CA2831395A1

CA2831395A1 - Fluid turbine with variable pitch shroud segments

Info

Publication number: CA2831395A1
Application number: CA2831395A
Authority: CA
Inventors: Walter M. Presz, Jr.; Michael J. Werle; Soren Hjort; Rune Rubak; Bo Lovmand; Soren Dalsgaard
Original assignee: FloDesign Wind Turbine Corp
Current assignee: FloDesign Wind Turbine Corp
Priority date: 2011-03-30
Filing date: 2012-03-30
Publication date: 2012-10-04
Also published as: WO2012135638A1; EP2691645A1; US20140030059A1

Abstract

One or more variable pitch airfoils in fluid communication with a rotor of a fluid turbine can control the amount of energy directed to the rotor, and further control the amount of energy generated by the turbine. Varying the pitch of the airfoils may provide a means of controlling the power output of a fluid turbine without the need to control the pitch of the rotor blades, and may further provide a means of mitigating the effects of wind shear on the rotor. Variable pitch airfoils may also include a means of controlling the active power, reactive power and SCADA, of a group of fluid turbines.

Description

FLUID TURBINE WITH VARIABLE PITCH SHROUD
SEGMENTS
RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent Application No. 61/469,133, filed March 30, 2011, and U.S. Provisional Patent Application No. 61/493,833, filed June 6, 2011. The entire contents of both applications are incorporated herein by reference in their entirety.
BACKGROUND

[0002] Conventional horizontal axis wind turbines (HAWTs) used for power generation typically have two to five open blades arranged like a propeller, the blades being mounted to a horizontal shaft attached to a gear box which drives a power generator. HAWTs often comprise blades with pitch control for the purpose of furling the blades into the wind to mitigate speed and torque on the generator. Blade pitch control provides a means of regulating the power output of an individual, or a group of turbines, and a means for protecting the turbine and electrical generation equipment from excessive wind speeds.

[0003] HAWT can experience asymmetrical loading resulting in oscillations that cause stress on the tower and can effect electrical generation equipment.

Further, large HAWTs can experience greater wind speeds in the upper regions of the rotor plane than in the lower regions of the rotor plane, which is known as wind shear. Other wind events can also cause various types of asymmetrical loading on the rotor plane. By furling the blades of a HAWT into the wind in the highly-loaded regions of the rotor plane, and out of the wind in the lesser-loaded regions of the rotor plane, oscillations can be mitigated.

[0004] In conventional large HAWTs, control of the pitch of blades has been used to control power generated by the rotor, to mitigate oscillations caused by wind shear, and to mitigate stress on the tower caused by such oscillations or by excessive wind speed.

BRIEF DESCRIPTION

[0005]
Embodiments include a shrouded fluid turbine in which a pitch of at least a portion of the shroud is variable or adjustable, and method of operating or using such a shrouded fluid turbine. For example, in one embodiment, a shrouded fluid turbine includes a rotor and turbine shroud with a ringed airfoil.
The ringed airfoil includes a plurality of pivotable airfoil segments, each pivotable airfoil segment having a low pressure surface in fluid communication with the rotor. Each pivotable airfoil segment is rotatable about an axis to change a pitch of the pivotable airfoil segment.

[0006] In some embodiments, the shrouded fluid turbine further includes a pitch control mechanism that alters the pitch of at least a portion of the ringed airfoil. The pitch control mechanism may be configured to continuously change a pitch of at least a portion of the ringed airfoil while the shrouded fluid turbine is in use. A pitch of each of the plurality of pivotable airfoil segments may be individually adjustable.

[0007] In some embodiments, the plurality of pivotable airfoil segments includes a plurality of outwardly curving airfoil segments, which may be referred to herein as outwardly directed mixing elements. The ringed airfoil may also include a plurality of inwardly curving airfoil segments, which may be referred to herein as inwardly directed mixing elements.

[0008] In some embodiments each of the plurality of pivotable airfoil segments may be pivotable coupled to a frame of the fluid turbine. In some embodiments, the ringed airfoil further comprises a plurality of arms, each arm coupled to, and configured to adjust a pitch of, one or more of the plurality of pivotable airfoil segments.

[0009] In some embodiments, the ringed airfoil comprises a plurality of mixing elements configured to create a plurality of mixing vortices downstream of the rotor. In some embodiments, the shrouded fluid turbine also includes an ejector with a second ringed airfoil downstream of the ringed airfoil having the plurality of mixing elements. The second ringed airfoil may include a second plurality of pivotable airfoil segments.

[0010] In some embodiments, the rotor is in direct communication with a generator. In some embodiments, the rotor is in communication with a generator via a gearbox assembly.

