Rotor blade and rotor having the same

Abstract

A rotor blade for a wind turbine includes a blade root, a blade tip, and a longitudinal length extending from the blade root to the blade tip. The rotor blade includes an inner portion having an inner portion length extending from a first inner portion end at the blade root to a second inner portion end, and an inner angle of attack decreasing along the inner portion length from the first inner portion end to the second inner portion end. The rotor blade includes an outer portion having an outer portion length extending from a first outer portion end proximate the second inner portion end to a second outer portion end at the blade tip, and an outer angle of attack constant along the outer portion length for a maximum lift-to- drag ratio of the outer portion at a design tip-speed-ratio and at a design wind velocity.

Description

ROTOR BLADE AND ROTOR HAVING THE SAMEFIELD

[0001] This application relates to the field of rotors and rotor blades for wind turbines.INTRODUCTION

[0002] Wind turbines convert kinetic energy of moving air particles into electrical energy. In operation, air flowing over the blades of the wind turbine generates a lift force. In horizontal axis wind turbines, the blades are oriented such that a torque component of the lift force acts to drive the blades to rotate about the horizontal axis. The rotational energy of the blades is then converted into electrical energy using a generator. Accordingly, rotor blades enable wind turbines systems to capture of a percentage of the wind energy spectrum providing a clean, sustainable energy source.DRAWINGS

[0003] FIG. 1 is a front view of an example rotor, in accordance with an embodiment;

[0004] FIG. 2 is a top perspective view of the rotor of FIG. 1 ;

[0005] FIG. 3A is a cross-sectional view of a rotor blade of the rotor of FIG. 1 at a first end of an inner portion thereof;

[0006] FIG. 3B is a cross-sectional view of the rotor blade of the rotor of FIG. 1 between the first end and a second end of the inner portion;

[0007] FIG. 30 is a cross-sectional view of the rotor blade of the rotor of FIG. 1 at the second end of the inner portion;

[0008] FIG. 3D is a cross-sectional view of the rotor blade of the rotor of FIG. 1 at a first end of an outer portion thereof;

[0009] FIG. 3E is a cross-sectional view of the rotor blade of the rotor of FIG. 1 between the first end and a second end of the outer portion;

[0010] FIG. 3F is a cross-sectional view of the rotor blade of the rotor of FIG. 1 at the second end of the outer portion; and

[0011] FIG. 4 is a chart showing an example angle-of-attack gradient of the rotor blade of the rotor of FIG. 1 from the root to the midspan.SUMMARY

[0012] In accordance with one aspect of this disclosure, a rotor blade for a wind turbine includes a blade root, a blade tip, and a longitudinal blade length extending from the blade root to the blade tip; an inner blade portion having: an inner portion length extending from a first inner portion end at the blade root to a second inner portion end, and an inner angle of attack, the inner angle of attack decreasing along the inner portion length from the first inner portion end to the second inner portion end; and an outer blade portion having: an outer portion length extending from a first outer portion end proximate the second inner portion end to a second outer portion end at the blade tip, and an outer angle of attack, the outer angle of attack constant along the outer portion length for a maximum lift-to-drag ratio of the outer blade portion at a design tip-speed-ratio and at a design wind velocity.

[0013] In any embodiment, a local aspect ratio of the rotor blade at any position along the inner portion length may be less than 4.

[0014] In any embodiment, the local aspect ratio of the rotor blade may increase along the outer portion length from the first outer portion end to the second outer portion end.

[0015] In any embodiment, the design tip-speed-ratio may be between 1 .5 and 6.

[0016] In any embodiment, at the design tip-speed-ratio, the inner angle of attack may be at least 40-degrees at the first inner portion end and at least 15-degrees at the second inner portion end.

[0017] In any embodiment, at the design tip-speed-ratio, the inner angle of attack may be between 40-degrees and 70-degrees at the first inner portion end and between 15- degrees and 25-degrees at the second inner portion end.

[0018] In any embodiment, at the design tip-speed-ratio, the inner angle of attack may be between 65-degrees and 70-degrees at the first inner portion end and between 20- degrees and 25-degrees at the second inner portion end.

[0019] In any embodiment, the inner portion length may be at least 50% of the blade length.

[0020] In any embodiment, the rotor blade may further include a transition portion having: a transition portion length extending from the second inner portion end to the first outer portion end, and a transition angle of attack, the transition angle of attack decreasing from the inner angle of attack at the second inner portion end to the outer angle of attack.

[0021] In any embodiment, the inner blade portion and outer blade portion may be integrally formed.

[0022] In any embodiment, the rotor blade may further include a leading edge, the leading edge extending the longitudinal blade length and continuously formed across the inner blade portion and the outer blade portion; and a trailing edge opposite the leading edge, the trailing edge extending the longitudinal blade length and continuously formed across the inner blade portion and the outer blade portion.

