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re: solar fan for greenhouse
10 oct 2004
mike c.  wrote:

>i currently use an ac power fan for cooling my greenhouse.  i would
>like to convert this fan to a solar powered fan.  i have two goals;
>learn more about solar power and stop using fossil fuels for my
>greenhouse (my wife and i are organic gradners and simple living
>practicaners).  i would like to convert the motor on the fan to dc,
>buy a photocell and get everything working.

consider a motorized vent actuator instead. how many cfm do you want
to move? two a ft^2 vents with an h' height difference and a dt temp
diff will move 16.6asqrt(hdt) cfm or about 16.6asqrt(h)dt^1.5 btu/h.

for example, a 14'x96' greenhouse covered with 80% shadecloth in full
sun (250 btu/h-ft^2) collecting 0.2x0.9x250x14x96 = 60.5k btu/h with
a 1'x96' opening at the ridge 7' above 2 0.5'x96' slots near the ground
might move 16.6x96sqrt(7)dt^1.5 = 60.5k btu, so dt = 14.3^0.666 = 5.9 f
above the outdoor temp, or less, counting evaporation from plants and
conductive heat loss through the glazing. 


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