4. 1 PV Module Design, Manufacture, Performance and Reliability
Summary / Abstract:
In recent years, glass/glass (G/G) module designs became an increasingly popular alternative to traditional glass/backsheet (G/B) modules promising greater lifetimes and the possibility of higher power output when used with bifacial photovoltaic (PV) cell architectures . Despite the rapid growth of G/G module installations, some key differences with the G/B modules have not been considered in early designs. Consequently, greater degradation rates are observed in field-deployed G/G modules compared to G/B and attributed partly to higher degradation of the encapsulants . In the majority of G/B module constructions, the encapsulant of choice is poly(ethylene-co-vinyl acetate)-based (EVA) due to its low cost, good optical and mechanical properties, and long-term field experience . However, G/G modules using EVA encapsulation exhibited increased browning and delamination due to the impermeability of the rear glass layer and higher operating temperatures . EVA is a thermoplastic copolymer formed of polyethylene (PE) and vinyl acetate (VA), transformed into an elastomer by cross-linking reactions during module lamination. By action of heat, UV radiation, and humidity, the degree of cross-linking further increases, releasing corrosive by-products (i.e. acetic acid) . In G/G constructions, the volatile acetic acid accumulates inside the module laminate and causes discoloration, metallic corrosion, and delamination . To avoid the detrimental by-products of EVA encapsulants, alternative non-curing encapsulants have been extensively studied as an encapsulation for G/G module constructions . Polyvinyl butyral (PVB) is a well-established encapsulant for building-integrated PV (BIPV). Polyolefin elastomer (POE) and thermoplastic polyolefin (TPO) encapsulants have the advantage of not containing the vinyl acetate groups associated with acetic acid generation, higher volume resistivity and lower moisture permeation . These encapsulants are already in the market and used in G/G PV modules, however, detailed studies and comparison of their material properties when subjected to different accelerated stress testing are still to be completed. In addition, different formulations of these encapsulants using UV additives and stabilizers modify the properties of the material, i.e. changing its crystallinity, UV cut-off wavelength and resistance to potential-induced degradation (PID) . In this work, we study and compare a set of commercial and experimental encapsulation formulations containing EVA, POE, TPO and PVB after UV, damp heat (DH), and sequential (UV followed by DH) accelerating testing. Material-based properties (unaged and after accelerated testing) of samples are examined to gain insight into the durability of these PV materials when used in G/G configuration.