On the Thermo-Electrical Modeling of Small Satellite's Solar Panels

The thermal modeling of small satellites (up to 100 kg) is crucial for predicting and managing their operation, and may present significant differences when compared to the thermal modeling carried out in relation to larger spacecraft. In this article, the modeling of their solar panels is underlined as one of these differences. The efficiency of spacecraft solar panels is affected by three main variables: the sun's irradiance, the cells' temperatures, and the operating voltage. Unlike the most common approach, which only takes into account the first two variables and decouples the thermal problem from the electrical one, the coupled thermo-electric problem of the solar panel efficiency as a whole is considered in this article. Using the UPMSat-2 mission as an example, the importance of considering the operating voltage in the thermal analysis of a mission is proven in the present article (for the prediction of temperature and generated power). A simple but accurate I-V (Current-Voltage) curve model to calculate panel efficiency is proposed for modeling the solar panels performance, this method being easily implemented in ESATAN^©.

I. Introduction

During the last decade, small satellites (too small to be the standalone payload) have become a relevant fraction of the spacecraft launched every year and continue to increase. A total of 244 new nanosatellites were successfully launched in 2018, and more than 400 were announced for 2019. Furthermore, the number of launched nanosatellites is expected to reach 700 in 2023 [1]. The main reason behind this success is a great cost reduction, which makes space-based commercial and scientific activities an actual possibility for many new actors that previously could not afford them. This cost reduction is achieved by means of miniaturization, which also implies a large reduction of mass and standardization of components. The power subsystem is a typical example of standardized parts and components. In almost every small satellite, this subsystem is composed of solar panels (see Fig. 1), a battery, a cable harness, and a printed circuit board used as power distribution control system [2]. It is obvious that an adequate knowledge of the solar panels’ behavior is key to efficient power management. Nevertheless, it should be underlined that proper knowledge of the solar panels’ performance is equally important for the thermal modeling of the small satellite, something which is usually left aside in the design of this type of mission, because a simplified model is used [3]. This relationship between power generation and thermal effects represents a coupled problem, as the electric power delivered by a solar panel depends on its efficiency which, in turn, depends on the operating voltage V, the solar irradiance G, and the temperature of the solar cells T [4]–[7]. Fig. 1.

Selex-Galileo SPVS 5-cell modules composed of Azur Space 3G28C solar cells (top-left). UPMSat-2 satellite [8]–[10] TVAC (Thermal VAcuum Chamber) testing at IDR/UPM Institute (bottom-left). UPMSat-2 during integration tasks at the Centre Spatial Guyanais of CNES (Kourou, French Guiana, February 2020) (right).