Submerged photovoltaics could recharge subs, underwater robots
Charles Q. Choi is a contributing editor for IEEE Spectrum.
This LED panel is powered by lithium-ion batteries, which were charged for two hours by solar panels submerged 10 meters underwater.
Nowadays, solar cells are typically found on sunny surfaces on land or aboard satellites in space. A new study suggests photovoltaics can also work below the ocean’s surface, generating useful amounts of electricity underwater even at depths of 10 meters.
“With submerged solar cells, submarines or autonomous underwater vehicles could achieve far greater range and mission endurance than they do today,” says Wen-Hua Zhang, dean of Yunnan University’s school of materials and energy in Kunming, China. Many other underwater electronic devices, such as sensors, cameras, lights, and communications systems “could also benefit from longer operating periods and much broader deployment coverage in the ocean, to support a subsea Internet of Things (IoT) in the future,” Zhang says.
More than 70 percent of Earth’s surface is covered by water, encompassing vast areas solar cells could potentially be placed. However, water strongly absorbs sunlight with long wavelengths—those greater than 630 nanometers, corresponding roughly to red and beyond to infrared, microwave, and radio wavelengths. This limits conventional solar panels, which can absorb sunlight with wavelengths ranging from roughly 400 to 1,100 nm, or from blue to infrared.
To overcome this challenge, Zhang and his colleagues designed solar cells that could make full use of shorter wavelength sunlight. They fabricated perovskite solar cells that cover the spectrum of light that does filter through five to 10 meters of water, which consists mostly of blue-green 400 to 600-nm wavelengths.
The scientists found that while their photovoltaic cells attained a light-to-electricity conversion efficiency of 17.08 percent under the conditions usually found on land, they achieved 34.71 percent efficiency when given the spectrum of light one would expect submerged 10 meters underwater. Most standard land-based solar panels achieve an average efficiency between 20 and 25 percent.
The researchers also integrated their cells into underwater mini-robots equipped with rechargeable lithium-ion batteries, deploying them near Weizhou Island in the South China Sea. They encapsulated these cells in glass, synthetic rubber, and epoxy resin to protect them from seawater. The robots could automatically maintain a fixed depth to help the scientists see how well the cells performed at different distances underwater.
At a depth of 2 meters, 115 square centimeters of these encapsulated cells could generate a modest 1,416 milliwatt-hours over two hours—roughly half the capacity of a rechargeable AA battery; at a depth of 10 meters, these cells could still generate 324 mWh of energy over two hours. The performance of these cells at 10 meters exceeded Zhang’s expectations by about three- to six-fold, he says.
Compared to conventional silicon solar panels, perovskite solar cells are cheaper, easier to make, and more efficient, making them a strong choice for underwater applications. But they also often rapidly degrade in performance under intense light and high temperatures. In contrast, the low temperatures and weak light typically found underwater could allow for more durable operation.
The researchers tested this possibility with lab experiments where the new cells were subjected to the kind of light seen underwater at 10 meters. Given the minimal level of degradation the researchers saw at standard operating temperatures of 25 °C—the annual average water temperature near Weizhou Island—they estimated the cells were capable of continuously operating for roughly 5.5 years before degrading to 80 percent of their original efficiency. (In contrast, standard perovskite cells begin to deteriorate after just one year of use on land, although researchers worldwide are investigating ways to significantly extend perovskite cell lifetime.)
“The submerged solar cells show application potential as a real solution for underwater energy,” Zhang says. “Massive deployment of submerged solar cells could extend photovoltaics from land and space into the ocean, addressing the urgent demand for durable offshore energy supply. Ultimately, this could support the growth of the blue economy and may open a new chapter for marine energy.”
Submerged photovoltaics face a number of potential obstacles, Zhang notes. For instance, ocean currents could make it difficult to deploy these devices. Seawater corrosion and encrusting from barnacles and algae could also reduce their long-term performance.
In the future, Zhang would like to see just how deep underwater solar cells can operate. He would also like to integrate them with energy storage devices to create all-in-one units to accelerate practical applications. In addition, standardized testing protocols are needed to analyze submerged photovoltaic performance; these could enable comparisons across different labs to advance the development of these devices, he notes.
The scientists detailed their findings on 11 September in the journal Joule.
Charles Q. Choi is a science reporter who contributes regularly to IEEE Spectrum. He has written for Scientific American, The New York Times, Wired, and Science, among others.
