Modeling of Solar Cells and Environmental Conditions for Space Microgrids

Abstract:
A significant increase of interest in space exploration has emerged during the last years by different sectors of society, such as the scientific and industrial communities. The current efforts for a new generation of space trips point towards Mars and the Moon, which is usually considered a strategic place to settle a transit base between the Earth and the red planet. However, the creation of lunar bases demands the use of cutting-edge technologies. Therefore, several research centers are committed to develop and improve the required space technology. For the case of photovoltaic (PV) cells, which is regularly considered the best energy generation solution to space trips relatively close to the Sun, the biggest issue lies in the extreme environment of outer space, which can heavily degrade them in a short period. Particularly, the radiation-induced degradation of PV cells due to nuclei particles is a very important challenge to be overcome. Besides, given the very high cost of launching experiments to the Space, highly accurate and fast models, with results having physical meaning, to represent not only the behavior of the PV cells but also the actual space environments have to be created to perform suitable ground-based tests. In this regard, several models have been proposed to understand the dependency of the PV cell performance on different variables and the nature of its degradation due to the impact of nuclei particles under different conditions. Furthermore, improvements to the cells by thermal annealing, illumination exposure, a forward bias for recovering, while coatings, nanostructures, Bragg reflectors, for hardening, etc., have been proposed. Besides, software packages like SRIM and SPENVIS are widely used to observe theoretically the distribution of defects across the PV cells created by the particle radiation. However, the state-of-the-art of PV cells modeling is mostly limited by mathematical approximations without physical meaning or numerical approaches with long processing times. On the other hand, the mathematical modeling of the radiation effects on the PV cells is mostly semi-empirical or even fully empirical with curve-fitting techniques. In this respect, this project presents a comprehensive review of different PV-models and studies dedicated to the radiation-induced degradation and shielding of the PV cells intending to pave the way for improved physical models in future studies. Besides, experimental results regarding the characterization of 39 triple junction (TJ) III-V based PV cells are introduced and analyzed. Moreover, a new physical model with high accuracy in a wide range of temperatures and solar irradiances, resembling outer space conditions, is proposed. Finally, a novel technique for identifying the illumination condition at the South Pole of the Moon is proposed taking advantage of the topographic data provided by the lunar orbiter laser altimeter (LOLA) experiment of the National Aeronautics and Space Administration (NASA). Finally, by integrating all proposed models, a conceptual framework for modeling the PV cells behavior in outer space environment is planned for future works. The review process identified important challenges that need to be addressed to allow an extended PV cells lifetime under space conditions. The proposed PV cell model showed a higher accuracy in a much wider range of light intensities and temperatures in comparison to the state-of-the-art approximations while being based on physical laws. Besides, a higher accuracy was observed not only in the maximum power point (MPP) but also in the whole range of voltages. Finally, the computation of the illumination profiles over the South Pole of the Moon allowed finding the most optimal arrangement of sites for a PV-based multi-microgrid system while allowing to test improved PV models for real conditions.

Funding: Mexican National Council of Science and Technology (CONACYT)