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solar closet questions and answers
4 jan 1996
>1/ if you have the thermal mass under a house isn't it going to be bloody
>hot in summer?

perhaps, but a) why put the thermal mass under a house?
         and b) ve haf ways to keep the sunspace from overheating in summer.

why not put the thermal mass, eg a solar closet, inside the house, sitting on
the ground, behind the sunspace... of course the house would have some thermal
mass of its own too. in the summer the sunspace and solar closet might be
quite cool, if they were aired out in the evenings, or shielded from the sun
by some greenhouse shadecloth hanging outside of the glazing, etc. or we might
want to keep the solar closet hot to make hot water for the house or to let it
serve as a thermal chimney at night, in order to ventilate the house when the
outside air is cooler, and store "coolth" in the thermal mass of the house
itself at night, then button up the house during the day, with no daytime
ventilation to the outside. keeping the sunspace hot in the summer might mean
tilted vs vertical glazing, or a horizontal shiny reflector in front of it.

>with any reasonable level of insulation it is still going to leak heat a bit.

sure, but the point is to design an overall system that will maintain a 
comfortable temperature. vents, grapes, runner beans, clematis, and overhangs
can all help keep the sunspace cool in the summer too. the heat that leaks
into the house during the day can usually be vented out of the house at night.

for example, suppose the house is 2,000 ft^2, two stories, ie about 30' x  30'
x 16' tall, with average r30 walls and ceiling, and suppose it has gypsum
drywalls, 1/2" thick, as the main thermal mass of the house, which have a
thermal mass equivalent to about a half pound of water per square foot of
wall, and suppose it is 90 f outside during the day and 70 f at night.

if the house has the same sort of ventilation night and day, and no internal
heat gain, it will average out the outdoor temperature swings, to have an
average internal temperature of 80 f. what can we do to keep the house at
80 f max, 24 hours a day? this house has an exterior surface area of about
1,000 ft^2 for the ceiling and 500 ft^2 for each wall, a total of 3,000 ft^2,
so a/r is 3,000/30 = 100, ie if the outside temp is 1 degree f warmer than
the inside temp for an hour, the house will gain 100 btu of heat.

in this example, the heat that leaks into the house during the day through
the walls and ceiling will be about 12(90-80)100 = 12,000 btu, which could be
removed with 2 hours of air conditioning with a 6,000 btu/hour 1 kw window
air conditioner, or removed by night ventilation... how much, using seat of
the pants calculations?

1 btu will heat 55 ft^3 of air 1 degree f, which means that 1 cfm of airflow
with an air temp difference of 1 degree f will remove about 1 btu/hour. we
need to remove about 12,000 btu in 12 hours, or 1,000 btu/hour with a 10 f air
temp difference, which means the fan (or solar closet induced ventilation)
has to move an average of 100 cfm at night. a very small fan. here's another
rule of thumb for convection airflow induced by heat: if you have a space
that is h feet tall, and the air at the bottom is tb degrees f and the air
at the top is tt degrees f, and there are vent holes for airflow at the
top and bottom of the space that have an area of av ft^2 each, the amount 
of air that will flow through those holes in cfm will be approximately 

   q (cfm) = 16.6 av sqrt(h(tu-tt)).

so how big do the holes have to be to make 100 cfm of air flow at night,
if the house is 80 f and the outdoor air is 70 f and the distance from
the top to the bottom vent hole is 8'? let's turn this equation around:

   av = q/(16.6 sqrt(h(tu-tt))) = 100/(16.6 sqrt(8'(80-70))) = 0.67 ft^2.

about 100 in^2, not very big, 10" x 10"... of course we want these holes
to open up automatically at night and close during the day, eg with motorized
dampers with thermostats or passive plastic film backdraft dampers that only
allow air to flow into the house through the bottom hole and out of the house
through the top hole. 

suppose we used the solar closet as a "fan" to move this air. suppose
it were 120 f instead of 80 f? then we have 

   av = 100/(16.6 sqrt(8(120-70))) = .3 ft^2, ie 43 in^2 or say, 10" x 4". 

the vents could be still smaller, or there would be more airflow for
the same size vents. 

what does the thermal mass of the house have to do with this? 

