Sea temperature to power underwater robots
Researchers working for NASA have come up with a way of exploiting the difference in temperature between surface sea water and ocean depths to power robotic submarines
Jack Jones and Yi Chao, working at Caltech, have suggested exploiting the expansion and contraction of a pentadecane wax as it melts and freezes. Using the same principle, they have also suggested using liquid pressurised carbon dioxide as it evaporates and condensates.
Sea surface temperature is between 15°C and 20°C over much of the earth's surface. But at 300m this falls to between 4°C and 7°C. This would cause the wax to change volume by about 8%. Melting could be used to generate a pressure of about 20bar. This is sufficient to push hydraulic fluid into bellows, which would in turn, rotate a generator through a rack and pinion mechanism.
The carbon dioxide system envisages liquid carbon dioxide stored in the hull. When the vehicle is near the surface it would evaporate to produce a pressure of 57bar filling a bladder with pressurised gas. Upon descent, the cooled hull would reduce the gas pressure to around 44bar. At this point a control valve would be opened, allowing carbon dioxide from the pressurised bladder to expand through a turbine, generating electricity.
More information from iaoffice@jpl.nasa.gov Ref NPO-43500 for the wax system and ref NPO-43304 for the carbon dioxide system
