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

>...on a lark i put together a poor-man's trombe wall...

"trombe walls" have thermal mass. sounds like you built an air heater.
even better. congratulations!

>basically i bought some cheap black fabric and hung that on the exterior
>of the wall (about 16 x 10' worth, cost about $15).

what kind of fabric? black aluminum window screen works. home depot sells
4'x25' rolls for $25.

>i then bought a 25 x 15' roll of 4 mil clear poly... and loosely attached
>it to the wall, over the fabric.

these work best with an air gap between the poly and the fabric and
a vent supplying room air to fill that gap. the room air gets heated
as it flows up and back (north) through the fabric. it then flows back
into the house through a vent at the top to the north of the fabric.
this arrangement keeps the air near the glazing cooler, with less heat
loss through the glazing. 

>i then cut two 10x10" holes in the wall... one up high and one down low.

a little small for natural circulation, which would need about 2% (3 ft^2)
of the wall surface. and holes at diagonal wall corners tend to work better
than holes in a vertical line, which may only collect heat from a vertical
strip of wall.

>two grainer fans were installed in the upper hole, controlled by
>a thermostat switch mounted on the outside wall (inside the collector).
>poly flaps were installed on the interior as dampers.

small fans. a more efficient collector might have 5-10 cfm/ft^2,
more like a $12 20" window box fan.
 
>this things looks like hell, but works pretty good although i've only
>had it up several days.  this morning was a good test.  after a good
>snow last night, and an air temp of 10 deg f at 9am, the fans were
>already on, sending 150+ cfm of 68 deg f air into the room.

why would you want to waste fan power collecting 68 f air?

>by 11am i was up to 85 deg f, with an outside air temp of 17.  certainly
>the snow reflection helped.  earlier in the week, at around noon with the
>air temp in the low 50s, i was getting 112 deg f.  and since the wall
>itself is heating up pretty good, i'm getting some additional heating
>benefit that way.

not much, if it has any insulation. btw, polyethylene film has little
"greenhouse effect," ie it doesn't trap heat well, with a high longwave
ir transmission. your wall-warmer would be a lot more efficient with
a single layer of dynaglas or replex polycarbonate plastic. about $2/ft^2
from greenhouse suppliers, with a 10-year guarantee. 

>all and all not a bad experiment.  certainly the performance exceeded
>the 4x8 solar air collectors/heaters i had made and tried (although
>i'm sure i didn't do the best job making them).  and you can't beat
>the cost either.  i suspect the poly will be unusable each season, but
>it is recyclable and the cost of replacement is minimal.

greenhouse poly film has uv inhibitors and a 4-year guarantee. hanging
some greenhouse shadecloth over it in summertime would make it last
longer and keep the house cooler.

>and screw damage to the wall could be minmized by installing a "frame" of,
>say, 2x4s onto the house, which could be painted, caulked, etc.  the
>lumber/plastic "edges" of the collector could then, each year, just be
>screwed into this "frame."

greenhouse suppliers sell aluminum extrusions that make it easy to change
the poly film.
 
>'not much room for thermal mass, but i suppose i could stack a layer
>of bricks up a ways on the exterior to help in that regard.

thermal mass _inside_ the house can help. on the outside, it would cripple
the thermal performance.

nick




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