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re: insulated trombe wall
4 may 1999
jordi llovera  wrote:

>...i will build a solar closed, in the garage wall.

hey! (a "closet," as in a place to store things, like heat :-)

>it will be made like an insulated trombe wall (40m2) with a lot of
>55 gallon drums.

no sunspace to help it work? 

>from inside to outside i build this layers: concrete (10"), insulation
>polystyrene (between 2 and 4"), air (about 4") and simple glass.

serious concrete. that will store heat too. the 4" air gap sounds a
little small (if 8' tall, height/15 = 6") for natural convection, and
you might melt the polystyrene, if this really works well or the
airflow unexpectedly stops when the sun is shining ("stagnation.")

>my question is: what i put to absorb the sun?
>- a continuous metal sheet
>- a metal sheet plenty of holes
>- a metal grille

i'd say 1-2 layers of dark wide insect screen, the kind used in house
windows, made of metal or fiberglass. steve baer likes 4-5 layers of
dark-painted metal stucco lath.

>and what kind of paint i must use?
>- usual black ?
>- selective black ?

i'd use "usual black" or maybe "high temperature black," the kind used
on hot metal like charcoal grills or car engines, but the screen might
be dark enough already... 

>if i need selective surface or selective paint, where can i find it?

i don't know. this might make a worthwhile performance improvement,
if it were not very expensive. 
 
>does the radiation heat losses be important or are small in front of the
>conduction heat losses.

the mesh absorber in the air gap will reduce the radiation loss, which the
glass will block as it heats up and loses heat to the outdoors by convection.
with enough absorber surface to keep the absorber-air temperature difference
small and block reradiation, radiation loss should be less than convection.

also, with a mesh absorber the airflow path can be arranged so cool slowly-
moving air entering the heater is between the mesh and the glass, and solar-
warmed air is on the other side of the mesh, after it flows through the mesh.
this reduces the heat loss from the glazing.

>i would know how to dimension the holes in the top and bottom...

you might make them each about 2% of the area, if using natural convection.
you could estimate the heat storage efficiency as a function of hole size.
larger holes mean a lower temperature in the air heater during the day,
which lowers heat loss and raises collection efficiency, but they also mean
more heat loss through the holes at night, which lowers storage efficiency. 

i guess you need a one-way damper somewhere to prevent reverse airflow
at night. the holes might be smaller and the dampers less delicate and
the heat loss less if using a fan or two with a high temperature rating. 

nick

nicholson l. pine                      system design and consulting
pine associates, ltd.                                (610) 489-0545 
821 collegeville road                           fax: (610) 489-7057
collegeville, pa 19426                     email: nick@ece.vill.edyou

computer simulation and modeling. high performance, low cost, solar heating
and cogeneration system design. bsee, msee. senior member, ieee. registered
us patent agent. web site: http://www.ece.vill.edu/~nick 




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