Wind Turbine Blade

The use of windmills to generate electrical power is well known. Not only windmills but other kinds of wind-driven devices have been used or proposed for generating electrical power. Thousands of small wind-driven generators were in operation in rural areas in the 1930's and 1940's to supply power to farmers before power lines were built. Such systems were of relatively simple construction and included a small generator directly driven by a two-blade propeller of conventional construction having a span of a few feet. Because of widely varying wind velocities, alternating currents would have been of varying frequencies, so the generator was connected to charge a bank of batteries and the power used as direct current. Where it is desired to supply and use alternating current in large quantities, the system becomes much more complex. Since a generator inherently supplies alternating current at a frequency varying with its rotational speed, a number of schemes have been proposed for controlling the feathering of propellers to keep their speed relatively constant, or within a narrow range of speeds such that other means may be used to drive the generator at a constant speed.

Where such windmills are used to generate power in commercial quantities to be supplied to power lines, economic considerations point to efficiencies in building very large windmills of diameters in excess of 40 meters. The large cost of such systems puts a considerable premium on efficiency of the installation. Used in this sense, efficiency refers to the ability of the windmill blade to extract the maximum amount of energy for the range of wind velocities experienced. To do this successfully, the windmill should be structurally capable of withstanding high wind velocities such as in the order of 70-80 kilometers per hour without requiring total feathering and shutdown of the system. It should also include feathering means for protecting the windmill from wind velocities in excess of values determined to be safe for the equipment.

One of the problem areas with windmills having a horizontal shaft is that since the wind blows substantially perpendicular to the direction of blade rotation and impinges on the blades from immediately outboard of the hub to the blade tips, there is a very great difference in blade velocity from one radial position to another. It has long since been determined that windmill blades are, in general, most efficient when constructed with an airfoil-like cross-section. With the differences in velocities along the blade, however, it is almost inevitable that one part of the blade or another will be in a condition of stall. Stalling frequently begins near the root of the blade even when the blade is rotating at relatively high speed. The stall region may then tend to migrate along the blade radially until the blade no longer has sufficient power to carry the load, and the entire blade will then stall. It is known in some designs to put twist in the blade from the hub to to the tip avoid or minimize stalling of the blade. There is, therefore, a need for a windmill blade configuration which is as efficient as possible, which minimizes stalling, and which is rugged and durable enough to operate and thereby generate power at substantial wind velocities and resulting high rotational speeds.

Applicant has designed a blade for a wind turbine which has considerable strength, which is efficient, and which provides the desired degree of lift along its entire length from the portion nearest its hub to its tip. At the inside or root end of the blade where it adjoins the hub, the blade is quite wide and has a cross section which is designed to provide a high lift at low speed. At the outside end, the blade tapers to a minimum thickness and length and has a low lift cross section similar to that of a typical stationary airplane wing which allows a maximum lift coefficient of about 1.25 to 1.5 before stall occurs. This low lift section extends over approximately the outboard one-fifth of the blade. From the root to this point, approximately one-fifth of the way from the outside end, the blade tapers smoothly from the maximum high lift configuration to the low lift cross section described above. The point at which the transition to a conventional low lift cross section occurs is subject to some variation depending upon the size and normal operating speed of the wind turbine with the transitions desirably occurring at the radius at which air speed does not exceed about 240 kph. The described high lift configuration may be similar to that observed in a large aircraft when it is about to land and has its flaps at maximum deflection and (sometimes) slots deflected, thereby allowing high aerodynamic anges of attach which provide a maximum lift coefficient near the hub of approximately 3.0 before stall occurs. The exact angular downward deflection of the leading edge contour which is similar to slots is significant but is of less importance than the greater deflection at the trailing edge. In the case of the windmill blade and using long known principles of lifting bodies, the deflection is outwardly, toward the wind, such that the wind is deflected by the blade first in a direction generally parallel to the surface of the blade and then is deflected by the trailing edge back toward its source and only turns back again toward its original direction in time to again be deflected by the next blade coming around. Very little air actually flows across the blade near its root and over a circle whose radius is about one-third of the length of the blade, but due to less inertia and thereby less resistance of the moving air mass envelope outside the blade periphery, this air tends to move outwardly along the blade before it flows across. In this way a substantial pressure differential is maintained across the inboard portion of the blade, and the air which would otherwise flow across this portion adds adds increased dynamic pressure to that flowing across the outer portion where the velocity is much higher, thus adding the blade's overall efficiency.

FIG. 1 is a perspective view illustrating a wind turbine supported on a tower having turbine blades made according to this invention.

FIG. 2 is a diagrammatic view of one of the turbine blades of FIG. 1 with a number of cross sections shown turned 90 degrees.

FIG. 2a is an enlarged view of one of the cross sections shown in FIG. 2.

FIG. 3 is an enlarged view of another of the cross sections shown on FIG. 2 .

