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RE: Constructing a greenhouse

  • Subject: RE: [ferns] Constructing a greenhouse
  • From: "Winter, Wim de" <Wim.dewinter@wur.nl>
  • Date: Wed, 29 Sep 2004 21:47:00 +0200
  • Content-class: urn:content-classes:message
  • Thread-index: AcSlbVhr8i4jRr+SSoWifgbX1lMm7gA7NoJ8
  • Thread-topic: [ferns] Constructing a greenhouse


The greenhouse you're describing uses water-cooling in a way that is
thermodynamically efficient, but not so from a point of view of water
management. The mist in the greenhouse is carried away by convection before it
has evaporated and so before it has consumed its heat of evaporation. This is
a quick way of cooling in excess of water, but having seen Namibian pictures
showing all (semi-)desert I might venture that a less water consuming system
should be considered. After all, in an 18 cubic meters' enclosure at 100%
humidity you'd only need 1.7 g of water evaporating per degree temperature
decrease (18m3, 60g H2O/m3, 600 cal/g evaporation energy). The heat flux into
the system is determining the water consumption.

Assuming the greenhouse is well shaded, the heat flux largely depends on the
materials used in the construction. Suppose the goal is to depress the
environmental temperature of 430C by 10 degrees. How much cooling capacity is
needed to maintain this temperature difference?

The greenhouse measured 3x3x2 m (lxwxh) so the total area would be 33m2.
The heatflux at 10K difference is found by

phi = U.A.(t1-t2) = U x 33 x 10 = 330 U [Watt]

U is the transmission coefficient which can be found for various materials.
For single glass, plastic sheets, corrugated plastic and the like it's about
4.5 to 6.2 W/m2.K. Isolation glass is as low as 1.2W/m2.K

So, taking an average of 5, the flux would be 1650 W or 1650 J/s.
Evaporation energy of water is 2.24 kJ/g so you'll need 0.74 ml/s or 2.65 l/h
Not too bad, only 31 l when you need it 12h/day.

Assuming you don't open the door and no extra losses through the construction.
Double glass/plastic should be able to half the amount of cooling needed.

Rethinking this, the fridge should be constructed outside the enclosure, e.g.
the roof. Wouldn't that exclude the roof fromn the total surface area?


Wim de Winter
"there's always a theoretical solution"

By the way... Is cooling realy necessary, or is stopping the evaporation
sufficient? I'm not sure about long term stays at 430, but a short time at
least is harmless.


-----Original Message-----
From:	owner-ferns@hort.net on behalf of Bob Needham
Sent:	Tue 9/28/2004 4:35 PM
To:	cfao@iway.na
Cc:	ferns@hort.net
Subject:	Re: [ferns] Constructing a greenhouse
After re-reading your post again more carefully, I have one additional idea.
If the air temerature is hotter outside than inside, you will need misters
at the ridgeline (on a different valve), as then, the airflow will reverse;
i.e. air will come in through the roof vents, be cooled by the ridge misters,
contract, and sink, flowing out through the ground-level vents.

Sorry. I teach Science, Physics & Math, but my "paying-attention" skills are
low, just ask my wife! ;^>

> You need openable/closeable vents near the ground, and openable vents right
> up at the top of the ridgeline. The ridgeline vents should be LARGE. Make
> all the way down the ridge on both sides, and easily openable (hand-crank
> garage-door opener/motorized). They should reach ~25% from the ridge to the
> eaves. If motorized, they can be set up on a thermostat to open when the
> temperature passes a certain point, and close again when it cools off. To
> help even more, put misters down just inside of the ground-level vents.
> The evaporation will cool the air, and natural passive convection will draw
> the too-warm air out the ridgeline vents. No fans needed...
>                  ---/\--- <-- Ridge vents
>                    /  \
>                   /    \
>                  /______\
>                 /|      |\
>                  |      |
>                  |  ___ |
>                  |  | | |
>                  |  | | |<-- Ground-level vents
> ---------------------------------------------------------------------
> Support hort.net -- join the hort.net fund drive!
> http://www.hort.net/funds/

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