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Passive Solar

Post in 'The Green Room' started by Thor, Jan 27, 2013.

  1. Thor

    Thor New Member

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    Loc:
    Central Pa
    I have a house that if my calculations are correct sits 40 degrees east of solar south. I know there is a lot of what ifs but as a general rule can I still gain some passive solar heat by adding some windows to the southern wall and tiles to a concrete slab floor ? Is the 30 degree max just for heat gain in the summer ? If that is the case then it seems to me that a over 30 degree to the east would be better then a house set westerly over the 30 degree recommendation. House is heated by a wood stove and I am wondering if passive solar would work in the shoulder season and those 50 degree winter days ?

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  2. GaryGary

    GaryGary Feeling the Heat

    Joined:
    Jul 12, 2010
    Messages:
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    Loc:
    SW Montana
    Hi,
    A good way to look at this is use PVWatts version 1 : http://rredc.nrel.gov/solar/calculators/pvwatts/version1/

    You can run it for a tilt angle of 90 degrees (vertical), and an azimuth of 180 degs (due south), and print the monthly Solar Radiation numbers.
    Then change the azimuth to 140 degrees, and run it again.
    Compare the Solar Radiation numbers for each month to see how much you lose.

    It looks to me like for PA, in mid winter you are down about 16% for the 40 degree off south (compared to true south).

    It will get you nice sun early in the morning because its pointed more east -- so, put the kitchen or wherever you spend mornings on that side of the house.

    How well this works depends mostly how much 'south' glazing you are talking about, and even more on how good a job you do on the thermal envelope -- insulation, sealing, ... Also, some kind of thermal shades on all that south facing glazing to cut night heat loss is important.
    You might also want to have a look at Low Thermal Mass Sunspaces: http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm#LowMassSS

    Gary
  3. Thor

    Thor New Member

    Joined:
    Jan 12, 2013
    Messages:
    18
    Loc:
    Central Pa
    Thanks Gary,
    So that would give me 86 % with the 40 degree east setting. Do you think this is beneficial for heating purposes ? I have been watching the tract of the sun over my house for a couple of years but only recently started to research passive solar heating. I have 2000 sq ft house of which two of the rooms are on the southern walls. The wall is 40 ft long and I can add approximately 40 sq ft to the existing 3 windows. From some of my reading it seems like with an over 30 degree southern exposure you have to worry about over heating in the summer as well as a less then Ideal heating potential . Couldn't you just make some sort of mechanical shading on some of the windows for the summer if this is the case ? I am thinking a white board or shutter allowing air space behind it. Also for my calculations I used solar noon not true south. For the 30 degree recommendation are you suppose to use true south ?
    Are you suppose to use special windows for glazing ?
    What are some recommendations for thermal shades ? Are we talking heavy drapes or something a little more elaborate ?
    How beneficial are passive heaters like this one http://www.motherearthnews.com/Do-It-Yourself/1977-09-01/Mothers-Heat-Grabber.aspx. ?
  4. Circus

    Circus Member

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    Jan 11, 2013
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    124
    Loc:
    EC Wisconsin
    True south windows (thermally speaking) are a wash, if your lucky. They'll lose more at night than gain during daylight. Facing the SE means a lot less gain, losses stays the same and no summer shading from the roof. Better view though.
  5. GaryGary

    GaryGary Feeling the Heat

    Joined:
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    Loc:
    SW Montana
    Hi,
    Before I forget to include this, here is an interesting idea from William Shurcliff's book:
    http://www.builditsolar.com/Experimental/ShurcliffPart1/S89pt5.htm
    He argues that with the right glazing/reflector arrangement that that a 45 degrees off south rectangular house layout may be the most solar effective.
    By the way, this Shurcliff boook -- "Low-Cost Solar Heating -- 100 Daring Schemes Tried and Untried" by Dr. William Shurcliff is my all time favorite solar book -- even though it was done back in 79, its full of great and innovative ideas.
    http://www.builditsolar.com/Experimental/ShurcliffPart1/TOC.htm

    I do think that going to more south glazing of the right type and with thermal shades would be a worthwhile heating gain.

    The increased summer exposure does seem like a valid concern to me, and some form of sun control (preferably something on the outside) would be good to address potential summer overheating. Lots of shading schemes here: http://www.builditsolar.com/Projects/Cooling/passive_cooling.htm#Shading
    The one with the "Exterior Roll Up See Through Shades" is my house, and I can say that they work really well, and basically allowed us to avoid buying into a central AC system.

    You would want to use windows designed for solar gain. You want windows with a high SHGC. The http://efficientwindows.org/ is a good place to look at different window technologies.

    Passive solar is more than a BTU gain or loss decision. It will likely help your heat bill by a modest amount -- houses designed for passive solar from the ground up might get a solar fraction of 0.5 or even more, but I'd guess that you might might be talking more like 0.2 (this is just a guess). But, more than that, there is a bit of a lifestyle change that you may or may not like. The house will be brighter and will have a more open feel -- I like this a lot, but some people don't like it a lot :)

    On the thermal shades, we use the Symphony Shades of the cellular type with side tracks to keep air from leaking around the edges -- I think that other outfits make them as well, http://symphonyshades.com/comfortracks.html These have worked well for us, and they add something like R3 or R4 to the window.

    Another approach would be to integrate solar air heating collectors into the wall space between the existing windows. These could be thermosyphon collectors with dampers to prevent flow at night. They would be completely silent and maintenance free.
    I use these on my shop: http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm
    Obviously, you would want something with a nicer finish on the inside for a living space, but the design could be about the same. These are very efficient, fully automatic, and dead simple. They have the advantage that they don't have the larger heat loss at night that solar windows have.

    Your house may not be suited to it, but I also like the idea of a low mass sunspace along the 'south' wall -- its a very efficient solar heating collector and does not have the large night time heat loss, since the sunspace just goes cold at night. And, you get some extra space that can be used for all sorts of things.
    My favorite one: http://www.builditsolar.com/Projects/Sunspace/MikeSunspace/MikeSunspace.htm

    I think that the Mother Earth heat grabber is a great project to do to learn solar. But, unless you can do several of them, or the space to heat is small, they are kind of small to make a lot of difference. That said, lots of people build them, and the ones I've talked to think they work great and that they learned a lot.

    The statement about true south windows being a wash thermally with night losses offsetting day gains I do not believe to be true. This question came up once before, and I used RESFEN to look at several locations in the lower 48 and they were all net gainers. RESFEN is a free download, and it lets you look at gain and loss through windows for a full year using an hourly simulation with your local weather.

    I think that you could try modeling your house in RESFEN and see what adding good solar windows would get you -- I think that it can handle the 40 degree off south and it may even do thermal shades.


    Gary
  6. Thor

    Thor New Member

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    Loc:
    Central Pa
    Although you are getting away from the passive part, if you insulate those windows at night or cloudy days can it be beneficial ?

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