Feasibility study on the implementation of planar inductors onto c-Si solar cells through numerical simulations

Abstract

With the integration of solar panels in urban areas, where shading is a common issue, it is desired to realize more shade-resilient designs. A DC-DC boost converter is an electronic device that can be used for maximum power point tracking (MPPT). When implementing MPPT at sub-module level, the shade resilience of the module can be increased. This thesis aims to investigate the feasibility of implementing planar inductors into c-Si solar cells using the numerical simulation software COMSOL Multiphysics®. Through simulations, the inductance and resistance values for various planar coil geometries were obtained. Subsequently, the feasibility of using these coils in a sub-module DC-DC boost converter was investigated. For this study, a specific case was assumed where a Gen 2 Cell - 160 mm c-Si cell is used with a DC-DC boost converter with a duty cycle of 0.5 and a current ripple of 20%. Given the parameters from these devices, the minimum required inductance of 1.20 μH for a frequency of 200 kHz and 2.40 μH for a frequency of 100 kHz were obtained. The series resistances that can be added to the solar cell while still maintaining high efficiency, range from 1.0 mΩ to 4.8 mΩ, for a 1% to a 5% power loss, respectively. This is all for single-cell power conversion. Two different approaches have been examined; the screen-printed coil(s) approach and the screen printing thickness exceeding coil(s) approach. The former studies coils that do not exceed the maximum thickness achievable with screen printing. As such, these coils could be simpler to implement into a solar cell production line than coils with a higher thickness. In this research, for thicknesses below 300 μm a best-case scenario is studied where the skin effect is neglected. The latter studies a larger range of thicknesses, not taking any specific production method into account, but the skin and proximity effect is taken into account. The screen-printed approach is limited to a thickness range of 10 μm to 200 μm, thus simulated by the 2D model. For the air-core single-cell inductors for DC-DC boost converter applications, the inductance and resistance values were either below the 1.20 μH threshold or above the 4.8 mΩ threshold, respectively. These thresholds change depending on the number of series-connected cells. The inductance of a planar inductor can be boosted up to 200% when adding a magnetic material in a sandwich structure, however, this will also increase the costs of the application. In a boosted-inductance case, the 3-turn coil with a thickness of 200 μm, and a 5% power loss is the only configuration that is feasible for a 200 kHz DC-DC boost converter application, with an inductance of 1.569 μH and a resistance of 3.45 mΩ. By increasing the number of coils per cell, the power dissipation in the coils can be reduced. Using this approach, six feasible topologies can be created. One of those is the nine parallel-connected 8-turn coils with a thickness of 200 μm, reaching an inductance of 1.210 μH and a resistance of 3.186 mΩ, allowing for a 200 kHz DC-DC boost converter application without any magnetic inductance increase. All six options are feasible for integration on a single cell using screen printing, however, the actual resistances will be higher due to the skin and proximity effect, and single-cell integration will be too expensive for industrial applications. When studying thicknesses above the screen printing limit, only the 4-turn coil with a thickness of 300 μm, a magnetically increased inductance of 2.718 μH, and a resistance of 4.01 mΩ, is feasible for the 100 and 200 kHz frequencies. This case however does neglect the skin effect. For the 3D model and thicknesses above 400 μm, the skin effect significantly impacts the resistance. When series connecting multiple cells to a single coil, the only feasible option is the eight-cell connected 8-turn coil for a thickness of 200 μm and 300 μm, both reaching a magnetically increased inductance of 10.431 μH and resistance of 23.33 mΩ and 15.56 mΩ, respectively.