I downloaded a good book about wind energy systems
by Dr. Johnson.
Please see the discussion below:
'Wind Energy Systems'
An extract of the book:
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Chapter 1 Introduction 1 17
The main advantages of the Savonius are
a very high starting torque and simple construction.
The disadvantages are weight of materials
and the difficulty of designing the rotor
to withstand high wind speeds.
These disadvantages could perhaps be overcome
by good engineering if the turbine efficiency were
high enough to justify the engineering effort required.
Agreement on the efficiency of the Savonius turbine
apparently has finally been reached a half century
after its development. Savonius claimed an efficiency
of 31 per cent in the wind tunnel and 37 per cent in
free air. However, he commented:[10]
The calculations of Professor Betz gave 20 % as
the highest theoretical maximum for vertical airwheels,
which under the best of circumstances could not
produce more than 10 % in practical output.
The theoretical and experimental results failed to agree.
Unfortunately, Savonius did not specify the shape
and size of his turbine well enough for others to try
to duplicate his results.
A small unit of approximately
2 m high by 1 m diameter was built and tested
at Kansas State University
during the period 1932-1938[6].
This unit was destroyed by a high wind, but efficiencies
of 35 to 40 % were claimed by the researchers.
Wind tunnel tests were performed by Sandia
on 1.5 m high by 1 m diameter Savonius turbines,
with a maximum efficiency measured of 25 %
for semicircular blades[1]. Different blade shapes
which were tested at the University of Illinois
showed a maximum efficiency of about 35 %[5].
More Savonius turbines were tested
at Kansas State University, with efficiencies reported
of about 25 %[13, 4].
It thus appears that the Savonius,
if properly designed, has an efficiency nearly as good
as the horizontal axis propeller turbine or the Darrieus
turbine. The Savonius turbine therefore holds promise
in applications where low to medium technology
is required or where the high starting torque is
important. A chart of efficiency of ifve different turbine
types is shown in Fig. 8. The efficiency or
power coefficient varies with the ratio of blade tip
speed to wind speed, with the peak value being
the number quoted for a comparison of turbines.
This will be discussed in more detail in Chapter 4.
It may be noticed that the peak efficiencies of
the two bladed propeller, the Darrieus,
and the Savonius are all above 30 %,
while the American Multiblade and the Dutch windmills
peak at about 15 %.
These efficiencies indicate that
the American Multiblade is not competitive
for generating electricity, even though it is almost
ideally suited and very competitive for pumping water.
The efficiency curves for the Savonius and
the American Multiblade have been known
for a long time[6, 10]. Unfortunately, the labels
on the two curves were accidentally interchanged
in some key publication in recent years,
with the result that many authors have used
an erroneous set of curves in their writing.
This historical accident will probably take years
to correct.
----
So it seems to be the case that Mr. Savonius
was anyway right and people willing to build these
really simple turbines will get much better power from
these machines than many people have believed.
- Hannu
