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Researching for practical entry into home wind-gen I came
across this formula:
Optimum TSR = 4Pi/n (where n is the number of blades)
In theory then if Pi = 3.14 and you have 3 blades, your
optimum TSR will be 4.187; or in other words your blade
tips need to be doing just over 4 times the wind speed, for
maximum wind energy to mechanical energy conversion
efficiency.
Does this apply only to axial flow HAWTs or is it a general
rule for all turbines?
Hope this blows past some efficient gurus! Thanks in
advance. Nat."Where there is no vision, the people perish..." King Solomon.
oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686
Posted: 08:30am 15 Aug 2009
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As no-one else has answered this I will give it a try.
Optimum anything in wind generators is a false hope. Everything has to be compromised. If you design every part to be optomised, you end up with a beautiful lawn ornament that may deliver little power to your batteries.
It is all about compromising every part of the system to achieve the best wind loading into the alternator and then power into the batteries.
Optimum TSR is when you can match the wind to the load. This is not a magical figure, it is a dynamic everchanging value.
It is usual to aim for about 6. It rarely is 6, but that is about the best value to design around. It gives a combination of torque and speed.... so when we drift around this figure, we can get something out the other end..... However, it is the impedance of the alternator driving the load that will determine if this TSR can be achieved, over achieved or stalled.... and it will go between all three at different wind speeds.... if your lucky.
If you design for a high TSR, and can't develop enough torque to break out of stall, you get nothing. If you design for low TSR, you will have a to build an over large alternator to deal with the low rpm to get decent volts/rpm at reasonable current.
The best news I can give you is to follow a proven design for the first one to get the feel for all the variables, and how they change dynamically as the wind changes and the state of charge of the batteries changes. It is far from simple.
In short.. don't try and re-invent the wheel...it is a tough act to follow.
There is good information on this site to build F@P style systems, and Hugh Piggotts books are gems. Otherpower has specific plans for 10' machines on the web for free and oodles of sobering information on wind generating... and it's pitfalls.
If you think that generating power of your own is cheap, or will save you bundles compared to the grid.... forget it. If you want a challenging hobby or are off grid, then proceed with caution. It is worth the effort, but don't go it alone.... use someone else's hard won knowledge first. No one question or bunch of questions can answer it all.... you need to do it....but follow someone else who has done it sucessfully first.
It is not easy, but nothing good ever is.
The formula you quoted will give you a well running prop, but may drive nothing properly... it is the whole system working in unison successfully that will give you the results you need.
.........oztulesVillage idiot...or... just another hack out of his depth
Thanks for the reply oztules. Although out of country at
present, I'll be back in Oz next month.
It seems I should detail my design constraints:
1. Low speed cut in. Mean annual wind 4.5m/s.
2. Robust & low maintenance: bearings biennial okay.
3. Quiet: inaudible standing underneath.
4. Rated 1.5kw+ @ 170rpm (20kw @ 170rpm has been done).
5. Cheap: using 80% recycled materials.
6. Very low bird strike risk (large avian population).
7. Able to cope with turbulence.
8. Grid-tied.
You may agree that the above constraints render an axial
flow traditional HAWT unfeasible. Now to quote:
"It is usual to aim for about 6. It rarely is 6, but that
is about the best value to design around. It gives a
combination of torque and speed.... so when we drift
around this figure, we can get something out the other
end..... However, it is the impedance of the alternator
driving the load that will determine if this TSR can be
achieved, over achieved or stalled.... and it will go
between all three at different wind speeds.... if your
lucky. " End quote.
I am looking at proven designs but not many "back-shed"
ones. There have been many papers, models, prototypes and
the real deal up and running in recent years. It is no
longer necessary to have high RPMs to generate
electricity. It is no longer justifiable to have cogging
torque causing a problem, when numerous studies have
shown different ways of reducing it to being negligible.
Currently I'm looking at how practical it is to increase
wind velocity, and to decrease or shut off wind
completely as required, using an external housing.
Accepted that this will increase the material cost, but
are the benefits worth it? Increasing velocity by 1.7 -
2.2 has been achieved. Double the velocity can mean 8
times the power output, so could be very economical.
The next step is to do some 3D modeling and simulate the
flow control, trying to use the KISS principle for the
mechanics. Then build the external housing on a rotating
platform, and measure the difference in wind velocity to
the ambient wind. Then I might have a few more facts on
which to base the turbine design. Then when the turbine
is built and installed, we can do some torque tests. As
some of the studies have demonstrated that the use of
FEMM gave accuracy to the PMA design, I think I'll use it
to downgrade a commercial 20kw design to 5kw. As said,
"there's no point reinventing the wheel." Regards, Nat."Where there is no vision, the people perish..." King Solomon.
GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127
Posted: 01:33am 18 Aug 2009
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The volume of air that the blades pass through relates to the amount of power. The method above of increasing the velocity will result in a reduction of total available power due to inefficiencies. The wind turbine machine to convert the wind energy to electricity may be smaller, but the total physical machine size and the total capture area and original volume of air will be larger. The power is proportional to the mass flow rate of air past a point.
Quote:
"The method above of increasing the velocity will result
in a reduction of total available power due to
inefficiencies." End quote.
I didn't give a method. There are various methods:
www.flodesign.org use a stator, rotor & diffuser like a
jet engine.
www.racecom.com use "directors" increasing volume/speed.
www.warp-eneco.com use "saddle ridge" stackable platforms
with two 13kw turbines on each platform (yawable/static
alternate). 10mw+ are their normal installations.
These are commercial systems that have a smaller
footprint than traditional systems. They do not have any
fantastic new technology, but are combining designs from
other fields that work well for their application.
"Where there is no vision, the people perish..." King Solomon.
oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686
Posted: 08:47am 18 Aug 2009
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Best of luck in your venture.... but I can't help but think you will be disappointed. Gordon is on the right track with this in my opinion....
In short it may become unmanagable (size and engineering wise) very quickly... for little if any return.
Extra height can double your windspeed , and be in laminar air too... huge bonus. Not too sure the practicality of a decent size director up a tall tower... not for me. I worry enough about just a propellor.
.............oztules
Village idiot...or... just another hack out of his depth
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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