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Posted by Gary on October 3, 2006, 4:23 pm
Jeff wrote:

Hi Jeff,
It actually specifically says it blocks IR.
The idea is to provide lighting without as much heat gains.



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Posted by SJC on October 4, 2006, 5:05 am
Yeah, I am pretty sure...




Posted by Jeff on October 4, 2006, 6:20 am
 SJC wrote:

<URL: http://en.wikipedia.org/wiki/Barra_system  />

   Much of light's energy is in the visible spectrum which is converted
to heat. So less less reradiated loss with an IR block but a bit more
incoming. This is roughly how selective surfaces work, they reradiate
less IR, but they also absorb less IR. They are good visible absorbers
but poor IR absorbers, particularly far IR.

I'd think that without the IR screen would be preferrable for most
active collectors but where losses are more critical the IR block could
help (as in passive systems). It's not so clear cut to me... I think it
matters most how far into the IR it cuts. Near IR is good, far IR is
mostly losses. I would think the IR guard lexan is tilted more toward
near (and hence bad), but don't know.

   Note also that low iron glass passes more IR.




Posted by schooner on October 4, 2006, 10:01 am
 Moving more air through your collector and thus reducing the overall
collector temp will also help with heat loss.  The hotter the collector the
more chance of loss.  If you use a larger fan on the unit you can keep the
collector cooler and although you have a lower temp gain through the
collector you make up for it on the increased airflow.  You want to maximize
the BTU output, not just the temp gain across the panel.


Posted by daestrom on October 5, 2006, 12:15 am

The only limit to this is when it's really cold outside (say, -5F).  You
want the collector to at *least* be hot enough to deliver energy to the
house.  If the collector can't manage at least ~70F with zero air flow
because of too many heat losses (conductive, convective, IR radiant), then
you're pretty well shutdown until warmer weather.

Granted, most folks don't have to worry about *that* cold, but then again,
many of us do.  So a 'visible light in' and 'no IR out' surface may be just
the thing.


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