On Dec 9, 1:26*am, Sleepalot wrote:
"Dave Liquorice" wrote:
On Sun, 7 Dec 2008 15:49:07 -0800 (PST), Weatherlawyer wrote:

So why don't they use more smaller blades higher up? It is in the 50
meter+ level the real winds begin. If the blade spends half its time
below 20 then it isn't very efficient and is subject to tremendous
flexing stresses.

Presumably because the amount of available energy is related to the swept
area?

Oooh, I know this one...

P = rho d^2 v^3

P = power (W)
where rho = air density (1.2 kg/m^3)
d = blade diameter (m)
v = wind speed (m/s)

Poo!

That's all maths to me. What are the wind speeds and how does the
number of vanes affect the system?

Weatherlawyer wrote:

On Dec 9, 1:26*am, Sleepalot wrote:
"Dave Liquorice" wrote:
On Sun, 7 Dec 2008 15:49:07 -0800 (PST), Weatherlawyer wrote:

So why don't they use more smaller blades higher up? It is in the 50
meter+ level the real winds begin. If the blade spends half its time
below 20 then it isn't very efficient and is subject to tremendous
flexing stresses.

Presumably because the amount of available energy is related to the swept
area?

Oooh, I know this one...

P = rho d^2 v^3

P = power (W)
where rho = air density (1.2 kg/m^3)
d = blade diameter (m)
v = wind speed (m/s)

Poo!

That's all maths to me. What are the wind speeds

For a 1m dia. generator (with lots of approximations)

P = v^3 Watts

So, for wind speed...

mph m/s v^3
10 5 125 Watts
30 15 3375 Watts
50 25 15625 Watts

and how does the number of vanes affect the system?

Ah, it's not for a pratical system: really, it's telling you the
energy available in the wind.
For a wind generator, wind speed is *everything*.


--
Sleepalot aa #1385

On Dec 9, 7:28*pm, Sleepalot wrote:
Weatherlawyer wrote:
On Dec 9, 1:26*am, Sleepalot wrote:
"Dave Liquorice" wrote:
On Sun, 7 Dec 2008 15:49:07 -0800 (PST), Weatherlawyer wrote:

So why don't they use more smaller blades higher up? It is in the 50
meter+ level the real winds begin. If the blade spends half its time
below 20 then it isn't very efficient and is subject to tremendous
flexing stresses.

Presumably because the amount of available energy is related to the swept
area?

Oooh, I know this one...

P = rho d^2 v^3

P = power (W)
where rho = air density (1.2 kg/m^3)
d = blade diameter (m)
v = wind speed (m/s)

Poo!

That's all maths to me. What are the wind speeds

For a 1m dia. generator (with lots of approximations)

P = v^3 Watts

So, for wind speed...

mph *m/s *v^3
10 * * *5 * *125 * *Watts
30 * * 15 * 3375 * Watts
50 * * 25 * 15625 Watts

and how does the number of vanes affect the system?

Ah, it's not for a piratical system: really, it's telling you the
energy available in the wind.
For a wind generator, wind speed is *everything*.

But the number of vanes decides the area swept, no?

My point with the large vanes is that the height changes remarkably
therefore so does the wind pressure. It must be rather a strain on the
machine.

"Weatherlawyer" wrote in message
...

snips
On Dec 9, 7:28 pm, Sleepalot wrote:
For a wind generator, wind speed is *everything*.

Weatherlawyer But the number of vanes decides the area swept, no?

No. The swept area (for the purposes of Sleepalot's example) is the disc
defined by the radius of the blades, not the area of the blades themselves.
I agree it's counter-intuitive, but for the large scale turbines used for
wind generation a few slim blades (typically three) are more efficient than
a lot of smaller ones. I think it's to do with the interference between
blades, each blade having to operate in the slipstream of its neighbours, so
adding more blades offers diminishing returns from the extra material and
cost. It also explains why the turbines are widely spaced on the ground. I
have no idea whether the old-fashioned "prairie" wind pumps were efficient
for their scale, or whether they were just designed intuitively or perhaps
simply cheaper to make and install.

Weatherlawyer My point with the large vanes is that the height changes
remarkably therefore so does the wind pressure. It must be rather a strain
on the machine.

Indeed, but that's taken into account in the engineering, and is one reason
why there are relatively few manufacturers of the very largest blades, it's
high-tech stuff.

