Navy Submarine Runs Eternally On Thermal Power From Ocean Currents

SOLO-TREC Deployed off the Coast Of Hawaii NASA/JPL/U.S. Navy/Scripps Institution of Oceanography

From Popular Science:

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.

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