[0023] In accordance with one aspect of this disclosure, a rotor for a wind turbine includes a hub having a first hub end, a second hub end, a hub sidewall extending longitudinally from the first hub end to the second hub end, and at least two rotor blades circumferentially spaced apart around the hub. Each rotor blade has a blade root connected to the hub sidewall, a blade tip, and a longitudinal blade length extending radially outwardly from the hub from the blade root to the blade tip; an inner blade portion having: an inner portion length extending from a first inner portion end at the blade root to a second inner portion end, and an inner angle of attack, the inner angle of attack decreasing along the inner portion length from the first inner portion end to the second inner portion end; and an outer blade portion having: an outer portion length extending from a first outer portion end proximate the second inner portion end to a second outer portion end at the blade tip, and an outer angle of attack, the outer angle of attack constant along the outer portion length for a maximum lift-to-drag ratio of the outer blade portion at a design tip- speed-ratio and at a design wind velocity.

[0024] In any embodiment, a local aspect ratio of each rotor blade at any position along the inner portion length may be less than 4.

[0025] In any embodiment, the local aspect ratio of each rotor blade may increase along the outer portion length from the first outer portion end to the second outer portion end.

[0026] In any embodiment, the design tip-speed-ratio may be between 1 .5 and 6.

[0027] In any embodiment, at the design tip-speed-ratio, the inner angle of attack of each rotor blade may be at least 40-degrees at the first inner portion end and at least 15- degrees at the second inner portion end.

[0028] In any embodiment, at the design tip-speed-ratio, the inner angle of attack of each rotor blade may be between 40-degrees and 70-degrees at the first inner portion end and between 15-degrees and 25-degrees at the second inner portion end.

[0029] In any embodiment, at the design tip-speed-ratio, the inner angle of attack of each rotor blade may be between 65-degrees and 70-degrees at the first inner portion end and between 20-degrees and 25-degrees at the second inner portion end.

[0030] In any embodiment, the inner portion length of each rotor blade may be at least 50% of the blade length.

[0031] In any embodiment, a moment of inertia of the rotor is less than 0.001 .

[0032] In any embodiment, a normalized inertia number of the rotor less than 0.005.

[0033] These and other aspects and features of various embodiments will be discussed in greater detail below.DESCRIPTION OF VARIOUS EMBODIMENTS

[0034] Numerous embodiments are described in this application, and are presented for illustrative purposes only. The described embodiments are not intended to be limiting in any sense. The invention is widely applicable to numerous embodiments, as is readily apparent from the disclosure herein. Those skilled in the art will recognize that the present invention may be practiced with modification and alteration without departing from the teachings disclosed herein. Although particular features of the present invention may be described with reference to one or more particular embodiments or figures, it should be understood that such features are not limited to usage in the one or more particular embodiments or figures with reference to which they are described.

[0035] The terms "an embodiment," "embodiment," "embodiments," "the embodiment," "the embodiments," "one or more embodiments," "some embodiments," and"one embodiment" mean "one or more (but not all) embodiments of the present invention(s)," unless expressly specified otherwise.

[0036] The terms "including," "comprising" and variations thereof mean "including but not limited to," unless expressly specified otherwise. A listing of items does not imply that any or all of the items are mutually exclusive, unless expressly specified otherwise. The terms "a," "an" and "the" mean "one or more," unless expressly specified otherwise.

[0037] As used herein and in the claims, two or more parts are said to be “coupled”, “connected”, “attached”, “joined”, “affixed”, or “fastened” where the parts are joined or operate together either directly or indirectly (i.e., through one or more intermediate parts), so long as a link occurs. As used herein and in the claims, two or more parts are said to be “directly coupled”, “directly connected”, “directly attached”, “directly joined”, “directly affixed”, or “directly fastened” where the parts are connected in physical contact with each other. As used herein, two or more parts are said to be “rigidly coupled”, “rigidly connected”, “rigidly attached”, “rigidly joined”, “rigidly affixed”, or “rigidly fastened” where the parts are coupled so as to move as one while maintaining a constant orientation relative to each other. None of the terms “coupled”, “connected”, “attached”, “joined”, “affixed”, and “fastened” distinguish the manner in which two or more parts are joined together.

[0038] As used herein and in the claims, a group of elements are said to ‘collectively’ perform an act where that act is performed by any one of the elements in the group, or performed cooperatively by two or more (or all) elements in the group.

[0039] Some elements herein may be identified by a part number, which is composed of a base number followed by an alphabetical or subscript-numerical suffix (e.g. 112a, or 112i). Multiple elements herein may be identified by part numbers that share a base number in common and that differ by their suffixes (e.g. 112i, 1122, and 112s). All elements with a common base number may be referred to collectively or generically using the base number without a suffix (e.g. 112).

[0040] Wind with high frequency fluctuations, referred to herein as gusty or unsteady wind conditions, contributes significantly to the wind-energy spectrum. Gusts are often defined as a sudden change in wind speed with amplitudes of 30% to 50% of the freestream air flow. The range of gusts contributing to the wind-energy spectrum generallylast between about 1 second and 30 minutes, or more particularly between about 30 seconds and 3 minutes, or even more particularly about 1 minute. However, despite unsteady wind conditions contributing significantly to the wind-energy spectrum, traditional wind turbines are designed to operate in steady wind environments, wherein wind conditions are generally sustained for 1 or more hours up to 10 or more days. As a result, where traditional wind turbines are installed in unsteady wind environments, the traditional design is sensitive to unsteady fluctuations of the incoming air flow, and the power outputs of these systems are correspondingly unsteady under such conditions. For example, traditional small wind turbines, which are often installed in urban environments where wind is gusty, perform poorly throughout a substantial portion of their operational life.

