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re: whacky low-tech "solar wall" idea/s???
4 feb 2003
puncturevine  wrote:

>i have a 15' wide x 10' high (peaking to a gable roof at 15')
>south-facing wall i'd like to pull some solar heat from...

how big is your house? nrel says january is the worst-case month for
house heating in wichita, where seldom is heard a discouraging word and
1270 btu of sun falls on a south wall and 780 falls on a horizontal
surface on an average 29.5 f day with a 39.8 average daily max.

>this is what i was thinking: loosely attach (e.g. with grommets and screws)
>a big piece of dark material over the wall...

porous material, like the conserval wall? "loosely attached," with lots
of air leaks at the edges?

>you'd heat the wall behind it up pretty good, getting some... conduction
>through it to the inside (this is a 6" frame wall with standard roll
>fiberglass insulation).

if it's 35 f on a clear day in wichita, and 250 btu/h of sun falls on
a square foot of dark south wall behind an air gap and a single layer
of r1 "dynaglas" polycarbonate with 90% solar transmission, you might
collect 225 btu/h. if the air between the wall and the glazing is 100 f,
you'd lose (100-25)1ft^2/r1 = 75 btu/h, for a net gain of 150, or 28k
btu/h for your 187.5 ft^2 wall. you might use a 28k/(100-70) = 938 cfm
fan to move the air at 400 fpm through the 12x938/(15'x400) = 2" (or
larger) air gap.

on an average jan day, you might collect 0.9x1270 = 1143 btu/ft^2 and
lose about 6h(100-35)1ft^2/r1 = 390, for a net gain of 753, ie 141k btu
for the wall, about the same as the heat in a gallon of oil.

>i'd think you'd be heating up the air between the dark material and
>the wall pretty good.

with a dark material and no air leaks, you might have something like this
(viewed in a fixed font):

            1/7       r20
    35 f ---www-------www--- 70 f house air
                  |
       250 btu/h  |
           ---    |
      |---|-->|---
           ---

with a thevenin equivalent like this:

            1/7       r20
    70.7 ---www-------www--- 70 f house air

making the air outside the wall about 70 f would save about
6h(65-35)187.5ft^2/r20 = 1688 btu/day, a gallon of oil every 77 days.

>that air would rise and i would think be collected under the 2' soffits,
>eventually 'pooling' at the gable peak (depending on wind conditions of
>course).  one would think you could stick in a gable-end vent or louvre
>(something i should do anyway for summer venting), and with some ducting
>and a fan on the inside attic side, pull in a fair amount of that hot air to
>your room.  you would then just remove your dark material for the summer.

or open a vent at the top and bottom, or leave it closed and hot all year
and add some fin-tube pipe near the top to heat water for showers. argo
industries #1560 fin-tube has 5.2 btu/h-f-ft of 140 f water to 65 f air
thermal conductance. an 8' piece with 2" fins on 3/4" copper costs $15.67.

>i suppose you could also increase efficiency by recirculating some room
>air by cutting a vent near the bottom of the wall, which would end up
>pulling some inside air out.

good idea.

>even if this were reasonably successful it seems like much less work
>(and fewer holes in the wall) than multiple stand-alone solar air
>collectors.

and cheaper as well. you might improve the efficiency (and darken the
wall) by putting a layer of black aluminum window screen between the
wall and the cover. 

nick




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