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!!!!!! Thermal solar collector panel

 
pollinator
Posts: 370
Location: Greybull WY north central WY zone 4 bordering on 3
70
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Well the home made solar thermal panel has been on the front of the house for 3 months so it is time to report.



Some background first.  I live in an earth berm passive solar house built in 1984.  It has a full basement and 3 of the upper walls are concrete with insulation on the outside. The south wall standard insulated wood frame with foam insulation on the outside.   With a family of 4 living it, it uses a about 500 gallons of propane a year.  That runs hot water, clothes drier, cook stove and auxiliary heat.   If heated on any sunny day in Dec or Jan the day time indoor temps upstairs run between 78 and 85 typically with the heat having turned off at 72.  If company is present and ovens running it can easily go over 90 Night time it coasts till about 10 pm to midnight on that heat and then heater begins kicking on occasionally.  If it has been unheated the whole season to that point the interior temps at around Christmas 3 different years have been running morning temps of 48 degree to 50 degrees climbing to the low 70s on sunny days.  Even in our zone 4, borderline zone 3 climate with nothing broken or left open I think it would run all winter without ever freezing a pipe or a plant.  Once the interior temp drops below ground temp it stabilizes and seems to mostly sit there with the ground heat from the basement making up the bit the passive doesn't.  I assume if carried all winter the house would slowly get colder as more heat was pulled.  But I have never quite made it to the new year before I gave up and started heating.  But in those we can be doing 20 and 30 below days and still not dropping below 48 degrees inside with no heat yet that season.   And heated so it starts warm it can still easily reach 80+ on sunny days with 40 below out.  So the house does decent.  

But it does have problems.  1.  It still needs auxiliary heat to stay comfortable.  2. It gets too warm in the late summer early fall.  The sun starts sneaking in the clearstory windows August 21.  Till that time they have been shaded by sun angle.  So for the next 2 months the house will be too warm.  3 In the spring the eaves shade the window to soon and the house runs cold for a about a month or two.

So the plan was to add an active air based collector to the house with the hope that I could get it to run passively without needing a fan.  Pull the air from the basement floor level up thru the collector and push it back in an openable clearstory window and hopefully do this on pure convection although if a fan is needed that is fine too.  In the fall use it to convectively pull more heat into the basement.  To warm the basement slightly.(most days I can dump the excess heat up stairs out upper windows.)  Thus storing a bit of fall heat for the winter.  Then thru the winter the hope was I could bring in more heat to reduce auxiliary heating while at the same time pulling warm air to the basement to use the basement walls to increase heat storage.  Then in the spring because most of the collector is below the shadow from the eves it should add spring heat to compensate for losing the passive solar.  In summer just open it to vent to the outside top and bottom and let it chimney to keep it from getting too hot inside.  Possibly at a later date add a hot water preheat system to it for summer use.  By using the top of the existing escape hatch from the basement bedroom and going back in the window  the only real changes to the house were shortening the escape hatch door, some caulk on the siding and a few holes in the siding if it didn't work and I wanted to take it down.  Door is simple to replace, caulk holes and scrape and paint remaining caulk if it fails.  And of course size was determined by the size of the windows I was given.

So now for the collector.  Probably nearly everyone has seen the articles or videos on pop can solar heaters. These are the same thing built differently. There are dozens of types of do it yourself  air thermal solar panels. Simple black box, Pop can, window blind, sofit vent, rain gutter down spout, window screen and more.

