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a passive window solar collector
17 dec 1995

would someone like to experiment with a convective air loop window heater
with no dampers, that makes heat for a house during the day, and insulates
the window at night? this might make for an interesting paper or science fair
project...

windows are big heat leakers, compared to most walls. malcolm wells used to
leave very nice looking interior foam/stucco panels in place all winter over
the windows of part of his underground office in cherry hill, nj. my friend
tom minimizes his heating bill dramatically by pushing plain foamboard into
most of his house windows in the winter. but it would be nice to have a small
transparent opening in the foamboard to let some daylight and solar heat in
from the south windows... perhaps like this:


	      view from room                   side view


                                           |   wall       |
                                           |              |
       ---------------------------   ---   |--------------|
      |  air  |    foam    | air  |        |/    .      <== cool air in 
      |   in  |    board   |  in  |        |     .        .
      |---------------------------|   |    |     /---------
      |       |     air    |      |        |     |      warm air out ==>
      |       |     out    |      |   |    |     |
      |        ------------       |        |     |   a|   |
      |        vent area av       |   |    |     |  a |   |
      |                           |        |     |  a |   |
      |                           |   |    |     |  a |   |
      |                           |        |     |  a |   |
      |            foam           |   |    |     |  a |   |
      |            board          |        |     |  a | f |
      |                           |   |    |     |  a | o |
      |                           |        |     |  a | a |
      |                           |   h    |<-g->| a  | m |
      |                           |        |     | a  |   |      room
      |       window area a       |   |    |     | a  | b |
      |                           |        |     | a  | o |
      |                           |   |    |     | a  | a |
      |                           |        |     | a  | r |
      |                           |   |    |     | a  | d |
      |                           |        |     | a  |   |
      |                           |   |    |     |a   |   |
      |                           |        |          |   |
      |                           |   |    |\        /|   |
       ---------------------------   ---   |--------------|
                                           |              |
                                           |   wall       |
                                           |              |


steve baer suggests trying out a very simple passive system in which the sun
shines through an outer layer of glazing, then through an inner layer of
glazing onto an absorber surface. there would be an air gap of dimension g
between the outer and inner glazings, and another air gap between the inner
glazing and the absorber. as i understand it, cool house air to be heated
would somehow be persuaded to flow  down  between the cold outer glazing
and the inner glazing, then  up  through the air gap between the inner glazing
and the absorber. this tends to be more thermally efficient than a system in
which solar heated air is in contact with the cold outer glazing. there would
be some insulation behind the absorber, between the absorber and the room.
no backdraft damper would be needed, because at night the cold air would form
a stagnant pool that stays behind the insulation, since cold air sinks. 

the width of gap g determines the airflow resistance, in part. steve suggests
that g might want to be on the order of h/15. the vent area av might want to
be about 5% of the window area. the absorber might be one or more layers of
black aluminum window screen, with an absorbing area of something like 5 times
the window area, large enough so that most of the heat from the absorber is
transferred to the flowing air, vs having a small-area hot absorber that loses
a lot of heat to the glazings by radiation. one measure of success is glazing
and absorber temperatures, the lower the better. one way to measure these
temperatures is to use exergen d501 scanning thermometer (about $1000.) one
way to look at the airflow is to blow some smoke into the collector. the 
useful heat output of the collector is proportional to the product of the
air velocity and the temperature difference between the input and output air.
steve suggests that 50% solar collection efficiency is a good target.

perhaps we will see a commercial product someday, a clever reversible storm
window, in which the summer screen doubles as a winter solar absorber.

nick

some useful rules of thumb:

q (cfm) = 16.6 av sqrt(h delta t), for a chimney, in which
               av is the smaller vent area in square feet,
	       h is the chimney height in feet, and
	       delta t is the input/output air temp difference (f).

u = 0.174e-8 ac (t+460)^4  btu per hour is re-radiated by an ac ft^2 surface
               at temperature t (f).  

full sun is about 300 btu/ft^2/hour.

1 btu will heat about 55 ft^3 of air 1 degree f.

steve would doubtless be happy to hear of such experiments, at 

zomeworks
p o box 25805
albuquerque, nm 87125



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