2011-11-28 | Editor : John Landers 7632 pageviews

The Solar Glass and Reflector Value Chain

Solar modules require tempered solar glass to protect interior components against the elements. In thin film applications, glass function as a substrate for the deposit of the charging material, such as silicone. Coated glass forms the foundation for reflector technology used in concentrated solar plants collectors, parabolic troughs, central receivers and parabolic engines.

Reflectors also work for concentrated photovoltaics. An anti-reflective coating improves the transmission rate of sunlight, which increases efficiency.

Commercial Glass Composition

Solar glass has the same basic chemical make up as most commercial glass. The chief element consists of Silicon Oxide orSiO2, a derivative of sandstone. The first step requires heating raw material, including sand, sodium carbonate, dolomite limestone sodium sulfate and other components in float glass furnace, at temperatures about 1,500 degrees Celsius (C). The molten glass undergoes a solidification process on molten tin. As the mixture gradually cools, it flows off the molten tin and forms float glass.

Float glass requires additional refinement to reach the standards required for the solar glass manufacturing process. The manufacture reheats the float glass above 1560 degrees C. to create annealed glass. This produces anneal glass, which breaks into large pieces when fractured.

The next step heat-treats annealing glass in an annealing oven at a temperature of 580 to 620 degrees C. The glass goes through an electronically controlled cooling, process at a specific rate based on the thickness of the glass and produces tempered glass. Tempered glass has up to six times the strength of annealed glass, and when broken—shatters into small fragments.

An important element for creating solar glass concerns the removal of the iron impurities found in most commercial glass. Iron salts, contained within the SiO2,   impairthe sun’s transmission through the glass.

The final step involves application of thin coatings materials on one side of the glass. The coating provides the anti-reflection characteristics of the glass, which enhance conductivity and provide self-cleaning capabilities.

Solar Glass Characteristics

The solar industry measures three values for glass used in solar applications: transmission, mechanical strength and specific weight. Transmission concerns the measurement of the energy emitted from the light for a certain glass and glass width. The industry measurestransmission in three ways:  solar transmission, light transmission and PV transmission.

Solar transmission considers the ratio of total energy from the total light spectrum (300- 2,500-nanometer or nm wavelength.) Light transmission -- the ratio of the energy covering the visible spectrum (380 – 780nm.) PV transmission covers the ratio of the total energy from AM 1-5 source---- quantum efficiency of the average crystalline silicon solar cell. AM 1-5 refers to irradiation. Quantum efficiency measures the modules sensitivity to light.

Untreated commercial glass has a transmittance rate of 83.7 percent. This means 16.3 percent of the sunlight fails to strike the photovoltaic material. Some energy loss results from the iron impurities in the glass, but also partly due to reflection off the glass surface.

In terms of mechanical strength, glass has significant innate strength—it localized stresses that cause issues—because of the tendency to quickly brittle facture. Manufacturers measure mechanical strength in accordance t with the direction of the applied force. Solar modules manufacturers measure the glass tensile strength as expressed in Pascal (Pa) or unit of pressure.

For comparison sakes, anneal glass has a mechanical strength of 45 Megapascals (MPa) compared to tempered glass, which has a mechanical strength of 120MPa. The density of glass for the average crystalline module consist of 3mm and makes up about 67 percent of the solar panel’s weight. Thin film solar panels require two laminated layers of 3-millimeter (mm) glass and equates to approximately 96 percent of thin film panels.

Flat Glass for Manufacturing Solar Modules

Crystalline and thin film solar panels require flat glass. Photovoltaic module manufacturers use a pattern glass. Thin film panel modules make the substrate and clear back glass from float glass. The plate glass process molds patterns, such as diamonds or mattes, into the glass surface by sending it through engraved rollers. The pattern enhances the lamination process, and increases the non-binding attributes of the glass. It also creates a better appearance for solar modules.

Eighty percent of the PV glass market consists of pattern glass. Pattern glass has a basic solar transmission over 91.4 percent. The anti-reflective coating increases total solar transmission above 91.4 percent. The high cost of crystalline polysilicon modules PV installation necessitates the use of high quality solar glass. According to Guardian, 90 percent of the solar glass consists of pattern glass.

