Savonius rotor blade construction particularly for a three bladed savonius rotor

A Savonius vertical axis wind turbine rotor, that is effective yet relatively easy to manufacture, includes spokes, vanes, and fasteners. Each spoke has a hub having a central opening, three arcuate ribs extending radially outwardly from the hub with inner (concave) and outer (convex) surfaces, and channels defined in at least one of the inner and outer surfaces. The vanes of sheet material generally conform to an inner or outer surface of a rib and have openings aligned with the channels. First fasteners pass through the openings and cooperate with second fasteners in the channels. Two of the spoke pieces may be joined by a bridging piece, and two spoke pieces by a clamp. A Savonius or helical rotor includes a generator, and a drive connecting the generator and rotor. The drive automatically increases the effective gear ratio between the generator and rotor as the speed of rotation increases.

CROSS REFERENCE TO RELATED APPLICATION

This application relates generally to the technology in co-pending application Ser. No. 11/113,176 filed Apr. 25, 2005, and specifically claims some of the features disclosed but not claimed therein. The disclosure of Ser. No. 11/113,176 is hereby incorporated by reference herein.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a particular construction of Savonius rotor blade, a particular Savonius vertical axis wind turbine rotor, and a drive for a driven element operatively connected to a Savonius rotor which automatically increases the effective gear ratio between the driving and driven elements in response to rotational speed of the driving element. The Savonius rotor blade, and rotor, according to the present invention have numerous advantages over prior art Savonius blades and rotors. In particular, because of the unique construction of the blades according to the present invention, a three bladed Savonius rotor is provided which can be expected to operate much more smoothly and effectively than conventional two bladed Savonius rotors, and be constructed in an overall better manner.

In the following specification and claims the following terms have the indicated meanings:“Cp” or “maximum power coefficient” means (as it normally does): Turbine torque times turbine rotational speed divided by freestream dynamic pressure times freestream velocity times the turbine swept area; or proportional to maximum power divided by swept area [that is Cp=P/[1/2 A ρ v3] where P=power, A=swept area, ρ=the density of air (about 1.2 kg/m3at sea level and 70 degrees F.), and v=wind velocity].“Tip Speed Ratio” or “TSR” means (as it normally does): blade tip speed divided by wind speed. A drag rotor cannot have a TSR greater than one.“Curvature” of a blade means: The ratio of the radius of the blade to the depth. The smaller the ratio, the more pronounced the curvature.“Skew factor” of a blade means: The maximum curvature depth location along the radius of a blade. The larger the skew factor, the closer the maximum curvature depth is to the free end of the blade.“Aspect ratio” means (as it normally does): The ratio of the length (height) of a rotor (or individual blade of a rotor) to its diameter.“Effective gear ratio” means: The rpm ratio between a driving and a driven component, whether gears or some other mechanical structure (such as chains and sprockets, pulleys and belts, cones and belts, etc.) are used to provide the operative connection between the driving and driven components.“Operatively” means (as it normally does): Any connection or engagement that allows the components connected or engaged to function as designed.

Although from the time of filing his first patent application in 1924 (see canceled FIG. 6 of GB published specification 244,414) Sigurd Savonius—the inventor of the Savonius rotor—contemplated a three bladed version as well as two bladed versions, more than eighty years later there are few [e.g. seeEnvironmental Building News, Vol. 13, #4, April, 2004, p. 7, “Solar and Wind-Powered Outdoor Lighting from MoonCell”] commercial versions of the three bladed version. Perhaps because extensive wind tunnel testing by Sandia Laboratories in 1977 [Blackwell et al, “Wind Tunnel Performance Data For Two And Three-Bucket Savonius Rotors”, SAND76-0131, July, 1977] concluded “The maximum power coefficient of the two-bucket configuration is approximately 1.5 times that for the three-bucket configuration” [Id. At p. 31], there has been almost no attempt to optimize a three bladed Savonius rotor. Conversely, there has been a great deal of work done on optimizing two bladed configurations [for example see Khan, “Model And Prototype Performance Characteristics Of Savonius Rotor Windmill”, Wind Engineering, Vol. 2, No. 2, 1978, pp. 75-85].