The window screen panels are winning all of the comparisons that I have seen. They are using multiple layers cheap fiberglass window screen as the collector surface. The window screen apparently won't take the heat and sun  long term and gets gray and fuzzy over time. The recommended cure for this is simply to paint the screens with flat black bbq spray paint to start with. It apparently protects the resins that hold the screens together just enough to keep them from self destructing over time. Originally they ran the screens parallel to the box and the inside of the box was painted black. Over time they changed both of those. The screens were found to work better in on a diagonal starting out at the glazing at the bottom and tapering to the back wall of the collector at the top. That way all the air is forced to go thru each layer of screen and in the process the hottest air is pulled away from the glass helping to keep it hot. The internet experimenters also found rather than painting the box black that lining it with foil reflected the light back out so it hit the screens again heating the back side of them. Comparisons there were showing a 3% to 5% gain in a foil lined box vs a black box. Screen collectors win on both heat transfer and lack of air flow restriction basically every where I looked. One of these panels in a vertical wall mount like I wanted was showing a measured 123 cfm air flow on natural convection with no fan.
This last piece of information lead to my plan. The plan is the windows gifted provide roughly 17% more glazing surface area to the house. The advantage of active panel design over passive is that you don't have the night time glazing losses subtracting from house heat. And if I can keep the active panel passive in operation no needed power to operate. Disadvantages include house getting hotter rather than directly transferring the heat to the mass, potential need for a fan. Now if I can push this heat down into the basement because of all the concrete for mass heat storage down there I can store heat and hopefully level the household temps some. So the plan is to pull colder air from the basement by hopefully natural convection going out the top of the basement bedroom escape hatch and going back in the clearstory window. This way no major permanent mods to the house. The collector will heat the upstairs, suck air out of the basement and there by hopefully pull all the air down warming the basement in winter mode. In fall mode it will pull heat down and simply heat the basement increasing the stored heat for winter. It will be one time heat but free. In late spring open the vent at the top of the collector and use it to pull air out letting cooler outdoor air settle in the basement cooling it for the summer. Summer mode vent both top and bottom of collector to the outside while closing the house off and let it chimney the heat away. Might provide domestic hot water heat for 4 months or so if done correctly.

So the collector box would normally be made with either 2x4 sides or 2 x 6 sides.  But to have enough room to open the window under the collector glazing I was looking at needing 2x8 sides to keep enough room to open the window inside the collector.  Total outside box width is 36 1/2” and the collector from  ground floor level goes up 14 feet.  The box goes 2 feet below ground level to go in thru the top of the basement escape hatch doorway.  The screens are cheap fiberglass screen painted with bbq paint.  For this collector I used 3 cans and probably should have done a 4th one. The back is uninsulated 1/2” plywood.  I skipped insulating it figuring the fact that it was on an insulated wall would serve.  The foil liner is standard commercial grade food foil from Costco.  I glued it with silicone sealant because all the other glues I could find broke down with heat.  The silicone should take 450 degrees which is about all the wood would take anyway.  The screens were attached at the sides with 1x2 furing strips wrapped in foil.Where the screen mounting cross bar hit the glass I moded the strip to go close to the glass and stapled on strip of rubber inner tub to actually go out and touch the glass to form a air seal.  The windows were left in their wood frames and mounted on weatherstrip and screw directly to the front of the collector.  The goal was to have all light coming into the collector go through 2 or 3 layers of window screen coming in, hit the foil and bounce back and then go through the same layers out bound.  The screens were 7 feet long because that was the standard roll length at the local hardware store.  Every joint had multiple seals on it.  Some combination of glue, caulk or weather strip.  The back of the collector where it mounts to the wall was done on weatherstrip to space it out a tiny bit from the siding.  This is to allow a small amount of air flow behind the collector to try and be sure I didn't trap moisture in the wall.  Because of where it sits on the wall it completely covers one 16 inch stud cavity.  It is heavily sealed to where it meets the wall around the window with multiple weatherstrips and heavy caulking inside.  And the box is wraped in tyvek.  The goal was no air leaks.

So now for what was learned.

First of the expected problems that did indeed occur.  
1. The house did get hotter  up stairs and especially near the ceiling under convection operation.  And this heat does very little directly for night time holding power as it has to conduct into the mass which gives poor storage.
2. Whole collector box cools off at night to outdoor temperatures. Minimal mass, sealed off from the house and a really good black body radiation surface virtually guarantees this right thru the glass. The very things that makes it a good absorber make it a good emitter too.
3. .

Second in operation under convection with good sun the air flow varies in the range of roughly 60 to 120 cfm.  120 cfm will result in one air change per hour to the basement.  I started out simply estimating air flow but was given the proper equipment to measure it for Christmas.  With good sun near noon, snow on the ground to reflect a bit more light into the collector the air leaving the basement is 58.8 to 60 degrees and the air coming in the top is 95 to 110 degrees with an air flow close to 120 cfm.  That is with no fan.  In early January the air flow was just barely noticable at about 10:15 am to 10:30 am and by 11 am was hitting peak flow.  It starts to fall off again about 2:30 pm and has basically stopped by 4 pm.  If the ceiling fan is run to keep the room air mixed instead of stratifying those temperatures mostly hold over most of the day.  But if the air is allowed to stratify the air flow drops and the air coming in the window can climb to the low 120's

