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re: solar barn -- "first sun"
29 feb 2004
gary  wrote:

>> p.e. drew gillett"  writes:
>>
>> > i have some suggestions you can forward...
>> >
>> > 1) include plastic dampers at bottom vent as well
>>
>> 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...
>
>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?).

an old-fashioned punk stick? or cigarette smoke from a 3'x1/4" glass tube
to cool it...

>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.

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.

>as an aside, i have been looking for something that behaves like smoke (but
>is non-toxic) to use for flow visulization.

norman saunders uses milkweed. people sometimes test air heaters with smoke
bombs, looking for air leaks to the outdoors...
>>
>> > 2) lower vents to floor level or provide ducting down to floor level so
>> > coldest air is scooped off floor during daytime... (note how the inlet
>air temp is rising during your tests while the floor remains cold.)...

>> maybe this barn is better without heat storage...
>
>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?

more or less.

>and, that this will also have the effect of lowering the daytime high
>temperature in the barn?
 
yes. that could be undesirable.

>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.

you might need a lot more barn insulation for that to be useful.

>...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'd say no, if you are only in the barn during the day.

>i thought about the 5 gal buckets, because i see recyled ones around town
>for $1 each, so this would be cheap.  is there a better storage container -- 
>maybe an 15 ft long 12 inch dia pipe?

or a 15' long greenhouse polyethylene film air duct with a few inches
of water inside. 

>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.

sure. dark green can be almost as good.

>> > 5) 24 hours of labor at 10 is 240 so my rule of labor equals materials
>> >for construction projects still applies.  $4/sg.ft installed is still
>> >a very good low price.  remember that at best such a system would deliver
>> >1 gallon of fuel oil per sq.ft/year so payback is over 5 years but still
>> >very good. more realistic performance is probably half that
>
>around here, the equivalent of a gallon of fuel oil would be (140k btu/95k
>btu heat content) * ($1.35/gal propane) = $1.98, and, if it does deliver
>this per sq ft, the year total would be ($1.98)*(160 ft^2 of collector) =
>$317 per year. this would be a year or two payback?

drew is counting labor.

>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
>framing.

sounds good, with a lightshelf.

>> 10 sig=1.741e-09'stephan-boltzman constant
>> 15 for t=.2 to .8 step .2'screen transmission...
>> 180 eff=(250-(tg-30)*2-(tg-30)*gg30)/250'solar collection efficiency...

>> 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
>>
>
>is the last column the total percentage of solar energy absorbed by the set
>of screens?

no. it's the ratio of useful energy output to energy input.

>should the screens be spaced apart from each other by a bit (say 1/4 inch)?

that might lower the airflow resistance a bit...

>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.

sounds interesting...

nick




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