9 Solar Module Technology
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which one glass sheet is usually adopted as a substrate for the deposition of the
semi-conducting/absorber layers. Thin film modules generally make use of a
single layer of encapsulant rather than two layers as for c-Si.
9.2.2 Materials
9.2.2.1 Front Covers
Most modules make use of a transparent glass sheet as front cover. In general, a sodalime glass with a low iron (Fe
2+ ) content (so-called “solar-grade glass”), which
reduces the absorption of light, is used. To improve its mechanical stability and
protect the module against impacts (hail, transport, handling, etc.) the glass sheet
needs to be tempered (or heat-strengthened
3 ). A standard glass thickness is 3.2 mm;
although, for cost and weight reasons,
4 efforts are posed to reduce the thickness of
the glass. BIPV products, which may be exposed to high wind loads make use of
thicker glass (4–6 mm or thicker). Depending on its composition, glass blocks a
considerable part of the UV-B radiation (280–320 nm wavelength), but only a part
of the UV-A radiation (320–400 nm wavelength).
Some special products (e.g. flexible thin film modules) use transparent polymers,
such as ETFE (ethylene tetrafluoroethylene) as front covers. These polymers are
highly transparent and can provide an excellent barrier against water ingress, but are,
at the moment, also very costly.
9.2.2.2 Rear Covers
Most solar modules have a composite polymer sheet as backsheet generally formed
by a multi-stack structure of three layers with an overall thickness of 280–400 μm.
A typical backsheet structure is made of PVF/PET/PVF where PET (Polyethylene terephthalate) is a polyester, which isolates the module electrically and provides mechanical stability, whereas PVF (polyvinyl-fluoride, also known with the
commercial brand Tedlar
® ) protects the internal circuit (and PET layer) from weathering agents (moisture, dust, UV, etc.). Several other materials are used to manufacture backsheets. PVF (in the outer and inner layers) is often replaced by PA
(polyamide), by other fluoropolymers, such as PVFD (polyvinylidene difluoride), or
3 There are two different types of heat-treated glasses: tempered and heat-strengthened. With tempered glass, the cooling process is accelerated to create higher surface compression and/or edge
compression in the glass. This is the process that makes the glass four to five times stronger and
safer than annealed or untreated glass. With heat-strengthened glass, the cooling process is slower,
which means the compression strength is lower. Heat-strengthened glass is approximately twice as
strong as annealed, or untreated, glass.
4 Glass weighs approximately: ~2.5 kg/mm/m 2 .
227
which one glass sheet is usually adopted as a substrate for the deposition of the
semi-conducting/absorber layers. Thin film modules generally make use of a
single layer of encapsulant rather than two layers as for c-Si.
9.2.2 Materials
9.2.2.1 Front Covers
Most modules make use of a transparent glass sheet as front cover. In general, a sodalime glass with a low iron (Fe
2+ ) content (so-called “solar-grade glass”), which
reduces the absorption of light, is used. To improve its mechanical stability and
protect the module against impacts (hail, transport, handling, etc.) the glass sheet
needs to be tempered (or heat-strengthened
3 ). A standard glass thickness is 3.2 mm;
although, for cost and weight reasons,
4 efforts are posed to reduce the thickness of
the glass. BIPV products, which may be exposed to high wind loads make use of
thicker glass (4–6 mm or thicker). Depending on its composition, glass blocks a
considerable part of the UV-B radiation (280–320 nm wavelength), but only a part
of the UV-A radiation (320–400 nm wavelength).
Some special products (e.g. flexible thin film modules) use transparent polymers,
such as ETFE (ethylene tetrafluoroethylene) as front covers. These polymers are
highly transparent and can provide an excellent barrier against water ingress, but are,
at the moment, also very costly.
9.2.2.2 Rear Covers
Most solar modules have a composite polymer sheet as backsheet generally formed
by a multi-stack structure of three layers with an overall thickness of 280–400 μm.
A typical backsheet structure is made of PVF/PET/PVF where PET (Polyethylene terephthalate) is a polyester, which isolates the module electrically and provides mechanical stability, whereas PVF (polyvinyl-fluoride, also known with the
commercial brand Tedlar
® ) protects the internal circuit (and PET layer) from weathering agents (moisture, dust, UV, etc.). Several other materials are used to manufacture backsheets. PVF (in the outer and inner layers) is often replaced by PA
(polyamide), by other fluoropolymers, such as PVFD (polyvinylidene difluoride), or
3 There are two different types of heat-treated glasses: tempered and heat-strengthened. With tempered glass, the cooling process is accelerated to create higher surface compression and/or edge
compression in the glass. This is the process that makes the glass four to five times stronger and
safer than annealed or untreated glass. With heat-strengthened glass, the cooling process is slower,
which means the compression strength is lower. Heat-strengthened glass is approximately twice as
strong as annealed, or untreated, glass.
4 Glass weighs approximately: ~2.5 kg/mm/m 2 .