[Greenbuilding] A combined solar closet in Pittsburgh
nick pine
nick at early.com
Sun Sep 29 13:27:43 CDT 2013
Young Ben would like to live inexpensively on a farm near Pittsburgh...
http://pittsburgh.craigslist.org/fod/4062701580.html offers a 12' by 56' 2
bedroom mobile home with a new steel roof, a front kitchen, and a large
bath, in very good condition for $3,900.00.
http://pittsburgh.craigslist.org/cto/4039312791.html lists "Antique CLASS
C - 22' MOTOR HOME for sale - pictures sent to serious inquires - inspected
and ready-to-go anywhere you want. Mechanically VERY RELIABLE, needs TLC
(tune-up) - everything works. Great for camping, hunting, or just fix it up
for the ultimate tail-gator mobile. Comes with kitchen, refrigerator, stove,
oven, microwave, furnace, holding tanks (for gray and black water), full
bathroom, overhead bed, couch, chair and dinette table. It has a 350 small
block engine (that is very reliable), and transmission also very reliable,
no slips or bangs, very smooth. Very easy to drive and is still being driven
daily. Selling for $1500, or best offer."
Here's one for $100... http://pittsburgh.craigslist.org/for/4056583831.html
"1974 Holly Park mobile home for sale. Must be moved. Serious inquiries
only; sold as-is. You move it. Needs repairs and attention, but would make a
great hunting cabin."
Ben has also been offered the free use of a pop-up camper. None of these
structures have much thermal insulation, but they could be solar-heated if
enclosed in a strawbale structure inside an inexpensive plastic film
greenhouse. Ben might be happy living inside a strawbale structure without a
pop-up camper, with a strawbale bed and a strawbale table and a sawdust
toilet.
An 8'x12'x8'-tall room with about 536 ft^2 of R55 dry 2' strawbale walls
would have thermal conductance G = 536/55 = 9.7 Btu/h-F... 10 cfm of fresh
air would raise the conductance to about 20. Keeping it 70 F on an average
31.5 F December day in Pittsburgh would only require (70-31.5)20 = 770
Btu/h.
http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/US/code/pvwattsv1.cgi
says 1.7 kW/m^2 of sun falls on a vertical south wall and 1.91 falls on a
south wall with a 45 degree slope on an average December day with a 35.1
average daytime temp. Suppose a 6 cent/ft^2 4-year 14'x24' R1 sloped
polyethylene film south wall with 90% solar transmission and 100% longwave
IR transmission connects the top edge of the south room wall to the ground
10' south of the room wall and transmits 0.9x14'x24'x1.91x317 = 185K Btu and
keeps the room 70 F for 24 hours and radiates heat from the ground inside
the wall and the south room wall at absolute Rankine temp Ta to an outdoor
temp Ts (R) and 185K = 24h(70-31.5)20 + 6hx0.1714E-8V(Ta^4-Ts^4)480ft^2,
where V = 1/4 is the approximate view factor of the radiating surfaces. (A
more expensive and durable polycarbonate south wall would block longwave
IR.)
Duffie and Beckman's 2006 Solar Engineering of Thermal Processes book has
equation (3.9.2) for sky temperature Ts =
Ta[0.711+0.0056Tdp+0.000073Tdp^2+0.013cos(15t)]^(1/4), where Ts and Ta are
in degrees Kelvin and Tdp is the dew point temp in degrees Celsius and t is
the number of hours from midnight.
Ta = 35.1+460 = 495.1 degrees R, ie 495.1/1.8 = 275.1 degrees K. NREL says
the humidity ratio w = 0.0030 pounds of water per pound of dry air on an
average December day in Pittsburgh, which makes the partial pressure of
water in air Pa = 29.921/(0.62198/w-1) = 0.145 "Hg, which makes the dew
point Tdp = 9621/(17.863-ln(Pa)) = 486 R, ie 486-460 = 26 F, ie -3.3 C. With
t = 12 hours (noon), cos(15t) = -1, so Ts =
275.1[0.711-0.0056x3.3+0.000073x3.3^2-0.013)]^(1/4) = 253.2 K, ie 1.8x253.2
= 455.7 R, ie -4.3 F, 39 F degrees less than the air temperature.
... 185K = 24h(70-31.5)20 + 6hx0.1714E-8V(Ta^4-455.7^4)480 makes 166.5K =
1.234E-6(Ta^4-455.7^4), so Ta = 649.5 R, ie 190 F. A quarter-cylindrical R1
film wall with a 10' radius and a 24' length and 377 ft^2 of surface with
190 F air inside and 35.1 F (495.1 R) air outside would lose
6h(190-35.1)377ft^2 = 350.4K Btu/day to the outdoor air, which is more than
the solar input, so 190 is too hot.
If 185K = 6h(T-495.1)377 + 1.234E-6(Ta^4-455.7^4), ie T = 576.9 -
5.46E-10(Ta^4-455.7^4). Plugging in T = 540 R on the right makes T = 554.0
on the left. Repeating makes T = 549.0, 550.8, and 550.2 R, ie 90.2 F, warm
enough to solar heat the room on an average day.
If the room is 70 F at dusk and 60 at dawn, 18 hours later, and 60 =
31.5+(70-31.5)e^(-18/RC), time constant RC = -18/ln((60-31.5)/(70-31.5)) =
60 hours = C/G, and C = 60hx20Btu/h-F = 1200 Btu/F of room temp thermal mass
with lots of surface can store average-day heat, eg 1200/5 = 240 8"x8"x16"
hollow concrete blocks, but they would take up a lot of space in the room.
Putting 2 750 ml glass bottles of Pellegrino water into the holes in each
block would raise its capacitance to about 9 Btu/F and lower the number of
blocks to 1200/9 = 133.
