Which is better - lots of windows, or passive solar collector?

Discussion in 'Alternative Energy' started by cc-rider, Dec 17, 2006.

  1. cc-rider

    cc-rider Baroness of TisaWee Farm Supporter

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    On the south side of my (proposed) log cabin, I have the following 2 options. Which is a better plan?

    The cabin is wider than it is deep, with the wide side being on the south. One story.

    I can put lots of glass and sliding glass doors on the south side. Direct gain solar?

    or

    I can put a sliding glass door on the south, and then build panels that fasten to the south side. The panels are just 6" frames with glass fronts, and black plastic tubing filled with water on the inside. The back of the panel would be foil-faced to reflect the light back to the tubing. Theoretically, the glass heats the plastic tubing and holds the heat. There are top and bottom vents cut into the south side of the cabin. The cold air goes from the cabin into the glass collector mounted on the cabin, its warms and rises, and then goes back into the cabin from the top vents. The top vents will have a way to close them when the collector isn't warmer than the inside air (manual at this point). Also, maybe put a non-electric fan in there, too, to help blow the air into the cabin. This is all conjecture at this point....don't know if it will actually work.

    The question is, though...... which is better - direct gain from lots of windows (and good insulated curtains), or a quasi-solar collector on the entire south side of the house (plus one sliding glass door)?
     
  2. SolarGary

    SolarGary Well-Known Member

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    Hi,

    Having the wide side of cabin face south is good.

    Direct Gain:
    Direct gain solar (through windows) is quite efficient at collecting. During the time the sun is shinning, windows have greater collection efficiency than regular solar collectors do. The reason for this is that most of the losses from solar collectors come from the absorber getting hot, and radiating and convecting heat out the collector glazing. For a direct gain wndow, the "absorber" is the room contents, and these will run cooler than a collector absorber, and lose less heat to the outside. The place where direct gain windows don't do so well as collectors is that they lose a lot of heat at night, so if you use a lot of direct gain, you really want a good way to insulate the windows well at night.

    For direct gain, you also want windows with a high SHGC (Solar Heat Gain Cofficient) -- this is a measure of how much solar radiation they let into the room. 70% is good -- there are some windows out there that haver very low SHGC's -- avoid these -- they are made to reduce heat gain in hot climates. The SHGC is usually listed on the window label.

    For the direct gain, you should have an roof overhang over the window so that it blocks the high summer sun, but lets the low winter sun in. There is a good overhang design tool here:
    http://www.builditsolar.com/References/SunChartRS.htm#Overhangs
    Without an overhang or some way to reduce summer heat gain, you will cook in the summer.

    You also want some thermal mass in your floor. If you don't have enough thermal mass in the cabin, it will overheat, and you will find yourself venting the heat out in the afternoon. You want enough mass to prevent afternoon overheating, and to store the heat for later. The best place for thermal mass is the floor where the sun shines on it directly.

    The other nice thing about direct gain is that you get lots of natural lighting.

    Collectors:
    I would recommend that you NOT build the collectors as you describe. By putting the thermal mass (water filled tubes) inside the collector you greatly increase its nightime heat loss rate. Put the thermal mass inside the cabin. Keep the thermal mass inside the collector as small as possible. Thermal mass located inside the collector will radiate its heat out to the cold night sky very very rapidely after sunset -- not what you want.

    This is the collector I use on my shop/barn:
    http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm
    Its cheap and effective.
    The simple poly backdraft dampers solve the problem of backflow at night that you mentioned -- they work really well.
    For a cabin, the glazing could be the sliding glass windows you mention. I would also consider these types of glazing:
    - Sliding glass window panes in wood frames -- these can often be obtained cheap from glass suppliers as pull outs or blemished or miscut.
    - Twinwall polycarbonate glazing -- this is the stuff that a lot of greenhouses use. Make sure its polycarbonate and that it has a UV coating on the outside to protect it from sun degradation. This stuff is easy to handle in large sheets, and is very nice to work with. I used it on my "Solar Shed" project:
    http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/solarshed.htm
    Each of those sheets is 4X12.

    A nice feature of these collectors for a cabin is that no electricity is requred to run them -- its all natural convection.

    The cost of this kind of collector is only about $2.50 to $4 per sqft -- it will have a one year payback in cold climates.

    For thermal mass with this collector, I would look at water filled tubes or containers mounted up near the ceiling on the south side, right near where the hot air vents from the collector come into the room. This area runs hot, and will get the water container quite warm. Maybe even warm enough to heat water for a shower.

    The back wall of these collectors will work better if its a dark color (does not have to be black -- any medium dark color is OK). This will absorb light that gets through the screen absorber. It will heat the air in the collector just like the absorber does -- this is good.

    Be sure to follow the ground rules in the article regarding the depth of the collector and the size of the vents. The only thing moving the air through the collector is the bouyancy of the heated air -- this is not a strong force, so you need a good open flow path through the collector. If you don't have an open flow path the collector will run to hot, and have high losses out the glazing.

    This kind of collector could be built right into a 2X6 stud wall. This would drop the cost of the collector to near 0 $ per sqft over the basic wall cost. With this arrangement, the twinwall galzing becomes the wall's siding. Let me know if you want more details on how this might work.

