There is a short, but fascinating article in Popular Science about new idea in propulsion for submarine and other maritime uses.
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. The motor generates electricity and recharges the batteries, which power a pump. The pump can change the float’s buoyancy, allowing it to move up and down the water column.
This has potential for all sorts of systems, and if it can economically scaled up to where it could replace a Nuke plant, then we’ve freed ourselves of all sorts of restraints. Additionally, there could be autonomous, self-powered acoustic arrays for passive detection which could maneuver over large areas as the tactical or strategic situation shifted.