Researchers Identify Promising Generators and Materials for Underwater Ocean-Observing Vehicles

In 2023, National Renewable Energy Laboratory (NREL) researchers investigated a new concept for harnessing energy from ocean thermal gradients. Their novel approach integrates materials that absorb a large amount of heat while changing from liquid to solid or solid to liquid (known as phase change materials, or PCMs) with generators that convert that heat into electricity. Together, PCMs and these generators have the potential to power uncrewed underwater vehicles and aquaculture farms. 

Ocean thermal gradients are the differences in temperature between the ocean's cold, deep water and warmer surface water. Ocean thermal energy conversion (OTEC) systems use this temperature difference to produce electricity. 

The use of thermoelectric generators (TEGs) with PCMs in uncrewed underwater vehicles is an unconventional OTEC system that does not require a power generation cycle (the process of compressing and heating a fluid to transform it into gas, which is used to drive a turbine). Instead, this system connects multiple TEGs that are located between hot and cold sources. The temperature source could be warm or cold ocean water. Any increase or decrease in temperature is absorbed by the PCM and harnessed by the TEGs. During the energy generation process, heat flows from the hot to the cold side of the TEGs, producing direct power. The PCM maintains the constant internal temperature of the TEGs by absorbing heat as the PCM changes from liquid to solid or from solid to liquid. 

After performing a detailed assessment, the NREL team concluded that organic PCMs—which include paraffins, fatty acids, and polyethylene glycol, all possessing the ability to absorb and release large quantities of heat—have the required characteristics to most effectively harness energy from ocean thermal gradients. ("Organic" is one of the classifications of PCMs based on their chemical composition.) These materials can also be tailored to various geographic locations and seasons so that more energy can be harvested. Researchers also evaluated different TEGs that are commercially available, identifying five different designs that could be coupled with the recommended PCMs. 

Using these results, the team recommended different combinations of PCMs and TEGs for specific seasons and locations. Their work produced optimal combinations for the North and South Atlantic, the eastern and western Gulf of Mexico, and the North, Central, and South Pacific.

Ocean thermal gradients have great potential to power uncrewed underwater vehicles in deep-water research, investigation, and exploration. They could also meet a portion of the energy requirements for aquaculture farms. By enabling self-powered uncrewed underwater vehicles, the combination of PCMs and TEGs may help devices collect more and better data. They could also be integrated with aquaculture farms to decrease carbon dioxide emissions by replacing fossil fuel consumption. This work ultimately advances the Water Power Technologies Office's Powering the Blue Economy™ initiative by providing power at sea to underwater vehicles that can better monitor the oceans.