Phase changes in what materials work in the greenhouse!

[Bobberman]

If we find cheap materials that freeze at lets say 40 degrees will be a god send for a solar greenhouse! When a liquid freezes it gives off a 100 times as much energy as a one degree drop in tempreture of a equal volume of the liquid! This is very confusing in the fact that freezing seems to mean cold but just think in reverse!
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The temp in a greenhouse during the day is usually very warm and cold at night! If you had a substance that melted at 40 degrees it would need 100 times as much energy to melt to a liquid during the day but when it freezes as the temp drops it would give this energy back to the greenouse as it freezes! Get my point? Suggestions on liquids or waxes that will freeze between 40 to 60? jello is sugar water and freezes in the frig at 40 degrees!

[Dillbert]

are you trying to "find" these kinds of things?
in the solar world - check for eutectic salts and/or phase change materials

it's the ole' HS science thing "Latent Heat of Fusion"

technically one can "construct" a phase change material that acts at a specific temperature. which is nice, but because it exists in a lab beaker does not mean it is a commercial product.

many of the phase change materials are not "stable" - they do not maintain their performance over the long term. and, they tend to be expensive.

here's an interesting DIY theory:
http://www.builditsolar.com/Experimenta ... terial.htm

[Bobberman]

I was wondering if honey would work. I herd that honey last for years and you can heat it to make it flow better but at 50 degrees it freezes & gives off heat! I know honey is expensive but would it work!

[nnyr]

How hot does it normally get in your greenhouse during the day in PA? How cold at night? What are the temps that you would like to maintain in your gh? I'm also collecting information about greenhouses and would love to know about yours. We have a phase change material that melts at 70 to 73 degrees F, but do temps inside your gh get that high in the daytime during winter?

[Dillbert]

nina - surely you're kidding.... you people make phase change materials and design systems and you don't know how hot a greenhouse gets in the winter?

well, really hot. cook the plants hot. at north 40' lat.


Bobberman -

regards honey - don't confuse "solidify" with "freezing" - it's not the same thing.

depending on water content honey truly "freezes" in the low 20'sF
the specific heat (BTU/lb it can hold) is about half that of water.

[Bobberman]

Thanks for the info. My greenhouse gets in the 80' even on a cold sunny day but the new one is more insulated . I expect it to top 90 which will keep my water drums warmer and maybe I will not need any heat. The phase change material that melts at 70 will freeze slowly and give off heat for a long time I expect! What most people don't understand is that it takes 50 to 100 times the energy to melt than it does to drop or raise 1 degree after it melts! That means that a 50 gallon drum will act like 100 50 gallon drums during the phase change!
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I remember last year when it was 10 degrees for a week and my greenhose with only the water heat with 500 gallons of water stayed at 30 degrees. I could not figure why till I thought about the phase change as the water started to frees! The phase chance was giving off enough heat to keep it from going below 30! My plants all survived that I kep all winter!

[nnyr]

That's great that you kept your gh temps warm enough to see your plants through the winter! I've encountered a lot of people here that also use barrels of water for thermal mass and works well fro them. The PCM is ideal if you're wanting higher temps in the gh. Since it changes phase at around 72 F, that is the temp it'll maintain. If your gh heats up into the 80's during winter days, that's plenty warm enough to charge the PCM and it'll release the stored heat for later at night when temps drop. Check out: http://www.suncatcherdesigngroup.com/projects These guys are also from NC and they design passive solar, growing spaces and also use barrels of water. They're growing tomatoes in February!! Thanks for sharing!