Thermodynamics -- Comparing Materials with large differences in Heat Conduction

[Steven Bolgiano]

Hi Folks,
I'm 70, so I often confuse myself! Ha !

My question is: Do materials both absorb and release heat equally?
So when I evaluate different materials for thermo efficiency, does a copper pipe's surface absorb solar heat at the same rate it gives off heat?

My question is specific to a solar water heated, anaerobic digester project already under construction. The digester itself is a soft tank, tightly sitting inside a frame and acrylic panels, ... heated from underneath with solar heated water.
The plan for the sake of economy, is to use 3/4" cPVC pipe in a clear sealed "cabinet", for collecting the solar rays, that connects adjacently, to another pattern of 3/4" type N copper under the tank to heat the tank.
Attached are some slightly obsolete images, I just used to grab quickly, ... but they may help to visualize


So if there is a large disparity between the Collector (cpvc) and the Radiators'(copper) conduction rates, ... am I wasting time using copper for the radiator, because of that disparity?

I was hoping using copper as the Radiator, would use the heat from the cPVC with the optimum efficiency.

As I understand the three methods of heat transfer, any comments, corrections:
Conduction - Heat transfer by molecular contact
Convection - Heat Transfer through density differences
Radiation - Heat transfer by wave motion (no material required)
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Conduction operates by contact
Convection operates by density
Radiation operates by waves
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[russ_watters]

My question is: Do materials both absorb and release heat equally?
So when I evaluate different materials for thermo efficiency, does a copper pipe's surface absorb solar heat at the same rate it gives off heat?

My question is specific to a solar water heated, anaerobic digester project already under construction. The digester itself is a soft tank, tightly sitting inside a frame and acrylic panels, ...

Welcome to PF.

No; the conditions of absorbing and releasing heat tend to be very different from each other, so the rate is typically different.

So if there is a large disparity between the Collector (cpvc) and the Radiators'(copper) conduction rates, ... am I wasting time using copper for the radiator, because of that disparity?

Maybe, but it is really difficult to determine something like that ahead of time. Ultimately the system will reach an equilibrium where the rate of absorption of heat from the sun and release of heat from the copper will equalize. So each component of the system affects the total heat transfer rate.

I assume you are using water as a heat transfer fluid? What sort of temperatures are you looking for? (both the temperature in the digester and ambient). Do you know how much heat you need?

 

[Steven Bolgiano]

Apologies for delaying responding!
I'm on the road and missed your message.
I will follow up with more info in another reply... But the surface of the bladder tank (in contact with the copper tube radiators) cannot exceed 60C, and the digestate within the tank cannot exceed 50C.
In the warmer seasons (mid-atlantic coast), no heating of the tank is needed. My project is exploring through solar heating, the feasibility of extending the months anaerobic digestion can be used in regions that experience a cold weather.
I do think you're correct that there is a equalization that will occur between the two materials, that would be near impossible to predict. But I think it's safe to say that you're dealing with a cPVC that absorbs and releases heat energy significantly slower than the copper that absorbs the heat and releases it much more efficiently.
Because with this project nearly no solar water heat is required in the summer and in the coldest part of the winter, the heat that can be created from solar energy is only enough to keep the biodigester from going dormant, but not enough to create a significant amount of methane... I need to start talking to people about thermal couples and thermal switches so that I can reroute the hot water to a storage system when I don't need it going to the digester.
Clear as mud I bet huh? Ha! Thanks so much for that answer, although it may have not been specific I think it was correct