Now if a common box fan running on high is placed in the upper window to suck air into the house from collector the air flow measures just a bit over 450 cfm and the incoming temperatures are running 82 to 85 for a roughly 22 to 25 degree gain in temperature.  If you punch that into the calculator correcting for elevation that gives roughly 10,000 BTUs per hour from the collector peak flow.   Under convection it is roughly 2/3 of that.  So using the heat energy to drive the convective cycle plus the added losses out the glass because the inside of the box is hotter gives those losses.  They can also be felt.  Standing in front of the glass outside under convection I can feel the heat radiating onto my face.  If I turn the fan on and wait 10 minutes that radiated heat feeling disappears.  So it will work poorly in a purely passive mode and better fan driven.  So getting quiet efficient fans into it will be part of the future changes.

Third there were a bunch of surprises that would change the future designs or thinking.

1. The first one was where I wasn't heating the house yet I had forgotten to take into account that the basement is warmer than the upstairs in the morning so the first air the collector drags down is actually colder than the basement cooling it rather than heating. Running the heater would cure it. But for now since I want to experiment without any heat I have found that throttling the air inlet in the basement to about 1/2 to 1/3 seems to help slightly. As the over all house temp is slowly climbing it is less of a problem.  Another year if the collector can run all fall hopefully much of this will go away and it goes away the minute I light the heater and start adding auxiliary heat.
2. Next surprise was simple understanding. The directions say for best convection flow the inlets and outlets should nearly equal the collector. Since I was basing my design on either 3 1/2" or 5 1/2" deep collectors but was being forced to go 7 1/2" to have enough room for the window to open under the collector glass I based my thinking on the smaller cross sections. 3 1/2 x 33 1/2 is 115 sq inches. So it is less than 1 square foot. So the duct into the collector is 29 x 5.5 for 1.1 square feet. The collector box without the window screen furring strips in it is 33 1/2 x 7.5 for 1.7 square feet. The outlet in thru the window is 24" by just over 5.5" plus a triangle on either side as the window angles out. The 2 triangles add up to another rectangle roughly 18" x 5.5" for an inlet of 1.6 square feet. That means the inlet is the biggest restriction. I have found that chopping it in half adds roughly 4 degrees to the convective outlet flow into the house but that the flow volume actually rises most of the back in spite of the restriction.
3. I was surprised at how saggy screens get when hot in the collector in spite of being assembled with as much stretch as I could reasonably get on them.
4. I didn't expect the late day circular flow inside the collector because of the shadow. This one might not happen in a shallower collector.  But with  the box that deep and under convective flow I can feel the cold air coming down from the shadowed side of the box into the room while the sunny side draws up.
5. I didn't expect as much die off as the hot air up high piles up late day. This means I will have decent flow for a shorter time. Plan had been for 5 to 6 hours. Looks like 4 to 5 will be as good as it really gets.  Running the ceiling fan to mix the air in the room helps.
6. I was surprised by how much space the furring strips wasted in the collector box. It was what most of the builders were using and cheap.
7. Next unexpected one is the amount of moisture it condenses each night. It is filled full of moist household air on any sunny day and then at night freezes it on the glass.(maybe some on the wood but as best I can feel reaching out the upper window it seems to be mainly on the glass. Which makes sense as the glass should cool slightly faster and be slightly colder making it the more ideal spot to collect.   The fact that my box is deeper than common amplifies the problem because my air volume is greater.   But this means a catch basin moisture trap needs to be added to the design or the water will eventually ruin the bottom of the collector.  Measured moisture has been running roughly ½ a cup per day of operation.  Looking at how the water collects each pane of glass should have its own catch basin at the bottom of it.  A drain trap could go outside to drop it outside but it would need a u-trap to seal it from air leak and that would freeze every night so the tubing would need to be flexible enough to handle it.  But more likely here the best thing to do is bring the tube inside and simply run it into a bucket to collect the water.  Use the water in the bucket as the trap to seal the air leak.
8. Next surprise has been how little clouds it takes to make a noticeable hit in collector performance.
9. Another surprise is how much cold air flow comes down if I forget to close it after dark for what is a sealed box meaning all heat loss is thru the glass.  Air flow down is nearly 20 cfm and the air flow into the basement is 48 to 52 degrees .(this measurement is really questionable as it is right on my anemometer's bottom reading range) I realize that the collector is designed to absorb and therefore emits well, but this has me looking seriously at how to cover houses big clearstory windows with some sort of insulated shutter.  The collector windows are good double pane sealed with no sign of moisture inside so the seal is still intact and I am sure my box is sealed tight which means this is basically all radiative losses.  So that means the household windows will lose nearly equally.
10. Another bit of surprise was the gain from having snow on the ground so the collector gets a bit of reflected light too.

