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gbwillson

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Reply with quote  #1 

Hi gang-

I ran a quick experiment using different reflective materials and how they may improve the solar heat gain of a collector. 

The reflective surfaces tested were:

 Nothing-Have to have a baseline, ya know…

Bright satin white-to best emulate snow

Reflective mylar-from a thin emergency space blanket

A glass mirror

 While others, including mclark, and Kevin, have tested reflectors by setting a them in front of a collector versus no reflector, I wanted to test the potential of different reflective surfaces that are likely to be used as reflectors. This test allows me to see just how much potential each surface has compared to others at the same time. It would not be practical to test this way using full sized reflectors in front of identical collectors. So...

 I did the next best thing. On two different days with different weather conditions, I placed a sheets of flashing vertically with a assorted reflectors flat on a table in front of it. I allowed the setup to sit undisturbed for 30 minutes before testing. 


IMG_1078.jpg 

IMG_1070.jpg 

 

Black Surface—10˚F, Hazy Sun

None-61.3F ————

White-83.8+36.7% Temp increase

Mylar-111.5+81.9%

Mirror-113.2+84.7%

 

Brown Surface—40˚F, Clear Skies

None-100.4F————

White-110.6+10.2% Temp increase

Mylar-120.3+19.8%

Mirror-141.7+41.1%

Please note that in no way expect a reflector to suddenly increase my performance by the percentages listed above. But as mclark noted when he laid out some mylar in front of his collector, a 40% increase is certainly possible.
http://simplysolar.supporttopics.com/post/snow-versus-mylar-reflector-comparison-7302220?pid=1286240075

 A couple of items of note are that the mylar film I used was about as thin as you can get, and thus, was unable to created a smooth, consistent reflection. So measuring the reflected surface was less consistent than the mirror. There were very inconsistent reflections. I think the mylar, if smooth, will perform as well as a mirror, at a fraction of the cost and weight. And mylar will likely need to be replaced every few years. Thicker mylar will last longer, and would be easier to create a mirror-like surface as long as the substrate was smooth. 

 The last thing that stands out to me is the mylar/mirror comparison is on a sunny day versus a hazy sun day. On a sunny day the mirror clearly outperforms the mylar. But on a hazy day, the results were very close. The hazy sky made the visible reflections somewhat hard to see. And this may have negated some of the disadvantage of the inconsistent mylar reflection.  And while we have a few sunny days in Winter, there are likely to be a lot more partly cloudy days. So mylar may win out in less than clear, sunny conditions.

Any reflector is better than no reflector. It can allow you to either build a smaller collector for your heat needs, or increase the performance of existing collectors at a fraction of the cost of building a larger collector. Even the cheapest mirror costs about $1.25 per sf. A decent 2 mil mylar can cost less than $.10 per sf. 

 

Greg in MN


stmbtwle

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Reply with quote  #2 
Very interesting (and informative) test.  I still think I'd go with white.  While there is a lot of scattering with white, I think it would be more forgiving of indirect sunlight than the mylar or mirror.   The sun moves, and in order to get full performance from the mylar or the mirror they would have to be larger than the collector, probably at least double.  

If you still have the setup, I'd suggest you turn it 45° to the sun and run the test again, and compare the two.

I think that the reflector with a vertical collector might be sort of "self adjusting" as the sun progresses further north in the spring.  If the reflectors are oriented for winter when they're needed most, their effectiveness will automatically diminish in the summer.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
KevinH

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Reply with quote  #3 
Was that the space blanket type of Mylar?  Some of that has such a thin coating that if held up to a light you can see through it.  The thicker Mylar that is used for plant growing and comes in rolls is highly reflective and lays flatter.  EDIT: Reading Greg's post again, he said it was a thin space blanket.

As a reminder there is more info on reflectors on BuilditSolar, including ideas on how to use reflectors to increase the heat gain through house windows.
http://www.builditsolar.com/Experimental/Reflectors/Main.htm
And search for "reflector" on this site.

I adjust my reflectors a few times during the heating season as the sun angle changes.  As the sun is getting higher now I have to raise the side away from the collector.

The part of the reflector farthest away from the collector provides the least benefit.  When the sun is at enough of angle, the light reflecting off the farthest away part doesn't hit the collector.  You can widen the reflector at the outer areas to increase the amount of time the reflector is fully hitting the collector (as in C and D below), but have to take into account the cost of doing that.
Reflector Size.JPG 

A surface that scatters the light more, like white paint or snow, would spread out the reflection more throughout the day, however, it is also reflecting some of the light away from the collector.

Kevin H
MN



stmbtwle

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Reply with quote  #4 
White does scatter some of the light away from the collector, but if the reflector is larger than the collector, parts of the reflector that are not in direct line will scatter other light towards the collector, and I think would give a more consistent output. This won't happen with a mirror.

Then there's the cost issue. White paint is cheap, and pretty durable.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
KevinH

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Reply with quote  #5 
White did well in Gary's test where he measured the light intensity.  It wasn't as sensitive to the angle of the reflector surface which is a benefit.  To really measure the total benefit of each you would have to track it over the course of a full day which would take a lot of measurements and changes between the two test reflectors.

A mirror would have a big gain peak mid day.  White would probably have a smaller peak mid day, but would give gains that are spread out over a wider range of time.  White would also scatter the light more evenly over the collector, whereas a mirror would focus the light on more distinct parts of the collector.  A way to spread out the gain over time with a Mylar reflector is to curve the reflector a little.

However they are done, reflectors are a low cost way to boost the output of collectors (when the collector is not angled perpendicular to the sun).

Kevin H
MN
gbwillson

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Reply with quote  #6 
Kevin-
The mylar I used for my test is about was thin as you can get. And you CAN see through it. So either mount it on a light colored substrate, or use a thicker mylar. Many of the mylar rolls online come in a 3mil thickness, which is as thick as a really heavy duty trash bag. Some of the reflective mylar rolls have a texture that would break up the mirror-like surface. Or could you simply crinkle up the mylar before mounting it to break up the surface reflection? The reflection from the unfolded mylar space blanket shows quite a few irregularities. 

Willie-
Paint is cheap, but snow is even cheaper![biggrin] 

Greg in MN


stmbtwle

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Reply with quote  #7 
It is, but shipping it to Florida is VERY expensive...

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
dbc

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Reply with quote  #8 
Wow - I wish I had seen this thread a couple weeks ago.  Have to pay more attention to the Experimentation and Performance section.  This is great information.

Now I don't feel so bad about my plywood reflectors drooping at the ends (after 2 days!); the white, now-convex top surface may even help with early/late reflection.  Not that I planned it that way.

I got a tip from a friend that hydroponics supply stores are a good source for mylar; going to check that out.

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