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re: greenhouse glazing
9 oct 1998

>the lexan i had was... clear and seems to intensify the sunlight
>and heat.  it made summer cooling more difficult.

so you only had 1 clear layer, with an r-value of about 1 and about 90%
solar transmission? this wouldn't "intensify" sun, although it might feel
like that in the summer, when sun power enters the greenhouse and gets
converted to heat which leaves by conduction, convection, and radiation.

the glazing's r-value includes all three of these losses. it's measured
around room temperature, and it can be used to predict heat loss in that
temperature range, using "ohm's law for heatflow": u = (ti-to)a/rv, where
u is in btu/h, ti and to are indoor and outdoor temps in degrees f,
a is the area in square feet, and rv is the us r-value. 

a square foot of surface perpendicular to the sun's rays receives about
300 btu/h. if 90% of that solar power shines through the glazing, ie
270 btu/h, and the outdoor temperature is, say 86 f (the average daily
max in july in philadelphia) and the greenhouse has no vents, so the
heat can only leave through the glazing, 270 = (ti-86)1ft^2/r1, so the
indoor temp ti = 86 + 270 = 356 f. well, not really, but hot. radiation
would limit the temperature to something closer to 130 f. 

the greenhouse that covers the front of my house is 32' long x 16' high
x 12' deep. it gathers about 32x16x300x0.9 = 138k btu/h in full sun, and
1 cfm of airflow with a 1 f temperature difference moves about 1 btu/h
of heat, so keeping my greenhouse at most 10 f warmer than the outdoors
in full sun requires about 13.8k cfm of airflow. that might come from
a 100 watt ceiling fan removing air to the outdoors during the day, or
a passive vent with area av, where 13.8k = 16.6 av sqrt(h dt) = 210 av,
so av = 66 ft^2, eg an 8x8' vent at the top and the bottom.

covering the glazing with greenhouse shadecloth that blocks 80% of the sun
(which costs about 15 cents per square foot, unlike grapes, trumpet vines,
honeysuckle, runner beans, kudzu, etc.) reduces the gain to 54 btu/h-ft^2,
so the inside temp would be about 86 + 54 = 140 f in peak sun, with no
venting. with a 4x8' vent at the top and bottom and a solar heat gain of
32x16x54 = 28k btu/h = cfm dt = 16.6 x 4'x8' sqrt(16 dt) dt = 2125dt^1.5,
dt = 6 f, ie the greenhouse would be 92 inside at noon in july, with 80%
shading and venting, ignoring the heat loss through the glazing.

some thermal mass could store nighttime coolth for the next day. on an
average july day in phila, nrel says 1890 btu/ft^2 of sun falls on a
horizontal surface, and 820 falls on a south wall, for a daily greenhouse
heat gain of 12x32x1890x0.9 + 16x32x820x0.9 = 1031k btu in full sun or
0.2x1031 = 206k with 80% shading. the average july daytime temp in phila
is about 81 f, so if the greenhouse air has a constant temperature t, it
gains about 12h(81-t)1000ft^2/r1 = 12k(81-t) btu from the warm outdoors
during the day, with no daytime venting.

the average daily min temp in july in phila is 67.2 f. stacking 80
55 gallon drums 5-high along the back wall of the greenhouse and filling
them with water in summertime (and emptying them in the winter, to allow
the greenhouse to heat the attached house more efficiently), and venting
the greenhouse at night to cool the drums to say 70 f, makes the drum temp
rise by dt degrees by dusk, where 80x55x8 dt = 206k+12k(81-t) and t is
about 70 + dt/2, so dt = 8 f, and the greenhouse might be 78 f by the
end of an average july day.

a pond inside the greenhouse might work even better, with more heat loss
by evaporation (the average dew point in july in phila is about 65 f) and
conduction to 55 f soil, and less water and no drums. it might also grow
fish or store rainwater or help keep plants or bare solar water heating
collectors inside the greenhouse from freezing in wintertime. some water
plants or a fountain or ceiling fan above the pond might increase its
evaporation and heat transfer rates. how big would the pond have to be
to keep the greenhouse less than 70 f on an average july day?


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