July 30, 2013
The Academy students met with Dr. Sheila Bailey to hear about her work at NASA. Sheila Bailey has spent her life working on photovoltaics (PV). Sheila completed her PhD in England and then completed a post doc in Australia before returning to the United States, where she started working as a lecturer, and raised three children.
At NASA, in addition to being a union representative, she works in the PV branch and is involved in PV research for both air and spacecraft. In her 28 years at NASA, she has been able to travel across the world due to her research, and is also the founder of the Photoluminescence, Spectrometry, Nano-characterization, and Scanning Electron Microscopy labs, which she thinks of as her “toys.” She also ran the 4th WCPEC conference in Hawaii in 2006.
Sheila’s job, put simply, is to make a better solar cell. This means creating a solar cell with better performance, one that lasts longer, and one that’s cheaper. Satellites are always power constrained in their design, and even though solar panels are large and have complex unfolding mechanism, 95% of space missions are powered by them, making them the foundation of satellite power systems. The power system on a satellite usually composes around 20-35% of its total weight, and PVs drive many aspects of satellite design. Solar cells, for space use, have the advantages of being reliable and have a long usage history, do not use any consumables, and are relatively light weight.
Up until the 1980’s solar cells were primarily crystalline silicon based, with 15-17% efficiencies. Since then, three types of solar cells have evolved: thin films, silicon, and crystalline multi-junction, the latter of which are primarily used in space. Multi junction solar cells stack multiple cells on top of each other that are each optimized in absorbing a specific band of light, from ultra violet to infra-red, however, more junctions means more complexity. Still, many avenues are available for increasing solar efficiency, such as with metamorphic growth, more junctions, concentrator designs, and quantum confinement. Sheila has specifically worked on increasing solar cells through the use of quantum dots, and could demonstrate a 0.2% increase in efficiency and better end of life performance.
Solar cells in space primarily degrade due to space radiation, the strength of which depends on the orbit a satellite is in. The largest solar arrays ever deployed in space are on the international space station (ISS), with a 250 kW power rating, and would take up most of a football field in size. They were designed for a 15 year mission life, and are expected to degrade by 20% in performance over that time. Performance of solar cells is different in space than on earth, as there is no atmosphere to contend with.
Sheila concluded her talk by discussing the unique challenges faced by the solar powered Mars Exploration Rovers in the form of accumulation of dust on their solar cells.