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Solar heat box

Post in 'The Green Room' started by Billy123, Mar 19, 2011.

  1. GaryGary

    GaryGary Feeling the Heat

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    Hi,
    This is a somewhat unusual design.
    As they mention, most collectors of this type would place the absorber panel over the top of the baffles and run the air behind the baffles. There would be no air flow between the glazing the absorber. This is the traditional back pass collector.
    They place the absorber behind the baffles and run the airflow between the absorber and the glazing.

    This seems questionable to me in that you are scrubbing high speed and hot air over the glazing -- it seems like this would result in increased heat loss out the glazing. This is bad in that the heat loss out the glazing is nearly all of the collector heat loss -- its basically the reason that typical air heating collectors run at 40 or so percent efficient instead of 90% efficient. So, generally you want to do things avoid heat loss out the glazing.

    This is a long way of saying that I think the screen collector is probably more efficient. But, I've never tested the two side by side, so I could certainly be wrong.

    Another factor to think about is that the back pass collector (a collector with baffles to force the air to pass over the full absorber surface) had a lot more pressure drop than the screen collector. This means a bigger fan and nosier fan that uses more power if you want to achieve the same flow rate.

    Gary

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  2. fishingpol

    fishingpol Minister of Fire

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    Gary,

    Thanks for your replies. I have some thinking to do. I'm reading more on BIS and will consider my options for a collector. I am going to take notice of temperature on the south side of the house. I did make a small collector today using poly-iso foam foil tape and a piece of regular glass. I mounted it vertically to the side of the house put in small air holes on the top and bottom to prevent overheating. I stuck a Weber remote meat probe thermometer into the side that gave me constant readings in the house. Highest temp was 140* in the collector in the afternoon. Not bad for a 40* day with wind and clouds. I will make a slightly larger one as I have some old storm door panels. When I make the collector, I will probably incorporate the supply air from my basement to travel inside a duct within the collector to reduce heat loss. It will travel from the top, down along the (in)side in aluminum round duct and to the bottom into a baffle to disperse flow. It will be separated from the main collector with a foam partition wall. The window it will be going through is higher than collector so I may also incorporate a fan to move the air through.


    I did read through your $2k collector for domestic and radiant floor heating. It was very interesting with a ton of info. I am mechanically inclined and everything you posted makes sense and was easy to follow.

    Have you heard of any issues of poly-iso foil de-laminating from the heat?

    Do you recommend painting the poly-iso flat black? It is a little glossy and possible reflective.

    Thanks ,

    Jon
  3. GaryGary

    GaryGary Feeling the Heat

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    Hi,
    I've not had any trouble with the polyiso delaminating or degrading -- it appears to stand up to collector temps well.

    There are several types of face sheets used on polyiso -- a lot of it has a sort of aluminum foil for the face sheet.
    I use a lot of the Atlas R-Board, which is polyiso with a face sheet that appears to be a thin sheet of fiberglass. It comes in grey and takes paint very well. Makes a nice inside insulating shutter: http://www.builditsolar.com/Projects/Conservation/HalfShutter/HalfShutter.htm

    On the $2K water heater, the absorber is opaque, and basically no sun gets to the polyiso behind it, so it just left it the grey it comes in.
    On the screen collectors, I've been painting the polyiso behind the absorber black to absorb the light that gets through the screen.
    I used to use the Rust Oleum flat black BBQ paint, but this is a little expensive and time consuming for larger surfaces, so I had Home Depot mix up a gallon of flat black latex to see how it would hold up inside the collector. It seems to be doing fine even when the collector is stagnated.

    You do want to be careful about materials used in collectors -- a lot of materials don't hold up will at collector temps. Particularly important for air collectors since you are breathing and smelling anything that out gasses.

    Gary
  4. fishingpol

    fishingpol Minister of Fire

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    Ok, thanks for the info Gary.

