Field testing of thermoplastic encapsulants in high‐temperature installations

Abstract

Recently there has been increased interest in using thermoplastic encapsulant materials in photovoltaic modules, but concerns have been raised about whether these would be mechanically stable at high temperatures in the field. Recently, this has become a significant topic of discussion in the development of IEC 61730 and IEC 61215. We constructed eight pairs of crystalline-silicon modules and eight pairs of glass/encapsulation/glass thin-film mock modules using different encapsulant materials, of which only two were formulated to chemically crosslink. One module set was exposed outdoors with thermal insulation on the back side in Mesa, Arizona, in the summer (hot-dry), and an identical module set was exposed in environmental chambers. High-precision creep measurements (±20 μm) and electrical performance measurements indicate that despite many of these polymeric materials operating in the melt or rubbery state during outdoor deployment, no significant creep was seen because of their high viscosity, lower operating temperature at the edges, and/or the formation of chemical crosslinks in many of the encapsulants with age despite the absence of a crosslinking agent. Only an ethylene-vinyl acetate (EVA) encapsulant formulated without a peroxide crosslinking agent crept significantly. In the case of the crystallinesilicon modules, the physical restraint of the backsheet reduced creep further and was not detectable even for the EVA without peroxide. Because of the propensity of some polymeric materials to crosslink as they age, typical thermoplastic encapsulants would be unlikely to result in creep in the vast majority of installations. 566

Trial-run of a junction-box attachment test for use in photovoltaic module qualification

“…Failures were typically observed sooner for the PO melt in Phoenix. As might be expected, the creep test is therefore better correlated to the field results in Phoenix, the climate for which the creep test was prescribed [10]. One additional delamination is observed for the PE tape in Golden.…”

“…All fielded modules were rack mounted at 45°. The front glass surface of the fielded modules was painted with a black primer (P/N 245198, Rust-Oleum Corp.), with a measured absorptance similar to that of a PV module [10]. The mock modules were not producing electricity, and therefore may reach a slightly greater temperature than rack mounted PV.…”

“…In a recent study [10], temperature data were also obtained using a thin-film T-type thermocouple taped to the back of PV modules. In that study, an ~15°C range was found to exist across a module, from a center location to the frame.…”

“…The King model suggests that the T max of 110°C may be briefly achieved for roof-mounted modules during record hot weather (55°C ambient) in Riyadh, Saudi Arabia. Roof-mounted modules are generally ~15°C warmer than rack-mounted modules [10], [13], [14].…”

“…The temperature on the backsheet is typically 1°-2°C cooler than the cell temperature [15], i.e., the temperature typically represented in models [10], [13], [14]. Thermography was performed on the specimens fielded in Golden on one of the hottest days in July, 2013.…”