Next-Gen Solar Power for Underwater Vehicles
Solar cells can be a useful power source for autonomous underwater vehicles combined with rechargeable batteries. However, limitations in conventional silicon-based materials have long slowed successful deployment. NYU research highlights promising alternatives.
Autonomous underwater vehicles (AUVs) play a critical role in ocean research, from deep-sea exploration and underwater mine-hunting to coastal surveys, environmental monitoring, and security surveillance. However, short battery life and demands for continuous onboard power have limited the long-range operations of AUVs.
This solar-powered autonomous underwater vehicle, called SAUV II, was developed by Falmouth Scientific. Image used courtesy of NOAA
According to the National Oceanic and Atmospheric Administration, weight and available power are significant factors in the effectiveness of power sources for underwater activities. Most AUVs traditionally used special types of batteries, but scientists have since introduced new power sources such as fuel cells and rechargeable solar technology. Some marine vehicles, such as gliders, can reduce energy demands by harnessing gravity and buoyancy for propulsion.
Solar cells can harvest light as deep as 164 feet with high efficiencies, according to a 2020 study published in the journal Joule. Underwater solar systems can produce power with up to 65% efficiency in clear waters. However, more advanced wide bandgap semiconductors would be required to maximize power generation.
In addition to power sources, connecting underwater vehicles to remote satellites, radio frequency communications systems, and underwater sensors is another challenge to consider with surfacing and battery charge requirements. Solar cells can power fixed sensors and communication devices. They can also support long-range operations when combined with oceanic thermal energy conversion (OTEC) technologies.
New York University (NYU) researchers recently examined the many challenges and opportunities of solar-powered AUVs in a paper in Nature Photonics. Underwater solar energy generation can work together with batteries to provide a more balanced solution, but conventional silicon solar cells lack strong performance because water absorbs near-infrared light better than visible light. Converting the energy also presents challenges.
Researchers Propose Silicon Alternatives
The researchers pinpointed the main reasons silicon solar cells don’t work as well underwater. For one, they absorb red and infrared light, which can only penetrate shallow water.
The researchers studied examples of successful applications and solar harvesting materials showing promise as alternatives to silicon. Subjects such as cadmium telluride (CdTe) and gallium indium phosphide (GaInP) variants offer high efficiency and can be optimized based on water conditions. Advanced cells using perovskite and organic materials are another potential option.
Beyond absorber materials, solar cells face other challenges. NYU cited an example in biofouling, in which organic matter like microorganisms and plants reduce solar cells’ access to light. Past experiments found it takes only 30 days for biofouling to accumulate across more than half of the underwater vehicle’s surface.
The researchers also identified a few methods engineers could use to improve solar cell testing, especially in areas without water access. For example, they can use LED lights to simulate light spectrums at varying depths. Using this technique, researchers found silicon solar cells beat their counterparts in shallow waters but weren’t as efficient in water over 6.5 feet. The same NYU researchers published a study in 2022 that used LEDs to reproduce underwater solar irradiance spectra at different depths. They found GaInP solar cells exhibited efficiencies close to 54%, surpassing silicon and CdTe performance.
Several companies and research groups have deployed solar-powered AUVs over the years. For example, the Massachusetts Institute of Technology’s PEARL prototype was designed as a floating serving platform that harvests solar energy to recharge AUVs and link high-bandwidth low-Earth orbit satellite systems.
Video used courtesy of the Massachusetts Institute of Technology
In another example, the Autonomous Undersea Systems Institute and manufacturer Falmouth Scientific developed the SAUV II solar-powered autonomous vehicle, capable of surface operation or deeper missions as far as 1,640 feet. The vehicle uses lithium-ion batteries for endurance and in conditions with minimal solar radiation. The onboard battery system has a capacity of 2.4 kilowatt-hours.
SAUV II’s 10.7-square-foot solar panel layout can collect 300 to 900 watt-hours daily. The vehicle can run autonomously for weeks or months, though missions typically operate at night while solar energy charges the batteries during the day.
In 2010, Falmouth Scientific supplied a version of SAUV to the University of Tokyo for tectonic plate research.
The solar-powered SAUV II design. Image used courtesy of the Autonomous Undersea Vehicle Applications Center (Figure 1)
Though not an AUV, Liquid Robotics’ Wave Glider is another marine vehicle utilizing solar energy. The autonomous uncrewed surface vehicle (USV) uses waves for propulsion and features an additional architecture using stored solar energy. The solar system can also recharge batteries that power the glider’s sensors. Its maximum solar collection is 225 watts.