Solar Attic Fan

Harbor Freight had their 5 Watt solar panel on sale for $39.99.  That's $10 cheaper than their regular price.  Since I have a Harbor Freight credit card, I get gift cards as part of their cash back program.  I had a $20 gift card, so my actual out of pocket expense was $20.  A 5 Watt panel works out to be 416 mA worth of power in full sunlight.  The formula for Power is P=I (current) x E (voltage).  In this case, 5 Watts = I x 12V, so I = 5/12, or 416.6 mA.

I had several 12 Volt muffin fans.  The largest one is 6" in diameter, but is rated at 500 mA.  The solar panel will spin it, but not up to full speed.  The voltage drops to about 8 Volts.  The next largest fan I had was a 3.5" 12 Volt computer fan.  It was rated at 180 mA, well below the rating of the solar panel.  Consequently, the fan instantly jumped to full speed, moving quite a bit of air.  I attempted to connect two small fans in parallel, but once again the voltage dropped.  I suspect the panel puts out less than the rated 5 Watts.  Nevertheless, it runs one fan nicely.

Before mounting the fan & panel, I tested several configurations.  Once I found the ideal setup, I grabbed my drill, screws, and some silicone RTV.

I mounted the solar panel right next to the gable end vent.  The power cord drops through a hole I drilled in the roof.  I used a liberal amount of silicone RTV on the power cord and screw heads to prevent leaks.

This is the inside of the gable end vent.  Man, is it hot in here!  The ridge vents aren't very effective.

The plate with the fan is simply screwed into place.  It fully covers the gable vent.

Just to see how hot it is in the attic, and if the fan made any difference, I mounted a temperature data logger on one of the rafters.  I'll post the data later.

The solar panel is almost invisible on the roof.  If you look closely, you can see it right near the ridge line.  The other two items on the roof are skylights.

Once it was finished, I went back on the roof and put my hand near the gable vent.  Felt like a dryer vent.  The fan moves a lot of air for its size, and the air was hot.  Good sign! 

So, for $20, I have a solar attic fan.  It only runs when the sun is out, but that's when it's hottest.  It's quiet, costs nothing to operate.  If I can later find a larger 12 V fan that draws 300 mA or less, I'll give that a try.