Subject: Today's high, in the solar attic

From: [email-masked] (Nick Pine)
Date: 23 Oct 1995 17:47:04 -0400

127.4 F, under the new polycarbonate roof, with no attic airflow. 

I hope this will improve a bit when I start sealing up the air leaks,
making surfaces darker and adding some shadecloth and insulation up there.

Nick

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From: [email-masked] (Nick Pine)
Date: 24 Oct 1995 07:28:09 -0400

Andrew McKegney  <[email-masked]> wrote:
 
>If your attic has any wood , paper or non-fireproofed wood fibers in it 
>you could create a problem for yourself.

It has all that stuff. Wide pine floorboards, cedar shingles on the north
side, dacron ropes around the rafters, holding up the second floor, etc.

>If these materials are exposed to high temperatures for long periods of time,
>they develop phyric (?) compounds that can eaisly catch fire.

I wonder how high these high temperatures have to be? It seems unlikely to me
that the attic will catch fire, since it has little insulation. That would be
interesting, tho. A pyric victory, in solar air heater design. I'd worry more
if this were a concentrating system. Howard Reichmuth, P.E., says he almost
melted a pair of good rubber boots, by accidentally standing in the line
focus of his parabolic concentrating greenhouse, 20 years ago. His advice:
"Don't stand in the focus!" 

The attic will have some cooling, eventually, in the form of blowing some
of the warm air down to the rest of the stone house, to store heat in the
stone walls, surrounded by foam insulation on the outside.

>This is why the idea of building solar panels of wood went up in flames.
>Literally.

Sounds like suburban folklore to me. Altho I could imagine that happening in
a well-insulated, stagnated, liquid solar collector, if the pump failed. That
kind of collector is expensive and goes on the roof, in the wind and the snow,
with antifreeze and heat exchangers and complicated control panels. I'm not
very interested in those things. If I insulated it _really_ well, perhaps
my attic could use some 180 F passive vent openers to open the doors under the
turbine vents, if the temperature gets too high up there. Solar Components
121 Valley Street/Manchester, NH 03103-6211 $56 #13040 Solarvents are said to
start opening at 188 F and be fully open at 212 F, altho the last one I tried
20 years ago didn't work very well. Perhaps you are not supposed to boil them
long in a pot full of water on the stove. Jade Mountain (800) 442-1972 sells
$54 #FC115 Thermofor vents, and Steve Troy says they work well, lifting 15
pounds 15", but the catalog says "select a temperature between 55-85 degrees." 

BTW, I've decided to hang a $70, 32' x 16' layer of 80% greenhouse shadecloth
on top of the transparent part of the attic next summer, using a couple of
pulleys and ropes. This should make the attic a lot cooler, and make the
clear, corrugated, polycarbonate glazing last a lot longer. It won't interfere
with the view that much. I had it hanging over the whole south front wall
of the house this summer, and the worst part was the moire pattern it made
as I looked through the screened part of the windows from the inside. From
the outside, it just looked black. A new look in houses. Trendy, perhaps.

>Better consult your local fire department before you go much further.

I don't think they know much about this sort of thing. I gave them a copy
of my solar closet paper a while ago, and suggested that they modify their
firehouse accordingly. The firehouse has a very nice white stucco south-facing
wall, with no windows, but I don't see any changes yet. I suspect they don't
even know about Ohm's law for heatflow. The south wall of the Ursinus College
building next to the firehouse is completely shaded by big evergreens close 
to the wall, and it has a lovely porch upstairs that is crying out to be
glazed in, after the evergreens are cut down, but that hasn't happened either.
I guess Ursinus College doesn't know much about Ohm's law for heatflow either. 

>A point to consider. Unless you have something to store the heat in, 
>you'd be better off installing a skylight directly into the living space. 

Sounds like a good idea for daylighting, perhaps with a reflective sunscoop,
aimed south, but I'd think one would want a large amount of vertical glazing
to collect low-angle winter sun, and one needs some way to avoid having all
the house heat disappear out the glazing at night, or during periods of
cloudy weather.

