Solar power works best when it has full access to the sun—not 9 meters underwater. That being said, the U.S. Naval Research Laboratory (NRL) has found that photovoltaics can operate at up to 9 meters underwater, harbor water that is. While others have developed solar arrays that float on water, this may be the first effort to completely submerge solar cells to test them underwater. 

The research lab was testing out whether cells could still absorb sunlight at depths to eventually power autonomous systems and sensors. Between using different cells types and advances in electronics, Phillip Jenkins, head of NRL Imagers and Detectors Section of the Electronics Science and Technology Division, found that they could. 

Jenkins tested numerous cells including silicon photovoltaics but found that the most promising PV cells for underwater deployment are gallium indium phosphide PV cells. “They are a little different from III-V cells developed for space and concentrator cells. They are essentially the ‘top’ cell in a monolithically stacked multi-junction cell used for space cells or concentrators,” he said. The cells will be more expensive than common silicon PV cells. But once they are manufactured at volume they should be about the price as space cells, according to Jenkins.

That being said, don’t expect underwater gargantuan solar farms anytime soon. During the test, PV cells were mounted at 9 meters underwater in a harbor. At that depth they were capable of producing 7 watts per square meter, much less than a conventional module in ambient conditions. Still, under other water conditions, like salt water or pond water, the cells could yield different results.

Still, that’s enough to power remote sensors and other autonomous systems, according to Jenkins. “Modern electronics continue to lower total energy used per instruction cycle.  We are now at the point where producing power measured in milliwatts is useful for sensor systems,” he said. “I could imagine these things working on just a few cells.”

This was just the first step in the research. “Step 1 is to figure out if there’s anything worth going after,” Jenkins said. “We’re saying there is somethings worth going after. The next step is to build a power system and do a long-term test.” Later tests will explore the viability of cells in conditions overtime and will look into how they would be affected by salt or silt in water, for instance.