This could be more reliable, if the heater channel can work with either
damper stuck open, and it might add thermal resistance when closed, but
it would also add airflow resistance, and it's more difficult to fit or
replace than the top damper. A barn with horizontal purlins vs vertical
studs might easily have one continuous damper strip vs 20, with less labor
and less leakage when closed...
Maybe this barn is better without heat storage...
I've been looking into that. See calc below, which doesn't account for
the first screen layer also keeping cool air near the glazing, and more
screen layers adding more sun-air conductance and airflow resistance...
Simply rotate the barn :-)
Maybe Drew means "upper half," with a lightshelf to bounce some sun up
under a reflective ceiling...
10 SIG=1.741E-09'Stephan-Boltzman constant
15 FOR T=.2 TO .8 STEP .2'screen transmission
20 TG@:TS0:TA 0'initial values
40 GG30=4*SIG*(460+(TG+30)/2)^3'radiation conductance from glass to outdoors
50 GGS=4*SIG*(460+(TG+TS)/2)^3'radiation conductance from glass to screen
60 GGA=4*SIG*(460+(TG+TA)/2)^3'radiation conductance from glass to absorber
70 GSA=4*SIG*(460+(TS+TA)/2)^3'radiation conductance from screen to absorber
100 TG=NUM/DEN'glass temp (F)
130 TS=NUM/DEN'screen temp (F)
160 TA=NUM/DEN'absorber temp (F)
170 IF ABS((TA-TAL)/TA)>.001 THEN TAL=TA:GOTO 40'iterate to 0.1%
180 EFF=(250-(TG-30)*2-(TG-30)*GG30)/250'solar collection efficiency
190 PRINT T,TG,TS,TA,EFF
Screen glass screen absorber solar collection
xmsn (frac) temp (F) temp (F) temp (F) efficiency (frac)
.2 37.86897 99.60938 84.81229 .9106325
.4 (4 layers) 41.59563 102.5794 96.12272 .8678353
.6 (2 layers) 46.36161 105.8859 107.3936 .8127683
.8 (1 layer) 52.25522 109.4545 118.7363 .7439739
Some great ideas here -- my thanks to Drew and Nick
My impression is that the plastic damper seals pretty well, so I'm not sure
that adding a bottom one is necessary. I will try and see what kind of flow
comes from the bottom vents at night (maybe with smoke?).
I think that I could also do a bit better a job on the top vents (I put them
in a hurry) -- e.g. give them a well defined hinge line, a good smooth
surface to seal against, experiment with different weights/types of plastic.
As an aside, I have been looking for something that behaves like smoke (but
is non-toxic) to use for flow visulization. Anyone have any ideas for this.
I know some of the wind tunnels use vaporized kerosine, but this sounds
messy and complex.
I could do this pretty easily, and it would also allow incorporation of a
bottom plastic damper that can be accessed easily (if needed). I just want
to confirm that the idea here is to effectively use the concrete floor to
store heat so that the nighttime temperature does not fall as much? And,
that this will also have the effect of lowering the daytime high temperature
in the barn?
I have been thinking about an alternative heat storage plan. Right now, the
heated air from the collector (up to 120F) exits the top vents, and a good
bit of it goes up and heats the cavities between the first few 2nd floor
beams to prettty high temperatures (90F ish). I am thinking about putting
some form of water heat storage in the 1st or 2nd floor beam cavity. For
example, I could hang 15 5gallon, closed top plastic pails in the cavity for
about 600 lb of water. I would insulate above the water pails to lessen
heat transfer to the 2nd floor. Based on hanging a 1 gallon water can in
this space for the last couple days, I think that the water might heat to
somewhere in the high 70's on a sunny day, and that it might cool to
somewhere in the 40's at night (depending on outside temp etc.). This would
give about 18000BTU of stored heat, which seems like it might be enough to
make a few degrees difference by morning. Does this make sence??
I thought about the 5 gal buckets, because I see recyled ones around town
for $ each, so this would be cheap. Is there a better storage container --
maybe an 15 ft long 12 inch dia pipe?
