Author Topic: Bidirectional HAWT  (Read 8983 times)

0 Members and 1 Guest are viewing this topic.

lifer

  • Guest
Bidirectional HAWT
« on: October 12, 2015, 02:28:15 AM »
Hello,

As my location has a specific wind pattern (N-S and S-N only), it just crossed my mind if I could build a static bidirectional (no free turning) HAWT, by putting two regular three-blade assembly back to back on the same shaft.

Of course, there should be some space between them (0.5m?) as I want to put two bearing systems (one at each end of the shaft) to increase the overall stability.

Maybe I should mount them with some "phase shift", to minimize the bad influence of each other.

Actually, I want to build three such a "dual hawt" in line, as I can't build a bigger one (due to city policy constraints).

By the way, the rotor diameter will be around 2m. Do you think it could be a feasible solution?

rockhaggis

  • Newbie
  • *
  • Posts: 2
  • Country: scotland
Re: Bidirectional HAWT
« Reply #1 on: October 12, 2015, 06:34:44 AM »
Im no expert ... but would not the blades in effect cancel each other out?

Would it not be better having fixed a single blade in position to have a switching system, mechanical or electronic that swapped polarity depending on wind direction.

Plus another advantage of having a fixed orientation setup... which I will be exploiting with my VAWT ... is funneling. Directing more air flow through the blades ... along these lines ...

http://www.alternativeconsumer.com/wp-content/uploads/Ross/2009/Fall_Winter09/WindTamer__1.jpg
 

lifer

  • Guest
Re: Bidirectional HAWT
« Reply #2 on: October 12, 2015, 08:42:59 AM »
I have no experience with HAWT rotors (I'm just like you, I was only playing with VAWTs) but I thought there could be a specific position where the passive blade assembly stays "in the shadow" of the active one.

Actually, the main reason for this fixed orientation setup was to mechanically couple those three HAWTs to drive a single axial flux generator. I've already made this big (>3kW) generator hence I want to use it somehow.

Funneling is great, indeed. I read about that outer ring before and I've always wondered if one could mount that cylinder on top of the blade tips (that is, to rotate along with the blades). It could be even easier with my intended setup as there will be two blade sets to hold the cylinder. Daydreaming?!

Harold in CR

  • Hero Member
  • *****
  • Posts: 564
Re: Bidirectional HAWT
« Reply #3 on: October 12, 2015, 10:11:29 AM »

 
Quote
mount that cylinder on top of the blade tips (that is, to rotate along with the blades).

According to Marcellus Jacobs, as I read his information, back in the late 70's, When a prop/blades are turning, the air, instead of passing through the prop/blades, actually is thrown out the length of the blades. He stated, after starting with a water pumping windmill, he experimented with lesser and lesser amounts of blades, and concluded that 3 blades was the preferred most efficient system.

 I found interviews with Mr Jacobs on several different magazines and references from those magazines.

 From experience, when I was up my tower, trying to stop my 16' prop from running away, I could actually light a cigarette while standing behind the prop while it was running at near 150 RPM's.

 Don't ask why I did such a crazy thing, but, I was successful in saving the machine.

clockmanFRA

  • Hero Member
  • *****
  • Posts: 952
  • Country: fr
    • Renewable Energy creation
Re: Bidirectional HAWT
« Reply #4 on: October 12, 2015, 12:46:43 PM »
The below worked well for several thousand years in China.

The design was simple and was used as a water pump/lift for irrigation.

9211-0

I have seen records somewhere of ancient huge funnel walls on China's Plains, for directing the constant Winds on to just the down wind side.
Everything is possible, just give me time.

OzInverter man. Normandy France.
http://www.bryanhorology.com/renewable-energy-creation.php

3 Hugh P's 3.7m Wind T's (12 years) .. 5kW PV on 3 Trackers, (8 yrs) .. 9kW PV AC coupled to OzInverter MINI Grid, back charging AC Coupling to 48v 1300ah battery

lifer

  • Guest
Re: Bidirectional HAWT
« Reply #5 on: October 12, 2015, 01:58:40 PM »
If we're talking about wind concentrators / wind lens principles, the ancient ones looks like that:



A more recent alternative:



.. or state of the art ones:

9214-2

But I was talking about this "outer ring" used for some HAWT turbines:



.. and I was wondering if that ring could be tied to blade tips (instead of being in a fixed position) to rotate along them.



lifer

  • Guest
Re: Bidirectional HAWT
« Reply #6 on: October 12, 2015, 02:27:13 PM »
From experience, when I was up my tower, trying to stop my 16' prop from running away, I could actually light a cigarette while standing behind the prop while it was running at near 150 RPM's.