If a three bladed configuration of a Savonius rotor is optimized, the three bladed version can have advantages over and at least be competitive with two bladed versions. In addition to operating more smoothly, it can be just as easy or easier to manufacture; can have a Cp as great as, or greater than, two bladed versions with the same aspect ratio; and self-starts more easily. An important factor in the optimization of a three bladed Savonius rotor is the skew factor, something not even recognized as a result-effective variable for three bladed Savonius rotors in the prior art. It has been found that a high skew factor (e.g. at least about 0.6, preferably over about 0.7, and most preferably about 0.75-0.85), along with significant curvature, results in a rotor with a Cp about 2-5 times greater than those with similar curvatures but lower skew factors, e.g. 0.25 or 0.5 (about 0.5 being the common skew factor for three bladed Savonius rotors).

According to one aspect of the present invention there is provided a Savonius vertical axis wind turbine (“VAWT”) rotor comprising: A plurality of spokes, each spoke comprising a hub having a substantially central opening, three at least partially arcuate ribs extending substantially radially outwardly from the hub with inner and outer surfaces, and a plurality of channels defined in at least one of the inner and outer surface of each rib. A plurality of vanes of sheet material generally conforming to an inner or outer surface of a rib and having openings therein operatively aligned with the channels. And first fasteners passing through the openings into the channels and cooperating with second fasteners provided within the channels to securely hold the vanes to the ribs, so that the vanes assume an at least partially curved configuration presenting alternately a substantially concave and substantially convex curvature to wind as the rotor rotates about a substantially vertical axis.

The openings in the ribs are preferably non-tapped, and preferably the first fasteners comprise bolts and the second fasteners comprise nuts. Preferably, each spoke is in three pieces each piece comprising a hub segment and an arcuate generally radial rib. Two of the spoke pieces may be joined by a bridging piece, and two of the pieces may be joined by a clamping mechanism which draws the pieces toward each other to reduce the size of the central opening. Desirably a central shaft extends between the hub central openings, the clamping mechanism clamping the spoke hub to the central shaft. In one embodiment the clamping mechanism comprises a first fastener receiving element operatively connected to one of the spoke pieces at the hub segment, and a second fastener receiving element operatively connected to another, adjacent, spoke piece at the hub segment; and a fastener extending between the fastener receiving elements for drawing the elements toward each other to effect clamping.

More generally, each hub defines a clamp adapted to cooperate with a shaft so that the hub is securely affixed to the shaft. The clamp may be as described above, that is comprises surfaces of the hub defining a substantially radial slot in the hub communicating with the central opening; first and second fastener receiving elements on opposite sides of the slot and operatively connected to the hub; and a fastener extending between the fastener receiving elements to draw the surfaces of the hub together.

Preferably, the vanes generally conform to the outer surfaces of the ribs and are operatively connected thereto. Also, preferably each of the ribs has a free end opposite the hub, and a supporting element [e.g. strut or bar] extending between a central portion of the rib and a portion adjacent the free end thereof which increases the strength of the rib. Where three spoke pieces are provided, the rib of each spoke piece has a free end opposite the hub segment, and a supporting element extending between a central portion of the rib and a portion adjacent the free end thereof which increases the strength of the rib, and typically the spoke pieces are substantially identical.

The invention also relates to a substantially rigid spoke piece for a Savonius wind turbine comprising: a hub segment having an arcuate extend of roughly about 120 degrees and defining with two other spoke pieces a substantially circular opening; and a generally radial rib having a substantially convex surface and a substantially concave surface. The rib of the spoke piece has a free end opposite the hub segment, and preferably a supporting element extending between a central portion of the rib and a portion adjacent the free end thereof which increases the strength of the rib.

According to another aspect of the invention, a VAWT is provided comprising: A Savonius rotor comprising a plurality (preferably two or three) of blades having generally convex and concave surfaces operatively connected to each other, or a helical rotor. A driven element (such as an electrical generator or alternator, as disclosed in U.S. Pat. No. 6,172,429; a propeller, such as disclosed in co-pending application Ser. No. 10/443,954 filed May 23, 2003, a pump, etc.). And, a drive operatively connecting the driven element to the rotor; the drive automatically increasing the effective gear ratio as the speed of rotation of the rotor increases. [The maximum effective gear ratio is preferably at least about 10:1 when the driven element is a generator or alternator.] The Savonius rotor preferably further comprises at least one substantially vertical shaft operatively connected to the blades. Desirably, the drive directly senses rotor speed, or speed of an element operatively connected to the rotor, and does not and need not directly sense wind speed.

