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Passive Solar Heater Box Design

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Posted by altzone on June 20, 2005, 8:41 am
 
Newbie alert...
I'm looking at designing and building my own passive solar space
heater, like the one shown here:
http://www.aaasolar.com/design/homemain.html

I have a few questions and was wondering if anyone has done any
research and already has typical figures:

1) How much efficiency improvement does "Low iron" glass give over
ordinary sheet glass? I suspect it won't be a HUGE amount, and probably
not worth the extra cost when I could use ordinary glass and simply
increase the surface area? The Low iron glass here in Australia looks
to be very exspensive, about 10 times the price of regular glass.

2) I have read other information that says that a larger thermal
heatsink inside the box can help improve the efficency. I was think
along the lines of attaching aluminium heatsinks with fins to the
underside of the absorber plate to increase the surface area.

3) Is the spacing between the glass and collector plate important?

4) Would it be better to series connect multiple units, i.e. air blows
from one box into anothr box and then another, increasing the heat each
time. Or would it be better to simply have one large unit with an
efficient sink-air transfer mechanism?

What other stuff do I need to take into account?

Any help, pointers or links to any reseach or data would be
appreciated.

Thanks
Dave :)


Posted by Anthony Matonak on June 20, 2005, 11:04 am
 
altzone@gmail.com wrote:

As I recall (and I could be wrong) I think it's a bit less than 10%.
It's not worth the extra cost for these things. In fact, you might
look and see if you can scrounge old patio doors from glass installers
or the like. In a pinch, you can even use those polycarbonate or
translucent fiberglass panels they sell in home improvement stores.
You lose a little bit more efficiency going with translucent panels
but since they are so much cheaper, you can just make the collectors
larger.


On this newsgroup I've heard someone strongly suggest using fiberglass
furnace filter media painted black. The trick is to get the best
heat transfer from the absorber to the air. Aluminum is fairly costly,
unless you recycle a lot of aluminum cans (which is labor intensive)
so I'd suggest finding something less expensive. Worst case, you can
just leave out the absorber and paint the back of the thing black and
it'll still work, just slightly less well.


The fellow recommending fiberglass media suggested mounting it at
an angle so that the air flows through it from front to back. Otherwise
any spacing the keeps the collector from touching the glass will help.


The higher the temperature rise within the unit, the less efficient
it'll be. It loses heat to the outside through the glass, sides, ducts
and so forth. You basically want to run them as cool as possible. That
means you run them in parallel, not series.

Some random links...
http://www.homepower.com/files/hotairhp72.pdf
http://www.motherearthnews.com/library/1979_November_December/Solar_on_a_Shoestring_Mother_s_Corrugated_Collector
http://www.motherearthnews.com/library/1977_September_October/Mother_s__Heat_Grabber_
http://www.cps.gov.on.ca/english/plans/E9000/9731/Q-9731L.pdf

Anthony

Posted by David L. Jones on June 20, 2005, 11:20 am
 Anthony Matonak wrote:

Just what I suspected, excellent!
I'll look to scrounge some plain glass, although the polycarb stuff
sounds good from a robustness point of view. I was going to experiment
with some of that stuff anyway.


Again, as I suspected, transfering the heat to the air would be the
key, which is why I thought of the finned heatsink idea to maximise the
surface area.
Yes, aluminium looks to be an expensive solution.
I'll look into the other stuff.


I got the impression this cavity should be "sealed"? I guess to reduce
the heat loss back through the glass.


Of course...

I'm figuring the bigger the heatsink and the larger the surface area
from collector to air is the key (and airflow of course) is the key to
keeping the thing cool.


http://www.motherearthnews.com/library/1979_November_December/Solar_on_a_Shoestring_Mother_s_Corrugated_Collector

http://www.motherearthnews.com/library/1977_September_October/Mother_s__Heat_Grabber_

Thank heaps for all that Anthony, most appreciated!

Regards
Dave :)


Posted by gary on June 20, 2005, 2:41 pm
 
Hi Dave,

Pilkington glass quotes 92% transmission for there low iron Optiwhite, and
only 2% less for their regular clear.   I think that most of what you will
find around
is more like 85%.  Avoid glass with high iron content-- it will have a
greenish tint from
the iron, and will transmit poorly.

I have had good luck with the corrugated Polycarbonate (eg SunTuf) that
sell around here for $ per sqft.  Some people use the Polycarbonate
twinwall (eg Macrolux etc.).  It is more flexible, and needs to be supported
properly -- see the links at bottom of post.
If you use Polycarbonate, make sure it has UV protection -- it will last a
very long time if it does.  It also has a very good temperature capability
compared to most other plastics.


You don't want a lot of "heat sink" inside the collector -- this will just
give
it more thermal mass, and it will increase the delay between the time
the sun first shines on it, and when it starts producing heat.  You want
something
that absorbs well (most anything that is black), and transfers the heat to
the air well.

I have used 3 layers of ordinary black window screen, and had
good performance.  The air flow path must be arranged to make
the air flow through the absorber for this arrangement.  This is pretty
cheap, easy to
install, you don't have to worry about painting it, and it won't melt or
smell.



I think(?) its not terribly critical -- try to avoid high velocity air near
the glass.


No, use a parallel arrangement.  You will get more efficiency if you heat
lots of air a little bit, than if you try to heat a little air a lot.  This
is because
the losses go up (and efficiency down) with hotter air.

Good airflow path is important -- see refs below.
Good transfer of heat from absorber to air.
Not too leaky.
If the air is going into a living space, watch out for things that will
smell
when heated (there are lots of these).
Use silicone for caulking and sealing (its a good high temperature
material).


Quite a few plans here:
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm
Useful ref info here:
http://www.builditsolar.com/Tools/tools.htm
These are extensive excerpts from a very good book on air collectors that
the
authors have made available:
http://www.builditsolar.com/Projects/SpaceHeating/SolAirHtSysBook/SolAirHtingBk.htm

The book is out of print, but still available (and cheap) at
bookfinders.com.

Good Luck -- Gary

--- If you build a collector, think about sending some photos,
description... to BuildItSolar.com


--
gary@BuildItSolar.com
www.BuildItSolar.com

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Posted by David L. Jones on June 20, 2005, 11:18 pm
 gary@BuildItSolar.com wrote:

http://www.builditsolar.com/Projects/SpaceHeating/SolAirHtSysBook/SolAirHtingBk.htm

Will do, was going to publish the final design anyway to let others
benefit if I come up with something good.

Thanks heaps Gary.

Regards
Dave :)


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