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rustythread

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Reply with quote  #1 
Newcomer here. I've browsed many of the solar threads here, but am still lacking a lot of knowledge and understanding.  Your forgiveness of dumb questions will be much appreciated.    
  I have a metal-skinned barn / workshop 30' x 60', with the long sides on the north and south.  The south wall  has a span of 30' x 10' uninterrupted by windows or doors, and I am thinking about a solar hot air collector of approximately 28' x 8' in that space.  Insulation (which I am still finishing up) is 1 1/2" styrofoam set between the wall purliins, covered in the interior by a 6-mil barrier and paneling.  Since I am both older than dirt and likely to be working by myself, I'd like to have something both physically simple to build and not likely to need a lot of maintenance. 
   The building presently has no heat, and is pretty unusable once the concrete floor gets down below freezing; of course, when the air then warms up outside, I can find all the low-altitude air infiltration by the pools of condensed water on the floor.  Anything which will make the shop usable more of the winter without a huge power bill will be considered a gain.
   A.  I'm considering an exterior frame of  2x4 steel studs, framing a flat-black-painted 28' x 8' section of the metal wall, with  a 2-screen assembly in front of the wall, with an 8" or so top-center duct and fan, and two 6" or so intakes mounted 1 each in the bottom corners.  No baffles or turns in the air flow in the collector  
     **Any comments or caveats or additional design necessities from the gurus??
    B.  Probable dumb question:  All the screen assemblies I have found so far seen to have the short top & bottom frame members between the screens, in the same plane as the airflow space.  Is there any info or theory on whether if might / might not be prefereable to have the top & bottom of the screen frames on the outside of the screen structure, so that the air passage between the screens is unimpeded although the flow on the outsides of the screens would have the additional turbulence / restriction?

   C.  Assuming, based on experience here, that there willl be more overcast days than clear days during the heating season, should I be looking at a non-UV-protected glazing  in order to pick up what ever I can from UV making it thru the overcast when the visible light does not?
   D.  I have a 120V AC motor & squirrel cage  from a central air conditioner (that AC setup covered approx 1100 sf of 100-year-old brick home); I hope it will be in the ballpark for size and CFM.  Comments / caveats?

Doubtless I will have more, and hopefully better-informed, questions as I go along.  No pictures, since the camera is still lost in the stuff not yet unpackee frim the move.

Thanks in advance for any comments.



gbwillson

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Reply with quote  #2 
Hi Rustythread-

Welcome! Never worry about asking questions on this site. Most of us like to tinker, or have a curiosity or a desire to learn about solar. And after spending some time reading up on this site, your head is no doubt spinning from all the information. 

And I have a few questions I'd like to ask to help us better understand your situation.

1. Where are you located?
2. You mentioned the walls are insulated, is the ceiling and how high is the ceiling?
3. Is the styrofoam wall insulation directly touching the metal walls?

And as to your probable dumb question...I'm not understanding, so maybe I'm the confused one. There are two basic screen configurations. The first is the basic 2-screen, where the air passes THROUGH the two layers of screen that sit at an angle in the collector box. The second is the ZeroPass or ZP, where the air passes BETWEEN the two layers of screen and the screen layers are parallel to the glazing.

For a collector of this dimension, your fan MAY move enough air, depending on the final build. The goal is to collect as much heat as possible, and have enough air moving through the system to keep the output temps moderately low for maximum BYU output. So if your fan turns out not to be inadequate, the output temps may be too high, and therefore wasteful, you could always add a 2nd booster fan. Larger setups like your can benefit from two fans as one fan can be used when output temps are low, such as early and late in the day. But when the output temps are high, the 2nd fan can increase airflow for maximum efficiency. 

As for glazing, UV protection is always preferred. Any plastic glazing will last longer and won't yellow over time. 

Greg in Minneapolis[wave]


rustythread

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Reply with quote  #3 
Greg, thanks for the reply

I'm in Southern Illiinois, about halfway between St Louis, MO and Evansville, IN.; just off I-64, about 5 miles east if the I-57 and I-64 intersection.

The peak of the shop roof is about 20' from the floor, or about 10' higher than the walls.

The roof appears to have insulation comprising about 1/2" of fibreglas and foil on both sides.

The styrofoam wall insulation does touch the metal skin (except for where the vertical corregations in the metal create air spaces.
 The dumb question was re the ZP configuration.

Air leakage is complicated by the fact that there is a 10 x10 rollup door at one end and a 10 x 10 slider on the other.  I'm working on reducing the leakage around the doors.  Also still have to put vapor barrier on the taller sections of the building ends--slowed by lack of taller ladder and having to move rather carefully (only 10% vision in one eye, 0% in the other makes for slow progress).
gbwillson

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Reply with quote  #4 
Rusty-

You live in a good location for many of the supplies needed for your collector. Menards is where I purchase almost all of my supplies, and thankfully most are frequently on sale. It's nice to have all that ceiling height. But you might want to upgrade that at some point as heat rises. As you are not in climate noted for frigid winters, I would wait on adding ceiling insulation. Just remember that all that warm air will cool rather quickly with so little insulation. Imagine trying to heat a tent... With such a high ceiling, a couple of fans would be helpful to move the rising warm air backdown to the floor.  Another option would be to build a false ceiling at the 10' height, insulate it, and use the upper "attic" for storage. It would help to keep the warm air down where it's needed. Heck, you could build special work rooms with all that extra space. 

The reason I asked about the styrofoam touching the metal wall was that styrofoam has a low melting point and if you paint the outside metal siding black it will get very hot and possibly off-gas, if not melt the styrofoam. Stagnated collectors can rise in temp well over 200˚F. Easy fix would be to add ½" of polyiso insulation to the back of the collector box, or replace the insulation on the inside of the barn wall with polyiso where the collector is going to be mounted. 

While it's kinda hard to insulate a roll up door, you could cover it with insulation during the winter. You may be able to insulate the sliding door and still have it operational. 

About the ZP screen frames, there are no hard and fast rules as far as to the way the frames hold the screens. But I like the idea of making the screen gap as smooth and flat as possible. My testing last spring showed the optimal space between the two screen layers to be 1⅜". Personally, I don't recommend using the stud track, or at least the 3⅝" deep version. It's too shallow, at least for my current ZP. It doesn't have enough room to increase the screen gap to the optimal. I also had issues with rust where the plywood back attached to the stud track. If you end up attaching the unit directly to the metal wall, that may not be an issue. Just make sure you have enough room for any insulation, screen gap and space between the upper screen and the underside of the glazing. I assume you will be building the unit on the side of the garage, rather than building it, and then mounting it. If so, you could use almost any materials, as the large, finished weight will never have to be lifted. If you do use wood, I suggest cedar boards, as they will hold up better over time.

One thing I would STRONGLY suggest is to have the airflow on a 8'x28' collector running sideways, from end to end. Your will collector far more heat than if you have the airflow running vertical. The air has far longer to pick up heat. And with a ZP of this size, you will collect a LOT of heat. So in one end and out the other...

Greg in MN
rustythread

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Reply with quote  #5 
reg: 

Thanks again. much appreciated.

   The doors:  On the slider at the east end, I have added the 1 1/2" styrofoam between the 2 x 4 frame members, and stapled 6 mil vapor barrier over it.  The west end roll-up door appears like it may have had some sort of insulation kit added--the door's about 2" thick but light-weight.  I still need to reduce air flow around the edges.

  The wall and styrofoam cooking:  the 2 x 4 purlins are also flat against the steel skin--if I manage to replace the styrofoam with polyiso, will the wood also be problematic if the heater stagnates?

   Horizontal vs. vertical orientation of collector:  How would you recommend placing / sizing the intake/exhaust ducting?  12 x 12" with intake at lower corner and output at upper corner on far end?  8" round ?  12" round?  

   What about having the fan in "push" mode and putting it at the bottom intake instead of pulling at the outflow?  Would be easier for me to install and maintain.

   Ceiling height:  Unless I add insulation to the present rafters, or build partition walls as well as sub-ceilings, I'm kinda stuck on ceiling height due to the height of some of my macninery and the fact that I have a 1 1/2 ton chain hoist hung from the one of the bent frames (the barn frame consists of 7 bents on 10-foot centers; each bent is made up of steel I-beams from recycled mobile home frames.  Hell for stout.  Prior owner-builder was a welder by trade.) 

   I do plan to add one or two ceiling fans hung off the roof peak to recirc the heated air.  I don't  mind variations in temperature so long as I can store enough heat in the machinery and concrete floor to keep the place above freezing.  It would be even better if I can heat enough to run a dehumidifier without it freezing up--maybe a snap switch on that, with the dehumidifier mounted 8 or so  feet above the floor...

Thanks again for your thoughts

rustythread

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Reply with quote  #6 
Thinking further about the potential overheating of existing styrofoam in the south wall, it appears that my choices are:
      1. remove the paneling, wiring, vapior barrier, and styrofoam, replace the styrofoam with pollyviso, and put back the vapor barrier, paneling, and wiring,
     or 2.  give up the idea of using the wall as an absorber and build a whole box on the exterior with  2 x 6 studs and 1" or  1/2" polyviso in addition to the screen assemblies. 
 
   If I do it all on the exterior, looks like the only differences in material & construction would be the 2 x 6 rather than 2 x 4 studs and the additional weight of the larger studs and the polyviso on the exterior rather than between the purlins. Lot less labor doing it all exterior, vs. rebuildint the interior wall.

    

Have I overlooked anything?
gbwillson

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Reply with quote  #7 
RustyT-

As the wall is finished inside I think an outside build makes the most sense. You could still partially build the collector box inside, out of the wind. And depending if you have any help, you could mount the box sides one at a time. Bert, and others built their collectors that way. You likely could get by using the exterior wall for the back of the collector. Simply add polyiso to the wall once the box wall has been mounted. Don't mount it too low to the ground though. Moisture might cause issues as well as critters looking for a new home. With a collector so large I'd normally suggest 2x6 walls, but since this would be permanently mounted, maybe 1x6 material could be used. But I would use cedar, if possible. Last longer, and critters don't like it. 

The thing is with a ZP collector so large you will need a LOT of airflow. I'm talking about 500+ CFM output. A 4x16 ZP needs at least 400 CFM out the tailpipe so you will need even more airflow. One of the many features of the ZP is you could probably mount the intake anywhere within the intake manifold. It takes very little air movement to overcome the buoyancy of warm air. The high volume of air swirls like crazy in the intake manifold and the only place for the air to go is through the screen slot. I have my ducts mounted in the center of the manifold top to bottom, but not for nay particular reason. And as the output volume of air is so high, you can direct the air in any direction within the garage, including down towards the floor. Just remember, a breeze might feel warm or even hot when your hand is right in front of the output duct. But several feet away, it will feel far less comfortable.

I have tried the ZP both pushing and pulling air through the collector. Pushing works better, for the simple reason that it evenly distributes the air into the screen slot. With a single fan pushing, you will have to design a way to keep the glazing from bulging, as this allows air to bypass the screen gap directly against the cold manifold, greatly hurting performance. My friend Krautman added a second, booster fan near the exhaust. This not only boosted output, but greatly reduced the bulge in the glazing.

And I've never heard of using a dehumidifier in the winter. Cold climates normally have to ADD humidity to the air in winter. Just having air movement in the garage will help dry things out.

Greg in MN
rustythread

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Reply with quote  #8 
Greg:
   Height of bottom above ground is contemplated as  12-18" (bottom of siding is about 12" above grade).

   Reasons for dehumidifying:  1.  reduction of airborne moisture which would condense on machinery and floor during cooler periods, and 2.  drying lumber stored in shop.  Since the typical day here Feb-April is overcast and precipitation, any time I can get the  temp up enough to run a dehumidifier I will likely want to do so.

"intake manifold" ?  I may have alreqdy hit my level of incompetence.  Are we talking about some sort of plenum or other space along the top / bottom of the screen assembly?  or just the box enclosing the fan ?

thanks again!
gbwillson

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Reply with quote  #9 
Rusty-

Come to think about it, my garage is usually wet from all the slush and snow brought in by the truck. And it seems like everything seems like it's "sweating" unless it's really cold. So if I had anything other than a few crap tools in my garage a dehumidifier might be needed. But you would need a really big one as your garage has a lot of space. And you might have to bring in the container each night if the garage gets too cold. 

The manifold, or plenum is simply the area where the air enters or exits the ZP collector. So on my 4x16 ZP, I have about a 10" wide area that runs from top to bottom at each end of the collector, with the screen gap frames in between. So the air enters the intake manifold, swirls like crazy, before exiting through the screen gap. The air travels between the screen layers picking up heat as it moves towards the exhaust manifold, where the air swirls around before exiting through the exhaust ducting. The manifold only needs to be a little bigger than the intake and exhaust ducts, so if you were to use 10" ducts, you would be fine with a 12" wide manifold.

Greg in MN
rustythread

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Reply with quote  #10 
Greg, et al:

Another question, re the overall design:

    Is there some generally-useful figure or range of figures for desired airflow in terms of how many complete changes of air per minute or per second  in the heater?  For example, in the net cubic footage of the interior of the heater is 5" x 28ft x 8ft = .417 x 28 x 8 = ` 95 CF, and I need to have the air in the heater for no more than ten seconds for a turnover of 6 x per minute,  I'm ( I think)  looking at a need for 570+ cfm.  If the desired air dwell/turnover time is only  1 second, I'm looking at a need for 5,700 CFM.  Am I making any sense at all?

Thanks again
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