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re: solar assisted a / c
15 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...

>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
>
>...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

a 32'x32' greenhouse with a larger fan might double this:
 
20 data 78.1,86.6,0.0158,1550
30 data 69.1,79.4,0.0114,1370
40 cfm=3560'fan airflow (cfm)
50 l=32:w=32'greenhouse length and width (feet)
60 gh=800'house conductance (btu/h-f)
70 for month = 1 to 2'september and october in new orleans
80 read tavg,tmax,wa,hsun'tavg, tmax, humidity ratio, sun on ground
90 td=(tavg+tmax)/2'average daytime temp (f)
100 sunpower=.9*l*w*hsun/8'sun power (btu/h)
110 tg=td+sunpower/cfm'greenhouse temp (f)
120 t=80'initial house temp est (f)
130 term1=17.863-log(29.921)
140 term2=(2*cfm+gh)/1000
150 term3=60*cfm*.075
160 term4=(cfm*tg+td*(cfm+gh))/1000
170 t1=(term4+term3*wa)/term2
180 t2=t1+term3*.62198/term2
190 th=9621/(term1-log((t1-t)/(t2-t)))-460
200 if abs(t-th)>.01 then t=th:goto 190
210 pi=exp(17.863-9621/(460+t))
220 wi=.62198/(29.921/pi-1)
230 p=4.5*cfm*(wi-wa)
240 print tg,t,wi,p
250 next month

greenhouse    indoor        humidity           pounds
temp (f)      temp (f)      ratio              per hour
 
132.5073      84.19831      2.602676e-02       163.8327    september
118.5826      76.60541      2.008044e-02       139.0606    october

greenhouse polyethyelene film comes in 32'x100' rolls with a 4-year
guarantee. it costs about 5 cents/ft^2. a lot of it's made in texas.
to minimize the greenhouse heat loss, we might put black plastic on
the ground and hang a layer of shadecloth above, with the fan above
that, pushing outdoor air into the greenhouse which warms as it passes
down through the shadecloth and flows into the lower house windows.

nick




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