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Huh? I thought energy was proportional to the _square_ of the velocity. After all, we're not talking relativity here.

-Harry Wertmuller

- Thread starter HarryWertM
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- #1

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Huh? I thought energy was proportional to the _square_ of the velocity. After all, we're not talking relativity here.

-Harry Wertmuller

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tiny-tim

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Hi Harry!Reading about renewable energy on the Internet …

I googled "energy is proportional to the cube of the velocity", but only got two hits, neither of them relevant.

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Maybe it's just an error ! :)

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russ_watters

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You need to substitute the first equation into the third...1 CFM varies Directly with RPM (CFM 1 / CFM 2 = RPM 1 / RPM 2)

2 SP varies with the SQUARE of the RPM (SP 1 / SP 2) = (RPM 1 / RPM 2)2

3 HP varies with the CUBE of the RPM (HP 1 / HP 2 = (RPM 1 / RPM 2)3

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LURCH

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rcgldr

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Huh? I thought energy was proportional to the _square_ of the velocity. After all, we're not talking relativity here.

-Harry Wertmuller

Here is how I think of this surprising, but true, fact:

1. The energy per molecule (or mole, or gram, or whatever) of air is proportional to the

2. The number of air molecules per unit time, that pass through the windmill blades, is proportional to the air velocity.

Multiply the quantities in these two statements and we find:

The power (i.e. energy per unit time) in the air passing through the windmill is proportional to the

- #8

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the column of air in 1 second= v

Huh? I thought energy was proportional to the _square_ of the velocity. After all, we're not talking relativity here.

-Harry Wertmuller

the volume of air= va (a= cross area)

the mass of air=vad (d=density)

the kinetic energy= 1/2 m v v

=1/2 vad v v(vad=m)

= 1/2 ad (v v v)

hope you got the answer

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http://en.wikipedia.org/wiki/Rayleigh_distribution

windmills on average operate at only ~35% of their maximum design power. So a 1.5 MW windmill on average produces only about 500 kW.

Bob S.

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