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ZL3AI > APRDIG 21.06.03 20:20l 261 Lines 10248 Bytes #999 (0) @ WW
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From: ZL3AI@ZL3VML.#80.NZL.OC
To : APRDIG@WW
Subject: Re: Field Day Water Power
From: David VanHorn <dvanhorn@cedar.net>
Date: Tue, 17 Jun 2003 16:59:43 -0500
X-Message-Number: 28
>a comment before we all get kicked off of here...
:)
>Hardly any difference at all. A good prop is also a good turbine. It is
>almost 100% reversible (under identical design conditions)..
All the windmill engineers would disagree with you.
>You are comparing a prop (which has a tip-speed ratio of 8 or more) to a
>windmill (which has a tip-speed ratio of 1 or less). This is apples and
>oranges. The prop was designed for high speed (100 to 300 MPH, high RPM,
>and low torque). THe windmil was designed for low speed (7 to 30 MPH,
>very low RPM and very high starting torque (remember, they had to pump
>water up and down)... Hence totally different designs.
Well, that depends on the design. High solidity designs are low tip speed,
but the 2-blade and 1 blade are low solidity that operate with the tips at
.2-.3 mach.
http://www.webpal.org/webpal/b_recovery/3_alternate_energy/electricity/wind/blas
de/bladedesign.pdf
http://www.webpal.org/webpal/b_recovery/3_alternate_energy/electricity/wind/blas
de/windrotord.pdf
>There are many very efficiient wind turbines that use blades that are
>identical to props, but they need higher winds, and very low
>starting loads. But once a 2 blade wind turbine is up to speed, it
>sucks all the available energy out of the wind just like a fat windmill
>does at 1/10th the speed. So there is no difference, as long as you
>propely engineer the blades to the given conditions.
Right, but you'll end up with about 2/3 of the total available energy. (you
made me pull out a book!)
For a waterwheel design, the closest windmill equivalent is the savonious
rotor. I've seen heli blades used in a Darrieus design, but that's a very
different animal. Not likely to produce much useful power underwater.
>Even a Helocopter with a failed engine will auto-rotate its unpowered
>blades to act like a turbine to slow its descent and cushion its
>inevitable crash...
Right, they will go, but that doesn't mean they are extracting anything
like the max available energy. They also are not starting from a
standstill. They dissipate a lot of energy as drag, in that mode. The
problem is that the apparent wind vector ends up on the wrong side of the
airfoil, unless it's a symmetrical airfoil, and I don't know why you'd use
a symmetrical airfoil on a helicopter, unless you planned to hover upside
down. (models do, real ones probably don't)
>For a given speed of water, and a given pressure, and a given RPM, a
>bladed "thingy" should work about the same whether it is driving the
>water, or the water is driving it...
Airfoil designers disagree.
Some of the subtleties are explained here.
http://www.mh-aerotools.de/airfoils/windmill.htm
I did see an interesting device, essentially a tethered gyrocopter, used as
a generator. That has possibilities.
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Subject: Re: Field Day Water Power
From: David VanHorn <dvanhorn@cedar.net>
Date: Tue, 17 Jun 2003 17:04:54 -0500
X-Message-Number: 29
>So are good wind turbine blades.
Yes, but the curve is to the other side.
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Subject: Re: Field Day Water Power
From: Alex <kr1st@amsat.org>
Date: Sun, 15 Jun 2003 01:43:29 -0400
X-Message-Number: 30
Amazing. A discussion, or expression of a view, on hamradio is considered
off topic.
73,
--Alex KR1ST
(consistence n. firm coherence in applying principles or a policy; my
extension: ...exercised at convenience and dependent on who violates
policy)
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Subject: Re: Field Day Water Power
From: Bob Bruninga <bruninga@usna.edu>
Date: Tue, 17 Jun 2003 18:38:25 -0400 (EDT)
X-Message-Number: 31
On Tue, 17 Jun 2003, David VanHorn wrote:
>>Hardly any difference at all. A good prop is also a good turbine. It is
>>almost 100% reversible (under identical design conditions)..
>
>All the windmill engineers would disagree with you.
My 100% here means ALL "pump type devices" will work backwards as a
turbine. I did not mean to imply that they were 100% efficient that way.
They wont work as well as for what they are designed, but they will be
within the same ball park which is my point.
I was disagreeing with your original premise where you were saying they
were totally different and comparing a 32 bladed windmill to a 2 blade
prop.. Yes, totally different, but not because of Pushing versus pulling,
but because of RPM and Torque. There are plenty of very high efficiencyy
2 bladed prop wind turbines.
>Right, but you'll end up with about 2/3 of the total available energy.
