwater cables. The main challenge moving forward, the researchers say, is to develop a reliable manufacturing process. Their recent tests of the system also underscored the importance of designing new buoys that respond to waves in the best way for generating power. And they’ll need to design electronics that, by varying the voltage across the polymer, can modify the stiffness of the system to adapt to different weather conditions.
The system’s first commercial applications will likely be in systems for powering navigation, communications, and sensor buoys, and these could come within two years, Kornbluh estimates. But it could be five to ten years before the system can be ramped up for large-scale electricity generation.
“It’s very exciting,” says Ray Baughman, a professor of chemistry at the University of Texas at Dallas. “It’s a promising direction for harvesting energy, not only for remote devices in the ocean but also perhaps for larger-scale energy harvesting.”
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