Subject: Re: A zero-fossil-fuel studio?

From: Mike Youso <[email-masked]>
Date: 12 Mar 1996 00:54:26 GMT

A major concern with any thermal storage system is the moisture from 
condensation on the surface of the thermal storage modules if a remote 
storage bank is used.  If the storage bank gets much above 60% rh some 
rather nasty organisims will be growing in the air supply system with 
possibly serious considerations for IAQ.  The most succesful thermal 
storage uses direct-gain solar mass walls rather than ducting warm moist 
air to a storage bank.  Water modules work well in the lower 48, and 
gypsum board is better than masonry.  Two or three layers of drywall is a 
good compromise since only the first 1.5" to 2" is diurnally active.

a zero fossil fuel studio is quite feasible almost anywhere south of the 
arctic circle.
*

---

From: [email-masked] (Nick Pine)
Date: 12 Mar 1996 07:26:35 -0500

Mike Youso  <[email-masked]> wrote:

>storage bank is used.

I agree this is a good thing to avoid. I don't see a problem with moisture
condensing on these containers of water while the containers are heating
the house on a cloudy day, do you? Would moisture condense on the containers
of water as they are being heated by sun-warmed air? Perhaps, if the air is
moist. But the air should be fairly dry in the winter, if the house is
reasonably-well ventilated and the sun heats it up. Do you think condensation
will occur in this system, or are you just saying it's a concern? If so,
I wonder under what conditions might it occur, and how might we avoid it?

>If the storage bank gets much above 60% rh some rather nasty organisms
>will be growing in the air supply system with possibly serious considerations
>for IAQ. 

I wonder how the humidity would increase to 60%? It seems that, say, 32 F air
at 100% relative humidity will have a much lower RH at 68 F, and on an average
day in December, this solar closet should be about 130 F inside.

>The most succesful thermal storage uses direct-gain solar mass walls
>rather than ducting warm moist air to a storage bank.

Again, where does the moist air come from? During charging, the air in a
solar closet is completely self-contained. There is no mixing with house air...

And when you mention "successful" direct gain solar mass walls do you mean
Trombe walls? 

A few years ago, I spent some time explaining to a local architect, a more
technical person than most, who had taken a few engineering courses on the
way to architecting, that a "Trombe wall" with some dark-colored insulation
on the outside and an air gap between the insulation and glazing and some
to the inside of the house that opened up during the day was a lot more
efficient at collecting and keeping solar heat in the house than a plain old
"traditional" Trombe wall, with masonry right behind the glass, with no
insulation. Here's what I said:

  A modified "Trombe wall" with insulation on the outside, and 1 ft^2 of south-
  facing single-glazed area and an R-value of 20, will receive about 1000
  Btu/day of heat on an average 32F December day, where I live. If the room
  behind it has a constant temp of 70F, and the sun shines 6 hours a day,
  on the average, the energy that leaks out of the glass will be about 6 hours
  x (70F-32F) x 1 ft^2/R1 = 228 Btu during the day, and 18 hours x (70-32) x
  1 ft^2/R20 = 34 Btu at night, a net gain of 1000 -228 -34 = 738 Btu/day.
  Simple, no? (~750 Btu, net, with double glazing, which passes less sun.)

  A standard unvented Trombe wall (Table IV-14b of Mazria's book says vented
  ones don't work much better) with a very large uninsulated thermal mass
  right behind the glass and an R-value of, say 2 (roughly 1' of masonry),
  would have an average temperature at the outside wall surface of about
  32F + R1 x (70F-32F)/(R2+R1) = 45F, if there were no sun. If you add a
  heatflow of 1000 Btu/day of sun to that model, falling on the outside
  of the wall, the outside wall surface will have an average temperature
  of about 45F + 1000/24 x (R=2/3) = 72.4F, which contributes 24 hours x
  (72.4F- 70F) x 1 ft^2/R2 = 29 Btu/day to the room behind the wall.

  So the "improved Trombe wall" above, (actually an air heater with the
  thermal storage inside the house) is _more than 25 times as efficient_
  (738/29) at collecting and keeping heat in the room behind it, than
  the usual Trombe wall. This is a bit oversimplified...

Trombe walls are also thermal disasters during long strings of cloudy days.
When the sun goes in for a week or two, they lose their stored heat in less
than a day, and then leak house heat badly, dramatically raising backup
heat or other solar thermal storage requirements.

>Two or three layers of drywall is a good compromise since only the first
>1.5" to 2" is diurnally active.

Seems to me that a few "accent drums" full of water sprinkled around
the house might be cheaper...
 
>a zero fossil fuel studio is quite feasible almost anywhere south of the 

We seem to agree on that :-)

Nick