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re: comparing a super-house with super-windows vs. a sunspace house
9 nov 1996

hi carl,

>i appreciate your contributions to these groups.

good :-)

>while i don't necessarily agree with your general concept of a low-cost,
>unheated solar-clost it obviously has merits worth considering.

an unheated solar closet? not our concept. and our concept is very specific,
not at all general, altho specific implementations can take a wide variety of
architectural forms, with equivalent thermal performance. perhaps you are
talking about a low-thermal-mass sunspace that gets cold at night, something
like a $1/ft^2 transparent lean-to tent covering a part of the south side
of a house, or a $100/ft^2 aluminum or redwood and glass sunspace. it's the
same configuration, from an engineering point of view, but it can have many
architectural forms, with different costs, real-estate taxes, aesthetics, etc.

a solar closet is diametrically different from a sunspace, as physics prof
dr. paul bashus and i described it in our paper. it's a high-thermal-mass
energy storage element, and its internal temperature changes slowly. it stores
solar heat for about a week without sun. it might live in the sunspace, with
its own glazing. it definitely stays warm overnight, completely surrounded
by insulation. someone just told me that years ago, he added a 15' cube of
water on to his house in new jersey, for seasonal heat storage. a solar closet
is sort of like that, but smaller and easier to build, and easier to get
heat into and out of, using air, and it only provides heat during cloudy
periods, while the sunspace provides heat for the house on a statistically
average winter day, with an average amount of sun. the sunspace takes care
of the statistical mean, and the closet takes care of statistical deviations.

the national renewable energy laboratory's solar radiation data manual for
buildings lists monthly weather data collected over the last 30 years for
239 us cities, with statistical means and deviations. it says a south wall
in philadelphia receives an average of 1000 btu/day of sun in january, with
a standard deviation of about 7%.

from a less statistical point of view, a sunspace is like a stereotypical
husband who only works when the sun is shining, and moves fast, a yin person
who has no memory for tomorrow. the solar closet is his yang farmer wife, who
puts every penny of her egg money into the cookie jar, and doesn't spend a
penny on heat until a cloudy week comes along, when she leaves her husband
out in the cold (sniff) and keeps her human children warm in the house, her
highest calling. she thinks only of future rewards and delayed gratifications, 
and has few concerns for today, moving slowly, tilling the soil protected
by the sunspace.

the larger the solar closet, the lower the r-value of the house can be, and
still be 100% solar heated. david boyer's 20 x 100' greenhouse with r1 walls
(2 layers of polyethylene film) was over 80% solar heated last winter, with
a propane heating bill of $1500 vs $8000 the year before. the only thing we
did was to add lots of thermal mass, 200 55 gallon drums full of water, about
a hundred thousand pounds of water. with r10 walls, we might have the same
performance with only 10,000 pounds of water, but the greenhouse was already
there, and the drums were free, and david mainly wants to grow plants and
people, not build solar structures.

what would the r-value of the walls of a 32'x32'x16' tall sunspace house
have to be, for 100% solar heating, if the sunspace contained an 8' tall x
8' deep x 16' long solar closet, under a deck? it might have 55 gallon drums
4 deep x 8 wide x 2 high inside, 64 of them, about 32k pounds of water 
completely surrounded by r20 insulation, with an air heater over the south
wall of the closet: a single 16'x 8' layer of polycarbonate glazing with an
air gap between the glazing and the south wall closet insulation, and openings
in that wall at the top and bottom to let solar warmed air into the closet
during the day, with an insulated damper to cover the top opening at night.
what would the average water temperature be, inside that closet?

the sunspace might be 68 f on an average december day, filled with house air 
flowing into the sunspace from an opening in the house wall near the bottom.
that 68 f air might travel upwards and sideways, from south to north through
a mesh absorber, eg a piece of 50% solar-absorbing green greenhouse shadecloth
with sun shining on and through it, hanging a few inches from the house wall.
that air might become warmer, say 120 f in that thin space, and re-enter the
house through an opening at the top with a motorized damper and a couple of
thermostats to keep the house warm on a statistically average day. 

meanwhile, the sun would also be shining through the sunspace glazing and
then into the solar closet glazing, inside the sunspace, with no shadecloth
in front of that. approximately 16'x8'x1000btu/ft^2/dayx0.9x0.9 = 103k btu
of sun would enter that solar closet on a statistically average december day,
where i live, with no reflecting pool. the air heater would warm closet air
which would circulate through the solar closet, heating the drums full of
water inside. each 2' diameter x 3' tall drum has a surface area of about
25 ft^2, so 64 of them have 1600 ft^2, vs the 128 ft^2 of solar glazing,
a large 12.5:1 ratio, which means the drum temperature would be about the
same as the air temperature, when the sun is shining.

on an average december day in philadephia, the solar closet might lose heat
to a 68 f sunspace during the day and a 36 f sunspace at night. i assume an
average solar collection day is 6 hours long in december in phila. after a
long string of statistically average weather days in december, if the slowly-
changing water temp in the closet is t, ignoring the floor, we have 

103k btu = 6hr(t-68)128ft^2/r1    for the south wall, during the day
        + 18hr(t-36)128ft^2/r20   for the south wall at night
        +  6hr(t-68)128ft^2/r20   for the deck, during the day
        + 18hr(t-36)128ft^2/r20   for the deck, at night
        +  6hr(t-68)128ft^2/r20   for the end walls, during the day
        + 18hr(t-36)128ft^2/r20   for the end walls, at night
        + 24hr(t-68)128ft^2/r20   for the north (house) wall
         = 6(t-68)128 + 6(t-68)256/20 + 18(t-36)384/20 + 24(t-68)128/20
	 = (768+76.8+153.6)(t-68) + 345.6(t-36) = 998.4(t-68) + 345.6(t-36)
         = 1344t - 80333, so

t = (103k + 80333)/1344 = 136 f.

if the drums can provide heat for the 68 f house until they reach 78 f, we
can store (136-78)32k = 1.86 million btu in the closet. on a average cloudy 
day, our sunspace house with about 3000 ft^2 of walls and ceilings with an
average r-value r might lose 24hr(68f-36f)3000ft^2/r = 2.3 million/r btu.
it might need about 11.5 million/r btu to stay warm inside for 5 days in
a row with no sun. if 1.86 million = 11.5 million/r, then r = 6.2. one might
even build the whole house out of r8 windows, except for the r20 south wall,
if one were rich and crazy. houses with sunspaces and solar closets do not
have to be superinsulated and airtight to be 100% solar-heated.

