Flame treatment in photovoltaic applications
Flame treatment is a surface treatment technique used to modify the surface chemistry of several substrates (mainly polymers), making them polar, so improving their surface energy and making them wettable, so ready for following coating/laminating steps. If it is true that flame application and its benefits in the field of flexible packaging are well known, it is also true that not so many people know about the application of this treatment method and its plus in the field of photovoltaic applications.
Here is given a brief summary of this topic.
PV Module and its composition
Solar panel refers to a panel designed to absorb the sun’s rays as a source of energyfor generating electricity or heating. A photovoltaic (in short PV) module is a packaged, connected assembly of typically 6×10 solar cells. Solar Photovoltaic panels constitute the solar array of a photovoltaic system that generates and supplies solar electricity in commercial and residential applications. Each module is rated by its DC output power under standard test conditions, and typically ranges from 100 to 365 watts. A photovoltaic system typically includes a panel or an array of solar modules, a solar inverter, and sometimes a battery and/or solar tracker and interconnection wiring.
Most PV bulk silicon PV modules consist of a transparent top surface, an encapsulant, a rear layer and a frame around the outer edge. In most modules, the top surface is glass, the encapsulant is EVA (ethyl vinyl acetate) and the rear layer (back-sheet). Let we briefly introduce each layer:
· The front surface of a PV module
must have a high transmission in the wave-lengths which can be used by the solar cells in the PV module. In addition to its reflection and transmission properties, the top surface material should be impervious to water, should have good impact resistance, should be stable under prolonged UV exposure and should have a low thermal resistivity. In most modules the front surface is used to provide the mechanical strength and rigidity, therefore either the top surface or the rear surface must be mechanically rigid in order to support the solar cells and the wiring. There are several choices for a top surface material including acrylic, polymers and glass. Tempered, low iron-content glass is most commonly used as it is low cost, strong, stable, highly transparent, impervious to water and gases and has good self-cleaning properties.
· An encapsulant is used to provide adhesion between the solar cells,
the top surface and the rear surface of the PV module. The encapsulant should be stable at elevated temperatures and high UV exposure. It should also be
optically transparent and should have a low thermal resistance. EVA (ethyl vinyl acetate) is the most commonly used encapsulant material. EVA comes in thin sheets which are inserted between the solar cells and the top surface and the rear surface. This sandwich is then heated to 150 °C to polymerize the EVA and bond the module together.
· The key characteristics of the rear surface (back-sheet) of the PV module
are that it must have low thermal resistance and that it must prevent the ingress of water or water vapor. In most modules, a thin polymer sheet is used as the rear surface. Some PV modules, known as bifacial modules are designed to accept light from either the front or the rear of the solar cell. In bifacial modules both the front and the rear must be optically transparent.
· A final structural component of the module
is the edging or framing of the module. A conventional PV module frame is typically made of aluminum. The frame structure should be free of projections which could result in the
lodgment of water, dust or other matter.
PV Module back-sheet and its service life issues
During its lifetime, a solar module will be exposed to the environment and challenged by UV radiation, temperature cycles and mechanical loads, including snow and wind.
The backsheet, is a critical component in a solar module, since it is responsible for protecting and electrically insulating the module itself.
The defects of the most concern for backsheets include cracking, yellowing and delamination. The visual aspect of the solar modules as this can be an early and first indicator of failure, that can lead to more serious problems. While modules containing backsheets, that have cracked and delaminated represent actual failures of the backsheets, yellowing of the backsheet known to be an early potential predictor of backsheet failure.
Typically, encapsulant yellowing is a result of UV light exposure. The encapsulant is formulated with cross-linking agents, free radical scavengers and UV blockers to prolong the life of the module. Although the EVA can yellow on its own, these additives have also been known to yellow.
This yellowing phenomenon can reduce transmission by up to 5%, directly impacting power. It is typically associated with degradation of the polymer which, in severe cases, can result in the embrittlement of the backsheet. Day-night thermal cycles on a brittle backsheet can lead to cracking and/or delamination, resulting in loss of electrical protection of the module.
