Hybrid phase change material based thermal gradient energy system for powering uncrewed underwater vehicles

Several phase change material (PCM)-based systems have been developed to harness ocean thermal gradient energy for a long duration operation of uncrewed underwater vehicles (UUVs). However, the current state-of-the-art PCM-based thermal gradient energy systems, which generate 6–10 kJ per profile, cannot fully power high-energy-consuming UUVs. Key challenges are the low energy-conversion efficiency inherent to thermal gradient energy systems and the high energy consumption associated with the buoyancy engine employed in UUV profiling. In this work, a hybrid (buoyancy regulation and electricity generation) thermal gradient energy system is proposed for powering UUVs. The hybrid system leverages direct buoyancy change, converting ocean thermal energy into hydraulic energy to change UUV buoyancy, which minimizes energy requirements, and enhances energy conversion efficiency. Key system parameters including the accumulator size and working volume for power generation were analyzed and optimized, leading to a theoretical thermal-to-hydraulic energy conversion efficiency of 1.4 %. A hydraulic-to-electrical energy conversion system was modeled using MATLAB/Simulink to optimize the hybrid system before it was implemented. Experimental results demonstrate that the hydraulic-to-electrical energy conversion efficiency of first line hydraulic system was 58.9 %, surpassing that of state-of-the-art hybird-type thermal gradient system. To supply power for various types of UUVs, the required mass of PCM in the hybrid system was also analyzed to guide the design of the thermal gradient system for powering UUVs.