[0041] When there is a sudden increase or decrease in wind, such as a gust or stop in the air flow, a rotor of a traditionally designed wind turbine may experience a loss of lift, resulting in a loss of power output and thus a decrease in the overall efficiency of the system. The loss of lift may be attributed to separation of flow on the suction side of the blades of the rotor which, when attached, is responsible for the lift force. That is, since traditional wind turbines are designed to operate within a narrow range of steady wind conditions, when the wind drops below or a gust exceeds that operational design range, flow on the suction side of the rotor blades may separate, and available wind energy may thereby be lost.

[0042] Disclosed herein is a rotor, generally referred to as rotor 100, which addresses the above-described problems. Referring to Figures 1 and 2, in the illustrated example, the rotor 100 includes two rotor blades 102. Each rotor blade 102 is divided into an inner blade portion 104 and an outer blade portion 106. As described in greater detail subsequently, the outer blade portion 106 is designed for a maximum lift-to-drag ratio, and thus maximum power output, while the inner blade portion 104 is designed for improved efficiency across varying wind conditions. In particular, the inner blade portion 104 has a bio-inspired design to maintain a stably attached leading edge vortex, and thereby maintain lift and power output, across varying wind conditions. Accordingly, a wind turbine (e.g., a small wind turbine) utilizing a rotor designed in accordance with this disclosure and operating under unsteady wind conditions (e.g., in an urban environment) may have an increase in annual energy output of 30% to 40% or more.

[0043] Referring still to Figures 1 and 2, the rotor 100 includes a hub 108 having a first hub end 110i, a second hub end 1102, and a hub sidewall 112 extending longitudinally from the first hub end 110i to the second hub end HO2. The hub 108 has a longitudinal axis defining an axis of rotation 114 of the rotor 100. The rotor 100 further includes two rotor blades 102. As shown, the rotor blades 102 are circumferentially spaced apart and symmetrically distributed around the hub 108. While two rotor blades 102 are shown in the illustrated example, any number of rotor blades 102 may be used. This may depend, for example, on the size of the hub 108 and / or the cross-sectional size of the rotor blades 102 where the rotor blades 102 connect to the hub sidewall 112.

[0044] Each rotor blade 102 includes a blade root 116, a blade tip 118, and a longitudinal blade length 120 extending from the blade root 116 to the blade tip 118. Each rotor blade 102 is divided into the inner blade portion 104 and the outer blade portion 106 (see e.g., stippled lines in Figure 1 showing imaginary dividing lines). The inner blade portion 104 has an inner portion length 122 extending from a first inner portion end 124i at the blade root 116 to a second inner portion end 1242. The outer blade portion 106 has an outer portion length 126 extending from a first outer portion end 128i proximate the second inner portion end 1242 to a second outer portion end 1282 at the blade tip 118. The rotor blades 102 further include a leading edge 130 (i.e., the first part of the rotor blades 102 to encounter the wind during rotation) and a trailing edge 132 opposite the leading edge 130. The leading and trailing edges 130, 132 extend the blade length 120 from the blade root 116 to the blade tip 118.

[0045] In any embodiment, the inner blade portion 104 and the outer blade portion 106 can be integrally formed. In such embodiments, the leading edge 130 and the trailing edge 132 of the rotor blades 102 can be continuously formed across the inner blade portion 104 and the outer blade portion 106. Continuously forming the leading and trailing edges 130, 132 may advantageously provide a smooth transition between the inner blade portion 104 and the outer blade portion 106. A continuous, smooth leading edge 130 and trailing edge 132 may minimize aerodynamic losses and ensure optimal energy conversion. In some embodiments, a transition portion 134, described in greater detail subsequently herein, can be provided between the inner and outer blade portions 104, 106. The transition portion 134 may further smooth the transition of the leading and trailing edges130, 132 between the profile of the inner blade portion 104 at the second inner portion end 1242 and the profile of the outer blade portion 106 at the first outer portion end 128i .

[0046] Similarly, in any embodiment, the rotor blades 102 and the hub 108 can be integrally formed. Alternatively, the rotor blades 102 and hub 108 can be separately manufactured and the rotor blades 102 can be subsequently connected to the hub 108 through known techniques (e.g., mechanical fasteners, welding, thermal welding). In the example shown, each rotor blade 102 is integrally formed with a portion of the hub 108, and the portions of the hub 108 are subsequently connected to form the rotor 100. The rotor blades 102 and the hub 108 can thus be manufactured and assembled using any suitable means. For example, where the materials are metallic, the rotor blades 102 and the hub 108 may be separately or integrally machined. As another example, where the materials are polymers, the rotor blades 102 and the hub 108 may be separately or integrally produced by additive manufacturing processes such as injection molding or 3D printing.

