In order to fully utilise the potential of solar energy, the absorption and reflection of sunlight is crucial. Solar glasses and mirrors are thus some of the most important components of a solar system. They are at the front line of receiving the solar energy that is converted into electricity. The properties and characteristics of these glasses and mirrors determine how much sunlight is effectively processed by the system. For solar glasses, effective transmission is key while for mirrors, it is their ability to reflect. With improved technologies, the demand for these components is evolving. As the demand for solar power increases, more projects will be set up, which, in turn, will require more solar modules and thus more solar glass.
Structurally, a solar photovoltaic (PV) glass is positioned on top of silicon-based solar cells. It serves the dual function of optically transmitting solar energy from the sun while protecting solar cells underneath. Solar glasses are most often used in solar panels with silicon cells as well as thin-film cells. In thin-film technology, glass also serves as the substrate upon which the PV material and other chemicals are deposited. Pattern glass is mostly used as front glass in crystalline modules while float glass is used for both substrate and back glass in thin-film modules. The solar glass base is often paired with an anti-reflective (AR) coating or other chemical coatings to enhance performance.
As bifacial modules are gaining traction, glass-glass panels have emerged as a new trend. These panels have a glass sheet at the rear end as well, similar to the dual-faced solar cells in bifacial panels. These panels are different from the typical glass-foil solar panels, which consist of a glass plate, with solar cells behind it and synthetic foil on the back. Glass-glass panels have more transparency on both ends of the panel and thus generate more electricity. However, glass-glass panels suffer from problems such as delamination, cracking, corrosion and high power degradation. Glass is also the basis for mirrors used to concentrate sunlight in solar thermal projects, although new technologies sans glass are emerging. Flat glass is commonly used in solar applications.
Coatings and characteristics
Some of the major types of solar glasses based on their coatings and characteristics are AR coated glass, transparent conductive oxide (TCO) glass and tempered glass. AR coating aims to reduce the amount of sunlight reflected off solar panels, thus increasing the amount of light absorbed in cells and delivering higher output. Flash tests performed by industry players have resulted in up to 3 per cent increase in the performance of solar panels as compared to non-coated panels. According to DSM Advanced Solar, about 250 million solar modules across the world, amounting to a capacity of up to 70 GW, have AR coating. Besides AR coating, another technique to reduce reflection is by roughening or texturing the surface. This method is usually adopted in regions where AR coating is not enough to withstand weather conditions, or are too expensive. However, in this case, the panels is more likely to gather dust and dirt. This could lead to advantages of reduced reflection being outweighed by losses incurred due to increased top surface soiling.
TCO glass is also one of the key materials for manufacturing thin-film solar cells. It is produced by the atmospheric pressure chemical vapour deposition process or other processes. This type of solar glass has high transmittance, adjustable sheet resistance and substantial weather resistance. Thin-film cells consist of a stack of extremely thin photosensitive layers sandwiched between a top TCO coating and a back contact. TCO coating can be used with a number of thin-film PV technologies, including amorphous silicon, a combination of amorphous and microcrystalline silicon, cadmium telluride, copper indium (gallium) diselenide and dye-sensitised solar cells.
Tempered glass is a highly preferred option for monocrystalline, polycrystalline or amorphous solar panels. Tempered, low-iron-content glass is most commonly used as it is low cost, strong, stable, highly transparent, impervious to water and gases, and has good self-cleaning properties. Also known as safety glass or toughened glass, tempered glass has up to six times the strength of a normal plate glass. Flat plate glass is used in amorphous or thin-film solar panels as well as in cheaper varieties of monocrystalline and polycrystalline panels. Tempered glass is considered to be safer than other glass sheets. Upon breaking, it shatters into smaller pieces instead of long sharp shards as in the case of normal glass plates. Although tempered glass makes a solar panel durable, it adds to the weight of the panel and may cause balance issues.
In order to generate electricity from solar energy, mirrors are used as solar thermal reflectors in a collector to concentrate the irradiation in a small area. This is usually done on a PV material or a thermal receiver. Typically, the reflectance of solar mirrors is less than one due to the various losses in the process. The desirable properties of a solar mirror are high solar and specular reflectance, low capital costs and durability.
There are different types of solar mirrors available in the market. Some of these are thick glass, thin glass, aluminium front and laminate. A solar mirror contains a substrate with a layer for reflecting solar energy, often along with an interference layer. The solar installation may consist of a planar mirror or parabolic arrays of solar mirrors to increase the concentrated reflection factor. Various types of materials are used for making solar mirror. Glass is the oldest choice and is used to manufacture two kinds of solar mirrors – first-surface and second-surface. In first-surface mirrors, the glass is coated with protective and reflective films positioned on the front side. In second-surface mirrors, the reflective film is positioned on the back side.
Market scenario and outlook
There are several players across the world in the glass and mirrors market. For many glass companies, the solar glass segment is only a small part of their business. AGC Solar is a global glass manufacturer, supplying front glass for solar modules as well as mirrors for concentrating devices. Pilkington is the flat glass business vertical of the NSG Group, which has developed self-cleaning solar glass. Guardian is a major glass manufacturer. Its products include pattern and float glass as well as mirrors for solar applications. Companies such as Guangdong Golden Glass even specialise in glass for BIPV modules. Other significant players in the market for solar glass are Xinyi Solar, Almaden, Anci Hi-Tech, Irico Group, AVIC Sanxin, Huamei Solar Photovoltaic Glass, Taiwan Glass, Saint-Gobain, Interfloat, Xiuqiang, Topray Solar, Henan Yuhua Solar Glass and Targray Solar.
While companies such as Guardian, Saint-Gobain and Pilkington manufacture both glass and mirrors, some firms are more focused on manufacturing reflectors. Examples of such companies are Germany-based Flabeg and Alanod, Italian company Almeco, TG Yueda Solar Mirror (a joint venture of the Taiwan Glass Group and the Jiangsu Yueda Group). The solar mirror market has been growing slowly and steadily over the past few years due to good macroeconomic conditions and more concentrate solar projects being developed. However, due to the large-scale expansion of solar PV projects, the market for solar glass is more active than the market for mirrors.
Driven by the increasing uptake of bifacial modules and larger panel sizes, the demand for solar glass has outstripped supply. The pace of manufacturing capacity addition for solar glass is slower than the rise in demand and thus the costs have also been relatively high. With more projects being implemented, the demand for modules and the pressure on glass manufacturing have increased in the previous year.
A large portion of panel and glass manufacturing is located in China, which is experiencing constricted production due to shortage of solar glass. According to the China Photovoltaic Industry Association, the country is the largest manufacturer of solar PV products. In 2019, China accounted for almost 80 per cent of the world’s solar panel production capacity. The solar manufacturing segment, including glass and mirror manufacturers, took a hit in operations due to the Covid-19 crisis. The dip in glass manufacturing was further pushed by the reduced availability of raw material, energy and other production costs.
Going forward, the segment expects an upturn as both the manufacturing and project development industries make up for lost time and focus on increasing revenues through higher sales and greater capacities.
By Meghaa Gangahar