One key to harvesting the ocean’s clean energy—at least a little of it—may lie in static electricity. A team of researchers in Portugal has now successfully used it to run small generators inside a navigational buoy, powering the sensors and lights that the buoy uses to collect data and aid sailors. Though the project’s scale is small so far, the researchers say it is an important proof of concept for a technique that could supplement existing attempts to harness the power of waves, as well as other kinds of naturally occurring motion.
Oceans are an appealing target for renewable energy generation. Waves alone produce 32,000 terawatt-hours of natural energy per year—for reference, the entire world uses around 23,000 terawatt-hours annually. And there is also the power of currents, tides and thermal energy. But despite decades of research, the motion of the ocean has proved difficult to harness. Wave patterns are unpredictable, seawater corrodes metal generating machinery, and waves’ energy is simultaneously dispersed across three dimensions (up-down, forward-backward and left-right).
In part because of such challenges, the electrical output from several nascent, large-scale wave power projects has lagged behind predictions. The Portuguese researchers instead focused on something smaller and more manageable: powering navigational buoys, which often incorporate lights to guide boats and sensors to monitor ocean conditions. The team turned to so-called triboelectric nanogenerators, or TENGs, which convert motion into an electrical current using static electricity—the same principle as rubbing a balloon on a fuzzy sweater to generate charge. At each TENG’s core are two surfaces, just a few square centimeters in area, that can easily become positively or negatively charged. Atop these two stacked surfaces, the researchers placed 10 stainless steel balls, about 12 millimeters in diameter, that are free to move around. When their container tilts, the balls roll around and rub the two surfaces together. This builds up a static charge, which can be converted into electricity to power a battery.
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“We developed these novel devices that convert rhythm and mechanical energy into electrical power,” says Cátia Rodrigues, a nanotechnology Ph.D. student at the University of Porto in Portugal. She delivered a presentation about her team’s wave-powered buoy last week at an American Institute of Physics conference that was held online. “The devices are low-cost. They reach high power densities [with] high efficiencies,” Rodrigues says, adding that TENGs continue to perform well even when waves are small and infrequent.
TENGs can generate power from any form of motion, but Rodrigues and her collaborators have focused on testing various TENG prototypes to optimize them for the specific conditions of wave motion. In their most recent tests, she and her colleagues wanted to see which setup would produce the most electricity the most consistently: placing all the balls together in a round basin shaped like a shallow bowl or creating individual “tracks” for each ball like swimmers in the lanes of a pool.
Working in a hydraulics lab at the University of Porto, the team tested designs for TENGs embedded in a one-eighth-scale replica of an oceanic buoy. They placed the model in a wave pool and simulated the five most frequent wave patterns that occur in the seaport in nearby Figueira da Foz, Portugal.
TENGs were invented by a researcher at the Georgia Institute of Technology in 2012. The new study marked the first time they have been tested under such realistic wave conditions, Rodrigues says. And it was a success: the swimming-lane-esque TENG design produced a maximum output of 230 microwatts—enough to power small devices such as medical implants. It also converted energy more consistently under different wave conditions than the bowl design did. Rodrigues says the output could be boosted by incorporating multiple TENGs or adding nanoparticles to the surfaces underneath the metal balls, increasing the materials’ capacity to gather charge.
TENGs may offer a solution to a key problem that has stymied other ocean energy technologies, says Andrew Hamilton, engineering division chair at California’s Monterey Bay Aquarium Research Institute, who was not involved in the new work. The ocean, he says, is a high-force, low-speed system: it contains a vast amount of energy, but that power is widely distributed. As a result, traditional spinning generators often require more energy to electricity than a small patch of ocean can provide, and other attempts to develop wave-powered buoys have been flawed. Monterey Bay’s own buoy project generates power by using the difference in motion between the water’s surface and a platform suspended dozens to hundreds of meters below. But to work at great depths, this requires a long cable that takes damage from breaking waves and underwater currents. In 2017 a navigational buoy in India powered itself with an oscillating water column system: waves alternately filled and emptied a partially submerged chamber, accelerating air into and out of the column. The fast-moving air then turned a turbine to generate electricity. But this method produces potentially problematic loud noises, and it only takes advantage of the vertical motion of a wave.
A TENG’s small size helps it avoid both of these pitfalls. Rodrigues says its compactness is one of its perks, allowing researchers to easily combine TENGs with other electricity-generating methods such as solar panels or different kinds of wave-energy harvesters. Based on the success of their wave pool trials, the researchers plan to modify their TENG prototype and install it in a full-scale buoy in Figueira da Foz. Hamilton notes that an open-ocean test may present challenges that cannot be simulated in a wave pool. “Anything you design for year-round use in the ocean, you have to design it for the storm that’s statistically likely to happen every 100 years,” he says. He explains that this type of extreme weatherproofing often makes a device bulkier, less maneuverable and less durable over time because the added surface area provides more opportunities for wear and tear.
Rodrigues is not daunted. She says she is studying TENGs’ performance not just when they are in the ocean but also under other “harsh conditions,” including when they are placed inside groundwater extraction wells—and sewn into the insoles of shoes. These wide-ranging applications are why, in the future, she expects to see TENGs “everywhere.”