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Heat conductivity of polyethylene...

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#21 toonces

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Posted 06 February 2021 - 02:27 PM

Not very much.  The one I'm looking at is the same width, but about 4 feet longer.  Those bottom panels are about 33"tall, the bottom frame is another 5".The drums are 34 5/8".  Then the sides start to curve in.
 
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Edited by toonces, 06 February 2021 - 02:34 PM.

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#22 miccullen

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Posted 06 February 2021 - 02:33 PM

you might be taking up a lot of valuable real estate in such a small space, unless you might use them literally to hold up benches


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#23 toonces

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Posted 06 February 2021 - 02:37 PM

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#24 miccullen

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Posted 06 February 2021 - 03:26 PM

to me the ground  level would be used too, not just the shelves


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#25 toonces

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Posted 06 February 2021 - 04:23 PM

to me the ground  level would be used too, not just the shelves

Definitely.


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#26 davelew

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Posted 06 February 2021 - 08:40 PM

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?

.

The equation for conductive heat transfer is heat transfer power = (thermal conductivity) *(temperature difference) * (area) / (thickness). The area of the polymer is big enough relative to the thickness that I don’t expect much temperature rise over the barrel material.

The choke point is probably the barrel-to-air thermal interface. Heat transfer there is heat transfer power = (thermal conductivity) *(convection coefficient) * (area) * (temperature difference)

Convection coefficient will range from 2.5 to 25 W/(m^2-K). Note the absence of the thickness, which means air-to-barrel is a few hundred times worse than the through the barrel heat transfer.
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#27 toonces

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Posted 06 February 2021 - 10:42 PM

you might be taking up a lot of valuable real estate in such a small space, unless you might use them literally to hold up benches

Well, that was part of the plan.  Until I started looking at a radiator heat transfer.  Even then, I would still use whatever space on the tops of the drums is available.  Polyethylene tops would be easier to modify for plumbing said system for more efficient use of drum top real estate.


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#28 toonces

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Posted 07 February 2021 - 12:06 AM

.

The equation for conductive heat transfer is heat transfer power = (thermal conductivity) *(temperature difference) * (area) / (thickness). The area of the polymer is big enough relative to the thickness that I don’t expect much temperature rise over the barrel material.

The choke point is probably the barrel-to-air thermal interface. Heat transfer there is heat transfer power = (thermal conductivity) *(convection coefficient) * (area) * (temperature difference)

Convection coefficient will range from 2.5 to 25 W/(m^2-K). Note the absence of the thickness, which means air-to-barrel is a few hundred times worse than the through the barrel heat transfer.

If the two (poly vs cold rolled steel) are compared...  In the first equation, the thermal conductivity for CRS is 100 times that of poly.  The temp difference would initially be the same, the area would roughly be the same, but the thickness of CRS is half that of the poly.  So, the heat transfer of CRS will be 200 times greater than that of poly.

 

Like I said previously, the air temp heating the barrel is going to be smaller than the direct illumination of the sun.

 

Now, getting the stored heat out is issue.  The thermal conductivity of CRS is about 50.  And the surface area of one steel drum is about 19400 cm2.  And there are going to be 8 of them.  Thermal conductivity of aluminum is about 225.  Copper is about 380.  So the surface are of a Al radiator would have to be 34600 cm2 to get all of the heat out in one pass.  20490 for Cu.

 

Still having trouble figuring this out.  Some sleep might help...


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#29 Patrick C.

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Posted 07 February 2021 - 12:20 AM

This is something to be engineered, not scienced.  If you're going to run power for a pump, you may as well put in a small heater.

 

 

 

6 Ways To Keep Your Greenhouse Warm In Winter

https://www.growell....-warm-in-winter


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#30 Patrick C.

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Posted 07 February 2021 - 12:23 AM

This one mentions water, but just having it in there is enough to get the benefit.   

https://www.totallan...m-in-the-winter


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#31 davelew

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Posted 07 February 2021 - 05:45 AM

If the two (poly vs cold rolled steel) are compared... In the first equation, the thermal conductivity for CRS is 100 times that of poly. The temp difference would initially be the same, the area would roughly be the same, but the thickness of CRS is half that of the poly. So, the heat transfer of CRS will be 200 times greater than that of poly.

Like I said previously, the air temp heating the barrel is going to be smaller than the direct illumination of the sun.

Now, getting the stored heat out is issue. The thermal conductivity of CRS is about 50. And the surface area of one steel drum is about 19400 cm2. And there are going to be 8 of them. Thermal conductivity of aluminum is about 225. Copper is about 380. So the surface are of a Al radiator would have to be 34600 cm2 to get all of the heat out in one pass. 20490 for Cu.

