A Navy-funded thermal engine bobbing off the coast of Hawaii is accomplishing a rare feat -- it produces more energy than it consumes. Though it's not quite a perpetual motion machine, it could provide scientists or the Navy with a perpetual presence on the seas. The engine is attached to an unmanned underwater vessel, called SOLO-TREC (for Sounding Oceanographic Lagrangian Observer -- Thermal RECharging), and uses the energy of the ocean to derive a practically limitless energy supply.
SOLO-TREC is outfitted with a series of tubes full of waxy phase-change materials. As the float encounters warm temperatures near the ocean's surface, the materials expand; when it dives and the waters grow cooler, the materials contract. The expansion and contraction pressurizes oil, which drives a hydraulic motor.