9 Solar Module Technology
229
Thermoplastics include PVB (polyvinil butyral), TPU (thermoplastic
polyurethane), TPO (thermoplastic polyolefins), and Ionomer. Materials as
EVA (ethylene-co-vinyl acetate) and POE (polyolefin elastomers) used by the solar
industry are specially formulated to become thermosetting polymers.
Over the years, EVA has become the best choice for the industry, because—
without being an optimal material–it combines overall satisfactory properties at a
reasonable cost and a long-track record of deployment in the field.
As of today, in fact, over 90% of the solar modules manufactured and deployed
globally contain EVA as encapsulant. On the other hand, the attempt to shorten process times [6] and the current interest in glass/glass structures (for bifacial modules)
is opening up doors to the adoption of alternative materials. Some market analysts
see promising trends for polyolefins (as an alternative material to EVA) with an
estimated market share of ~30% of the market in 2030.
One drawback of EVA in fact is that, upon exposure to UV (in combination with
heat and moisture), it tends to photo-degrade, generating acetic acid. This acid will
reduce the transparency of the encapsulant and may lead to the corrosion of the
interconnects (ribbons and fingers) and of the solar cell. Due to the “breathability”
of polymer backsheets, conventional glass/foil structures tend to release acetic acid,
partly mitigating the potential impact of acetic acid generation.
In more solid sealing structures (i.e. less permeable structures) such as glass/glass
ones, acetic acid will remain trapped in the inner structure potentially posing a serious
threat to the long-term performance of the modules. For glass/glass modules, therefore, the replacement of EVA by polyolefins (or other encapsulants) can potentially
lead to longer service life-times.
Additionally, in a small market share (<5%) of modules, PVB (polyvinylbutyral)
is employed as encapsulant material. PVB is a material used to manufacture safety
glass and is widely employed in the automotive and building industry [7]. Because
of its mechanical stability and safety properties, PVB is mainly used to manufacture
BIPV (Building Integrated PV) modules consisting of glass/glass structures.
Over the long run, the resistance to weathering and the stability of an encapsulant
largely depend on the additives used in its formulation. If we focus on EVA, as an
example, several additives are added to the base material or resin. These include:
a. Thermally-activated peroxydes used to promote cross-linking at elevated temperatures;
b. Silanes used to promote adhesion between EVA and inorganic surfaces such as
glass;
c. UV absorbers that reduce the effects of UV radiation;
d. Antioxidants.
Over the last thirty years, the formulation of EVA (and of other encapsulants) has
improved considerably. Consequently, the stability of these polymers has increased,
especially with regards to problems of discoloration (yellowing or browning), photothermal degradation, hydrolysis and reduced adhesion.
When selecting a polymer foil as an encapsulant for the manufacturing of solar
modules, the more relevant properties are:
229
Thermoplastics include PVB (polyvinil butyral), TPU (thermoplastic
polyurethane), TPO (thermoplastic polyolefins), and Ionomer. Materials as
EVA (ethylene-co-vinyl acetate) and POE (polyolefin elastomers) used by the solar
industry are specially formulated to become thermosetting polymers.
Over the years, EVA has become the best choice for the industry, because—
without being an optimal material–it combines overall satisfactory properties at a
reasonable cost and a long-track record of deployment in the field.
As of today, in fact, over 90% of the solar modules manufactured and deployed
globally contain EVA as encapsulant. On the other hand, the attempt to shorten process times [6] and the current interest in glass/glass structures (for bifacial modules)
is opening up doors to the adoption of alternative materials. Some market analysts
see promising trends for polyolefins (as an alternative material to EVA) with an
estimated market share of ~30% of the market in 2030.
One drawback of EVA in fact is that, upon exposure to UV (in combination with
heat and moisture), it tends to photo-degrade, generating acetic acid. This acid will
reduce the transparency of the encapsulant and may lead to the corrosion of the
interconnects (ribbons and fingers) and of the solar cell. Due to the “breathability”
of polymer backsheets, conventional glass/foil structures tend to release acetic acid,
partly mitigating the potential impact of acetic acid generation.
In more solid sealing structures (i.e. less permeable structures) such as glass/glass
ones, acetic acid will remain trapped in the inner structure potentially posing a serious
threat to the long-term performance of the modules. For glass/glass modules, therefore, the replacement of EVA by polyolefins (or other encapsulants) can potentially
lead to longer service life-times.
Additionally, in a small market share (<5%) of modules, PVB (polyvinylbutyral)
is employed as encapsulant material. PVB is a material used to manufacture safety
glass and is widely employed in the automotive and building industry [7]. Because
of its mechanical stability and safety properties, PVB is mainly used to manufacture
BIPV (Building Integrated PV) modules consisting of glass/glass structures.
Over the long run, the resistance to weathering and the stability of an encapsulant
largely depend on the additives used in its formulation. If we focus on EVA, as an
example, several additives are added to the base material or resin. These include:
a. Thermally-activated peroxydes used to promote cross-linking at elevated temperatures;
b. Silanes used to promote adhesion between EVA and inorganic surfaces such as
glass;
c. UV absorbers that reduce the effects of UV radiation;
d. Antioxidants.
Over the last thirty years, the formulation of EVA (and of other encapsulants) has
improved considerably. Consequently, the stability of these polymers has increased,
especially with regards to problems of discoloration (yellowing or browning), photothermal degradation, hydrolysis and reduced adhesion.
When selecting a polymer foil as an encapsulant for the manufacturing of solar
modules, the more relevant properties are: