Welcome to the Solar Collector
Brainstorming and Development Page!


 

Home

Hot Air Collector

Hot Water Project 1

Hot Water & Space Heating

Solar Electric

Solar Construction 101

FAQs

Best Collectors

Simply Solar
Sign up Calendar Latest Topics
 
 
 


Reply
  Author   Comment   Page 5 of 28     «   Prev   2   3   4   5   6   7   8   Next   »
gbwillson

Avatar / Picture

Registered:
Posts: 2,275
Reply with quote  #41 
I'd like to see a bit more than the 5/16" screen frame to keep the screen from the polyiso. If you add the little reinforcing plate, add the head of the bolt and the 5/16"screen frame, you will be pretty close to ½", which is about as close as I'd go. But the truth is, my tests showed little to no air movement outside the screen gap, so it may not make much difference. But I still think a bit more space below the back screen will allow the air around it to pass around it better as it picks up not only heat from the sun, but heat and reflection from the painted polyiso. Not much free space to play with in any case.

Also, make sure you add a little something to reinforce the screen frame corners. Krautman cut little metal triangles out of flashing and then used tiny screws to hold things together. Those plastic corners will only soften in the heat. But having the two layers of screen is a great idea, and that might give you more options for adding stability to the screen frame.

Greg in MN

dbc

Registered:
Posts: 249
Reply with quote  #42 
If I raise the screens up 3/16 inch, I will have 1/2 inch from the back screen to the polyiso and 13/16 from the front screen to the glazing.  The screen frame assembly will then be 3/16 above the polyiso and 1/2 inch below the glazing.  That should still work OK for fitting a piece of 1/2 inch wide J-channel or something across the top, between the screen frame and the glazing.  If I need something under the bottom, Home Depot has some old-style wood lath.  I could drill clearance holes for the bolt heads.  Or I could just rest the bolt heads on some little aluminum plates if a small air gap under the back screen is acceptable.  Trying to limit the amount of wood inside the insulated box.

Bert - I remember some discussion on a couple threads about the clearance from screen to glazing being more critical than spacing at the back, so I started there.  I was also trying to keep it easy to build.  Either way, now's the time to decide.

Greg - Did Krautman use the standard plastic corners along with the triangular gussets?  I will look at his pictures again.  I got some flat corner braces, but I think a little gusset may fit better.  Also - how did he hold the screen in the frame?  I got some sisal twine to use in place of the vinyl spline.  I can also crimp the frame in a few places and/or dab on some epoxy.  The sisal is rough and fibrous; I am hoping it will hold the screen tight.
gbwillson

Avatar / Picture

Registered:
Posts: 2,275
Reply with quote  #43 
Don-

I agree that it is better to keep the screen assembly farther away from the screen as is practical. Given your very limited space, I think you are doing it the right way, as the glazing is going to be far colder than the polyiso. Even though very little air will be passing outside of the screen gap, the cold glazing will still suck up the heated screens from being in close proximity. That may also possibly be one of the reasons Krautmans ZP build sometimes did better than mine in really frigid weather. My top screen was only ¾" away from the glazing, so it may have been sucking up some heat from the screens. But since he used 1x6 boards for the sides of his collector, he has about 2" above his screens. It wasn't enough to make me want to change from a thin design though since I still have to move my collector seasonally. Maybe on the next build...

Krautman did use the plastic corners, and the frames were quite flimsy, hence the reinforcement. Since your two frames will be connected, I don't see why you can't use the flat corner braces. They would be stronger than the homemade flashing triangles he used. He also used the vinyl spline to hold the screen. I think he either put a few dabs of glue or crimped the spline groove  in places, but I can't remember. Not sure how well vinyl spline will hold up, but if he can keep the temps down inside the ZP, they may hold. He didn't use anything to hold the screen frames in place other than gravity, as his collectors are/will be tilted back at a 60˚ angle.

Greg in MN
dbc

Registered:
Posts: 249
Reply with quote  #44 
Started working on the screens.

But first - I finally got the project up off the floor with the trusty 45 year old Workmate and a second modern imitation:

Table, 080816.JPG 

I built the frames out of 5/16 material.  Frames are 85 1/2 x 21 1/4, which fits pretty tight into the collectors.  I added a crossbar to keep it from bowing in during screen installation; more on that below.  Here are some pictures:

Frame, 080116.JPG 

Corner, 080116.JPG 

Crossbar clip, 080116.JPG 

I thought about making some small triangular gussets to stiffen the corners, but I would have to install them after the screen.  I wasn't sure I could keep the frame square, so I used the corner braces instead.  One drawback was that they made it harder to get a smooth 'drape', but they seemed to work OK.

Bigger problem may be that the frame still bowed together almost 1/4 inch between the crossbar and the corners.  I may have to add 2 more crossbars to keep the long sides straight(er).  Here is a frame with the screen installed:

Finished screen frame, 080816.JPG 

The screen is fairly tight - about like a normal window screen, but it still bowed in some.  That 'hairy' stuff in the groove is sisal rope (need to trim the loose ends).  I tried both sisal and jute; the sisal was 1/4 inch (too big) so I had to separate the strands . The jute was too small, so I tried braiding 3 strands.  Both held the screen tight, although the sisal was a little bit tighter, possibly because it is rougher.  Here is a closeup of a test piece with sisal, and some shots of the two types:

Screen with sisal, 080816.JPG  Sisal strands, 080816.JPG  Jute and Sisal, 080816.JPG 

Both cords want to unravel a bit at the ends, so I will dab some paintable silicone on the corners.

Next I think I will try a frame with 3 crossbars.  Overall, I am not super impressed with the screen frame system, but I am sort of committed now, so I will probably stay with it on this project.  Looking for other ideas.

dbc

Registered:
Posts: 249
Reply with quote  #45 
Forgot to mention in previous post - the completed screen shown has 2 layers of charcoal fiberglass screen.  This will be one of the back screens.  The front screens will only have one layer.

Before I bolt the front and back screens together (see earlier post), I plan to tape over the plastic corners with foil tape and paint everything with flat black spray paint.
gbwillson

Avatar / Picture

Registered:
Posts: 2,275
Reply with quote  #46 
Don-

The screens look good. I assume you are going to paint or at least darken the spline material. Painting would work, and would even help hold the spline in place if you painted it and pushed it into the groove before it dries. Soaking the cord in fabric dye would also work to darken it. Having the screens as tight as possible using the extruded aluminum screen tracks will be tricky as you have seen. But if, as you say they are as tight as a normal window screen, you should be fine with your vertical install. Where  screen sag can become a problem is an angled collector, since the screen material will likely sag a bit over time. You can always check your screen gap by using a phone's camera stuck up inside the two duct openings at the ends of your collector. 

I like how you reinforced your corners too, with the corner angles. What size and type of screws did you use? They look like regular sheet metal screws. I do like the idea of attaching the front and back screen layers together to give them added strength. Due to the length of your collector overall I'm not surprised your screen frames bow in a bit on the long sides. As the screen frames are 7' long, I wonder if it would be better to make them 3½' long instead? Twice as many to make, but much easier to manage overall. Just a thought...

Anyway, it's great to see your project coming along. And like Bert, you might be one of the few crazies out there that are looking forward to colder weather so you can test your creation.

Greg in MN
dbc

Registered:
Posts: 249
Reply with quote  #47 
Greg,  I used #6 x 3/8 self-drilling sheet metal screws with #6 flat washers to fasten the corner braces.  The braces come with some nice little screws that have  countersunk heads, but they are so long they would go completely through the frame.  I wanted to just go through one side, so there wouldn't be sharp points sticking out.  The screws I used are just short enough with the flat washers.  It takes a light touch with the drill, because the frame material is thin; the screws are easy to strip.  For the 2 holes closest to the corner, I drilled a 5/64 pilot hole, since these 2 screws also go through the plastic corners.  I will post a picture of the screws in my next report.

My biggest concern with the screen frames bowing together is creating a leakage path around the sides.  How critical is this?  I tried to make clearance there as tight as possible without actually sealing it with silicone, etc. I didn't want to do that, in case I need to take the screens out later.

I could make the frames shorter, although I already made all of them full-length, and Home Depot is telling me they no longer plan to carry the 5/16 screen frame in the stores.  They still had a few of the smaller kits, (up to 48 inch), but I had to drive to another HD 25 miles away to find the last 4 84 in. pieces (should have bought enough the first time).  Anyway, I think best solution now is to add 1 or 2 more crossbars, which I have enough material for.  Result should be about the same.  I always think twice about putting anything in there that will reduce the solar aperture.  Don't know which is worse - blockage from a couple 3/4 in.crossbars or a slight gap around the edge of the screen frame.  One drawback I am finding with a narrow collector is that the structural 'overhead' (insulation, plywood, etc.) is nearly the same, but the reduction in absorber area is proportionally twice as great.  My screen frames are 21 1/4 in. wide.

I do plan to paint the sisal twine, along with the screen and the frames.  Probably paint both sides, but the sun-facing side for sure.  I want to do that before I bolt the front and back frames together so the bolt threads stay clean.

As for looking forward to cold weather, I don't know if I am that desperate - yet.  More hoping I can finish this thing before the cold returns.  Wish I was faster like Bert, but alas, I am slow.
gbwillson

Avatar / Picture

Registered:
Posts: 2,275
Reply with quote  #48 
Don- 

Not sure how much a small gap along the sides would make, but any additional cross braces won't make a difference in the air flow as they are outside the screen gap. I think he also used a few dabs of glue alone the ends to help hold the spine in place. I suppose you could add some foam or weather stripping to fill the side gap. But I like the additional bracing idea better.  I asked Krautman about his screen spline. He used the standard vinyl spline, but covered the spline in the groove with foil tape before painting. He liked the corner reinforcement angles you used. His frames did seem quite wobbly from my recollection. I'd expect he will add some bracing before the final install.

I'm a little surprised your material costs for the narrow collector. I would have thought the basic costs would have been roughly half of a 4' collector. I would expect the labor would stay roughly the same. Not that the costs are high, since the basic design is so simple. In any case, the ultra narrow 2x16 will be a great comparison with several other ZP collectors that have double the height. While I would expect less BTU's than a 4'x16', I'm thinking it will be more than you would expect for a collector half the size. It will also be a good test for someone that has limitations and needs a long, narrow collector. In any case, you can always sell the unit and get back most, if not all of the material costs. 

Greg in MN
dbc

Registered:
Posts: 249
Reply with quote  #49 
Sorry if I wasn't clear, but I agree with you about the material costs and labor.  I was referring to the amount of area taken up with the collector frame itself + insulation and side reinforcement vs. the area of the actual screen absorber.  I don't regret the decision to go 2x16 - it was the best fit for my available space, and I am eager to see how well it works too.  Also, the 2x8 sections are really easy to pick up and move around.

I like Krautman's idea to tape over the spline.  That can only help keep everything in place, and will probably look better than the raw 'hairy sisal' even if it's painted.  Maybe the normal vinyl spline is OK taped over.

I think I will add more cross bars on the remaining screens (only built 1 so far) and may even do the first one over.  Heck, it's only August.
gbwillson

Avatar / Picture

Registered:
Posts: 2,275
Reply with quote  #50 
Okay, I see your point a bit clearer now. With each of your sections being 2x8, I bet you can pick them up with no problem. I wish I could since I have to move mine seasonally. My 4x8 sections are too wide for me to get a good grip. I can lift a section, but setting one down in a gentle manner, that doesn't involve caving in one of the corners, I can't do. So I attached a pair of wheels to the bottom of each section. I had made a couple of dollies that clamped to the bottom, but direct attachment was far better.

Krautman wanted me to ask how you attached your cross members to the frame. It looks like a metal tab bent to fit in the spline groove. His cross pieces were attached to the frame using sheet metal triangles attached with small screws. Yours look much cleaner

Greg in MN
Previous Topic | Next Topic
Print
Reply

Quick Navigation:

Easily create a Forum Website with Website Toolbox.

 

web statistics