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re: first attempt at solar air heater
2 feb 2002
bvaughnjr  wrote:

>in just a few hours spread over a couple of days i threw together a test
>4'x8' solar air heater connected to the building through a partial window

i've been thinking along the same lines, wondering how to solar heat a
40'x60' metal barn about 25 miles west of phila. why use fans or plywood
or insulation, vs a layer of glazing over the darkened wall? the glazing
could be fairly airtight to outside air, with a slot in the wall at the
top and bottom and plastic film dampers over the slots. glaze the whole
wall, or some large fraction, with an airtight perimeter, but no plywood
or insulation or lots of little boxes with backs. if it's big, it might
be very cost-efficient ($/btu) with less thermal efficiency (btu/ft^2).

well, ok. thermostats with motorized dampers or fans might be nice for
better room temp control.

my biggest obstacle in this endeavor is the barn owner, a horse person
with very specific ideas about how barns should look. they need white
(vs dark) walls, with light blue trim and no glazing over the walls, and
a pretty cupola with a fox weathervane on top, above blue-gray asphalt
shingles. also, she says, horse barns don't need heat. (this alternates
with complaints that water and fingers freeze inside the barn.) 

last year she accepted the idea of blocking the 2' horizontal continuous
soffit vents with 60 2'x2' pieces of plywood to keep the great winds from
whistling up through the continuous ridge vent when it's 10 degrees below
zero outside, but that was only acceptable because it was invisible. 

>i know it's only "toy" size but if i can build one of these in a few hours
>for $120 (all new materials) why can't we get some low cost and efficient
>commercially produced ones? 

imo, you need to simplify your design first: eliminate the plywood,
insulation, fans, controls, electricity and furnace filters. look
for a simple way of attaching the glazing with a reasonably airtight
and durable perimeter (pressure-treated or cedar 1x6s with 4x4 wood
blocks inside the picture frame for angle brackets, and spray foam
inside the frame to seal it well to the barn wall? you might attach
the glazing with 1x2 cedar or 1/2"x1/8" aluminum cap strips.)

>it is located in eastern pennsylvania in a morton metal building

have you spoken with morton? in 1980, they had a program aimed
at solar-heating their buildings... 

>in my first tests i closed off the 10'x10' room
>where the vents entered.  here's some info. and initial numbers:
>morton steel building 24 x 30
>r19 walls, r38 ceiling <-- nice :-) how about airtightness?
>10 x 10 room being heated;
>solar air heater 4x8; 
>6" r4 insulated ducts; polycarbonate glz 
>105 cfm fan (18 watt); snap disc 110/90; 

why use fans and electricity to force air through tiny insulated ducts
when you can make the "ducts" a lot larger with no insulation and use 
natural airflow by just sealing the glazing to the wall?

>r4.2 back;
>* o <----                  *  air path bottom left to right; up;  
>* out                   ^  *  then top right to left; 
>*******************   ^  *  furnace filter absorbers; air flow
>* in                      ^  *  through filter 8 times; 4 each leg;
>* o --->                   *  window plug with 6" in/out ducts.

you have a sheet of some r4.2 stuff against the original barn wall. why?
seems to my we are far better off sealing the glazing to the barn wall
than letting cold outdoor air whistle between the back of the collector
box and the barn wall. why does the box need a back, vs the barn wall? 

then (going south), you have some furnace filter material, then glazing.
the air flows out of the barn into the south glazing cavity, then up and
back through the filter (8 times, with serpentine partitions???) keeping
cooler air near the glazing is good, but this seems very complicated. 

some black window screen might be useful as a mesh absorber, with no need
for a fan to protect it from high summer temps. or use plastic shadecloth
inside the glazing, and vent the box and hang another layer of shadecloth
over the outside in summer to keep the inner shadecloth from shrinking.
you might even try poly film glazing, with a fan to make a slight vacuum 
to keep it from flopping much in the wind.

>rm=room temp across room from solar air heater vents

what was the outdoor temp? how were the other 3 walls of
the 10'x10' room insulated from the rest of the building?

>         [air heater] 
>      rm  output
>11:00 55    115.0
>12:00 58    110.8
>01:00 61    117.7
>02:00 64    111.5
>max   66
>09:30 51    77.0
>10:00 53    97.2
>10:30 55   108.0
>11:00 56   112.7
>11:30 58   116.3
>max   63
>09:00 48   83.7 
>09:30 52   90.0 
>10:00 54   99.7
>10:30 56   99.7
>11:00 57 110.9
>11:30 59 122.6
>12:00 61 121.5
>12:30 63 124.6

it looks like the collector is working, but the room is coolish.
more collector surface would help, then maybe some heat storage... 
you might put some vertical thermal mass in a box near the inside
of the south wall and store heat at t > 70 f. 

good luck!


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