Black roof tiles or black metal sheet best for collector.

 Hi Folks,

In a nutshell, I have a collector which pipes (aided by fans) the hot
air from the cavity into a stone store. Currently, the collector is
actually the shed's felt roof painted black. While this works, I don't
believe it's the best option.

I have plenty of old roof tiles (cement molded ones) which I could use
to lay down on the roof and paint them black. Being cement, they would
store a lot of heat as well and gradually cool as the sun fades. Would
they be better of a 1-2mm steel sheet be better at collecting (not
transferring) heat?


TIA


Cheers,
Crispin

  crispin.proctor@gmail.com wrote:

I don't think it will make much difference as a collector.

Since you didn't mention glazing, I assume there is none. Then what you
should be concerned with is moving heat into the store than leaving it
on the roof where it can more easily lost.

http://www.fsec.ucf.edu/en/publications/html/FSEC-PF-337-98/

   Has some  data.

   What is under the felt?


   Jeff

 
Thanks for the link - will have a read.

Currently there is a single layer of corrugated pvc. I will be adding
a second shortly to aid insulation.
As for moving the heat into the store, when it is all setup and
working, as the temp hits 35c (configurable) a series of fans will
(progressively) move the heat into the store and back into the
collector (Exhaust from store is back into collector as I figure it
will still be warmer than ambient in winter). As soon as the temp
drops below 30, fans will turn off all together. There is a target
temperature that the fans will try and maintain in the collector.

Cheers,
Crispin

 



I would be careful about a 2nd layer of PVC.  It has a fairly low
temperature capability, and with two layers, the inner layer will run
hotter, and (I think) is quite likely to have a problem with the
temperature (that is -- melt or permanently deform).
Using polycarbonate glazing (SunTuf is a common one) would give you a
lot higher temperature capability.

I agree with Jeff that you don't want storage IN the collector -- it
should be low thermal mass.

An important thing with air collectors is making sure you get flow
over the full surface of the absorber without dead or low flow areas.
Usually this would call for some type of baffling to get the air to
flow over the full area.
A couple resources that might be helpful:
http://www.builditsolar.com/Experimental/AirCollectors/AirCollectors.htm
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm#Active
The "Solar Air Heating Systems" book listed at this link has a lot of
good air collector design info.

Gary

 On Tue, 17 Jun 2008 07:18:14 -0700 (PDT), gary@builditsolar.com wrote:


Doesn't triple wall polycarbonate exhibit all these features if you
could make the inside of the bottom wall matt black? The outer wall is
both a green house and insulator, the walls have low thermal inertia
and the longitudinal cells ensure air movement in the right direction.

I was thinking of creating a warm roof by adding 100mm celotex
(polyisocyanurate foam) over the rafters and cladding with triplewall
polycarbonate just like this will a simple manifold and fan to
circulate the air. My orientation isn't optimum at SE facing but I've
got to put a roof on anyway. So it may as well be a solar collector.

AJH