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What color house?

Discussion in 'Photovoltaics' started by Vladimir Vladimirovich Nabokov, May 27, 2008.

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  1. I assume a dark color will absorb the heat and awhite will reflect heat, so if
    you live in the Midwest, what's a good compromise?

    Pure white would seem perfect in the summer to keep out the solar heating, but
    in the winter you would welcome the heat gain. Am I stuck with middle gray, or
    is there an established scientific answer?
     
  2. Guest

    What happened on cloudy days?
    NREL says 930 Btu/ft^2 falls on a south wall in Des Moines on an average
    24.4 F December day, so 1 ft^2 of white R20 south wall with a U2 airfilm
    conductance would lose about 24h(65-24.4)1ft^2/(R20.5) = 47.5 Btu/day.

    We could model a dark south wall like this, viewed in a fixed font:

    930/24h = 38.75 Btu/h
    --- 20
    |--------|-->|------------www--- 65 F
    --- |
    |
    1/2 |
    24.4 -----www-------

    which is equivalent to:

    1/2 20
    ---------www-------------www--- 65 F
    |
    | 24.4+38.75/2 = 43.78 F
    ---
    -
    |
    -

    with a heat loss of 24h(65-43.78)1ft^2/20.5 = 24.8 Btu/day.

    With an 8'x48' wall, the difference is 8x48(47.5-24.8) = 8698 Btu/day.
    If a single-glazed air heater gains 0.9x930 = 837 Btu/ft^2 and loses
    6h(80-24.4)1ft^2/R1 = 334 on an average December day, for a net gain
    of 503, the dark wall is equivalent to 8698/503 = 17 ft^2 of air heater.

    Nick
     
  3. mike

    mike Guest

    Don't remember all the details, but some kid won the national science
    fair prize for a roof that looked like a stairway.
    Black on the vertical surfaces and white on the horizontal surfaces.
    Average reflectivity went down as the sun moved south for the winter.
     
  4. Guest

    PE Norman Saunders patented "solar staircase" has mirrors for treads
    and transparent risers...

    But shading based on detailed summer sun angles isn't as effective as
    80% vertical shadecloth over the outside of a window, since a lot of
    summer sun is diffuse, eg 65% on a south wall in Austin in August.

    Nick
     
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