Autonomous Underwater Vehicles (AUVs) are becoming increasingly important for military surveillance and mine detection. Most AUVs are battery powered and have limited lifetimes of a few days to a few weeks. This greatly limits the distance that AUVs can travel underwater. Using a series of submerged AUV charging stations, AUVs could travel a limited distance to the next charging station, recharge its batteries, and continue to the next charging station, thus traveling great distances in a relatively short time, similar to the Old West “Pony Express.”
One solution is to use temperature differences at various depths in the ocean to produce electricity, which is then stored in a submerged battery. It is preferred to have the upper buoy submerged a reasonable distance below the surface, so as not to be seen from above and not to be inadvertently destroyed by storms or ocean going vessels. In a previous invention, a phase change material (PCM) is melted (expanded) at warm temperatures, for example, 15 °C, and frozen (contracted) at cooler temperatures, for example, 8 °C.
Tubes containing the PCM, which could be paraffin such as pentadecane, would be inserted into a container filled with hydraulic oil. When the PCM is melted (expanded), it pushes the oil out into a container that is pressurized to about 3,000 psi (≈20.7 MPa). When a valve is opened, the high-pressure oil passes through a hydraulic motor, which turns a generator and charges a battery. The low-pressure oil is finally reabsorbed into the PCM canister when the PCM tubes are frozen (contracted). Some of the electricity produced could be used to control an external bladder or a motor to the tether line, such that depth cycling is continued for a very long period of time.
Alternatively, after the electricity is generated by the hydraulic motor, the exiting low-pressure oil from the hydraulic motor could be vented directly to an external bladder on the AUV, such that filling of the bladder causes the AUV to rise, and emptying of the bladder allows the AUV to descend. This type of direct buoyancy control is much more energy efficient than using electrical pumps in that the inefficiencies of converting thermal energy to electrical energy to mechanical energy is avoided.
AUV charging stations have been developed that use electricity produced by waves on floating buoys and that use electricity from solar photovoltaics on floating buoys. This is the first device that has absolutely no floating or visible parts, and is thus impervious to storms, inadvertent ocean vessel collisions, or enemy sabotage.
This work was done by Jack A. Jones and Yi Chao of Caltech, and Thomas Curtin of NATP 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:
Innovative Technology Assets Management
JPL, Mail Stop 321-123
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.
NPO-46985
This Brief includes a Technical Support Package (TSP).
Submerged AUV Charging Station
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Overview
The document outlines the technical details of a Submerged Autonomous Underwater Vehicle (AUV) Charging Station developed by NASA's Jet Propulsion Laboratory (JPL). This innovative system utilizes a phase change material (PCM) to generate and store electrical power while operating underwater, making it particularly suitable for AUVs that require recharging during missions.
The charging station operates by leveraging temperature differences in the ocean. The PCM is submerged in hydraulic fluid and, when heated above 10°C, it melts and expands, pushing hydraulic fluid through a series of valves to drive a turbine generator. This process converts thermal energy into electrical energy, which is then stored in a battery. Once the PCM cools below 10°C, it solidifies and contracts, allowing the hydraulic fluid to return to its original chamber, ready for the next cycle. This thermal cycling mechanism is crucial for efficient energy conversion and storage.
The document also discusses the buoyancy control aspect of the system. By venting to an external bladder at depths around 300 meters (450 psi), the system can create buoyancy to lift the submersible back to the surface. This feature allows for effective depth cycling, with 15% of the energy used for buoyancy control and 85% for power generation.
Additionally, the mass of the system is detailed, indicating that the total mass includes fixed components (3.5 kg for electronics and 1.5 kg for plumbing) plus variable components that depend on the energy produced. For instance, a system generating 1 watt-hour (w-hr) of energy per dive has a total mass of 17.5 kg, while one producing 5 w-hrs has a mass of 67.5 kg.
The document emphasizes the potential applications of this technology beyond underwater vehicles, highlighting its relevance in various technological, scientific, and commercial fields. It serves as a part of NASA's Commercial Technology Program, aiming to disseminate aerospace-related developments for broader use.
Overall, the Submerged AUV Charging Station represents a significant advancement in underwater energy solutions, combining innovative thermal management with practical applications for autonomous systems in challenging environments.
