Air Collector - Black Heat Paint vs Selective Paint?

 For a solar air collector is there an advantage to using selective
paint for the collector over normal heat paint?  Does selective paint
offer that much improvement and hold up better over time?

  Mark Langille wrote:


I'm no expert, but the higher the temperature the collector operates at
the more heat that is reradiated and the more important a selective
coating would be. So this would be less important for forced air solar
than domestic hot water.

   Selective paints themselves reradiate more than a selective coating
like black chrome. In other words, consider your application and look
for a solution.

   I just wanted you to have something to mull over before a more
experienced hand answers. This appears to be a low volume newsgroup.

   Cheers,
Jeff


   Does selective paint

 

When you look at the science of selective coatings, it would *seem* like an
easy choice.  They re-radiate a lot less than they absorb, so should get
much hotter.  A simply black body absorbing sunlight will re-radiate more
and more as it heats up until it radiates as much as it absorbes.  For
simply coatings that have an emissivity equal to their absorption, this max
temperature is <200 F though.  A high tech coating could improve on this to
get higher temperatures.

But, with all that said, those numbers and such *assume* a couple things.
First, that no convective/conduction losses occur.  Second, that there is no
medium between the surface and the sunlight.  But that isn't typical
collector construction.  So putting a *glass* plate over a collector will
reduce the incoming sunlight slightly, but reduce the re-radiated losses
dramatically.  So a simple black surface *under glass* should get much
hotter simply because it can't re-radiate as much energy back out.

Air collectors don't really need to get that warm.  They can be kept
relatively cool and maximize the amount of heat gained by absorbing as much
sunlight as possible (flat-black surface) and high air flow rates.  If the
collector surface is kept cool with a high air flow rate, the re-radiation
is minimized.  Of course, you don't want the air to lose heat to the outside
through contact with cold glass either.

So I've wondered if a second cheap plastic film to make a 'double-glazing'
with only one layer of glass could work well.  Lower conduction/convection
losses due to the insulating air gap, and higher transmittance of light
through just a single pane of glass.

Sorry, got to rambling a bit there, not sure I answered your question
really.

daestrom

 So selective paint really just allows you to up the max temperature of the
collector correct?  So if that is the case you would need to increase
airflow to properly cool it.

 schooner wrote:


The emisive/reradiated loss rises as the collector temp rises. Normal
glazing (and I don't know how you would make a one way IR reflective
glazing) will simply let this escape back, no matter how many layers. It
only helps with the convective/conductive loss.

   I think the interesting question is at what temp does the emissive
loss excede the convective loss for single pane. I would think this
would be well above 100 degree F (closer to the max temp limit mentioned
above).

   Also, it looks like the selective paints must be applied very carefully.

   No real knowledge here, so take it like a "W" stay the course speech.

   Cheers,
Jeff