Materials Challenges in Photovoltaic Energy Generation in Space
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Introduction Photovoltaic energy conversion has been used since the onset of space technology. Vanguard I, launched in 1958, was the first satellite to be powered by p on n monocrystalline Si solar cells, which had been developed at Bell Labs several years prior.1 With increasing efficiency, solar cells have become virtually the only viable source for space power and have been used for low-intensity, low-temperature missions far away from the sun (e.g., to particular comets2 or the asteroid belt).3 The solar cells are incorporated into a solar array, which is comprised of all components necessary to deliver the required power to the satellite. The environmental boundary conditions of space do not allow for a multitude of design solutions. In addition, once a given design has proven to work successfully in space, this knowledge becomes an important asset. Therefore, the basic solar array design has remained virtually unchanged.
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The radiation environment in space mandates a front-side shielding of the cells, usually in the form of a thin glass slide, the cover glass. In order to reach the desired voltage and current, the solar cells are interconnected in series and in parallel, while the inclusion of diodes protects the cells from reverse voltage. The cells are mounted on rigid panels composed of an Al honeycomb core with carbon fiber face sheets, so that they survive the mechanical loads during launch and maintain a constant sun-facing position in orbit. Larger arrays are composed of several individual panels connected by hinges due to the need to mount them in a folded configuration on the satellite sidewall during launch. In addition, a yoke is required to prevent shadowing of the deployed array by the satellite body. Figure 1 shows a photograph of a state-of-the-art array for telecommunication satellites in a geostationary orbit. Both wings combined, each
18.5 m long, provide 16 kW of electrical power over the course of a lifetime of 15 years, while weighing only ≈250 kg. These performance figures highlight the importance of low resistive, redundant wiring on the array rear side. The cumulative currents of individual solar cell sections amount to several hundred ampere for bus voltages between 50 and 100 V. In terms of materials, the solar array is one of the most demanding locations on a spacecraft. Unlike inside the main satellite structure, the materials remain completely unshielded and are therefore exposed to the highest environmental loads. Among the vast number of materials challenges arising, the scope of this article is limited to the core materials of photovoltaic energy generation in space, the solar modules comprised of solar cells, the cover glass, and encapsulating materials. Figure 2 provides a cross-sectional view through this assembly.
Solar Cells in Space With little room for change in the basic array design, despite some alternative concepts such as flexible solar arrays, the materials challenges to reach and to surpass the performance figures quoted in Figure 1 are, to a large extent, re
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