What would I change for a future design or do I need to add now.
1. Probably the most important would be to automate this.  Anytime I forget to seal the collector off at night either top or bottom I see losses and amount of time it takes to manually babysit the collector is surprising.  Have the aurdino and now need 4 temperature sensors and 2 to 5 servo or stepper motors to control the system and fan(s) to go in the collector.  Outdoor temperature, collector temperature, upstairs and down stairs temperatures.  A minimum of 2 locations need to be automated.  The window into the top of the collector and butterfly valve into the bottom.   Ideally long term it would likely be desirable to control doors into the collector outside top and bottom as well and to control another window in the house to vent hot air in the fall.
2. Need to better insulate the collector.  I was leaving that part till I knew better what I wanted for controls.  So for now the sides of the collector box are only 2X8 material with a layer of tyvek over the outside. Eventually I would like to get insulation and siding on them too.
3. Need to open the collector back up and install a water catch basin system on both panes of glass to keep the wood work inside  the collector dry.
4. Need way better seals around the butterfly door.  As temperature and moisture vary the current duct and door change shape too much and that is making for a poor seal.  Should think about some sort of magnetic seals here to get a positive seal.
5. The online directions argue that either 2 or 3 layers of screen is the correct.  But I have one short stretch of 4 simply because how screens worked out.  Looking at the light reflected off the foil and feeling how much heat makes it back out to me I am going to say it should be a minimum of 4 layers so any light hits 4 layers then is reflected back out thru the same 4 layers.
6. Sagging screens in the heat let them touch each other so heat gain will be reduced and possible air flow reduce.  The cure for this would be a scaffolding of wires running basically vertically under the screens with springs at the top and tied at the bottom on the same angle as the screens. Space the wires on roughly 8” to 12” spacings.  Current thinking is to gather a bunch of springs off an old bed spring to use as tension(paint them black of course) and run aluminum electric fence wire as the wire.
7. The furring strips used to hold the screens on the side use a lot of room reducing cross sectional area for air flow and would make spacing the screens 4 deep more difficult.  So I think for a future build I would like to try using the extrude aluminum strips that they make screen frames out of  that anchor the screen with a rubber spline.(will the spline take the heat?)  I would turn the strips so the spline faced the sidewll of the box and the screen was also pinched to the box by the strip as well as held by the spline.  Drill holes thru to screw it to the box.  It might even be possible to add little belville washers under the screws to let the strip angle just a bit so it could provide adjustable cross tension a bit.  While it would be far easier to simply buy the strips in a length needed salvaging a bunch of old house(or trailer house) screen frames might provide the material to do this as salvage.  It would probably be shorter pieces and a bit rougher to install is all.
collectorweb.jpg
passive air heater
passive air heater
 
C. Letellier
pollinator
Posts: 370
Location: Greybull WY north central WY zone 4 bordering on 3
70
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Have added 6 120mm square 12V muffin fans to drive the collector actively.  With a rated air flow of 108 cfm @ .28 inches of water pressure and 5 watts each that is a theoretical 648 cfm and 30 watts.  Measured air flow is actually a bit higher so I assume my back pressure is less.  Also the fact that short term I am running off an old car battery and a battery charger means slightly higher than 12V to the fans so that might also help account for the higher flow.  12V was chosen with the long term goal of going photovoltaic on this part as the ability to run if the grid is down would be nice.  By moving the noise of the fans down stairs there is almost no noise up stairs.  It is reduced to a barely audible hum.  And even down stairs standing right beside the fans they only make about 1/2 or 2/3 of the noise the box fan did.  And being a basement bedroom that would be unoccupied most days the noise shouldn't be a bother.  And if it is used during the summer at night to push air out to help cool the basement it would be easy bearable to sleep with.

I spent a bunch of time looking for quiet high efficiency fans for this and was lucky enough to find the most efficient fan I found(previous years model) in surplus for $5 each so there is only $30 cost in the fans and no freight because it was combined with a business order that paid the freight.

