Whether or not a solar panel manufacturer will exist in the long term is largely dependent on its ability to incorporate product changes in a timely manner without compromising quality.
Lesser quality materials may result in failures in the field. Considering that module materials will remain fixed in production for less than two years (and often much less), how are good material changes differentiated from the bad?
This may seem simple, since the IEC publishes minimum design qualification and safety tests (the so-called “IEC retest guidelines”) that must be passed to incorporate a change into an already certified product. But, passing only boilerplate tests can leave the product’s reliability in question. Enhanced test methods should be used to identify materials worthy of assimilation into the product bill of materials. The following provides a summary of additional points to consider when screening changes to a few key module subcomponents.
Solar Cell
One of the most decisive factors for quality and performance of solar modules is the solar cell. Testing screens for solder-joint reliability, robustness of cell metallization to corrosion, hotspot failures from localized shunts, persistently elevated cell temperatures from reduced active area cells, high-system, voltage-induced degradation (PID) and cosmetic defects.
EVA Encapsulation
EVA remains the most commonly used encapsulant in PV module production. EVA bonds all the material layers into a uniform laminate, provides an important electrical barrier to leakage current through the glass side of the module, and allows high light transmission into the solar cell.
Testing ensures that new encapsulants will maintain or improve upon module PID resistance; withstand long-term exposure to sun and moisture without discoloration, delaminating or loss of optical transmission; and have low acetic-acid-formation rates, which prevent cell metallization corrosion.
Since most encapsulants will pass the basic IEC, CSA and UL testing requirements, enhanced material screening methods also offer a graduated scale for choosing among newly available materials.
Backsheet
The majority of PV modules produced today still employ a glass/encapsulant/backsheet structure. The backsheet protects against electric shock and electrical faults by providing a mechanically robust, high-dielectric barrier over the back surface of the panel.
Most backsheets derive their electrical properties from a layer of polyester (PET) at their core, 100 μm to 300 μm thick. This core is typically surrounded with an outside layer designed to protect the PET core from UV light degradation and an interior layer that promotes adhesion between the backsheet and EVA encapsulant.
In addition to electrical insulation, typical backsheets have a much lower water vapor transmission rate (WVTR) compared to EVA. This slows the permeation of moisture into the panel, although the package remains breathable resulting in daily and seasonal variation in module moisture levels.
Hydrolysis degradation and UV-induced degradation of PET result in easily measured reductions in backsheet ductility, yellow index and adhesion strength. As such, retained elongation and tensile strength of backsheet and backsheet/EVA adhesion strength are key figures of merits used to determine backsheet acceptability. High-performance backsheets will have retained mechanical properties ≥70% of pre-exposed values and ΔY.I. ≤5.
In practice, excessive loss of ductility from UV and humidity exposure results in brittle fracture of the backsheet under subsequent thermal cycling or mechanical-loading conditions. These cracks completely compromise the dielectric strength of the backsheet, resulting in an unsafe panel that presents a shock hazard, may be prone to ground faults and is susceptible to excessive water transmission to the cells through the backsheet cracks.
Junction Box
The junction box provides a strain-relieved transition from the interior laminate conductors to flexible leads terminated with quick connectors. The junction box also houses bypass diodes or, with increasing frequency, more advanced safety/energy conversion electronics.
Conclusion
The above examples of solar cell, EVA, backsheet and junction box supplemental testing requirements are intended only as a small indication of the months of testing required to differentiate among various material options. However, there is no magic test protocol to identify the correct material choice.
Diligence, hard work and continual incorporation of teachings from field failures into our testing protocols results in a product continually evolving to higher quality for our customers.
By Sam Tsou is senior vice president and CFO at Motech Industries.