    Jon
  5. Amaralluis

    Amaralluis Member

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    how well does those collectors work in the winter in the colder zones like up here in the Maritimes?
    what kind of btus can one expect to achieve with this?
  6. GaryGary

    GaryGary Feeling the Heat

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    Hi,
    I live in Montana, and the collectors do well here -- we have days where the high is below zero (that's zero F :)

    This is my shop heating collector at -20F:
    http://www.builditsolar.com/Projects/SpaceHeating/SolarGarageCollector/GarCol20F.htm

    If you want a quantitative answer, this collector calculator will tell you how much the efficiency and heat output drop with temperature:
    http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm#efic
    Just run with a mild (say) 40F, and get the efficiency and output, and then run it again with a colder temp to see how much it changes.
    One thing to bear in mind is that when you look at outdoor temps, you want to look at the high for the day, since that is when you are doing the collecting. And, you want to look at average outdoor temps for each month of your heating season -- not just the coldest month. You can get good averages at this nice new weather site: http://weatherspark.com
    Don't guess at the average temps -- most people guess way to low. If you ask someone who lives here in Bozeman what the average January high temp is, they will probably guess around 0F, but in fact its 30F.

    Gary


    Gary
  7. fishingpol

    fishingpol Minister of Fire

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    I built up a collector yesterday using 1/2" poly-iso. Roughly 24" x 28" using a 1/4 piece of glass from an old storm door panel. All pieces sealed with foil tape with two 1/2" holes at the bottom and two at the top so there was some movement of air. My highest reading was 168*. I'm sure I could realize higher temps in the collector if the supply was ducted from the basement air at about 65* rather then outside air at 45* The sky was pretty clear with a very thin haze of clouds. The depth of the box was 6" deep with no absorber. The mock-ups are getting bigger. It would be nice to vent that into the basement in the winter. Wheels are turning...
  8. GaryGary

    GaryGary Feeling the Heat

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    Hi,
    You actually want to move air through the collector fast enough so that it does not get too hot.

    You want about a 50 to 60 F temperature rise from inlet to outlet.

    The hotter the collector runs, the less efficient it will be as it will lose more and more heat out the glazing.
    The 50 to 60F temp rise is good in that the air coming out of the collector is hot enough to feel warm, but not so hot as to lose too much heat out the glazing.

    To measure collector output you need to measure both the temperature rise through the collector and the flow rate through the collector. The heat output is proportional to the product of these two, and anything that improves this product makes the collector more efficient. Measuring just the maximum temperature a collector can produce does not really tell you anything about how well its doing -- except that you know that if the outlet temp is quite high, that the collector is losing a lot of heat out the glazing and won't be efficient.

    Some more on measuring collector efficiency/output:
    http://www.builditsolar.com/References/Measurements/CollectorPerformance.htm

    Gary
  9. fishingpol

    fishingpol Minister of Fire

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    Gary,

    I will read the link. The final product will have some sort of fan moving air through. I was also thinking of using "blast gates" that are available through wood working catalogs that can be slid opened or closed to adjust the flow of air. I was actually thinking a fan would send the air through too quickly and not pick up all the heat potential.



    http://www.rockler.com/search_results.cfm?srch=usr&filter=blast gate&submit;.x=15&submit;.y=12

    Thanks for another tip to help with the final design.

    Jon
  10. Billy123

    Billy123 Member

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    Can you post some pictures of that? You have a fan?
  11. fishingpol

    fishingpol Minister of Fire

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    Sorry Billy, playing catchup here. I am building a slightly larger one in a cellar window. I'll get some photos up soon. I just scored some metal shelves that someone was throwing away. Those will be painted black and put on the back of the collector over the foam panel. Gotta love free stuff.
  12. fishingpol

    fishingpol Minister of Fire

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    Ok, first picture is the solar collector that I built from poly-iso panels and a storm window.

    Second picture is the window with the left panel being removed for the ducting to the basement. More to follow.

    Attached Files:

  13. fishingpol

    fishingpol Minister of Fire

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    Next is Poly-iso over 1/2" plywood secured to window frame. Duct holes were cut with neighbors hole saw bit. I will have a horizontal baffle separating the input(bottom hole) and output(top hole) to slow air down a little to heat up. I don't see the screen absorber working here as it is limited space. I know there is a shadow from a small overhang, but in the winter it is barely covering it.


