This system supplants a related, previously proposed OTEC system.

An ocean thermal energy conversion (OTEC), now undergoing development, is a less-massive, more-efficient means of exploiting the same basic principle as that of the proposed system described in Alternative OTEC Scheme for a Submarine Robot” (NPO-43500), NASA Tech Briefs, Vol. 33, No. 1 (January 2009), page 50. The proposed system as described previously would be based on the thawing-expansion/freezing-contraction behavior of a wax or perhaps another suitable phase-change material (PCM). The power generated by the 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 depths, measuring temperature and salinity.

The PCM Expands and Contracts upon melting near the ocean surface and freezing at depth, respec- tively. The expansion and contraction causes the hydraulic fluid to flow cyclically through the hydraulic motor to drive the alternator to charge the battery.
At one phase of its operational cycle, the previously proposed system would utilize the surface ocean temperature (which lies between 15 and 30 °C over most of the Earth) to melt a PCM 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 PCM. The melting or freezing would cause the PCM to expand or contract, respectively, by about 9 volume percent. The PCM would be contained in tubes that would be capable of expanding and contracting with the PCM. The PCM-containing tubes would be immersed in a hydraulic fluid. The expansion and contraction would drive a flow of the hydraulic fluid against a piston that, in turn, would push a rack-and-pinion gear system to spin a generator to charge a battery.

The present system was conceived because the previously proposed system was found to be too heavy and inefficient for the intended application. The main difference between the present and previously proposed systems is that in the present system, the flow of hydraulic fluid drives a hydraulic motor instead of a piston.

The operational cycle of the present system involves three phases of flow of the hydraulic fluid and is best understood by reference to the figure. In phase 1, near the ocean surface, valve 1 is held open and valves 2 and 3 are held closed, and the expansion of PCM upon heating to >10 °C pushes the hydraulic fluid through valve 1 into the bellows in a gas-spring/bellows chamber, charging the chamber to an absolute pressure of about 3 kpsi (≈21 MPa). In phase 2, valve 2 is opened, allowing the pressurized hydraulic fluid to flow through the hydraulic motor and into the bellows in a second, lower-pressure gas-spring/bellows chamber. Upon completion of this flow, valves 1 and 2 are closed and valve 3 opened in anticipation of phase 3. In phase 3, which takes place upon cooling to <10 °C at depth, contraction of the PCM upon freezing reverses the pressure gradient in the plumbing, causing the hydraulic fluid to flow from the second gas-spring/bellows chamber back to the chamber containing the PCM.

This work was done by Yi Chao, Jack Jones, and Thomas Valdez of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category.

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:

Innovative Technology Assets Management JPL Mail Stop 202-233 4800 Oak Grove Drive
Pasadena, CA 91109-8099
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Refer to NPO-45404, volume and number of this NASA Tech Briefs issue, and the page number.

This Brief includes a Technical Support Package (TSP).

Improved OTEC System for a Submarine Robot (reference NPO-45404) is currently available for download from the TSP library.

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