But it seems more interesting to combine daily and cloudy-day heat storage
in a single higher-temp glazed Solar Closet
http://www.ece.villanova.edu/~nick/solar/solar.html with 8'x8' of R1
vertical polycarbonate south air heater glazing over an insulated south
wall. If the room is 70 F for 16 hours per day and 60 F for 8 hours and we
count Ben's 300 Btu/h body heat for 8 hours and ignore the south room wall
heat gain from the 90.2 sunspace air during the day, we need to store
8h((60-31.5)20-300)+16h(70-31.5)20 = 14.5K Btu of average-day heat. With
lots of surface, we can store this heat in C Btu/F of daily mass cooling
from T (F) to 60, with (T-60)C = 14.5K, ie T = 60+14.5K/C.
If the air heater glazing transmits a constant 0.9^2x1.7x317/6h = 73
Btu/h-ft^2 of sun with a 90.2+73xR1 = 163 F Thevenin equivalent temperature
and the daily mass warms to T = 163-(60-163)e^(-6/RC) F in 6 hours of sun,
and we replace T with 60+14.5K/C, 60+14.5K/C = 163-(60-163)e^(-6/RC), ie
14.5K/C = 103(1-e^(-6/RC), ie 141/C = 1-e(-6/RC), and an R = 1/64 glazing
resistance makes 141/C = 1-e(-384/C), ie C = 141/(1-e^(-384 /C)). Plugging
in C = 160 on the right makes C = 155.1. Repeating makes C = 153.9, then
153.7 Btu/F, eg 31 hollow sparsely-stacked concrete blocks cooling from
60+14.5K/155 = 154 to 60 F on an average night.
But wait! An 8"x8"x16" hollow concrete block with 2 4"x4"x8" holes has 384
in^2 for the solid faces + 192 in^2 for the two faces with holes + 256 in^2
for the holes, totaling 832 in^2, ie 5.78 ft^2. With a 1.5 Btu/h-F-ft^2
slow-moving airfilm conductance, one block's airfilm conductance is 8.67
Btu/h-F, and 31 blocks have a 269 Btu/h-F film conductance.
If the blocks absorb 14.5K/6h = 2417 Btu/h from hot air, the air will be
2417/269 = 9 F warmer than the blocks. And the air in the air heater has to
be warmer than the air near the blocks for thermosyphoning to occur. One
empirical chimney formula says cfm = 16.6Asqrt(HdT), where A is the area of
the chimney opening in square feet and H is the chimney height in feet and
dT (F) is the temperature difference between the chimney and outdoor air.
With 6"x8' slots at the top and bottom of an 8' tall air heater, A = 4 ft^2
and H = 8'. Heatflow in Btu/h is cfmxdT, approximately, so Btu/h = 2417 =
16.6x4xsqrt(8)dT^(3/2) makes dT = 12.9^(2/3) = 5.5 F. With 2 1 ft^2 vents to
the room and an 8' height difference, 770 = 16.6x1xsqrt(8)dT^(3/2) makes dT
= 16.4^(2/3) = 6.4 F.
Given these effects, the daily heat store has a smaller temp swing, so it
needs more capacitance... If T = 66.4+14.5K/C =
148.5-(66.4-148.5)e^(-384/C), ie 14.5K/C = 82.1(1-e^(-384/C), ie C =
177/((1-e^(-384/C)). Plugging in C = 200 on the right makes C = 207 on the
left. Repeating makes C = 210.0, then 210.9 Btu/F, eg 42 hollow concrete
blocks cooling from 135.4 to 66.4 F. Or fewer blocks, since 42 would have
more surface than 31, with smaller charging and discharging temp drops.
Heating the room directly for 6 hours with 90.2 F air from the sunspace
instead of air from the daily store would also reduce the number of blocks
required in the daily store. And a thermosyphoning air-air heat exchanger
could help. If 31.5 F air falls down through the corrugations of 64 ft^2 of
8 mm Coroplast and 10 cfm of 65 F air rises up between the Coroplast faces
with possible condensation and freezing, NTU = AU/Cmin =
128ft^2x0.75Btu/h-F-ft^2/10Btu/h-F = 9.6, and E = 9.6/10.6 = 0.906, and Tco
= 31.5+E(65-31.5) = 61.9 F, and Tho = 65-E(65-31.5) = 34.6 F, with 46.7 and
49.8 F average air temps in the hot and cold chimneys and a 3.1 F difference
between the averages. Not much, for thermosyphoning air. So maybe this heat
exchanger needs 2 small DC fans that only run when the indoor RH or CO2
concentration exceed 60% or 1000 ppm, with an Arduino controller that can
also shut off the cold air fan if condensation in the outgoing air passage
begins to freeze, eg http://www.youtube.com/watch?v=wexdNx_StRc
Without these refinements, the room needs 5x14.5K = 72.5K Btu for 5 cloudy
days in a row, with an approximate solar heating fraction of 1-2^-5 = 0.97.
This could come from 72.5K/(135-70) = 1108 Btu/F of cloudy-day mass with
lots of surface cooling from 135 to 70F, eg 2 vertically-stacked 450 Btu/F
steel 55 gallon drums with plastic film liners surrounded by a layer of
rocks inside a 3'-diameter x 6' tall cylindrical welded-wire ag fence
gabion.
If the drums are well-insulated with strawbales, trickle-charging them hot
won't require much daily overflow hot air from the daily air heater, which
could first heat the daily store to 135 with thermosyphoning air and a
passive one-way plastic film damper, then heat the cloudy store behind the
daily store with more thermosyphoning air and another film damper.
For more fun stuff, see http://pineassociatesltd.blogspot.com/
Nick
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