The NASA Solar Probe Plus (SPP) mission will fly into the Sun’s corona, reaching as close as 9.86 solar radii from the center of the Sun. Power generation for the spacecraft will be provided by two solar array wings, which are being designed and built by JHU-APL and Emcore. SPP will get closer to the Sun than any previous mission, meaning that the solar arrays will need to operate reliably under unusually high irradiances and temperatures, a situation that introduces interesting challenges for the array design.

A schematic of the Solar Probe Plus spacecraft under (a) low irradiance near the Earth, and (b) high irradiance near the Sun.
As the spacecraft travels between the Earth and the Sun, the ambient solar irradiance levels vary widely, between 0.97×AM0 and 495×AM0. Nevertheless, the array requirements for power production and waste heat dissipation remain comparatively constant throughout the orbit. In the JHU-APL concept, the irradiance is controlled by varying the flap angle that the arrays make with the Sun line, thereby varying the light’s angle of incidence (AOI) onto the panels. In addition, the spacecraft is equipped with a thermal protection shield (TPS) that is oriented towards the Sun, creating umbra and penumbra regions into which the arrays can be retracted as needed to further reduce the irradiance. Each array wing is made of a larger primary section oriented collinearly with respect to the boom, and a smaller secondary section at a fixed angle with respect to the primary. As shown in the figure, in the high near- Sun ambient irradiance, the AOI is high, the arrays are located primarily within the umbra and penumbra of the TPS, and the secondary array supplies most of the power required. In the lower near- Earth irradiance, the AOI is near normal, the arrays are located outside of the TPS’s shadow, and the primary array supplies most of the required power. As an added thermal mitigation measure, the array substrate temperature is actively controlled by water cooling, nominally to

The SPP array design optimization process is a highly iterative search through the trade space of allowed array geometries that meet the mission requirements. The goal is to find specific array configurations (i.e., length/ width of the primary/secondary arrays, angle between them, etc.) that allow for the most benign possible array operating conditions, in terms of irradiance, operating temperature, and angle of incidence. The optimal array geometry that emerged from this study allows the array to stay below a 20×AM0 irradiance and a 70° AOI, respectively, throughout the orbit. The qualification test requirements chosen add significant margin onto those nominal values expected in flight.

The high radiant flux environment of SPP implies that thermal considerations are of crucial importance. Adequate heat sinking to the substrate is particularly important for minimizing the cell operating temperature. There are two types of solar array designs being considered. One is essentially a modified version of the standard 1×AM0 array, with some of the materials replaced by relatively more thermally conductive versions. The other is a more radical departure from the traditional approach, using a cell assembly similar to that of terrestrial concentrator (CPV) receiver packages. The two SPP designs have significantly lower thermal resistance than the standard one, which translates into lower operating temperatures. For example, at 33.7×AM0, the open-circuit cell temperature as estimated by a 1D thermal model is 240 °C for the standard, 172 °C for the SPP semi-standard, and 154 °C for the SPP CPV-inspired approach, assuming a substrate temperature of 150 °C in all cases. The semi-standard and the CPV-inspired approaches have both been successfully tested on small but otherwise flight-like coupon versions of the array, as part of parallel TRL5 qualification sequences.

This work was done by Andreea Boca of Emcore Photovoltaics. For more information, visit https://info.hotims.com/49741-122.