danf's article was
a little discouraging for me
as i struggle with my design.
if anyone has a functional unit
now is the time to stand to.
DanF's article is very nice.
But the picture on the page 21
describing the power of different
wind machine designs seems to be
a wrong one especially
concerning Savonius rotors.
The correct knowledge
is available from G. Johnson.
http://www.eece.ksu.edu/~gjohnson/
---
I wanted again to point out here
the same thing, which I've
posted on this site
already earlier.
----------------------------
Most of the people
have got a wrong opinion
about the power coefficient
of Savonius rotor.
-----------------------------
According to American
research it is actually
quite good.
Unfortunately
the efficiency is put
wrong in most text books
about wind energy.
See below.
-----
From:
Wind Energy Systems
by Dr. Gary L. Johnson November 20, 2001
http://www.eece.ksu.edu/~gjohnson/
----
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 five
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.
------
------
The correct picture of
the power curves of the different
wind machines
is published on the next
page of Jonson's book.
Please see it there.
---
Also it is not correct
to say that that all successful
electricity producing
modern wind machines are HAWTs.
For example 'Windside' from Finland
is a VAWT.
At present I know
only about one a little bit bigger
installed machine of this type.
The machine is in Raisio, Finland.
It has got two helical rotors
as coupled together giving
the rated power 50 kW.
That machine seems
to be quite efficient.
For example during one year
that machine produced
about 400 kWh/square meter.
It was the best wind machine
installed in Finland
during that year.
The best HAWT type produced
about 300 kWh/square meter
during the same year.
- Hannu
