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re: searching for comments on omsolar space heating system
22 may 2001
steven tjiang  wrote:

>...can one build a solar trough easily with items from home depot?
>for $500?

i think so. commercial plastic film greenhouse kits with polyethylene
film stretched over curved bows on 4' centers cost about 50 cents/ft^2.
h. schwarz & sons of wilmington de (800) 523-3500 sells a 14'x96' kit
for $649, without the plastic film. stuppy in kansas city (800) 733-5025
may still sell kits for 55 cents/ft^2. 

we might slice one in half down the middle lengthwise (conceptually--
would it squirm like a lobster?) to make something like this solar
trough reflector, with parabolically-curved half-bows on 4' centers
and an open south side and 2 layers of film stretched over the bows,
polyethylene on the outside and reflective mylar inside that, between
the poly and the bows.

the poly film comes in very large pieces, up to 40'x150' in folded rolls,
and it is more weatherable than the mylar, which comes in smaller widths,
eg 54". i've glued it to poly with a paint roller and axle grease.
home depot doesn't sell the films yet, but they sell 2x4s :-) 

>i'd like to see the plans and materials list.

you might make a 12'x12'x12' reflector with

4 16' 2x4s with a dozen kerfs         $20 
64' of galvanized strapping             6
3 12' 2x4 purlins                       9
350 ft^2 of 4-year poly film           18
350 ft^2 of 2 mil mylar                35

about $100 for the basics, plus deck screws to attach the strapping
inside the kerfed 2x4s to keep them curved, etc. this would be a 3-sided
standalone structure with reflective endwalls. i've also kerfed and
welded 18' steel 2x3 unistrut bows. more money and labor, but stronger. 

>but i am constrained quite a bit.  narrow lot: 50'x150'
>shading from neighbors reduce solar exposure on the first
>floor.  this means i have to collect solar heat from the roof,
>the only surface assured of good solar exposure.

you might build a solar trough into the attic with corrugated
polycarbonate (eg dynaglas) for the south roof and parabolically-
curved north rafters and mylar glued under the north roof osb. 
a different geometry might fit better here, eg 8'x8'x24' long,
in some dramatic attic space.

>> >i plan to build a home of about 2500-3000 ft^2
>> >in the san francisco bay area

>> that's one of the easiest climates in the us for solar house heating.
>> nrel says it's 48.7 f (vs 30.4 in phila) on an average january day,
>> 1,050 btu/ft^2 of sun falls a south wall (vs 1,000 in phila) and 680
>> falls on a horizontal surface. a 1-axis ew tracker can gather 2.5 kwh/m^2
>> (793 btu/ft^2) of beam sun. a fixed solar trough can do almost as well
>> in january...
 
>48.7f:  i assume this is the average temperature on an average january day
>for san francisco  how did you find this out?

http://rredc.nrel.gov... 48.7 is the 24-hour average from data gathered
from 1961 to 1990, with an average daily min and max of 41.8 and 55.6 f,
and record min and max of 24 and 72 f.

>actually san jose climate matches that of my area (30 miles south of sf)
>a little better.

sf is the closest nrel weather station. probably good enough.

>> say it's a 2,048 ft^2 32'x32' 2-story house with 8% of the floorspace
>> as r4 windows with 50% solar transmission, 4% on the south side, and
>> 2% on east and west sides, with 0.5 air changes per hour. suppose
>> we use 600 kwh/month of electricity...

>> with r16 insulation (4" sips)... the house needs 24h(65-48.7)399
>> = 156k btu of heat on an average january day.

>> with 50% solar transmission, the south windows provide 43k btu, with
>> 0.5x82ft^2x445 = 18k from the east and west. electrical use adds 600/30
>> = 20 kwh/day, ie 68k btu, so the house only needs 156k-43k-18k-68k = 27k
>> btu of extra heat on an average january day. (it wouldn't need any extra
>> heat with r25 6" sip walls.) it needs 156k-68k = 88k btu on a cloudy day.

btw, that's another option. use r25 walls and forget all this solar stuff. 

>> we might keep the house 70 f during the day and 60 f at night, when it
>> needs about (60-48.7)399 = 4500 btu/h of heat, of which 68k/24h = 2.8k
>> btu/h comes from electrical usage. a 4"x1024ft^2 hydronic slab with
>> 4"/12"x1024ft^2x25 = 8500 btu/f and a 1024x1.5 = 1500 btu/h-f conductance
>> to house air and a 6 hour time constant might be 60+(4500-2800)/1500
>> = 61 f at dawn. absorbing 43k+18k = 61k btu/day of solar heat from the
>> windows, its temp rises 61k/8500 = 7.2 f during the day. with no direct
>> sun on the slab, the house air would be about 61k/6h/1500 = 6.8 f warmer
>> than the slab, ie about 61+7.2+6.8 = 75 f at dusk. some direct sun on
>> the slab would lower that.

>i did not understand this last calculation, staring with "with no direct
>sun". why would the slab be warmer than the slab?  why would direct sun
>lower the air temperature?

the slab stores 61k btu in 6 hours, at a rate of about 10k btu/h, like
the i in e = ir. the thermal conductance between the slab and slowly-
moving room air is about 1.5 btu/h-f-ft^2, or 1500 btu/h-f for 1000 ft^2,
so r = 1/1500, and e = 10k/1500 = 6.7 f, ie the room air has to be about
6.7 f warmer than the slab to move 10k btu/h into the slab.

with direct sun on the slab, the slab may be warmer than the room air,
vs cooler, so its airfilm resistance heats the room air. if the slab is
61+7.2 = 68.2 f just after dusk when the house needs about 4.5k-2.8k
= 1.7k btu/h of heat, the house air might be 68.2-1.7k/1500 = 67.1 f.

>> we might use a fixed reflective solar trough in an attic...

>my neighbors would kill me if i put a 12' x 12' water trough on my roof.

the trough would go inside the attic, with a transparent south roof and
a reflective north roof. your building inspector or insurance agent might
kill you, too. a brave architect or pe might defend you.

>> the house needs 5x72k = 375k btu for 5 cloudy days in a row. with an
>> 80 f minimum water temp after 5 days, we need at least 375k/(180-80)
>> = 3750 pounds or 469 gallons of 180 f water cooling to 80 f, eg a 4'
>> plywood cube lined with a single layer of epdm rubber, somewhere
>> inside the house.
 
>i don't understand where the 72k btu comes from.  i would have though it
>was 88k btu from your previous calculation.

just keeping you on your toes :-)

nick




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