>The mass in the living space will absorb some of the heat...

Indeed it will. We should build more stone houses, surrounded by polyurethane
foam. There was one described in a recent Mother Earth News, built by two
brothers with wheelbarrows--13 million pounds of rocks, including the stone
roof, but they forgot the foam on the outside. Concrete furniture also helps,
in ordinary houses. 

>Remember maximize the insulation before trying to use the insolation!

Perhaps someone should tell David Boyer about that. His 2,000 ft^2 commercial
greenhouse in Sassamansville, PA is doing fine at the moment, with a single
layer of R0.7 polyethylene greenhouse film surrounding a lot of poinsettias,
which are sitting on top of 200 55 gallon drums full of water. He doesn't seem
worried about fire, either. I think he understands Ohm's law for heatflow.

Thank you for your concern, Andy.

Nick

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From: [email-masked] (Nick Pine)
Date: 25 Oct 1995 01:29:04 -0400

Dan Settles <[email-masked]> wrote:

>Hi Nick,

Hi Dan :-)

>Glad to see you are still sharing information on your solar experiment.

I'm happy that there is more than one of them underway: David Boyer's
greenhouse, the small solar house on top of the science building at the
local college (donations still welcome :-), and the roof of my house. Now
that the small solar house is up on the roof and working, the next thing
I want to do is to bring down some hot air from my attic with a fan, and
add a lean-to sunspace on the front of my house, using some curved steel
pipes and plastic film glazing, standard commercial greenhouse materials.
 
>I was wondering if you are approaching the upper temperature limit for the
>polycarbonate.  I can't remember what that upper limit is, but I was
>wondering if you may start degrading your roof if it gets much warmer?

I don't know what the upper limit is either, but nothing's sagged or melted
yet, and the fires are under control :-) The manufacturer says that a
continuous temperature of 130 F is no problem.

Nick

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From: [email-masked] (Nick Pine)
Date: 25 Oct 1995 05:09:01 -0400

>polycarbonate...

Three more thoughts: 

My polycarbonate plastic roof is very thin, a single layer 0.020" thick, and
the plastic itself has a fairly high thermal conductivity, so it will
probably have a uniform temperature that is halfway between the attic air
temperature and the outdoor temperature, like a paper cup used to boil water
over a candle.

Even if the attic were perfectly insulated, the attic air temperature has
a natural limit, based on radiation losses. If the peak solar power into
a square foot of glazing were 300 Btu/hour, and there were no thermal storage
in the attic, and the outdoor temperature were 30 F, and all of the solar
power left through the glazing by radiation (it doesn't, in this case--
Table 4.2 of _Greenhouse Engineering_ (3rd revision, August, 1994, published
by the Northeast Regional Agricultural Engineering Service at Cornell,
152 Riley-Robb Hall/Cooperative Extension/Ithaca, NY 14853-5701) lists the
solar and infrared transmissivities of polycarbonate as 0.85 and 0.01,
_better_ than a single layer of window glass, listed at 0.85 and 0.02--
meltdowns are less likely with polyethylene film :-), which has solar and
infrared transmissivities of 0.92 and 0.81) we have a maximum attic air
temperature of roughly 122 F, from the following formula:

300 = 0.174 x 10^-8 ((Ta+460)^4 - (30+460)^4).

If the R-value of the inside air film under the plastic were 2/3, and the
plastic itself and outside air film were perfectly conductive, and the plastic
passed no infrared radiation at all, the maximum attic air temperature would
be closer to 478 F, using Ohm's law for heatflow:

300 = (Ta-30)/R0.67.

Double glazing would be more dangerous.

Nick

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From: [email-masked] (Nick Pine)
Date: 27 Oct 1995 08:18:27 -0400

Nick Pine <[email-masked]> wrote:
 
>... closer to 478 F, using Ohm's law for heatflow:
 
>300 = (Ta-30)/R0.67.

