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solar greenhouse questions
1 mar 2000
jt writes: 

>i have a small greenhouse, 500 sq. ft., that i'd like to add drums of water
>to, but i need to know how many drums? ...this is an insulated wooden
>greenhouse that is glazed with glass, an airspace, and fiberglass roofing
>on the inside = r2 ??  the north wall is around r12 as are the east, west,
>and south walls up to about 4' where they meet the glass. 

so the main heat loss is about 500 ft^2 of r2 glazing, ie 250 btu/f of
conductance, plus some air infiltration, eg 150 btu/h-f at 2 air changes 
per hour, say 400 btu/h-f of thermal conductance.

>...the benches are already built in and configured in such a way that
>the barrels would fit underneath but would receive little direct sunlight,
>so they would be heated primarily by the air???

they need a lot more surface and thermal mass than dark drums in
the sun, if heated by warm air, and warm air rises, so you probably
need a fan to bring down warm air from the greenhouse peak, or an
old auto radiator and fan near the peak, with a pump and a big tank.

>everyone i've talked with so far has just advised me to add as much
>water capacity as possible.   sounds good but i need to know if it is
>really worth my time and money -- 55 gal drums are not free out here
>in ca and can cost as much as $10-15 each.

other options are "water walls" made with plastic film drum liners and
2x4s and welded-wire fence, as described in anna edey's solviva book,
or 1 gallon milk jugs on shelves. you can fit 137 jugs in a 4'x4'x3'
tall ("4 drum") module on 3 shelves made with 4 concrete blocks and 72'
of 2x4s, 2 4' "joists" over the blocks and 8 4' horizontal floorboards
per shelf, with the jug tops sticking up 1/2" through gaps in shelves
on 11" vertical centers. this uses about $56 worth of materials at
25 cents per jug. they need shade, since the plastic quickly degrades
in the sun. 

drums hold more water, but jugs have a higher surface to volume ratio,
which minimizes the air-water temperature difference. the shelves can
also store other things. 

more soil helps too, elevated, eg on benches. some people make raised
greenhouse planters by cutting 55 gallon drums in half lengthwise and
filling them with soil. then again, compost makes heat and co2, as do
chickens, rabbits, and goats... 

>we use the greenhouse to start vegetables for our small farm,
>so we really only need it from early feb. through may.  during
>this time period, the daytime temp. averages in the greenhouse
>around 80-90f (could be hotter but we open doors cause the plants
>don't like it) and night time temp in the greenhouse averages 
>between 40 - 45f.   ideally, i'd like the night time temp to be
>about 50-55, so an increase of 10 degrees is all i need.

that's a lot, given the high glazing conductance. if it's 40 f
outdoors and the greenhouse would stay 105 f inside for 6 hours
on an average day with no ventilation, it's collecting about
6h(105f-40f)400+18h(45f-40f)400 = 192k btu/day of solar heat.
a huge number of drums would only make the constant day/night
temperature 40+192kbtu/(24hx400btu/h-f) = 60 f.

here are some calcs using fewer drums, and milk jug modules...

10 tout=40'average outdoor temp (f)
20 tmin=45'min greenhouse temp (f)
30 tmax=105'max greenhouse temp (f)
40 cong=400'greenhouse conductance (btu/h-f)
50 ein=6*(tmax-tout)*cong+18*(tmin-tout)*cong
60 for nd=8 to 32 step 8'number of 55 gallon drums
70 a=25*nd'drum area (ft^2)
80 rt=1/cong+.67/a'thevenin resistance (f-h/btu)
90 tt=tout+ein/6/cong'thevenin temperature (f)
100 c=450*nd'thermal capacitance (btu/f)
110 rc=rt*c'time constant (hours)
120 cost=15*nd'drum cost ($)
130 num=tout+(tt-tout)*exp(-18/rc)-tt*exp(-24/rc)
140 den=1-exp(-24/rc)
150 tdw=num/den'dawn water temp (f)
160 tdg=tout+(tdw-tout)/(rt*cong)'dawn greenhouse temp
165 tdug=tdw+(tt-tdw)/(rt*1.5*a)'dusk greenhouse temp
170 print nd,tdw,tdg,tdug,cost
180 next nd
190 for nm=1 to 4'number of heat store modules
200 a=301*nm'container area (ft^2)
210 rt=1/cong+.67/a'thevenin resistance (f-h/btu)
230 c=1096*nm'thermal capacitance (btu/f)
240 rc=rt*c'time constant (hours)
250 cost=56*nm'module cost ($)
260 num=tout+(tt-tout)*exp(-18/rc)-tt*exp(-24/rc)
270 den=1-exp(-24/rc)
280 tdw=num/den'dawn water temp (f)
290 tdg=tout+(tdw-tout)/(rt*cong)'dawn greenhouse temp
295 tdug=tdw+(tt-tdw)/(rt*1.5*a)'dusk greenhouse temp
300 print nm,tdw,tdg,tdug,cost
310 next nm

