A proposed system for exploiting the ocean thermal gradient to generate power would be based on the thawing-expansion/ freezing-contraction behavior of a wax or perhaps another suitable phase-change material. The power generated by this system would be used to recharge the batteries in a battery-powered unmanned underwater vehicle [UUV (essentially, a small exploratory submarine robot)] of a type that has been deployed in large numbers in research pertaining to global warming. A UUV of this type travels between the ocean surface and various depths, measuring temperature and salinity.

The Wax Would Expand and Contract upon melting near the ocean surface and freezing at depth, respectively. The expansion and contraction would cause the hydraulic fluid to flow cyclically against the piston to periodically drive the generator to charge the battery.
This proposed system would be an alternative to another proposed ocean thermal energy conversion (OTEC) system that would serve the same purpose but would utilize a thermodynamic cycle in which CO2 would be the working fluid. That system is described in “Utilizing Ocean Thermal Energy in a Submarine Robot” (NPO-43304), imediately following this brief. The main advantage of this proposed system over the one using CO2 is that it could derive a useful amount of energy from a significantly smaller temperature difference.

At one phase of its operational cycle, the system now proposed would utilize the surface ocean temperature (which lies between 15 and 20 °C over most of the Earth) to melt a wax (e.g., pentadecane) that has a melting/freezing temperature of about 10 °C. At the opposite phase of its operational cycle, the system would utilize the lower ocean temperature at depth (e.g., between 4 and 7 °C at a depth of 300 m) to freeze the wax. The melting or freezing causes the wax to expand or contract, respectively, by about 8 volume percent.

The operational cycle is best described by reference to the figure. The wax would be contained in tubes that would be capable of expanding and contracting with the wax. The wax-containing tubes would be immersed in a hydraulic fluid. Near the ocean surface, the expansion of the wax upon heating to >10 °C would push hydraulic fluid into a bellows in a chamber pressurized to about 200 bars (about 20 MPa). Valve 1 would then be opened, allowing the pressurized hydraulic fluid to push against a piston that, in turn, would push a rack-and-pinion gear system to spin a generator to charge a battery. Next, valve 2 would be opened, allowing the hydraulic fluid to drain into a fixed-volume container. Later, upon cooling to <10 °C at depth, the contraction of the wax upon freezing would cause hydraulic fluid to flow from the fixed-volume chamber into the chamber containing the wax tubes, thus completing the cycle.

This work was done by Jack Jones and Yi Chao of Caltech for NASA’s Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

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Refer to NPO-43500, volume and number of this NASA Tech Briefs issue, and the page number.

This Brief includes a Technical Support Package (TSP).
Alternative OTEC Scheme for a Submarine Robot

(reference NPO-43500) is currently available for download from the TSP library.

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This article first appeared in the January, 2009 issue of NASA Tech Briefs Magazine.

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