Sneak Peak Video of the 
New Solar Hydrogen Home DVD
Coming SOON!

Download Over 100Meg of
FREE Hydrogen Video
Ride in the Famous H2 Geo
Click Here

re: dc exhaust fan help
28 feb 2004
dave  wrote:
>i am starting to gather some items to make a solar powered exhaust fan
>for my attic...

why would anyone do that, vs simple gable and soffit vents?

say it's 90 f outdoors and 250k btu/h falls on your tr (f) 1000 ft^2
r1 south roof with 80% solar absorption, and it loses 2 btu/h-f-ft^2
by convection and gr = 4x0.1714x10^-8(460+(tr+90)/2)^3 btu/h-f-ft^2
by radiation to the outdoors. if tr = 130 f, gr = 1.27 btu/h-f-ft^2. 

if the north roof is also 1000 ft^2, and the attic air temp is ta, and
your solar fan moves 1000 cfm, we have something like this, viewed in
a fixed font:

              tr            ta
     ---      |   r1/1000   |   r0.5/1000
|---|-->|-----*-----www-----*-----www----- 90 f
     ---      |             |
 200k btu/h   |             |
              |             |   1/1000[cfm]
90 f ---www---               -----www----- 90 f,

which is equivalent to this:

         1/2750  |   1/1000gr
163 f ----www----*----www---- 90 f, 

so tr = 163-2.5x10^-9(tr-90)(460+(tr+90)/2)^3. plugging in tr = 130 on
the right makes tr = 144.5 on the left, then 136.8, 141, 138.7, and 140,
so tr = 140, approximately, and ta = (140+270)/4 = 102.5 f.

you can also make ta=102.5 with an a ft^2 gable vent 8' above an a ft^2
soffit vent, where 1000 cfm = 16.6asqrt(8(102.5-90)), so a = 6 ft^2...


I got ALL of these 85 Solar Panels for FREE and so can you.  Its in our Ebook

Site Meter