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re: solar greenhouse
6 mar 1999
dave gower   wrote:

>hi gang.

hi dave,

>i've just got a solar greenhouse going.

congratulations :-)

>the idea is to keep it above freezing all during an eastern ontraio winter.

not easy, i'll bet. one big problem with greenhouses seems to be that
plants need light, so they need lots of solar glazing that is usually
not well-insulated at night, and growers don't like to waste space or
create shade with extra solar heating equipment inside the greenhouse. 

i wonder if this site has canadian weather data...

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>   it's 40 x 8 and attached to my house. has 11 4x8 double-pane lexan
>panals mounted vertically.

you might get about 11x4x8x0.8x311btu/h = 88k btu/h of peak sun
through those 11 panels...

>...when the sun's shining, warm air is blown through a 120-ton rock bed
>under the floor by 2 350 cfm air fans, one pulling warm air off the
>ceiling the other pulling cold air from the bottom of the rocks.

that's a lot of rocks, but not much airflow. seems like 700 cfm would
allow an empty greenhouse temp to rise about 88k/700 = 125 f above the
rock bed temperature, or twice that if the fans are in series, so a lot
of the heat would be wasted on a sunny day. i wonder how much power your
fans use, and if the rock bed is insulated above. have you measured
their static pressure? 

an insulated above-ground box of sealed containers of water might have
less resistance to airflow, so it would need less fan power, and it might
be 3x smaller, since water stores 3x more heat than rocks by volume.

>  so far my experience is mixed, partly because the insulation isn't
>complete and partly because i only got it working in late january, when
>it was pretty well frozen up.

i had that problem with a sunspace with too much moisture. it froze on
the inside of the glazing and reflected most of the sun out. bright sun,
frozen sunspace. a better vapor barrier on the ground fixed the problem.  

>  looking to share ideas/info with anyone who has built or is thinking
>of building something similar.   

if i were building a greenhouse, i'd look into filling the cavity
between two layers of glazing with tiny soap bubbles at night, a la
thermatics of montreal, or maybe add on a section whose only purpose
was to collect and store heat for the greenhouse proper... 

a local grower and i put 200 55 gallons of water inside his 20x100'
half-cylindrical plastic film greenhouse, under the benches. no special
airflow or insulation for them. that raised the average air temp in
the greenhouse about 5 f, and reduced his propane heating bill from 
$8k to 1500, he says... it seems to me those drums would have worked
better in a big box off one end of the greenhouse, something like this:

                   100'                  36'
     |                            |  drums in   |
     |        greenhouse          |   solar     | 20'
     |                            |  closet?    |

insulating the north half of the double poly greenhouse makes the
overall thermal resistance about 5pi/r1.2 = 13 btu/h-f per linear
foot, so it needs about 24h(70f-30f)13 = 12.5k btu per linear foot
to stay warm over a 30 f day. where i live, the greenhouse gathers
about 5pix1620x0.8 = 20.3k btu per linear foot, so that seems ok,
except on a cloudy day. keeping it warm for 5 cloudy days in a row
takes 5x12.5k = 62.5k btu, which might come from 62.5k/(120f-40f) =
780 pounds of water cooling from 120 to 40 f, about 1.7 55 gallon drums
per linear foot of greenhouse, or 175 for a 100' greenhouse. they
might be stacked 2-high in a box that's about 12'wide x 36'long
x 8' tall, almost as big as the original greenhouse :-)

maybe it could also be used for drying herbs and vegetables...

at night, or on a cloudy day, the greenhouse needs 52k btu/h of heat.
if air leaves the box 10 f warmer than it enters, that means about
5200 cfm of airflow, which seems doable with a couple of grainger's
$26 4ch71 117 watt 3200 cfm 20" window box fans. 


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