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Thermal Mass Formula?
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Author:  Carolinajim [ Jan 15th, '09, 10:09 ]
Post subject:  Thermal Mass Formula?
How do you calculate the volume of water which is required to keep a greenhouse above freezing?

For example; greenhouse volume is X and sunlight hours are Y requires a water volume of Z to maintain temperature above freezing overnight.

Thanks,
Author:  Hex [ Jan 15th, '09, 16:17 ]
Post subject:  Re: Thermal Mass Formula?
Hi Jim
Its not that easy to calculate but in simplified terms, heatloss is geared to surface area versus temperature difference, modified by insulation R-value. (A x Td/R)
Once you have the heatloss figured you`ll know how much heat you`ll need to add to stay above freezing. Its possible the amount of water required will be more than the greenhouse can physically hold.
Author:  KudaPucat [ Jan 15th, '09, 17:15 ]
Post subject:  Re: Thermal Mass Formula?
many things other than water retain heat, and radiate it though the night.
Growing trees or low shields (that don't block the sun) will decrease the heat radiated by the greenhouse during the night.
Also the amount of wind will effect it's cooling (as air conducts crappily but convects very well)
there really are too many factors.
I do know: an exposed greenhouse will lose much heat at night.
I would suggest: Thermally insulate as best you can.
Reduce radiation as best you can whilst still allowing for solar radiation.
add a practical amount of water.
perhaps think about covering the GH with spaceblanket type material during the especially cold nights? (only practical if your GH is small) I have seen anti frost curtains on cherry orchards before. They're a 'tent roof' a-frame style thing, on 3 wires.
This is pulled across every night (and thunder storm) and removed every morning. Some are even motor driven.

I know this hasn't answered your question at all, just trying to think laterally about the root cause of your problem
Author:  Food&Fish [ Jan 15th, '09, 17:42 ]
Post subject:  Re: Thermal Mass Formula?
Real cool here last night after2 hot days over night ft water went from 23 to 20.5 havent checked today
Author:  Carolinajim [ Jan 16th, '09, 03:03 ]
Post subject:  Re: Thermal Mass Formula?
Hex wrote:
Hi Jim
Its not that easy to calculate but in simplified terms, heatloss is geared to surface area versus temperature difference, modified by insulation R-value. (A x Td/R)
Once you have the heatloss figured you`ll know how much heat you`ll need to add to stay above freezing. Its possible the amount of water required will be more than the greenhouse can physically hold.


Thanks Hex, Great points!

How about a formula for time to freeze a cubic meter of water from say 4 C.

Thanks!

Water trivia...for those so inclined http://www.townofpendleton.org/pdf/Wate ... _Facts.pdf
Author:  Carolinajim [ Jan 16th, '09, 03:16 ]
Post subject:  Re: Thermal Mass Formula?
KudaPucat wrote:
I know this hasn't answered your question at all, just trying to think laterally about the root cause of your problem

Actually those were great pieces of information. Thanks!

I'm just trying to design a low input system for my climate. The water will continue to be circulated by an air flow pump run off a cheap aquarium air pump powered by solar.

I'm already using additional coverage with plastic sheeting of my "guinea pig" taro and avocado plants...successful so far in my prototype scaled down 1000 liter system.

This Spring I intend to dig a 1.3 X 3 or so meter deep pond which will be the sump for my air flow pump system. Along the north wall of my greenhouse I plan have approximately 2000 liters of water in barrels...nice slow moving system which gently circulates the water. To cool in the summer and warm in the winter using solar/ground heat.
Author:  johnnie7au [ Jan 16th, '09, 07:45 ]
Post subject:  Re: Thermal Mass Formula?
Quote:
How about a formula for time to freeze a cubic meter of water from say 4 C.


It depends how cold the ambient temperature is. The colder it is the faster the water will freeze.

A formula for how much energy would need to be removed to freeze a cubic meter of water from 4 deg C is easy to provide.

Energy = (mass H2O x latent heat of fusion H2O ) + ( Mass H2O x Specific heat H2O x Temp change)
Author:  TCLynx [ Jan 16th, '09, 08:44 ]
Post subject:  Re: Thermal Mass Formula?
Perhaps look for info from an HVAC forum about BTUs and such might help you figure our some of the parts of the equation.

