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re: air conditioning:  doing it cheaper
30 jun 2003
>you might make an apartment less massy and desirably increase its thermal
>resistance to outdoors by gluing lots of $11 4'x8'x1" sheets of atlas
>energy-shield double-foil polyisocyanurate "r7.2" inside the drywall...

otoh, as steve baer says, thermal mass is good for natural cooling, as in
this average july day in phila, with 67.2, 76.7, and 86.2 f min, 24h, and
max average temps (view in a fixed font):

                   1/gapt                1/gdry
    ----------------www-------------------www----------- t drywall temp (f)
   |   |           1/200         |       1/6000      |
   |   |                         |                   |
   |   |    /      1/gfan        |                   |
   |    ---.  .-----www----------                  i |
   |     fan on    1/3000 (cfm)                      |
   |+    at night                                    |
  ---                                          cdry --- 2000
 | ~ | ta = 76.7+9.5sin(wt)                         --- btu/f
  ---       outdoor temp (f)                         |
   |                                                 |
   -                                                 -

10 pi=4*atn(1)'basic fun
20 w=2*pi/24'angular frequency 
30 gapt=200'apartment conductance to outdoors (btu/h-f)
40 gfan=3000'approx 3k cfm fan conductance (btu/h-f)
50 cdry=2000'drywall capacitance (btu/f)
60 gdry=6000'drywall air film conductance (btu/h-f)
70 dt=0.1'time step (h)
80 t=76.7'initial drywall temp (f)
90 for d=1 to 2'simulate for 2 days
100 for h=0 to 23 step dt'in 6 minute time steps
110 ta=76.7+9.5*sin(w*h)'ambient temp (f)
120 if ta>t then gs = gapt else gs = gapt+gfan'conductance to outdoors
130 i=(ta-t)/(1/gs+1/gdry)'heatflow into drywall (btu/h)
140 t=t+dt*i/cdry'new drywall temp (f)
150 if d<2 goto 210
160 if h>11.2 and h<11.6 goto 190'pick interesting results
170 if h>18.7 and h<19.1 goto 190
180 goto 210
190 if gs>200 then fan = 1 else fan = 0
200 print h;fan;ta,t
210 next h
220 next d

hour      fan  outdoors    indoors 

11.20001  0    78.67514    78.24278
11.30001  0    78.43121    78.2446   max indoor temp
11.40001  1    78.18611    78.2385   fan turns on
11.50001  1    77.93998    78.20735...

18.70004  1    67.3591     67.46273
18.80004  1    67.40762    67.45697  min indoor temp
18.90004  0    67.46251    67.45703  fan turns off
19.00004  0    67.52373    67.45768...

the fan would run for about 7 hours per night. we could estimate the
expected yearly cooling and heating energy of a conventional ac system
and furnace with and without smart night and day ventilation schemes with
a tmy2 simulation using nrel data for every 8192 hours of a typical year
in phila, turning the fan off and the ac on if the house becomes too hot
or humid (say more than 60% rh) or too cold, or the drywall temp becomes
less than the dew point, threatening condensation on the indoor surface.

this could work better with bigger fans and more thermal mass, eg
some water-filled poly film ducts between floor joists or rafters.

nick




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