Below is the picture of the fans in the inlet duct out of the basement.  As this was the experimental rigging the wiring needs to be neatened up and weather stripping added around the fan bar to reduce leakage back around the fans.  The steel rod across the middle of the picture is the pivot for the butterfly door that closes the bottom end of the duct off.  The door is out to give more room for the install and was left out to take a clearer picture of the fans.  As you can see the fans only block around 1/2 the duct and I know from experimenting in Jan. that the convection operation still works with half the duct blocked off with only slight rises in collector temperature and very slightly reduced flows.  So having the fans in the duct will only marginally affect gains if the collector is run in convection mode.



 
C. Letellier
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Posts: 370
Location: Greybull WY north central WY zone 4 bordering on 3
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Fans in duct
duct-fans.jpg
heater fans
heater fans
 
C. Letellier
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Posts: 370
Location: Greybull WY north central WY zone 4 bordering on 3
70
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And now the report on success so far.  I quit heating the house a month early and the collector has mostly made up the difference.  A few cool days with the house getting down to 61 degrees 2 days in that time but having the collector doing active has mostly made up for the clerestory windows being shaded by the eves of the house and by the lower solar gain in the time just before that.  And the 2 cold days were partly my fault for forgetting to close the drier vent.   While it hasn't quite reached the dream goals it has been fairly successful at reaching the expected primary goal of shutting down heat sooner in the spring.  If I get it automated so I don't forget it etc I really think it may do as much as 6 weeks early most years.

 
C. Letellier
pollinator
Posts: 370
Location: Greybull WY north central WY zone 4 bordering on 3
70
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Well it is time to report again.  The collector has been in and running a bit over 11 months.  It is the 11th of November in WY and we just had a blizzard last night with 30 mph winds and temperatures around 0 F.  I started the collector running a 10 AM  Outdoor temperature 28 degrees.  I still haven't heated the house this fall.  Between the passive design of the house and the active collector my worst morning no heat has been 58 degrees.  I started this morning with 62 degrees upstairs and down.  But 10 AM when I started the collector the passive had brought the upstairs temperature up to 63.4 degrees.  On passive collector operation I had .66 feet per second which translates to 44 cfm passive flow just starting the day. and 73.1 degrees coming in the top for basically an 11 degree rise.  Probably should have started the collector a bit earlier.  Turning the fans on brings me 9.48 feet per second  which translates to 625 cfm.  All running off a salvaged computer power supply off the 12V leg.  It is now 11:40 and the air incoming to the collector is still 62 in the basement.  The outlet temperature is now 83 degrees with at least 625 cfm flow

Thru the summer months having the fans so I could force ably exhaust the basement air at night was beneficial in cooling the house.

I started heating the basement in mid Aug to store heat.  I found I could raise the temperature of the basement about 4 degrees during the day.  Over night it maintained about 0.9 to 1 degree gain per day and the concrete ate the rest by morning even if I had the house closed up so no cold air came in.  Gray days of course there was little to no gain.  Colder fall this year with more rain and gray than last year and the house is still warmer than last year at this time because of the collector and the stored heat in the basement.  Dumping the heat to the basement during the late summer did keep the house a bit cooler.  But I found once I had the basement starting the the low 70 that meant it was getting too warm to comfortably go to bed(since I have typically slept in the basement.)  So I both gained and lost comfort.  I find the basement floor is still staying right at earth temp and nothing I do seems to change it much.  The walls are insulated on the outside but the floor isn't.  But more importantly I simply can't pull the warm air down to it good.  So if I were to build a new home there would be some way to put that heat under the basement floor and not just on top of it.(think it would be water based instead of air)  Have learned that 6 air changes an hour is about the minimum that will effectively transfer the heat to the mass of the basement.  Which is coming out right at the number that the earth battery greenhouse people are recommending too.  They are saying 5 to 10 air changes an hour typically

Things to change for the future.

In the collector.
1.  Still need to get the moisture collection system inside the collector
2.  Still need to automate it.(have had a bad case tomorrow disease here combined with not finding part of the parts I need cheap)  I can also make small gains insulating it too and that has been waiting on me getting the wiring for the automation run.
3.  Possibly add more layers of screen.  

In the house/system over all
3.  I find that if I am running the house closed up which is necessary to run without heat that I have to be very careful or it starts to get musty.(no large pans of boiling water, no occasionally air drying clothes inside etc)  So adding another collector designed purely to heat incoming air I think needs to be on the goals list.  Much lower flow needed so I think I could run it on pure convection.  If I want to be able take potentially -40 degree air and bring it in at 70 that is a 110 degree rise.  While that is rare that should be the criteria.  The current collector lifting from 8ft of dead air(height of the basement) would make enough flow.  Remove the need to lift the dead air and it easily should be able to back pressure some when not running at 40 below.
4.  Having felt the collector loss at night when I know it is a well sealed box has me seriously looking at how to add insulated shutters to cover the 4 big clearstory windows of the passive solar.

