MadSci Network: Engineering

Re: Can I get free power from attic fans?

Date: Wed Jan 30 12:06:46 2008
Posted By: David and John Free, Post-doc/Fellow, MFA, MFA
Area of science: Engineering
ID: 1189912990.Eg

Hi Jim

Yes, if you would be happy with a few microwatts!

Most attic fans are driven BY USING electricity. To get power from them 
you need to drive them FASTER than the mains does. Or you can switch them 
off and let the rising hot air turn the fans. That would generate a few 

A typical motor is around 50% efficient so the maximum power you could in 
fact extract from an electrically-powered fan (by placing a similar fan 
to be blown round by its air) would be 25% of the power used by the 
electrically-powered fan. So you would be losing energy, not gaining it 
(and the house would get hotter!)


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