the house has a thermal mass of about 1/2 pound of water per square
foot of wall and ceiling, or 1/2 x 3,000 = 1,500 lb of water, ie 1,500
btu/degree f of thermal mass, total. if we want the maximum temperature
inside the house to be 80 f, with the conditions above, how cool do we
have to get the walls of the house at night? we cool them off at night,
and they heat up when the heat leaks into the house during the day...
as the house heats up 1 degree f, the walls will absorb 1,500 btu of heat.
we decided that during the day, the house will absorb about 12,000 btu
of heat, which will raise its thermal mass temperature, ie its temperature,
by 12,000/1,500 = 8 f, so to keep the house temp below 80 f at all times
during the day, it has to have a maximum temperature of 72 f at dawn.
if we added more thermal mass to the house, it could be warmer at dawn
and still stay below 80 f during the whole day. let's try that, say an
extra layer of drywall, poof. now we have 3,000 btu/degree f, so the house
only needs to be 76 f at dawn... 

in one solar closet house example, we suggested 20 55 gallon drums full of
water for thermal mass. if this were used for house cooling, with the closet
vented at night and completely shaded and insulated during the day, the
thermal mass available for cooling would be about 10,000 pounds of water,
so the closet could keep the house below 80 f during the day if its temp
at dawn were roughly 80 f - 12,000/10,000 = 80 - 1.2 = 78.8 f. we would want
to look at the heat transfer rate here too.

>2/ is there any way you could have the sunspace physically separated from
>the thermal mass?

you mean more separated? sure. the easiest way is to put the thermal mass
above the sunspace, as in the attic warmstores of norman saunders, pe,
so hot air from the sunspace naturally flows up to the thermal mass and
heats it during the day, and the heat stays trapped up there in the thermal
mass like an igloo, with the entrance below the heated space, since warm air
rises. but then you have to have a stronger attic or second floor to hold up
the thermal mass, which tends to be heavy, and that can be a more dangerous
situation in earthquake-prone parts of the world (altho it might make the
house more fireproof, with some sort of sprinkler system, and provide a more
natural water and hot water supply, starting with rainwater), and you have to
somehow bring some warm air back down to the house to provide heat for the
house when the house needs heat, which probably means using a fan, rather
than natural convection, even tho natural convection can make this whole
system work if it is on the ground floor, in principle.
 
a vaulted stone ceiling with foam on the outside of the stone would be nice,
with a ceiling fan to bring the heat down to the living space... a cathedral
or monolithic dome... we could modify notre dame. foam the roof, add some
ceiling fans, fill in the south buttresses with polycarbonate or polyethylene
film glazing to make an inexpensive sunspace...

we can also put the thermal mass on the same level as the sunspace, or
underneath it, as in a low-thermal-mass solar attic, with the thermal mass
in the basement. this usually requires a fan or a blower to make it work,
but if the ducts that connect the low-thermal-mass sunspace and the high-
thermal-mass heat battery are large enough in cross section, eg 2'x 2' across,
large fans can be used instead of more power-hungry blowers, and the
electrical fan power used can be reasonably small. 

>3/ is there any way you could have the sunspace/thermal mass separated from
>the house?

sure. but it is not easy to move heat over large distances. in the case of
hot air, if the ducts are small in cross section, the air velocity and fan
power need to be higher, and if the ducts are larger, they cost more and
take up more space and leak more heat through their walls, even if those
duct walls are insulated.

it's easier to move hot water than hot air over a distance, but then you have
energy efficiency and economic and complexity penalties in converting the
hot air to and from hot water at each end. i suspect that putting a solar
furnace out in the yard may not make much sense in todays's economics, if
that's all the structure does, since oil is so cheap. but then, putting
solar water heating panels or pv panels on the roof may not make much
economic sense either... chacun a son gout special.

>4/ would the air passing over the thermal mass and then into the house pick
>up the smell of hot plastic? (assuming water is being used for thermal mass
>and said mass is stored in plastic containers.)

i don't know. i doubt it, at these temperatures.

>could you use a heat exchanger to avoid this problem?

i suppose so, but that sounds like a complication to be avoided.
 
>5/ what about using phase change materials in the heat store? (admittedly
>more complex.)

last time i looked, they seemed to be expensive, vs water, and glauber's salts
need to be stirred up mechanically once in a while to keep working. they also
had a limited temperature range over which they act, vs water. suppose you did
have some super material with a very high thermal mass per unit volume--how
would you get solar heat into and out of it? you need surface area for that,
eg thin layers of material eg wallboard or ceiling tiles, but then the heat
battery is not compact, and it has little insulation, and you have to live
inside the heat battery, so you can't charge it up to a high temp on a sunny
day... i haven't looked at this lately.

