Modeling and Optimization of a Phase Change Material-Based Ocean Thermal Energy Harvester for Powering Uncrewed Underwater Vehicles

As oceans cover over 70% of the planet's surface, they represent a large reservoir of resources that remain vastly untapped. Uncrewed underwater vehicles (UUVs) are becoming a key technology for ocean exploration. Ocean thermal gradient is a permanent and reliable energy source that can be used to power UUVs using phase change material (PCM)-based thermal engines. When using PCM-based thermal engines to power UUVs, there are different energy conversion stages: thermal, hydraulic, kinetic, and electrical, dependent on a wide variety of parameters. Thus, optimization of the overall energy conversion is still a challenge for powering the increasing energy demanding UUVs for long missions. The goal of this study is to propose a PCM-based ocean thermal energy harvesting system for powering float-type UUVs such as the SOLO-II float. This reduces the cost of battery replacement and expands the float's mission time. For this purpose, we developed a theoretical model consisting of hydraulic and electrical systems, designed to provide the electrical power needed by the UUV. The hydraulic and electrical systems are implemented using matlab/simulink. Parameter values from the literature and an accumulator size of 3.78 L are used. The mass of PCM calculated for the energy harvesting system is 5.73 kg, providing a theoretical volume change of 0.78 L. Varying the value of the electrical load connected to the electrical generator, the developed model can, theoretically, provide 13.66 kJ of electrical energy, which is more than 1.5 times the energy requirement per cycle for the SOLO-II float.