Hygro-thermo-mechanical modeling of thin-walled photovoltaic laminates with polymeric interfaces

A three-dimensional hygro-thermo-mechanical computational framework for photovoltaic (PV) laminates as well as its numerical implementation are established in this work. Aiming at an efficient thermo-mechanical modeling of thin-walled structures with polymeric interfaces, the solid shell element, which incorporates the enhanced assumed strain (EAS) method and the assumed natural strain (ANS) method for the alleviation of locking pathologies, and the interface element with thermo-visco-elastic cohesive zone model using fractional calculus are formulated. Besides, the finite element (FE) implementation of moisture diffusion in the 3D setting along the polymeric interfaces is also derived with the consideration of spatial and temporal variation of diffusivity due to its temperature and material decohesion dependencies. Given the difference between the time scales of moisture diffusion and thermo-mechanical problems, a staggered scheme is proposed for the solution of the coupled hygro-thermo-mechanical governing equations. Specifically, the relative displacement and temperature fields are firstly solved from the thermo-mechanical analysis, and then projected to the FE model of moisture diffusion to determine the diffusion coefficient for its subsequent analysis. The proposed method is applied to the simulation of three international standard tests of PV modules, namely the damp heat test, the humidity freeze test, and the thermal cycling test, and numerical predictions are compared with analytical solution for the damp heat case with a constant temperature boundary condition, as well as experimental electroluminescence (EL) images obtained from the thermal cycling test with the cyclic temperature boundary condition. A very satisfactory consistency demonstrates the effectiveness and reliability of this modeling framework.