[0047] Any material having a degree of flexibility and a mass-to-strength ratio sufficient to withstand the aerodynamic forces experienced by the rotor blades 102 designed in accordance with this disclosure under unsteady wind conditions may be used. Suitable materials for the rotor blades 102 and compatible with machining include metals such as, for example, aluminum, titanium, and steel, and metal alloys including the same. Suitable materials for the rotor blades 102 and compatible with additive manufacturing include polymers such as, for example, Acrylonitrile Butadiene Styrene (ABS), Polypropylene (PP), Polycarbonate (PC), Nylon (PA), and Polyethylene (PE), and fiber- reinforced polymers. Additive manufacturing processes and the polymer materials suitable for use therein may advantageously provide a fast, efficient, and cost-effective means of repeatably and accurately manufacturing the rotor 100. The advantages of additive manufacturing may be compounded where the rotors 100 are for use with small wind turbines (e.g., where the rotor blades 102 have a blade length 120 less than 50 centimeters, or more particularly less than 25 centimeters), since production may be easily scaled to produce a large number of small rotors (in contrast to traditional wind turbines, which generally use a small number of large rotors to service a similar area).

[0048] The rotor blades 102 can have any blade length 120. For example, the blade length 120 of the rotor blades 102 can be at least 5 centimeters, 10 centimeters, 15 centimeters, 20 centimeters, 25 centimeters, or more. The maximum blade length 120 may depend, for example, on the chord, thickness, and material properties of the rotor blades 102 across the blade length 120. The chord and thickness may themselves depend on the particular airfoil profile used. Accordingly, the maximum blade length 120 may depend on the particular airfoil profile and material used for the rotor blades 102. That is, material used and airfoil profile along the blade length 120 may dictate the cantilevered load of the rotor blades 102 alone (i.e., the mass distribution of the material along the blade length 120) as well as the load on the cantilevered rotor blades 102 from the wind, where the wind load includes bending induced by the lift component of the wind (i.e., in plane in Figure 1 , causing rotation) as well as bending caused by the thrust component of the wind (i.e., out of plane in Figure 1 ).

[0049] As described, the geometry of the rotor blades 102 and the material used will determine the mass distribution along the blade length 120. In operation, the mass distribution may dictate the moment of inertia of the rotor 100, which may be mathematically characterized as a function of mass and radius of the rotor 100. Accordingly, as mass and / or blade length 120 of the rotor blades 102 increases, inertia of the rotor 100 may similarly increase. Where the rotor 100 has a low moment of inertia, the rotor 100 may be expected to react quickly to a gust, causing a tip-speed ratio of the rotor to increase. Conversely, where the rotor 100 has a high moment of inertia, the rotor 100 may be expected to experience a lag before reacting to the gust, causing the tip-speed ratio to decrease (and the rotor to experience dynamic stall). In either scenario, the result may be a reduction in the performance of the rotor system.

[0050] The rotor moment of inertia may be used together with changes in flow inertia to quantify the ability of the rotor to extract gust energy (i.e., to determine the power output of the rotating system) operating under unsteady wind conditions. In particular, the normalized inertia number is a non-dimensional number representing a ratio of the moment of inertia of the rotor to the inertia of the wind (i.e., gust acceleration). The normalized inertia number describes quantitatively the influence of the rotor inertia on the dynamicperformance of energy-extracting rotor system in unsteady flows. Referring to equation (1 ) below, the normalized inertia number, / *, is given by: l* = l (a / p R4U2) (1 )

[0051] In equation (1 ), I is the moment of inertia, U«, is the incoming flow speed in the rotor frame of reference, a is gust acceleration, R is radius of the rotor, and p is density of air. As shown by equation (1 ), / ‘can be increased by either increasing the rotor moment of inertia (i.e. , effective size and mass), the rotor size (i.e. , blade length), or by increasing the gust acceleration. Similarly, / ‘can be decreased by decreasing any of these parameters.

[0052] The speed of the rotor response to changes in wind speed, which scales with / *, may determine the overall performance of the rotor 100 during gusts. For example, rotors 100 designed in accordance with this disclosure and having a moment of inertia that is less than 0.001 Kg.m2and / or a normalized inertia number that is less than 0.005 may be sufficiently responsive to changes in wind speed such that the rotor 100 passively maintains a design tip-speed-ratio (described in greater detail subsequently herein) at which the rotor blades 102 are designed to operate. The rotor 100 can have any other moment of inertia to yield any other normalized inertia number that is less than 0.005, or more particularly 0.004, or more particularly 0.003, or even more particularly less than 0.0025. Rotors 100 having a moment of inertia and / or a normalized inertia number as described may exhibit improved passive regulation of the design tip-speed-ratio and thus improved maintenance of optimal efficiency.

[0053] In addition to use in designing a rotor system that passively regulates the design tip-speed ratio, the normalized inertia number may be used in designing a rotor system that actively regulates the design tip-speed ratio. Actively maintaining the design tip-speed-ratio can include, for example, using Maximum Power Point Tracking (MPPT) control mechanisms known in the art. Characterization of the independent response of the rotor to changes in the flow speed (i.e., the passive response) can be used for developing effective MPPT control mechanisms to better ensure that the rotor 100 operates at its optimal tip-speed ratio during varying wind speeds. That is, knowing the passive rotor response can help avoid over / under correcting using active measures. Reliance on active control mechanisms to maintain the design tip-speed-ratio can thus be reduced oreliminated by adjusting the normalized inertia number, using the parameters discussed above, and thereby adjusting the passive responsiveness of the rotor 100 to changes in wind speed. This may obviate the need for sophisticated and costly active control mechanisms upon which traditional wind turbines are reliant.

