MIL-HDBK-1003/13A
Table 2-3. A Comparison of Various Materials Used for Collector Covers.
Solar
(Shortwave) (Longwave) Index of Weatherability[a]
Glazing Type
TransmittanceTransmittance Refraction and Durability
(%)
(%)
White Crystal
91.5
2
1.50
Excellent
glass
Low-Iron Tempered
88-89
2
1.51
Excellent
Glass
Low-Iron Sheet
88-89
2
1.51
Excellent
Glass
Tempered Float
84.3
2
1.52
Excellent
Glass
Fiberglass Rein-
85-89
5-10
1.54
Good
forced Plastic
(FRP)[b]
Sheet Acrylic
88-92
10-25
1.49
Average to Good
Sheet
82-90
10-20
1.59
Fair to Good
Polycarbonate
FEP Teflon
92-96
25-26
1.34
Good
Polyester Film
80-87
20-25
1.64-1.67
Fair to Good
Polyethylene Film
88
80
1.64
Poor
[a]
Durability and weatherability of many plastics is still to be determined
by field use. Some testing by NBS (Technical Note 1132) suggests that
over a 24 month period glass covers will lose about 0-2% of their
transmittance and plastic covers about 5-13%. Accumulated dirt
accounted for another 4% average loss over the same period on both types
(however, this dirt could be washed off). Of the plastic covers, FEP
Teflon seemed to have the least weathering (0.5 to 4.3% loss) and the
most dirt accumulation (5.4-7.7% loss).
[b]
Common trade names - Kalwall Sunlight and Tedlar-Clad filon.
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