Solar Glass & Mirrors
Glass is used in photovoltaic modules as layer of protection against the elements. In thin-film technology, glass also serves as the substrate upon which the photovoltaic material and other chemicals (such as TCO) are deposited. Glass is also the basis for mirrors used to concentrate sunlight, although new technologies avoiding glass are emerging.
Chemical Composition of Glass
Most commercial glasses are oxide glasses with similar chemical composition. The main component is Silicon Oxide, SiO2, which is found in sandstone.
Annealed Glass: The components are heated in a furnace at temperatures above 1560°C and cooled down slowly after the forming process, resulting in annealed glass..
Tempering: Glass is heat-treated by heating annealed glass to ~620°C and then rapidly cooling by airflow. As a result, tempered glass is about 4 times stronger than annealed glass. In addition, tempered glass breaks into small fragments, reducing probability of serious injury.
Iron Impurities: Most glass contains iron impurities in the form of iron salts within the silicon oxide that impair tthe transmission of light through the material. Sources for low iron glass include low iron sand and limestone. To produce low iron flass, furnaces must be designed to handle higher melting and refining temperatures.Coating: Thin layers of coating may be deposited on one side of the glass for anti-reflection, improved conductivity or self-cleaning.
For solar applications the main attributes of glass are transmission, mechanical strength and specific weight.
Base-line commercial glass has a solar transmission of 83.7%. I.e. 16.3% of the sun's energy do not even get to the PV material. The energy loss is due - in equal parts - to reflection on the surface and absorption within the glass due to iron impurities.
Specific WeightThe density of glass is about 2,500 kg/m3 or 2.5kg/m2 per 1mm width. Typical crystalline modules use 3mm front glass, whereas thin-film modules contain two laminated glass layers of 3mm each for front and back. As a result, assuming 3mm glass, 96% of the weight of a thin-film module and 67% of a crystalline module is glass!
Mechanical StrengthGlass has great inherent strength. However, as it can not not reduce localised stresses, it is subject to rapid brittle fracture. There are a number of measures for mechanical strength depending on the direction of the applied force. For the purpose of solar modules, the most significant measure is the tensile strength, a measure of pressure expressed in Pa (Pascal).
Flat Glass for Solar Applications
Solar applications require flat glass. So-called Pattern Glass is mostly used as front glass in crystalline modules, whilst float glass is used for both substrate and back glass in thin-film modules.
The Solar Glass Challenge
The objectives for solar glass are:
Choice of Glass
|Rationale||Choice of glass|
|Crystalline Silicon Module||High cost of photovoltaic materail per area requires top of the range solar glass||Pattern Glass with transmission > 91.4%, plus antireflective coating, resulting in total solar transmission > 94%|
Amorphous Silicon, CdTe
|Lower cell efficiency and cost per area do not warrant the marginal costs for ultra clear glass||89% float glass|
|Thin-film CIS / CIGS||Higher cost of pv material per area warrant cost for higher quality glass||Low iron float glass, solar transmission > 90%. Plus a cotaing of Molybdenum to optimize conductive characteristics of the CIS and CIGS layer. Molybdenum is a TCO (Thermal conductive oxide).|
The reflector should have high solar reflectance and good specular reflectance properties. Similar to the definition of transmission in glass, for mirrors it is the ability to reflect:
|Solar Reflectance||Ratio of total energy that is reflected from an AM1-5 source.|
|Specular Reflectance||Measures the ratio of the energy of the direct light that is reflected from an AM1-5 source. This is an important measure, as some light is reflected as diffuse light and can not be focused.|
|Precision of curvature||The precision of the mirror is usually expressed in the percentage of the energy of the reflected light that hits a target area around the focal point. For instance: 99.5% within 70mm, 98.8% within 60mm and 95% within 40mm target.|
Household mirrors have a reflectivity in the 80% range. For solar mirrors, 93% would be an excellent value. However, net reflectivity must also take into account the cleanliness of the mirror. Typical value for cleanliness is 96%, which would have to be multiplied by the mirror's specular reflectivity.
The aim for solar mirror is for high specular reflectance over and extended lifetime.
|Type||Company||Description||Specular Reflectance||Cost [$/m2]|
|Thick Glass||Flabeg||5mm silver-coated glass. Parabolic (or other non-flat) shapes can be acheived through hot bending at 700°C.||94%||40|
|Thin Glass||Naugatech||Stability may be an issue at 1mm.||93 - 96%||15 - 40|
|Aluminium Front||Alanod, Almeco (Vegaflex)||Aluminzed polished aluminium reflector with nano composite oxide protective layer. Weighs 6.8 kg/m2 - less than thick glass. But lower reflectance.||> 87%||< 20|
|Laminate||Reflec Tech||Silverized polymer film on a polymer substrate, laminated to aluminium. Needs a hard coat for required strength.||94%||20 - 30|