A high rate of backside yellowing was observed in polyester-based backsheets, while frontside yellowing occurred most prominently in PVDF-based backsheets.
Yellowing typically represents degradation of the polymer that can lead to backsheet cracking, which can represent a safety hazard for anyone conducting operations or maintenance procedures.
For standard modules that use EVA encapsulation, for the backing can be used a layer of Tedlar composite (Tedlar-Polyester-Tedlar), known as TPT.
Tedlar is the DuPont tradename of a film of polyvinyl fluoride, PVF, laminated with polyethylene terephtalate (PET), typically two layers of PVF on either side of a core layer of polyester. This backsheet, presents the lowest occurrence of the above mentioned defects, performing well with no major defects. TPT backsheets for PV modules present excellent strength, weather resistance, UV resistance and moisture barrier properties.
These properties significantly improve module life, allowing module warranties up to 25 years vs. 10 years warranties of specially treated PET film laminates.
So the use of TPT backsheets avoids the need to replace PV modules as frequently.
Flame treatment application for improving TPT adhesion properties
Notwithstanding the above TPT excellent properties, achieving a good adhesion of TPT laminate to EVA encapsulant can be an issue, especially when you have to do with O-Tedlar (oriented Tedlar); several producers use acrylic primer to improve this adhesion (in the past requested peeling strength between Tedlar and EVA layers used to be higher than 8N/cm, while actually this limit has been increased up to 40N/cm), but the effect of the primer tends to disappear after few months and its price is high, not to mention environment issues.
An other method to improve the adhesion is hot calendering (Tedlar and EVA are laminated thanks to combined action of heat and pressure exerted by pinching rolls) but, in this case, lamination speed is limited to 10 ÷ 15m/min (33fpm ÷ 49fpm).
An interesting solution to this adhesion issue is given by flame treatment.
Applications of this surface treatment technique are well known in the field of polyolefins films and 3D components, or on paper and paperboard, for preparing the surfaces (treating them) of these materials to following coating processes.
TPT lamination with EVA
Not so well known are the benefits, in terms of adhesion, coming from flame for the specific application of TPT lamination with EVA.
In following image is shown a typical flame treatment installation for Photovoltaic applications for rewinding speeds of 50mpm and higher.
Typically Flame Intensity values [kcal/m2], a parameter that represents the amount of m2 energy given by flame to the web, in flame treatment applications, vary in the range 2.5 kcal/m2 to 3.5 kcal/m2 depending on web type; when it comes to PVF polymers, then a much higher energy output is necessary.
Flame intensity values of 20 kcal/m2 and higher becomes necessary for correctly treating surface Tedlar webs preparing them for following lamination with EVA.
In order to have available this amount of energy, a double burner configuration (DDBS) for flame treatment system is necessary. As shown in above drawing, the system is formed by two burner heads, each of them capable of 140kW per meter, as output power. Big diameter cooling roll, adjustable in temperature, allows to get web surface without surface optical defects and prevents any web shrinkage.
Below is reported an experimental result (please refer to following page images). Tedlar surface of a TPT laminate has been flame treated at a speed of 50mpm (164fpm); TPT has been then laminated with EVA, at around 150C, for a fixed time frame and under specific pressure/vacuum conditions at the two faces of the laminate.
Then a peeling test, at 100mm/min as speed, has been run.
Very high adhesion (124N/cm as average value) has been achieved, during this peeling test, as shown below, between the Tedlar surface and the EVA surface.
Flame Treatment technology can be successfully applied in photovoltaic applications
Flame Treatment technology, widely applied in the field of flexible packaging can be successfully applied also in photovoltaic applications.
This surface treatment methods represents, in fact, an interesting solution, both in terms of surface properties and of productivity, for making rid of adhesion issues between oriented Tedlar and EVA layers forming a PV module back-sheet.