Thermal Energy of the Ocean Powers New Autonomous Vehicle

NASA, U.S. Navy, and Scripps Institution of Oceanography at UC San Diego researchers have successfully demonstrated the first robotic underwater vehicle to be powered entirely by natural, renewable, ocean thermal energy.

The Sounding Oceanographic Lagrangrian Observer Thermal RECharging (SOLO-TREC) autonomous underwater vehicle uses a novel thermal recharging engine powered by the natural temperature differences found at different ocean depths. Scalable for use on most robotic oceanographic vehicles, this technology breakthrough could usher in a new generation of autonomous underwater vehicles capable of virtually indefinite ocean monitoring for climate and marine animal studies, exploration, and surveillance.

“The present thermal engine shows the great promise in harvesting ocean thermal energy,” said Russ Davis, an oceanographer at Scripps and project co-principal investigator. “With further engineering refinement, SOLO-TREC has the potential to augment ocean monitoring currently done by the 3,200 battery-powered Argo floats.”

The international Argo array measures temperature, salinity and velocity across the world's oceans. NASA and the U.S. Navy also plan to apply this thermal recharging technology to the next generation of submersible vehicles.

Researchers at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. and Scripps Oceanography completed the first three months of an ocean endurance test of the prototype vehicle off the coast of Hawaii in March.

People have long dreamed of a machine that produces more energy than it consumes and runs indefinitely," said Jack Jones, a JPL principal engineer and SOLO-TREC co-principal investigator. "While not a true perpetual motion machine, since we actually consume some environmental energy, the prototype system demonstrated by JPL and its partners can continuously monitor the ocean without a limit on its lifetime imposed by energy supply."

SOLO-TREC draws upon the ocean's thermal energy as it alternately encounters warm surface water and colder conditions at depth. Key to its operation are the carefully selected waxy substances known as phase-change materials that are contained in 10 external tubes, which house enough material to allow net power generation. As the float surfaces and encounters warm temperatures, the material melts and expands; when it dives and enters cooler waters, the material solidifies and contracts. The expansion of the wax pressurizes oil stored inside the float. This oil periodically drives a hydraulic motor that generates electricity and recharges the vehicle's batteries. Energy from the rechargeable batteries powers the float's hydraulic system, which changes the float's volume (and hence buoyancy), allowing it to move vertically.

So far, SOLO-TREC has completed more than 450 dives from the ocean surface to a depth of 500 meters (1,640 feet) and is reporting temperature and salinity profiles three times per day. Designers control the depth to which the instrument with built-in ballast dives through its hydraulic system. Its thermal recharging engine produced about 1.7 watt-hours, or 6,100 joules, of energy per dive, enough electricity to operate the vehicle's science instruments, GPS receiver, communications device and buoyancy-control pump.

SOLO-TREC is now in an extended mission. The JPL/Scripps team plans to operate SOLO-TREC for many more months, if not years.

--Robert Monroe

(Portions of text provided by NASA/JPL)