Effect of Cell Electrical Mismatch on Output of Crystalline Photovoltaic Modules

Abstract

The importance of energy supply and demand has been emphasized over the past few years. Renewable energy without regional bias continues to attract attention. The improvement of the economic feasibility of renewable energy leads to the expansion of the supply, and the global supply of solar modules is also rapidly increasing. Recently, the price of polysilicon for solar modules is also rising significantly. Interest in recycling waste modules is also increasing. However, the development of cost-effective treatment technology for solar modules that have reached the end of their commercial useful life is still insufficient. We are going to propose the standards necessary to restore and reuse so-called waste solar modules in a more eco-friendly and economical way. A crystalline solar module is an aggregate of individual solar cells. The technology is stable and has good durability. The efficiency of crystalline solar cells has dramatically improved in recent decades. The grade of cell that was mainly used two or three years ago will be discontinued soon. Therefore, electrical mismatch of the cells occurs while repairing an old-manufactured module with recently produced cells. In this paper, we experimentally verify how the increase in cell mismatch affects the module output. We intend to suggest the range of acceptable mismatches by analyzing the tendency. First of all, we repaired and restored the module in which all the existing cells were discontinued after about 10 years of production. The replacement cell had 16.94% higher output than the existing cells. After restoring the module, it was confirmed that the electrical mismatch loss of the cell in this range was very small, about 1.69%. Second, the mismatch loss was confirmed by manufacturing a module by mixing the two cells. The difference in output between the two cells was 5.56%. The mismatch loss compared to the predicted value based on the output of the individual cell and the actual value was very small, less than 0.76%. The long-term reliability results through the DH 1000 hr experiment on the sample that simulated the situation of repair, and the rest of the samples also showed a decrease in output up to 1.13%, which was not a problem. Finally, we hypothesized that a series-connected array should be constructed by reusing modules with different output classes. By cutting into 1/4, 1/3, and 1/2 of cells of the same grade, various unit module samples composed of 0.5 cells to 2.0 cells were manufactured and the output was measured. Electrical mismatch loss was tested by serially combining each unit module at various mismatch ratios. It was confirmed that the output loss in the three or more samples similarly exceeds about 10% with the mismatch ratio of 50% as the starting point. In the previous study, when the mismatch ratio was 70%, the output loss was about 17.98%. The output loss was 18.30% at 86.57%, 17.33% at 77.33%, and 14.37% at 75%. Considering that it is a value measured in a wide range, it is a result that is quite consistent with the results of previous studies. When the cell output difference was less than 50%, the electrical mismatch of the cell had no significant effect on the module output. When it exceeds that, a sudden output loss of 10% or more begins to occur. Consequently, the mismatch range of compatible cells should be less than 50%. If it exceeds that, not only output loss but also safety problems may occur due to heat generation. We can offer a range of interchangeable cell output power when crystalline solar modules are repaired and reused. By recycling modules with different outputs, you can provide a standard for those who want to use it by composing an array. By extending the lifespan of a solar module once used, it is expected that the generation of waste can be reduced from environmental point of view and the resources required to manufacture a new module can be saved from the resource-circulation point of view.