Solar Cell Glass: Hardness of glasses

[nickfarfol]

Hello,

I am choosing glasses for a novel solar cell design, and part of the criteria is if the glass will be able to withstand hail and other things (?) falling from the sky, since this glass will be the cover glass for the entire solar cell. I can gather information on the glass including Knoop/Moh's hardness, Young's modulus, etc., but 1. I'm not sure which of these structural properties translates to being able to withstand things like hail and 2. If it's hardness, how hard is hard enough/how can I determine this?

Any help is greatly appreciated. Thanks.

 

[EdStainless]

It is a combination of strength (proportional to hardness) and the structural stiffness of the entire unit.
If it flexes you can't make it strong enough, you need a balance of the two.
But fist you should be looking at the optical properties of the glass, you don't want to block usable wavelengths of light.

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Plymouth Tube

 

[nickfarfol]

Thanks. I am looking at a wide variety of properties with the glass, including transmission range, transmission percent, thermal conductivity, etc. The one I don't know is how to determine if it will withstand hail or anything like that. The cell is only 5x5 cm so it doesn't need to support that much weight, but I just need to know about how the hardness translates to ~~"resistance to being destroyed by hail."

 

[IRstuff]

Well, seems to me that you want to define what the hail looks like, before you can determine whether you'll survive an impact. What impact velocities, etc.? What statistical percentage of impacts to survive?

If it's only 5 cm across, then the obvious answer is to increase the thickness of the glass. A 1/8-inch thickness would probably resist the majority of impacts, barring those that would crush everything around your solar cell. Note also, the smaller gap between the cell and the cover glass, the less opportunity for breaking the cover glass.

TTFN
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[TVP]

nickfarfol,

The short answer is that there is no single property like hardness or modulus that translates into x amount of resistance to hail. Impact conditions, whether it is from hail, a dropped hand tool like a hammer, or something else entirely, is a complicated, dynamic event governed mostly by fracture mechanics. Fracture of brittle ceramics like glass depend a great deal on the presence of microscopic flaws, and designing for impact either requires knowledge of the statistical distribution of these flaws (within a part, lot-to-lot variation, etc.) or considerable product testing under realistic conditions.

 

[sreid]

Take a look at Gorilla Glass.

http://en.wikipedia.org/wiki/Gorilla_Glass

 

[jedward]

Look for tech information by John Pepi who has studied this topic re damage to glass
separate comment: I do not think there is usually a gap above the solar cell, clear adhesive used

 

[TomDOT]

Which structural property?

Impact testing.

Something along the lines of ASTM F3007 (though it's specific to architectural laminated glass, it gives you the idea. Drop a steel ball from a known height and see what happens!)

 

[glass99]

Way late to the discussion here, but:
- you want glass both as thin as possible and low iron for optical performance
- tempering will increase impact resistance, as will chemical strengthening. Tempering is much cheaper.
- Partly in response to the needs of the solar industry, the tempering machines now available are designed for thin glass. I don't have a number at hand, but I believe they can temper down into the 1-2mm thick range for relatively large panels.
- Alumina silicate glass with chem strengthening like Gorilla or Willow glass would be the Cadillac for this application, but is likely to an order of magnitude more expensive than tempered soda lime glass.