Harvesting Power from the Ocean

A new technology could generate electricity from waves.

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.

Wave power: A buoy that generates electricity from the motion of waves. The clear cylinder in the middle of the buoy (see bottom image) contains a roll of rubberlike material that stretches and contracts as the buoy bobs up and down, separating and bringing together electrodes. Credit: SRI International

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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.

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.

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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.