Modelling and analysis of the impact of module design, materials and location on the annual yield of a crystalline solar module

Abstract

Solar modules are typically sold according to their nominal output power measured under standard test conditions (STC). Furthermore, the design of solar cells and modules is usually optimized separately for these conditions. However, outdoor conditions differ strongly from the standard test conditions, such as angular incoming light of varying intensity and spectrum as well as varying cell temperatures. The assessment of the performance of different module designs under outdoor conditions requires long field exposure of the modules. However, this conflicts with the short development cycles of new module designs and material changes. This thesis presents a methodology to predict the annual yield of crystalline silicon solar modules for varying module configurations and environments. This enables a rapid virtual prototyping of new modules designs under realistic conditions to support the short technology development cycles. Twelve factors that affect annual module performance are quantified, starting with solar cells in air under STC, and finishing with a complete module under realistic conditions. The interaction of optical, thermal and electrical effects in the module are considered and combined with an angular, spectral and time-resolved light source. Consequently, this Cell-to-Module Yield (CTMY) model is validated for crystalline silicon heterojunction solar cell modules and an agreement of the annual yield within 0.7 % is found. Applying the CTMY model, this thesis systematically investigates the impact of specific module design properties on the annual yield. For example, the severe benefits of silver-coated V-grooved backsheet structures can be substantial over- or underestimated under STC compared to the detailed CTMY analysis. The front metallization of a crystalline silicon solar cell is usually optimised for the current generated by a cell surrounded by air. Inside a module compound, the cell current changes due to cell interconnection and the possibility of light recycling on fingers and interconnectors. Therefore, this thesis optimizes the metallization for operation inside a module under STC and then compares the annual yield for six of the best performing module designs. It is found that applying a light redirecting film (LRF) on top of a planar interconnector ribbon achieves the highest yield for full- as well as for half-cut cells. Further, this work investigates the impact of different glass textures, such as planar, V-grooved and pyramid textures as well as anti-reflection coatings. One key finding of that pyramid glass textures outperform planar glass in combination with planar ribbons, however, when LRF ribbons are used, conventional anti-reflective coated planar glass achieves similar performance. A less understood part of the cell-module compound are angular optical effects occurring at the interface between module embedding and cell surface. In this work, special focus is placed on multicrystalline silicon cell texturing, such as plasma texturing, and metal catalysed chemical etching. Most importantly, it is found that the wide spread in angular absorption in air observed for all different cell surface textures is significantly reduced after module embedding. Consequently, the optical advantages of black silicon structures in air are minimized at module level. In summary, optimizing the cell and module design separately for a maximum performance under STC does not result necessarily in an optimum performance under field exposure. Specifically, for module configurations using 'non-symmetrical' elements, such as half-cut cell designs or V-grooves on backsheet materials, the performance can notably vary with installation and environmental conditions. An aligned cell and module design, and an optimization for realistic environments has therefore a strong potential to reduce the levelized cost of electricity.