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Posted by nicksanspam on April 5, 2008, 12:38 pm
 


    http://www.iedu.com/DeSoto/Projects/  and
    http://www.iedu.com/DeSoto/Astro/
 
You or Gary might add an ap note with an indoor vertical duct or closet
to let hot ceiling air return to the lower part of an air heater without
mixing with room air, with some shiny mass under the ceiling and a skirt
ssss around it to contain the hot air next to the mass, and a backdraft
dampers ddd and a fan or a motorized damper controlled by a room temp
thermostat in the skirt. Like this, viewed in a fixed font:

                             |     |
                             |     |
--------------------------------------------
          sfd                 d          |
      ==> sa d  ceiling mass d           |
          sn  d             d -----      |
          s            |     |     |     |
                       |     |     |     |
                       |  r  |  h  |  a  |
                       |  e  |  o  |  i  |
                       |  t  |  u  |  r  |
                       |  u  |  s  |     |
                       |  r  |  e  |  h  |
                       |  n  |     |  e  |
                       |     |  w  |  a  |
                       |  d  |  a  |  t  |
                       |  u  |  l  |  e  |
                       |  c  |  l  |  r  |
                       |  t  |     |     |
                       |     |     |     |
                       |  |   -----      |
                       |  v              |
                       |                 |
-----------------------------------------

If 1 ft^2 of R2 twinwall polycarbonate with 80% solar transmission transmits
200 Btu/h in full sun on a 30 F day and the average air heater temp is 130 F
and it loses 50 Btu/h to the outdoors, 10 ft^2 of 2-sided ceiling mass with
a 30 Btu/h-F air-mass conductance can be 125 F. So a 4'x8' air heater might
heat 320 ft^2 of mass, eg 10 4'x8' radiant barrier sheathing osb trays with
4" of water in 30" greenhouse poly film ducts hanging a few inches under
the ceiling, surrounded by a 1' skirt to make a heat trap.

Nick


Posted by Morris Dovey on April 5, 2008, 2:32 pm
 
nicksanspam@ece.villanova.edu wrote:


Great minds think alike. :-D

I have a couple of drawings showing how I envision this approach
in a directory at

   http://www.iedu.com/DeSoto/Projects/ThermStore/

I took the closet approach, and (because I'm particularly partial
to passive systems) decided to find a non-powered method of
controlling whether heat is gated into and out of the storage
closet.

At some point, if there seems to be wider interest, I'll
incorporate the drawings into normal web pages...

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/

Posted by nicksanspam on April 6, 2008, 12:40 pm
 

The first 2 drawings are nicely simple :-) A bypass mode can keep
the store hotter and make collection more efficient, vs a system
with average-day heat supplied from the store.
 

Used windshield wiper motors and thermostats are cheap and accurate
and they can work with a very small average power...

It's hard to store and retrieve overnight heat from a single drum. On a 20 F
average January day in Des Moines (when 1090 Btu/ft^2 falls on a south wall),
a tiny house with a 49 Btu/h-F conductance needs I = (50-20)49 = 1470 Btu/h
to stay 50 F at dawn. If this comes from 1 drum with 19 ft^2 of sidewall and
28 Btu/h-F-ft^2 of slow airfilm conductance, it needs Tmin = 50+1470 = 102.5
at dawn. Tmax = 102.5+16hx1470 = 157 at dusk. We could do this with 2610 ft^2
of R2 air heater glazing with 80% solar transmission :-)

20 GI
30 I0*G
40 TMINP+I/28
50 TMAX=TMIN+16*I/450
60 RC=-8/LOG((TMAX-238)/(TMIN-238))
70 A=2/(RC/450-1/28)
80 PRINT A

2610.478

OTOH, we can store 16hx1470 = 23.5K Btu of overnight heat in 23.5K/(70-50)
= 1176 Btu/F of house mass that cools from 70 F at dusk to 50 at dawn. If
the house needs (60-20)49 = 1960 Btu/h to be 60 F average over an 8 hour
collection day and a 100 F A ft^2 heater gains 0.8x1090/8h = 109A and
109A = (100-20)A/2+1960+23.5K/8h, A = 71 ft^2.

We can do this with less ceiling mass and perfect room temp control, with
more glazing. Collecting (8h(70-20)+16h(50-20))49 = 43.1K Btu at 5390 Btu/h
with 96 ft^2 of glazing makes T = 238+10.8K/96 = 126 F on average. With lots
of airflow and mass surface, we only need 23.5K/(126-50) = 309 Btu/F, eg
309 pounds of water in shiny trays under a ceiling.

Nick


Posted by Morris Dovey on April 6, 2008, 2:54 pm
 nicksanspam@ece.villanova.edu wrote:

Air-only storage won't do much for anyone. The drawings without
drums were just to capture the basic flow concept.


That's true. My personal preference is to find ways to make
things as simple,  reliable, and cost-effective as possible. If
it doesn't actually /require/ an electric motor, a solenoid, a
thermostat, wiring, and a lifetime supply of electricity - then I
won't include these things in the design.


I'm inclined to agree. The entire width of the panel should be
filled with drums - and I'm inclined to better like smaller drums
stacked two-high.


Shiny trays under the ceiling sounds pretty kinky - I'm not sure
it'll catch on here in the heartland. 309 pounds of water in them
is only about 38 or 39 gallons - roughly 70% of the capacity of a
single 55 gallon drum.

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/

Posted by nicksanspam on April 6, 2008, 7:03 pm
 

Or a stack of shallow trays supporting water in poly film ducts.


It could be a high price to pay, aesthetically-speaking.
(Doesn't Frank Gehry live in the heartland?)


Yes, with an infinite surface, or more water, with less surface:

20 FOR NTRAYS = 1 TO 4
30 A=4*8*NTRAYS'one-sided surface (ft^2)
40 G=3*A'2-sided thermal conductance (Btu/h-F)
50 TMINP+1470/G'min water temp (F)
60 TMAX6'initial max temp (F)
70 C = 23400/(TMAX-TMIN)'thermal mass (Btu/F)
80 RC=C/G'time constant (hours)
90 TMAX6+(TMIN-126)*EXP(-8/RC)'new max temp (F)
100 IF ABS(C-CL)>.1 THEN CL=C:GOTO 70'iterate
110 DEPTH*C/A/62.33'water depth (inches)
120 PRINT NTRAYS;C;RC;TMIN;TMAX;DEPTH
130 NEXT

485.2478  5.054664  65.3125   113.5336  2.919428
346.5029  1.804702  57.65625  125.1881  1.042344
330.3709  1.147121  55.10417  125.9337  0.6625439
324.2509  0.844403  53.82813  125.9945  0.4877029

We might use 2 trays with 346 total pounds of water.

Nick


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