Vawt Thoughts

[vawtman]

First off i dont understand why vawts like the Lenz would be made to a wider diameter vs height.I would think more height vs diameter would be better for his turbine.Ed thoughts?

 Speed is what we are looking at to generate power.If you increase the diameter just for swept area sake most of the wind just blows through and the turbine runs too slow in common winds and puts more stress on the upwind blade.Seems like common sense to me(thats another story).Then when the wind blows real hard the blades will have alot to chew on and self destruct.

 Ive been inside my turbine in 30mph winds and I bet an anemometer wouldnt spin if so jerky from pulses.Gotta try that.

 Has far has wasting limited resources goes(ron).I remember Zubbly stating once."it keeps me out of the tavern" I could blow more money in a couple hours in the bar than i spent so far.

 In ending there becomes a rpm  at a certain speed depending on the amount of blade material on the periphery that a vawt will go into stall because it has overpowered the wind and not overspeed.

 My thoughts and not giving up just because it wont work.ITS FUN

[windstuffnow]

  Vawtman, I think the overspeeding problem might be more detremental to the construction of the turbine.  My roof top will overpower the alternator in a 25 mph wind and starts to run away.  I've clocked it at over 600 rpms with the alternator shorted.   With the Lenz wing design there is an inherent drag as the wind increases but, as well, the downwind side is all drag.   So even if the upwind side is adding drag in the increasing wind speed its also producing lift as well as catching the wind on the downwind side.  Even though the drag increases, lowering efficiency, it doesn't offer enough resistance to keep it at a reasonable rpm in a 40+ mph wind.   Going to a larger diameter can accomplish this by running at a much lower rpm.  It also need to be much stronger as the centrifugal forces are much higher even at lower rpms.   Unfortunately, it doesn't cure the "run away" problem as would a furling system.   The larger diameter machine would require a fairly hefty braking system or an added load such as a centrifugaly controlled eddy current generator to maintain a reasonably high load on the machine and mainting a safe rpm.   An 8ft diameter x 8ft tall L2 in a 40 mph wind would be producing about 10 hp at the shaft, not something easy to deal with.   In storm winds of around 60mph your talking about 35 hp ( more than my 4cyl diesel in my tractor ).   Even though its running at considerably slower speeds it still has the potential to break things.

  If your working with smaller turbines its easier to go with a smaller diameter for the rpm gains because its much easier to build a direct drive alternator for them.   The larger diameter turbines require an overdrive to some extent, only because its much more practical than building an extreemly large, very expensive alternator to run at those low speeds.   The redrives I've experimented with aren't that bad in the efficiency range but they do need to be strong ( as mentioned in the HP range for high winds ).   Still, pricing a nice cog belt system with a smaller alternator is far less expensive than huge discs, high magnet counts and many pounds of wire.

  It is a whole bunch easier to toss together an HAWT with the high speed blades, smaller alternators and the furling systems all worked out... I guess for me its simply the challenge and its different.  Their more fun to watch spin and in high winds they intensify your adrenalin rush as you ponder the speed at which it will eventually fatugue and break.  

  I'm just rambling on here, its cold and extreemly windy outside and I'd rather be in the shop working...

.

[Ungrounded Lightning Rod]

Speed is what we are looking at to generate power.

Actually, horsepower is what you are looking for to generate power - RPM times torque.

It's convenient to have high speed at the gap of the genny - the higher the speed there the more power you can get from a given set of magnets.  But building a genny - or a genny plus transmission - that is matched to the torque and RPM of your turbine is a separate issue from designing the turbine to collect power.

If you increase the diameter just for swept area sake most of the wind just blows through and the turbine runs too slow in common winds and puts more stress on the upwind blade.

You'd think that, but it's wrong.

The key is that the blades are MOVING.  They affect (and draw power from) the air for a significant distance upwind and downwind of themselves, not just the tiny bit of air that actually touches them.  So when they move across ("sweep") an area, they draw power from a slug of wind the crosswind width of the area (plus a tad) and for a significant up/downwind depth.  While they're out of the way the slowed slug blows downwind and another comes in from upwind.  Then another blade comes by and pulls power from THAT slug.

Design your blades that they come around just as, or just before, the old slug clears the active zone and you get to pull power from all the air that passes through the swept area.  As the wind speeds up the blades speed up in proportion so the geometry of the blades and slugs remains the same.

Similarly, scale up both the radius and the blade width and you continue to sweep power from the scaled-up cross section.  The speed goes down, but the torque goes up to more than compensate.  Double your radius while retaining the same height and you cut your speed in half and multiply your torque by four.  (One factor of two on the torque comes from doubling the lever arm, the other from doubling the width of the blades.  Scale up by 2 in all three dimensions and your speed still goes down by x2 but now your torque goes up by x8.

That's why, though a turbine may only block light through a tiny amount of the cross-section, it pulls power from all of it.  (It also is why, when it's spinning under load, it has drag like it was nearly a solid barrier across the whole cross-section.)

[Ungrounded Lightning Rod]

One factor of two on the torque comes from doubling the lever arm

which doubles the width of the slug from which a blade draws power, as well as doubling the torque for a given force on the end of the lever,

the other from doubling the width of the blades

which doubles the depth of the slug from which a blade draws power, as well as doubling the force the blade puts on the end of the lever.

[Stonebrain]

It is maybe worth to mention that tsr and efficiency

of a rotor depends on solidity.

I think,for every type of blades there is a optimum solidity

I solidity is too high,every blade will not have 'clean' airflow.

If solidity is too low,the blades will harvest only part of

the energy ffrom the moving air.

So,ffor any type off blades,it's worth to do some 'research'

to ffind out this optimum solidity.

For darius-lift-type rotors this 'solidity approach' is quit

straightforward.For 'savonius-drag-type' rotors,where the solidity

is often over 100% it's more complex and an 'optimum'design is

very much dependent on the shape,which is diffficult to quantiffy.

Just a few thoughts from mine.

cheers,

stonebrain

[vawtman]

Hello Ed

 I just plugged some numbers into Reg's calculator for an 8ft x 8ft swept area.

 I set the efficiency at the Betz limit and the windspeed at 15m/s the highest it would go and it spit out 14kw.WOW

 Now i see why the geared up 5hp conversion didnt effect the rotor speed in higher winds.

 Thanks for the info.and no i dont expect 14kw my goal has been 2kw.

[vawtman]

ULR Thanks for your reply.

 Solwind turbine claims to go into stall at 25m/sec im pretty sure they mean rotor speed not windspeed.

 http://www.solwind.co.nz/vertical.html

 The blades on mine are 5.5in thick with a chord of 24in.

 Now im thinkin when i add 2 more blades the solidity would be~ .3, 8ft of airfoil with a perimeter~25ft.

 Is there a calculation out there that could predict stall for this?

 That little turbine in the background seems to reach its max rpms in 30mph and seen 60mph winds.

 Maybe ill just have to suck in my gut and throw another 50bucks of my limited resources and add the blades and see.LOL

[Norm]

  Speaking of solidarity...

remember the one that had himself

convinced that the spaces between

the blades was wasted space....

I don't think any of us ever convinced him

otherwise

           ( :>) Norm.

[vawtman]

Speaking of solidarity... Wasnt that a polish union thing?Givin ya crap Norm and just kidding.

However i think unions are a waste of space.