Still having trouble figuring this out. Some sleep might help...

Imagine two resistors in series with a voltage source. The first resistor is .001 ohms. The second resistor is 1 ohm. Improving the first resistor will have an effect on the overall circuit whose significance will approach the natural logarithm of unity.

The first resistor is thermal resistance from the inside of the barrel to the outside. The second and larger resistance is thermal resistance from the outside of the barrel to the air.

Edited by davelew, 07 February 2021 - 05:46 AM.

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#32 dondewey

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Posted 07 February 2021 - 05:56 AM

.

The equation for conductive heat transfer is heat transfer power = (thermal conductivity) *(temperature difference) * (area) / (thickness). The area of the polymer is big enough relative to the thickness that I don’t expect much temperature rise over the barrel material.

The choke point is probably the barrel-to-air thermal interface. Heat transfer there is heat transfer power = (thermal conductivity) *(convection coefficient) * (area) * (temperature difference)

Convection coefficient will range from 2.5 to 25 W/(m^2-K). Note the absence of the thickness, which means air-to-barrel is a few hundred times worse than the through the barrel heat transfer.


This is what I was trying to say.
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#33 davelew

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Posted 07 February 2021 - 08:24 AM

Here's an example:

 

Assume water in the barrels in 20 C, and the air in the greenhouse is 10 C.  Assume the barrels are 54 cm diameter and 87 cm tall, and water is only contacting the curved surface of the barrel (top is in air, bottom is on the ground with low heat transfer).  Assume barrel thickness is 3mm.  Total power flow is 143 watts of heating from inside the barrel at 20 C, to outside the barrel at 19.434 C, to the air at 10 C.

 

Power from inside the barrel to outside:

143 watts = (0.566 C temp difference) * (0.5 W/m/K thermal conductivity) * (1.515 square meters surface area of the round part of the drum ) /(.003 meters thick)

 

 

Power from outside the barrel to the air

143 watts = (9.434 temp difference) * (10 w/m^2/K convection coefficient) * (1.515 square meters surface area of the round part of the drum )

 

 

(power from inside-to-outside has to equal power from outside-to-environment at steady state, otherwise energy will accumulate on the outside of the barrels)

 

So, the temperature rise due to thermal conductivity of the polyethylene is 5.66% of the overall temperature rise.  You could go to steel or aluminum and turn that 6% into something smaller, but there's not a lot of meat on the bone for improvement there.  You'd be better off spending that money getting a small battery operated fan to blow on the barrels to get heat from the barrels into the environment more quickly, because forced airflow will increase that 10 W/m^2/K number for the convective heat transfer coefficient.


Edited by davelew, 07 February 2021 - 08:28 AM.

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#34 toonces

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Posted 07 February 2021 - 10:22 AM

This is something to be engineered, not scienced.  If you're going to run power for a pump, you may as well put in a small heater.

 

 

 

Amperage needed for a small pump is much, much less than needed for a heater.  


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#35 Dave McG

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Posted 07 February 2021 - 11:31 AM

Amperage needed for a small pump is much, much less than needed for a heater.

How about a small 12V bilge pump hooked up to a battery with a solar charger? You are gong for circulation, not any real head pressure.
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#36 toonces

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Posted 07 February 2021 - 12:02 PM

How about a small 12V bilge pump hooked up to a battery with a solar charger? You are gong for circulation, not any real head pressure.

It's an idea, especially since the current contender for purchase comes with a solar powered circulation fan.


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#37 Patrick C.

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Posted 07 February 2021 - 12:27 PM

An air circulating fan on its own would probably get you most of the benefit, and also help get rid of the dank smells.  Unless that's what you are planning to grow in there :)


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#38 davelew

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Posted 07 February 2021 - 01:50 PM

An air circulating fan on its own would probably get you most of the benefit, and also help get rid of the dank smells. Unless that's what you are planning to grow in there :)


This.
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#39 AspenLeif

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Posted 07 February 2021 - 01:58 PM

Why wouldn't you do a similar setup to one of those thermal heat sink heaters.  It would require 2 pumps though.  Bury an insulated tank (or it could be above ground).  During the day, one pump pumps water from the tank through a series of black PE pipe loops on the roof, heating the water in the tank all day.  Then at night, that pump shuts off and the 2nd pump turns on, putting that water through the heat transfer coil in the greenhouse.  


Or, I guess you could do it with a single pump and a series of valves.  


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#40 SnowMan

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Posted 07 February 2021 - 02:08 PM

Maybe at the end of the day... We're all over engineering this... And by the time he builds our system he can just buy a couple years of b energy needed to heat it. Buncha drunks
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