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Solar panel work on the international space station

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Posted by DavidM on June 12, 2007, 12:51 pm
It was interesting to have a science story in the breakfast news
program, but it was no more than an excuse to show some nice space video.

Apparently NASA astronauts have taken their first space walk of the
year, they needed to replace solar panels. Any body know if they are
still using silicon up there, or have they adopted one of the new
technologies? If so which one?

DavidM newsNO@SPAMdjmorgan.org.uk

Posted by R.H. Allen on June 12, 2007, 7:31 pm
DavidM wrote:

They're using crystalline silicon for the International Space Station.
Different projects use different PV technologies, though, depending on
individual project requirements, and plenty of satellites are using
gallium arsenide or exotic multijunction cells. I don't know for sure
why they chose silicon for ISS, but it has (or will have) the largest
solar array ever put into space in terms of both physical size and power
output, IIRC. Using silicon enabled them use a bifacial solar cell
design, so they can get power from light hitting both the front and the
back of the solar cells, and I imagine it minimized a significant cost
as well -- last I checked (though it *was* quite a long time ago),
space-qualified silicon cells cost about a quarter of their more exotic
brethren. I know cost isn't usually an issue with space projects, but
NASA has had a number of ISS budgetary issues and it wouldn't surprise
me if it was a factor here.

Posted by rlsusenet@NOSPAMPUHLEEZschnapp on June 12, 2007, 11:50 pm
 R.H. Allen wrote:

For space-based hardware, I expect that launch cost is more important
than manufacturing/qualification cost.  Typical quotes are $0,000/lb to
orbit on the shuttle, incremental cost.

Perhaps the mass of the supporting structure of the panels dwarfs the
mass of the panels?  I dunno.

I suspect an analysis was done, comparing GaAs to Si panels.

More likely, the choice was made for engineering reasons, e.g., perhaps
Si panels are more amenable to the required deployment strategy.  Or
perhaps the bifacial attribute you mentioned was critical to the
construction sequence or flight modes, somehow.

Posted by R.H. Allen on June 13, 2007, 5:28 pm
 rlsusenet@NOSPAMPUHLEEZschnapp.org wrote:

Sure, but that's an argument in favor of silicon cells since they're
lighter. That's a change from awhile back, but they're making them so
thin now that they're super light, while GaAs and triple-junction cells
sit on relatively heavy germanium substrates. I checked some Spectrolab
spec sheets to get some numbers (the percentages are efficiencies):

Spectrolab K6700B (Si)   13.7%  28 mg/cm^2  ->  1.3 kg/kW
Spectrolab K6700BWT (Si) 14.2%  55 mg/cm^2  ->  2.9 kg/kW
Spectrolab GaAs 140 m   19.0%  80 mg/cm^2  ->  3.1 kg/kW
Spectrolab TJ 140 m     24.5%  84 mg/cm^2  ->  2.5 kg/kW

The K6700BWT cells are the bifacial cells that AFAICT are the ones used
on ISS. They *are* a bit heavier than the triple junction (TJ) cells,
but a whole heck of a lot cheaper. They're also used in an 8 x 8 cm size
on ISS, which is 30% larger than Spectrolab's largest GaAs cell and
about twice the size of the largest triple-junction cells available from
either Spectrolab or Emcore. Fewer cells, of course, translates into
fewer interconnects (i.e., lower failure probability) and possibly less
support structure.

I believe that's correct. Don't quote me on this, but I believe I've
read that the ISS solar panels weigh about 45 kg/kW -- compare that to
the 2.9 kg/kW for the K6700BWT cells that I think it is using. FWIW, I
think at least some of them -- as well as a lot of other ISS parts --
were lifted into orbit with Russian rockets that are much cheaper than
the Shuttle.

Any of those sounds reasonable to me. As I said, I don't really know for
sure what the basis for the decision was, so I'm just speculating anyway....

Posted by MWAZ on June 16, 2007, 5:13 am
Hide quoted text -

Don't forget the technology time lag.

The original design of the ISS goes back quite a ways. When the
decision on
the solar power system was made silicon cells were a mature well
established product. multijunction gallium arsenide cells were too new
the production capacity was simply not available. Even if capacity had
available the manufacturing yield was low and the cost would have
exorbitant. In addition, panel manufacturing had its own learning
curve to
go through with different bonding and interconnect techniques needed
the more fragile GaAs cells. Today the cell/panel of choice would be
the now
proven GaAs multijunction. The engineers made the best decision at the


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