@Harold, that means there should be no influence if anything (be it a second blade set) is positioned behind the active blades? There's no air flow through the blade swept area?

Harold in CR

  • Hero Member
  • *****
  • Posts: 564
Re: Bidirectional HAWT
« Reply #7 on: October 12, 2015, 04:59:33 PM »

 Lifer, as I found, there is very little air passing through the swept area. This creates a low pressure area and that is what influences the tail rising UP hill as it furls. Gravity would keep the tail on a level plain.

 We had a Chevy Luv pickup we ran around with. The hood was hinged above the radiator, near the front bumper. The latch was broken in a previous crash, so, we never bothered to fix it.

 When a Large box truck or semi passed us, going in the opposite direction, that hood would lift, sometimes 8 " or more from the truck creating the low pressure area as it went by.

 I'm no scientist, physicist, weathercist or anything. I just know what I experience and try to reason out why.

 From Boating experience in my entire life, Volvo make a dual prop system on their outdrives. One set of blades was COUNTER rotating from the direction of the other. This is pulling water INTO the second and counter rotation creates more force from changing the rotation of the water.

 Mr Jacobs reasoned that too many blades caused TORQUE  for slow pumping of water, but not sufficient RPM's to run a generator.

 My thinking about that ring at the blade tips, if all that air is passing ALONG the blades, where does it go when it strikes that ring ? It HAS to become turbulent, ??

 Two props fixed with no yaw, would probably work, fastened apart and had the same pitch to catch the wind from different directions, but, I would rather let the mill yaw and accept the wind at a direct path not at an angle if it can't yaw ?

MattM

  • Hero Member
  • *****
  • Posts: 1167
  • Country: us
Re: Bidirectional HAWT
« Reply #8 on: October 12, 2015, 07:21:06 PM »
I'd think you'd want the ring to spin and the center to divert the mass out to the ring.

stofanel

  • Jr. Member
  • **
  • Posts: 51
  • Country: us
Re: Bidirectional HAWT
« Reply #9 on: October 13, 2015, 11:59:37 AM »
The ring around a propeller/windmill transforms the machine into nothing more than a shrouded propeller/windmill. These devices are slightly more efficient than unshrouded props because they cancel some of the vortex tip losses. The same mechanism makes wingtip winglets useful in combating induced drag on airplanes. That being said, the added weight and complexity makes shrouds impractical for most propellers and wind turbines. It is much cheaper to increase the swept area (slightly increase blade size) than to add a shroud.

lifer

  • Guest
Re: Bidirectional HAWT
« Reply #10 on: October 13, 2015, 02:11:42 PM »
I was thinking of using that ring (cylinder) to increase the overall stability, by connecting the two back-to-back propellers together. I could use a simple steel (circular) wireframe for that but if there's any advantage in using a steel sheet cylinder instead I'll do it that way.
 
However, the main question still remain: what about having two back-to-back propellers on the same shaft? Are they going to cancel each other? By what degree?

lifer

  • Guest
Re: Bidirectional HAWT
« Reply #11 on: October 13, 2015, 02:16:51 PM »

lifer

  • Guest
Re: Bidirectional HAWT
« Reply #12 on: October 14, 2015, 11:08:45 AM »
According to this theory Counter-Rotating Wind Turbines, I might kill two birds with one stone. ;)

Anyway, both rotors have to be upwind thus it doesn't meet my bidirectional requirement.

And here comes another question (excuse my lack of understanding about HAWT working principles): what happen when the wind strikes from behind (assuming that a HAWT is mounted in a fixed position)? Does it turns in reverse direction? Does it simply rest?

stofanel

  • Jr. Member
  • **
  • Posts: 51
  • Country: us
Re: Bidirectional HAWT
« Reply #13 on: October 14, 2015, 11:38:13 AM »
Regarding the counter-rotating blades:

The Betz limit on wind turbine efficiency applies to ALL propellers and turbines, regardless of design. The formulas used to derive turbine efficiency analyze the airflow in front of the turbine (which can be HAWT, VAWT, or anything in between) and the airflow behind the turbine. Therefore, the method or the machinery used to achieve the energy extraction is completely irrelevant. So any claim of a machine extracting even 25% more power than a well designed conventional wind turbine is completely bogus.

