Review on phase change materials and thermoelectric generators for ocean thermal gradient applications

Ocean thermal energy has the potential to address the challenge of balancing long-term operation and high-frequency monitoring due to the limited energy storage capacity of batteries for unmanned underwater vehicles (UUVs). Several emerging technologies to power UUV, profiling floats, and other ocean applications, including the thermoelectric generators (TEGs), phase change materials (PCMs), and memory shape alloys have been proposed. However, most of these technologies are still in an early stage of development and have not been deployed on a commercial scale. In this study, we conducted a comprehensive review on TEGs and PCMs to harness ocean thermal gradients. This review is focused on TEGs' and PCMs' characteristics, properties, and progress to address technical limitations to advance towards the deployment in ocean power applications, such as ocean thermal energy-powered UUVs at a commercial scale. First, the paper provides a thorough review of the different PCMs, TEGs, focusing on enhancing their thermophysical properties. Then, based on their technical attributes and the ocean thermal gradient potential in the United States, a selection of PCMs and TEGs is proposed for different locations and seasons. The ocean thermal gradient offers a stable annual storage potential of approximately 1.09 × 1019 MJ, with about 7.63 × 1017 MJ being harnessable with an efficiency of 7 %. Despite PCM-based UUVs is one the most promising alternatives for ocean thermal gradient applications, the low heat transfer rates, low energy conversion efficiency (between 0.14 % and 0.6 %), and low energy storage density (around 0.26 Wh/kg) are the main challenges for effective commercial scale applications. This study identifies key PCMs suitable for ocean thermal applications, including organic, inorganic, and eutectic PCMs, with melting enthalpies ranging from 100 to 250 kJ/kg. It also evaluates various TEGs for different oceanic locations and seasonal variations. Recent advancements in integrating PCMs with TEGs, through enhanced materials and innovative designs, have significantly improved energy conversion performance, increasing thermal conductivity by up to 15 times using metal foams and 10 times using nanomaterials. The integration of PCMs and TEGs offers a feasible alternative to power future UUVs, profiling floats, and other ocean power technologies, including aquaculture farming and water desalination.