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Power-User

Join Date: Dec 2010
Location: 1° North Singapore
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### Solar Collector

12/04/2011 4:34 AM

In this part of the world, although it is 1 Degree North, though the sky is bright for many hours , it is cloudly most of the time. I have read that solar collector is not effective with diffused light, therefore using parabolic solar collector for solar power generation is a bad idea.

Any one has any idea just how much reduction for parabolic solar collector under cloudly condition vs clear sky condition ?

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Guru

Join Date: Apr 2010
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#1

### Re: Solar Collector

12/04/2011 9:05 AM

(Adapted from Wikipedia): The Sun as seen from Earth has an apparent diameter of about 0.5°, so it covers a solid angle of about 0.20 deg2 or square degrees, thus it covers a fractional area of approximately 0.0047% of the total celestial sphere which is about 6×10−5 steradian.

So if the clouds scatter the sunlight over your local hemispherical sky, that means that the equivalent area in the sky equal to the sun's area would only have about 0.0094%, or call it 0.01%, of the sun's unobstructed energy. And that's assuming all of the sun's light came through the clouds (instead of some being reflected back into space).

So at most a parabolic collector would produce about 0.01% (0.0001) of the energy on a cloudy day as it does on a sunny day. A flat solar cell array might be a better choice.

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#2
In reply to #1

### Re: Solar Collector

12/04/2011 9:45 AM

Your calculation seems very different from real life experience: cloud covered sun does not feel like it is 0.01 % of direct sun.

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#3
In reply to #2

### Re: Solar Collector

12/04/2011 11:33 AM

The figure given is probably not far off. If you wish to perform your own experiment, try using a magnifying glass to focus light on a cloudy day. You'll find what you get is *far* less concentration of light at the focal point than on a sunny (cloudless) day. Since concentrating solar collectors only transfer heat at the focal point, such a collector would fail to pick up the vast majority of solar flux from an obscured (cloudy) sky.

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#4
In reply to #2

### Re: Solar Collector

12/04/2011 11:47 AM

Your body is (probably) not parabolic, so you are experiencing the scattered energy from the sun coming from all angles over the two pi steradian hemisphere of the sky.

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#8
In reply to #1

### Re: Solar Collector

12/05/2011 6:56 PM

This is the extreme case where the reflector is perfect and focuses just the disk of the sun on the entire solar cell. I would think that in reality, the concentrator might focus on a larger piece of sky.

If it's usually cloudy, I believe you are better off to forego focusing optics in order to capture all the light that you can.

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#5

### Re: Solar Collector

12/04/2011 10:38 PM

My solar panels will run as little as 1% or as much as 10% of their sunny day production on a cloudy day; depends how cloudy. I don't know of any way to efficiently collect sunlight on a cloudy day. Try searching insolation.

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#9
In reply to #5

### Re: Solar Collector

12/05/2011 9:38 PM

This is a piece of useful real life data, thx !! It confirm what I read .

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Guru

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#6

### Re: Solar Collector

12/04/2011 10:58 PM

I presume you mean trough collectors. There are other collector shapes that might be more suitable. You will not get as much concentration as a parabolic but they collect from a large area of the sky so even if it is cloudy, they will collect some heat. One is called the cusp collector and gets light from a full 180 degrees and another one is a trough shaped like a seashell. For a link to a great pdf about your different choices, Search for "Tapas Kumar Mallick Chapter 1 Photovoltaic Concentrators and Building Integrated Photovoltaics" and it is a 4.8 megabyte pdf. I am sure it will help you make a suitable choice. (the link was too long to past in here). I have sent this pdf to several people in the

last year and they sounded pretty enthusiastic (but no firm results back yet).

I think it is really worth finding out how wide the "average" cloud cover defuse the "average" solar influx in different areas because this has a huge bearing on what collector works best. Case in point, Calgary gets far better sun in winter than Victoria. We get a gray rainy monsoon in winter and the sun might be gone for weeks on end. Calgary gets sunny subzero weather for much of the winter but lots of people I know say it is preferable to the crap weather we get in winter here in lovely Victoria.

Brian

Good Answer (Score 3)
Guru

Join Date: Dec 2005
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#7

### Re: Solar Collector

12/05/2011 9:19 AM

As others have posted, concetrating collectors are not effective for diffuse light sources.

If your looking to capture solar heat for water heating, you might want to research evacuated tube technologies and coatings for the cloudy conditions you describe. Flat plate collecters may also be a preferred option to a parabolic type collecter in you particular climate.

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#10
In reply to #7

### Re: Solar Collector

12/05/2011 9:52 PM

I was considering for steam electricity generation

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#11
In reply to #10

### Re: Solar Collector

12/06/2011 9:11 AM

For solar steam generation, use the assumption when the sun is not directly shining on the collector system the steam generation will stop.

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