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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 |