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.
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.