The design and application of a vertical axis wind rotor is reported that operates as a two stage turbine wherein the wind impinging on the concave side is circulated through the center of the rotor to the back of the convex side, thus decreasing what might otherwise be a high negative pressure region. Successful applications of this wind rotor to water pumps, ship propulsion, and building ventilators are reported. Also shown is the feasibility of using the energy in ocean waves to drive the rotor. An analysis of the impact of rotor aspect ratio on rotor acceleration shows that the amount of venting between rotor vanes has a very significant effect on rotor speed for a given wind speed

Vance, W.

English

NASA Technical Reports Server

VERTICAL AXIS WIND ROTORS - STATUS AND POTENTIAL
W. Vance
Advanced Concepts Division
Science Applications, Inc.
La Jolla, California
Except for a rather inventive period in the 1920's, the approach
taken to extracting power from the wind has been that of using blades
or vanes rotating about a horizontal axis with the plane of the blades
essentially perpendicular to the wind velocity vector. The two devices
shown in figure 1 were patented almost 50 years ago and have as a com-
mon element a vertical axis of rotation. The first device, patented by
Msr. G. J. M. Darrieus in 1931, has received some recent study by the
National Aeronautical Establishment in Canada; the second was developed
and patented by Mr. S. J. Savonius in 1929. These rotors share a per-
formance characteristic which differentiates them from the horizontal
axis wind rotors, namely, their ability to operate equally well regard-
less of the direction of the wind. This characteristic is important
because it permits the rotor to extract the energy of a given wind or
gust instantaneously regardless of any rapid changes in wind direction.
Considering that the energy available from the wind is proportional to
the cube of the velocity, the feature of not having to take time to head
the machine into the wind may well provide additional energy extraction
capability over that of a horizontal axis rotor. It is also likely that
the elimination of a heading control and servosystem will tend to reduce
acquisition and maintenance costs and improve reliability.
Although a number of applications were developed for the vertical-
axis rotor, the concept never became popular. Horizontal axis machines
were improved over the years and have received substantial attention,
perhaps largely due to the availability and advance of propeller theory.
We believe that the time is right to take a hard look at the vertical
axis machines to see if recent advances in aerodynamics, structures, and
materials technology might not place these concepts individually (or
perhaps in combination) in a favorable light in comparison with the
horizontal axis wind rotors.
To maintain brevity, we will concentrate on the S-rotor for the
remainder of the presentation. The configuration of the original S-rotor
shown in figure 2 resulted from some 30 or more wind tunnel and field
tests conducted by Savonius wherein he varied some of the parameters of
the rotor.
Essentially, the device operates (at least during part of its
96
https://ntrs.nasa.gov/search.jsp?R=19740008661 2020-03-23T10:56:44+00:00Z
rotation) as a two stage turbine wherein the wind impinging on the concave
side is circulated through the center of the rotor to the back of the con-
vex side, thus decreasing what might otherwise be a high negative pressure
region. The flow is indicated in figure 2.
Savonius applied his wind rotor to water pumps, ship propulsion, and
building ventilators, all with some success. In addition, he also showed
the feasibility of using the energy in ocean waves to drive the rotor.
This last application was developed subsequentially as an ocean current
meter and is available commercially. Very good current measurement capa-
bility exists in a region of from 0.05 to 5 knots.
In reviewing the work that has been done on vertical axis rotors, we
have concluded that there are a number of development alternatives that
should receive some attention from the standpoint of both test and analysis.
Figure 3 indicates some of these alternatives. The effects of aspect ratio
(the ratio of rotor height to diameter) and the number of vanes will be
discussed in detail below. The issue of the profile of the rotor has not
been investigated, at least in terms of large (50 ft high or greater)
machines. Questions have arisen concerning whether more of the area of
the rotor should be at the top to catch the higher wind speeds or whether
the area should be at the bottom to provide a more uniform torque distri-
bution along the height. The rotor camber and thickness distribution also
need to be optimized. Our own limited amount of test data have indicated
that the amount of venting between the rotor vanes has a very significant
effect on the rotor speed for a given wind speed.
Figure it presents some of the results of a preliminary analysis of
the impact of rotor aspect ratio on rotor acceleration. Most of the
rotors in use have relatively low aspect ratios (refs. 1 to 3). If we
look at the rotor's ability to accelerate as defined by the ratio of the
torque on the rotor to its polar inertia, it can be shown that this char-
acteristic improves in proportion to the square root of the aspect ratio
as aspect ratio increases. Clearly, there must be limits to this trend
due to structural or other considerations. Furthermore, constant-speed
performance may impose other requirements.
Test data are shown in figure 5, which indicates the static torque
obtained for the two- and three-vaned rotors shown as a function of wind
direction. The torque diagram for the two vaned S-rotor has a consider-
able irregularity that, could make it difficult to start under some orien-
tations. The addition of the third vane smoothes the torque diagram to
some degree and apparently increases the torque per revolution, but also