Jeez, doesn't ANYONE look at the numbers in these postings?! It's been
almost a week now, and nobody's noticed the simple and glaring math error
above. Nobody gets any points for finding this mistake. The maximum attic
air temperature above should be T = 0.67 x 300 + 30 = 230 F, not 478 F,
which is 300/0.67 + 30. One of the nice things about USENET is how people
so kindly offer corrections, when one makes a mistake, but that useful
process does not seem to work well when we use numbers. How depressing.

Our solar closet house model is now overheating, because of some lack of
forsight on my part. Yesterday when it was 70 F outside, the house was 104 F,
the 1,200 pounds of water in the solar closet was 81 F (still warming up
very slowly :-) and the sunspace was 154 F. So today I'm adding a small
cooling fan with a thermostat, inter alia, to bring in some cold winter air
during the day. This could be done with less energy, using the stack effect
and winter winds. We will be measuring and recording windspeeds shortly. 

Yesterday, the average power used in this 2' x 4' x 8' "house" was about
20 watts, counting power for the three fans and the modem and the data logger
itself, which has been taking readings every 2 minutes for the last 24 hours.
The data logger also measures its own power. The maximum solar intensity was
about 900 watts/m^2. The pyroheliometer sensitivity was insufficient to
record starlight, and we observed no significant solar radiation at night :-)

If we have some thermal mass up near the ceiling of the house, we can also do
night ventilation for summer cooling--low-energy air conditioning: keep the
house buttoned up during the day, then ventilate it at night, when the outside
air is cool, to cool the thermal mass of the house, which will very gradually
warm up over the next day. The 1994 Van Nostrand (?) book by Architect/Engineer
Professor Baruch Givoni, _Passive and Low-Energy Cooling of Buildings_,
indicates that this technique works well in many parts of the world.

Steve Baer estimates that there are very few days in Philadelphia, even in
August, in which the night temperature fails to get down to 74 F. We can test
that weather hypothesis, and find a cumulative distribution of summer night
minimum temperatures, and experiment with this simple form of cooling, now
that we have this cooling fan. Steve thinks this will require about 1 cfm/ft^2
of house, which is a lot of ventilation. We want to connect the fans to the
Data Trap soon, so we can control them in more interesting programmable ways.

Perhaps some gentle reader can do a small theoretical investigation of this,
and post the results. How much thermal mass do we need in the house, eg some
containers of water near the top of the house, and how much surface area does
the thermal mass have to have, and how long does our 6.8 watt, 55 cfm fan have
to run each night, if the daily night min is 74 F, and the daily max is 94 F,
in order to keep the house at 80 F max, given that it has 72 ft^2 of R20 walls?
What will the daily temperature swing be?

By the way, contributions to this project are still welcome. If someone sends
us a some money, we might give them the phone number and password for the
house, so they can call up the Data Trap and observe how things have been
going, using a modem with simple help menus. Perhaps it can be accessed via a
web page. If someone sent us more money, they could do their own experiments
remotely, eg write some new real-time fan control algorithms and measure
the results. We could even put in a Trombe wall, if somebody wanted to measure
its performance, or fill up the solar closet with cement blocks instead of
sealed containers of water, or put some fin tube pipe near the ceiling and
an insulated container of water on the roof for a solar closet water heating 
experiment, using a warm water convective loop.

This would be a little bit like renting a part of the space shuttle, but much
less expensive, and administratively simpler :-) It is more like what Howard
Reichmuth, PE, is now doing by modem from Hood River, Oregon. Howard also has
a Data Trap and a modem and a solar heating experiment underway somewhere in
Hawaii. He probably has a project schedule that requires a site visit in early
February. I'll likely be going to Lewiston, Maine, instead :-)

So please feel free to send money, even tiny amounts, to: 

Nicholson L. Pine                      System design and consulting
Pine Associates, Ltd.                                (610) 489-0545 
821 Collegeville Road                           Fax: (610) 489-7057
Collegeville, PA 19426                     Email: [email-masked]

We will put it to good use.

Nick