[Greenbuilding] Power Vent for Solar Air Heater

[conservation architect]

Thanks Laren

The info on fan for thermo-siphon air collector control and temperature is exactly what I was looking for.  



Laren's comments:

Okay....So you may not be building a Solar air heater.  What will

you be using as your absorber?  What kind of "plastic" will you be

using?  Polyethylene is transparent to IR, so only has an Rvalue of

around 0.83   TwinWall polycarbonate (also "plastic") can have

an Rvalue of R2.  With your partial sunny situation, that can

make the difference between heat and no heat.



My comments:

I do take pause per your input.  I am proposing to use 6mil poly that we purchased for a hoop house a few years ago.  This is unused off the same roll.  I called the supplier and learned it has a solar transmittance of 92%.  I had assumed all the insulating value was similar to a single layer of glass where I thought all the insulating value was the air film on each side.  Does solid glass material actually have insulating value?  In any case, it does trap and convert light to heat in the hoop house that I am sure is not very tight.  My proposed installation would be much tighter.  I have dark wood siding that should absorb light to convert to heat.  Behind siding is a 3/4" air space over continuous 3" rigid insulation.  The reason I thought it might produce a significant amount of heat is that I feel the heat at my South facing windows.  The intention is to only circulate air during the productive times which you advise to cut on fan circulating air into room at 90F and cut off at 80F.  At night it will get cold fast with no thermal mass  and warm up quickly when sun returns.  This would be outside the thermal envelope and therefore not subtract heat from the house.  This is the advantage over passive windows are thermally weak areas in thermal envelope.  The big unknown is how much do the tree branches diminish the solar gains.  



Eli 

LarenCorie LarenCorie at axilar.net 
Thu Sep 19 13:33:29 MDT 2013 

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LarenThanks for info.  This is exactly what I was looking for.  I do take pause at the discussion about the effective R value and transmittance.  I am  From: "conservation architect" <elitalking at rockbridge.net>

> Can any on the list recommend a through the wall power vent to
> achieve active flow in air heater. If not satisfied with the production,
> I will remove and reuse the materials.  I would like it to activate when
> 80F is achieved that reduces the outward heat losses of increasing
> to 90F before passive flaps can work.  Motorized damper for
> return air may also be useful.

Hi Eli;

  You do not want the fan to activate at 80F.  When the fan starts
its will lower the collector temperature. You are better to have it start
at 90F (or more) and have the fan drop the collector temperature to
80F or so. Use a cooling snap switch. It will have an on temperature
that is higher than its off temperature, so that when the fan drops the
temperature, it will not turn it off until it truly can not supply useful 
heat.
Use the fan on the lower end, so that it remains cool. If you can put
a passive backflow damper there too, fine.  But, if you have to fit it
all within the depth of a wall, you might need to put the backflow
damper in the top vent.  Depending on the collector size and the
capacity of the air mover, you might use a bathroom or kitchen
vent fan, that has a noise rating.    Noise can be an important
aesthetic and customer satisfaction factor, for simple, through
the wall type, Solar air heater.

> I am putting plastic over the parts of the South wall that are
> not windows.  Because I live on a steep hill in a wooded site,
> I am not sure about the reduction in solar available.   However,
> from my South windows, I know that I feel that warmth.  I am
> framing such that if it proves affective, I can replace plastic with
> permanent glass when plastic wears out. If it proves not adequately
> productive, I will dis-assemble materials to repurpose.

Okay....So you may not be building a Solar air heater.  What will
you be using as your absorber?  What kind of "plastic" will you be
using?  Polyethylene is transparent to IR, so only has an Rvalue of
around 0.83   TwinWall polycarbonate (also "plastic") can have
an Rvalue of R2.  With your partial sunny situation, that can
make the difference between heat and no heat.

 -Laren Corie-

From: conservation architect 
Sent: Wednesday, September 18, 2013 7:05 PM
To: greenbuilding at lists.bioenergylists.org 
Subject: Power Vent for Solar Air Heater

Can any on the list recommend a through the wall power vent to achieve active flow in air heater.    If not satisfied with the production, I will remove and reuse the materials.  I would like it to activate when 80F is achieved that reduces the outward heat losses of increasing to 90F before passive flaps can work. Motorized damper for return air may also be useful.  

I am putting plastic over the parts of the South wall that are not windows.  Because I live on a steep hill in a wooded site, I am not sure about the reduction in solar available.   However, from my South windows, I know that I feel that warmth.  I am framing such that if it proves affective, I can replace plastic with permanent glass when plastic wears out. If it proves not adequately productive, I will dis-assemble materials to repurpose.  

Thanks Eli 
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