>(you made me pull out a book!)
If my junk "what-you-have-on-hand" pump run backwards is 2/3rds as good as
a properly engineered turbine, then I am very happy with it and it is
"working" as a turbine which is my only point.
>>For a given speed of water, and a given pressure, and a given RPM, a
>>bladed "thingy" should work about the same whether it is driving the
>>water, or the water is driving it...
>
>Airfoil designers disagree.
>Some of the subtleties are explained here.
>http://www.mh-aerotools.de/airfoils/windmill.htm
Ah, but I think we are saying the same thing. I fully agree that there
are subtleties and they will affect performance, but I was never claiming
"optimum" efficiency. I was claiming that a Pump will work like a
turbine in almost all cases when the flow, and pressure and RPMs are
similar... If it works within 2/3rds of perfect, then I consider that a
home-grown solution obtained for free is a "working" implementation...
Bob
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Subject: Re: Field Day Water Power
From: Bob Bruninga <bruninga@usna.edu>
Date: Tue, 17 Jun 2003 18:40:35 -0400 (EDT)
X-Message-Number: 32
On Tue, 17 Jun 2003, Jim Sanford wrote:
>This is where I disagree, Bob. Look at the difference between a turbine
>(steam or gas) and a centrifugal pump. Yes, a pump can probably be made
>to work as a water turbine, but I think a proper design will work MUCH
>better.
Ah, I agree. Now all we are quibbling over is what "much" means... and
free versus $600 to $900...
<grin>
BOb
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Subject: Re: Field Day Water Power
From: David VanHorn <dvanhorn@cedar.net>
Date: Tue, 17 Jun 2003 18:18:09 -0500
X-Message-Number: 34
>My 100% here means ALL "pump type devices" will work backwards as a
>turbine.
Depends on how loose you're willing to be with the definition of "work".
Spin, yes. Generate useful output power, maybe. Payback on investment,
almost certainly not, though that's probably not important here.
>I was disagreeing with your original premise where you were saying they
>were totally different and comparing a 32 bladed windmill to a 2 blade
>prop..
I wasn't
A 2 bladed heli rotor is designed differently than a 2 bladed windmill
rotor. Neither will do the other's job well, though they both can be made
to "work", if you're willing to define that loosely enough.
I've built model turbines with symmetrical heli blades. They are pretty
impressive until you load them. They usually need an external start, and
once started, quickly spin up to where their drag losses equal what they
are taking out of the windstream. But when you load them, even lightly,
they die.
>If my junk "what-you-have-on-hand" pump run backwards is 2/3rds as good as
>a properly engineered turbine, then I am very happy with it and it is
>"working" as a turbine which is my only point.
The 2/3 figure is the theoretical max that an optimally designed wind
turbine (or water turbine) can extract from the stream. Non optimal
designs are more likely <10%
>Ah, but I think we are saying the same thing. I fully agree that there
>are subtleties and they will affect performance, but I was never claiming
>"optimum" efficiency. I was claiming that a Pump will work like a
>turbine in almost all cases when the flow, and pressure and RPMs are
>similar... If it works within 2/3rds of perfect, then I consider that a
>home-grown solution obtained for free is a "working" implementation...
I think you'll be unpleasantly surprised.
For one thing, the nozzle design is important in a pelton wheel, but a
centrifugal pump doesn't even have one.
Some designs reverse better than others. A centrifugal pump is shaped like
a pelton wheel, but it isn't a pelton wheel. A pelton wheel makes a lousy
centrifugal pump BTW.
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Subject: Re: Field Day Water Power
From: "Mike Yetsko" <myetsko@insydesw.com>
Date: Tue, 17 Jun 2003 20:49:35 -0400
X-Message-Number: 35
>My 100% here means ALL "pump type devices" will work backwards as a
>turbine. I did not mean to imply that they were 100% efficient that way.
>They wont work as well as for what they are designed, but they will be
>within the same ball park which is my point.
Actually, some won't. I've seen pumps that had NO impeller blades.
There were disks that spun. The water filled the chamber from the middle,
and the friction (I guess you'd call it that) of the disk when they spun
was what flung the water out to the rim, which had an ring type collector.
The pump was designed as a 'debris' pump. In other words, if something
got in there that fit through the pipe, there was nothing in the pump to
'catch' it or slice it up. No impellers at all. Worked surprisingly
well.
If you reverse flow there are NO blades to catch the water flow and
direct it. And there was VERY little tangental movement of water at
the rim of the disk, as there was a curcular 'chamber'. Reversing flow
just means you get water flowing over a disk to a center exit pipe.
Mike
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