    Second picture is the sheet metal shelves that I cut to size and painted high-heat black. I will be working on the vinyl trim frame this weekend. I have already cut a vinyl sill for the bottom, sides will be today. Glazing will be after that with a vinyl trim cap last. I have to locate a small DC fan motor and PV panel to power it.

    Three questions:

    1.) Anyone know how to stack pictures one above the other so I can put wording between each picture instead of side by side?
    2.) Anyone know where to get an inexpensive PV panel to power a small fan?
    3.) Can cheap solar outdoor lights be used? I read that they have to be wired together to get adequate output to start fan.

    Attached Files:

  14. Billy123

    Billy123 Member

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    You peaked my interest. Keep us posted.
  15. bucketboy

    bucketboy Member

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    In january my daughter and i built a pop can heater, for 2 reasons, it was a school project to start with then i claimed it to heat a space in our home when she was done with it. The tough part, we made it up as we wnt as no real plans were avaliable during our net search. we overcame some common problems, sealing all the seams on the containing box, sealing the cans together, paint blistering on the cans, air movement through the box. All the hype that you can use computer fans, or duct fans is rubish. we have a 4 x 8 heater the duct fans do not push against resistance in the collecting manifold. Computer fans do not have enough cfm volume to make the generated heat circulate in the space your heating. We finially overcame all the shortcomings and expenses of trying different fans and went with a small furnace fan and the unit works great . we do achieve very high temperatures before the fan comes on but during the running of the fan we get 60 to 70 degrees of temperature rise and comfotablly heat a 20 x 20 finished space with no other heat source to 75-76 F for 6 hours a day temperture starts to drop off around 330 and settles at low 60 s overnight Our unit is 32 sq feet of collection area, it has about 44 hours of construction time and a cost of the unit to build was $400. we have started rounding up materials to build 3 more this summer and i suspect it will assist the temperature in the house particularly in shoulder season. These are great ideas to assist in raising heating costs. i m sure there are lots of folks that could or have tweeked the unit design and operation but an average do it yourselfer can build this unit sucsessfully and enjoy the free heat for years. Also the fan is hooked to an 80 watt solar panel and a storage battery ( Not in the 400 building costs) as there was no plug avaliable in the area of installation.
  16. begreen

    begreen Mooderator Staff Member

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    Sounds like an awesome project bucketboy. Did you document it with photos somewhere on the web? How is its performance on cloudy days?
  17. bucketboy

    bucketboy Member

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    Thanks, my daughter and I took pictures along the way , i have some, i will attempt to post to show you. they work poorly on cloudy days but there are alot of sunny days in the winter to take advantage of. Overall i am pleased with the operation, now that we have a few of the BUGS worked out
  18. fishingpol

    fishingpol Minister of Fire

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    Bucketboy,

    Do you think the computer fan would be enough to move air through a smaller collector? I just need some type of fan to move air without letting the collector get too hot loosing heat through the glazing. Gary from BIS made the suggestion above. Where did you get your panel from? That was an encouraging post you put up there. If mine works out, I will look for parts for a larger one to build this year...
  19. jebatty

    jebatty Minister of Fire

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    In the early 80's I built the equivalent of a solar heat box, but mounted it on the roof: 4' x 8' with corrugated steel collector painted black. I ducted air from the main level of our house into the bottom of the collector, air flow across the bottom of the collector to the other end, across the top side of the collector, and then ducted back into the basement. I used a 50 cfm bathroom exhaust fan for the blower and a snap-disc thermostat in the collector to cycle the fan. On a clear sunny day, even in January with -20F outside air temp and 70F air in, I got 120-140F air out in full sun. This was at 45N latitude, and a really low sun angle. The collector failed after 4 years due to moisture condensation inside the collector which rotted the wood components. This was little more than a plywood box with fiberglass insulation on the bottom and a polycarbonate greenhouse glazing panel.

