The recipe is nanotechnology
Algae-repellent coatings are already widely used in the maritime sector. Around 90 per cent 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,” Søren 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 micrometres to nanometres, 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,” Søren 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 around 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 amount 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 under water 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.
