alt.energy.renewable

Steve Ackman wrote:

>>> Think Trombe Wall.
>>>http://en.wikipedia.org/wiki/Trombe_wall
>>
>> Staggeringly inefficient...
>
> Compared to the plain igloo doghouse, or compared
>to the brick floor that's shaded from the sunlight
>and insulated from the room?

Perhaps both.

Article: 7630 of alt.energy.renewable
From: nick@vu-vlsi.ee.vill.edu (Nick Pine)
Subject: Trombe wall efficiency
Date: 18 Jun 1995 10:29:44 -0400
Organization: Villanova University

Dr. Bill (bill@brenta.eepo.com.au) writes:

>I have heard of an architect in Sydney who asked one of our leading solar
>designers which side of the house should he put the solar greenhouse...
>he thought south was "better"...

At least he asked. A lot of US architects think they know it all. They are
artists, at least, at worst, arrogant SOBs who won't learn. They get defensive
when you start to talk about performance and numbers. Who was it that said,
a few hundred years ago, "When a man is wrong and will not admit it, he
almost always becomes angry"? Women do this too, nowadays :-)

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 insulation on the outside
and some passive plastic film dampers to the inside of the house, that
opened 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. I said:

A single-glazed south-facing square foot of "Trombe wall" with insulation
on the outside and an R-value of 20 will receive 1000 Btu/day of sun 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 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 somewhat oversimplified, of course...

And do you know what the architect said? "I agree with you completely, but
if you do that, you will violate the integrity of the traditional Trombe wall,
which has a magical, wonderful way of *flywheeling*, and transporting the
heat through the wall, so it is available at the other side *precisely* when
it is needed, the next morning!" And he went on and on about this conceptual
delight, this conceit, completely ignoring numerical performance... :-)

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.

Dr. Bill continues:

>Sometimes I despair, but then I think there's only one way to go. Educate
>educate educate.

A long row to hoe.

>If anyone can see any reason why we can't apply our scientific knowledge
>of the sun and some climatology to building design and materials performance
>let me know, I'll argue with you in private.

You may be preaching to the choir. Altho I'm amazed that so many people, even
in these newgroups, are still so interested in Trombe walls, or their passive
solar equivalents, like high-thermal mass sunspaces. A lot of people are still
willing to settle for high-cost, low-performance passive solar house heating
techniques with a 30% yearly savings in backup fuel costs over a 20 year
payback period, vs. warmstores, solar closets, sunspaces and transparent
siding, which really can save close to 100% of the space and water heating
energy needed for a house, with a 1-2 year payback period of a year or two.

Nick