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re: water-wall greenhouses
10 nov 1998
i wrote:

>with an extra layer of glazing over the water wall, we might have something
>like this electrical analog:
>
>                            tg = 71.6 (initially)
>                            |
>              r1.2/25ft^2   |   r1.7/16ft^2
>    30 f --------www---------------www------------tw = 122.3 (initially)
>           exterior glazing     ww glazing   |     
>              resistance        resistance  ---   c = 2x16x64 
>              = 0.048           = 0.104     ---     = 2048 btu/f
>             \________ r = 0.152 ______/     |
>                                             -
>
>rc = 0.152 f-h/btu x 2048 btu/f = 311 hours for the water, so after 5
>cloudy days we might have tw = 30+(122.3-30)exp(-120/311) = 92.8 f, with
>a heatflow through the wall glazing of (92.8-30)/0.152 = 413 btu/h and a
>greenhouse air temperature tg = 30+413x0.048 = 50 f. still too cold.

we might let air flow under the wall insulation and glazing on that fifth
30 f cloudy day, making the total water wall thermal resistance something
like r0.67/32ft^2 = 0.021, the wall's heatflow (98.2-30)/0.069 = 991 btu/h
and the greenhouse air temperature 30+991x0.048 = 77.6... 
 
>...a less-wasteful scheme with a lot less thermal mass might have
>bubbles partially filling the waterwall glazing cavity on cloudy days.

this "series regulated thermal power supply" might use 5 times less mass,
only 2 4x8' 2-ton water wall modules for the entire 32x16x16' curved
greenhouse, which requires about 5dx24h(70f-30f)800ft^2/r20 = 192k btu of
heat over 5 30 f cloudy days, with bubble-filled glazing and perfectly-
insulating north and endwalls. the final water temperature might be
122f-192kbtu/8,000btu/f = 98 f.

each module could have a thermal resistance of about r0.67/120ft^2 with
internal airflow, 0.0028 f-h/btu for two, if surrounded by an insulating
skirt with a hinged top raised by a passive thermofor vent opener (eg
jade mountain's $54 fc115 "classic," which can move 15 pounds 15" at an
adjustable temperature between 55 and 85 f.) on a very cold -10 f day,
after 5 cloudy days in a row, with 0.025 f-h/btu of bubble-filled house
glazing resistance, the wall's heatflow would be (98f-(-10))/0.0278 = 3.9k
btu/h and the greenhouse temperature could be -10+3.9k(0.025) = 87 f.

nick




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