Engineers have long dreamed of tapping into the vast quantities of renewable energy available in the motion of ocean waves, but designing apparatuses to harness such power efficiently has so far proven difficult. Borrowing a technique from optical physics, a team has shown a new way to concentrate waves, tripling their height and allowing for better extraction of their energy. The results appeared in Physical Review Letters.
The constant churning of the ocean should in principle be a useful source of electricity—a mechanical lever moving back and forth with the waves could run a turbine that produces power. The Bureau of Ocean Energy Management, an agency within the U.S. Department of the Interior, estimates that the total recoverable energy available along the U.S. coast could be 1,170 terawatt-hours/year, or roughly one-third of the country’s total energy needs.
During the 1970s oil crisis, interest in all kinds of renewable energies spiked, but wave power development has since lagged behind solar and wind. More than 1,000 different wave energy converter designs have been proposed, but researchers have yet to converge on the optimal form.
Some versions consist of simple buoys moving up and down, while others are hollow floating boxes. Still other types of concentrators send waves up a small ramp and then allow them to drain through a hole and run a turbine, explains mechanical engineer Robert Thresher of the National Renewable Energy Laboratory in Golden, Colorado. None is inexpensive and reliable enough to see the given technique more widely deployed, he says.
In 2012, physicist Huanyang Chen of Xiamen University in China, who was working on using metamaterials to concentrate microwaves, ran into Zhenyu Wang, a hydraulics and ocean engineer at Zhejiang University in Hangzhou, at a scientific meeting. “We have a totally different background,” says Chen. “But when we came together, it was a perfect combination.”
The same equations describe both water waves and electromagnetic waves, so Chen was able to use his optics expertise to help design a new type of concentrating tank. The device consists of 50 thin metal sheets arranged in circle like vertical flower petals, allowing water to flow through them from any direction. A small open area in the central meeting point of the sheets has a floor raised relative to the outer edge of the device.
Water waves are focused through the slits and sent up small ramps. The difference in height scrunches the waves and raises their height. The team built two designs, a small 70-millimeter-diameter version that doubled wave heights and a larger 43-centimeter device that succeeded in tripling the amplitudes of certain waves, allowing them to impart more force on a mechanical lever that generates electricity. Not all waves received the same boost—the devices were optimized for certain frequencies: 4.95 and 7.05 Hertz for the small version and 1.5 Hertz for the larger one. But even those outside this range received some increase in their height. Chen says their next iteration should work for a broad range of frequencies.
As an added bonus, the design also suppresses any back-scattering waves. In many other wave concentrating devices, these waves often travel in an unintended direction and counteract the power-generating waves.
Thresher, who was not involved in the work, says the design is clever, especially since its circular symmetry can capture waves coming in from any direction. But, he says, wave power devices need to demonstrate more than added efficiency—most of the cost comes from building a rigging that can withstand the harsh oceanic weather, including extreme wave events that overwhelm a power station. “That’s the trick,” he says. “How do you withstand the storm waves?”
Chen says the team is now working on a version that is around 10 times bigger than their larger device. He hopes to test in near-shore shallow waters where inclement weather is less of a problem.