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Heat conductivity of polyethylene...

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#1 toonces

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Posted 05 February 2021 - 02:05 PM

Considering getting a greenhouse.  And thinking about putting 55 gallon drums, filled with water (with some level of antifreeze in them) to collect heat storage for overnight.  Was thinking about refurbed steel with some cheap liners, but found some really cheap refurbed black polyethylene.  Will polyethylene transfer the heat?

From what I an gather, the poly is anywhere between 2 and 3.5mm.

 

Anybody know?


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#2 toonces

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Posted 05 February 2021 - 02:20 PM

I'm looking online at the thermal conductivity of high density Polyethylene.  It's 0.50 W/(m K).  Steels vary between 11.6 for NiCr, to 66 for tungsten steel.  So, even at the lowest, the conductivity is 20 times lower.  

 

Hmmm.  Maybe poly isn't the way to go...


Edited by toonces, 05 February 2021 - 02:38 PM.

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#3 toonces

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Posted 05 February 2021 - 02:26 PM

Thermal conductivity of cold rolled steel, what one manufacturer says they use is  51.9.  

 

yyyyyyeeeeeeaaaaaahhhhh....


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#4 SnowMan

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Posted 05 February 2021 - 02:31 PM

I bet a small pump and some kind of radiator or heat exchanger would increase your efficiency a ton. Over just passive barrels
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#5 toonces

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Posted 05 February 2021 - 02:33 PM

I bet a small pump and some kind of radiator or heat exchanger would increase your efficiency a ton. Over just passive barrels

Hmmmm.  I'd have to run power out to it some how....


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#6 dondewey

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Posted 05 February 2021 - 02:39 PM

Conductivity is not sufficient. That's just heat movement through the material. You need to consider both how well it absorbs heat and how well it transfers it to find out if conductivity matters much at all.
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#7 porter

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Posted 05 February 2021 - 02:39 PM

Quit being cheap and just buy regular condoms.
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#8 HVB

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Posted 05 February 2021 - 02:46 PM

Hmmmm. I'd have to run power out to it some how....


Solar
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#9 Patrick C.

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Posted 05 February 2021 - 03:01 PM

Are you actively heating the water, or just planning to use the barrels as a kind of thermal flywheel?  I don't think the conductivity will matter much.  How well you limit heat loss out of the greenhouse is probably more important.  

 

Just get one steel and one poly drum and see how the temperature in them compares.


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#10 3rd party JKor

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Posted 05 February 2021 - 03:59 PM

Typical carbon steel is around 40 W/m-K, HDPE is probably even less than 0.5, depending on the grade it'll be 0.3-0.5.
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#11 miccullen

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Posted 05 February 2021 - 05:56 PM

My high school's greenhouse used sealed water filled 5 gallon buckets (steel) as it's "heat sink", around the whole perimeter.


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#12 toonces

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Posted 05 February 2021 - 06:14 PM

My high school's greenhouse used sealed water filled 5 gallon buckets (steel) as it's "heat sink", around the whole perimeter.


Sort of what I had in mind.
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#13 toonces

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Posted 05 February 2021 - 07:43 PM

I bet a small pump and some kind of radiator or heat exchanger would increase your efficiency a ton. Over just passive barrels

Help me pit with this.  I'm not getting my head around what this would look like.


Quit being cheap and just buy regular condoms.

I need something that big for your mom...


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#14 SnowMan

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Posted 05 February 2021 - 07:47 PM

Help me pit with this. I'm not getting my head around what this would look like.


I'm thinking as redneck as a car radiator with a small fan pushing air across it. Small pump pushing water into the hx with a gravity return. Tie the barrels together somehow... Supply from one end of the train, return to the other



When the air temp is warmer than the water you'll be storing heat in the water. When lower harvesting into your space.
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#15 Dave McG

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Posted 05 February 2021 - 08:21 PM

Which has a better coefficient, the polyethelene barrel or the air around it? I remember the arguements on the verde place about using SS tubing as an immersion chiller due to its poor thermal conductivity when compared to copper. On our local board a M.E. who designed heat exchangers chimed in. The short answer was to identify the choke point, not to worry about each component.
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#16 toonces

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Posted 05 February 2021 - 09:19 PM

Which has a better coefficient, the polyethelene barrel or the air around it? I remember the arguements on the verde place about using SS tubing as an immersion chiller due to its poor thermal conductivity when compared to copper. On our local board a M.E. who designed heat exchangers chimed in. The short answer was to identify the choke point, not to worry about each component.

When the zeroth order design is a black 55 gallon barrel filled with water to absorb, and store heat from the sun during the day, and then release that heat at night, I think the barrel is the choke point.  Or, am I missing something?


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#17 Dave McG

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Posted 06 February 2021 - 06:18 AM

I didn't see that they were getting heated in the sun, I thought that being indoors they would just try to equalize with ambient temps. Without pulling out all my old thermodynamics books my gut says that the plastic won't make a huge difference over the steel.
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#18 toonces

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Posted 06 February 2021 - 09:03 AM

I didn't see that they were getting heated in the sun, I thought that being indoors they would just try to equalize with ambient temps. Without pulling out all my old thermodynamics books my gut says that the plastic won't make a huge difference over the steel.

That was my initial thought.  But, then I realized there's at least two heat vectors: direct illumination, and heat from the greenhouse effect.  I suspect the first one is the most prominent.


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#19 toonces

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Posted 06 February 2021 - 12:35 PM

I'm thinking as redneck as a car radiator with a small fan pushing air across it. Small pump pushing water into the hx with a gravity return. Tie the barrels together somehow... Supply from one end of the train, return to the other
 

 

Threw this together so we would be on the same page...

CWi1uj3.jpg

 

Several questions.  Where's the best place to put the pump?  What flow rate?  The most drums I could get into the intended space would be 8.  Along with the piping, radiator, about 450 gallons.  Cycle that in 8-12 hours?  So, about .5 - 1.5 gpm?  What horsepower/wattage would the pump motor have to be rated at?

 

Tying the barrels together is a big somehow.  Not certain the bungs are threaded with anything available in PVC.  Same for any small car radiator.  Would need at least a couple of venting/priming valves.  

 

 

When the air temp is warmer than the water you'll be storing heat in the water. When lower harvesting into your space.

This might be the easiest part.  

I bet a small pump and some kind of radiator or heat exchanger would increase your efficiency a ton. Over just passive barrels

And this is the predominate question.  How *much* would it increase?  And is that increase worth the hassle and cost?


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#20 SnowMan

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Posted 06 February 2021 - 01:21 PM

Yeah that's what i was thinking. I'd pump into the radiator so you know it's as full as you can have it.

For connecting the barrels, can you elevate them slightly like steps? Put a bulkhead fitting at the top of each one and a small piece of hose and they could overflow into each other?
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