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re: solar hydronic greenhouse questions
12 jan 2001
rick solinsky wrote:
>> you might fill the space between two layers of poly glazing with 12" of
>> tiny soap bubbles at night, and put an 18"x16' $6 water-filled round poly
>> duct on the ground under a bench inside a 24" air-filled duct, and blow
>> warm air from the peak down through the 24" duct during the day to warm
>> the water and dehumidify the greenhouse.
>i'm not sure i understand what you are doing....
covering the greenhouse with two layers of poly film, with the space between
the layers filled with air during the day to let the sun shine in (normal
practice for commercial greenhouses) and insulating the greenhouse by
filling the space between the films with tiny cold soap bubbles at night
(abnormal practice :-)
you might make a watertight trough with the films near the ground, fill it
with a 10% soap solution, put a 2" pvc pipe with 1/16" holes in the trough,
and push some air through the pipe with a shop vac when you want to make
bubbles. let air flow out of the film cavity through a 2" opening at the
top that's covered with window screen, and turn off the shop vac when a
microswitch senses bubbles pushing on the screen. turn off the shop vac
and turn on small blower to fill the space between the films with air
during the day.
><18"x16' $6 water-filled round poly...> what is $6??
greenhouses often use inexpensive polyethylene film circular ducts to
move air. rimol greenhouse systems (800) rimol gh sells 18" duct for
39 cents per foot, ie $6.24 for a 16' piece. put it inside a 24" duct
(52 cents/ft), fill the inner duct with water, and blow air through
the space between the inner and outer ducts to warm the water. israeli
greenhouses use water filled ducts on the ground between the plants
to keep them warmer at night, but they don't put them inside a larger
air duct, as far as i know.
they do use "brine dehumidifiers," boxes filled with some kind of
concentrated salt solution (licl?) that absorbs water vapor from
warm moist air during the day, which warms the salt solution.
how do they reconcentrate it?
>to warm the water and dehumidify the greenhouse. >
>what are you doing here?? how or why would it dehumidify the greenhouse??
plants make lots of water vapor (about 1 pound per square foot) on
a sunny winter day, so greenhouses need some sort of dehumidification
in order to avoid becoming swamps with mildew and mold and rot. the
usual way to do this is to bring in some cold dry outdoor air, heat it,
and blow it out of the greenhouse, along with some water vapor, but
that takes a lot of energy, and it makes co2 enrichment difficult.
raising the level of co2 from 330 ppm (outdoors) to 1000 ppm (an office
with people inside) can increase the growth rate of tomato plants by 50%.
compost inside the greenhouse can raise the co2 level, but not if fans
used for cooling or dehumidification are constantly circulating outdoor
air through the greenhouse. (btw, you can also pump in the soap bubbles
to shade the greenhouse and keep it cool on a sunny day.)
allowing water vapor to condense on a cold surface inside the greenhouse
wastes less energy. some growers let vapor condense on the inside of the
glazing, which is treated to avoid dripping. this works best on greenhouses
with a peak at the top, vs circular bows, so the water runs down the sloped
film instead of dripping onto the plants from the underside of the horizontal
surface near the peak.
some growers blow air through rock beds. if you blow warm moist greenhouse
air through a duct surrounding a cool water duct on a sunny day, it will
condense water vapor on the outside of the water duct (which can run off
through some weepholes in the outer duct or be collected and reused) and
warm the water inside the duct. the warmed water can keep the greenhouse
warm at night, using the fan for temp control. you may still get some
condensation on the inside of the glazing during the day, if that surface
is cooler than the water inside the duct.
with no outdoor air exchange, water vapor from greenhouse air condenses on
the coldest interior surface, at the dew point temperature, which limits
the max greenhouse humidity. table 4-6 of the 1994 nraes-33 greenhouse
engineering book shows interior surface temps and max humidities as a
function of outdoor temperature, for a 60 f greenhouse with 1 and 2 layers
of film cover:
--1 film layer-- --2 film layers--
outside surface max rh surface max rh
temp (f) temp (f) (%) temp (f) (%)
47 f 52 72 57 89
27 39 45 52 74
7 26 26 47 61
-8 16 20 43 53
the first 2 columns might apply if you blew some greenhouse air between
the north wall films...
nick
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