[0054] In view of the foregoing general description of the rotor 100, the design of the inner blade portion 104 and the outer blade portion 106 of the rotor blades 102 will now be described in greater detail.

[0055] Samara trees rely on wind to disperse their seeds over large distances to ensure successful reproduction. After falling from the samara tree, samara seeds (e.g., maple seeds) autorotate around their centre of mass, producing large aerodynamic forces (i.e., lift), which increase their descent time. A higher descent time allows wind to carry the seeds for much larger lateral distances, ensuring effective propagation of the samara seeds including in unsteady wind environments. The aerodynamic forces may be attributed to the formation of stable leading-edge vortices (LEVs) on the suction side of the rotating seed wing, which may further be attributed to various physical and aerodynamic properties of the seed. These properties may allow for robust autorotation and resistance to perturbations from unsteady wind conditions.

[0056] Various characteristics akin to those of a samara seed may be applied to the rotor blades 102 to achieve improved efficiency in unsteady wind environments. In particular, the tip-speed-ratio, angle of attack distribution, and aspect ratio of samara seeds are parameters that may each contribute to, and collectively play a role in, LEV growth and stabilization, and thus in lift generation. Accordingly, the application of like parameters to the design of the rotor blades 102 may similarly contribute to LEV growth and stabilization, and thus in lift generation and power output, in unsteady wind environments. While the example embodiment described herein is designed for low wind speeds, it is noted that it may also perform well in high wind speed conditions. Further, a rotor may be designed specifically for high wind speeds in accordance with this description for even better performance under such conditions.

[0057] Referring again to Figures 1 and 2, the inner blade portion 104, also referred to as the bio-inspired portion, is designed using parameters similar to those of a samaraseed. The inner blade portion 104 may be designed to operate at a design tip-speed-ratio between 1.5 and 6, which is the tip-speed-ratio range for autorotating samara seeds. The tip-speed-ratio, , is given by equation (2) below:4 = VTIP / VWIND (2)

[0058] In equation (2), VTIP is the speed of the rotor blade 102 at the blade tip 118 and VWIND is the speed of the incoming wind. As wind speed is variable, equation (2) shows that the design tip-speed-ratio may be maintained by regulating the speed of the rotor blade 102 at the blade tip 118, such as by passive regulation and / or active regulation using the normalized inertia number as described previously herein. The angle of attack distribution of the rotor blade 102 is, in part, dependent on the tip-speed-ratio. Therefore, maintaining a relatively constant design tip-speed-ratio may also simultaneously maintain a relatively constant angle of attack distribution. As such, the normalized inertia number may be used to passively (or optionally, actively) maintain two parameters (i.e. , tip-speed-ratio and angle of attack) that may otherwise vary with changes in the wind speed.

[0059] Figures 3A to 3F show changes in an angle of attack a between various cross-sections taken along the blade length 120 of the example rotor blade 102 at the design tip-speed-ratio. As shown, the angle of attack a is the angle between the chord line 136 of the rotor blade 102 (i.e., an imaginary line from the leading edge 130 to the trailing edge 132) and the resultant wind force VRESULTANT (i.e., from incoming air flow and air flow resulting from rotation of the rotor 100). In order: Figure 3A shows the angle of attack a at the first inner portion end 1241 taken along line 138 of Figure 1 ; Figure 3B shows the angle of attack a mid-way between the first and second inner portion ends 124i , 1242 taken along line 140 of Figure 1 ; Figure 3C shows the angle of attack a at the second inner portion end 1242 taken along line 142 of Figure 1 ; Figure 3D shows the angle of attack a at the first outer portion end 128i taken along line 144 of Figure 1 ; Figure 3E shows the angle of attack a mid-way between the first and second outer portion ends 128i , 1282 taken along line 146 of Figure 1 ; and Figure 3F shows the angle of attack a at the second outer portion end 1282 taken along line 148 of Figure 1 .

[0060] As shown in Figures 1 , 3A, 3B, 3C, and 4, the inner blade portion 104 has an inner angle of attack distribution along the inner portion length 122. The angle of attack adecreases along the inner portion length 122 from the first inner portion end 124i to the second inner portion end 1242. The distribution can be similar to that of an autorotating samara seed. For example, at the design tip-speed-ratio, the angle of attack a can be at least 40-degrees at the first inner portion end 124i and at least 15-degrees at the second inner portion end 1242, or more particularly between 40-degrees and 70-degrees at the first inner portion end 124i and between 15-degrees and 25-degrees at the second inner portion end 1242. In the example illustrated in Figures 3A, 3B, and 3C, and as graphically shown in Figure 4, at the design tip-speed-ratio, the angle of attack a of the example rotor blade 102 is between 65-degrees and 70-degrees at the first inner portion end 124i (i.e., at r / R = 0 in Figure 4) and between 20-degrees and 25-degrees at the second inner portion end 1242 (i.e., at r / R = 0.5).