In one embodiment the drive comprises: A first sprocket operatively connected to the at least one shaft. Different size at least second and third sprockets, smaller than the first sprocket, and operatively connected to the driven element. A chain operatively connecting the first sprocket and one of the second or third sprockets. And a transmission comprising a centrifugal force responsive derailleur which automatically shifts the chain between the second and third sprockets. Especially where the driven element is a generator or alternator, the first sprocket and the third sprocket provide an effective gear ratio of at least 10:1, and the first sprocket and the second an effective gear ratio of less than 10:1.

While plural shaft versions of the Savonius rotor according to the invention—such as shown in co-pending application Ser. No. 10/854,280 filed May 27, 2004 (the disclosure of which is hereby incorporated by reference herein)—and other versions with spillover are within the scope of the invention, multiple shafts and significant spillover are not usually necessary when practicing the invention. That is, the Savonius rotor according to the invention may comprise a single shaft, with each spoke comprising a hub surrounding the shaft and operatively connected thereto to substantially preclude movement with respect to the shaft, the ribs extending generally radially outwardly from the hub.

It is a primary object of the present invention to provide an easily constructed and effective Savonius rotor having a wide variety of uses and used in a wide variety of manners while operating smoothly for effectively driving a number of different driven elements including a generator or alternator. This and other objects of the invention will become clear from a detailed description of the invention, and from the appended claims.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1schematically illustrates, generally by reference numeral10, an exemplary Savonius rotor for a VAWT. While the rotor10may be constructed using top and bottom discs as is common for Savonius rotors, preferably the rotor10includes a shaft11, and three blades12. While a single shaft11is preferred, a double shaft, or multiple shaft, embodiments may also be provided, as in U.S. patent application Ser. No. 10/854,280 filed May 27, 2004. Since it is the configuration of the blades12that is a major feature desirably illustrated therein,FIG. 1shows a top plan view. However, it is understood that the rotor10will have the same basic construction as illustrated inFIG. 5.

In the embodiment ofFIG. 1, the blades12are formed by a plurality of spokes13that are axially spaced along the shaft11, only one spoke13visible inFIG. 1since it is a top plan view. Each spoke13comprises three generally radially extending ribs14having a desired curvature and skew factor according to the present invention. In theFIG. 1embodiment, the ribs extend outwardly from a central hub15, which surrounds the shaft11and is operatively connected thereto (such as ultrasonically welded thereto, secured by mechanical fasteners, connected in the manners disclosed in co-pending application Ser. No. 11/113,176, or otherwise operatively connected thereto) The blades12further comprise vanes16which are shown as transparent material inFIG. 1, and extend substantially vertically between axially spaced, substantially vertically aligned, ribs14.

The vanes16may be of any sheet material suitable for use in a Savonius wind turbine, including modern engineered sail cloth such as Pentex (modified, low stretch, polyester). InFIG. 1, for clarity of illustration, the vanes16are shown made of transparent flexible material, such as Pentex, wrapped around the ribs14and the free ends thereof stitched or otherwise affixed to the main body of the vanes16to hold them taut and in operative position. However it is preferred that the vanes16are of relatively rigid sheet material (though the sheets themselves are flexible). That is, the vanes16may be aluminum, titanium, carbon fiber or other composite material, polycarbonate (transparent or opaque, transparent being particularly appropriate when the rotor12drives a boat propeller), or other suitable material having characteristics (particularly strength, weight, and manufacturability) comparable (including superior) to polycarbonate or aluminum For example the vanes16may be of the materials specified in co-pending application Ser. No. 11/113,176.

The radius of each blade12/rib14is the distance17from the center of the shaft11to the outer tip of rib14. The depth of each blade12/rib14is the maximum depth18thereof. The curvature of the blade12is the ratio of the radius17to the depth18. In theFIG. 1embodiment, the curvature is about 2.5:1. The skew factor is the location19at which the maximum depth18is located along the radius17. In theFIG. 1embodiment the skew factor19is about ¾, or about 0.75, that is the maximum depth18is located at about ¾ of the distance (the radius17) from the center of shaft11to the tip of rib14.

