Well, it's not exactly perpetual motion or energy, but it's getting closer than most such attempts. Plus, instead of being backed by some sort of random crackpot scientist, this particular project has the backing of NASA and the U.S. Navy. So, what is it? (Take a deep breath.) The Sounding Oceanographic Lagrangrian Observer Thermal Recharging (SOLO-TREC) autonomous underwater vehicle. Says Jack Jones, a JPL principal engineer and SOLO-TREC co-principal investigator:
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.