[0061] As shown in Figure 4, the rate of change of the angle of attack from the first inner portion end 124i to the second inner portion end 1242 is non-linear. In the example shown, the rate of change decreases moving from the first inner portion end 124i to the second inner portion end 1242. For example, the angle of attack a decreases by about 30- degrees in the first half of the inner blade portion 104 (i.e., from between 65-degrees and 70-degrees at r / R = 0 to between 35-degrees and 40-degrees at r / R = 0.25), and only by about 15-degrees in the second half of the inner blade portion 104 (i.e., from between 35- degrees and 40-degrees at r / R = 0.25 to between 20-degrees and 25-degrees at r / R = 0.5). A decreasing rate of change may beneficially smooth the transition from the inner blade portion 104 to the outer blade portion 106 for which, as described subsequently, the angle of attack may be constant. The particular rate of change of the angle of attack may depend on the blade length 120, the inner portion length 122, and / or the maximum and minimum angle of attack of the inner blade portion 104. The maximum angle of attack may further depend on the size of the rotor blade 102 as well as the size of the hub 108 to which the blade root 116 attaches.

[0062] In traditional wind turbine design, the stall regime of a rotor refers to a condition where the angle of attack of the rotor blades becomes too high, causing a disruption of the smooth air flow over the rotor blades (i.e., breakdown of the LEV on the suction side of the rotor blade) and thus a decrease in lift and, accordingly, a decrease in power output. Where the angle of attack is greater than 12-degrees, the rotor blade isoperating in the stall regime. Accordingly, contrary to traditional turbine design, the inner blade portion 104 operates substantially in the stall regime, beyond the 12-degree threshold.

[0063] Further, in traditional wind turbine design, deep stall refers to an extreme condition where the angle of attack of the rotor blades becomes so high that the rotor blade is engulfed in turbulent air flow (i.e. , complete breakdown and separation of the LEV from the suction side of the rotor blade) and thus lift, and thereby power output, is substantially or completely lost. Where the angle of attack is greater than 45-degrees, the rotor blade is operating in deep stall. Accordingly, contrary to traditional turbine design, a portion of the inner blade portion 104 operates substantially in deep stall, beyond the 45-degree threshold.

[0064] Despite exceeding both the 12-degree threshold for the stall regime and the 45-degree threshold for deep stall, a rotor blade designed in accordance with this disclosure may nonetheless maintain a stably attached LEV. That is, the tip-speed-ratio and angle of attack distribution (regulated by the tip-speed-ratio and the normalized inertia number) and the aspect ratio (described subsequently) of the inner blade portion 104 as described herein may collectively enable the inner blade portion 104 of the rotor blades 102 to operate entirely in the stall regime, and at least partially in deep stall. The combination of these parameters may stabilize the LEV in both the stall regime and in deep stall, which may beneficially result in higher lift and enable the rotor blades 102 to operate efficiently and maintain steady power output across unsteady wind conditions. This design practice may be understood as pushing the rotor blades 102 their operational limits without destabilizing the LEV.

[0065] In traditional turbines, when the rotor experiences a sudden change in wind speed, a corresponding sudden change in angle of attack occurs, which can lead to break down or separation of LEV. A sudden change in wind speed can therefore disrupt or eliminate the circulatory forces of the LEV on traditional rotor blades and can thus cause a sudden change of the output of low-inertia rotors. In contrast, manipulating the flow around the rotor’s blades using the parameters described herein, and hence the circulatory forces, may mitigate the effects of sudden changes in wind speed on the rotor’s response. That is,a rotor designed in accordance with this disclosure may render LEV stability insensitive to gusts (i.e., the strength and stability of LEVs may be self-regulatory via these mechanisms).

[0066] At high angles of attack, the LEV may remain stably attached when the local radius is shorter than four chord lengths. Where the local radius exceeds four chord lengths, the LEV may separate from the inner portion 104 of the rotor blades 102 at high angles of attack. Accordingly, to further stabilize the LEV at high angles of attack, a local aspect ratio of the rotor blade 102 at any position along the inner portion length 122 may be less than 4. The local aspect ratio (i.e., of the airfoil at any cross-section along the blade length 120) may be understood as represented in equation (3) below:ARLOCAL = RLOCAL / CLOCAL (3)

[0067] In equation (3), RLOCAL is the local radius (i.e., measured from the axis of rotation 114 to the location of the cross-section) and CLOCAL is the local chord length of that cross-section (i.e., the length of the imaginary straight line from the leading edge 130 to the trailing edge 132). The aspect ratio of the rotor blade 102 may control spanwise flow along the blade length 122, which drains the LEV. By draining the LEV, spanwise flow may therefore allow the LEV to stabilize. However, as the aspect ratio increases, the spanwise flow may decrease, resulting in the flow on the airfoil section of the rotor blade becoming 2- dimensional. Accordingly, if the spanwise flow is weak, the LEV may continue to grow until it breaks down and detaches from the rotor blade 102. This may ultimately result in the rotor blade 102 entering deep stall. Therefore, in the example shown in Figures 1 to 3, the local aspect ratio of the rotor blade 102 at every position along the inner portion length 122 is less than 4.