[0011] Another embodiment includes a shrouded fluid turbine including a rotor defining a rotor plane and a ringed airfoil. The ringed airfoil has a plurality of fluid contact surfaces pivotable to change a unit mass flow rate through at least a portion of the rotor plane.

[0012] One embodiment includes a method of operating a shrouded fluid turbine. The method includes providing a shrouded fluid turbine having a rotor, and a ringed airfoil including a low pressure surface in fluid communication with the rotor. The method also includes altering a pitch of at least a portion of the ringed airfoil.

[0013] Another embodiment includes a method of operating a shrouded fluid turbine having a rotor and a shroud with a low pressure surface in fluid communication with the rotor. The method includes measuring at least one variable associated with operation of the shrouded fluid turbine, and altering a pitch of at least a portion of the shroud based on the measured at least one variable.

[0014] In some methods, the ringed airfoil, or the shroud, may comprise a plurality of pivotable airfoil segments, and altering a pitch of at least a portion of the ringed airfoil, or of the shroud, may include changing a pitch of at least one of the plurality of pivotable airfoil segments.

[0015] One embodiment includes a method of controlling a power output of an array of shrouded wind turbines, each shrouded wind turbine including a rotor and a shroud having a low pressure surface in fluid communication with the rotor. The method includes measuring a reactive power of the array, and altering a pitch of at least a portion of the shroud of at least one of the array of shrouded wind turbines based on the measured reactive power to augment or reduce the reactive power of the array.

[0016] Some embodiments described in the present disclosure relate to a shrouded (e.g., ducted) fluid turbine including a rotor and a ringed airfoil having a particular structure, and to mixing elements engaged with such ducts. A
ringed airfoil with mixing elements surrounds a rotor and is known as a turbine shroud, a second shroud is in fluid communication with the mixing elements of the turbine shroud and is known as the ejector shroud. The turbine shroud is a ringed airfoil that may include of inward and outward curving elements that each have an airfoil cross section. The ejector shroud is a ringed airfoil that includes of an annular ring with an airfoil cross section. In some embodiments, the shrouds are comprised of airfoil segments arranged in a polygon. The present disclosure relates to a configuration that comprises articulated, variable pitch controlled shroud segments. By varying the pitch of airfoil segments that comprise the shrouds, the force of the fluid stream on the rotor can be controlled. Controlling the force of fluid flow over the rotor provides a means of controlling the torque on the generator and electrical generation components, a means of controlling the power output of individual turbines or of a group of turbines and a means of mitigating the effects of oscillations caused by wind shear. Controlling the fluid flow in this manner is a means of controlling the speed of the rotor without necessarily having to control the pitch of the rotor blades.

[0017]
Altering the pitch of at least one shroud segment can provide a means of shading the rotor-swept area in such a manner as to reduce the effect of sun shadowing, also known as shadow flicker, on the ground.

[0018]
Pitching of shroud segments may be employed to break up ice on the surface of the shroud segments.

[0019] The power output of the rotor/generator system may be controlled by changing the pitch of one or more shroud segments. Sensors may measure rotor torque, generator current, or other indicator of power output. In some embodiments, the power-output indicator is compared with a fixed-reference range to determine if the power output is within the acceptable range. If the power output is without the acceptable range, a power-error signal is generated and shroud segments are articulated to correct the power error, adjusting the power output to a value within the acceptable range. Controlling power output in this manner allows for the set of shrouds to be configured in such a manner as to gradually increase speed during start-up, reduce speed during shutdown, during low-voltage-ride-through, or to generate a minimum, or a reduced, amount of power in excessive wind conditions, thus allowing for continued optimal power output during a wide range of operating conditions.

[0020] In some embodiments, tower stress is prevented by measuring tower base moment or indicators thereof including tower top acceleration, tower tilt or rotor power output; and responding by pitching shroud segments in such a manner as to maintain constant or reduce the tower base moment. In a similar manner, tower oscillations can be dampened.

[0021] Often it is desirable to curtail power production in a wind park, producing less than 100`)/0 of the potential power output so as to de-rate a wind turbine or group of wind turbines or to operate with what is known as a spinning reserve. Shroud segments can be pitched in such a manner as to provide a spinning reserve.

[0022]
Individual shroud segments can be pitched in order to mitigate asymmetric loading, including wind-driven asymmetric loading, nacelle tilt or yaw loading, or blade loading caused by tower shadow. Shroud segments are pitched in response to blade load, blade bending, tip acceleration or nacelle tilt loading, or by monitoring the load vs. rotor azimuth for each blade continuously, to reduce speed in a specific area of the rotor plane.

[0023]
Individual shroud segments can also be utilized to apply a yaw moment to yaw the turbine upwind or downwind accordingly, to reduce the overall power output of the rotor/generator and/or to deflect wind to other turbines in a wind park so as to provide equal power output from each turbine in the park.

