Computational modeling of viscoelastic backsheet materials for photovoltaics

Backsheet is the outermost layer of the photovoltaic (PV) laminate which consists of polymers such as Polyethylene terephthalate (PET) or Polyvinyl fluoride (PVF). The viscoelastic response of these materials significantly affects the durability of the PV module. In this study, the viscoelastic response of commercially available backsheet materials is experimentally characterized and computationally modeled. An extensive viscoelastic experimental study on backsheet materials is carried out, considering the temperature-dependent properties to characterize the mechanical properties. Based on an experimental campaign, small-strain viscoelastic models based on the Prony-series (PS) and Fractional Calculus (FC) are herein proposed. The form of the constitutive equations for both models is outlined, and the finite element implementation is described in detail. Following the identification of the relevant material parameters, models are validated with experimental data, showing good predictability. A comparative study of model responses under different loading conditions is also reported to assess the advantages and disadvantages of both models. Such an extensive experimental study and constitutive modeling will help design and simulate a more comprehensive digital-twin model of PV modules, as illustrated by the benchmark problems.