Maximum Power Point Tracking for Bifacial Solar Cells: An Adaptive Approach Based on Grain Size

Maximum power point tracking (MPPT) methods based on a constant fraction of open-circuit voltage (FOCV) or short-circuit current (FSCC) offer relatively simple means of estimating the operating point of photovoltaic cells. This study examines their applicability to bifacial polycrystalline silicon solar cells and assesses how silicon grain size affects the factors used in these methods. A three-dimensional columnar-grain model of an n–p–p+ bifacial cell was used, with an empirical relationship connecting minority-carrier diffusion length and grain size. Photocurrent, photovoltage and electrical power were obtained for front-side and simultaneous double-sided illumination. Current–voltage and power–voltage characteristics were used to locate maximum power points, and the fractions relating maximum-power-point voltage and current to their open-circuit and short-circuit counterparts were evaluated across grain sizes from 10 to 300 µm. The simulations indicated that, for a given grain size, the fractions varied little between the two illumination modes. However, both fractions changed with grain size, indicating that fixed coefficients may be unsuitable across different grain structures. Using the simulated data, correlation equations were developed to estimate grain size from open-circuit photovoltage and then determine a grain-size-dependent voltage fraction. These equations were incorporated into a proposed adaptive FOCV tracking procedure represented by a flowchart. The results support using the constant-fraction principle for the two illumination modes within the model, while highlighting the potential value of grain-size adaptation.