    Good luck on your project.
  20. fishingpol

    fishingpol Minister of Fire

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    We are very fortunate to have such an array of materials available that were not around 20 years ago. Imagine 20 years from now what materials will be available. Thanks for these encouraging stories.
  21. bucketboy

    bucketboy Member

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    we had poor results with the computer fan circulating the air, in fact the paint blistered off the cans. as mentioned before , i m sure that there are folks that can modify the design to suit their situation and materials avaliable to them. we had a terrible time with trial and error to come up with something that worked adaquately. the snap themostat turns the fan in and off. the unit we built works well in our envirorment (southern ontario) one thing you need to keep in your mind when searching the net for ideas, take everything with a grain of salt. we found lots of ideas, but the one thing no one on the net shared was the pit falls people also closely gaurded their mistakes and bragged of the sucess. the collectors will give you the 150 f output if left in the sun, the trick was to circulate the heat into the room and exchange it back into the collector. pop cans worked as we were able to get them from the school cafeteria. we tried high heat bbq paint but opted for motor paint . washing the cans is important as was scuffing them to paint. the net sugested silicon to join the cans, we used high heat morter (fire place type and expensive) also sealing the box was important to ensure the input air goes throught the cans to be returned to the room heated. We used double pane glass the i was able to get from a recycle depot. to ensure good operation we had to measure the angle of the sun and build a frame to mount the collector on. we also hard piped the unit into the house and insolated the heat line. we found 4 in duct works but 6 inch give more volume of air. so if your going to build a collector, give it some serious thought to avoid some of the costly , time consuming errors we underwent.

    good luck
    bucketboy
  22. fishingpol

    fishingpol Minister of Fire

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    I finally had some time to finish the collector before the cold sets in. I am not a numbers and formula guy, but more of a figure what will happen and then build it. The glazing is just clear acrylic with silicone caulk around the perimeter. Inside I have a dc computer fan on the outlet, connected to the solar panel. I put a snapstat in, but not using at this time since the fan won't spin if the sun is not in reasonable direction to the panel. The fan is very quiet and it blows directly in the area of my workbench. I do realize that this is not going to heat my basement that much, but I am collecting heat and it is cheap BTU's over time that I would otherwise not be gleaning. I made the absorber screen of aluminum, and kind of wished I had doubled the screen layer. When I put a piece of air filter over the intake, the temp does go up a few degrees, probably from the air flow slowing down just a little. Overall this has been a fun project, and I'll tweak things as time goes on. I would have made a larger collector, but I need to get some free or low-cost glazing. I will find some at some point. I have enough material and insulation to make a larger one.

    The sun is effective from about 11:00 am to 3:30pm. I have a better location, but time will tell if I build one over there. This fall has been a bit overcast, so I didn't get the full potential. Maybe once the cold is here and cold fronts push through with clear skies, it will be more consistent. Input temps are around 62* and outlet temp at its' peak has been 84*. I plan on putting a small stick in front of the collector as sort of a sundial to cast a shadow and see if the highest output temps are when the sun is directly perpindicular to the panel.

    I could put a larger collector, actually a huge collector above the window, but the house bumps out a little and could pose a challenge running insulated ducts down a few feet to the basement. But I may figure it out. The collector does not change the look of the house at all, and I think it is quite clean looking.

    Attached Files:

  23. woodgeek

    woodgeek Minister of Fire

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    I'm confused by this project. You replaced a window that had passive gain with a collector of the same area with active gain. Aren't you right where you started, in terms of input? Overall, you might be getting less input since the air behind the collector pane is warmer than it was (higher parasitics), but your losses when the sun is not shining are also lower.

    What am I missing?
  24. fishingpol

    fishingpol Minister of Fire

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    This was an experiment to see if I can gain a higher temperature using the collector. I used the window as a base to work with without modifying the side of my house to see if the location is good. I would like to double or triple the area of the collector or even the design. This is new waters to me, and I don't have a background in solar heating, so it is just an experiment. The window may have better solar gain, but I am trying this out to see what works and what doesn't.

    The window is at grade level, so it has been covered on the inside for privacy so there was little solar gain to begin with.
  25. woodgeek

    woodgeek Minister of Fire

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    now I gets it.

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