Now having researched a number of things that didn't make sense like how rapidly the collector shuts down and why I don't get more gain late and early I will try and get write ups on those in here next.
 
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do you have an hrv in the house? it can be really expensive with the exchanger core or as simple as a long tube with a smaller corrugated aluminum duct inside it with muffin fans pushing opposite ways for incoming and outgoing air.
 
C. Letellier
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David Baillie wrote:do you have an hrv in the house? it can be really expensive with the exchanger core or as simple as a long tube with a smaller corrugated aluminum duct inside it with muffin fans pushing opposite ways for incoming and outgoing air.



No I don't.  If the heater is running it pulls enough to keep it from being a problem even with a family of 4 living here.  In the last 35 years it hasn't been a problem for the most part.  But now that I am trying to get rid of that heat and therefore the air draw I will have to do something long term.  While my parents were alive my mother always tried to make it to Thanksgiving before lighting the heater and made it about 50% of the time.   The house is so tight that it is necessary to open a window or a door to keep from back drafting the heater(or occasionally the water heater)  With the heater running the house gets too hot on a good sunny winter day so opening doors or windows in later afternoon was common.  So there was never a problem.  Now I am trying to use the collector to add a bit of heat and more importantly take the excess to the basement to use the basement for additional thermal mass to reduce heating.  But that means opening doors and windows less or not at all so I will have to do something.  Current thinking is to add another collector coming in another existing crank open window to bring additional air and at the same time add an air to air heat exchanger and a heat bell to recover heat from the drier.  Still mentally designing on that one.  Coming up with a bunch of good window glass is the first step.  Still trying to figure out how to implement the european style water ground loop HRV with it to.  No heat pump but simply a large ground loop.  The slope of the ground behind the house would be ideal for one aimed up to the west.  Problem is that is completely away from the best places to put in the air exchangers.  I know long term I have to do something and I am paying attention to it short term while I learn.
 
C. Letellier
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Now for a discussion of screen and drag and screen angle.  The drag info is my own thinking while I was trying to decide screen size and angle.  The screen angle stuff I am going to try and recreate a discussion on a users group that I read while I was researching collectors that I can't seem to find my way back to.

First lets start with a picture of the screen and then a graphic to explain the thinking.  While I realize this is fiberglass window screen and has no wires for expediency I will call them wires for this discussion

So look at the picture and measure.  You will find the wire is roughly .15 to .2 the width or height of a cell that we will call 1.  For now lets say it is .2 If you refer to the screen #1 graphic you will see that the total air passible area of a 1x1 cell of screen is 1(total area of square) -.2 x 1 (horizontal retangle)  - .2 x 1(vertical rectangle) + .2 x .2(need to add back 1 of the area where the 2 rectangles overlap) = 64% of any given area of screen is air passage.  So if I want the screen to have the same area for air flow as my box how long must my screen be.  Lets pretend this is a 5 1/2" deep collector box.  So width x 5.5 (cross sectional area of collector box) = width x length x .64(cross sectional area of the screen.  Width cancels giving us a screen length of just over 8.5 inches.  Now because we are going thru all those little holes each will add drag so we probably want way more.  So if the screen is just 3 ft the cross sectional area of the screen be over 4 times greater.  Now that should serve to reduce the drag.  Now to allow for opening the window inside it my box was deeper and I used 7 ft lengths of screen I still have roughly 8 times the cross sectional area getting thru any screen compared to the cross sectional area of the box.  Most of these collectors are either 3 1/2 or 5 1/2 deep.  So even allowing for all the small holes any individual screen will add very little drag.  

Now on to a discussion of screen angle.  The screen can be angled out or angle in and each has its advantages.  Orginally the screens went parallel to the air flow.  The problem with this is some air can completely avoid touching a screen.  By angling the screens you force the air flow to break into little tiny pieces and basically every part of it touches or comes close to the screen to scrub heat off it.  If you look at the screen #2 drawing you will see sun rays and the horizontal screen wires in 3 orientations.  Notice that far less light makes it thru the screen angled out at the top while the one angled in at the top lets most of the light thru.  If the screen is perpendicular to the light the maximum light makes it thru while as it approaches parallel it it begins blocking all the light as the wires begin to shade each other.  The primary goals here are maximum utilization of surface area and keeping the heat away from the glazing.  When you look at the screen #3 pictures your first response and mine is that the first picture is far better because keeps the heat away from the glass. and nearly all the heat is produced in the first screen.

