On Jul 20, 9:50*am, dpb wrote:
On 7/20/2011 8:03 AM, Home Guy wrote:
...

When you look at an ordinary fan, it has large blades that occupy a
significant portion of the cross-sectional swept area.

When you look at a wind turbine, the blades are very thin, occupying a
very minimal amount of swept area, allowing much of the wind energy to
flow right through or between the blades.

If a fan has fan blades that are designed to *efficiently move air*,
then why won't that same basic blade design also be *efficiently moved
by air* ?

Size has a lot to do with the design limitations.

Interestingly enough, the efficiency of adding blades is relatively
small; a one-blade rotor is nearly as efficient as two and the third is
even less of an increase.

While it doesn't go into a lot of technical detail, the wiki article
outlines some of the basics of the various competing factors that go
into modern generator blade design.

Limiting is more the physical characteristics required for survival and
control and related cost and the efficiency obtainable within those
restrictions as opposed to only the efficiency (altho modern designs run
probably nearly 80% of theoretical Betz limit of kinetic energy
extraction which is roughly 60% of input field KE.

I've not read the article for a while to see what, if anything has been
added/updated, but had the link bookmarked--

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

--

The trick is balancing the one blade model. Interestingly the same
things apply to boat propellers. It also occurred to me there is
another example of powered thin blades, helicopters.