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re: sizing a heating system
16 jun 2000
news  wrote:

>thanks.  had a decent look this time and all is much clearer. 

good.

>the application of a turning the solar closet into a solar attic...

we seem to be talking about two different things here.

>is a much better idea, as many houses just don't have
>the south facing walls to install a sunspace...

it's a tradeoff. most houses have "south facing walls," more or less
(45 degrees off is still fairly efficient, for vertical walls), but
they may be shaded by trees or other structures in wintertime, or 
too close to the edge of a lot for building onto. 
 
>the attic is generally wasted space as should be used to full use.

otoh, a sunspace can add inexpensive floorspace to a house and
provide a place to read a newspaper on a sunny winter afternoon
or grow things, whilst allowing more attic storage.

>norman saunders' houses have a store of water in the attic...

that's a problem with earthquakes and insecure psyches.

>but he heats the water in a rather more indirect manner
>and one that may interfere with the looks of a building.

perish the thought. will form follow function when oil runs out?
why not sooner? he suggests glazing the entire south wall, with
an air gap behind that (perhaps clear corrugated polycarbonate
"solar siding"), and a dark insulated wall with some windows in it.
warm air rises up through the gap to the attic. a layer of dark
window screen in the gap increases thermal efficiency.

walls are better than sloping surfaces for collecting low-angle
winter sun, especially with snow on the ground, and they overheat
less in the summer.

>an air heater behind a glazed roof surface heating a water store behind a
>superinsulated wall behind is just the same.

the roof would be sloped, the water would be in the attic, and the attic
would lose some useful floorspace. you'd need fanpower to move the warm
air down to the house. i'd say keep your sunspace and solar closet
on the ground...

>...a problem i see is keeping the double glazing clean on the roof.
>dirt could impair performance.

single glazing can work. one old study showed that the dirtiest and
dustiest imaginable window only passed 6% less sun than the cleenest. 

>using it to heat hot water is a little more difficult though.

with a low temperature, lots of little unpressurized containers,
and lots of plumbing air air to water to air heat transfer... 
 
>"i've been favoring concentrating solar attics lately, for some of the
>reasons you mention..."  nick, would you like to elaborate?

like i said, make the south roof transparent, and line the parabolic
north roof with aluminized mylar, which costs about 10 cents/ft^2 in
4' wide rolls. that makes a large reflective parabolic trough... at
dawn and dusk, the sun shines onto a focal line f feet from the north
wall, where y^2 = 4fx, y is the ridge height above the attic floor, and
x is the distance from the north wall to the ridge. for instance, a
12' high ridge over the middle of a 24' wide attic floor makes f = 3'.

at noon in wintertime, the focal line moves back towards the north wall,
so the collector target only needs to be 3' wide, or maybe 4', given
some diffuse reflection. it might be as simple as a 30" round poly film
greenhouse air distribution duct laid flat on the floor near the wall
with a shallow layer of water flowing through, over a dark shallow tray,
in case of leaks. concentration raises the collection temperature and
lowers the collector heat loss. 

>what do you recommend as the best method of constructng an efficient air
>heater.

polycarbonate plastic seems nice, with some dark shadecloth behind it
to increase thermal efficiency. cool air flows from the southern house
wall into the gap between the shadecloth and glazing at the bottom, then
rises up and flows back through the shadecloth horizontally from south
to north (in the northern hemisphere) as it becomes warmer and flows
back into the house through a hole near the top of the south wall. 

you could build a small box with sides and insulation behind it, but
why do that, if it's going to be screwed over an insulated house wall?
modifying a large section of a house wall seems more cost-effective.

nick




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