How much do contrails reduce the energy from solar cells?

skephu

Senior Member
A recent paper by Austrian scientists seeks to answer this question:
Weihs P, Rennhofer M, Baumgartner DJ, Gadermaier J, Wagner JE, Gehring JE, Laube W.
Potential impact of contrails on solar energy gain.
Atmospheric Measurement Techniques. 2015 Mar 5;8(3):1089-96.

It contains very interesting data. This figure shows the solar irradiance measured on a clear-sky day (purple curve) compared with a day with lots of persistent contrails (blue curve):
upload_2015-12-26_19-11-2.png

It is interesting that during most of the first half of the day, the solar irradiance on the "contraily" day is overall higher than on the clear-sky day. This is because the white contrails in the sky reflect sunlight and make the sky brighter. But when a contrail passes directly under the sun it creates a shade, and irradiance reduces by as much as 72% for those few minutes when the contrail passes. On this day, the authors counted 49 contrails, and 22 passed between the sun and the observer. By the afternoon, however, a cirrus haze formed from the contrails, and the overall irradiance was lower on average than on the clear-sky day. Still, it was observed that when a contrail approaches the Sun's position, it slightly increases the irradiance because of its reflectivity, so there is an increase in irradiance before and after a contrail passes the sun.

The authors used 3 different types of solar cells to produce energy, and measured the energy production on clear-sky days and contraily days. This table shows the energy losses on days with persistent contrails:

upload_2015-12-26_19-26-45.png

So contrails reduced the total energy production by about 0.4% on average; on the worst days, the loss was 2-3%. But in some cases, the contrails actually caused an increase in irradiance; the highest increase was about 1%.

They also present a graph showing on how many days there were persistent contrails in each month:
upload_2015-12-26_19-29-46.png

In the summer, persistent contrails appeared on less than 10% of the days, but in January, persistent contrails were observed on 35% of the days.

The authors conclude that if 7 to 8 contrails pass between the Sun and a solar cell per hour then a considerable decrease in the energy gain can be expected, and the situation will worsen as air traffic increases.

Still, with only a 3% reduction even on the worst day, it's really not that bad, and if the average is only 0.4% then it is practically unnoticeable.
 
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George B

Extinct but not forgotten Staff Member
I assume this study if performed in the Southwest of the US might yield significantly different results. However, these results are fascinating.
 

George B

Extinct but not forgotten Staff Member
How do you think the results would differ in the SW US?
Well, I would suspect there would be fewer naturally cloudy days than in Europe. More ambient sunlight overall on most days. The number and frequency of persistent and non persistent contrails may well be different as well.
 

Jay Reynolds

Senior Member
I'm not able to view the paper due to low bandwidth, but besides George's suspicions a possible confounding
with comparing a persistent contrail day with a clear-sky day might be that persistent contrail days often tend to coincide with natural cirrus. Maybe they accounted for that
 
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