i like water for thermal mass, in sealed containers in one compact, well-
insulated high-temperature space, with lots of insulation in the house, so
the heat battery does not have to contain too much water. i also like the
idea of combining a solar thermal store and a warm wastewater treatment
system, eg in a few septic tanks. septic tanks are very cheap where i live,
about $600 for a 5' deep x 6' wide x 12' long, 1500 gallon septic tank with
a sealed lid. it might make sense to stack a few of those up out in the yard,
say a structure 12' high and 8' wide and 12' long, surrounded by 6" of
fiberglass insulation on all 5 sides, all round, with glazing on the sun-
facing side. this would do a dandy job of sewage treatment, since biological
reaction rates double every time we raise the temperature of the system
10 degrees c, up to about 55 c.  but how would you get the solar heat out
of that structure into the house? cover the underside of the ceiling with
fin tube pipes? bury some copper pipes in sand or cement between the two
tanks stacked on top of each other?

>6/ if you put the thermal mass in the attic, what sort of extra building
>requirements to deal with added weight?

again, this isn't the way i'd build a house. i'd put the thermal mass
on the ground, but... we would need stronger than normal ceiling joists
to put it in the attic, eg prefabricated plywood or 2 x 4 trusses...
not too expensive, but different from the usual way of building houses.
stacking up 55 gallon drums full of water 2-high all over the attic floor
makes for an attic floor loading of about 275 lbs/ft^2 vs a more normal,
say, 50 lb/ft^2. if the attic floor joists were on 16" centers, supported
every 12', we might have something like this, following charlie wing's
book, _from the walls in_ (p. 39, little brown, 1979):

  1. f = 1,200 psi fiber stress in bending, for the wood (eastern hemlock)

  2. w = 275 psf uniformly distributed load
     o.c. = 4/3 ft on-center spacing 
     l = 12' clearspan

  3. w = w x o.c. x l = 275 psf x 4/3 ft x 12 ft = 4400 pounds
                        total uniformly distributed load

  4. m = w x l/8 = 4400 x 12 ft x 12"/ft/8 = 79,200 in-lb bending moment

  5. s = m/f = 79,200/1,200 = 66 in^3 minimum beam section modulus

  6. pick a joist thickness or breadth, say b = 1.5".

     then the depth d needs to be at least  d = sqrt(6s/b)
                                              = sqrt(66/1.5) = 16.2"

i suppose that some sort of 16" deep trusses might work here, or we might hang
the attic floor from the roof rafters with wood members or cables. i'm not a
structural engineer, and i'd put all this weight on the ground, if for no
other reason than that it would be easier to get the hot air out of the
thermal mass into the house without using a fan...

>7/ what about passing hot air from the house across pipes containing cold
>water as a cooling mechanism? (not quite on the topic but i thought i'd
>throw it in.)
 
seems like that would work, with say, a fan coil unit and some 55 f well
and 80 f air. for that matter, why not use a basement or crawl space
floor? blow up some cool air into the upper part of the house from near
a basement floor, with a vapor barrier under the floor. how much basement
floor area a do we need, in the above example? roughly speaking, 1000 btu/hr
= a (80-55)/r1, if the basement floor is 55 f. a = 40 ft^2, 5'x 8', not much.

>8/ any idea how long a plastic container heated to 170f will last?

no, perhaps a long time if the drum is not pressurised, but 170 f seems
quite warm for a system like this. radiation losses would limit the drum
water temp to about 130 f at most i'd think, if the collector has no
$elective $urface. 
 
>9/ say you put the sunspace under a (north facing for sh) verandah. does
>this affect energy collection? (i'm thinking of incident sun angles and
>whether or not we need an exposed upper face.)

that should work fine. what matters in wintertime is mainly how much
vertical glazing there is, since the sun is close to horizontal in the
middle of the winter. snow or a white surface in front can significantly
increase the solar input of vertical glazing by reflecting more sun
onto the glazing. this doesn't work so well with tilted glazing.

>10/ what's the biggest space currently being heated by a sunspace/thermal
>mass combination?

i don't know... 
do we count the new nrel visitor's center with its trombe wall? :-(

>11/ is anyone making them commercially?

"them," as in solar closet and sunspace kits? not that i know of, but
it sounds like a very good idea, especially if the whole shebang can
go together like an erector set, and be shipped ups (rolls of thin
polycarbonate glazing come to mind), except for a few common materials
that could be obtained locally.

>12/ how to make the outside of the sunspace/thermal mass aesthetically
>pleasing?

hire an architect, and watch him or her carefully, so that he doesn't make
the aesthetics the end-all and be-all of this :-) a lot of people are
very good at making things aesthetically pleasing...