FIG. 2is another exemplary embodiment of a Savonius rotor component. In this embodiment, for clarity of illustration, the vane material16is not shown, only the spoke and shaft. The reference numerals inFIG. 2correspond to comparable structures inFIG. 1. As in theFIG. 1embodiment, in theFIG. 2embodiment the centers of the blades12are arcuately spaced substantially uniformly from each other around the shaft11, e.g. about 120 degrees.

In theFIG. 2embodiment, the curvature is about 5:1, and the skew factor19is about 0.75. The radius17is less than in theFIG. 1embodiment, meaning that for a particular length (height) of rotor10the aspect ratio of theFIG. 2embodiment will be greater than for theFIG. 1embodiment.

In actual testing of rotors constructed substantially according to theFIGS. 1 and 2embodiments, using a plurality of spokes13spaced along the axis defined by shaft11, with an aspect ratio of theFIG. 1embodiment of about 0.85:1 and an aspect ratio of theFIG. 2embodiment of about 1.2:1, both the rotors12ofFIGS. 1 & 2had a Cp of about 0.04. This compared to a Cp of about 0.008 for a rotor having a curvature of about 2.5:1 but a skew factor of about 0.25 and an aspect ratio of about 0.67; a Cp of about 0.022 for a rotor having a curvature of about 2.5:1 but a skew factor of about 0.5 and an aspect ratio of about 1.2:1; and a Cp of about 0.019 for a rotor with a curvature of about 5:1 but with a skew factor of about 0.25 and an aspect ratio of about 0.85:1. The Cp of theFIG. 1embodiment would very likely have been greater than that of theFIG. 2embodiment if the aspect ratios had been the same. This is because it is known in the art that for a Savonius rotor generally Cp increases as aspect ratio increases, at least up to an aspect ratio of about 3:1 (seeMother Earth News, Issue No. 28, July/August 1974 “More on The Savonius Super Rotor” by John Boll). Thus it can be concluded that a high skew factor with high curvature is particularly desirable for the blades12of a three bladed Savonius.

The blades12desirably have a curvature of greater than about 7:1 (preferably about 2:1 to 5.5:1) and a skew factor of greater than about 0.6 (preferably about 0.7 or greater, e.g. of about 0.75-0.85). Also, it is preferred that the aspect ratio of the rotor10be at least about 0.8:1, preferably at least about 2:1, e.g. about 3:1.

As with essentially all wind turbine rotors, the Cp of the rotors of the invention are at their maximum within a certain range of TSR. For example, the rotors of bothFIGS. 1 & 2will have their maximum Cps when the TSR is between about 0.2 and 0.45, gradually ramping up from a TSR of 0, and gradually ramping down form a TSR of about 0.45.

FIG. 3Ais a top plan view of one spoke21of an exemplary rotor22(seeFIG. 5) according to the present invention. The spoke21has a configuration similar to that of the spokes in FIGS. 4 & 5 of Ser. No. 11/113,176 only specifically adapted for a particularly desirable three bladed Savonius rotor22(FIGS. 4 & 5).

In theFIG. 3Aembodiment, for ease of manufacture, the spoke21is constructed in three major pieces27which may be identical, or almost identical (that is, substantially identical), and in use are arcuately spaced about 120 degrees from each other. [Alternatively, but less desirably, the entire spoke21can be formed in one piece.] Each piece27includes a rib23and a hub segment24. The hub segments24when aligned and substantially abutting—as in FIG.3A—define a complete hub, which in turn defines an open center area25. In the preferred embodiment illustrated, the open center area25is substantially circular having substantially the same diameter as a single shaft (26inFIG. 4) which it receives and is operatively connected (e.g. clamped) to. The skew factor19of the ribs23actually illustrated is about 0.78, and the curvature is about 2.6:1, and the aspect ratio of the rotor22ofFIG. 5made therefrom is about 2.67:1. The individual pieces27may be laser, water jet, or otherwise cut from sheets or plates of steel, aluminum (e.g. about ¼ to ½ inch thick), titanium, carbon fiber, or the like, or may be molded, or otherwise formed. Because of the high curvature and skew factor of the ribs23, preferably a supporting element28is also integrally formed as part of each piece27. The element28, which preferably is a strut or bar as illustrated, extends between a central portion of the rib23and a portion adjacent the free end of the rib23spaced from the hub segment24. The strut or bar28increases the strength of the rib23while minimizing the amount of material of the piece27.

