# Collecting sunlight



## fishhead (Jul 19, 2006)

I have an idea for using sunlight similar to the sun tubes that collect light and transport it into a home.

Is there any kind of proven shape that captures sunlight from say 100 degrees of sun travel (9 am to 3 pm) and directs it downward into a specific point like into a pipe?

Maybe I'm making this too complicated. I do know that there are solar powered generators that use a bank of mirrors to target a single collector where it heats up a salt solution or something. They may track the sun so that would be more complicated than I would like to go. 

I want to be able direct sunlight straight down and do it all day long even as the sun moves across the winter sky.


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## Harry Chickpea (Dec 19, 2008)

There are shapes that use multiple reflections to more or less accomplish that, but you miss a point. Imagine that you have a perfect inverted triangle where all energy impinging on the top side of the triangle will transfer down to the point at the bottom. Now imagine the sun shining down on it from various angles. If you draw parallel lines from the top of the triangle indicating the sunlight, when the sun is directly overhead a cross section of those lines will be as wide as the triangle top. Draw lines to the sun at a 45 degree angle to the top, and a cross section of the rays will show a line that is significantly shorter than the triangle top.

In short, you can't get away from some form of tracking without incurring losses.


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## fishhead (Jul 19, 2006)

Harry Chickpea said:


> There are shapes that use multiple reflections to more or less accomplish that, but you miss a point. Imagine that you have a perfect inverted triangle where all energy impinging on the top side of the triangle will transfer down to the point at the bottom. Now imagine the sun shining down on it from various angles. If you draw parallel lines from the top of the triangle indicating the sunlight, when the sun is directly overhead a cross section of those lines will be as wide as the triangle top. Draw lines to the sun at a 45 degree angle to the top, and a cross section of the rays will show a line that is significantly shorter than the triangle top.
> 
> In short, you can't get away from some form of tracking without incurring losses.


I don't follow. Do you have any pictures?

I will have to accept loss because of the nature of my project.


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## cfuhrer (Jun 11, 2013)

If the inside of the inverted triangle was reflective/mirrored, would that improve the percentage of light that successfully traveled through the triangle when the sun was not shining directly down it?


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## Harry Chickpea (Dec 19, 2008)

Here is an illustration. The triangle is actually a concentrating funnel for sunlight, open at the top. Since it doesn't rotate to follow the sun, when the sun is directly overhead it works at capacity, but as the sun angle gets lower, the amount of sunlight entering the funnel decreases significantly. The two green lines are the same size. See how the green line that is the full cross section of the incident rays on the right only covers a little more than half of the incident rays when the sun is directly overhead? 

A solar concentrator is effectively a flat panel of the size of the opening of the concentrator.


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## fishhead (Jul 19, 2006)

Okay. Now I understand. Thank you.

It looks like my best option is to find a happy middle based on the angle of the sun at the most critical period(s) which will probably be Dec and Jan or Feb. Then I can position the face of the collector so that it is perpendicular to the sun at noon. The collector could have 3 faces (10 am, 12 pm, 2 pm) instead of one face (10 am-2 pm). That might be simpler than trying to make one that combines all the angles.


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## wy_white_wolf (Oct 14, 2004)

google "parabolic trough solar collector"

I think that is what you're looking for. But they will go way over 100F if designed properly.

WWW


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## ace admirer (Oct 5, 2005)

there is little energy available in morning or evening sunlight, At low angles the light has to travel through a greater distance of our atmosphere which is laden with dust and water vapor. not to mention the high angle reflection of sunlight off the atmosphere itself. but what you are considering would extend the usable visible light itself.


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## Harry Chickpea (Dec 19, 2008)

fishhead said:


> Okay. Now I understand. Thank you.
> 
> It looks like my best option is to find a happy middle based on the angle of the sun at the most critical period(s) which will probably be Dec and Jan or Feb. Then I can position the face of the collector so that it is perpendicular to the sun at noon. The collector could have 3 faces (10 am, 12 pm, 2 pm) instead of one face (10 am-2 pm). That might be simpler than trying to make one that combines all the angles.


