Transparent coating can prevent growth of algae on underwater solar cells

DTU researchers have developed an important coating for underwater drones and solar cell platforms. The coating will not adorn the drones in different colors, but rather act as a self-polishing layer that keeps algae and other biofouling away from the drones’ built-in solar cells when they are on missions below sea level.

The transparent coating was designed by Professor Søren Kiil from DTU Chemical Engineering, who in collaboration with Assistant Professor Narayanan Rajagopalan, Laboratory Manager Claus Erik Weinell, and a group of students. It allows the sun’s energy to pass through, while also preventing algae and other fouling materials from shielding the solar cells.

SOS from the US Navy

Underwater drones are used for a variety of tasks across the world’s oceans, including surveying the seabed, surveillance and other inspections. But for the drones to function, they must be able to absorb solar energy when the drones position themselves on the ocean surface.

The Office of Naval Research, which develops science and technology programs for the U.S. Navy and Marine Corps, approached DTU and some companies and asked each of them to come up with a solution to the problem.

“Within a few weeks, I came up with an idea and put together a team. Through experimental studies and trials at our test station in Hundested Harbor, we have proven that the idea works. Our coating functions for up to three months without the need for mechanical cleaning of fouling,” Kiil says.

The research team sent transparent panels with the newly developed coating to Florida, where the Office of Naval Research tested the coating by placing the panels under water with subsequent evaluation on solar cells.

The recipe is nanotechnology

Algae-repellent coatings are already widely used in the maritime sector. About 90% of commercial shipping uses chemically active ship bottom coatings (anti-fouling coatings), which consist of complex chemical compositions with microscopic particles. The major challenge was therefore to figure out how the coating could also be used on solar cells.

“Making a coating that is both transparent and stays free of biofouling is difficult. You can hardly add anything to the coating, because it prevents the light from passing through,” Kiil explains.

Therefore, it was a balancing act for the research team to find the perfect composition. When their newly developed coating comes into contact with the seawater, a self-polishing effect begins where the outermost particles in the coating gradually dissolve and are replaced by new active particles that act as a kind of shield against fouling. This way, throughout the life of the coating, new particles will constantly be ready to fight fouling as the top layer of particles dissolve.

With existing methods, a pigment layer would block the sun’s rays, so the researchers had to find a new method. The recipe turned out to be inspired by a 20-year-old model simulation article.

The model predicted that by making the already small particles in the coating even smaller, from micrometers to nanometers, the particles would become sufficiently small to not leave behind a pigment-depleted porous layer that would prevent sunlight from reaching the solar cells. This allows the solar cells to absorb energy, while the particles keep the fouling away.

“We reduced the size of the particles and the amount of ingredients enough for the coating to become transparent. It turns out that nanoparticles of copper(I) oxide and zinc oxide are particularly effective against fouling because they are tiny. Even in ultra-low concentrations, they are present in the coating in very large numbers with very little space between the individual particles. Barnacles and algae therefore experience the coating as a repellent barrier,” Kiil says.

Other solar cells at sea

Not only the U.S. Navy can benefit from the invention: In the North Sea, solar cells float with the movement of the waves as part of future green energy solutions. Although most solar cells are found on roofs and in solar farms, square panels that harness the sun’s energy to produce electricity can also be placed on or in the ocean.

Currently, the global capacity of floating solar cells is only about 4,000 megawatts (MW), but this is expected to increase to 30,000 MW by 2030. In comparison, the total capacity—including land-based solar cells—reached 2 million MW in 2024. Although the number of floating and underwater solar cells is just a drop in the ocean compared with the total capacity, they offer the opportunity to utilize open ocean space and existing infrastructure, such as in between offshore wind turbines.

However, as with drones, underwater solar cells will also be blocked by fouling, and there is a risk that waves will cause favorable conditions for fouling on the floating solar cell platforms. Therefore, the DTU researchers also see a potential for using the new coating on solar cells underwater and on the sea surface. However, right now the next task is to develop a coating that also works on underwater sensors and cameras, which requires an even more complex coating.