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Solar heat for greenhouse at night question

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Posted by john on June 8, 2008, 9:04 pm
Over the winter I put in a hobby size greenhouse for the wife (approx.
8.5'W x 12.5'L x 8.0'H w/ Gambrel roof and 4mm double wall polycarbonate
(P.C.) sides.)  In an attempt to keep this thing from being too much of
a money pit, I would like to collect heat during the day to use for
night time heating.  I already have four 4'W x 10'L thermal panels that
were given to me (that I need to test also.)  BTW, we live in sunny
Texas north of Dallas.  Most of the time we have too much heat to
contend with here, but a few nights in the winter it gets in the low 20s
and sometimes lower (I know you guys up north call that beach weather,
but for us its d****** cold.)  I do have some electric heat for back up
and am working on a small gas heater too.  Ideally the G.H. temp can be
maintained at night with the solar and the elec/gas used only during
extreme weather.

How do I calculate the heating capacity of my thermal panels.

I also need to set them up (saw horses in the yard) to leak check.  If I
measure deltaT at a given water flow rate, OAT, angle of inclination
(laying flat on saw horses), latitude, Julian date (season),(what else?)
is there an easy formula to extrapolate the efficiency of the panels for
winter operation?  The thing is, in another month you can almost boil
water on the side walk here...

Another issue is where to store all this heat until I need it.  I have
seen ideas ranging from insulating barrels to buying cheap water heater
tanks.  This will be a closed system, so I am not adversed to using an
antifreeze chemical.  (Someone mentioned an antifreeze in another thread
sometime back that was more Eco friendly than ethylene glycol.)

[According to the manufacturer's formula, the Greenhouse requires 3kw
during extreme conditions (20 F) to maintain temperature.]

Suggestions, comments, ideas all are welcome.


Posted by Jeff on June 9, 2008, 5:00 pm
john wrote:

Solar data:


  There's a wealth of solar data at NREL in different formats, hunt
around if that isn't what you need.

Efficiency will be higher for lower delta T. You might figure about 50%
as a good starting point.

Search Gary's site for EPDM liner home made tanks.


   My thoughts are that you have enough collector to keep your little
greenhouse toasty. You should get 100,000 BTUs plus per day from the
collector.  That would give you 10K BTUs/hour for 10 hours. The double
wall glazing cuts the losses roughly in half. I would think the solar
would give you an added 30F or so for the cold night. Someone else might
have a better guess at the conductance of the green house.


  This will be a closed system, so I am not adversed to using an

Posted by Steve on June 10, 2008, 2:34 am

When I was a kid my stepdad had a 100' x 40' greenhouse in the Oregon.  We
were lucky enough to find a cheap source of 5 gallon buckets with lids.  We
filled the buckets with water and stacked them two high and put trays of
plants (we called them "flats") directly on top of the buckets.

The idea was that the water would absorb heat from the greenhouse during the
day and re-radiate it at night.  I can't say just how effective the
arrangement was since we didn't do any careful measurements, but I do think
it made a significant difference.

If it warms up during the day on those few days where it gets down to 20 F
at night, it may be enough to add a bunch of thermal mass to keep things
from cooling down in the greenhouse too much at night.  It also give the
added benefit of keeping things from getting too hot during the day on those
hot days.


Posted by john on June 11, 2008, 3:06 am
 Good information, thanks to all.

Guess I have some reading to do now...


john wrote:

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