Even below the water's surface, researchers are harnessing renewable energy to power systems that stalk the ocean to watch ships, monitor the weather and report on climate change.

Whether for submarines or sensors, researchers need to get electricity to remote parts of the ocean and use it judiciously.

"There are very few options for a sustainable power source underwater," said Phillip Jenkins, head of the imagers and detectors section at the U.S. Naval Research Laboratory. As a result, scientists are finding ways to scavenge what little energy there is while using less of it for systems that often need to function underwater for days or months at a time.

Jenkins worked with a team of Navy researchers to develop a photovoltaic panel that works underwater down to 9 meters. They presented their findings at the IEEE Photovoltaic Specialists Conference last month.

"These cells use a semiconductor material, gallium indium phosphide, which has a higher band gap energy than [conventional] silicon solar cells," he explained. "This allows for the higher conversion efficiency of the blue/green light, which penetrates through water."

Why go through all the trouble of capturing meager sunlight underwater? Jenkins said electricity from these photovoltaic panels could power offshore sensor networks that monitor ocean chemistry, water quality or distant vessels, instead of running them off batteries or running a cable from land. This gives sensor networks more flexibility in terms of where and how deep they can be installed.

"There are a number of applications where high efficiency is more important than lowest cost," Jenkins said.

High-tech 'gliders' to inspect the sea
In theory, there is enough useful light reaching depths as far as 20 meters. But as the waters sway and surge above the solar panels, light is bent and twisted, shifting in intensity like the sun shining through a swimming pool and limiting how effective the panels can be.

"That does play some havoc with being able to string these cells together," Jenkins said.

Researchers are also looking into harvesting energy from temperature differences in the ocean. Teledyne Webb Research, a company that develops ocean gliders, is designing a glider that can travel 40,000 kilometers in the ocean over five years by harnessing temperature gradients for energy.

Ocean gliders are cruise missile-shaped autonomous vehicles that swim by changing their buoyancy instead of using a propeller. As the glider rises or falls in the water, its wings direct some of the movement forward, tracing a sinusoidal path through the water. In the process, the glider can use its sensors to take readings on ocean salinity, density and biomass, transmitting that information when it surfaces periodically.

Clayton Jones, senior director of technology at Teledyne Webb, explained that the glider uses a wax that freezes and thaws based on its surrounding temperature. As the wax freezes, it expands like ice and changes the volume inside the glider, which makes it rise. When it reaches the surface, the warmer waters melt the wax.

"That gets you to go up and down for free," he said.

Eventually, the company hopes to use its thermal system to also produce electricity to drive the glider's sensors and communications systems. Battery-powered versions of their gliders have already posted impressive results. In 2009, researchers at Rutgers University sent a glider across the Atlantic Ocean, from New Jersey to Spain, powered by a single battery charge. The 93-inch Scarlet Knight completed the journey in 221 days.

"The reason the gliders have such long endurance is that they're not going very fast," Jones said.

$100,000 a pop, and sharks love 'em, too
In fact, the gliders are slow enough to be vulnerable to barnacles and occasionally deal with remoras, fish that usually attach to sharks to scavenge scraps for meals.

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