Being able to say and particular amount of water at 70 f can keep a particular size greenhouse from freezing on a 25 F night but the bigger issue will be can you recover the water temp the next day to be ready for the next cold night. That is kinda the problem we are having down here this week where we are having cold nights and it has been cloudy the past two days so the solar heaters arn't doing much good.
Author:  Hex [ Jan 16th, '09, 08:45 ]
Post subject:  Re: Thermal Mass Formula?
Phase change adds a nice twist to things :wink:
Author:  Carolinajim [ Jan 16th, '09, 09:12 ]
Post subject:  Re: Thermal Mass Formula?
:shock: I will probably need some more help. Oops left out ambient temp.

Ambient temp is -10C

Thanks, we'll hit near our bottom temps for this area tonight but it should warm to 0 C and then back down to -10 the next night. So, this will be a great test for my little prototype system.

I'm not really trying to keep the greenhouse temp up just trying to create little microclimates for survival of the tropical "guinea pigs" an avocado and a taro plant.
Author:  hydrophilia [ Jan 16th, '09, 14:46 ]
Post subject:  Re: Thermal Mass Formula?
Carolinajim wrote:
:shock: I will probably need some more help. Oops left out ambient temp.

Ambient temp is -10C

Thanks, we'll hit near our bottom temps for this area tonight but it should warm to 0 C and then back down to -10 the next night. So, this will be a great test for my little prototype system.

I'm not really trying to keep the greenhouse temp up just trying to create little microclimates for survival of the tropical "guinea pigs" an avocado and a taro plant.


I have a friend here with a tomato plant that is nearly 2 years old while all others in town have been killed off by a multitude of hard frosts. It survives with the help of large adjacent concrete buildings next to it which warm it at night rather than leaving it exposed to cold sky. We often get hard frosts while the air only reads freezing or slightly above due to exposure to clear cold skies. Under an evergreen or next to those buildings there is no frost.

You can figure that a BTU warms 1 lb of water 1 degF, but that is not really helpful. I think what you need is a way to prevent the surface of the leaves from reaching freezing. You could do this by warming the air (least efficient), having water tanks that radiate heat (a wall at 70 degF will make a huge difference), with a radiant heater shining IR on the plants, a radiant reflector (space blanket) as suggested, or you could mist the plants with water to keep them above freezing.
Author:  Carolinajim [ Jan 16th, '09, 23:26 ]
Post subject:  Re: Thermal Mass Formula?
The temp outside is 25F and the greenhouse is now at about 65 degrees. Took the plastic draping off of the barrels and no freezing and no damage to the tropicals.

Interestingly, the condensation on the inside of the greenhouse is frozen. There is a pretty good wind out of the north. The low this AM before sunrise was 20 degrees and it was about 25F inside the greehouse. Don't know what the micro climate temp was around the barrels under the plastic sheeting but guessing between 40 and 45F.

I added about 15 gallons of groundwater to my system which is about 55F.

The airlift pump functioned fine with no freezing at all...eventhough the outlet was exposed to the ambient greenhouse temp.

Away from the micro climates and conventionally planted, the chinese cabbage is wilted as are the brussel sprouts.
Author:  hydrophilia [ Jan 17th, '09, 02:55 ]
Post subject:  Re: Thermal Mass Formula?
Carolinajim wrote:
Interestingly, the condensation on the inside of the greenhouse is frozen. There is a pretty good wind out of the north. The low this AM before sunrise was 20 degrees and it was about 25F inside the greehouse. Don't know what the micro climate temp was around the barrels under the plastic sheeting but guessing between 40 and 45F.


Citrus growers protect their crops at times with sprayed water. The air/sky/environment has to suck heat out of the water to turn it to ice, just as with your condensation. From the view of the plants, that section of sky (with the freezing condensation) was at 32degF rather than 10degF or 15degF or whatever. The heat of enthalpy for liquid water>ice is 143 BTU/lb, so you get a lot of protection from a small amount of freezing water.
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