I don't feel like the system has a lot of excess capacity to give up day
heat and store it for a higher temperature the next morning, but I still
have some weather stripping and insulating to finish up, and I can gain some
collector efficiency through some of the changes suggested below -- this
should make for some excess capacity.
I thought about this during construction, but rejected it mostly on
aesthetic grounds. When I take it appart to add more screening, I might
paint it a darker color -- it looks like some of the darker shades have
absorbtion coeficients that approach black.
Around here, the equivalent of a gallon of fuel oil would be (140k BTU/95k
BTU heat content) * ($.35/gal propane) = $.98, and, if it does deliver
this per sq ft, the year total would be ($.98)*(160 ft^2 of collector) =
$17 per year.
This would be a year or two payback?
Having tried to heat the garage with a portable propane heater, I don't
think this is too far off.
After installing the better temperature sensors that I can record, I plan to
do a carefull set of economic calculations.
I don't like payback period as a measure of economic goodness. I think
that, if I can take some money out of my stock market investment pool that
(on a long term historic average) might make 8%, and invest it in a solar
project like this one that makes 25+% per year, then this is a good deal.
The solar project is also probably less risky and is tax free.
Not to beat this to death, but from the point of view of someone who decides
they need to heat thier barn, there is a choice of 1) put $00 into some
kind of simple furnace system + pay fuel bills for life, or 2) put $00 into
a solar heater and never pay a fuel bill (depending on how critical the
heating is for cloudy periods). I think a lot of people would take door 2
if it was offered to them??
I'd be happy with the SE facing collector wall if it wasn't for that
mountain to the East -- maybe we can move the mountain instead of rotating
the barn :-)
I like the idea of replacing the a part of the solid wall with fixed pane
windows that would let more light in. I am leaning toward adding a couple
of 5 ft high by 22 inch wide windows -- one near the East end and one near
the West end. I can do this between the existing studs without a lot of new
I did use oversize holes, and the screws that SunTuf sells with EDPM washers
under the head. But, I also felt it was necessary to run a light bead of
silicone seal along the places where the panels joined, and at the panel
edges. The panels deflect very easily, and without the sealant I think the
leakage rate would be too high.
I don't plan to let the panels stagnate. When I close off the vents
connecting the panel to the garage, I will open the summer vent panels in
the collectors top and bottom sills.
I think if I were doing this again, I would take a closer look at the glass
vs Polycarbonate trade.
Bozeman weather pretty well takes care of keeping snow in front of the
Thanks Drew -- Gary
Is the last column the total percentage of solar energy absorbed by the set
Should the screens be spaced apart from each other by a bit (say 1/4 inch)?
I could add two screen layers to the end bay, then I could measure
temperatures and flow rates for one screen, two screens and three screens.
Talcum powder is good, but it does settle out fairly quickly. The best
stuff easily available is banana oil. It is used for testing respirators for
good fits, and you can order it from the usual safety supply catalogs.
Makes a light white smoke and smells like bananas. You can also
see if there is a party supply store around and buy one of the smoke
machines. Cheap ones run about $5 each, and the pint or so of
smoke juice that comes with it should be far more than you will need
for this sort of thing. If you go that route, experiment outside with the
machine first, they take a while to warm up and are a bit balky in
operation to get what smoke level you want.
An old-fashioned punk stick? Or cigarette smoke from a 3'x1/4" glass tube
to cool it...
A higher hinge or lighter weight might improve flow. Again, you might
prop them open during the day and see if the exit air temp decreases
or the exit air velocity increases.
Norman Saunders uses milkweed. People sometimes test air heaters with smoke
bombs, looking for air leaks to the outdoors...
More or less.
Yes. That could be undesirable.
You might need a lot more barn insulation for that to be useful.
I'd say no, if you are only in the barn during the day.
Or a 15' long greenhouse polyethylene film air duct with a few inches
of water inside.
Sure. Dark green can be almost as good.
Drew is counting labor.
Sounds good, with a lightshelf.
No. It's the ratio of useful energy output to energy input.
That might lower the airflow resistance a bit...