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efficiency of direct gain windows as solar collectors - Page 2

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Posted by bkreamer on November 12, 2006, 9:43 am

Gary wrote:

Gary, Nick addressed your question, rather than your implied concern.
If, given the goals of creating heat while saving money over a 10 year
period, you want to know whether to install a solar collector or a
window, you could ask him that.  We would both learn something from his

But I'll give you a loose comparison in anticipation of that.

A window into a room may be a slightly better solar collector than a
dedicated solar collector, but only from a limited perspective.  That
is, when comparing side by side installations, monitoring for the 5 or
6 best solar hours of a day may show the window to have gained more
energy than the solar collector.  But, in cold and windy weather, what
happens during the rest of the day and night gives the solar collector
the edge.

Both a window and a solar collector "leak" some energy while they are
collecting, but on a good solar day there is a net gain.  The essential
difference between a solar collector and a window is that while they
both leak energy during solar collecting hours, the window leaks energy
24 hours of every winter day, while the solar collector leaks energy
only during its 5-6 operating hours.  During non-collecting hours, the
solar collector loses practically no energy.

While windows have greater losses than solar collectors, these can be
mitigated, as you suggest, by insulating at night.  The way to have
your "window cake" and eat it too, is to make "window plugs".  A
solar collector is really an "auto-window with self-tending window

Incidentally, if a window already exists, the window provides a view
and you've already paid for it.  You would install a collector
through the wall next to the window, rather than replacing the window
(or install it beneath the window, or across the window sill, etc.).
That way you have two solar devices, the collector and the window.

People with "passive solar houses" quickly discover that it pays to
take an active role in preventing nighttime losses.  At night, or right
after you come home from work, you might plug your window openings with
window plugs made from 1" blue Styrofoam board, edged with 3/4" x 1"
wood strips (ripped from a 1 x 4 board).  If you intend to leave them
in for long periods, add 3M V-seal around the perimeter.

Start by fitting a rectangular frame of 3/4" x 1" wood to the inside of
your window frame, using one ring nail per corner.  Diagonally brace
the frame while it's in the window.

Remove the frame and lay it on top of the 1" Styrofoam on the floor.
Mark the foam inside the frame with a pointy marker pen, leaving a
strong 1/16" clearance all the way around.  Use a sharp utility knife
to cut out the foam.  Glue the foam into the frame with yellow glue
thickened with wood flour.  Cover one or both sides with muslin or some
other attractive cloth (use spray adhesive).

Put the window plugs behind the couch during the day, or velcro them to
the wall or ceiling.

Posted by nicksanspam on November 12, 2006, 11:06 am

Dynaglas "solar siding" comes to mind...

That would also be true of a window on a low-thermal-mass sunspace with
an insulated wall between the sunspace and the living space and a way
to control airflow between the sunspace and living space.

A 2-story house might have windows above each other that act as vents
for some solar siding between them.

Based on history, most people tire of that role.


Posted by nicksanspam on November 12, 2006, 10:53 am

I think so, like a crab trap, but I don't think this lowers the emittance.
And collector plates are usually black, so they absorb well. OTOH, rooms
have lower temperatures and very large collector flow rates :-)

Soap bubbles.


Posted by most.sly on November 12, 2006, 5:24 pm
Gary wrote:


Why buy air collectors if it's new construction?  I would think it's
more economical to have custom built sun space and air collectors with
new construction.  Let me describe a friends self built home with
custom built in air collectors here in Alamosa, (actually one of the
homes described in Zaugg's book).  It has a south facing sun space with
air collectors on the inside wall of the space.  The collectors feed
closed loop ducting going up through the attic through the low
efficiency roof collector, down through storage and garage areas on the
N side of the house,  and through the concrete floor slab surfacing
back to the collectors.  A thermostat in the attic controls fans.  The
house has something like R40 walls with R60 ceiling insulation.  After
3 cloudy days in a row a wood stove is used.  For summer the collectors
are covered and power switched off to the fans.  The collectors are
covered at night as well.  I don't know the full details of
construction but I'll ask if you like.

- cameron

Posted by SJC on November 12, 2006, 5:51 pm
   One of the projects they mention on the mylar sheet site is making roller
blinds out of reflective mylar. You could have roller blinds automatically
roll down at night to provide an air barrier and a radiant barrier.





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