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The US Navy has funded Danish research into preventing biofouling and maintaining visible light transmission in underwater solar cells. The superior solution uses ultra-low concentrations of nanosized pigments soluble in seawater. Power generation remained close to 100% after 13 weeks underwater.
Researchers funded by the U.S. Navy have developed a novel coating for underwater solar cells that prevents biofouling while preserving visible light transmission. Biofouling, the attachment and growth of organisms, can reduce the optical efficiency of solar cells. These cells are used in autonomous and unmanned underwater vehicles for naval surveillance, oceanographic research, and other applications.
“The need for this study is due to the lack of fouling-resistant coating systems that maintain light transmission on their own,” the researchers explain. “Existing coatings require periodic human intervention, such as mechanical cleaning and wiping techniques, to maintain their effectiveness, which is a tedious and laborious process. The proposed self-polishing technology can eliminate the need for mechanical cleaning and offer a more effective and less laborious solution.”
The novel solution uses ultra-low concentrations of seawater-soluble nanosized pigments, such as cuprous oxide (Cu2O) and zinc oxide (ZnO), combined with an organic biocide and a fast-polishing binder. The team explained that when these coatings are exposed to seawater, the pigment particles dissolve, creating a porous layer that allows seawater to diffuse into the coating.
“This leached layer allows dissolved biocidal compounds to diffuse out of the coating and into seawater,” they noted. “The binder matrix of the leached layer reacts with seawater ions and forms soluble compounds in a controlled manner. To balance this and establish a higher or lower polishing rate, self-polishing coatings use copolymers that may or may not hydrolyze in seawater. As hydrolysis continues, the eroded polymer and dissolving pigment fronts expose fresh layers of acrylate polymer and pigments, resulting in a self-polishing effect with a more or less stable leached layer thickness.”
The research team tested different binder systems for the coating, using various particle mixtures. They used silyl acrylate (SA) alone or in combination with rosin (SA-R) in a 70:30 weight ratio for the binders. The particle mixtures included nano-sized cuprous oxide (NC), nano-sized zinc oxide (NZ), the organic liquid biocide SeaNine 211 (SN), micro-sized cuprous oxide (MC) and micro-sized zinc oxide (MZ).
The coatings were applied to 6 mm thick, 200 mm × 100 mm smooth, clear polycarbonate substrates. The coated panels were immersed in Hundested Harbour, Denmark, for two and a half months and inspected and photographed at two, six and ten weeks.
"The combined action of NC, NZ and SN in an SA-R coating provided significant fouling resistance over the 12-week exposure period, attributed to the synergistic rate of dissolution effects and the rapid polishing rate of the SA-R binder," the researchers said.
This champion coating had a pigment volume concentration (PVC) NC ratio of 0.04%, PVC NZ of 0.08%, and a solvent-free (SN) coating base weight percentage of 3%.
“The same formulation showed complete polishing by week 12, after which the coating began to foul, implying a high polishing rate of approximately 1.4 µm/day, exceeding conventional rates of 5-15 µm/month (0.15-0.5 µm/day),” the researchers noted. “Further testing with the formulation was conducted by the Office of Naval Research (ONR) in Florida, where the coating showed exceptional resistance to biofouling in warm seawater (22ºC to 30ºC) under extreme fouling conditions. Solar power generation remained near 100% efficient for 13 weeks.”
The researchers noted that nanosized versions of Cu2O and ZnO are about 20 times more expensive than conventional micron-sized versions. However, they noted that the low volume required in the developed solution keeps formulation costs relatively low.
Researchers at the Technical University of Denmark described their work in “ Self -sustaining antifouling coating for underwater solar cells ,” recently published in Progress in Organic Coatings . |
