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efficiency of direct gain windows as solar collectors

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Posted by Gary on November 11, 2006, 2:45 am

Has anyone seen published measurements for the efficiency of south facing
windows  as solar collectors?  Efficiency curves are easy to find for regular
solar collectors, but I have not been able to find efficiency data for direct
gain windows as collectors.

I am inclined to think that if you compare an actual solar collector to a
window, both having the same glazing, same area, and the same orientation, that
the window will have a higher collection efficiency?  But, I have been unable to
find any data to support this.

(I realize that the window (if not insulated) will lose more heat at night)

To put the question another way, if I were to replace this direct gain system
with actual solar collectors mounted in the same place, would I gain more heat
or less heat?




"Build It Yourself" Solar Projects

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Posted by Jeff on November 11, 2006, 4:40 am
Gary wrote:

The Passive Solar Energy Book says simply that *all* the incoming light
is converted to heat.

   It's all about calculating losses:

Short-Cut heat Loss Estimating

Single glazing

1 exposed wall  2+ exposed wall   thermal wall store  adjacent greenhouse

8.1             13.0             8.0                  7.4

Double Glazing or insulating Shutters

6.4             9.7              6.3                  5.7

BTU/day-sq ft floor degree F

   You can, sort of, think of your collector as being a "Space Adjacent
to an Attached Greehouse", ie, the last column... Roughly a third less
losses. These estimates are for a "well-insulated space".

   Hope that is of some help...

   Primed the collector boxes today with shellac, hope to paint them
early tomorrow, before the weather turns in the afternoon!



Posted by nicksanspam on November 11, 2006, 8:09 am

Just subtract the loss from the gain? On an average 22.4 F January day with
1020 Btu/ft^2 of sun on a south wall in Billings, MT, an American Craftsman
6068-2 6'x80" U0.48 sliding glass door ($69 each at Home Depot) with 63%
solar transmission would gain 0.63x1020 = 643 Btu/ft^2. If it's 70 F indoors,
it would lose 24h(70-22.4)1ft^2xU0.48 = 548, for a net gain of 94.

But maybe you want an "efficiency graph" for a window in the same form as
a collector efficiency curve, with efficiency on the vertical axis and dT/I
on the horizontal axis. The door above would have E = 0.63 with dT = 0. In
254 Btu/h-ft^2 "full sun" with dT/I = (70-22.4)/254 = 0.1874, the gain is
160 Btu/h-ft^2 and the loss is 22.8, so E = (160-22.8)/254 = 0.54, ie 54%.
But collectors often have real curves vs straight lines, with small squared
terms that window manufacturers don't publish...

Sounds likely, since a room temp is likely to be lower than a collector temp
and a room with a large indoor surface and small glazed surface may have
a higher absorptivity than a shallow collector, based on geometry. Duffie
and Beckman's formula 5.11.1 says room absorptance Aroom = a/(a+(1-a)Aw/Ai),
where a is the indoor surface absorptance and Aw and Ai are the window and
indoor room areas. If Aw << Ai, Aroom approaches 1, regardless of a. Examples
5.10.1 and 5.11.2 on pages 225 and 231 of the 3rd edition of SETP compare
the absorptance of a solar collector and a room under similar conditions,
and the room wins.


Posted by Gary on November 12, 2006, 2:54 am
 Thanks Nick, Jeff, Cameron

nicksanspam@ece.villanova.edu wrote:

Right -- I was interested in being able to directly compare the performance of a
commercial solar collector to a direct gain system on the same basis.  I was
hoping that someone might have done some testing -- but, the analysis seems

The door above would have E = 0.63 with dT = 0. In

Those examples are really interesting.  Its nice that rooms absorb really well
even though they may not be made from materials having a high absorption.  I
guess this is the result of most reflected energy eventually being absorbed by
another room surface (maybe after several reflections)?

It all makes me wonder if a cost effective solar collection strategy would be to
have simple windows (maybe even single pane) with high transmittance (maybe
90%), and include a good window insulation system for times when the sun is not
shining (maybe R10 could be achieved).  The simple windows with insulation
scheme might cost less than a fancy R5 window, and would collect with much
greater efficiency, and may also lose less heat over a full day than the fancy
R5 window?

It seems like the pro list for such a system compared to external commercial
solar collectors:
- Less expensive (commercial collectors at $0 per sqft + fans, pumps, ducts,
and controllers  vs  a simple window at $(?) per sqft for a basic window + $?
per sqft for a good insulation system for windows)

- Less complex - no ducts, fans, pumps, controllers, -- no maintenance, no

- A very minor change over the basic house shell that has to be there anyway.

- Greater collection efficiency, and no distribution losses.

- Daylight -- pleasant to live in and saves lighting electricity.

A lot of these are the same advantages that apply to a regular passive system,
but I think that the using a combination of simple windows with high
transmittance plus a good window insulation system make the advantages stronger.

I suppose the con list might include the fact that someone has to invent the
good window insulation system :)

Cameron -- I see your point, but if you think of it in the starting from a blank
slate (building a new house), and the owner not building the collector but
buying it, the situation (it seems to me) tends to tilt toward the passive
approach.  I think it depends a lot on where you are starting from.


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Posted by bkreamer on November 12, 2006, 9:39 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.

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