An OTEC thermodynamic cycle would be divided into surface and depth phases.
A proposed system would exploit the ocean thermal gradient for recharging 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 proposed system is related to, but not the same as, previously reported ocean thermal energy conversion (OTEC) systems that exploit the ocean thermal gradient but consist of stationary apparatuses that span large depth ranges.
The system would include a turbine driven by working fluid subjected to a thermodynamic cycle. CO2 has been provisionally chosen as the working fluid because it has the requisite physical properties for use in the range of temperatures expected to be encountered in operation, is not flammable, and is much less toxic than are many other commercially available refrigerant fluids. The system would be housed in a pressurized central compartment in a UUV equipped with a double hull (see figure).
The thermodynamic cycle would begin when the UUV was at maximum depth, where some of the CO2 would condense and be stored, at relatively low temperature and pressure, in the annular volume between the inner and outer hulls. The cycle would resume once the UUV had ascended to near the surface, where the ocean temperature is typically ≥20 °C. At this temperature, the CO2 previously stored at depth in the annular volume between the inner and outer hulls would be pressurized to ≈57 bar (5.7 MPa). The pressurized gaseous CO2 would flow through a check valve into a bladder inside the pressurized compartment, thereby storing energy of the relatively warm, pressurized CO2 for subsequent use after the next descent to maximum depth.
Upon descent, the outer hull would become cooled — possibly to a minimum temperature as low as about 4 °C at a depth of about 300 m. The cooling would reduce the pressure of the CO2 remaining in the annular volume to about 44 bars (4.4 MPa) or less. Then a control valve would be opened, allowing CO2 from the pressurized bladder to expand through a turbine, thus producing electricity for recharging the battery. After flowing through the turbine and the control valve, the CO2 would enter the annular volume, where it would be condensed at low temperature and pressure, completing the thermodynamic cycle.
This work was done by Jack Jones and Yi Chao of Caltech for NASA’s Jet Propulsion Laboratory.