Thermal glider takes to the sea

Meghan McCallum

Autonomous underwater gliders are one of the most important tools in oceanographic research, collecting detailed data needed over large stretches of ocean and time.
The most obvious method of collecting data at sea is from the deck of a boat, but this is often prohibitive, not only in the effort needed on the part of oceanographers, but also in cost; oceanographic research vessels cost somewhere in the range of $20,000 per day on the ocean.
A convenient solution is the use of robotic gliders. These torpedo-like tubes with wings are only big enough to accommodate a small bay of instruments as well as the propulsion system. The gliders stand in contrast to the more traditional, propeller driven, underwater robots.
Rather than push themselves along with propellers, the gliders translate vertical buoyant force into horizontal force using movable wings. Much like a submarine, the glider is able to control its buoyancy by taking in and pumping out water, and it turns with the help of a vertical rudder.
Traditional gliders save grant money almost immediately, though they start out looking rather costly at $100,000 to build. However, they require only two days at sea, a drop-off and a retrieval, plus the cost of recharging their batteries, in order to carry out a continuous data-collection mission for an entire month.
Most of the gliders used by researchers today change their buoyancy with battery-powered mechanical pumps that move water from inside the glider’s pressurized hull outside into the ocean, and back again, thus changing the glider’s volume without changing its mass.
In a new thermal glider, developed by researchers at Woods Hole Oceanographic Institution and at the University of the Virgin Islands, the change in buoyancy draws its power from the thermal layering of the ocean.
Wax-filled tubes convert the thermal energy of the ocean into a mechanical energy. Near the surface of the ocean, where the temperature is higher, the wax expands, pressurizing gas which can be expanded later to do work. As the glider sinks deeper, the surrounding temperature drops, compressing the wax and readying it for the warmer temperatures of the surface.
The obvious advantage of this mode of propulsion is that it uses no battery power to actually move the glider. The only things that require electric power are the sensors and the rudder. This has the potential of extending the lifetimes of data collection missions to long periods, with the only limiting factor being the small current drawn by the few electrical components remaining onboard.

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