Steve P

On Dec 10, 10:00*am, "Steve Pardoe" wrote:
"Weatherlawyer" wrote in message

...

snips
On Dec 9, 7:28 pm, Sleepalot wrote:

For a wind generator, wind speed is *everything*.

Weatherlawyer But the number of vanes decides the area swept, no?

No. The swept area (for the purposes of Sleepalot's example) is the disc
defined by the radius of the blades, not the area of the blades themselves.
I agree it's counter-intuitive, but for the large scale turbines used for
wind generation a few slim blades (typically three) are more efficient than
a lot of smaller ones. *I think it's to do with the interference between
blades, each blade having to operate in the slipstream of its neighbours, so
adding more blades offers diminishing returns from the extra material and
cost. *It also explains why the turbines are widely spaced on the ground. *I
have no idea whether the old-fashioned "prairie" wind pumps were efficient
for their scale, or whether they were just designed intuitively or perhaps
simply cheaper to make and install.

Weatherlawyer My point with the large vanes is that the height changes
remarkably therefore so does the wind pressure. It must be rather a strain
on the machine.

Indeed, but that's taken into account in the engineering, and is one reason
why there are relatively few manufacturers of the very largest blades, it's
high-tech stuff.

There is a lot of stuff that is counter-intuitive in meteorology.

Not least the above case. Why would the vanes interfere for example in
a flow that is perpendicular to their rotation?

I would have thought a block of vanes would capture all the force
available. In fact if I were deigning one I'd use the Italian
tunnelled airscrew used on some early monoplane designs.

The sort of thing still used in some jet engines.

Weatherlawyer wrote:

On Dec 9, 7:28*pm, Sleepalot wrote:
Weatherlawyer wrote:
On Dec 9, 1:26*am, Sleepalot wrote:
"Dave Liquorice" wrote:
On Sun, 7 Dec 2008 15:49:07 -0800 (PST), Weatherlawyer wrote:

So why don't they use more smaller blades higher up? It is in the 50
meter+ level the real winds begin. If the blade spends half its time
below 20 then it isn't very efficient and is subject to tremendous
flexing stresses.

Presumably because the amount of available energy is related to the swept
area?

Oooh, I know this one...

P = rho d^2 v^3

P = power (W)
where rho = air density (1.2 kg/m^3)
d = blade diameter (m)
v = wind speed (m/s)

Poo!

That's all maths to me. What are the wind speeds

For a 1m dia. generator (with lots of approximations)

P = v^3 Watts

So, for wind speed...

mph *m/s *v^3
10 * * *5 * *125 * *Watts
30 * * 15 * 3375 * Watts
50 * * 25 * 15625 Watts

and how does the number of vanes affect the system?

Ah, it's not for a piratical system: really, it's telling you the
energy available in the wind.
For a wind generator, wind speed is *everything*.

But the number of vanes decides the area swept, no?

I'm not an engineer. I'd guess that more vanes means more
torque at low wind speeds. It's similar to early aircraft: they
didn't have a lot of forward speed, so they needed a lot of wings
to get enough lift.

My point with the large vanes is that the height changes remarkably
therefore so does the wind pressure. It must be rather a strain on the
machine.

Oh yes, the stesses involved on the blades of a large prop must
be horrendous - just as a vertical dive threatens to rip the wings
off an airplane.


--
Sleepalot aa #1385

On Wed, 10 Dec 2008 10:00:03 -0000, "Steve Pardoe"
wrote:

I agree it's counter-intuitive, but for the large scale turbines used for
wind generation a few slim blades (typically three) are more efficient than
a lot of smaller ones. I think it's to do with the interference between
blades, each blade having to operate in the slipstream of its neighbours, so
adding more blades offers diminishing returns from the extra material and
cost. It also explains why the turbines are widely spaced on the ground. I
have no idea whether the old-fashioned "prairie" wind pumps were efficient
for their scale, or whether they were just designed intuitively or perhaps
simply cheaper to make and install.

Modern wind turbines work by generating aerodynamic lift on one side
of the blade in a similar way to an aeroplane's wing. The most
aerodynamically efficient design would be a single blade but the
unbalanced stress would be an engineering nightmare. Most wind
turbines have 3 equally spaced blades as this design is balanced
around it's axis orf rotaton, and has a constant turning moment for
when the wind changes direction.

The old-fashioned "prairie" wind pumps and traditional wind mills work
on drag rather than lift. They are much less efficient and designed
mainly by trial and error, it's largely a case of more blades is
better.