The prototype of this mill has a high angle and thus a low TSR - already a loser.  But it also has no twist.  So the bulk of the drive is occurring on the middle and inner regions (which have smaller swept area than the outer regions and thus little power to collect) while the outer portion of the blade is actually acting as an air brake.

The comparison is between this inefficient blade with the holes plugged, and the same blade with the holes open, allowing airflow from the hub to exit near the ends.  This airflow is apparently intended to give the blade a push, something like a water sprinkler or a hero turbine, or make the flow remain laminar when it would have otherwise peeled off due to the improper angle of attack.  But what it's actually doing, IMHO, is breaking the suction in the outer section, making it act as a MUCH LESS EFFICIENT airbrake.  Sure it works better than the same non-optimum blade in its solid form.  But it's nowhere near what a good twisted blade design would do.

Also:  Air collected at the hub has to be accellearted sideways as it travels toward the tip, and this retards the motion of the blade.  To get that momentum back will be an interesting play.

I doubt designs of this, or ANY, form, will ever produce a significant improvement over a properly-designed, high-TSR, twisted blade.  You need to find a flaw in Betz' work to do that.

Also,  Ram Jets Tips.

Regards

This is an interesting concept - however please consult the following sources and extra material:

1: The famous Dr Ulrich Hutter who as the then leader and chief designer of the German firm of Schempp-Hirth in the period just after WW2 designed and built a 3meter single-blade high-speed wind turbine using this suction principle.
[source:  "Wíndkraft gestern und heute" - Karl Handschuh - Oekobuch Verlad Germany 1991 - ISBN 3-922964-33-8

2: The famous French-British Enfield-Andrieu 100 kW 25 meter diameter two-bladed wind turbine from the late 1950's and later re-erected and successfully operated for many years in Algeria.
[source:  "Vetroenergeticheskiye agregaty" Ya.I Shefter - Mashinostroyeniye Press Moscow USSR/Russia 1972.
translated as - "Wind-powered Machines" - Ya.I. Shefter - NASA TT F-15,149 USA 1974.

3. : Different research papers by Sandia USA/Canada for Darrius-rotors - includingthe report - "Vertical Axis Wind Turbine Power Regulation Through Centrifugally Pumped Lift Spoiling" - P.C. Klimas, JF Sladky - Sandia USA 1985.
4. :  And of course the different experiments with ram-jet blade-tips for lift-off rotors for different types of aircraft - done in the USA and UK during the 1950's and 1960's - inspired by aircraft design-concept work done in Germany during the latter part of WW2.

5:  A paper from the middle of the 1990's by the German Prof Dr. Ing. Ludwig Elsbett [of diesel engine fame] showing an illustration of a possible technology in - "Das Kohlendioxid-Verursacherrecht - Co2 ist der wertvollste Rohstoff des Lebens".
[source:  "Der Pflanzenol-motor fur die Naturwirtschaft" - yearly edition Elsbett-Umwelt-Technik - Roth Germany]

6:   Recent interesting work done by Prof. Salter from Edinburgh University in Scotland UK - concerning a specialized type of Darrius rotor-technology - and presented at a conference in Crete in 2002.

I have earlier written that there are some
other research papers indicating that
you might somehow utilize air currents
from the middle to the tips of the blades.

Betz limit is based on the idea that
the air goes straight through the turbine
and that idea is probably not valid.

An extract from my previous post
http://www.fieldlines.com/comments/2004/4/26/93024/1936/13#13

************
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.

...

*********

I'm not sure, if it is the
best way to put the air going
through the blades.

I've been supporting the idea that
it might help if you'll make
the blades wider on the tips than
on the roots.
(Contrary to the common practice.)

But I hope that further research
will be done on this jet principle, too.