Note: Descriptions are shown in the official language in which they were submitted.
1212046
Balanced Stress Vertical Axis Wind Turbine
Abstract of the Disclosure
Curved airfoils are mounted on members attached to a vertical
shaft. The ends of the airfoils point outward from the shaft,
The members are attached to the upper an lower section3 of
the airfoils and converge as~hey approach the haft. This
minimizes the shaft length above the turbines upper most
bearinc and provides a balanced condition that distributes
force in such a manner which reduces airfoil stress.
The invention relates to vertical axis wind turbines having
airfoils radially disposed about their axis of rotation, wherein
19 airfoil stress is balanced against reaction forces. The length
of-t~e shaft above the upper most bearing of a vertical axis
wind turbine;having no support structure above its airfoilsi
is usually one half the length of its airfoils This results
in a large bending moment on the shaft 7 hence its diameter
must ,be made large for the power it handles. In this type of
turbine members are used to connect the airfoils to the vertical
shaft. In the vertical axis wind turbine using a troposkein
shape, a supporting structure such as guy wires must be
employed or a very thick shaft used. In the present invention
airfoil stress due to centrifugal forces is balanced against
reaction forces set up in the members used to connect them to
i shaft.To provide this condition these members must -
converge as they approach the s'haft.This will permit the use of
a relatively short shaft above its upper most bearing,thus
reducing shaft diameter and bearing size.
An objective of the invention is to reduce airfoil stress as
com~r9d to what it would be in a vertical axis wind turbine
using straight airfoils.
Another objective is to reduce shaft diameter and bearing
size.
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An ob~ectlve of the invention is to provide a vertical axis wind turbine
whlch has a relatively small diameter vertical shaft.
Another objective is to reduce airfoil stress as compared to what it
would be in a vertical axis wind turbine using straight airfoils.
Also another ob)ective is to reduce airfoil stress when the wind turbine
is parked.
Still another objective is to provide a cost effective wind tubine.
Fig 1 LS a front view of the present invention showing a
balanced strews vertical ax;s wind turbine.
Fig 2 ls a plan view of fig. 1 through section A-A.
Fig. 3 is a vector diagram of forces on the wind turbine of fig, 1.
Fig. 4 is a partial front view of a vertlcal axis wind turbine employing
reinforced interconnecting members.
Fig. 1 shows a vertical axis wind turbine 1 ~av:ing an airfoil 2 attached
by members or arms 3 and 4 to shaft 5. Airfoil 8 is similarily attached to
shaft 5 by members 6 and 7. Shaft 5 rotates in bearings gand 10 which
are mounted on tower ll.Members 12 and 13 are attached to airfoils
2 and 8 respectively and are also connected to shaft 5; they are optional.
Member 14 is attached to members 3 and 6, it is also optional.
The operation of alrfoils 2 and 8 is identical, hence airfoil 2 will only
be dlscussed.
When wind turbine 1 rotates centrifugal forces tend to pull airfoil 2 away
from shaft 5. The middle section of airfoil 2 tends to buckle outward due to
this centrifugal force. MeTr.bers 3 and 4 tend to oppose the outward
buckllng of airfoil 2, since they converge on shaft 5. Members 3 and 4
also tend to come together as a result of centrifugal force. The upper
and lower ends of airfoil 2 away from lts points of attachment to members
3 and 4 also react against the bu ckling of airfoil 2 s middle section
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Members 3 and 4 maybe airfoils. thls will reduce drag and provlde some
lift .
Weight distrubution, the curvature of airfoil 2, the points of attachment
of members 3 and 4 and their angle with respect to airfoil 2 can be set to
provide a balanced condition. This will minimize stress.
Fig. 2 shows airfoil 2 attached by members 3 and 4 to shaft 5.The arrows
20,21,22 and 23 are force vectors. Airfoil 2 is subdivlded into end
sections 24 and 26 and middle section 25.Arrows 27, 28, 30 and 31 are
mo-nents and point 29 is the center of airfoil 2.
The vector 20 due to centrifugal action is greater near its end, slnce
they are further away from shaft 5 ;han airfoil 2s center. The end sections
24 and 26 tend to pivot about the points of attachment of members 3 and 4
respectively. This is due to the different strength of force vectors
acting on them. This results in moments 27 and 31 being developed
about the points of attachment. The centrifugal forces acting on center
section 25 tend to cause it to buckle outward producing vector 21.
