Ocean thermal energy application technologies for unmanned underwater vehicles: A comprehensive review

Ocean thermal energy offers the possibility to solve the contradiction between long-term and high-frequency observation and finite power of unmanned underwater vehicles. There are very limited reviews on unmanned underwater vehicles powered by ocean thermal energy. This review is primarily dedicated to a comprehensive overview of ocean thermal unmanned underwater vehicle development, and an indication of future developments. Firstly, this paper provides a review of the current state of ocean thermal energy utilization in vehicles. The application principles of phase change material thermal harvesting system are concluded, and several typical PCM-based unmanned underwater vehicles are listed. Then, several theoretical issues based on this type of vehicle have been discussed. Moreover, the basic principles of ocean thermal energy utilization approaches based on shape memory alloys, thermoelectric generators, thermodynamic cycle, and their potential application forms in vehicles are also reviewed. Finally, the challenges and difficulties involved in the application of ocean thermal energy in unmanned underwater vehicles are discussed as well. The results show that (i): PCMbased vehicles are still the most promising ocean thermal underwater vehicles; (ii): For this type of vehicles, there are still some problems to be solved, such as slow heat transfer rate, low energy conversion efficiency (less than 0.6%), low energy storage density (about 0.26 Wh/kg) and lack of synergy between motion and heat transfer; (iii) The other three types of ocean thermal UUVs are still in the conceptual design phase, and their basic application principles, as well as the exploration of methods to improve energy density and efficiency, are the most critical focuses at present.