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kevin5881

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Reply with quote  #1 
I am looking at building a screen collector on my new detached garage. This is my first attempt so I have some questions. My collector will be permanently mounted on the south wall of my building. The collector will be 24'w x 4'h. I am going to build the frame out of heavy gauge aluminum that was salvaged from another job. I plan to line the inside with cement board siding. I figure this will provide good thermal mass. I plan to insulate the sides but haven't determined which material is the best to use. I have a bunch of left over 2" blue board insulation but I am unsure how it would hold up to the high temps in the collector. My other concern is what to do with the collector in the summer. Will I have to cover the collector or ventilate it somehow to avoid extremely high temps building up in the collector? I plan to use twin wall for glazing with aluminum angle to trim it out. Any info is greatly appreciated.

SolarInterested

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Reply with quote  #2 
Hi Kevin and welcome to our forum.

It's generally thought that the mass in a solar collector should be kept to a minimum. See the link below:
The Zen of Passive Solar Heating Panel Design (link)

You definitely want to check the specs on your insulation as normal stuff will melt as per the following:
Be Careful About Insulation (link)
"The common polystyrene insulation board (this is the "pink", "blue" and "white" stuff) will not hold up to much above 130F, and the batch enclosure temperatures will exceed this, and melt the insulation.
An example: http://www.builditsolar.com/Experimental/pinkfoam.htm
Polyisocyanurate insulation board works well, and comes with an alum reflective surface already installed."

Mike

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Both temperature rise and airflow are integral to comparing hot air collectors
kevin5881

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Posts: 11
Reply with quote  #3 
Thanks for the link, I have some more reading to do. I plan to use this one as my test mule on my garage. If it works well, I will be doing something similar on the house. I live in an earth home so I have a nice south facing roof that will make a nice place to mount a panel.
gbwillson

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Reply with quote  #4 
Welcome Kevin!

Mike is correct about not wanting mass inside the collector box. While it might sound like a good idea to retain a little bit of heat after the sun goes down, it is not. The reason is that in the early morning when the sun first hits the collector glazing, the inside of the collector box will take far longer to warm up. You are essentally trying to warm up a block of ice. So the idea is to have a collector that will warm quickly first thing in the morning to start producing usuable heat. So at best, any gain from internal mass would be offset by the long delay in collecting usable heat in the morning. 

You CAN use extruded polystyrene inside a collector box. BUUUUUT, you would need to cover it with additional insulation that can handle the heat. At 2", you will add a lot of thickness to your collector. And that might make for the dimensions inside the collector box to be a bit smaller if you use standard material sizes, such as 4'x8' twin wall. 

You don't mention where you live, but if your collector is attached to the side of the house, only a small amount of overhang may be needed to shade the collector during the summer as the sun gets overhead. There have been many styles of covers used. Some are as simple as a tarps or blankets, sheets of plywood, even planted flowers and shrubs.

Again, welcome, and don't hesitate to ask lots of questions. Anything we know or learned was likely from our own mistakes or from asking lots of questions.

Greg in MN[wave]
kevin5881

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Posts: 11
Reply with quote  #5 
Thank you very much for that explanation. That actually makes a lot of sense. So my plan now is to install a layer of 3/4" foil faced insulation directly to the building sheathing. I can then install my trim around that. My trim allows for a depth of 2.25" behind the glazing. This seems it will work perfectly. I will have .75" of insulation, .75" of air gap, a screen, another .75" of air gap, another screen and then .75" of air gap between the screen and the glazing. As for shading, The overhangs will provide no shade for the collector. Will tarping the collector be enough. As an alternative, I thought of piping a damper in on the output side of the collector. This would allow me to run the fan to expel heat from the collector. Instead of dumping the heat into the building, the damper would divert it out side of the building. The problem with this is that I would be wasting energy all summer and if the fan were to fail, I'm not sure if there would be damage to the glazing or the collector.
gbwillson

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Reply with quote  #6 
Kevin-

Where are you located? From what you describe it sounds like you are building a 3-layer ZeroPass collector. I have been wanting to build one of those myself. Here is a great place to buy the screen frames, spline, corners, and even the screen material.

http://www.qualitywindowscreen.com

Screens are all they do. They have screen frames in different widths, thickness, and strengths, and colors. You can also purchase frames in 8' lengths, which will help eliminate waste. Big box stores only sell thin, lightweight frame pieces in 7' lengths, so for a 4' cross piece, you end up wasting 3'. Once you eliminate the waste, the prices are actually better than the big boxes, and better quality. And for a 24" collector, consider making the screen frames 4'x4' as long frames get very prone to twisting. 

I wouldn't mind seeing a little more space between the upper screen and the glazing, but that wholly depends on the depth and materials you are using for the sides of your box. And consider that if you use 5/16" or 7/16" thick screen frames you will have a an inch or more of cross section taken up by the thickness of the screen frames. So your 1.5" total screen gap may be far less due to the thickness of the center screen frame. Unless, of course, you plan a different way to frame your screens, in which case we want the dirty details. But thicker and wider frames will allow you to have your screens tighter and are far less likely to mow or twist. In hindsight, my last ZP had 3/4" between the upper screen and the glazing and I wish it was a bit more. It made it hard to work inside and I think the screens were too close to the cold glazing. What do you plan to use for ducting and fans? You will need large ducts and lots of CFM's moving through your collector. 

A lightweight or light colored tarp may still get hot, but perhaps not hot enough to cause major damage. I used fan fold 1/4" insulation. I had to paint more than one coat on them as the sun was still heating up the unit. So far though, they've been working quite well. It was cheap, folds flat, and gives me plenty of extra should it not last. But from what I can see, it should last several years. If you do add a diverter vent, it really doesnt take much air movement to keep things cool. The sun in the summer is nearly overhead, so little sunlight penetrates the glazing due to the sharp angle. 