[0068] The inner blade portion 104 can have any inner portion length 122. For example, in the illustrated example, the inner portion length 122 is about 50% of the blade length 120. In other examples, the inner portion length 122 may be at least 20%, 30%, 40%, 50%, 60%, 70%, or 80% or more of the blade length 120. It will be appreciated that changing the relative proportion of the inner portion length 122 to the blade length 120 may change the performance of the rotor 100. For example, a rotor wherein 100% of the blade length 120 of the rotor blades 102 has a similar tip-speed-ratio, angle of attack distribution, and aspect ratio to that of a samara seed has an efficiency of about 3-4%. However, it hasbeen found that a portion of blade length 120 (i.e., the outer blade portion 106) can have a different design from the inner blade portion 104 without destabilizing the LEV of the inner portion 104 while simultaneously improving efficiency of the rotor 100 in unsteady wind conditions.

[0069] Accordingly, as exemplified in Figures 1 to 3, the rotor blades 102 have a hybrid design. The inner blade portion 104 is designed as described previously herein to improve operational efficiency under unsteady wind conditions by maintaining a stably attached LEV. The outer blade portion 106 is design as described subsequently to achieve a maximum lift-to-drag ratio at the design tip-speed-ratio to improve power output. This hybrid design is advantageous since the LEV forms at the blade root 116 and the outer blade portion 106 contributes more to power generation than the inner blade portion 104 (i.e., due to torque increasing moving radially outwardly from the blade root 116 toward the blade tip 118). As such, this hybrid design may extract energy from wind with greater efficiency (30-40% or more), including under sudden changes in wind speed.

[0070] Referring to Figures 1 , 3D, 3E, and 3F, the outer blade portion 106 has an outer angle of attack that is constant along the outer portion length 126. That is, at the design tip-speed-ratio, the angle of attack a is constant from the first outer portion end 128i to the second outer portion end 1282. For example, the angle of attack a at the crosssection of Figure 3D (i.e., at the first outer portion end 128i) is the same as the angle of attack a at the cross-section of Figure 3E (i.e., mid-way between the first and second outer portion ends 128i , 1282), which is the same as the angle of attack a at the cross-section of Figure 3F (i.e., at the second outer portion end 1282). The constant angle of attack a may be the angle at which the particular airfoil used for the outer blade portion 106 has a maximum lift-to-drag ratio when operating at the design tip-speed-ratio and at a design wind velocity. The design wind speed may be, for example, the average wind speed of the area in which a turbine having the rotor 100 is to be installed.

[0071] Since the angle of attack will naturally change for a straight rotor blade across its length (i.e., due to a natural increase in local wind speed moving radially outwardly along the blade length), the rotor blade 102 may be formed with a twist in the outer blade portion 106 along the outer portion length 126 to maintain the constant angle of attack a at thedesign tip-speed-ratio. By maintaining a constant angle of attack a, circulation of the air flow on the suction side of the outer blade portion 106 may be similarly maintained constant, thereby maintaining the most efficient lift distribution of the outer blade portion 106.

[0072] The outer blade portion 106 can have any local aspect ratio along the outer portion length 126. In the illustrated example, the chord length is constant along the outer portion length 126 such that the local aspect ratio of the rotor blade 102 increases along the outer portion length 126 from the first outer portion end 128i to the second outer portion end 1282. In alternate embodiments, the chord length can decrease along the outer portion length 126 such that the local aspect ratio of the rotor blade 102 would increase at a greater rate along the outer portion length 126. Decreasing the chord length in this way may save material costs without negatively affecting lift generation of the outer blade portion 106. Decreasing the chord length in this way may also beneficially assist in maintaining more uniform circulation of the air flow on the suction side of the outer blade portion 106, minimizing drag and improving overall efficiency.

[0073] Optionally, as exemplified in Figures 1 and 2, the rotor blade may further include the transition portion 134. As shown, the transition portion 134 has a transition portion length 150 extending from the second inner portion end 1242 to the first outer portion end 128i. The transition portion 134 has a transition angle of attack, which decreases from the inner angle of attack at the second inner portion end 1242 to the constant outer angle of attack starting at the first outer portion end 128i .

[0074] The transition portion length 150 may make up a relatively small proportion of the blade length 120 of the rotor blade 102, such as less than 10% or less than 5%, for example. The transition portion length 150 may be any length suitable for providing a smooth transition between the angle of attack a at the second inner portion end 1242 and the angle of attack a at the first outer portion end 128i . This may depend, for example, on the difference between the angle of attack a and these respective ends (i.e. , the greater the difference, the longer the transition portion length 150 may be).

[0075] As mentioned previously, including the transition portion 134 in the rotor blade design may be conducive to continuously forming the leading and trailing edges 130, 132along the full blade length 120. The transition portion 134 may provide a continuous, smooth transition between the angle of attack a at the second inner portion end 1242 and the angle of attack a at the first outer portion end 128i , which may minimize aerodynamic losses and ensure optimal energy conversion.