>13/ say you had one of those dinky little air-303 wind-turbines. i guess you
>could hook this up to a resistive element in the thermal mass area and shove
>in some heat this way?

sure. but it wouldn't contribute that much heat compared to the solar heat
from a small amount of glazing: if the air-303 were putting out 375 watts,
24 hours a day (which would take a continuous 30 mph wind, vs about 150 watts
at 20 mph or 30 watts at 10 mph), it would contribute 375 x 3.41 x 24 = 30,690
btu/day of heat to the sunspace, about the same amount of net heat as 40 ft^2
of glazing, ie 5'x 8' of glazing, at a higher price and complexity. i'd rather
use this electricity to make my meter run backwards, or charge some batteries
if i lived out in the boonies. on the other hand, putting a small woodstove or
paper trash-burning stove in the solar closet might make a lot of sense.

>now, as i understand it, essentially we are trapping heat by warming air in
>a sunspace.

sort of. not trapping heat, exactly, just warming some air with it. then the
warm air flows into the house to heat it, and the sunspace warm air keeps
the glazing of the solar closet warm. the glazing of the solar closet is in
the back wall of the sunspace, so that inner glazing is exposed to that
sunspace warm air, and it loses less heat than if it were outdoors. the sun
shines through the sunspace glazing, then through the solar closet glazing,
to heat the air in the solar closet air heater, which air recirculates through
the solar closet, heating the sealed containers of water inside the closet.
meanwhile, the sun has heated the sunspace air to a lower temp, and some of
that air flows through the house to heat the house.

there are 3 air loops

  1) sunspace air flows through the house to heat the house on a sunny day,

  2) the hotter solar closet air heater air flows behind its separate smaller
     glazing inside the warm sunspace through the solar closet to heat sealed
     containers of water on a sunny day, and

  3) on a cloudy day, house air flows through the solar closet, where
     it is warmed by the containers of water and flows back out to keep
     the house warm. 

there is a 4th loop in a water heating system: warm water from an air-heated 
fin-tube pipe just under the ceiling of the solar closet (such as the pipe
used in baseboard radiators in houses and offices) rises up to move through
a conventional water heater ("geyser") on the floor above and then back down
to be warmed up by the fin tube pipe again, in a closed loop of pipe.

all of these loops can be driven by fans and pumps if desired, but in some
ways it's more elegant (and expensive, initially) to design the system so
that they don't need to use fans or pumps, just natural convection (warm water
and warm air rise.) this is like sailing vs. powerboating, a matter of
personal style or purity and perversity :-) it doesn't matter. both should
work fine...

>this warmed air is then used to heat a thermal mass in an area
>thermally decoupled from the sunspace other than by this warmed air transport.

the sunspace air is used to heat the thermal mass of the house itself,
when the sun is shining... air to heat the solar closet comes out of
the solar closet at the bottom, into the air heater cavity behind the
solar closet glazing, then rises up and is heated by the sun, then
goes back into the solar closet at the top of the solar closet glazing.

>(by thermally decoupled i mean that the only way energy gets from
>the sunspace to the thermal mass is by mass transport of warmed air.

true. but the warm air in the sunspace is not the same as the warm air in the
solar closet. solar closet air moves behind the inner solar closet glazing.
(we could build a system with only one glazing, but i think it would be more
expensive in cloudy climates, and it would be less efficient, i think, and
the total amount of glazing would have to be larger.) 

>this means we don't give a shit about what temp. our sunspace gets to at night
>because we are not relying on it for energy storage

pretty much true. unless there are plants therein, in which case we might want 
to controllably let some warm air leak back out of the house into the sunspace
at night to keep the plants from freezing. but you are on the right track here.

>and there is no means for energy to leak from the closet back into the
>sunspace.)

true. except through the good insulation.

>the thermal mass area - or closet - is well insulated. the solar closet has
>connections to a house such that it can exchange energy into the house.

true. two openings, one at the top and one at the bottom, to allow warm air
to flow out from the closet to the house at the top, and allow cooler house
air to flow into the closet to replace it and be warmed by the sealed
containers of water and flow out the top, back into the house.

>14/ where does the air entering the sunspace come from?

from the house, eg from a return air register near the floor of the house that
opens into the sunspace. or from a basement window or the lower part of a
partly open first floor window of the house, with a passive film backdraft
damper that only lets air flow from the house to the sunspace. warm air from
the sunspace would flow back into the house through an upstairs opening or 
window fitted with a motorized damper or fan controlled by a two thermostats--
one to enable the fan to turn on when the sunspace is warm, and one to enable
the fan to turn on when the house is cool. both would have to be "on" to
enable the fan to turn on, ie the fan would be in series with both
thermostats, electrically. 

controlling a motorized damper is only slightly more complicated. the kind
that uses no power when it is in a fixed position has one set of wires to
make it open more and another set of wires to make it close more. it might
be controlled with three thermostats, or two thermostats and a relay.

two passive thermostatic dampers with bimetallic springs, in series,
could also do this job, less accurately, but less expensively ($20) and
more naturally, with no electricity needed for controlling the house temp.

