environment-powered underwater vehicle

How cool is this! NASA-JPL's latest mailout tells of a robot called SOLO-TREC -- Sounding Oceanographic Lagrangrian Observer (SOLO) Thermal RECharging (TREC). JPL and Scripps Oceanographic Institute spent the last five years developing it and the last three months exhaustively testing it in the ocean off Hawaii. This neat little robot is powered by heat differences in the ocean itself. Because it gets its energy directly from the ocean it can embark on journeys that are not limited by the need to refuel.

At the moment there are 3,200 battery-powered floats drifting in the world's oceans -- the Argo array -- constantly sending back information about temperature, salinity and velocity. These could be supplemented by these small SOLO-TREC robots in the future. The next generation of submersible vehicles will likely use this technology too.

But here is the part I really like... how it works:

Different layers within the ocean have different temperatures -- lower is colder and higher is warmer. The robot has 10 external tubes containing waxy substances that melt and expand when in warmer water nearer the surface, and solidify and contract when it dives to colder water. The expansion of the wax pressurizes oil, which drives a hydraulic motor that generates electricity to recharge the robot's batteries. The batteries power the buoyancy system so that it can it move up and down in the ocean.

Read more here and here.