increases the polar inertia of the rotor, which may offset the increased
torque when starting under low wind conditions. Whether two or three
vanes will be optimum remains to be resolved. It is also likely that the
torque diagram for a rotating rotor may be considerably different from
that of the static case described.
The S-rotor may be located in any area where a horizontal axis rotor
might be sited. However, the nondirectionality of the S-rotor may be put
to use more effectively on sea coasts where the diurnal variation of the
97
wind could be readily accepted. In considering this basic application,
it occurred to us that it might be possible to generate an artificial
on-shore breeze through the appropriate use of solar energy in the desert.
Figure 6 shows a concept of such an artifice. A set of S-rotors are
placed circumferentially around a circular area whose surface is made
such that the air over it is heated to a higher temperature than the air
outside of it. A flow will be established from outside of the heated
area to replace the rising heated air. By locating the rotors in the
throats of suitably contoured areas, it may be possible to extract con-
siderable energy from the resulting accelerated air. It is recognized
that this.is an ambitious concept. In essence, we are trying to produce
our own wind in sufficient quantities to make a cost-effective power
system. Analysis and test techniques must be developed to verify the
feasibility of this system concept.
Another application of S-rotor might be -in remote areas such as the
one depicted in figure 6. In the Arctic and many other places in the
world, empty oil drums might be used for rotor vanes. In some underde-
veloped countries it may be possible to construct the rotor from indigenous
materials. The actual siting of the rotor in a village or base camp would
depend on knowing where strong winds persisted without regard to their
direction. This simplification coupled with low costs (for the rotor)
might make the S-rotor a valuable asset to the community. It should also
be noted that vertical axis rotors might be of considerable value in meet-
ing instrumentation and power needs for research on the surface of other
planets.
In conclusion, we believe that the potential of vertical axis rotors
has not been exploited in recent years and that a comprehensive program
including design, analysis, and test could yield devices of equal (if not
better) cost-effective.performance than that of horizontal axis rotors.
We further believe that applications of these rotors should be considered
simultaneously with their development to ensure the practical utility of
the wind machines.
DISCUSSION
COMMENT: There are two units currently being manufactured in Switzerland
which use this principle. One is a 50 watt unit; the other is a 250 watt
unit. The design is a slight variation of the Savonius rotor principle.
The larger unit has been in production now for about 5 years. They are
being used very successfully, particularly on top of radio towers where
a little power is needed for a booster amplifier to take the signal down
to the house. They worked very well for that purpose.
COMMENT: The company, Electro GMBH Company, also produces the 6-kilowatt
standard generators. The man that runs that company is very interested
in vertical axis design. He has experimented a lot with them. He has
come up with, it seems, a quite successful unit for a small-scale, very
simple in design and virtually no maintenance whatsoever, and no prob-
lems with regulations in high wind, and so on. There has been one built
98
as a matter of fact, out in the Scripps Institute of Oceanography not too
far north of where we are.
Q: You speak of one of these machines being 100 feet high. What is the
Largest model you know about?
A: I haven't heard of any near that size. An important question is
whether this type of rotor be scaled up to larger sizes? My answer
is: I don't know.
Q: How big you have seen any size.
A: About 15 or 20 feet high.
Q: In the thirties one about 100 feet high was constructed in New
Jersey.
VERTICAL AXIS WIND ROTORS
DARRIEUS ROTOR
S-ROTOR
BACKGROUND
PATENTED IN 1931 (US AND FRANCE)
CURRENTLY UNDER STUDY AT NATIONAL AERONAUTICAL
ESTABLISHMENT, OTTAWA, CANADA
CHARACTERISTICS
EFFICIENCY ~ 35%
TIP SPEED TO WIND SPEED ~ 6 TO 8
POTENTIALLY LOW CAPITAL COST
CURRENTLY NOT SELF STARTING
BACKGROUND
PATENTED IN 1929 (US AND FINLAND) BY S. J. SAVONIUS
CURRENTLY USED AS AN OCEAN CURRENT METER
OTHER APPLICATIONS SHOWN FEASIBLE
CHARACTERISTICS
TIP SPEED TO WIND SPEED ~ . 8 TO 1.8
EFFICIENCY ~ 31%
SELF STARTING
VERTICAL AXIS ROTORS OPERATE INDEPENDENTLY
OF WIND DIRECTION AMD THUS HAVE A POTENTIAL FOR
HIGH EFFICIENCY IN CHANGING WINDS
FIGURE 1
S-ROTOR DEVELOPMENT
CONFIGURATION
END PLATES PROVIDE
STRUCTURAL MEMBERS
CONDITIONS FOR 2 DIMENSIONAL FLOW
COMMON ASPECT RATIOS < 3
SHEET METAL VANES
FLOW CONDITIONS
PERFORMS SOMEWHAT LIKE 2 STAGE TURBINE
FLOW TO BACK SIDE OF ADVANCING VANE
REDUCES NEGATIVE PRESSURE
SUBSTANTIALLY CONSTANT AREA FOR
AIR FLOW
DEMONSTRATED APPLICATIONS
WIND DRIVEN WATER PUMP
WIND DRIVEN SHIP PROPULSION
BUILDING VENTILATORS
OCEAN WAVE DRIVEN WATER PUMP
OCEAN CURRENT METER
FIGURE 2
DEVELOPMENT ALTERNATIVES
Number of Vanes Vane Profile
Vane Thickness Rotor Configuration
Separation
UOverlap
FIGURE 3
100
EFFECT OF ASPECT RATIO ON ROTOR ACCELERATION
Sa
© 2
ACCELERATION •> AR1/2
0 1 2 3 4 5 6 7 8
AR
ASSUMED CONSTANTS
WIND VELOCITY
BEARING FRICTION
ROTOR AREA
ROTOR WEIGHT
AVERAGE LOAD 2-DIMENSIONAL
PROVIDING TORQUE FLOW
AVERAGE LEVER ARM/ROTOR RADIUS
FIGURE 4
STATIC TORQUE PROFILE
180°
18<F
Wind
270°
FIGURES
101
Iww8CQOzil§s
^§o<K ZHH^^ H H°^S^l«S K^ogsS.8wo§SH wO
COO
n
F
K§
2
 
<;
K
 
HAR
oK
S iO
T
 g
K
 
*
O
 
2
H
 
W
o
 g
K
 
O
fc
 g
0
 
g
K
 
W
W
 
j
»
 
3HO
CO
HO
HK
g£rig
 5S
^
s
o
S
 g
o
w
CO
CO
102


Vertical axis wind rotors: Status and potential

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server