re: solar heating the s.a.v.e. house
18 jan 1997
larry coutlee wrote:
>nick, not trying to be flippant but this post is full of "might" and "if
>they" and "maybe".
this system is more iffy than most, so i tried to describe it appropriately.
the "if they"s and "maybe"s acknowledge the fact that the students make the
decisions here, and there is more than one choice. the "mights" indicate that
it will probably take some experimentation to make this work.
the big-picture iffy part is the save house itself, which is almost anti-solar
by design :-) it's pretty weird to build a house like that and heat it with a
structure almost the same size next door, but the students seem to want to do
that, god bless 'em. it's not often that solar structures can be net producers
of heat. this one would be much smaller or even unnecessary had the save house
been superinsulated with fewer windows, 1/3 the heat loss, oriented east-west,
with a low-thermal mass sunspace and some sort of internal thermal storage.
i'm concerned about a couple of things here: firstly, it may be difficult to
somehow get air to flow down the south face of the shadecloth, through it
horizontally, south to north, and back up the north side to the 2 fan-coil
units, and secondly, a lot of heat may radiate from the shadecloth back out
the north and south faces of the sandwich, if it is made of polyethylene film,
which has little "greenhouse effect," ie it transmits longwave ir well, vs
glass or polycarbonate. the shadecloth layers shading each other should reduce
longwave ir reradiation, but if it turns out to be a problem we can add a few
more layers of shadecloth or change some of the glazing to polycarbonate or
add some insulation to the back side of the sandwich, eg some foamboard
near the floor.
as to the first problem, it seems to me now that this sandwich might work
better with some fin-tube pipe near the top, on the south side, and a number
of fans below the pipe blowing air down the north side. fin-tube pipe has a
thermal conductance of about 6 btu/hr-f-ft in still air, so to get the same
thermal conductance as 2 fan-coil units, ie 1600 btu/hr-f, we need about
267 feet of fin-tube pipe, about 8 32' pipes above the sandwich, which is
a lot. if the pipes are near the upper south edge of the sandwich, they should
induce a downwards convection current, so the air will be slowly moving, not
perfectly still. the conductance of the pipes should double (2+v/2) in 4 mph
air, so using 4 pipes in moving air seems simpler and more cost-effective
than using 8 pipes in still air.
a 32' fin-tube pipe has an area of about 8 ft^2. if they are painted a dark
color and the sides are exposed to the sun, they will collect 32 ft^2 of sun
directly, reducing the needed airflow conductance by 32/384 ft^2 to 1466
btu/hr-f. if the cross-sectional area containing the pipe is 5" wide x 32'
long, ie about 13 ft^2, and each fan moves 560 cfm, making air flow at 4 mph,
ie 350 ft/min requires 350x13/560 = 8 fans, eg 1 grainger 4c688 560 cfm 10"
diameter 110 volt 36 watt $60 fan every 4', or some pv-powered attic fans.
sizing the fans for a solar intensity of about 50% over a 6-hour january day
means the system will collect sun less efficiently in full sun at noon on a
clear day. a few sheets of dark-painted 1/2" drywall with a thermal mass of
0.5 btu/f-ft^2 and an rc time constant of about 1 hour would help smooth out
sunny peaks, at the expense of delaying solar collection at dawn and storing
some solar heat that would be lost after the sun goes down.
>do you have any projects that you have worked on that are real life things
>and not just "maybe" projects that have been proven to work?
sure. my house near phila, built in 1820, which used about 25 gallons of oil
last winter, david boyer's sassamansville greenhouse, which used $8,000 worth
of propane for heat two winters ago vs. $1,500 last winter, after we put 200
free 55 gallon drums full of water under the benches. and last winter's solar
closet structure on top of the science building at ursinus college, which had
a sunspace temp of 157 f at one point, according to our data logger, etc, and
this year's solar closet structure, 60 times bigger, which was 72 f inside a
couple of days ago when the sunspace was 102 f and the outdoor temp was 30 f.
low-thermal-mass thermally-isolated sunspaces were popularized as "solaria"
by the brace research institute in canada in the 70s and 80s. they can add
floorspace to a house, as well as heating it efficiently. harvard physicist
and solar researcher bill shurcliff writes:
it is hard to think of any other system that supplies so much heat
(to an existing house) at such low cost...
one could shorten the warm-up time of the enclosure and increase the amount
of heat delivered to the rooms by making the enclosure virtually massless--
by greatly reducing its dynamic thermal capacity. this can be done by
spreading a 2-inch-thick layer of lightweight insulation on the floor and
north wall of the enclosure and then installing a thin black sheet over
the insulation. then, practically no heat is delivered to the massive
components of floor or wall; practically all of the heat is promptly
transferred to the air. and since the thermal capacity of the 100 or
200 lb. of air in the room is equal to that of one fourth as great a mass
of water (about 25 to 50 lb. of water), the air will heat up very rapidly.
i estimate that its temperature will rise about 40 f. degrees in about two
minutes, after the sun comes out from behind a heavy cloud cover. at the
end of the day, little heat will be "left on base" in the collector floor
or north wall and, accordingly, the enclosure will cool off very rapidly.
new inventions in low cost solar heating--
100 daring schemes tried and untried
by william a. shurcliff, phd, physics
brick house publishing, 1979
solar closets look ok-in-principle to quite a few real experts in the field,
as well as the technical reviewers who accepted our paper describing the
concept and last winter's experiment for the world renewable energy congress
held in denver last june.
>i'm interested in following solar on a realistic scale.
good. there's little magic here for people who understand heatflow basics,
eg newton's law of cooling, but there don't seem to be many people like
that who are interested in solar house heating... it's a fertile field
for people with that understanding and open minds.
nicholson l. pine system design and consulting
pine associates, ltd. (610) 489-0545
821 collegeville road fax: (610) 489-7057
collegeville, pa 19426 email: firstname.lastname@example.org
computer simulation and modeling. high performance, low cost, solar heating and
cogeneration system design. bsee, msee. senior member, ieee. registered us
patent agent. solar closet paper: http://leia.ursinus.edu/~physics/solar.html
web site: http://www.ece.vill.edu/~nick