Differences between these materials for absorbing and radiating heat?

[Steven Bolgiano]

I have to design and assemble a solar water heater for a small Anaerobic Digester system.
It will have two connected sections, using pipe, ... one assembly section to collect solar heat, one assembly to radiate the heat.
Attached is an illustration. Please disregard the pipe arrangement design. This is just one of several design possibilities.

Please speak to comparative thermal efficiencies an draw backs of materials I am considering.

• What's the different comparative levels of efficiency to ABSORB heat, using either clear poly-carbonate (or acrylic) tubes, or CPVC, Galvanized, or "Black Iron pipe? (assume same diameter pipe for each).
• What's the different comparative levels of efficiency to RADIATE heat, using either copper, galvanized, or "Black Iron pipe? (assume same diameter pipe for each).

More details ... the Heat Collector arrangement is contained in an insulated box with clear top to take in sun's rays, and maintain optimum temps.
It circulates the heated water to the panel of Heat Radiating pipes, which heats a "bladder bag" full of anaerobically processing "digestate".

• Solar Collector: Does clear poly carbonate pipe absorb solar heat for its contained water significantly more efficiently than CPVC pipe?
  AND does it retain the heat acceptably as the water travels to the radiating pipe section?
• How much more efficiently does a a copper tube, "release" (transfer) heat though pipe walls to the "bladder bag" that is sitting on top and in contact with those radiating pipes... then a galvanized or "black iron" plumbing pipe?
  
  Thanks in advance!
  Steven

 

[jrmichler]

Rather than trying to write a book about collecting solar heat, I'll recommend some search terms:

Absorbing solar energy as heat: search solar absorber
Understanding the solar spectrum of heat: solar spectrum
Using the above knowledge to build a solar collector: solar collector

Heat transfer is an entire semester course in engineering school. Summarizing and simplifying, but not quite to the point of absurdity: The water transfers heat to the pipe, the heat conducts through the pipe, the heat conducts from the pipe to the bladder, the heat conducts through the bladder, the heat conducts from the bladder to the digestate, then the heat conducts through the digestate. Each and every one of those heat transfers has a resistance to heat flow. The total resistance to heat flow is such that the thermal conductivity of the pipe makes very little difference to the total heat transfer.

Because most of those thermal resistances are out of your control, the total heat transfer from water to digestate will be controlled by the area of pipe in contact with the bladder. Rather than trying to calculate total resistance to heat flow, I suggest just building something that looks like it should work. Measure the water temperature entering and leaving the bladder heat exchanger. Measure the temperature of the digestate. If the digestate temperate varies significantly at different locations, find a way to stir it. If the water temperature leaving the bladder heat exchanger is significantly hotter than the digestate temperature, add more pipes. If the water temperature leaves the bladder heat exchanger at only a little warmer than the digestate, you are done. You do no harm by making the bladder heat exchanger "too large". At worst, you would spend more than necessary for the pipes.

 

[Dullard]

I don't disagree with jrmilcher. I do think that his description skipped over an important fact (based on your apparent level of familiarity with heat xfer) :

How much solar energy absorbed by your 'collector' is dominated by the coating on the collector, rather than the base material. There are paints designed to produce maximum heat from the solar spectrum. There are pipe materials that will perform better/worse in terms of xferring that heat from the coating to the water, but that comes down to the temp difference between the coating and the water. That's an equilibrium problem.

It sounds as if the secondary xfer is not radiation - it is conduction. That's a whole different situation. Again, it will boil down (excuse the pun) to temp differences. Coatings don't matter (much). If you're looking for actionable information:

Use copper/aluminum. It will keep your temp diffs to a minimum. You may not need it, but... Paint the copper on the collector with a paint designed for solar thermal collectors. Search Florida Solar Energy Center (and about a million other places). If the cost of copper is prohibitive, there probably isn't enough margin in the job to make it worth messing with.