On 7/21/2011 8:00 AM, Home Guy wrote:
....
Similarly, wind that wants to move past the blade must push it
aside, and in doing so it will rotate the hub. The more surface
area you present to the wind (ie the wider the blade) the more
rotational force you transmit to the hub.
Explain what's wrong with my concept.
....
The interaction between blades and the competing design factors (weight,
strength, speed control, etc., etc., etc., ...)
A ceiling fan is built to keep the occupants of a room comfortable by
moving air gently. A primary design consideration is to minimize noise
while the fan rotates at low speed and to keep the construction costs,
and therefore the purchase price, low. Energy efficiency is not a
primary concern, because operation is inexpensive so most ceiling fans
incorporate blades that are comparatively inefficient drag devices;
rotating the pitched blades pushes air vertically out of the way. Wide,
flat blades are inexpensive to build and work well as drag devices. More
blades are better, up to a point, and the usual layout of four or five
blades is the result of balancing trade-offs between efficiency and expense.
OTOH, a wind turbine must capture the energy in fast-moving air and
rotate at relatively high speed. Slow rotation would increase the
torque and require heavier and more expensive drivetrain components.
For high-efficiency energy conversion lift-type turbine blades, similar
to airplane wings, of twisted and tapered airfoil shapes are used. The
blade design creates a pressure difference in wind—high pressure on one
side and low pressure on the other—that causes the blades to turn.
The reason for taper is the same as that for the shape of airplane wings
and/or props--Bernoulli lift/pull. The longer path over a wing surface
causes the velocity to rise and that lowers pressure on the upper
(behind in the case of the prop/blade) which "pulls" the rotor in that
direction for rotation.
A combination of structural and economic considerations drives the use
of three slender blades on most wind turbines—using one or two blades
means more complex structural dynamics, and more blades means greater
expense for the blades and the blade attachments to the turbine.
As noted before (and referenced in the Wikipedia article I bookmarked
earlier), the increase in effectiveness of two over only a single blade
fan is surprisingly little and the relative gain after that is smaller yet.
Also, again as noted, designs were within 75-80% of the theoretical
limit when I last had actual performance data some dozen years or so
ago; I'd expect continued refinements have pushed that to the upper
value or perhaps even higher for current and next-generation blades
(altho that's pretty closely held proprietary data, obviously, and not
readily passed out over the 'net). What I'm aware of is what vendor
provided to our electric co-op generation unit when evaluating the
build/purchase decision to meet the mandated "green" generation reqm'ts
coming. In the end, we chose to simply buy what we have to and keep
conventional low-cost generation in our pool to minimize our customer
costs as much as possible.
Following are some links that may be of interest; they don't delve into
the real intricacies of blade design; that's pretty complex but do have
some real-world design information and discussion of what actual design
efficiencies are, etc., etc., etc, ...
http://www.asr.org.tr/pdf/vol10no1p147.pdf
http://www.nrel.gov/docs/fy00osti/27143.pdf
http://practicalaction.org/docs/tech..._from_wind.pdf
http://www.bringaboutgreen.com/build...peed-ratio-tsr
http://www.raeng.org.uk/education/di...nd_Turbine.pdf
More than the above requires reading far more technical literature than
I'm prepared to try to reproduce for usenet; if you're really, really
interested, there are engineering texts but you'll need quite a lot of
background.
Probably one of if not the standard...
http://www.amazon.com/Wind-Energy-Explained-Theory-Application/dp/0470015004
Enjoy...
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