Volume 29, Issue 2 - Summer 2015

Glass Tech

Glass Properties of
Photovoltaic Applications
Considerations When Specifying Glass for PV in Architectural Applications

by Erin Roberts

Among structural materials, glass has many properties that make it uniquely suited for use in the design and fabrication of solar cells, modules and arrays.

But when specifying or fabricating glass for photovoltaic (PV) devices, there are a number of considerations that are important. Some of these characteristics are common to the familiar glass used in architectural and commercial applications; some are very different.

Types of Glass Specified

There are four major categories of glass specified for PV applications:

Annealed glass: Annealed glass is an option suited to some limited PV glass applications where cost is the driving factor and long-term reliability or strength is not. Generally, the term “annealed” glass refers to float glass that has not been through any additional heat strengthening processes. When broken, annealed glass tends to fragment into large and possibly dangerous shards. It should not be used in any application where breakage could cause injury, or when prohibited by building codes.

Heat-strengthened glass: Heat-strengthened glass is approximately twice the mechanical strength of annealed glass of the same thickness and composition. Mechanical strength values should not be confused with surface compression. In addition to being stronger than annealed glass, heat-strengthened glass may also have less optical distortion.

Fully tempered glass: Fully tempered glass is said to be approximately four times as strong as annealed glass of the same thickness and composition. In addition, it has the characteristic of rupturing into numerous, small, and relatively harmless fragments upon breaking–which is ideal in applications where safety is a concern.

Safety tempered glass: While there is no ASTM standard classification for safety tempered glass, there are two federal standards in the U.S. In these standards, tempered glass must comply in applications where human injury may otherwise occur in the event of glass breakage. Such applications include patio doors, shower doors and ground-floor windows in buildings. These standards, respectively known as ANSI Z97.1 (voluntary) and CPSC 16 CFR 1201 (mandatory), specify destructive testing criteria that must be met in order to have the glass certified as safety glass.


Cover Glass or Superstrate

Because the electrical output of any PV device is directly proportional to its ability to collect the maximum light energy, an optically clear material such as glass represents the ideal choice for cover glass applications. However, not all glass is created equal when it comes to allowing the maximum light energy (photons) to reach the photovoltaic collector circuitry behind it. The output efficiency is affected by these three basic properties of the glass:

Transmittance: The percentage of incident light (input) that passes through the glass (output);

Absorptance: The percentage of incident light absorbed by the glass as heat energy;

Reflectance: The percentage of incident light reflected off the glass surface.

Obviously, coatings that keep any amount of light from entering the glass should be avoided, as should any added tints or decorative materials.

The glass should be as flat and smooth as possible and free of any deformations in the surfaces or mid-plane of the glass. In addition, uniform thickness is a factor, since thickness variations can decrease transmittance efficiency.

Mechanical strength is of critical importance. Because the cover glass is exposed to the elements, it is subjected to many stressors, including temperature extremes, wind loads, hail and other impacts.


“While there is no ASTM standard classification
for safety tempered glass, there are two
federal standards in the U.S. with which tempered
glass must comply in applications where human injury
may otherwise occur in the event of glass breakage.”


Back Glass or Substrate

The so-called “back glass” of a PV cell module is a cover glass that providese structural support for the assembled unit and, in some cases, a decorative appeal. The type of glass used may be annealed, heat-strengthened or fully tempered, based on strength specifications, process and cost considerations. Because transmittance is not important, it can be fabricated from standard soda-lime float glass and produced with a tint, pattern or texture to fit into BIPV or spandrel applications. Even slight curvatures can be incorporated to add aesthetic appeal when desired. Attention has to be paid to ensure flatness at the edges to enable efficient fabrication of the laminated assemblies and assure the structural integrity of the assembled modules in the field.

To learn more, visit the “Tech Center” section at GANA’s website, www.glasswebsite.com.

AGG


Erin Roberts
is the director of marketing and communications for the Glass Association of North America in Topeka, Kan.


Architects' Guide to Glass & Metal
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