Page 97 - Applied Photovoltaics

sampled at random from a production batch that have been subjected to the
manufacturer’s normal inspection quality control procedures. Each module undergoes
a different sequence of tests to check the electrical, optical or mechanical construction
of the module type. A module type meets the qualification requirements if each
sample meets all of the following criteria:
1. There is no evidence of a major visual defect.
2. The degradation of maximum output power at STC is less than 5% after each
test and 8% after the sequence.
3. Insulation resistance and high-voltage tests are passed.

4. No sample exhibits any open circuit or ground fault.
The most important aspects of environmental protection are discussed below.

5.7 ENVIRONMENTAL PROTECTION
(After Treble, 1980; King et al., 2000 and Ecofys BV, 2004.)

The module must be able to withstand such environmental conditions as dust, salt,
sand, wind, snow, humidity, rain, hail, birds, condensation and evaporation of
moisture, atmospheric gases and pollutants, and diurnal and seasonal temperature
variations, as well as maintaining performance under prolonged exposure to UV light.

The top cover must have, and maintain, high transmission in the waveband 350–
1200 nm. It must have good impact resistance and a hard, smooth, flat, abrasion-
resistant, non-staining surface, which promotes self-cleaning by wind, rain or spray.
The entire structure should be free of projections, which could result in the lodgement
of water, dust or other matter.

Moisture penetration is responsible for the majority of long-term module failures,
with condensation on the cells and circuitry causing shorting or corrosion. Hence, the
encapsulation system must be highly resistant to the permeation or ingress of gases,
vapours or liquids. The most vulnerable sites are at the interface between the cells and
the encapsulating materials, and at all other interfaces between different materials.
The materials used for bonding must also be carefully chosen to be able to maintain
adhesion under extreme operating conditions. Common encapsulants are ethylene
vinyl acetate (EVA), Teflon and casting resin. EVA is commonly used for standard
modules and is applied in a vacuum chamber, as is Teflon, which is used for small-
scale special modules and which does not require a front cover glass. Resin
encapsulation is sometimes used for large modules intended for building integration.
Tempered, low iron content and rolled sheet glass is currently the most favoured
choice for the top surface because it is relatively cheap, strong, stable, highly
transparent, impervious and has good self-cleaning properties. Tempering helps the
glass withstand thermal stress. Low iron glass allows up to 91% of the light to
penetrate. A recent development is the availability of glass with anti-reflective
coatings applied by caustic processes or dip coating, resulting in up to 96%
transmission. Tedlar, Mylar or glass are commonly used for the rear of the module, to
act as a moisture barrier, but all polymers are permeable to some degree. Typical
short-term loss in performance of modules owing to dust accumulation and soiling in

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