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Re: Trombe wall efficiency



Nick Pine <nick@vu-vlsi.ee.vill.edu> wrote:
 
>>Is the air between the glazing and the insulation only at 70 degrees?
 
>To a first approximation... Not much more than that. I calculated the
>temperature rise for some "solar siding" recently, and it came out to
>be about 7 degrees F. To calculate this, you can look at the sunspace
>as a thermal chimney, and use the empirical formula CFM = 16.6 Av x
>sqrt((Tu-Tl)h), where Av is the vent area at the top and the bottom in ft^2,
>Tu and Tl are upper and lower sunspace temperatures in degrees F, and
>h is the sunspace height in feet. And you might figure that in full sun,
>the chimney is receiving about 300 Btu/ft^2/hour, and that 1 Btu heats
>up about 5 ft^3 of air 1 degree F (which tells you how much the sun
>warms a certain CFM, which in turn determines the CFM that flows.)

Oops. Let me do this again: this solar siding is about 16' high, and it has
a continuous 1 1/2" wide airspace at the top and the bottom, so a 1' wide
section would have cfm = 16.6 Av sqrt((Tu-Tl)h), where Av = 1.5/12 x 1'
and Tu-Tl is about U/cfm, and h = 16', and

U = 300 Btu/hr x 16 ft^2 - (68F-32F) x 16 ft^2/R1  =  4200 Btu/hr,
    full sun received      loss thru single glazing

so cfm = 539/sqrt(cfm) or cfm = 539^(2/3) = 66 cfm per horizontal linear foot
of siding, per 16' tall x 1' wide section of solar chimney. Not bad... But,
the temperature rise is about 4200/66 = 64F, not 7F, as I miscalculated above.
So if the air in the room behind it is 68F, it looks like the air will emerge
at the top of the siding at about 132F.

And the solar collection efficiency of this siding appears to be about
4200/(311 Btu/hr x 16 ft^2) = (66 cfm x 64F)/(311 x 16) = 84%. Not bad...

>In the design I just finished, it seems that the air leaving the house at
>68F (Tl) will return to the house at 75F (Tu.) The average air temp in the
>sunspace will be about 72F, but in this case, most of the warmed air that
>returns to the house will be North of some greenhouse shadecloth, not in
>contact with the cold glazing, and most of the unwarmed house air flowing
>out of the house will be South of the shadecloth, between the shadecloth
>and the glazing. So the air that is exposed to the cold glazing will be
>the 68F unwarmed house air, before it flows from South to North, sideways,
>through the shadecloth. This is good, but it doesn't matter much.

The above paragraph is still true, mostly :-)

Here's a small picture, a view from the East, say:

---------------       ^
132F <=O= <|   |      up        32F                    311 Btu/hr/ft^2
       |  <| ^ |                                 < --- Sun
   |   | ^<|   |<-Polycarbonate plastic
   |   |  <| ^ |
 house | ^<|<--|--80% shadecloth
  16'  z   z   z i                          South
       |  <| ^ |
   |   |<--|---|--Original house wall                  (clear enough? :-)
       | _<| ^ |
 68F ==I====>  |
-------------------------------------------
       |  3"   |

The plastic flap dampers are located at I and O, in the above picture,
and they look like this:

      .                      .
    p .                      .p
   p  .                      . p 
  p   .  <===  O             .  p  ===> I     
 p    .                      .   p
p     .                      .    p
      ^ hardware cloth ------^     
^                                 ^
|__ 1 mil polyethylene film ______|

The top one (O) will have a plywood door, hinged at the top, over the
plastic film, to close it off in summertime. When you close the upper
inner door, a link will open an upper outer door from the glazing cavity
to the outside, to make this work as a solar chimney in summertime,
cooling the room inside with some natural ventilation, and keeping
the sun off the original wall... If the house were air-conditioned,
the summer cooling air input could come from the outside, at the
bottom of the South wall. 

This house won't have a solar closet, but in general, a house with a
sunspace (or solar siding) and a solar closet might look like this:

   East view                 ^                      South view
---------------              up              ----------------------- 
               |d                           |     |                 |
               | d <--clear polycarbonate   |     |                 |
               |  d                         |     |    sunspace     |
    house      |   d       South            |     |                 |
 -  -  -   ----|    d                       |     |         --------|
          | SC |     d                      |     |        |  solar |
          |    |  SS  d                     |     |        | closet |
          |    |       d    srp?            |     |        |        |
--------------------------------        --------------------------------

               ^--dark surfaced, insulated wall   


--------------- ------- ..............
	  |    |       |              .
          | SC |       |              .
          |    |       |   shallow    .
          |    |  SS   |  reflecting  .       South             
   house   ----|       |    pool?     .
               |       |              .
               |       |              .
               |------- ..............
               |    ^  
               |    |__ dark, low-thermal-mass floor
---------------

Nick


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