To answer lifer's question:

For a VAWT, there is no such thing as in front or behind a VAWT. The machine is symmetrical with respect to a vertical axis. So we need to describe the airflow as moving towards the center axis or away from the center axis. So no matter which way the wins id blowing, it will always strike the turbine on the "front".

lifer

  • Guest
Re: Bidirectional HAWT
« Reply #14 on: October 14, 2015, 12:40:04 PM »
Regarding the counter-rotating blades:

The Betz limit on wind turbine efficiency applies to ALL propellers and turbines, regardless of design. The formulas used to derive turbine efficiency analyze the airflow in front of the turbine (which can be HAWT, VAWT, or anything in between) and the airflow behind the turbine. Therefore, the method or the machinery used to achieve the energy extraction is completely irrelevant. So any claim of a machine extracting even 25% more power than a well designed conventional wind turbine is completely bogus.

Their calculation looks like that: if Betz limit is ~60%, that means (theoretically) that 40% of the wind energy pass the turbine. If you put another turbine right behind the first one, you could extract another 0.6 X 0.4 = 0.24 (24%) thus you've got 60 + 24 = 84% of the wind energy for that swept area.

Of course, you should consider the turbulence & stuff, but still you could gain some more (at the expense of two rotors).

To answer lifer's question:

For a VAWT, there is no such thing as in front or behind a VAWT. The machine is symmetrical with respect to a vertical axis. So we need to describe the airflow as moving towards the center axis or away from the center axis. So no matter which way the wins id blowing, it will always strike the turbine on the "front".

Actually, my question was about HAWTs..

stofanel

  • Jr. Member
  • **
  • Posts: 51
  • Country: us
Re: Bidirectional HAWT
« Reply #15 on: October 14, 2015, 05:47:22 PM »

Their calculation looks like that: if Betz limit is ~60%, that means (theoretically) that 40% of the wind energy pass the turbine. If you put another turbine right behind the first one, you could extract another 0.6 X 0.4 = 0.24 (24%) thus you've got 60 + 24 = 84% of the wind energy for that swept area.

Of course, you should consider the turbulence & stuff, but still you could gain some more (at the expense of two rotors).


Actually, my question was about HAWTs..

http://emmanuel.branlard.free.fr/work/papers/html/2008ecn/actuatordiskb.png

I hope this picture helps. When a wind turbine obstructs a stream tube and extracts energy from it, the stream tube expands and slows down. Any residual energy recovery requires a secondary disk that is twice as big as the front disk and it must be sufficiently far away (approx 5-10 times blade diameter) as not to interfere with the front disk. In other words, one must place another turbine twice as big and sufficiently far away in the shadow of the first one in order to extract the remaining 59% of 33% of the energy left in the stream tube. 

Reversing wind direction on a HAWT keeps the windmill turning in the same direction. However, the twist of the blades is mirrored around the resultant velocity vector giving it the properties of a windmilling propeller. It will produce some power, but at a very low efficiency.

lifer

  • Guest
Re: Bidirectional HAWT
« Reply #16 on: October 15, 2015, 03:08:02 AM »
Reversing wind direction on a HAWT keeps the windmill turning in the same direction. However, the twist of the blades is mirrored around the resultant velocity vector giving it the properties of a windmilling propeller. It will produce some power, but at a very low efficiency.

So if I put two HAWT rotors back to back on the same driveshaft and design one turbine's blades to rotate to the right and the other ones to rotate to the left (thus all of them rotating on the same direction, being back to back) I could get a bidirectional (fixed position) HAWT?

The upwind rotor will produce the most amount of energy while the "shadowed" one will at least not interfere with the upwind one (and might produce some energy, too).

Am I getting your point right?

stofanel

  • Jr. Member
  • **
  • Posts: 51
  • Country: us
Re: Bidirectional HAWT
« Reply #17 on: October 15, 2015, 11:38:09 AM »
Counterrotating props (or windmills) will recuperate some angular momentum imparted by the front disk. However, this setup is never used in real life in order to improve efficiency. The numbers do not justify the cost. Like I mentioned before, it is far more practical to increase blade radius by about 5%-10% or so than to build another windmill blade behind the first. That's all the efficiency gains you are likely to get by going with counterrotating blades.

Some aircraft use conterrotating props to improve dynamic stability, but not to improve efficiency. All turbojet and turbofan engines use a counterrotating configuration in order to minimize (or perfectly cancel out) the angular momentum of the rotating machinery.

lifer

  • Guest
Re: Bidirectional HAWT
« Reply #18 on: October 15, 2015, 02:50:39 PM »
Many thanks for your kind explanations, but this time I was not talking about counter-rotating HAWTs but the setup I've mentioned on my very first post.

That is, the two HAWT rotors will be built on the same driveshaft (thus their blades should rotate simultaneous in the same direction). Is it going to work?

stofanel

  • Jr. Member
  • **
  • Posts: 51
  • Country: us
Re: Bidirectional HAWT
« Reply #19 on: October 15, 2015, 04:27:49 PM »
Many thanks for your kind explanations, but this time I was not talking about counter-rotating HAWTs but the setup I've mentioned on my very first post.