Referring to FIG. 1, a wind turbine includes an upright tower 10 supporting a platform 12 in a horizontal position above the tower. The platform is rigidly affixed to the top of the tower. A propeller 14 is supported on the tower adjacent the platform. The propeller has three radially extending blades 16 spaced apart equidistantly about a horizontal axis of rotation. The propeller includes a variable pitch propeller hub cowling 18 capped by a streamline nose 20. A root fairing 22 for each propeller blade couples into the hub. The propeller blades extend in a vertical plane adjacent the side of the tower, and the blades are relatively long, extending for a major portion of the height of the tower.

The propeller is coupled to rotating machinery supported on the platform 12. A skin 24 encompasses portions of a generator system, propeller feather control components, and associated framework, all mounted on the platform. These portions are described in greater detail in a copending application, Ser. No. 42,496, in which the present applicant is named as a coinventor, and they include a torque tube, hydraulic cylinder, gear box, motors, and attendant line and control circuitry.

The tower is mounted on a flat base 26. A circular track 28 at the periphery of the base guides the path of the tower as it rotates about a vertical axis through a pivot 30. Rollers 31 support the bottom of the tower for rotation on the base. The propeller blades rotate about the horizontal axis in response to the wind acting on the propeller. As the propeller rotates about its axis, the entire tower rotates about the vertical axis through the pivot 30 to constantly maintain the propeller upwind of the tower.

FIG. 2 is a diagrammatic view of a turbine blade made according to my invention with a number of cross sections shown turned 90 degrees to aid in understanding the detailed configuration of the blade. In this view the blade 16 is shown somewhat foreshortened with the scale as to blade width larger than its length. The blade structure may be formed of various materials including wood or metal or a combination covered with a suitable skin which may be of glass epoxy cloth or thin metal. Where the spar and ribs are of wood, the blade has been covered with a glass fiber fabric. The blade is formed of a main spar 34 attached by suitable means to a hub on the center line of the turbine to which is attached a large number of rib members defining the shape of the blade. The main spar could be of metal such as steel even if the rib members are of wood. Since the cross section varies substantially from the root end of the blade to the tip, these ribs have substantially different shapes. For the purpose of this description it will be presumed that a rib is located at each of the cross sections shown, although this may in practice not be quite true. There will normally be many more ribs than are shown in FIG. 2. For a 25-meter blade, normal practice would call for a rib approximately every 38 centimeters.

From FIG. 2 it will be observed that the cross sections a and b shown at the outward or tip end of the blade are in the form of conventional air foils like that of an airplane wing having a relatively low maximum lift coefficient of 1.25 to 1.5 before stalling and a relatively low angle of attack for maximum lift. The dashed line shown extending directly across the widest dimension of rib section a defines the chord of the blade at this point. See enlarged view, FIG. 2a. Section c is of slightly different shape from section b and represents a transition from a conventional air foil shape to a higher lift configuration. Section i which is nearest the hub is, of course, located at the point on the blade having the least velocity through the air. Parts of the blade farther from the hub have correspondingly greater velocities with maximum blade velocity at the tip. To minimize stalling of the blade, those parts having the least velocity at normal rotational speeds are formed of a high lift cross section (producing a maximum lift coefficient before stalling of 3.0) which is similar to the cross section of an airplane wing with both trailing edge flaps and leading edge slots deflected a maximum amount. As will be seen, this is the configuration of section i, shown enlarged in FIG. 3. Where applicant refers to an "airfoil" or an "airfoil-like" configuration, a cross-section like that of a typical airplane wing is meant where the leading edge is well rounded, one surface is curved more than the other, and both surfaces come together at a fairly sharp trailing edge, as shown in FIG. 2A.

In this enlarged view of section i, the cross section is shown with a dashed outline defining the configuration of the airfoil if it were conventional (like section a) and not deflected. The chord line is shown extending across this conventional section and also as it is rotated in the actual cross section in the deflected leading and trailing edges.

Section h, g, f, e and d have progressively less deflection until at about one-fifth of the distance from the tip, the ribs are not formed with deflection but are like conventional aircraft air foils which allow a maximum lift coefficient of perhaps 1.25 to 1.5 before stall occurs. The location along the blade where the transition between deflection and no deflection takes place will vary from blade to blade depending upon various operating conditions such as tip speed in normal use, rotational speed of the turbine, etc., but this transition should occur at the location where the velocity of the blade through the air is approximately 240 kph.

Due to relative velocity of the blade rotational speed versus the wind speed, resulting in higher aerodynamics angles of attach at the root area where the wind speed is the same but velocity due to rotation is less, stalling along a propeller blade tends to be progressive, beginning near the root and moving outwardly until ultimately the blade is installed and does not develop significant power. A standard method of avoiding stall is to twist the blade near the root. With the above described configuration, the blade has an effective aerodynamic twist without physical twisting of the blade.

While the above described blade has been described in terms of a particular structure, i.e., an elongated spar with rib members attached perpendicularly thereto to determine the cross-sectional configuration, this arrangement is appropriate to a very large blade, and alternative structure may be suitable as well, particularly where the turbine is of smaller diameter. Certain of the proportions, angles and dimensions named above are exemplary of one existing design involving a blade approximately twenty-seven meters long but are not necessarily critical except where indicated.