MadSci Network: Physics |
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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