[0076] While the above description provides examples of the embodiments, it will be appreciated that some features and / or functions of the described embodiments are susceptible to modification without departing from the spirit and principles of operation of the described embodiments. Accordingly, what has been described above has been intended to be illustrative of the invention and non-limiting and it will be understood by persons skilled in the art that other variants and modifications may be made without departing from the scope of the invention as defined in the claims appended hereto. The scope of the claims should not be limited by the preferred embodiments and examples, but should be given the broadest interpretation consistent with the description as a whole.

Claims

CLAIMS:1 . A rotor blade for a wind turbine comprising: a blade root, a blade tip, and a longitudinal blade length extending from the blade root to the blade tip; an inner blade portion having: an inner portion length extending from a first inner portion end at the blade root to a second inner portion end, and an inner angle of attack, the inner angle of attack decreasing along the inner portion length from the first inner portion end to the second inner portion end; and an outer blade portion having: an outer portion length extending from a first outer portion end proximate the second inner portion end to a second outer portion end at the blade tip, and an outer angle of attack, the outer angle of attack constant along the outer portion length for a maximum lift-to-drag ratio of the outer blade portion at a design tip- speed-ratio and at a design wind velocity.

2. The rotor blade of claim 1 wherein a local aspect ratio of the rotor blade at any position along the inner portion length is less than 4.

3. The rotor blade of claim 2 wherein the local aspect ratio of the rotor blade increases along the outer portion length from the first outer portion end to the second outer portion end.

4. The rotor blade of claim 1 wherein the design tip-speed-ratio is between 1 .5 and 6.

5. The rotor blade of claim 1 wherein, at the design tip-speed-ratio, the inner angle of attack is at least 40-degrees at the first inner portion end and at least 15-degrees at the second inner portion end.

6. The rotor blade of claim 1 wherein, at the design tip-speed-ratio, the inner angle of attack is between 40-degrees and 70-degrees at the first inner portion end and between 15-degrees and 25-degrees at the second inner portion end.

7. The rotor blade of claim 1 wherein, at the design tip-speed-ratio, the inner angle of attack is between 65-degrees and 70-degrees at the first inner portion end and between 20-degrees and 25-degrees at the second inner portion end.

8. The rotor blade of claim 1 wherein the inner portion length is at least 50% of the blade length.

9. The rotor blade of claim 1 further comprising a transition portion having: a transition portion length extending from the second inner portion end to the first outer portion end, and a transition angle of attack, the transition angle of attack decreasing from the inner angle of attack at the second inner portion end to the outer angle of attack.

10. The rotor blade of claim 1 wherein the inner blade portion and outer blade portion are integrally formed.11 . The rotor blade of claim 1 further comprising: a leading edge, the leading edge extending the longitudinal blade length and continuously formed across the inner blade portion and the outer blade portion; and a trailing edge opposite the leading edge, the trailing edge extending the longitudinal blade length and continuously formed across the inner blade portion and the outer blade portion.

12. A rotor for a wind turbine comprising: a hub having a first hub end, a second hub end, a hub sidewall extending longitudinally from the first hub end to the second hub end; and at least two rotor blades circumferentially spaced apart around the hub, each rotor blade having:a blade root connected to the hub sidewall, a blade tip, and a longitudinal blade length extending radially outwardly from the hub from the blade root to the blade tip; an inner blade portion having: an inner portion length extending from a first inner portion end at the blade root to a second inner portion end, and an inner angle of attack, the inner angle of attack decreasing along the inner portion length from the first inner portion end to the second inner portion end; and an outer blade portion having: an outer portion length extending from a first outer portion end proximate the second inner portion end to a second outer portion end at the blade tip, and an outer angle of attack, the outer angle of attack constant along the outer portion length for a maximum lift-to-drag ratio of the outer blade portion at a design tip-speed-ratio and at a design wind velocity.

13. The rotor of claim 12 wherein a local aspect ratio of each rotor blade at any position along the inner portion length is less than 4.

14. The rotor of claim 13 wherein the local aspect ratio of each rotor blade increases along the outer portion length from the first outer portion end to the second outer portion end.

15. The rotor of claim 12 wherein the design tip-speed-ratio is between 1 .5 and 6.

16. The rotor of claim 12 wherein, at the design tip-speed-ratio, the inner angle of attack of each rotor blade is at least 40-degrees at the first inner portion end and at least 15- degrees at the second inner portion end.

17. The rotor of claim 12 wherein, at the design tip-speed-ratio, the inner angle of attack of each rotor blade is between 40-degrees and 70-degrees at the first inner portion end and between 15-degrees and 25-degrees at the second inner portion end.

18. The rotor of claim 12 wherein, at the design tip-speed-ratio, the inner angle of attack of each rotor blade is between 65-degrees and 70-degrees at the first inner portion end and between 20-degrees and 25-degrees at the second inner portion end.

19. The rotor of claim 12 wherein the inner portion length of each rotor blade is at least 50% of the blade length.

20. The rotor of claim 12 wherein a moment of inertia of the rotor less than 0.001 .

21. The rotor of claim 12 wherein a normalized inertia number of the rotor is less than 0.005.

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