To facilitate clamping connection of the spoke21formed by the three pieces27to a shaft26(FIG. 4), openings29,30are formed in the hub segments24during cutting, molding, or other formation thereof, or drilled or punched after formation. While two different shapes/configurations of openings29,30are illustrated, other components may be designed and utilized which allow openings of only one configuration.

The openings29may receive pins—such as steel or aluminum pins31in FIG.4—therein. The pins31are force or friction fit in aligned openings32of one or more bridging pieces33(FIG. 4) preferably formed of the same material as the spoke21. While the bridging piece33is shown connecting only the right and left pieces27of the spoke21ofFIG. 3, other holes and pins can be associated therewith to connect to the central piece27of the spoke21. Alternatively, two other smaller bridging pieces34,35barely visible in dotted line inFIG. 4, with associated openings and pins (not shown) connect the leftmost piece27of spoke21to the center piece27, and the rightmost piece27to the center piece27, respectively. Other fasteners besides pins31may be utilized, and other connecting structures besides the bridges33-35and openings29shown.

In order to clamp the spoke21to the shaft26, fastener receiving elements (e.g. nuts)37are provided in the openings30, like in the FIG. 4 embodiment of co-pending application Ser. No. 11/113,176. The nuts37are internally threaded and welded, force fit, or otherwise securely inserted in the openings30or otherwise attached to the right and left pieces27of the spoke21ofFIG. 3. An externally threaded fastener38connects the nuts37to each other, and when tightened moves the nuts37toward each other to clamp the spoke21to the shaft26. Other conventional mechanical clamping components may be used instead of the nuts37and threaded fastener38.

The spokes21axially spaced along shaft26(seeFIGS. 4 & 5) are operatively connected to vanes40, preferably in the same manner as in co-pending application Ser. No. 11/113,176. That is, at spaced locations along each vane40where it will cooperate with a rib23of a spoke21are a plurality of openings41, designed to receive mechanical fasteners (for example bolts42, possibly with washers43between the bolt heads and vane40).

Cut or otherwise formed into the spokes21, particularly the ribs23thereof, are generally T-shaped channels45, having a stem portion46for receipt of a bolt42shaft, and a cross portion47for receipt of a nut48. This is most clearly seen in the enlarged segment of the rightmost rib inFIG. 3A. While first and second fasteners in the form of bolts42and nuts48are preferred, other conventional or hereafter developed fasteners may alternatively, or in addition, be provided. While the channels45may be provided in the inner (convex) surfaces of the ribs23, preferably—as seen FIGS.3&4—they are provided in the outer (concave) surfaces of the ribs23.

FIG. 3Bshows a spoke21′ which is a minor modification of theFIG. 3Aembodiment (like components are shown by the same reference numeral only followed by a prime). In theFIG. 3Bembodiment, the hub segments24′ are constructed so that the central opening25′ has the correct dimensions when the openings29′ in the center piece27′ are aligned with the openings29′ in the right and left pieces27′. Thus the pins31—see FIG.4—pass through the aligned openings29′, and through the openings32in a single bridge piece33, to hold all three pieces27′ together, e.g. for pivotal movement with respect to each other. When it is desired to clamp the spoke21′ to a shaft26, the same mechanism as seen inFIG. 4moves the left and right pieces27′ toward each other, and clamps spoke21′ to shaft26.

The construction ofFIGS. 3A,3B, &4is highly desirable since it allows the vanes40to be securely held to the spokes21, without likely high fatigue points, yet the connections may be made easily and inexpensively, and the construction easily assembled (and disassembled if desired) by unskilled labor. For example, the holes41and channels45may be punched, or laser or water-jet cut, and no drilling or tapping is necessary (although it may be provided in some circumstances).