Greg in MN

IMG_1102.jpg 



kevin5881

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Posts: 11
Reply with quote  #7 
Thanks for the tips. I live in north west Iowa. Winters get cold. Summers can get hot and humid. This building is my detached garage and it is block. For this winter it will be uninsulated. It is built into a hill and 3 and a half sides are below grade. Eventually it will get fully insulated. My goal is to keep the building as far above freezing as possible utilizing free heat from the sun. The building was built for storage only, I don't plan on working out of it for extended purposes.

A little more info on my build:
The aluminum that I have for the frame is bent into a z shape or basically 2 opposing 90s. I plan to mount it directly to the sheathing. When I am done it will resemble a large window frame. I will flash it just like a window and run J channel around it for my vinyl siding. On the inside, I plan to insulate it with the foil faced insulation. I will install it on the sides of the frame first and then to the portion of the back of the collector. I will probably end up seaming all the gaps with tin foil duct tape. I will then install some 3/4" lumber horizontally along the top, middle and bottom of the collector. This will hold the insulation and serve as my mounting point for my first screen. I will then install a second layer of lumber over the first and secure another layer of screen. Finally I will have a weather stripping of some type that will cover the entire perimeter and also down the center baton. The glazing will be drilled and screwed all the way down the center baton and around the perimeter. The perimeter will get aluminum angle that will flash and secure the glazing. Every 8' there will be a seam in the glazing. I plan to use some aluminum flat stock to secure and seam the panels at these locations. I haven't decided whether I will have the inlet and outlet on the same side of the collector. I fear this ma restrict airflow too much but it would make my interior ducting a little better. I may put the inlet and outlet on opposing sides but I fear this way would be harder to get air to move evenly through the collector. Any tips on this would be great. I have been reading so much over the past year and it is kind of making my head spin.

It all seems a little too simple so I am sure I am doing something wrong. Feel free to poke holes in my plan, that is what I am here for, to learn.

Here is the garage. The collector will go just above the garage door and run 24' of the buildings 28'. There are 2 small windows in the upstairs room that are hidden by house wrap in this picture. The collector will be just under those. garage.jpg 

kevin5881

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Posts: 11
Reply with quote  #8 
I haven't determined which way to go for a fan. I was thinking a 6" duct booster fan. I also have friends in the HVAC field and could find many options out of an old furnace if need be. I'm not going to get crazy with ducting the inside yet as I wan't to prove that the thing works before dumping money into the duct work. I am kind of winging that end of it. I do think eventually I would like to incorporate some type of damper to close when the fan isn't running. Hoping this will minimize the heat loss at night.
Gordy

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Posts: 137
Reply with quote  #9 
Kevin.

For the dampers I use Zip Lock bags as flap dampers. The air flow from the fan pushes them up, and when the fan shuts down the bags fall down and cover the holes. This works well as long as the collector is sealed from outside wind penetration. My collector is the main door and the storm door with a removable black felt collector between them, the storm door seal is NOT wind proof so at night we cover the holes in the steel main door with magnetic vent covers also. Not a big deal as the door is in the living room ;-)

There are pictures here somewhere ;-) Of a duct damper that works similar to the bag idea. It is nothing more than a sleeve of light weight tightly woven fabric inside the duct that is pushed open by the air flow and collapses shut when the fans shut down.

If you haven't, you may want to read through these links for air flow and duct sizing suggestions.

http://simplysolar.supporttopics.com/post/kraut-man-craig-and-his-zeropass-collector-7957395?pid=1294544363

http://simplysolar.supporttopics.com/post/don-cs-new-2x16-zeropass-collector-8071547?pid=1294525882

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Gordy,
Minnesota
gbwillson

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Reply with quote  #10 

Kevin-

How lucky you are to have Menards locally. Polyiso insulation and twin wall glazing can be tough, or very expensive, in some parts of the country, and around the world for that matter. Lowes sells 100' rolls of screen if you don't want to order online or in smaller rolls.

I don’t recommend drilling through the twinwall to secure it to the face of the collector. I did it on my very first collector and it was nothing but trouble. The holes started splits and cracks in the glazing, which let to leaks. I would suggest using your angle trim or some other method to secure the glazing.

Using flat stock as a baton between the panels has been done, but you really need to secure the batons so they don’t bulge outward when the unit is under pressure. Menards also sells an H-fitting to connect two pieces.

 https://www.menards.com/main/building-materials/roofing-soffits-gutters/specialty-roofing/6-mm-multilite-h-channel-8-clear/p-1444424082564-c-5819.htm?tid=-3025523965300103913&ipos=6

With the size of your heater, I wouldn’t suggest having the intake and exhaust at the same end in most cases. You will have plenty of heat without a return run. And a return would add a LOT of resistance! The 4x16 ZP units that have been built so far have been putting out 15,000+ BTU’s per hour on a sunny day. That’s more than 3 electric space heaters!  And such a run inside a collector as long as yours is will have even more heat output. Which brings up the airflow needed to move such heat out of the collector so it doesn’t get wasted. You will want ducts, and entry points for your collector to be larger than 6”. If you can’t move enough air fast enough through the collector box heat will be wasted through the glazing. You will likely need somewhere over 500CFM output from your exahust to operate your heater most efficiently. I like your idea of using the recycled furnace fan until you have seen the results yourself. And furnace fans make a great garage fan too. Get your hands on a 3-speed or variable speed fan if you can. Furnace fans normally have over 1000CFM output on high. Even so, you may need a booster fan to not only keep output temps low, which is most efficient, but also to prevent an excessive bulge in your glazing. But start with the furnace fan, and if your glazing bulges too much or your output temps are too high, consider adding a 2nd fan.


Greg in MN

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