The tension in members 3 and 4 is broken down into horizontal forces 23
and vertical forces 22. Vector 21 is opposed by vector 22 and moments
27 and 31 in sections 24 and 26.
In the middle section the vectors are longer near the points of attachment
to members 3 and 4 than near center 29. This results in moments 28 and 30
being developed about center 29 of airfoil 2. The more the curvature of
airfoil 2 the larger are the moments 28 and 30.
Moments 27, 28, 30 and 31 and vector 22 opposes ~.rec~or 21. They can be
made to balance each by the proper selection of parameters . thus
minimizing airfoil stress.
Outer sections 24 and 26 maybe eliminated. In this case moments 28 and
30 and vector 22 can be designed to cancel vector 21.
Also members 3 and 4 contribute to vector 21.
on To minimize airfoil stress no attachments should be made to the
outer enr1s of sections 2l- end 2~
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In order for vector 22 to exit members 3 and 4 must converge
on shaft 5. This reduces the length oE shaft 5 above the
upper most bearing 9. The moments on this shaft are con-
siderably less than that of a wind turbine using parallel
members to attach airfoils to its vertical shaft. This
allows the diameter of shaft 5 to be minimized, simultaneously
reducing the size of bearings 9 and 10.
The airfoils assume a simple curvature as a result of cen-
trifugal forces. If straight airfoils are employed they
would assume a compound curvature, due to centrifugal action.
Their end and middle sections would be pulled outward and
the points of attachment of the members would pull inwards.
This would result in higher stress than would be present
in the turbine of fig 1. In the parked condition the wind
forces on an up wind airfoil would not result in airfoil
buckling, as would be the case if troposkein or straight
airfoils were employed.
An airfoil parked down wind would tend to buckle outward.
This is opposed by the inward vertical force components
set up in the members to which they are attached. The
members in this situation are in tension, whereas in the
upwind case the members were in compression.
In general aerodynamic forces are less than centrifugal
forces. Parked wind loads way above cutout speed are inline
with aerodynamic forces. Also the frequency of occurance
of high wind speeds, which necessitates parking is low.
Members 12 and 13 are optional. If the airfoils 2 and 8
are sufficiently stiff these members will not be required.
However aerodynamic forces will cause some cyclic unbalance
to occur, since they are different in the down wind and
up wind positions of the airfoils. Merllbers 12 and 13 can
be employed to resist these forces. The turbines can be
designed such that when running these members are always
in tension.
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Members 12 ancl 13 could be made ridgicl compression members,
wheras members 3,4,6 and 7 could be tensile members only.
Stress can be balanced by proper design. Airfoil bending
stress however is very large when parked in heavy winds.
Member 14 maybe employed to balance the weight of airfoils
2 and 8 against each other. It would normally be in tension.
Although only two airfoils are shown, three or more airfoils
maybe similarily arranged. A single airfoil with a suitable
counter balance could also be employed.
Fig 4 shows a wind turbine 40 having an airfoil 41. Strips
42 and 43 are interconnected by rods 44 to form a ridgid
member 45 which will stand both compressive and tensile
forces. Similarily strips 46 and 47 are interconnected
by rods 48 to form a ridgid member 49. Strips 42,43,46
and 47 should have an airfoil cross section to reduce drag.
Airfoil 41 is connected to shaft 50 by members 45 and 49.
Member 51 connects the center of airfoil 41 to shaft 50.
Members 45 and 49 exhibit a minimal profile drag for a
structure that resists compression. They should be straight
as opposed to being arched, so as to minimize structural
requirements. By making members 45 and 49 ridgid they will
not buckle when a parked upwind airEoil is subjected to
a high wind, it will also minimize vibration. This is a -
perferred arrangement. Member 51 when used, may employed
to resist cyclic stresses by acting as a tensile member.
It could be made a ridgid compression member.
It is to be understood that the form of the invention herein
shown and described is to be taken as a preferred example
of the same and that various changes in the shape size and
arrangement of parts maybe resorted to without departing
from the spirit of the invention or the scope of the sub-
joined claims.
