Every year, nearly 5×1024 J of energy is provided by the sun and reaches the earth's surface. This amount is 10,000 times greater than the actual annual energy consumption of the entire world.
by Orietta León.
Among several sustainable energy resources, solar energy has recently evolved as the most important sought-after source of renewable energy due to the abundance of sunlight throughout the year and also technological advances in light energy harvesting.
Over the years, photovoltaic solar cells have managed to be the main source of solar energy use, as they are not only renewable but also safe and pollution-free. Photovoltaic arrays alone provide a relatively inexpensive method of producing electricity with high efficiency. However, factors such as high capital investment and the sensitivity of the glass surface hinder the efficiency of solar panels.
Still, the conversion efficiency of commercial PV modules is as low as 20%, which is attributed to the loss of reflection at the air/module interface and dust accumulation on the modules. As a result, the improvement of solar modules/panels has gained significant attention. This improvement is mainly due to the development of functional coatings for solar panels. Solar energy materials are chosen in such a way that the coatings possess key characteristics, such as absorption efficiency, electrical conductivity, transparency, wettability (i.e. hydrophobicity/hydrophilicity) among others, in order to maximize the performance of the solar panels. In general, the solar module materials should be of high transparency with excellent self-cleaning ability.
However, dust accumulation is one of the main causes associated with lower energy outputs since the power output has a strong correlation with incident solar radiation, and dust layers as well as debris behave as a barrier to incoming rays. A 4 g/m2 dust deposition layer can decrease solar efficiency by 40%. A study conducted found that despite heavy rainfall, the transmission of solar energy panels dropped to 87.6% from 90.7%. The effect of dust deposition on different types of PV panels was investigated and it was found that as the dust deposition density increases from 0 to 22 g/m2, the reduction in output power increases from 0% to 26%. However, it is important to note that dust differs in different parts of the world and factors such as panel orientation, wind direction, and dust composition have a collective effect on the layer of dirt that accumulates on solar panels.
A noticeable decrease in the efficiency of PV modules (e.g., optical properties) within 1-6 months of installation due to dust coverage has been reported in several experiments conducted outdoors in Saudi Arabia, India, Egypt, Libya, Poland, and Mexico. In desert and semi-arid regions where the maximum intensity of solar radiation is reached, it is evident that most of it is being lost due to the deposition of dust in the solar modules. At the same time, cleaning costs in these regions are very high, leading to increased operation and maintenance. These costs can be reduced by coating solar panels with materials that repel dust or make them easy to clean with water spray. Therefore, a solar panel material with excellent self-cleaning property is a sought-after material in the solar energy industry.
Self-cleaning coatings make it easy to remove dust from solar panels, which in turn increases their energy conversion efficiency. Typically, self-cleaning of solar panels is achieved through the use of natural energy, mechanical or electrostatic methods, and nano-film coatings.
Solar panel coatings to increase their self-cleaning property involve two types of films, superhydrophilic and superhydrophobic films. Superhydrophobicity is based on the Lotus effect and photocatalytic hydrophilicity is mainly based on TiO2. In fact, both surface characteristics facilitate the self-cleaning property of the underlying substrate. Superhydrophobic surfaces have a contact angle with water of more than ~150º forming spherical water droplets that roll on the surface and carry dust and dirt with them, while superhydrophilic surfaces have a lower contact angle with water (~5º) that facilitates the complete distribution of water on them taking dust particles with them as it flows.
There are numerous materials for the manufacture of superhydrophobic coatings such as fluorocarbons, silicones, carbon nanotubes, polymeric materials such as polystyrene, polyurea copolymers, polymethyl methacrylate, polycarbonate and poly(vinyl chloride). Superhydrophobic surfaces can also be obtained through organic and inorganic materials.
Artificial superhydrophobic surfaces are inspired by the lotus leaf and are prepared with a combination of micro and/or nanoscale structures using hydrophobic materials with low surface energy. This will be achieved by employing two strategies, either the hierarchical creation of structures (micro and/or nanostructures) on hydrophobic substrates, or by chemically modifying the hierarchical structured surface with materials of low surface energy. For a superhydrophobic surface, the dry phase should have lower surface energy than the wet surface. Moreover, to cope with the reduced surface energy, the shape of the liquid droplet becomes more spherical. The effect of surface energy can be adjusted by the roughness of the surface. Typically, liquid droplets form high contact angles with water on rough, low-energy surfaces, and low contact angles are achieved on high-energy rough surfaces.
Different methods are used for the preparation of coatings for solar panel applications, including solvent-based methods such as electrochemical deposition, layer-by-layer method, sol-gel method, chemical etching, and plasma etching.
Superhydrophobic hierarchical structures can also be fabricated, through techniques that use a gas-phase environment. These methods allow for precise and controlled rough surfaces that can be assisted through electrodes, including plasma etching and chemical vapor deposition.
The development of superhydrophobic self-cleaning surfaces using dry powder coating technology has been widely employed mimicking the structure of the lotus leaf. This approach carries out the coating process without the aid of water or solvents.
Despite having low hardness compared to inorganic materials, polymers have been widely used in self-cleaning coatings due to their surface properties, versatility and ease of formation, in addition to high toughness especially when combined with inorganic materials. As a result, polymer coating is a well-known method that lays out the procedures for producing new surface properties by polymerization. As an example, films formed by plasma graft polymerization can show controllable thickness, compositional uniformity, and dense membrane. Therefore, the plasma-induced graft polymerization technique has been used to prepare a variety of films, such as metal films, inorganic amorphous films, and organic films.
Moreover, surface roughness and porosity play an important role in the creation of superhydrophilic coatings. Some nanoscale porous microstructures and structures are thought to exhibit superhydrophilicity and their wettability can be adjusted by changing porosity and/or roughness. Most superhydrophilic coatings are made of TiO2 nanoparticles. Superhydrophilic surfaces are not usually easy to prepare, and manufacturing methods are typically substrate-specific.
Surface coating methods and materials tend to continually evolve as the need for better-functionalized materials arises. For solar panel applications, self-cleaning capability, photoactivity, and high transparency are some of the most sought-after attributes, however, features such as self-healing and antimicrobials have gained interest recently. Therefore, to generate films that are part of multiple functions, "smart" coatings have been developed that allow molecules to assemble according to their environment.
The function of self-cleaning coatings can be prone to deterioration due to adsorption of moisture or contaminants, and could be recovered by exposure to sunlight. They can also be damaged or even scratched by sand particles in the wind due to them being abrasive. Also decomposition under sunlight is another possible damage in an outdoor environment. These situations tend to lead to a decline in transmitance and affect wettability. In general, coatings have to be continuously redeposited and recovered, which is costly and inconvenient to achieve. Thus, manufacturing coatings with self-healing capabilities is believed to provide an efficient and economical way to maintain optical and other physical/chemical properties of coatings for practical applications.
In recent years, antimicrobial coatings have gained attention to prevent the accumulation of bacteria, especially on glass surfaces such as photovoltaic panels where the spread of bacterial growth colonies could greatly decrease their transmission and efficiency. In addition, this situation can be further aggravated by contamination with dust, sand, soot and pollen. There are several materials used for the development of such coatings, including silver, copper, and metal oxides for photocatalytic effects. These substances combined with superhydrophobicity and transparency make coatings multifunctional and smart to be extended to practical applications.
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