MadSci Network: Physics
Query:

Re: How big would blades on a homemade windmill have to be to power a 1.5V lamp

Date: Sat Jun 14 10:43:24 2003
Posted By: William Payne, , Electromagnetics, Altair
Area of science: Physics
ID: 1053972885.Ph
Message:

Interesting question, which spans different engineering specialties, from 
aerodynamics to mechanics to electrical.  The common thread here 
is "energy", or the time-rate of energy which is "power".

Starting at the end and working back, you need to know how much power your 
lamp uses.  Power P in Watts is the voltage V multiplied by the current 
I.  Obtain from the manufacturers rating, or you can measure V and I with 
a meter while a battery is powering the lamp.

The mechanical shaft power into the generator needs to be more than the 
electrical output, by the factor of efficiency.  Large generators can be 
over 90% efficient, but for a small one let us assume 50% efficiency, so 
plan on having about twice the shaft power in Watts as the power your lamp 
requires.

Now the wind power is P = 1/2 d A w^3  where
d = air density, typically 1.2 kg / m^3
A = area swept out by blades
w = wind speed

When doing the math be sure to keep your units in order!
This link will help
http://www.nrel.gov/doc
s/gen/fy01/NN0092.pdf

The efficiency of your blades is hard to predict, will leave that up to 
you.  Also when delivering power there is a question of ratios, that is 
speed versus torque and the equivalent ratio of current versus voltage.  
The simplest wind mill has a blade rotor that directly attaches to the 
generator, with the angle of the blades adjusted for a ratio of torque and 
speed that suits the generator.

Other interesting links
http://www.hydrogenappliances.com/
http://www.hydrogenappliances.com/TORQUE.html
http://www.nrel.gov/education/trac/
pdfs/w_pratt_module.pdf



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