View Single Post
  #22   Report Post  
Old 12-11-2007, 05:44 PM posted to rec.gardens.orchids
alpickrel alpickrel is offline
external usenet poster
 
First recorded activity by GardenBanter: Oct 2007
Posts: 23
Default BTU's per dollar

My guess would be that the cooler air down in the underground pipes
will not be drawn up into the greenhouse if there is cross venting
from inlet to exhaust vents in the greenhouse above that bypasses the
pipes unless the pipes have an external inlet. Cool air will settle
to the lowest point. A fan to push or pull it through the pipes seems
required without this external inlet.

The system is analogous to swamp coolers: in order to be effective
the air coming in must be forced through the cooling structure; be it
damp crushed clay filled pipes buried in the earth or soaked pads in
front of an inlet shutter. The trench of crushed clay pipes needs one
open end to be on the outside of the greenhouse and one to be on the
inside. Actually, I would guess two inlet openings and one exhaust
are needed. One inlet is outside the greenhouse to draw in air for
cooling as vents at the top of the greenhouse exhaust the hotter air.
A second inlet is needed (and the first one closed) when air inside
the greenhouse is circulated though the pipes to warm it to the
average mean temp during "heating".

Imagine a semi-passive cooling system where a fan powered by a solar
panel forced air through the cooling pipes. For cooling, you only
need the fan to come on when the sun is out anyway.

To be ultra effective as a heater makes it bit more complicated. I
would need to look into heat exchangers and heat exchange technology
because we are talking about a primitive heat pump here. Something is
needed to 'concentrate' the BTUs before moving them up into the
greenhouse air and releasing them.

I can see the idea of a passive system being of help if I imagine that
what the ground pipes are doing is drawing the mean average air
temperature held constant by the earth at about the 6 foot depth up
into the greenhouse. This way you start your base air heating BTU
requirements at 55 degrees or so and not at whatever the current air
temperature is outside the greenhouse. In addition, heated air
naturally rises. What you are doing with the pipes when you heat is
to increase the ground's heat radiating surface. Imaging a long or
deep cave with a plastic bubble setting on top of the opening that
traps the air heated to the underground mean average temperature.

In a VERY deep tunnel, would the mean average temperature rising up
from the bottom be magma?