run

no. of   min water     min air       max air       approx
drums    temp (f)      temp (f)      temp (f)      cost ($)

 8       52.40654      45.30194      90.92132      120
 16      54.46266      48.66028      80.62527      240
 24      55.65232      50.81957      75.42119      360
 32      56.42405      52.30266      72.2982       480

no. of   min water     min air       max air       approx
modules  temp (f)      temp (f)      temp (f)      cost ($)

 1       41.71559      40.90754      78.40425      56
 2       44.60019      43.18312      67.71125      112
 3       47.11097      45.48353      63.70967      168
 4       49.04439      47.39772      61.89867      224

the day/night temperature swing is a lot less with a bubblewall
filled with tiny r20 soap bubbles at night. say the greenhouse
air is 80 f max, during the day...

10 tout=40'average outdoor temp (f)
20 cong=400'greenhouse conductance (btu/h-f)
30 for nd=8 to 32 step 8'number of 55 gallon drums
40 a=25*nd'drum area (ft^2)
50 rt=.67/a'thevenin resistance (f-h/btu)
60 tt=80'thevenin temperature (f)
70 c=450*nd'thermal capacitance (btu/f)
80 rcd=rt*c'day time constant (hours)
90 rcn=(rt+1/cong+20/500)*c'night time constant (hours)
100 cost=15*nd'drum cost ($)
110 num=tout+(tt-tout)*exp(-18/rcn)-tt*exp(-6/rcd-18/rcn)
120 den=1-exp(-6/rcd-18/rcn)
130 tdw=num/den'dawn water temp (f)
140 tdg=tdw-(tdw-tout)/(rt+1/cong+20/500)*rt'dawn greenhouse temp
150 tdug=80'dusk greenhouse temp
160 print nd,tdw,tdg,tdug,cost
170 next nd
180 for nm=1 to 4'number of heat store modules
190 a=301*nm'container area (ft^2)
200 rt=.67/a'thevenin resistance (f-h/btu)
210 c=1096*nm'thermal capacitance (btu/f)
220 rcd=rt*c'day time constant (hours)
230 rcn=(rt+1/cong+20/500)*c'night time constant (hours)
240 cost=56*nm'module cost ($)
250 num=tout+(tt-tout)*exp(-18/rcn)-tt*exp(-6/rcd-18/rcn)
260 den=1-exp(-6/rcd-18/rcn)
270 tdw=num/den'dawn water temp (f)
280 tdg=tdw-(tdw-tout)/(rt+1/cong+20/500)*rt'dawn greenhouse temp
290 print nm,tdw,tdg,tdug,cost
300 next nm

run

no. of   min water     min air       max air       approx
drums    temp (f)      temp (f)      temp (f)      cost ($)

 8       70.9129       68.65427      80            120
 16      74.82656      73.50602      80            240
 24      76.38429      75.45278      80            360
 32      77.22111      76.50181      80            480

no. of   min water     min air       max air       approx
modules  temp (f)      temp (f)      temp (f)      cost ($)

 1       66.93311      65.59271      80            56
 2       72.61196      71.77973      80            112
 3       74.85598      74.25791      80            168
 4       76.05531      75.58931      80            224

nick




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