a)
I think that actually
the easiest solution
for HAWTs might be
to use many blades
something like 6 - 12
as Claus Nybroe is using.
Then the speed of the rotor is not changing
much even if the speed of the wind changes.
---
b)
Then I'm thinking according to our experiences
here and according to some other peoples'
experiences that it might be good to
use such kind of blades, which are wider
on the tips than on the roots contrary
to the common practice.
Recent theories such as the so-called GGS model
as well are supporting those views:
---
From an article:
Alexander N. Gorban
Professor and Deputy Director,
Institute of Computational Modeling,
Krasnoyarsk, Russia Assoc. Mem. ASME
Alexander M. Gorlov
Professor Emeritus,
Hydro-Pneumatic Power Laboratory
Northeastern University,
Boston, MA 02115
e-mail: amgorlov@coe.neu.edu Mem. ASME
Valentin M. Silantyev
Graduate Student,
Department of Mathematics,
Northeastern University,
Boston, MA 02115:
Limits of the Turbine Efficiency
for Free Fluid Flow
...
The most interesting finding
of our analysis is that
the maximum efficiency of
the plane propeller is
about 30 percent for free fluids.
This is in a sharp contrast
to the 60 percent given
by the Betz limit,
commonly used now for decades.
It is shown that the Betz
overestimate results
from neglecting the curvature
of the fluid streams.
We also show that
the three-dimensional
helical turbine is more efficient
than the two-dimensional propeller,
at least in water applications.
Moreover, well-documented
tests have shown that
the helical turbine
has an efficiency of 35 percent,
making it preferable
for use in free water currents.
@DOI: 10.1115/1.1414137#
...
The principal assumption of
the Betz model was that
the fluid flow remains rectilinear
when passing through the turbine
and maintains a uniform distribution
of the fluid pressure on the turbine.
Such a distributed load leads
to overestimating the forces
and torque applied to
the turbine and, as a result,
to overestimating the turbine's
power and its efficiency.
In reality, the fluid streams
are deflected from the rectilinear
direction near the barrier,
changing their motion to curvilinear
trajectories and reducing
their pressure on the turbine,
as can be seen in Fig. 1~b!:
By taking account of
the curvilinear trajectories
for the streams,
one obtains a more correct
turbine power and efficiency limit.
...
1.2 Suggested Model for Curvilinear Flow.
A new model - called
the GGS model for plane turbine
in free flow with curvi-linear
streams is shown in Fig. 1~b.
Comparison between the Betz
and the GGS models demonstrates
the following.
In the Betz model, Fig. 1~a,
the resultant force is
applied to each propeller
at the center of pressure,
which is a distance R/2
from the axis of the turbine,
where R is the turbine radius.
In the GGS model, Fig. 1~b!,
the resultant force is applied
at the center of pressure
that is calculated to be
a distance 0.37R from
the turbine axis,
much closer to the turbine shaft.
In both models,
the lift and drag components
of the resultant forces
develop the torque that
rotates the turbine.
It is easily seen that
the torque is greater
in the Betz model
than in the GGS model
as a result of the difference
in the lever arms.
Laboratory tests and
measured efficiencies
of operating turbines
often confirm that
the Betz limit is too high
for both hydraulic
and wind plane turbines.
In particular,
comparative performance
of various hydraulic turbines
in free flows shown
in Fig. 2 supports the
thesis that the Betz limit
highly overestimates the
propeller capacity
when used in the water.
The same comparison leads
to the conclusion that
the three-dimensional
helical turbine would
be preferable to any plane
propeller in free water flows.
The non-constrained helical
turbine has exhibited
an efficiency of 35 per-cent,
for example,
in well-documented
hydraulic tests,
and is superior to other
known hydraulic turbomachines.
The mathematical formulation
of the problem for plane turbine
in free flow, its definitions,
and exact solution
will follow after Section 2.
---
So:
if using horizontal axis
wind machines
I think that to make
the blade tips wide
might help.
---
c)
On the other hand I've
started to suspect that
in general well-designed
VAWTs might outperform
HAWTS...
There are other articles
besides the above
referred supporting this view.
Especially
the well-known (?) book
by Dr. Johnson,
which I have often mentioned
here is one
important text.
d)
I have started to think that
basically Hugh with his view
about 'no-iron' generators must be right.
If the magnets are cheap enough
something like a dual disk rotor
alternator for a slow
running multiblade or VAWT
might be the best
solution...
- Hannu
