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Indirect solar heat gain?

jinmtvt | Posted in General Questions on

Is there any data on indirect ( reflected ) solar heat gain available ?
I haven’t found anything using the search box, and only a few uninteresting papers with google.

I guess it would be very complex to calculate vs direct shg,
but is it influent enough to even bother ??

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jin,
    I don't know of any articles that explain how to quantify the phenomenon, but the phenomenon is real. You may be interested in reading an article that discusses the question: Windows That Perform Better Than Walls.

    In that article, I wrote, “According to Morgan Hanam, the manager for window services at Enermodal Engineering in Ontario, ‘In Canada, even if windows are evenly distributed between the north and south, it’s still worth putting in high-solar-gain windows on all sides – even considering air-conditioning loads.’ The reason is simple: even if sunlight never strikes a window directly, a north-facing window can gather a little bit of heat from reflected and ambient light.”

  2. heinblod | | #2

    Solar radiation can be meassured.

    And with the results the usefull thermal gain through a window can be calculated.

    Is the question relating to a building?

  3. jinmtvt | | #3

    Martin: i would've never find that article would you have not linked it .. nice info in there
    will do more search...

    Hein: not "a " particular building, but rather a general design question .

    Which type of solar radiation brings heat through windows ?? only IR ?

    Why aren't we using large foil reflectors on north side to "load" up north windows during heating season ? Gathering as much sun power as possible should be a priority nah ?
    I don't see how you can get cheaper than using a simple reflector !

    The north window article and some reading i did on CanMet website about preheating intake air using exterior metal cladding made me thought of why aren't we using ALL of the exterior south walls of abuilding to harvest the sun ??

    There must be a way to absorb some of that free heat while still insulating the walls,
    other than IGU ...
    though if the "16" panes IGU units we looked at a few weeks ago, ever get through at R50-R100
    and still have some SHG, we would not have to worry about that anymore :p

    Hein: what would be required to measure the north window reflected SHG ??
    Could be a fun experiment to measure it here in Quebec to see how much it could help.

    The surroundings of a particular setup/building will affect the quantity of reflected light to large extent i am pretty sure.

    There is not much to reflect on when a building is surrounded by flat land ( the winter angles are already very low ) does IR follow the complementary angle of reflection rule ??

  4. jinmtvt | | #4

    BTW: mr Stephen of Fibertech semmed to be reffering to results using HOT2000 software
    for north windows SHG calculation, i would be tempted to go on and ask CanMet or the reponsible group for HOT2000 on how their software/engine handles indirect SHG ...might be flawed as i have never seen any mention of this in their doc/papers .

    Also if mr Rosenbaum could join in an explain how he calculated the north SHG , i would be happy to read and learn more about it ! :)

  5. GBA Editor
    Martin Holladay | | #5

    Jin,
    Q. "Why aren't we using large foil reflectors on north side to load up north windows during heating season ?"

    A. The device you are talking about is called a heliostat. If you want one, you can buy one from a company called Practical Solar for $1,500. Read more about the technology in an article on the topic in Environmental Building News.

  6. heinblod | | #6

    The device to meassure global radiation is called "pyranometer"

    http://www.renewableenergyworld.com/rea/partner/kipp-zonen/products/solar-radiation-measurement-instruments

    To make use of passive solar heating paint the walls black. In Germany (where this effect was explored, Wuerzburg) the heating bill drops by around 8 % .

    Passive solar building designs frequently use a pond in front of the house to reflect some solar light towards the fascade. Cheap and efficient.

  7. jinmtvt | | #7

    Hein: thanks for the link.
    The pond is a great idea, except that during heating season here, all is covered with snow/ice,
    thus all of the ground is a very large refelctor, and it usually stays until haeating season is done for.

    Paint wall in black might be good for a "project house" but it is not allowed by code here,
    and i believe it is the same in many region. If i recall correctly, there is a maximim % of exterior walls that can be of black color, and it is very low.

    Paint walls black reduces the temp delta, thus bringing down the energy exchange ??
    is that how it works ??

    Martin, looks interesting ..will do some reading tonight :)

  8. heinblod | | #8

    " Paint walls black reduces the temp delta, thus bringing down the energy exchange ??
    is that how it works ?? "

    Yes. A warm external surface of a wall will reduce Delta T between internal surface and external surface.
    Dark stones like granite,basalt or slade will have the same effect.

    Since PV panels and ST collectors have become cheap they pay for themself when used as structural part of the building.
    No paint or stone would do that.......

    A white surface (snow,ice) reflects even better the light compared to a water surce.

  9. BobHr | | #9

    I wouldnt think painting would not be practical, tell me if I am wrong. The more a surface reflects the less is emits. Conversely the more it absorbs radiation the more it emits. The black wall would be absorbing for a short time in the winter since the days are short. Thus is would be emitting more radiant heat than it would absorb.

  10. BobHr | | #10

    I should add that would painting a wall black also lead to is dams. Warm air rising up a wall and in to the soffits causing snow to melt.

  11. BobHr | | #11

    I should add that would painting a wall black also lead to is dams. Warm air rising up a wall and in to the soffits causing snow to melt.

  12. jinmtvt | | #12

    Robert Hronek: this is only a matter of proper design and i don't personally consider anything without a
    "snow insulated flat roof" for new designs, which doens't require any soffits.

    I am unaware of the complete picture involved in using dark ( painted or not ) mettal exterior finish,
    but i guess that having some kind of sheet metal in dark color with some air space behind it will end up heating the air, and that is gathered energy that can be used, just as much as heating any liquid.

    I asked the question about indirect solar heat gains toward understanding window's overall performance attributes, and now we are steering toward solar collecting walls :p
    not that i am ain't interested in the later, but we are going off topic a bit here now.

    Also , i would be considering any cost effective methods that would use south walls ( and portion of west/east ) walls to gather heat solar energy during daytime, to be released during night time.
    If ones design SHG from main windows to be sufficient for day heating requirements,
    the remaining of the building walls should serve as collectors to store required energy for night use.

    since we are working in this direction,
    anyone can confirm that using current PV is much less efficient than direct heat collection from solar for heating purposes ???
    i've read approx 75% efficiency VS sub 10% with PV and associated electronics/conversions ...

    If we are going to use some solar energy to heat
    ( which is mainly what i am after for my local weather )
    why not do it directly instead of transforming it to electricity to store, with its current inefficiencies ??

  13. heinblod | | #13

    @ Jin Kazama:

    The g-value of your window glass pane will tell you what you want to know. I think.

    There are energy advisors out there who can calculate exactly for your individual structure how much solar energy will be collected and used.

    " If we are going to use some solar energy to heat
    ( which is mainly what i am after for my local weather )
    why not do it directly instead of transforming it to electricity to store, with its current inefficiencies ?? "

    More than 99% of the thermal energy used in your home to heat it is solar energy. Like it or not - your house is already very efficient in terms of solar heating. To cover the rest as well shouldn't be a problem a civil engineer.

  14. BillDietze | | #14

    Jin,
    Have you tried http://susdesign.com/windowheatgain/index.php?
    Use the window heat gain app. I chose a north facing window and set the ground reflectance to zero to get diffuse light (heat), then bumped the ground reflectance up to 0.65 to simulate light from slightly old snowy ground. The window heat gain application is fun to play with in this regard, I think it will get you ballpark numbers for the heating effects of indirect light through a window.

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