Solo-Trec enters the water in November to begin its three-month endurance test--powered only by changes in ocean temperatures. (Credit: NASA/JPL/U.S. Navy/Scripps Institute of Oceanography )

Researchers say they've taken underwater robotics to the next level, successfully running an autonomous robotic vehicle off the Hawaiian Islands for more than three months, powered only by energy harnessed from changes in sea temperatures.

The Sounding Oceanographic Lagrangrian Observer Thermal RECharging vehicle (or Solo-Trec, for short) is the product of a years-long joint research project by NASA, the U.S. Navy, and Scripps Institution of Oceanography, and the University of California at San Diego. The group said Monday that Solo-Trec is the first such vehicle to be powered entirely by the ocean's thermal energy.

Solo-Trec works by taking advantage of temperature changes as it moves between warmer, shallower areas to the cooler temperatures found in the depths. It's outfitted with 10 external tubes of a waxy substance known as phase-change material. In the warmth of the shallow waters, the substance melts and expands; in deeper waters, it cools, solidifies, and contracts. The expansion caused by warm temperatures pressurizes oil inside the device, which drives a hydraulic motor that, in turn, recharges Solo-Trec's batteries. This system has allowed the 183-pound vehicle to make more than 300 dives from the ocean's surface to a depth of 1,640 feet--as well as powering its buoyancy controls and scientific, GPS, and communication instruments.

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