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re: solar collector design for a solar closet?
8 nov 2002
rob  wrote:

>> one of the (largely unrecognized, even by me until lately) problems is 
>> that it's hard to store overnight heat for a house, starting with warm
>> air from a sunspace, unless the thermal mass in the house has lots of
>> surface exposed to the warm air. lots of insulation helps... 
 
>...thinking about storing water in 4 inch (unperforated) drain field pipe.

might make sense for the living space. with a lower required heat
transfer rate (as a trickle-charged cloudy day heat battery), the
closet can probably use plastic drums.

>...use a salt water mixture to increase the thermal capacity?

how would that help? we might evaporate water from a licl solution in
a cloudy day heat battery and condense it to warm the house on an
average day and sprinkle the condensation on a basement floor and
allow the licl solution to absorb water vapor that evaporates from
the basement floor on a cloudy day, as in a "chemical heat pump..."

>a lattice work on the top and bottom would hold them in place and allow
>air to circulate.  i think the pipe is aprox 4 in. diameter by 10 ft
>long and would hold almost 7.5 gallons of water.

sounds like these would be in a closet, but the extra surface may not be
needed there. the closet just has to keep itself warm on an average day,
with a thermal mass surface that's at least, say 10x the closet glazing
surface, eg 32 ft^2 of closet glazing and 320 ft^2 of drum surface, ie
12.8 drums.

the harder problem is storing overnight heat for the house from warm
sunspace air. if we live in the house, we don't want the house air to be
more than about 80 f on an average afternoon, and we don't want the house
to be much less than 60 on an average morning... 75 and 65 might be nicer.
if it's 20 f outdoors and 70 indoors, a superinsulated house with a
conductance of 200 btu/h-f needs 18h(70-20)200 = 180k btu of overnight
heat on an average day. storing that heat in 6 hours requires a heat
transfer rate of 30k btu/h.

with slowly-moving 80 f air and a 1.5 btu/h-f-ft^2 film conductance, an
infinite 70 f thermal mass needs 30k/(10x1.5) = 2,000 ft^2 of surface,
(a lot), eg 200 exposed 10'x4" pipes between ceiling rafters. otoh, if
the mass has infinite surface, the house needs 180kbtu/10f = 18k btu/f
of capacitance. those are the extremes. in between, we can figure out
the required capacitance with a finite surface, and vice-versa. it
usually turns out to be "very large."

btw, pipes between ceiling rafters have some advantages. they don't
take up floorspace, and if they are exposed, they can warm the room's
occupants by radiation while the room air stays coolish. during "charging,"
slowly-moving warm air from the sunspace near the ceiling can be warmer
than occupants would tolerate. we can store more heat and better control
the room air temp and "turn off the heat" (a slow-moving ceiling fan)
when the room is too warm or unoccupied. we can't do this if the heat is
stored in the walls. exposed pipes can also cool people in summertime.
you probably don't need that in montana, with an average daily max temp
of 85 f in july and a 0.0071 humidity ratio.

we could enclose the ceiling pipes by putting another ceiling below
them. less weird-looking, and that allows the air around them to be
warmer and faster-moving during charging, but it stops the desirable
radiant heating and also increases fan power. warmer sunspace air also
requires a little more sunspace glazing and can make a sunspace less
usable for people.

another alternative is to circulate sunspace air through some sort of
enclosed airshaft during the day, with heat storage in the airshaft.
that way, the air around the pipes can be warmer. a square spiral
staircase might enclose an airshaft, or the airshaft might enclose a
simple electric winch-elevator. the airshaft could be a central column
supporting a house. it might be 4'x4', with vertical pipes inside. 
they also come in 20' lengths. this would also allow better house air
temp control, by controlling the flow of house air through the warm
airshaft.

how many 4"x10' pipes would the airshaft need for the house above, if
the air inside were 100 f for 6 hours per day? ignoring the mass of the
house itself (drywall, etc.), 56 btu/f and 10 ft^2 at 1.5 btu/h-f-ft^2
makes rc = 3.7 h for each pipe, so tmax = 100+(tmin-100)exp(-8/3.7)
= 88.5+0.115tmin. the house needs (70-20)200 = 10k btu/h. tmin decreases
with increasing pipe surface. with n pipes, 10k = (tmin-60)10x1.5n, so
tmin = 60+667/n. if 180k btu = 56n(tmax-tmin), 3214 = n(88.5+0.115(60+667/n))
= 95.4n+76.7, and n = 33. tmin = 80.2, and tmax = 97.7. we could fit 36
pipes in a 30"x30" box with 3/4" between them. 

another alternative is to hang zomeworks "skymats" on the north wall of
the sunspace and thermosyphon warm water through some big ceiling pipes.
skymats are bare dark-colored plastic solar collectors. they are freeze-
tolerant, which is nice.

>> the tabs make this hard to read, but there appears to be only one
>> layer of glazing, vs sunspace and closet glazings... perhaps the sun
>> could make this a "frost-protected warm foundation" with no digging.
>
>does the solar closet collect heat directly from the sunspace or does
>it collect heat from a collector inside the sunspace?

the latter.

>aside from the extra living space, how does the sunspace improve the
>operation of the solar closet over a solar collector?

there can be a kind of "thermal cogeneration" that improves efficiency. 
the "waste heat" from the hot closet glazing ends up in the warm sunspace
air, which heats the house on an average day, when the closet just keeps
itself warm. 
 
>> >btw, here is some data for my area:
>> >
>>                    jan... dec 	year
>> 
>> >average high      33.1   33.0        56.2
>> >average low       11.9   11.9 	 29.8
 
>> looks pretty cold. how much solar energy falls on a square foot of south
>> wall on an average jan/dec day? what's the nearest town and state? 
>
>the town is dillon, mt in the sw corner.  helena mt or idaho falls, id
>would probably reasonable substitutes for dillon.  crest.org had the
>following info for helena, mt:  lat(n): 46.60 long(w): 112.00 *
>elev(m): 1188 [3898']
 
>flat-plate collector facing south at fixed tilt=90, solar radiation,
>kwh/m^2/day
>           dec     jan     feb
>ave        2.7     2.9     3.7
 sun        810     870    1100  btu/ft^2-day |
 avg. min   11.2     9.6    15.9 f            |
 avg. 24h   21.2    19.6    26.4 f            | nrel data
 avg. max   31.3    29.6    36.9 f            |
 avg. day   26.3    24.6    31.7 f during a 6h solar collection time?
 sun/dt     18.5    19.2    28.5   sun/(65-avg. 24h)
>min        2.0     2.1     3.0
 
>if i take the dec min at 2.0 kwh/m^2/day, that would be about 633.5
>btu/sq ft/day, right?

yes. norman saunders would say the worst-case month for solar house heating
is december, with 810 btu/ft^2 of sun on a south wall and an average 21.2
outdoor temp, which makes the sun/dt = 810/(65-21.2) = 18.5. february is
much better at 28.5. albuquerque has 48.5 in december. 

>the feb ave would be about 1172 btu/sq ft/day?

yes. 
 
>dillon had high solar potential because of the number of cloud free
>days.  that info is not available online though.

i guess altitude helps, as does dryness. we might predict this
from the "clearness index," kt = 0.44 for helena in december...

nick




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