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Success for wax powered robot sea glider

sloglider2.jpgSomewhere off the coast of Puerto Rico a silent robot is flying under the sea. Launched by researchers at the famous Woods Hole Oceanographic Institution the robot glider has now been at sea since December and crossed the 4,000 metre deep Virgin Islands Basin over 20 times.

Eventually oceanographer Dave Fratantoni envisages fleets of gliders, all collecting scientific data and beaming it back to researchers. “Gliders can be put to work on tasks that humans wouldn’t want to do or cannot do because of time and cost concerns. They can work around the clock in all weather conditions,” he says (press release).

Unlike previous designs, which relied on batteries, the new type ‘Slocum’ glider* is environmentally friendly. Its power comes from differences in sea temperatures.

LiveScience has the best explanation of how this works:

When it moves from cooler water to warmer areas, internal tubes of wax are heated up and expand, pushing out the gas in surrounding tanks and increasing its pressure. The compressed gas stores potential energy, like a squeezed spring, that can be used to power the vehicle.

The glider then uses this energy to change its buoyancy, becoming denser to glide into the depths, becoming less dense to glide back to the surface.

sloglider.jpg It changes its buoyancy by pumping oil around. Live Science again:

To rise, oil is pushed from inside the vehicle to external bladders, thus increasing the glider’s volume without changing its mass, making it less dense. The oil can be shifted inside to increase the density and sink the vehicle. A vertical tail rudder allows the glider to be steered horizontally.

Now the Woods Hole team are hailing the success of their test run.

Of course some batteries are still needed. Steve McPhail, of the National Oceanography Centre in the UK, told the BBC: “You still need to provide power for the sensors, for the data-logging system and for the satellite communications system to get the data back.”

However when Nature first mentioned the wax powered Slocum back in April** last year it was hoped that the wax expansion could charge a battery for these functions. It’s not clear if this is actually happening now. It’s also not entirely clear to me how the craft deals with differences in sea temperature.

Even allowing for these points, the successful test run is another step towards the day scores of Slocums cruise silently under our seas. This idea was first mooted way back in the 1980s, even becoming the subject of a science fiction story by Woods Hole’s Henry Stommel.

* named for the first man to sail single-handedly round the world, Joshua Slocum.

** in an article about a battery powered version that was being used to track whales.

Image top: Dave Fratantoni, Woods Hole Oceanographic Institution

Image bottom: John Lund, Woods Hole Oceanographic Institution


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