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re: solar assisted a / c
12 sep 2005
drew cutter wrote:

> i understand that dehumidifiers will be hard to come by because of 
> katrina. demand for these have gone way up.

solar-assisted fans might work faster with less electrical energy.
nrel says 1550 btu/ft^2 of sun falls on the ground on an average
78.1 f september day in new orleans, with an 86.1 daily max and
humidity ratio wa = 0.0158 pounds of water per pound of dry air. 

with 8 hours of sun and an average 82 f daytime temp, a $100 16'x16'
plastic film greenhouse with 1x3 bows and a $50 lasko 16" 90 w window
fan could collect 0.9x1550x256ft^2/8h = 44.6k btu/h, heating 2470 cfm
of 82 f air to 82+44.6k/2470 = 100 f.

with temp t and vapor pressure pi and w in a house with an 800 btu/h-f
conductance, we could remove p = 60x2470x0.075(w-0.0158) = 11115w-176
pounds of water per hour. if (100-t)2470 = (t-82)(2470+800)+1000p, ie
247-2.47t = 3.27t-268-p, p = 5.74t - 515 = 11115w-176, so t = 120.4-1936w.
pi = e^(17.863-9621/(460+t)) "hg and w = 0.62198/(29.921/pi-1) makes
t = 120.4-1204/(29.921/pi-1), ie pi = 29.921((120.4-t)/(1324-t)), so 
t = 9621/(14.46-ln((120.4-t)/(1324-t)))-460. plugging in t = 80 f on 
the right makes t = 74 on the left. repeating makes t = ... 78.79.

20 td=(78.1+86.6)/2'average daytime temp (f)
30 wa=.0158'outdoor humidity ratio
40 cfm=2470'fan airflow (cfm)
45 sunpower=.9*256*1550/8'solar heat (btu/h)
50 tg=td+sunpower/cfm'greenhouse temp (f)
60 gh=800'house conductance (btu/h-f)
70 t=80'initial house temp est (f)
80 term1=17.863-log(29.921)
90 term2=(2*cfm+gh)/1000
100 term3=60*cfm*.075
105 term4=(cfm*tg+td*(cfm+gh))/1000
110 t1=(term4+term3*wa)/term2
120 t2=t1+term3*.62198/term2
130 th=9621/(term1-log((t1-t)/(t2-t)))-460
140 if abs(t-th)>.01 then t=th:goto 130
150 pi=exp(17.863-9621/(460+t))
160 wi=.62198/(29.921/pi-1)
170 p=4.5*cfm*(wi-wa)
180 print t,pi,wi,p

indoor        vapor         humidity           pounds
temp          pressure      ratio              per hour

78.79059      1.006362      2.164776e-02       64.99785   september
71.0885       .7767595      1.657717e-02       57.5442    october

so it looks like we can remove about 8 gallons per hour or 64 gallons
per day this way in september, and 57 in october, with 1370 btu/ft^2
of sun and a 69.1 average and 79.4 max and wa = 0.0114. 


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