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Heating a greenhouse at night...

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Posted by john on September 2, 2008, 3:57 am
I built the wife a small 8.5 x 12.5 greenhouse to winter her plants in.
    I am building a small solar thermal collection and storage system to
use for heat on cold nights; trying to ween away from electric and gas
(seemed cheap at first, but project has developed a life of its own...)
  Someone suggested using underfloor radiant heating which seems like a
good idea as opposed to forced air, except I am familiar only with the
concept radiant heating, not the application and most of the
installation web sites I have visited show installation in either pier
and beam or slab; my little greenhouse has a floor backfilled with about
+12" of wood chips.  Putting the pipe in is not an issue, but where do I
start on figuring the pipe diameter, composition, spacing, flow rate,
distribution manifold (if needed?) water temp, etc.  Are balancing
valves needed on the return manifold? etc.  I see a lot of opportunity
for "I gotcha" and the repercussions of freezing the wife's plants are
just too horrible to consider...!

Any help or suggestions for this or alternative methods would be greatly

BTW, the electric heat size calculated for 20F night is about 3kw.


Posted by DG on September 2, 2008, 5:26 am

In the winter, for our small greenhouse we just use tubs of water in a
black plastic containers.  Put it southfacing in the winter and remove
it in the summer.

Not sure how long you want to use your greenhouse but we are in zone 9
and it works well even when we get to freezing (rare).

-=   http://www.flickr.com/photos/rosepetal236/    =-

Posted by nick hull on September 2, 2008, 11:05 am

I suggest you first look into thermal shutters for night, you can
conserve LOTS of heat that way.

Free men own guns - www(dot)geocities(dot)com/CapitolHill/5357/

Posted by Neon John on September 2, 2008, 6:32 pm
 Radiant heating is wonderful but I immediately see two problems here. One,
what/where is your thermal mass?  Two, where is your insulation to prevent
heat from conducting down into the earth.

With slab radiant heating, the slab forms most of the thermal mass.  With a
solar collector, it would heat up during the day and then give it up at night.
With no thermal storage, the heat would stop when the sun goes down.

Second, slab type systems that sit on dirt have some form of insulation
between the slab and dirt, normally some closed pore poly sheeting.  Without
the insulation, you'll end up sending a portion of your heat into the earth.

Here's an alternative suggestion.  Your objective is to keep the plants warm
so why not heat them directly?  Run the water tubing down the rows of plants.
Circulate the warm water through these tubes to warm the plants.  This assumes
that the plants are in pots.

If the plants are in dirt beds then simply zig-zag the tubing through the
dirt.  Enough heat will be given off by the dirt to keep the plants warm.

My late uncle was a rather famous orchid and camellia breeder.  He used the
later technique for his camellia beds.  He used semi-rigid electrical heaters
instead of water pipe but the principle's the same.  He used almost no aux
heat in his greenhouses.

At 3kW, you'll have to store considerable heat.  Assume the night is 12 hours
long, that 36kWh.  1 pound of water stores 1 BTU per deg F.  36kWh is 122,837.
If you could heat the water to 180 deg and let it cool to 80 degrees by the
end of the night, that's a delta-T of 100 degrees. 122,837/100 = 1,228 lbs of
water.  At about 8 lbs per gallon, that's 153 gallons.

Heating that much water to 180 during a short winter day is probably
optimistic unless you build a large array of collectors.  You'd also have to
arrange a tempering system to maintain the water circulating around the plants
at a safe temperature.  A tank that large would be kinda expensive, large and
a chore to insulate.

As an alternative, you might consider phase change storage.  phase change
means melting or freezing.  The two major advantages of phase change storage
are a) the temperature remains the same until all the medium is melted or
frozen and b) phase change requires many more BTU than simply changing the
temperature of the material.  

As an example, the heat of fusion of 1 pound of solid water (ice) is 143 BTU
per pound.  That is, it takes 143 BTU to turn 1 lb of ice at 32 deg into 1 lb
of water, also at 32 degrees.  To raise that water to 33 deg takes only 1
additional BTU.

Water is one of the best phase change heat storage mediums if you happen to
need your heat at 32 degrees :-)  There are other materials that freeze/melt
at more convenient temperatures.

Paraffin is an excellent phase change storage medium.  Alloys are available
that melt/freeze at anywhere from just above ambient to above the boiling
point of water. A number of companies sell paraffin and other phase change
materials conveniently micro-encapsulated in tiny spheres.  Fill a tank with
the spheres, circulate water through the interstices and you have your phase
change storage device.

If you're interested in this route, say so and I'll dig up some old info that
I have.  A few years ago I converted a client's RV water heater into a phase
change water heater.  That involved simply filling the water heater tank with
phase change spheres and employing an external diesel heater.  It tripled the
available hot water, which was his goal.


On Mon, 01 Sep 2008 22:57:53 -0500, john

John De Armond
See my website for my current email address
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
What do you call 4 Blondes in an Abrams?  Air Tank.

Posted by john on September 2, 2008, 8:47 pm
 I would be interested in that even if for no reason other than
intellectual curiosity.  Please tell me more.


Neon John wrote:

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