Yep. Take into account any local common weather patterns, such as clouds that don't burn off until noon, etc.


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## JeepHammer (May 12, 2015)

Not sure what you are getting at.

There is something called a 'Light Pipe' that uses a domed lens on top to direct sunlight down a highly polished tube into the home.
Virtually no heat, and is VERY efficient at transmitting light into the home.

I have 'Light Pipes' in my home, over the kitchen sink, in the bathroom, in the utility room.
The dome lens on top collects sunlight at any angle, transmits it to the polished tube and it reflects into the home.

You DO NOT want one of these where you want it DARK, like in a bedroom, bedroom closet, ect. since the outside security lights will keep you awake,
They are VERY efficient...
The outside security light is nothing but LED bulbs 40 yards from the lite pipe in the bathroom, and it's like a bright night light in there,
The kitchen is closer, and you can read by that lite pipe at night!

Good versions aren't cheap, can be a PAIN to install in existing housing, but I can HIGHLY recommend the better brands that are sealed so humidity doesn't condense in them and dull the mirror finish inside the pipe.


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Any piece of tubing will collect and transmit heat from the sun, simply paint it flat black.
Guys stack beer cans end to end (Ends cut out) to do this,
I've found that thin wall aluminum tubing is cheap, and you don't have to seal between cans... Same effect.

Cold air in one end, usually scavenged from the house,
Hot air out the other end where the sun has heated it.

A 'Window' in front of the tubes,
Light enters the window, hits the 'Black' tubing, changes wave length and becomes trapped since it can't escape the window anymore,
Heats the tubing, which heats the air inside the tubing.

Is this what you are talking about?

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There are 'Vacuum Tube' (Evacuated Tube) solar heating elements for water.
A thin tube inside for the water,
A larger tube outside to let the light in,
Heat travels in a vacuum much more efficiently, so the center tube gets VERY hot.

Haven't seen one for just heating air, but anything is possible...


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## ace admirer (Oct 5, 2005)

i think we have a local available brand called " solo tube"?


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## Bearfootfarm (Jul 13, 2006)

ace admirer said:


> i think we have a local available brand called " solo tube"?


Solatube

http://www.solatube.com/


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## Snowfan (Nov 6, 2011)

Not trying to hijack your thread, but I have a question. If you transmit natural light from outside, do you also necessarily transfer potentially unwanted heat? If so, can this be controlled? Can this heat be significant? Thanks.


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## Harry Chickpea (Dec 19, 2008)

Yes. Infrared filter (window tint). Yes.


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## fishhead (Jul 19, 2006)

I'm only trying to redirect sunlight not to produce heat.

I was wondering if a dome might collect light from all angles and redirect it into the tube. In my case a 1/2 dome would be enough because the sun doesn't travel across the sky as far in the winter.


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## Harry Chickpea (Dec 19, 2008)

Domes are not lenses. Most domes are plastic that don't refract light much at all.


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## wy_white_wolf (Oct 14, 2004)

fishhead said:


> I'm only trying to redirect sunlight not to produce heat.
> 
> I was wondering if a dome might collect light from all angles and redirect it into the tube. In my case a 1/2 dome would be enough because the sun doesn't travel across the sky as far in the winter.


Solar tube skylights have domes on them that are built like a fresnel lens to collect more sunlight than a normal dome would collect.

http://www.homedepot.com/p/ODL-10-i...gclid=CNPotJCRi8kCFZSBfgodD1YEuw&gclsrc=aw.ds


WWW


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## fishhead (Jul 19, 2006)

wy_white_wolf said:


> Solar tube skylights have domes on them that are built like a fresnel lens to collect more sunlight than a normal dome would collect.
> 
> http://www.homedepot.com/p/ODL-10-i...gclid=CNPotJCRi8kCFZSBfgodD1YEuw&gclsrc=aw.ds
> 
> ...


Thanks.

I was wondering if there was a shape that would redirect sunlight down from all angles. I just didn't know the shape I was looking for.


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