Amorphous Silicon and Cadmium Telluride-based panel’s use 89 percent float glass. The lower efficiency and area cost does not justify the higher cost pattern glass. Thin film manufacturers that use Copper Indium Selenide technology require low iron float glass, with solar transmittance greater than 90 percent. The solar glass requires a coat of Molybdenum—a thermal conductive oxide (TCO), which enhances the glass conductive.
Major Manufacturers and Challenges

The commercial float glass segment has four primary manufacturers: Asahi, NSG Group (Pilkington), Saint Gobain and Guardian. These companies account for over 60 percent of worldwide production of the high quality float glass. Because float glass used in solar applications make up less than one percent of the market for these companies.

Manufacturers tend to focus the bulk of their production capacity on construction and automobile glass. This could lead to supply pressures because manufacturers do not have productions lines specifically for solar glass. About 70 percent of the manufacturing costs of float glass raw material and energy consumption.

Transportation also constitutes a large portion of costs—up to 25 percent of the total cost. Glass manufacturers that locate their plant close to their customers have a major advantage over the competition.
The technological challenge for solar manufacturers consist of
·         Creating glass with higher transmission (ultra –bright) to increase solar module efficiencies
·         Manufacture glass with the mechanical strength to hold up to wind and snow
·         Laminating and coating technology depending on the application
·         Self-maintenance qualities to reduce maintenance expenses
Anti reflective coatings have the no coated technology, which increases the transmission of sunlight between 3 to 4 percent. The coating also eliminates the need for cleaning because the coating applied to the glass surface is self-cleaning.


Building Solar Glass Windows
Source: NREL

Other then construction glass, the industry does not have a suitable substitution for solar glass. Although, manufacturers have developed float glass that has transmittance qualities close to that of pattern glass, which some panel makers already employ in the production process.
Industry analysts expect glass, with antireflective coatings, to become the norm for both polycrystalline and thin film modules.

Solar Mirrors

Large-scale solar energy plants generate electricity  by positioning reflectors or mirrors to concentrate sunlight on PV panels or a thermal receiver,. Reflectors have two important properties: a high solar reflectance and good specular reflectance characteristics. The precision of curvature also affects the properties. Solar reflectance measures the total energy reflected from the sunlight.
 Specular reflectance measure the ratio of direct light reflection. Unlike diffuse light, the solar energy system recovers direct light. Precision of the mirror’s curvature concerns the percentage of the energy of the reflected light received at the target area—around the focal point, such as 99.2 percent within 75mm or 97.5% within 45mm.
For comparison sakes, the average household mirror has a reflectivity of about 80 percent. Attaining a 93 percent reflectivity for solar mirrors, represent significant progress. The net reflectivity includes the mirror’s cleanliness. Multiply the average 96 percent cleanliness value by the specular reflectivity to calculate the net reflectivity. The objective of manufacturers is to achieve a high specular reflectance over the life of the system.

Types of Mirrors

Mirrors or glass reflectors must meet the stringent manufacturing specifications for use  in concentrates solar power  (CSP)or concentrated photovoltaic systems (CPV). To obtain the parabolic or other non-flat shapes, the glass goes through a hot-bending treatment under heat of 700 degrees C. It has to meet the product manufacturers’ specification– especially strength. Solar thermal energy collector, made of long parabolic mirrors, have a bent or curve-like shape.
Collector tubes  runs along the focal point. The reflection of the sunlight off the mirrors multiplies as it strikes the  tubes, heating the liquid. Mirror, coated with silver or polished aluminum, must retain the proper strength required to support various processes and manufactured equipment.

Types of mirror used in concentrator technology  include:
·Thick glass, 5mm silver-coated glass
·Thin glass – 1mm thickness
·Aluminum Front consists of aluminized polished aluminum reflector with a nano composite oxide protector layer. It weighs less than thick glass, but has a lower reflectance.

One manufacturer, ReflecTech offers a polymer film technology, with a layer of silver,  which provides a mirror-like reflectance surface. The material, ReflecTech® Mirror Film, offers a suitable replacement for glass mirrors. The company claims the performance sturdiness compares favorably to mirrors. In addition, the polymer films will not break. Manufacturers use a pressure laminator to apply an aluminum tape to seal the seams and edges.

Key reflector manufacturers include Alanod-Solar, Almeco, Flabeg, Reflectech Inc and Sobel.

Conclusion

The solar energy industry continues to demand higher performance and lower costs as the technology moves toward grid parity with fossil fuels. As the solar industry continues to grow, the solar glass offers significant profit potential. The business model of traditional commercial glass firms do not align with the needs of the solar companies. New firms dedicated to solar glass and  mirror technology, will find this niche attractive.

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