Catching the Sun
Multi-junction solar cells
have become critical technology for the lives of spacecraft orbiting Earth and
circling other worlds. More efficient than single-junction silicon or gallium
arsenide cells, which absorb a fixed range of the light spectrum to generate
solar power, multi-junction cells rely on multiple cell layers to capture wider
variety of light wavelengths.
The double-junction (two
layers) and triple-junction (three layers) cells developed by Boeing's Spectrolab,
Inc. and the Air Force Research Laboratory have improved the power generation
capabilities of today's telecommunications and other spacecraft.
Traditionally, single-junction
solar cells convert about 19 percent of the light striking their surfaces into
useable power. Spectrolab's dual-junction cells have pushed that up to about
24 percent and the triple-junction cells reach up to 28 percent efficiency.
Nasser Karam, vice president
for advanced products at Spectrolab, said that the key to the multi-junction
cells is the ability to not only to collect light at different wavelengths,
but also to efficiently connect cells both optically and electrically.
Because the cells generate
more power over the same surface area as their standard, single-junction counterparts,
they are useful for satellite manufactures hoping to add capabilities to existing
spacecraft buses. The use of multi-junction cells has led to higher-power satellites
to provide various communications services such video streaming, direct broadcast
capabilities, as well as global positioning services.
Spectrolab's triple-junction
solar cells are the choice power source of NASA's robotic Mars explorers Spirit
and Opportunity. Past Mars missions, such as 1997's Pathfinder, relied on standard
gallium arsenide cells.
"The large increase in efficiency
is what led us to use them," said Richard Ewell, power subsystem element manager
for the current rover effort at NASA's JPL. "In this case, we get 40 percent
more output within the same area than we would get with strictly gallium arsenide
cells."
Karam said Spectrolab researchers
are looking to build even more powerful cells, including five-junction and six-junction
versions, in the future.
Spectrolab's multi-junctoin
solar cells.
Next page: Precision
GPS software system
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