Assessment of bypass diodes and cell interconnections in 1 kW-Class thermophotovoltaic modules under highly non-uniform irradiance conditions

Thermophotovoltaic (TPV) systems have recently achieved cell efficiencies exceeding 40%, highlighting their potential for high-performance thermal-to-electric energy conversion. However, scaling these devices to module level remains a major challenge, particularly under the strongly non-uniform irradiance and temperature distribution inherent to some practical TPV configurations. These non-uniformities induce electrical mismatch between cells, leading to significant performance losses that are not yet fully understood in large-area systems. In this work, we investigate mismatch mechanisms and mitigation strategies in a 1 kW-class TPV module comprising 2400 germanium cells arranged in a cylindrical architecture. The cells are grouped into 10-cell mini-modules, for which two interconnection schemes are analyzed: fully series-connected cells, and a parallel configuration consisting of two series-connected groups of five parallel-connected cells. The study combines experimental characterization of individual cells and diodes with electrothermal circuit modeling of the full module. Individual TPV cells are characterized through temperature- and irradiance-dependent I-V measurements, enabling the extraction of parameters for a temperature-dependent single-diode equivalent circuit model. These experimentally calibrated models are then integrated into a spatially resolved electrothermal simulation framework implemented in NGSpice to project module-level performance under realistic operating conditions. The results show that fully series-connected mini-modules outperform parallel configurations due to their reduced sensitivity to voltage losses. Bypass diodes significantly mitigate mismatch effects, with low-forward-voltage devices such as Ge diodes outperforming Si ones. An optimal number of bypass diodes is identified, balancing mismatch protection and voltage losses. These findings provide design guidelines for efficient large-scale TPV modules under realistic unfavourable operating conditions.