In use of the spokes21,21′ ofFIGS. 3A and 3B, an end of shaft26is placed in opening25,25′ and the spoke21,21′ (with pins31in place) is slid along the shaft26to the desired “vertical” (during ultimate use as a VAWT) location. There, the bolt38is rotated with respect to the elements37to draw elements37toward each other and narrow or close the slot between the right and left pieces27,27′. This causes the interior surface of the hub defined by hub segments24,24′ defining the opening25,25′ to tightly engage the shaft26so there is no slippage therebetween. Typically the vanes40are affixed to the spokes21,21′ after the spokes are placed in the desired position along shaft26.

In all of theFIGS. 1-5embodiments, flow directors may be provided at the top and bottom of the rotor, as illustrated in co-pending application Ser. No. 11/113,176.

If the vanes40, or at least the upper portions thereof, are made of flexible, collapsible, material, such as sail cloth, a conventional solenoid controlled clamp—shown schematically at50in FIG.5—may be used instead of the elements37,38. The clamp50may be responsive to a radio (or other remote) signal from an operator. Alternatively the clamp50may be a conventional quick release clamp that may be readily released manually by an operator.

FIG. 5shows one embodiment of a rotor22according to the invention mounted in a metal (e.g. steel) tower55. The tower55has three supporting legs56and three top cross pieces57connected to a central hub58. However any number (e.g. four or more) of legs56may be provided. The central hub58mounts a conventional bearing59for the shaft26.

At the base of the tower55is a bearing assembly60which mounts the bottom of the shaft26. The bearing assembly60may comprise both a thrust bearing and a load bearing. The rotor22drives a driven element, shown schematically at61inFIG. 5, such as an electrical generator or alternator, pump, or any other element which can be driven by a wind turbine. A drive—shown schematically at62in FIG.5—operatively connects the shaft26and driven element61with an effective gear ratio.

The drive62—shown in more detail in the schematic, exemplary, illustration in FIG.6—operatively connects the rotor22to the driven element61so as to automatically increase the effective gear ratio between26and61as the speed of rotation of the rotor22increases. One exemplary way this is accomplished is illustrated inFIG. 6. Note that the same drive62and driven element61may alternatively be used with a helical rotor, such as available from OY Windside Production Ltd. (see www.windside.com), or Windaus Energy Inc. (see www.windausenergy.com), or in U.S. Pat. Nos. 6,428,275 or 2,677,344.

InFIG. 6, the drive62comprises a first, large, sprocket63operatively connected to the shaft26for rotation therewith, and at least second and third smaller sprockets,64,65, respectively, operatively connected to a driven element61, such as a generator. A chain66connects sprocket63to one of sprockets64,65. A transmission67—shown only schematically in FIG.6—is provided to automatically shift the chain from the larger64of the small sprockets to the smaller65thereof when the speed of rotation of the sprocket63(or shaft26connected thereto) substantially reaches a predetermined level.

In one exemplary form, the transmission67comprises a centrifugal force derailleur which automatically shifts the chain66between sprockets64,65. Such a derailleur is commercially available under the trade designation “Auto Shift” in LandRider™ bicycles from Venture Cycle, LLC, Lutherville, Md. Since the “Auto Shift” derailleur automatically senses the speed of the chain66—which of course is directly related to the speed of the sprocket63and shaft26, and which is in turn generally related to the average speed of the wind acting on rotor22—no separate wind sensor is necessary. The “Auto Shift” derailleur also operates both ways, so that it downshifts back to sprocket65once the chain66speed falls below the predetermined level.

In the embodiment illustrated inFIG. 6, the effective gear ratio of the sprockets and the rotor speed at which the transmission67will shift will depend upon the size of the rotor22, the size of the sprockets63-65, the exact type of generator or other driven element61used, and other factors. In one example where the driven element61is a generator, the effective gear ratio provided by the sprockets63,64is about 7:1, and that provided by sprockets63,65is 10:1 or higher. If the rotor22reaches 50 rpm at a wind speed of about 5 mph, then the transmission67is designed to shift the chain66from sprocket64to sprocket65when it directly senses that the chain66reaches a speed comparable to a rotor22rotational speed of 50 rpm. Typically the rotational speed of shaft26which will result in the first shift will be between 10-50% of the expected maximum rotational speed of shaft26, with other shifts provided at higher speeds if desired until the maximum effective gear ratio is provided for the components involved. The increase in effective gear ratio may be incremental (as for gears and sprockets) or substantially continuous (as for cones and associated belts).