That is, the two HAWT rotors will be built on the same driveshaft (thus their blades should rotate simultaneous in the same direction). Is it going to work?

Regardless on what setup you use, you cannot break the Betz limit. Whether you use one rotor or two, or 10 for that matter, if they are within close proximity to each other (which they will be since they share the same shaft), they will act as a single actuator disk. So two rotors with 3 blades will act like a single rotor with 6 blades. Air disturbances caused by any object in the flow path propagate with the speed of sound. So the front rotor is influenced by the passing of the rear rotor blades behind it and vice versa.

For more clarification go back to the streamtube image:



The air does not care what kind of machine obstructs its flow. The Betz limit says that you can only slow the air down so much (i.e. extract only so much energy) before the pressure differential between the front and the rear of the turbine forces the airflow to go around the actuator disk instead of going through it. The optimal speed ratio between the streamtube inlet and outlet is 3. This sets the upper limits on the efficiency at 16/27, which is 59%. Try to block the air any further and the wind will go around the turbine instead of through it.

stofanel

  • Jr. Member
  • **
  • Posts: 51
  • Country: us
Re: Bidirectional HAWT
« Reply #20 on: October 15, 2015, 06:29:37 PM »
I actually gave this idea a little thought. One could design a bi-directional wind turbine, assuming that the blade airfoils are symmetrical. I have not run the numbers regarding what the twist distribution might be, for I am sure that there will be some serious compromises.

Simen

  • Sr. Member
  • ****
  • Posts: 479
  • Country: no
  • Grimstad, Norway
Re: Bidirectional HAWT
« Reply #21 on: October 16, 2015, 03:58:01 AM »
I Guess a Wells Turbine might do the trick? Wouldn't be as efficient as a traditional Hawt, though...
I will accept the rules that you feel necessary to your freedom. I am free, no matter what rules surround me. If I find them tolerable, I tolerate them; if I find them too obnoxious, I break them. I am free because I know that I alone am morally responsible for everything I do. - (R. A. Heinlein)

MattM

  • Hero Member
  • *****
  • Posts: 1167
  • Country: us
Re: Bidirectional HAWT
« Reply #22 on: October 17, 2015, 01:37:34 AM »
My first sheet metal turbine 'lawn art' used a simple 2x4 frame pivoting around threaded rod.  Each of the three blades were built to the same dimensions of a 2x6 and were five feet long with two part construction.  They were fastened to a 12" wide heavy plywood rotor.  Basically each blade was a simple wedge shape with a slip joint in the back.  It didn't turn well until a storm bent the pivot so the blade leaned back about 20° off vertical.  The thing actually spun bass ackwards at blazing speed, because air was catching that slip seam like a sideways cup.  That same storm had broke off the tail, but it self corrected effortlessly to match wind changes.  It was the only way to get any velocity out of a HAWT from all the experiments we did.  We never measured revolutions per minute, but it spun to a blur in lighter winds in the 15 mph range, just like the VAWT experiments.  Eventually we took it down because it was only eight foot off the ground and we didn't want anybody to get hurt. 
« Last Edit: October 17, 2015, 01:42:40 AM by MattM »

MattM

  • Hero Member
  • *****
  • Posts: 1167
  • Country: us
Re: Bidirectional HAWT
« Reply #23 on: October 17, 2015, 01:54:52 AM »
I was trying to clarify, but the thing won't let me edit...

My first sheet metal turbine 'lawn art' used a simple 2x4 frame pivoting around threaded rod.  Each of the three blades were built to the same dimensions of a 2x6 and were five feet long with two part construction.  They were fastened to a 12" wide heavy plywood rotor.  Basically each blade was a simple wedge shape with a slip joint in the back.  It didn't turn well until a storm bent the pivot so the blade leaned back about 20° off vertical.  The thing actually spun bass ackwards at blazing speed, because air was catching that slip seam like a sideways cup.  That same storm had broke off the tail, but it self corrected effortlessly to match wind changes.  It was the only way to get any velocity out of a VAWT from all the experiments we did.  We never measured revolutions per minute, but it spun to a blur in lighter winds in the 15 mph range, just like the HAWT experiments. Eventually we took it down because it was only eight foot off the ground and we didn't want anybody to get hurt.

Maybe instead of trying to extract energy on the same perpindicular axis you use an egg beater solution so that neither blade is exactly perpindicular.  Air should flow around each one and converge between them as it passes through.  Would be even more interesting as rear mounts.