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pianoman8020

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Reply with quote  #191 
stmbtwle,

I tried a shrink-film under the glazing and I will say I will never do it again. The film split at the smallest nick and it lasted about two weeks of operation probably due to the heat. It was hazy due to a talc type of powder used to reduce static and sticking together and was impossible to remove. I removed the film at the first cleaning and rebuild.

My collector uses a double screen with a 1.2" spacing between them. It's a ZP/DP arrangement with "VVVVV" shaped Cinefoil divider between the inner screen and the back insulated panel(2"). With back of the envelope calculation, 50% of the available light energy is adsorbed by Cinefoil and should be cooled in the best way possible, hence the double pass arrangement. 

FYI, If the available energy from the sun is 100 Lux you can make the following calculation.
100 Lux thru typical glazing is -11% or 89 Lux remaining.
89 Lux thru the top black aluminum screen is -25% or -22 Lux or 67.
67 Lux thru the second screen is again -25%  or -17 Lux or 50 Lux remaining to be adsorbed by the 'VVVVV" Cinefoil back panel.

The "VVVVV" acts as fins in the returning air path for the best possible heat transfer. I made the two screens and formed Cinefoil into a single removable unit. 

Jim from IL




stmbtwle

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Reply with quote  #192 
Thanks for the info on the film, I thought it might be something like that.
Your collestor sounds like a really effective setup. So I guess it's a zero/double pass. I made a cinefoil double pass a few years ago and it worked really well. The screens could only make it better!

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Willie, Tampa Bay
SolarInterested

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Reply with quote  #193 
Quote:
Originally Posted by pianoman8020
stmbtwle,

I tried a shrink-film under the glazing and I will say I will never do it again. The film split at the smallest nick and it lasted about two weeks of operation probably due to the heat. It was hazy due to a talc type of powder used to reduce static and sticking together and was impossible to remove.


I wonder if mylar film work work in this design. It is more durable, UV resistant and can handle some high temperatures as it's been used in Modified Trickle Down Collectors (MTD) 
http://simplysolar.supporttopics.com/post?id=6400627

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Both temperature rise and airflow are integral to comparing hot air collectors
gbwillson

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Reply with quote  #194 
I seem to recall that several years ago one of the newbies, who didn't "know" what you could and couldn't do, wrapped his entire collector box in essentially a clear plastic bag. I think he used a large window insulating kit. The kit contained a single long sheet which was enough to insulate several windows. Or in his case, enough to wrap his entire collector in clear film. His thought was that even with the glazing in place there would still be heat loss from the back and sides too, so why not wrap EVERYTHING. This would eliminate any of the issues with having to deal with a 2nd layer inside the glazing. 

Since we know no matter how hard we try, a collector WILL leak air. So if you will be pulling air through your collector, the exterior wrap will be pulled tight against the exterior or the box, minimizing the effect of capturing dead air for a bonus against heat loss. Spacers could be added to the collector exterior to retain a thermal air gap. On the other hand, if air passing through a collector is blown, especially a high volume design such as the ZP, a swollen plastic wrapped collector might look like bubble boy's luxury condo or a giant solar burrito. 

Greg in MN


Irishvoyageur

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Posts: 31
Reply with quote  #195 
Jim, are you able to provide a drawing or photo of your SAH design? Thanks, John
pianoman8020

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Posts: 48
Reply with quote  #196 

irishvoyaygeur

In your ZP/DP the VVVVV screen design being positioned next to the top covers would lose an excessive amount of heat at higher temperatures.  The VVVV or finned design would be better served in the return pass. 

It would be good to retain higher temperature air deep inside the collector when ever possible hence the reason for the double pass design. If the top screens were of a flat design 50% of the available light would be captured inside the collector. The addition of interior fins in the return path would seem to be a better arrangement (Cinefoil fins).  An insulated interior return path (R12) for hot air is a must for a Double Pass system. My system uses screen modules with 1.2” spacing between screens and 2” VVVV Cinefoil separator between air paths. The air path ratio is 2 to1.

The collector I put together incorporated as many of the best practices I could find on the Simply Solar blog. I must say that I am bias toward this design because that’s I did it.

 Here are some of the essential features that I think all Double pass systems should incorporate.

Finned double Pass collector research paper:

https://pdfs.semanticscholar.org/fea6/dc4b008f4c7205e66bb008196c0692f3bfce.pdf

Lessons learned:

Use heaver duty galvanized perimeter frames to avoid screw pull-outs and allow brackets and handles to be safely attached anywhere.  You will have more durable collector with a longer life span. 20 ga. Steel has a 104 pound pull out for a #8 screw.

https://www.clarkdietrich.com/products/curtain-wall-framing/structural-track

https://simplysolar.supporttopics.com/post/show_single_post?pid=1303645552&postcount=6&forum=268123

No foam seals should be exposed to the collector interior. Use silicone based seals or cover seal with aluminum tape. Use lightweight aluminum sheet (aluminum soffit roll) for flex seals.

https://simplysolar.supporttopics.com/post/show_single_post?pid=1302005988&postcount=233&forum=282856

Use 110 VAC blowers with a variable speed control. 12 or 24 VDC fans are not good at pushing air.

Try to use the lowest noise fan when possible.
Use cushion fan supports and/or cover it up whenever possible.

Use a programable temperature control for primary fan control, not a snap switch (use one as a backup alarm). You will be able to control the temp levels and on/off differential much better.

Inlet filters will reduce air flow by 20%. If used, double or triple the filter size unless you want to purposefully reduce your system air flow.

http://simplysolar.supporttopics.com/post/show_single_post?pid=1296825681&postcount=156&forum=268123

Tilted south facing collectors are 15 to 20% more efficient than vertical ones. If a vertical collector is used, a fold down reflector cover can double as summer season coverup.

System cost – US$ -- Two 4’ x 9’ collectors

Collectors          $1252

Supports               159

Ducts/Blowers      300

Tools                        89

Controls                   235

Total                      $2035

I have lot build pictures of the VVVV design but have not published yet.

Jim from IL

Irishvoyageur

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Posts: 31
Reply with quote  #197 
Thanks for the information. I will be interested in seeing your actual design when available.

The collector drawing I posted is not my design. It was based on a research paper that studied 4 different back pass type solar hot air collectors.  The illustration shown was the one with the highest efficiency. I was wondering if others have built a similar design and that's why I was interested in your collector.

Here is the research paper FYI.


https://www.ias.ac.in/article/fulltext/sadh/041/03/0369-0376


pianoman8020

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Posts: 48
Reply with quote  #198 
Irishvoyageur,


The research paper shown does not take into the effect cold weather exposure. They ran the tests in lab at room temperature in India. If the tests were run in a freezer at a reduce temperature (-10 deg, F) the results would be entirely different. All the heat would be radiated out the top cover.  Last winter it got down to -26 Deg. F.

I will try to publish my collector build some day. 

Jim from IL
Irishvoyageur

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Posts: 31
Reply with quote  #199 
Yes it would be nice to see the tests at colder temperatures. Though this would not likely change the order of which collector in this group of four worked best. Certainly not all the heat would be lost at the glazing with a double glazed collector.
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