Hi James
On (25 May 96) James di Paulo wrote to Anyone...
 Jd> I have a (hopefully) practical idea for heating a greenhouse with solar
 Jd> energy that I would like other people, perhaps people with some 
ractical
 Jd> experience, to comment on.
I haven't the experience, but you might be interested in comment-
 Jd> First, lower the greenhouse slightly, to reduce the amount of glazing,
 Jd> plastic, etc.
Most heat losses are through the walls [surprise! most people think roof. Not
so]. So a squat building makes sense, thermally.
Jd>  I am thinking that three feet would do.
                        ^^^^^
Are you a Vertically Challenged very small person? How will you work inside
such a tiny house??
                             
 Insulated side
 Jd> Put in a base of crushed rock and sand; then
 Jd> lay a "coil" of plastic pipe and cover this with more sand/gravel and a
 Jd> floor of paving stones.
Technical problem- how to make a good pattern for your "coil". At the feed 
nd
the water will be hottest. At the drain end the water will be quite cool. And
near the building walls the heat losses thru the walls will necessitate ample
heat there whereas the area inside the "coil" will tend to run hot because of
lowest heat loss.
The danger being, that if the "coil" is unsuitable, some plants will cook and
others freeze. I guess that as a first approach, the feed end should form a
coil around the perimeter of the floor, and after as many turns as you 
ntend,
the drain end comes out [cold] from dead centre and back to the external
"boiler".
To test the thing when running, a thermometer could be pressed into the 
round
at intervals to see if a uniform temp has been obtained. Surely it is an 
ven,
correct temp, that gives best plant development. If you find a bad result 
hen
dig out the coil and start laying again, or use a powerful pump so that there
is hardly any temperature difference between water in/ water out of coil, or
lay thin aluminium sheeting across the entire coil, or dump the coil idea
all together and use parallel flow instead of series flow. Parallel flow 
eans
lots of joints to be cut, and possibility of late leakage. Think hard!
 Jd> To heat the water, I think it may be possible to use black plastic pipe
 Jd> mounted on/in a movable plywood box, (surfaced with plastic marine 
paint,)
 Jd> facing the Sun, circulate the water through these and then through the
 Jd> pipes under the floor of the greenhouse.
Two problems I see with that- 1. heat loss from box would be a killer on cold
days. So double glaze the sun side, and insulate the box sides.
- 2. plastic pipe has thermal resistance higher than metal, and I wonder
whether the amount of pipe you can pack into the ply box will be able to
absorb sufficient solar energy. Copper or aluminium would probably be a 
etter
option, blackened of course! Three or four scrap auto radiators, sprayed 
lack
and boxed/glazed would be more appropriate than plastic pipe.
 Jd> I think it will be necessary to have a large storage tank in the 
ircuit,
 Jd> well insulated and installed partially or mostly underground for heating
 Jd> the greenhouse in the night. (Perhaps an insulated and covered swimming
 Jd> pool, sort of a very large stock tank, more or less.) The warm water 
rom
 Jd> this storage tank would then be circulated through the greenhouse floor.
Put the storage tank under the greenhouse! Use a hi temp cutout to block 
ater
flow to storage, if greenhouse overtemp thermostat operates.
 Jd> A propane and/or a wood/trash/paper fired hot water heater could be used
 Jd> in the rare times that there was no sun. Most days are sunny here, only
 Jd> very rarely do we see more than two or three completely overcast days
 Jd> in a row, summer or winter.
I'm no gardener, but think that your problems will include the need for
accurate temp control. The thing will exhibit great inertia at times when you
want to heat quickly, and great momentum! when the temp is rushing upward and
you want to stop the temp rise. Commercial greenhouses use quick response
automation and heater units I guess. So the gardener can take a short 
acation
at times. But maybe you plan to be there every day?
Second thoughts about auto radiators- if you use these with 12v thermatic
fans, these could be roof mounted to your glazing bars, and the fans
controlled by master thermostat. [These fans come with relays.]  The
radiators/ fans mounted in shallow double glazed boxes, to recirculate
greenhouse air NOT introduce fresh air.
Cheers....ALEC
... ........The wheel is environmentally unviable
--- PPoint 1.92
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