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This video about Spectrolab's spacecraft power systems mentions Juno several times and it looks like the Juno panels are shown at several points in the video. Multi-junction cells are discussed, they have higher efficiency than silicon and so were the obvious choice for Juno.

But I was confused when I saw this:

Spectrolab's solar cells are approximately 30% efficient in space and exceed 40% efficiency in terrestrial applications.

Why is there such a large difference in these values? Is photovoltaic efficiency defined differently for space and terrestrial applications?

Although I normally advise against taking a percent of a percent because it can be confusing:

$$ 1- \frac {\text{approximately 30%}} {\text{exceeding 40%}} \approx \text{25% less} $$

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above: Screen shot from the video found here:

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    $\begingroup$ Speculation: Additional layers of protection (e.g. coverglass) are needed in space to handle the U.V. and atomic oxygen, these absorb some light and reduce efficiency. Also, the wider operating temperature range may not be optimal for efficiency. $\endgroup$
    – pericynthion
    Aug 15, 2016 at 2:32
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    $\begingroup$ The 30% may be an end-of-life value, radiation causes the efficiency to decrease over time. $\endgroup$
    – 2012rcampion
    Aug 15, 2016 at 2:36
  • $\begingroup$ I've asked the related question Is photovoltaic efficiency defined and measured differently for space vs terrestrial applications? to keep the issues of ratings and measurement procedure separate. $\endgroup$
    – uhoh
    Aug 15, 2016 at 4:04
  • $\begingroup$ This may be caused by temperature, IIRC solar cells get less efficient at high temperatures. $\endgroup$
    – Hobbes
    Aug 15, 2016 at 7:29
  • $\begingroup$ @Hobbes Thanks - the need to allow for temperature variation within a ratings specification is noted by (at)pericynthion above. It's a good point though, and an important issue on Earth as well!. In at least one spacecraft they will be liquid cooled! $\endgroup$
    – uhoh
    Aug 15, 2016 at 7:43

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The 40+ percent efficiencies on earth are using solar concentrators, often with concentration ratios as high as 1000x. Most spacecraft including Juno use planar solar arrays without concentration. Record solar cell efficiencies as a function of time are published periodically by NREL and are available as a nice plot here: http://www.nrel.gov/ncpv/images/efficiency_chart.jpg.

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    $\begingroup$ A very nice plot indeed. Yep, the highest point there for 3-junction, non concentrator is 37.9% in 2014. OK you nailed it, thanks! $\endgroup$
    – uhoh
    Aug 15, 2016 at 22:58
  • $\begingroup$ Actually, you might include a reference to the comments about the additional protective layers (UV and energetic particles), efficiency losses at elevated (unregulated) temperatures in vacuum, and de-rating for end-of-life (15 years) performance loss to get that 37.9% record down to the >30% space rating. $\endgroup$
    – uhoh
    Aug 16, 2016 at 1:15
  • $\begingroup$ The link doesn't work, the NREL chart is now available at Best Research-Cell Efficiency Chart. $\endgroup$
    – Paul Rougieux
    Oct 23, 2020 at 7:39

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