Now the counter argument to this was that in taller collectors you wanted more layers of screen like shingles because heat transfer was also a function of surface area and the time the air spent in contact with a hot area.  He also argued that more even transfer helped so instead of trying to get 40 degrees off a first screen and 10 off the next 2 that it was more effective to get 20 off from each  Now looking at convection operation in this argument I added the farther down the air gets hot the more lift it has to drive convection.  My collector has screens for heat transfer 16 inches below the glazing.  If I was using the other slope I could only get 8.  So the question becomes one of where the heat is the greatest and which way it is being pushed.

Look at drawing screen #4.  I didn't get it quite right as it should be 3 layers of screen in every where.  So pretend it is 3 layers to the back.  If we ignore the diameter of the wires.(which mostly not show because it is the same everywhere the amount of sun that passes thru each layer will be the screens transmisivity x sin(angle between the sun and the screen)  So pretend for now those happen to be the .64 above, 30 degree sun angle and a 5 degree positive or negative screen angle for a 55 and 65 degree angles respectively.  So for screen out at first screen 48% of the available sun is absorbed by the first screen on the way in and 25% at the second and 13% at the third screen going in.  Then angle of incidence equals angle of refraction so it is headed back out thru the screens.  We have already gotten 86% of the available energy.    For the in at the top we get 42%, 25%, 14% for a total of 81% of the total energy.  Now because our angles have fliped the angles trade too.  So for out thru out at the top we have 7%,3%,1.7% for a total path of 97.7 with a 2.3% loss  So the third screen total is 21% absorbed, second 28% absorbed and first is 49.7% absorbed.    Going screen in it is 9%, 4.8% and 2.5% for a total path of 97.3% with a lost of 2.7%(must have a rounding error because the loss rate was identical in the example).  So here the third screen absorbed 23%, second 29.8% and 44.5%  So we have a 4.2% lower heat rate out next to the glass angling in and 1.8 % warmer 2nd screen and the third screen is also warmer by 2% in an angling in collector.  The heat more evenly distributed should give better heat transfer and the fact that the back side of the inner screens is heated more should give better surface area heat transfer.  That combined with the fact that the vertical wires will tend to push the heat in rather than pushing it out was the argument that convinced me to go with screen in at the top.  There was no side by side comparison on this.  
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C. Letellier wrote:

David Baillie wrote:do you have an hrv in the house? it can be really expensive with the exchanger core or as simple as a long tube with a smaller corrugated aluminum duct inside it with muffin fans pushing opposite ways for incoming and outgoing air.



No I don't.  If the heater is running it pulls enough to keep it from being a problem even with a family of 4 living here.  In the last 35 years it hasn't been a problem for the most part.  But now that I am trying to get rid of that heat and therefore the air draw I will have to do something long term.  While my parents were alive my mother always tried to make it to Thanksgiving before lighting the heater and made it about 50% of the time.   The house is so tight that it is necessary to open a window or a door to keep from back drafting the heater(or occasionally the water heater)  With the heater running the house gets too hot on a good sunny winter day so opening doors or windows in later afternoon was common.  So there was never a problem.  Now I am trying to use the collector to add a bit of heat and more importantly take the excess to the basement to use the basement for additional thermal mass to reduce heating.  But that means opening doors and windows less or not at all so I will have to do something.  Current thinking is to add another collector coming in another existing crank open window to bring additional air and at the same time add an air to air heat exchanger and a heat bell to recover heat from the drier.  Still mentally designing on that one.  Coming up with a bunch of good window glass is the first step.  Still trying to figure out how to implement the european style water ground loop HRV with it to.  No heat pump but simply a large ground loop.  The slope of the ground behind the house would be ideal for one aimed up to the west.  Problem is that is completely away from the best places to put in the air exchangers.  I know long term I have to do something and I am paying attention to it short term while I learn.



Most super-insulated homes have this issue, and the way they deal with it, is to install a bathroom fan, and then install a timer for it. Every 5 minutes on the hour or so, they turn on, bringing in fresh air into the home, but not too much fresh air. You can adjust the fan as you see fit.
 
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Love what your doing here and your reporting on it!

Have you considered aluminum screen?
It should be resistant  to heat induced sag.

