Leonardo C. Lopez

Simulation guided design of flexible photovoltaic laminates

Photovoltaic (PV) modules are manufactured using various PV cell technologies. The packaging requirements vary for each of the PV technologies and depend on their inherent thermo-mechanical properties and environmental stability of the cell. Thin film PV has the relative advantage of flexibility vis-à-vis their rigid and brittle competitors. This paper discusses the use of Finite Element Analysis (FEA) to develop a framework for guiding the design of flexible Building Integrated Photovoltaic (BIPV) shingles utilizing thin film PV cells. Microglass (0.2-0.3 mm thick glass) was chosen as the top barrier layer to exploit the flexibility of the thin film PV material. However, microglass alone does not provide sufficient protection to the shingle to meet regulatory impact resistance requirements. This paper discusses the use of FEA to guide the selection of reinforcing polymer layers and their placement around the microglass to enable microglass-based flexible laminates to sustain hail impact. FEA was used to guide the selection of reinforcing polymers and their placement in the laminate structure. Hail impact was modeled as static indentation and stresses were calculated in all layers of the laminate. This model was used to compare around 50 laminates generated through systematic variation of the polymer mechanical properties and laminate designs. Based on the analysis of the stresses generated in the microglass and other layers in these laminates, it is found that the optimal laminate design would consist of an elastomeric reinforcing layer above the microglass and a rigid polymer layer below it. An additional layer of a rigid polymer above the elastomeric layer can further reduce stresses in the laminate; however, relatively large thickness of this layer might be needed to get any significant stress reduction. The ideal location for the rigid reinforcing layer has been identified to be between the cell and the bottom barrier layer. The model predictions have been partially validated through hail impact testing.

Predicting the Reliability of Polyisobutylene Seal for Photovoltaic Application

Polyisobutylene (PIB) or butyl rubber has been used widely in applications such as construction materials, adhesives and sealants, agricultural chemicals, medical devices, personal care products, and fuel additives. Due to the unique low gas permeability, flexibility, and excellent weathering resistance, PIB or PIB based materials are frequently employed in photovoltaic (PV) industry as sealant to protect the electrical assembly in the package as well as moisture sensitive PV cells from aggressive environments. Long term behavior of the PIB sealant within the operating temperature range of the PV devices thus becomes a critical factor to the reliability of the device. In this paper, an experimental study of the temperature dependent fatigue behavior of a PIB based joint is presented. A finite element model capturing the joint region geometry is developed and an approach to estimate lifetime is proposed.

Reliability of crystalline silicon photovoltaic laminates: Test development and finite element analysis

The use of finite element analysis (FEA) and lab testing is gaining acceptance within the photovoltaic (PV) industry and is being increasingly used to “design-in” reliability into a product by investigating damage based on anticipated field conditions and environmental stressors. This paper presents a case study on the use of FEA to predict crystalline silicon (c-Si) cell fracture within a laminate subjected to bending loads (analogous to stepping loads during installation and loads experienced under rack-mounting and seasonal wind and snow conditions). Challenges related to development of a material model (that could be incorporated into the FE models) for c-Si cells are discussed. The use of electroluminescence (EL) as a diagnostic technique to detect fracture of c-Si cells within laminates is discussed. Finally, the use of in-situ Voc measurements during three-point bend testing in detecting failure events (in un-aged laminates and those aged under accelerated testing conditions) is also described. This could potentially be a new test method to investigate the effects of accelerated testing on the mechanical integrity of different parts of the electrical assembly such as the cells, weld and solder joints.