On-Orbit Performance Degradation of the International Space Station P6 Photovoltaic Arrays

This paper discusses the on-orbit performance and performance degradation of the International Space Station P6 solar array wings (SAWs) from the period of December 2000 through February 2003. Data selection considerations and data reduction methods are reviewed along with the approach for calculating array performance degradation based on measured string shunt current levels. Measured degradation rates are compared with those predicted by the computational tool “SPACE” and prior degradation rates measured with the same SAW technology on the Mir space station. Initial results show that the measured SAW short-circuit current is degrading 0.2% - 0.5% per year. This degradation rate is below the predicted rate of 0.8% per year and is well within the ±3% estimated uncertainty in measured SAW current levels. General contributors to SAW degradation are briefly discussed. Introduction The P6 solar power module (SPM), shown in Figure 1, was launched and installed on the International Space Station (ISS) in December 2000 and has continued to reliably meet ISS power loads. SPM power is generated by a photovoltaic array comprised of two solar array wings (SAWs). Each SAW has two flexible blankets populated with 8cm by 8cm, crystalline silicon solar cells. To achieve a nominal 160-V operating voltage, 400 solar cells are series connected to form a string. There are 82 solar cell strings on each SAW. SAW voltage is regulated by a sequential shunt unit SSU) that also contains sensors to measure SAW output current, shunt current, number of shunted strings and selected string voltages. However, SAW temperatures are not measured. The data are telemetered to the ground and processed in the Orbiter Data Reduction Center (ODRC) for subsequent retrieval and analysis. Since P6 activation, the electrical performance of these SAWs has been monitored. SAW electrical performance is affected by operating parameters, such as ISS flight mode, SAW pointing, and seasonal variations in environmental heating, as well as by environmental degradation. SAW performance degradation mechanisms include solar cell proton/electron radiation damage, contamination, meteoroid/debris impact damage and optical property deterioration (i.e., from UV darkening and plasma sputtering). Open-circuited strings and failed (short-circuited) by-pass diodes also degrade SAW performance. In the following sections, we will discuss the on-orbit performance and performance degradation of the P6 SAWs from the period of December 2000 through February 2003. Data selection considerations and data reduction methods will be reviewed along with the approach for calculating array performance degradation based on measured string shunt current levels. Measured degradation rates will be compared with those predicted by the computational tool “SPACE” and prior degradation rates measured with the same SAW technology on the Mir space station.