ENCAPSULATION OF PV MODULES USING ETHYLENE-VINYL ACETATE COPOLYMER ASA POTTANT - A CRITICAL-REVIEW

The primary purpose of this work is to review the literature about wha t is and is not known about using ethylene vinyl acetate (EVA) copolym er as the encapsulant (or pottant) material in photovoltaic (PV) modul es. Secondary purposes include elucidating the complexity of the encap sulation problem, providing an overview about encapsulation of PV cell s and modules, providing a historical overview of the relevant researc h and development on EVA, summarizing performance losses reported for PV systems deployed since ca. 1981, and summarizing the general proble ms of polymer stability in a solar environment. We also provide a crit ical review of aspects of reported work for cases that we believe are important. Failure modes resolved in the early work to establish relia bility of deployed modules and the purposes and properties of pottants , are summarized. Typical performance losses in large field-deployed, large-scale systems ranging from 1% to 10% per year are given quantita tively, and qualitative reports of EVA discoloration are summarized wi th respect to ultraviolet (UV), world-wide location and site dependenc e. The general stability of polymers and their desirable bulk properti es for solar utilization are given. The stabilization formulation for EVA, its effectiveness, and changes in it during degradation are discu ssed. The degradation mechanisms for the base resin, e.g., unstabilize d Elvax 150(TM), and stabilized EVA are indicated for literature datin g to the early 1950s, and the role played by unsaturated chromophores is indicated. The limited number of studies relating discoloration and PV cell efficiency are summarized. Observed degradation of EVA or the unstabilized base resin in the laboratory and examples used to measur e the degradation are summarized in sections entitled: (1) thermally-i nduced degradation; (2) photodegradation and photothermal degradation of EVA in different temperature regimes; (3) photobleaching and photod egradation of the UV absorber and cross-linking agent; (4) acetic acid and metal and metal-oxide catalyzed oxidative degradation; and (5) di scoloration and PV cell efficiency losses. Processing effects/influenc es on EVA stability an discussed in sections entitled: (1) EVA raw mat erials and extruded, uncured films; (2) thermal encapsulation processe s; (3) effects of lamination, curing, and curing peroxide on gel conte nt and chromophores formed; and (4) incomplete shielding of curing-gen erated chromophores. A summary is given for the limited number of acce lerated lifetime testing efforts and examples of erroneous service lif etime predictions for EVA are discussed. The known factors that affect the discoloration rate of several EVA formulations are discussed in w hich the reduction in rate by using UV-absorbing superstrates is a pri me example. A summary is given of what is and is not known about EVA d egradation mechanisms, degradation from exposures in field-deployed mo dules and/or laboratory testing, and factors that contribute to EVA st ability or degradation. Finally, conclusions about using Elvax 150 in EVA formulations are summarized, and future prospects for developing t he next-generation pottant for encapsulating PV modules are discussed.