>15/ the physical barriers between the sunspace and closet only open when the
>sunspace temperature is equal or higher than the temp. in the closet?

the barriers from the house to the sunspace open when the house is too cool
and the sunspace is warm enough. once the house warms up or the sunspace cools,
they close. the barriers between the solar closet and its air heater open
whenever the temp in the air heater is warmer than the temp in the closet.
this temp control is actually simpler--the "greedy algorithm"-- "make the 
solar closet as warm as possible--with no limit." vs. only heating the house
up to 68 f or 20 c.

>16/ how reliable are the simple dampers you mention in several of the posts?

the bimetallic spring thermostatic dampers (aka automatic foundation vents)
are very reliable, but they begin to close at say, 60 f and they are not fully
closed until say, 80 f, so they would make for fairly soft temperature control
in a house. if you turn the springs over these dampers work in the more usual
(opposite) sense, opening more as the air gets warmer, which would be useful
in the sunspace, in series with the sunspace to house damper, working in the
opposite sense, exposed to house air.  

some motorized dampers are very reliable, with tiny 2 watt motors and
expected lifetimes of 100,000 cycles, 300 years if they open and close
once a day...

backdraft plastic film dampers are fairly reliable, but somewhat delicate.
they should be inspected once every two or three months, it seems to me,
to check to see that the plastic film is not ripped or folded and stuck
to itself, stuck open, etc. the low temp plastic film dampers can be made
from dry cleaner bags and metal screenwire.

>17/ what constraints does relying on convection introduce?

warm air rises. so convection powered systems have to have everything
more or less on the same level, eg ground level, or even better, the
"heaters" should be below the things that are heated. an ideal solar
house might have the house at the top, the thermal mass below that,
and the sunspace glazing below that. the house should also have a cooling
damper that automatically opens to the outside if the house begins to get
too warm. this would waste some solar heat, and allow some ventilation,
and allow using less insulation between the thermal mass and the house.

>18/ can these be got around with fans?

of course. it's not too hard to calculate what the fan characteristics have
to be, in cfm and static pressure, and a fan-driven system will probably be
more economical... one of my favorite fans is the grainger 4c688 $60, 36 watt
10" 560 cfm fan with a stalled static pressure of 0.4" h20 and a max temp
rating of 149 f.

>19/ at what sunspace temp. will you be losing energy through the heat capture
>section of the sunspace at the same rate that energy is entering. (obviously
>this depends on incident radiation.)

well, suppose you have say, 300 btu/ft^2/hour of sun coming into the glazing,
(a direct beam of full sun) and the outdoor temp is say, 32 f. then if you 
are not taking out any energy from the sunspace to heat the solar closet or 
the house, energy in = energy out for the sunspace, and all of the sun that
comes into the sunspace heats up the sunspace, which heats up the glazing,
which heats up the outdoors again, so roughly speaking, ignoring radiation
heat transfer, for 1 ft^2 of glazing with an r-value of 1 that transmits
100% of the sun that falls on it and blocks 100% of the longwave ir that
tries to exit through the glass by radiation ("the greenhouse effect")

300 = (t-32)/r1, so t = 32 + 300/1 = 332. pretty simple, huh? :-)

radiation losses will probably limit sunspace temps to less than 150 f, if no
selective surface is used, since a 1 ft^2 black body at a temperature t (f)
emits 0.174 x 10^-8 x (t+460)^4 btu/hour by radiation. the outside world also
radiates a little energy back at the sunspace. a 32 f outside world would
radiate 0.174 x 10^-8 x (32+460)^4 = 102 btu/hour towards the sunspace,
so we have (counting only radiation heat transfer)

300 + 102 = 0.174 x 10^-8 x (t+460)^4 ==> t = 233 f.

but i think the real world temperature will be lower.

altho we have seen sunspace temps of 154 f in our test house, when we were
actually using the sunspace to heat the house and solar closet... this is
oversimplified. the glazing might pass 90% of the sun, with an r-value of 2/3,
not 1, which depends on the temp itself, and there's another factor of 0.88
in the radiation formula above, for the emissivity of the glazing, and there
is more solar energy coming in if there is snow on the ground, etc...

but you get the idea. hot :-)

nick



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