Normally at least a third sprocket69is also provided (almost any practical number may be provided, for example six or seven rear sprockets are used in a LandRider bicycle). The transmission67will automatically shift to the third sprocket69at a point where the TSR is at a certain level (e.g. about 0.25) to help maintain the rotor Cp near an optimum value.

While the particular drive62and transmission67described above provide a simple, reliable, mechanical system, other systems that are more complex and/or are electromechanical may be used instead. As one example a system as shown in U.S. Pat. No. 5,984,814 may be utilized. As another example, a conventional sensor which generates an electrical signal substantially proportional to speed may be mounted in association with the shaft26or any element operatively connected thereto. The electric signal so generated can be used to cause a solenoid, electric motor, hydraulic or pneumatic cylinder, or the like, to shift a chain between sprockets, or shift between driven gears driven by a drive gear connected to shaft26, etc. Any other conventional or hereafter developed mechanism for automatically changing the effective gear ratio between shaft26and driven element61in response to direct sensing of the speed of rotation of rotor22(or an element operatively connected thereto and moving at substantially the same speed) may alternatively be provided.

Using the drive62and transmission67according to the invention, it is possible to—without directly sensing wind speed (which may be highly variable and change too quickly)—change the resistance of a driven element connected to a Savonius rotor shaft in a manner proportional to wind speed. It is also not necessary to sense the generator input or output, although that can be done for other purposes. A Savonius rotor has high torque, but traditionally does not have high rotational speed, which is why it has not been in widespread use for generating electricity. However by increasing the effective gear ratio in response to the rotational speed of a Savonius rotor, and in a relatively simple manner, high generator output may be reached when the wind speed is high without stalling the Savonius rotor at low wind speed.

In order to change effective gear ratio to optimize Cp (that is keep the rotor within an optimum Cp range by adjusting the effective gear ratio in response to TSR), a conventional wind sensor for generating an electrical signal, rotor speed sensor for generating an electrical signal, and CPU may be connected to a CPU controlled transmission67. The CPU calculates TSR from the wind and rotor sensors, and then controls the transmission67to adjust rotor speed and thereby TSR to keep the TSR in the optimum Cp range.

FIG. 7illustrates a wind powered boat70that may use essentially the same rotor22as the rotor inFIGS. 4 & 5. The wind powered boat70comprises: A plurality of hulls71(the boat70is preferably a catamaran, trimaran, or other multi-hull). A propulsion mechanism—such as horizontal axis propeller72—operatively connected (e.g. by support73and shaft74) to at least one of the hulls71and between two of the hulls71. A Savonius vertical axis wind turbine rotor22having an aspect ratio of at least 2:1, and comprising: at least one substantially vertical shaft26; three blades (formed by ribs23and vanes40) operatively connected to the shaft26; and the blades23,40having a curvature of greater than about 6:1, and a skew factor of at least about 0.65. And, the rotor22is operatively mounted to at least one of the hulls71(e.g. by bearing75and supports76) and is also operatively connected to the propulsion mechanism72, e.g. by meshing bevel gears78,79.

The wind powered boat70—as the boat described in co-pending application Ser. No. 10/443,954—may have a manual assist80(such as a bicycle type drive for a propeller), a seat81, a rudder82, and a control stick83for operating the rudder82and operatively connected thereto, as by a lever. The hulls71may be connected together by cross pieces85, and the seat81operatively connected to the cross pieces85. The rotor22may be mounted to the rear of the seat81or in front of it (in which case the vanes40should be of transparent material). Alternatively, multiple rotors22and associated propulsion mechanisms72may be provided, one or more in front of seat81, and one or more in back of seat81.

All numerical values herein are approximate, and all narrow ranges within a broad range are specifically included herein (for example “about 0.75-0.85” means 0.76-0.856, 0.78-0.84, 0.745-0.80, and all other narrower ranges). While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment, it is to be understood that many modifications may be made thereof within the scope of the invention, limited only by the prior art, to encompass all equivalents within the scope of the appended claims.