The Slocum Glider, developed by Webb Research Corp. (WRC) can propel itself through water using only the differential temperatures of varying depths and a thermal engine.
The glider's thermal engine uses stored thermal energy in the form of pressure to change the buoyancy of the glider to dive and surface. As the glider moves into the cooler, lower-depth water, a proprietary type of wax, which is housed inside a cylinder with oil, freezes at 10C. As the wax freezes, it expands and forces the oil into another chamber of the glider referred to as the bladder.
“All materials expand and contract with temperature, so it’s just finding the right temperature, what’s normally called the phase change, when a material goes from a liquid to a solid,” says Tod Patterson, engineer for WRC.
When the glider needs to redirect itself back to the surface, the nose is readjusted by moving a mass within the glider controlled by a motor and a lead screw, and then the volume is changed to be more buoyant.
When the glider is on the rise back to the surface, it heats the wax with warmer-temperature water and the oil is then forced into a nitrogen-backed hydraulic accumulator running between 2 and 3,000 psi. The forced oil creates a pressure that is stored until the glider uses the pressure to change its volume and sink again.
“What you’re doing is changing the actual volume of the unit,” says Patterson. “It’s generally neutrally buoyant, so if you change the volume of it, if you shrank it, it would become more dense and would sink.”
The glider directs itself using a rudder that is moved by a stepper motor and uses a Hall effect sensor to determine the exact position of the rudder. Even though the propulsion of the glider is entirely driven by the thermal engine, it still requires a number of batteries to operate a GPS system, an iridium phone, the motors and sensing equipment.
“Each time it comes to the surface, it first gets its GPS location and then it makes the iridium phone call, calling into a computer network that you can access anywhere in the world through the internet,” says Patterson. “At the same time you can set it up to call your cell phone and give you a text message of the health and well being of the instrument.”
The glider most recently has been used by the Woods Hole Oceanographic Institution and has traveled thousands of kilometers in the Caribbean. The glider currently dives to depths of 1,200m in a sawtooth pattern. It takes about four hours to complete one dive/surface cycle and it weighs approximately 52 kg.