FEM-based development of novel back-contact PV modules with ultra-thin solar cells
With the availability of ultra-thin back contact solar cells, the question arises if they can be integrated into PV modules. Particularly the single-side metallization and joint architecture of back contact solar cells may cause critical stress. We develop a three-dimensional finite element model of a frameless 60-cell module with an electrically conductive backsheet to simulate the cell stress in terms of mechanical push load. The FEM model is validated by mechanical load tests of frameless modules. With the validated model we perform a variation of the cell and encapsulant thickness from 80 mm to 180 mm and 200 mm to 460 mm, respectively. Moreover we compare three different encapsulant materials, one Ethylene-vinyl acetate copolymer (EVA), one thermoset Polyolefin elastomer (POE-TS), one thermoplastic POE (POE-TP) and two different electrically conductive adhesives. The combination of 460 mm thick EVA and 180 mm cells shows the lowest first principal stress which, however, is far above the measured critical fracture stress of 134 MPa for the MWT cells used. We then modify the FEM model to simulate a framed glass-foil as well as a framed and frameless glass-glass module with the same material combination, 80 mm cells and 200 mm encapsulant. The framed glass-foil module shows a by 54% reduced deflection and a stress of 114 MPa, which is much smaller than without a frame (296 MPa). The lowest value is achieved for the framed glassglass module with 66 MPa. An interesting finding from the thickness variation is, that for cells below about 110 mm thickness the stress increases with increasing encapsulant thickness, while for thicker cells the stress decreases.