A group of scientists in Finland has developed a transparent, superhydrophobic film inspired by the leaves of the purple, which can also improve photovoltaic efficiency
Cleaning of solar cells, the passive approach
To obtain the maximum possible output from the plant, the photovoltaic panels must always be kept clean. Dust and dirt deposited above the module’s surface inevitably reduce the output power, leaving even 50% of the original output. Several approaches to solving the problem exist. One is to make the solar cells passively self-cleaning through the integration of special films or coatings.
An example is the transparent, superhydrophobic film made by a team of scientists from Aalto University in Finland. Thanks to a bioinspired approach, the group produced a biological-based film that facilitates the cleaning of the modules while increasing the conversion efficiency of the cells.
also read A wind-powered electrodynamic screen for clean solar panels
What is the Soiling Effect?
The accumulation of material on the light-gathering surfaces in solar systems has a precise name: Soiling effect. The name is used to identify any deposits of sand, pollen, mud, dust, snow, aerosol and other contaminants. An effect not so short since dirt can absorb, reflect and disperse part of the irradiation. And consequently to lower photovoltaic production.
How much? It’s hard to tell a priori and accurately. The soiling effect represents a site-specific problem that is strongly linked to the local climate. In temperate locations with moderate rainfall, it could cause losses of 5 percent in annual electricity production. In arid and desert conditions or in polluted megalopolises, it is easy to reach (and exceed) 40 percent.
Photovoltaic: the passive self-cleaning coating
There are several approaches to ensuring the cleanliness of solar cells. One of the most practical and fast solutions to the problem is to pre-treat the cells. This class of procedures includes self-cleaning coatings, films and materials applied directly to the surfaces of the modules to help them passively reject dirt. The current supply on the market, however, still faces several challenges. To begin with the difficulty of applying these films in such a way that they are suitable for the site-specific characteristics of the soiling effect. Not only. They must be able to boast of excellent optical transmittance – indicating the ability of a material to let accidental light pass through – not to obscure the cells’ active layer.
Industry research has shown that the combination of self-cleaning and high transparency can be achieved by adapting the surface microstructure on the outer layer of the solar cells. A “surface roughness” also contributes to the so-called dimness that extends the path of light in the photoactive material, translating into a greater conversion efficiency.
It’s a pity that having all these characteristics is very difficult. Superhydrophobicity is associated with a high level of roughness, which usually decreases transparency.
The new self-cleaning solar cells in perovskite This is where the Aalto University study comes into play. Scientists have developed self-cleaning solar cells in perovskite, integrating a new multifunctional “light management layer”. In detail, this layer was made by replicating the surface structure of the purple leaves on cellulose acetate (abundant and renewable bio-polymer). And covering it all with carnauba wax. Why the whore? Because the leaves of this plant reveal a surface covered with micrometre-sized slopes and a layer of epicuticular wax on a nanometer scale, capable of giving it a prominent superhydrophobicity. The bio-inspired layer, scientists explain, thus obtains self-cleaning properties.
How the waterproofing layer increases conversion efficiency
The surface water cleaning counteracts the accumulation of dirt on the photovoltaic panels. But this is not the only benefit. The special coating also has an optical function. The new films boast a high total transmission of about 94%. Not only. The layer also shows a gap of about 54% at a wavelength of 550 nm, which improves cell conversion efficiency by 6 ± 0.3%.
“We have demonstrated how porous-inspired surface structures introduce optical and self-cleaning properties in cellulose-based films, derived from renewable resources, making them interesting for solar panel applications,” explains Hamidreza Daghght Shirazi, Aalto University researcher and co-author of the study published in Communications Materials. “In this way, you could face two significant challenges in solar energy: maintain the cleanliness of the panels and maximize the absorption of light”.
