Subject: Re: Keeping the house cooler

From: [email-masked] (Nick Pine)
Date: 11 Apr 1997 11:06:49 -0400

How about a roof pond, say a flat EPDM rubber roof with standing seams every
16' and a 6" lip around the edge? Atlanta has a 24 hour average temperature
of 78.8 F in July, with an average daily max of 88 F and average daily min
of 69.5 (when dew begins to form, so evaporative cooling is nil.) A roof pond
covered with soap bubbles during the day, under 2 layers of poly film with
some shadecloth over that, perhaps, inflated and vented with outside air at
night, might have a thermal conductance of something like 2 Btu/hr-F-ft^2,
ignoring evaporation, so it might keep a 1-story 32'x32'house with a shaded
south wall and R20 insulation at temperature T, if the energy flowing into
the house over a day, 24h(78.8-T)1024ft^2/R20 equals the energy flowing out
of the pond, say 6h(T-69.5)1024ft^2x2, so T = 73 F. 

A sunspace containing a solar still (with a reflective cover and concentration
ratio of 2:1, heating a shallow trench of lithium chloride solution?) might
provide summer dehumidification, and additional winter solar heat storage in
the form of a few hundred pounds of concentrated LiCl solution, ready to
absorb water vapor and releasing some 1000 Btu/pound of water. (Or maybe part
of the roof pond should be LiCl?)

The average January temperature in Atlanta is 41 F, and an average of 1120
Btu of sun per day falls on a square foot of south wall, while an average of
820 Btu falls on a square foot of horizontal surface, eg a roof pond. If the
pond is covered with tiny cold soap bubbles at night (almost as good as
fiberglass insulation) it might collect about 32x32x820x0.8 = 672K Btu/day
of sun and lose about 6h(T-41)1Kft^2/R2 during the day and 18(T-41)1Kft^2/R20 
at night, making its average temperature T = 41 + 672K/3.9K = 213 F :-)
(Radiation loss would make it closer to 130 F, as would using some of this
heat to warm the house, altho that might better be done via a plastic sunspace
along the south wall, on an average day, with an average amount of sun.)

A pond like this would ensure the house roof never saw less than 32 F in
winter, since it would take about 4400 Btu to freeze a square foot of it
solid, at a rate of (32-(-10))1ft^2/R20 = 2 Btu/hr when it's -10 F outside,
ie 2,200 hours or 13 weeks with no sun at -10 F. It could also melt a lot of
snow on the roof, especially with wellwater keeping it warmer than 50 F. It
might make a good sprinkler system or gravity-feed rainwater source or large 
winter water heater, storing up to 4,000 gallons of water, and gathering 80
gallons a day where I live, with an average monthly rainfall of 4". 

Nick

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From: [email-masked]
Date: Sat, 12 Apr 1997 16:02:04 -0700

Nick Pine wrote:
> How about a roof pond, 

I was part of a group that retrofitted some surplus university buildings 
this way. I didn't like the design. It used a waterbed bag resting on a sheet 
of tin or aluminum over the roof joists, a strong frame, and hydraulically 
raised lid/reflector. The hydraulics were to be computer controlled but it 
never got that far.

My suggestion was to replace the tin or aluminum with plexiglas or 
something similar and to color the water. This would have all but 
eliminated daylighting by electricity while still collecting some heat. I also 
suggested that instead of big heavy insulated lid/reflected, that the bags be 
bunked--that is bag on top, supportive frame in middle, and bag on bottom. 
This way by pumping the water between bags you would have a great deal of 
control over the temperature.

BTW, the system was designed for summer cooling. Keep the lid shut 
during hot summer days, open at night and take advantage of night sky 
radiation. The bag absorbs heat from the building during the day.

Howard

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From: [email-masked] (Nick Pine)
Date: 14 Apr 1997 02:24:54 -0400

>raised lid/reflector.
 
I wonder where this project was, and where the insulation was.

Perhaps this system was only for cooling.

>My suggestion was to replace the tin or aluminum with plexiglas or something
>similar and to color the water. This would have all but eliminated
>daylighting by electricity while still collecting some heat.

There could be lots of light even while absorbing most of the sun, since
direct sun is about 10K footcandles, 200 times stronger than a well-lit
(overlit?) 50 FC office. Would colored light drive most people bananas?

>I also suggested that instead of big heavy insulated lid/reflected, that
>the bags be bunked--that is bag on top, supportive frame in middle, and bag
>on bottom. This way by pumping the water between bags you would have a great
>deal of control over the temperature.

I wonder about making the ceiling some sort of dark mesh, eg black aluminum
window screen or 80% black greenhouse shadecloth, under some large (8x8'?)
greenhouse polyethylene film pillows filled with tiny cold static soap bubbles
on a winter night (these bubbles can last for hours, and are almost as good as
fiberglass insulation) and larger, moving bubbles on a day with some sun.

Filling horizontal pillows with bubbles might be easier than trying to make
bubbles uniformly fill poly pillows in curved plastic film greenhouse walls,
without leaks, which was tried with some success in Arizona and New Hampshire
in the 70s. These days, we can probably make better bubble sensors to keep the
pillows filled, and somehow recycle the air that comes out at the top, to
prevent the bubble solution from becoming dusty.

Ceiling pillows 2' thick in the middle might contain 64 ft^3 of tiny bubbles
weighing about 20 pounds, and act as solar heat collectors during the day,
with a large thermal storage tank on/in the ground. The pillows might be under
a steep, fixed, monopitch roof sloping up to a translucent south wall, with
white paint or foil under the roof, and day/night modes reversed in summer.

Nick