    ---
    You might want to consider a mix of the direct gain windows and the thermosyphon collectors. You may find that having the full south wall in sliding glass doors is a little more light and glare than you want, and it also takes up all of the wall space. A mix of collectors and windows would still give you daylight. Just depends on how bright you want it in there. I use this one on my attached shop, and, I have to say, I like the bright open feel of all windows.
    http://www.builditsolar.com/Projects/SpaceHeating/SolarGarageCollector/garcol.htm
    (there is going to be an article on this one in the next issue of Home Power with a lot of good construction detail).

    Gary
     

  3. MELOC

    MELOC Master Of My Domain

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    what an awesome project!

    i do have a question about the collectors. how much copper piping is in the collectors? i was only able to see to top an bottom parts of the hot water portion of the collector. i assume there are vertical elements. is there one vertical per panel or more?

    i wonder how well 2 250 gallon steel fuel oil tanks would work for storage? i think i paid about $175 to $200 for my fuel tank. if i tackled a project like this, i would probably try to built a box around the tanks and insulate it.

    is your floor heat cycling very frequently? how well do you think it would work for a hot air exchange?
     
  4. SolarGary

    SolarGary Well-Known Member

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    Hi,

    I actually bought the absorber panels preassembled at this place:
    http://www.solarenergy.com/ws400CS.cgi?category=sh_absorbers.html&cart_id=
    Its not real clear from their pictures, but each 4 ft wide panel has 10 copper tube risers. the risers are welded to copper fins that are about 4 inches wide. The absorber plates were $260 for each 4X10 ft panel. This saved a whole lot of time, and probably resulted in a higher performance panel compared to making them. Given that the rest of the project was quite a bit of work, I'm glad I went that way. The whole collector ended up costing $8 per sqft -- this compares to $30 per sqft for commercial collectors, so you still save quite a bit of money.


    I think that the oil tanks would work fine as long as you insulate them well.
    Here is one system that uses a cleaned up used oil tank for storage:
    http://www.builditsolar.com/Projects/SpaceHeating/Paul/PaulSolShed.htm
    I've exchanged quite a few emails with Paul who built this system, and the oil tank is working out fine. In his case, he wants it to radiate heat into the space the tank is located in, so he only insulated the side that faces the outside wall. This way he gets both storage and a kind of radiator from the same tank.

    I'm stil working on the floor heat. I've got three of the radiant floor loops in, and I'm ready to install the radiant floor manifold and pump. I'm only about a day or two away from first heat! -- but, I think its going to have to wait until after Christmas.

    Today was clear and cold (-10F low, +10F high). I connected up the circulation pump that circulates water through the collectors and back to the storage tank, just to see how it would do on a cold but clear day. The tank's 500 gallons went from 108F up to 150F -- the equivalent of about 2 gallons of propane. I'm pretty happy with that :)

    Gary
     
  5. cc-rider

    cc-rider Baroness of TisaWee Farm Supporter

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    Wow....great links. Quick question....
    In the picture on page 31 of this link (page 2 of the pdf file, I think) http://www.builditsolar.com/Projects/SpaceHeating/SolarBarn.pdf
    It looks like the panels don't come down far enough to cover the vents. Is that only for illustration purposes? In the drawing, it looks like the panels go all the way down to the bottom sill, which would cover the vents.

    Oh wait... I just looked again and I think I answered my own question. What I'm seeing, and thinking is the panels, is the black window screen?

    The more I think about it, the more I think that I don't like the idea of logs. I can't insulate them (and they are only 6" thick). I can build a 2X6 framed cabin easier and faster, and probably MUCH more energy efficient...and build the solar collector into the framework while I build it. Would you recommend building the back wall with metal studs instead of wood? I don't need a building permit to build where I'm at in Ohio (but I don't know if that means that I don't have to follow SOME sort of code....)
     
  6. MELOC

    MELOC Master Of My Domain

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    cc rider,

    sorry i drifted the thread. i was just really impressed with gary's shed. :)
     
  7. cc-rider

    cc-rider Baroness of TisaWee Farm Supporter

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    Not a problem!!!!! I have to tell on myself.....(blush). I borrowed a copy of MEN to read while I was sitting on top of the hill last weekend and saw the article about the solor panel. I didn't realize OUR Gary was the same "Gary" that wrote the article, though. That article is what got my mind turning about how to adapt the solar panel to (what I thought, in my uninformed brain) make it more efficient. Before I actually would have built it, however, I intended to ask here for input, knowing that Gary was on the board and would answer me.

    Imagine my shock when Gary referred me to the MEN article HE wrote! When I pulled up the link, I called in my SO and said "look at this!!!!!!! Solar Gary is Gary from MEN!!!!!!!" Wow. How cool is that? I feel like we have a celebrity in our midst.

    Of course, I'm sure there are many others on this list, too, that are celebrities in their fields..... but it was just too cool that I was getting personal advice from Gary.

    There...... I've just contributed to the drift. We're even. :dance: :dance:
     
  8. SolarGary

    SolarGary Well-Known Member

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    Its a small world :)

    Please take any advice I give with a grain of salt -- I'm only a couple years into this solar stuff, and still learning and changing my mind about things almost daily.

    One thing I like about this Homesteading Today is that there are lots of knowledgable people here, so when I suggest something stupid, someone catches it :)

    Gary