Rotary kinetic fluid motors or pumps – Method of operation
Reexamination Certificate
2001-02-01
2003-08-05
Nguyen, Ninh H. (Department: 3745)
Rotary kinetic fluid motors or pumps
Method of operation
C415S004300, C415S004500, C415S908000
Reexamination Certificate
active
06602045
ABSTRACT:
BACKGROUND
1. Field of Invention
The present invention relates to the design of windmills. Such windmills are used to extract energy from the wind by mounting a propeller on a horizontal rotating shaft, and positioning it in a moving air stream. The rotating shaft is then connected either to a transmission or to a generator in order to convert the energy to a more useful form.
The energy available for wind power generation is directly proportional to the cube of the wind speed and to the square of the windmill propeller diameter. Paraphrasing the explanation found in
Aerodynamic Theory Vol.
4, (Durand, 1935), the following terms are defined: V is the free stream velocity, u is the velocity through the area swept by the windmill propeller, a is an interference factor relating the free stream velocity to the velocity through the area swept by the windmill propeller, expressed as u=V (1−a), R is the maximum radius of the windmill propeller, and &rgr; is the density of air. The drag is
D=
2
&pgr;&rgr;R
2
V
2
a
(1
−a
)
and the power is
P=
2
&pgr;&rgr;R
2
V
3
a
(1−
a
)
2
.
The efficiency of a windmill may now be expressed as the ratio of the power P to the work VD done against drag. The efficiency is simply,
&eegr;=(
V*D
)/
P
=1−
a,
and is maximized when a=⅓, which implies the efficiency is ⅔. Unfortunately, the available energy is reduced by friction and swirl velocity losses.
BACKGROUND
2. Description of Prior Art
The prior art consists of several configurations in which a windmill is placed in an air stream. The most commonly used method involves a propeller mounted on a rotating shaft supported by bearings mounted to a stationary (non-rotating) hub. A propeller is made up of 1 or more blades, twisted to describe a helical path, as they rotate with a hub on which they are mounted. The stationary hub is streamlined in order to minimize the disturbance to the flow and may house a generator and voltage controlling electronics. The stationary hub is mounted on a single pivot, and a conventional yaw control means is provided to point the windmill propeller in the direction of the wind. The single pivot in turn is mounted on top of a supporting structure which is high enough to insure the windmill propeller an unobstructed flow. The supporting structure is made of a steel cylinder or a steel structural-angle space frame and is mounted on a foundation. In the event the windmill is located where the wind comes predominantly from a single direction, the windmill's stationary hub would be fixed to face the propeller into the wind coming from that direction.
A disadvantage of the prior art is the high rotational design speeds of the conventional windmill propeller in relation to the free stream velocity. The speed ratio is defined as the tip speed of the windmill propeller divided by the free stream velocity. As the air passes through the area swept by the windmill propeller, it is slowed down, energy being transferred to the generator. The wake rotates in reaction to the windmill propeller torque. Analysis of the energy shows that the convected swirl is wasted. In order to reduce the convected swirl energy, high speed ratios are used which result in relatively high parasitic losses.
Several methods exist in the prior art to reduce the windmill's convected swirl energy and related energy loss. For example, a stationary propeller (stator) can be mounted either upstream or downstream of the rotating propeller. This stationary propeller will generate a counter swirl to cancel the convected swirl energy of the windmill's rotating propeller. However, this stator design involves friction losses associated with another propeller. Counter rotating designs are also found but incur added cost.
Prior art also includes vortex augmentation of the windmill wherein the propeller interacts with other aerodynamic surfaces. Referring to
Fluid Dynamic Aspects of Wind Energy Conversion,
(AGARDograph No. 243, by O. de Vries, July 1979), a delta wing was proposed to generate swirl to enhance the power generation of windmills. However, referring to
An Introduction to Vortex Breakdown and Vortex Core Bursting
, (National Research Council Canada, by J. L. Hall, 1985), a delta wing's leading edge vortex will tend to be unstable and break down into turbulence. The delta wing, therefore, is a poor vortex augmentation device.
The prior art has additional disadvantages. Conventional designs use a small hub size in order to maximize the area swept by the windmill propeller and also to avoid disturbing the flow. However, these windmill hub designs result in low relative air speeds near the hub. Since the relative speed of the air over the windmill propeller is the vector sum of the free stream velocity and the propeller's rotational velocity, the relative air speed near the hub is low compared to that near the tip. As a result, the airfoil sections of the windmill propeller near the hub are unable to extract significant energy from the flow. Some large hubs are known to exist in prior art. For example, the small windmills of less than fifteen feet in diameter used to charge batteries on sailing yachts have large hubs. These windmills have hub dimensions sized to use pre-existing “off-the-shelf” components. However, the large carefully engineered windmills have small hubs.
To summarize, typical windmills are designed for high speed ratios in order to minimize the energy lost to convected swirl velocity, which incurs relatively high parasitic losses. Typical windmills also use small hubs in order to maximize the area swept by the windmill propeller, which results in low relative air speeds near the hub and reduces the airfoil sections ability to extract energy from the air stream near the hub.
SUMMARY OF THE INVENTION
The present invention is a system and method for enhancing the generation of power with a windmill. It comprises a horizontal axis rotating propeller with a large hub, mounted on a lift-generating device which also acts as the support structure. By adding a lift-generating device around the support structure, lower rotational speeds for the windmill propeller are possible without the penalty of excessive swirl losses in the wake. The lower design speed reduces parasitic losses and improves the overall power output of the windmill. The present invention also uses a large hub to increase the relative speed of the air over the windmill propeller near the hub, further enhancing power generation. The use of a large hub also allows shorter blades to be used, which reduces blade stresses.
In the preferred embodiment, the supporting structure comprises a lift-generating device such as a wing. When placed in a flow, this lift generating device, or wing, sheds a tip vortex into the wake. The swirl generated by the wing support structure is equal and opposite to that of the windmill's rotating propeller, resulting in less total swirl convected downstream. As a result, the energy lost to swirl velocity is reduced, resulting in greater energy capture available to the windmill.
Note that a vortex shed from the tip of an efficient aircraft wing will remain substantially intact for some time. Witness the delay aircraft controllers place on small aircraft to land after a large aircraft has landed. The vortex augmentation resulting from an efficient aircraft wing is superior to delta wing vortex augmentation.
In the preferred embodiment, this wing would be much like an aircraft wing, and have a profile such as those in
The Theory of Wing Sections,
(Abbot and Doenhoff, 1959) wherein lift and drag may be determined from coefficients, and the shape of the airfoil sections determined from tables numerically describing camber and thickness. The wing support structure generates lift. Due to the three dimensional nature of the flow, a vortex is created at the tip. Swirl energy is convected downstream with the free stream flow. By designing a wing to produce swirl of equal and opposite strength to that of the windmill p
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