>i have a few questions about the following scheme though.  why does
>one scenario have a reflecting pond and the other does not?

most houses don't have ponds to the south, and i was trying to
compare a conventional house to something different, and better. 

>it seems that the pond could have been used in both to give a more
>fair comparison.
sure. that would reduce the amount of sun that falls on the sunspace by about
30% in december (-->, below), while increasing the lawnmowing, etc, so the
sunspace house would collect less sun. instead of collecting 5.44 times as
much sun as the super-house, the sunspace house would only collect about
4 times as much, which is still a lot more, and a lot more than needed for
100% solar heating, with reasonably-insulated walls and air infiltration. 

                        super-house         sunspace house

solar gain              120k btu/day        612k btu/day --> 471k btu/day

thermal loss             43k btu/day        123k btu/day

cloudy day loss          43k btu/day         20k btu/day

net energy gain          89k btu/day        489k btu/day --> 348k btu/day
                         (26 kwh/day)       (143 kwh/day)-->  102 kwh/day

>i also don't understand why the house has so much less window area than
>the lean-to.  if you assume that the house has a wall of windows on
>the south side it would have less glazing that an equivalent sized lean-to
>wall because of the structural supports, but if you started with the
>same size building, the lean-to would have a shorter south wall because
>of the slant of the roof over the 12 foot depth of your lean-to.

i'm trying to give the super-window-house a break. direct gain houses have
an optimal ratio of heat load to south window area. too many south windows,
which are poor thermal insulators, vs a house wall, and the thermal loss of
the house during the night or a cloudy week go way up, which kills the
performance unless you live in the sunny southwest. too few windows, and you 
have a superinsulated house with no solar gain. and those windows are very
expensive. sunspace houses have no such performance limitation. they can have
as much sunspace glazing as they want, since that glazing causes no thermal 
loss for the house at night, or on a cloudy day. the sunspace also eliminates
the average daytime loss from the south wall area covered by the sunspace,
but that doesn't mean much, since at high solar fractions, what matters most
is house performance on cloudy days. 
>i have a house that i selected because the rear of the house faces within
>20 degrees of south. 

nice to be free from aesthetic tyrants :-)

>i plan on adding a garage with a large room above it primarily for

that is to say, occasional use?

>i can't really consider your lean-to
>idea because it needs to be permanent and used for more than just heating.

nothing is permanent, but x s smith says their curved galvanized pipes should 
last for at least 40 years. the plastic film needs replacing and recycling
every 5-10 years, if it is covered with shadecloth in the summer. that might
take an hour or two, on a calm day. polycarbonate plastic is clearer, like a
window, and would last longer, perhaps 30 years if covered with shadecloth
in summer, but it is also more expensive, about $1/ft^2, and attaching it
is more work, since it only comes in 4' wide rolls, vs 40' wide rolls that 
only need be attached at the edges.  

is it better to build permanent structures, eg concrete monolithic domes
with a lifetime of some 2000 years, by their estimate, or to build things
with shorter lifetimes, and allow them to adapt or disappear over time? an
old californian specifically asked an architect to design him a house that
would not last much longer than his lifetime. suppose we covered the earth
with monolithic domes over the next few years, leaving a permanent legacy 
for our children, and their children, and so on. would their lifestyles be
different from ours? would they see this as an architectural disaster? would
they establish a superfund, with traveling concrete grinders to reduce them
to dust, so they could build more appropriate houses?

>the room hasn't been designed yet, but i'm considering having the roof of the
>room angle directly up to the south side so it's at its highest point there.

sounds good.

>amusingly, this style of roof is referred to as "lean-to".

sounds like a clerestory, that doesn't lean.

>the south wall would be mostly windows and have an automatic insulating
>shade that would open in the morning and close in the evening.

good luck. those things seem very expensive to me, and if they leak any air
around the edges, the r-value becomes a lot lower than what is claimed. and
how does the claimed r-value compare to an r20 wall?

>since it will only be used for entertaining it seems that it shouldn't need
>much heating if there is enough thermal mass to keep it warm into the late

something seems backwards here. if the room use is occasional, perhaps it
should have very low thermal mass, so you can heat it up quickly for parties
at night, eg with warm air from the house, and let cool off quickly. it
sounds like a fine sunspace, one that might even heat the rest of your house,
unless you fill it with thermal mass.

why not make the south wall of the garage below transparent, and let that
"sunspace" warm air flow up into your new room when the sun is shining, and
into the house, with no nighttime penalty? how about some clear corrugated
polycarbonate "solar siding," with a dark, low-thermal-mass surface and an
air gap behind it, or an old-fashioned glass garage door, or perhaps a
translucent fiberglass version? 


nicholson l. pine                      system design and consulting
pine associates, ltd.                                (610) 489-0545 
821 collegeville road                           fax: (610) 489-7057
collegeville, pa 19426                     email:

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:

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