May 18 2009
In the world of solar energy production, the more useable sunlight that can be directed into a photovoltaic module's silicon wafers, the more electricity can be produced. PVM designers and manufacturers, therefore, strive to increase the amount of solar energy their solar cells absorb.
To a large degree, the amount of useable light from the solar spectrum that enters the silicon wafer is affected by the encapsulant film that protects the wafer's surface.
For that reason, advances in solar encapsulate materials that resist discoloration are significant to solar panel manufacturers. Discoloration is yellowing or hazing that causes a reduction in the sunlight that penetrates to the silicon wafer, and the goal obviously is to reduce discoloration and extend the period of time during which the encapsulants are at their maximum clarity.
United Initiators, with facilities in Elyria, Ohio; Mobile, Alabama; Pullach, Germany; Syndey, Australia; and Shanghai, China, is the specialist in customized organic peroxides and inorganic persulfate initiators. Peroxides and other cross-linking agents are the sole focus of United Initiators' business, which is set up to supply products to solar panel manufacturers around the world.
To gain understanding about the role that crosslinking agents play in reducing discoloration in PVM silicon wafer encapsulates, it is helpful to understand the nomenclature.
"Peroxide" is the general classification of the organic materials used, while "initiator" is the functional name. People in the solar energy industry refer to peroxides and initiators as "crosslinking agents."
"The solar energy industry is ever on the alert for opportunities to gain increases in the light that penetrates to the silicon wafer to produce more electrical current," said Onofrio Palazzolo, United Initiators commercial manager. "Our customers gain satisfaction with increased performance of PVMs' silicon wafer encapsulates thanks to the crosslinking agents we manufacture for them."
Silicon wafer encapsulant yellowing is a complex issue that has been addressed with excellent results with new crosslinked resins.
Silicon wafers' EVA encapsulation must survive for 20 years of continued daily thermal cycling and retain those properties that are required for PVM performance. Among the various functional properties required of EVA encapsulates are maximum light transmission to the silicon solar cell operating wavelength. The EVA also must retain mechanical properties, maintain a required level of electrical insulation to protect against electrical breakdown and arcing, and remain chemically inert with encapsulated components such as the solar cells themselves, metallization, and electrical wiring.
A paper published in 1983 by the Jet Propulsion Laboratory (JPL Publication 83-35) documents that freshly cured and unaged A-9918 EVA has a total integrated light transmission of near 91 percent (not corrected for surface reflection losses, which would be in the order of 8 percent for normal incident light).
The goal is that even as they age, the EVA encapsulates should not change in transparency so as to reduce light transmission, which would then directly result in a decreased power output from the solar cells.
The goal for a PVM's power output is that it should not after 20 years decrease to less than 75 percent of the initial output. While power output can decrease due to such factors as surface soiling or a decline in the performance efficiency of the solar cells themselves, the EVA's light transmission properties are controllable and can be improved, thanks to advances in crosslinking chemistry that enables the encapsulate material to remain clear throughout its lifetime.
Peroxides have a proven track record in the polymer industry, which historically has used the same initiators. The TBPEHC product used for solar panels has become the standard.