I see it affixed horizontally at the top with a strip of wood or metal furring,the screen fabric   cut or folded to width,  and affixed at the bottom as in a similar way.
With tension between the top and bottom,  you might not need to affix the sides at all.

I have considered building a collector for my homes south side
With a first story of brick walls,  I would need insulation.
Have you considered lining your collector with foil faced insulation boards?
 
C. Letellier
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William Bronson wrote:Love what your doing here and your reporting on it!

Have you considered aluminum screen?
It should be resistant  to heat induced sag.

I see it affixed horizontally at the top with a strip of wood or metal furring,the screen fabric   cut or folded to width,  and affixed at the bottom as in a similar way.
With tension between the top and bottom,  you might not need to affix the sides at all.

I have considered building a collector for my homes south side
With a first story of brick walls,  I would need insulation.
Have you considered lining your collector with foil faced insulation boards?



As for aluminum screen yes it was considered.  No one ever mentioned the screens sagging together when hot with the fiber glass and it was considered the cheaper answer.  The one problem I see with metal screens is they are more porous so you will likely need more layers to get most of the sunlight.

As for attaching top and bottom unless you make it somehow adaptive you will still have the problem with it losing tension when hot.  Maybe attach the top and hang weights off the bottom?  When I was figuring out how to stop the sagging it was one of the options I looked at.  Finally decided I had lots of old bed springs around and that would be a better option for me.  As for not attaching to the sides the big thing is finding a way to be sure all the air has to go thru the screen rather than around it.  If it works great.  There are lots of answers.

As for insulation I very much considered it.  I skipped it for 2 reasons.  1.  Since I was screwing the collector to a heavily insulated wall it seemed foolish.  The tiny air gap I created using the foam  weather strip I don't think is much of a heat lost threat.  2.  I wasn't sure if it would work well enough to be worth it so not using the foam was an effort to save a few dollars.  Reminder here be sure you use the right foam.  Standard expanded/extruded polystyrene foams won't necessarily take the heat.  Lesson in that are delivered on a couple different youtube videos and a couple of the comparison sites.

If you are where you can do it I encourage you to try.  While it hasn't reached my dreams it has done well and I think would do so for nearly anyone who has good sun.  With last winter's mild winter it is hard to tell but right now I think it will pay off in 4 to 8 years.(remember I was given the glazing so if buying that too will add to pay off time)  As best I can tell it saved somewhere between 50 and 100 gallons of propane last year.
 
C. Letellier
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Several notes.  The collector has been in over a year now.  It is the third of Jan and I still haven't heated the house beyond solar.(remember I am northern WY and most people have been heating for months)  Roughly a month ago had 8 consecutive days with basically no sun and zero and slightly below nights with days climbing into the teens.  Ended up with the house doing 2, 51 degree mornings in that time and hardly warming at all.  Most of the heat stored in  the the basement walls thru the fall was exhausted during this time.  In the time since the weather has been slightly warmer than normal and in the  time since the basement basement has been bouncing in the 57 to 61 degree range in the morning before heat gain depending on sun.  On good days the house upstairs has reached 76 degrees several days and  the coldest morning has been 53 degrees upstairs.  So while not totally comfortable not that bad.  Moisture on the windows and a bit musty is still showing the need for an air to air heat exchanger.  Here is my best thinking as to how to build it but still just thinking.

Possible solar air to air heat exchanger design.

The biggest thing learned new is I am more convinced than ever that if the goal is to transfer heat from hot air to thermal mass that tons of air moved is a critical factor.  Plus I have found on days that the house got warm that I may actually want to run the collector a bit past when it stops making heat.  And the reason is to give more time to transfer the heat to the basement.  An extra hour or 2 of circulating the air and thus pulling the heat to the basement seems to be beneficial in spite of the fact that it actually cools the upstairs.  Moving that heat lower seems to result in a warmer house come morning.  But at the cost of a bit of warm house in the evening.  It has me wondering if I need to add more fan power looping the air to better share the heat with the basement?
 
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That's great!   ....you know I'm partial to convection only but this information shows people their house can be warmed in this fashion....and nobody is gonna freeze at 57 degrees or 51 degrees during a low.

I never got back on the wiki/research structure because I'm no longer sure that's really the style Paul wants here......and there are so few who are actually doing it for us to collaborate.

....anyways, lemme look at your possible desgin and get back (gotta work for the next four days so it'll be awhile)
 
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