[0024] An example shrouded wind turbine with a ringed airfoil turbine shroud and a ringed airfoil ejector shroud has been described in U.S. Patent Application Serial No. 12/054,050, which is incorporated herein in its entirety. Some embodiments provide a means of controlling the pitch of airfoil segments about the ringed airfoil for the purpose of controlling the power generated by the rotor, for mitigating oscillations caused by wind shear and for mitigating stress on the tower caused by such oscillations or by excessive wind speed.

[0025] These and other non-limiting features or characteristics of the present disclosure will be further described below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The following is a brief description of the drawings, which are presented for the purposes of illustrating the disclosure set forth herein and not for the purposes of limiting the same.

[0027] Figure 1 is a front right perspective view of an exemplary embodiment of a shrouded fluid turbine.

[0028] Figure 2 is a side cross-sectional partial exploded view of the fluid turbine of Figure 1.

[0029] Figure 3 is a side cross-sectional view of the fluid turbine of Figure 1.

[0030] Figure 4 is a side cross-sectional detail view of the fluid turbine of Figure 1 depicting a configuration with an outwardly curving airfoil segment angularly positioned for maximized, or increased, energy extraction at the rotor.

[0031] Figure 5 is a side cross-sectional detail view of the fluid turbine of Figure 1 depicting a configuration with outwardly curving airfoil segment angularly positioned for minimized, or decreased, energy extraction at the rotor.

[0032] Figure 6 is a front perspective view of another embodiment including fluid turbine having a turbine shroud with a first plurality of pivotable airfoil segments and an ejector shroud with a second plurality of pivotable airfoil segments.

[0033] Figure 7 is a rear perspective view of the fluid turbine of Figure 6.

[0034] Figure 8 is a side cross-sectional detail view of the fluid turbine of Figure 6 depicting a configuration with the outwardly curving airfoil segments and the ejector airfoil segments angularly positioned for maximized, or increased, energy extraction at the rotor.

[0035] Figure 9 is a side cross-sectional detail view of the fluid turbine of Figure 6 depicting a configuration with the outwardly curving airfoil segments and the ejector airfoil segments angularly positioned for minimized, or reduced, energy extraction at the rotor.

[0036] Figure 10 is a simplified, schematic, side cross-sectional detail view of an embodiment including a fluid turbine in depicting a configuration with the outwardly curving airfoil segments angularly positioned to maximize, or increase, the energy extraction at the rotor.

[0037] Figure 11 is a simplified, schematic, side cross-sectional detail view of the fluid turbine of Figure 10 depicting a configuration with the outwardly curving airfoil segments angularly positioned to minimize, or decrease, energy extraction at the rotor.

[0038] Figure 12 is a side cross-sectional detail view of another embodiment including a fluid turbine with an ejector shroud in the form of a ringed airfoil with a plurality of pivotable airfoil segments in a configuration with the outwardly curving airfoil segments angularly positioned for maximized, or increased, energy extraction at the rotor.

[0039] Figure 13 is a side cross-sectional detail view of the fluid turbine of Figure 12 in a configuration with the outwardly curving airfoil segments angularly positioned for minimized, or decreased, energy extraction at the rotor.

[0040] Figure 14 is a side cross-sectional view of a fluid turbine having a rotor in direct communication with a generator, in accordance with some embodiments.

[0041] Figure 15 is a side cross-sectional view of a fluid turbine having a rotor in communication with a generator via a gearbox assembly, in accordance with some embodiments.

[0042] Figure 16 schematically depicts a wind park including an array of wind turbines, in accordance with some embodiments.

[0043] Figure 17 is a flow diagram schematically depicting a method of operating a shrouded fluid turbine, in accordance with some embodiments.

[0044] Figure 18 is a flow diagram schematically depicting a method of operating a shrouded fluid turbine that includes measuring one or more variable associated with operation of the wind turbine, in accordance with some embodiments.

[0045] Figure 19 is a flow diagram schematically depicting a method of controlling a power output of an array of shrouded wind turbines, in accordance with some embodiments.

DETAILED DESCRIPTION

[0046] A more complete understanding of the components, processes, and apparatuses disclosed herein can be obtained by reference to the accompanying figures. These figures are intended to demonstrate the present disclosure and are not intended to show relative sizes and dimensions or to limit the scope of the exemplary embodiments.

[0047] Although specific terms are used in the following description, these terms are intended to refer only to particular structures in the drawings and are not intended to limit the scope of the present disclosure. It is to be understood that like numeric designations refer to components of like function.

[0048] The term "about" when used with a quantity includes the stated value and also has the meaning dictated by the context. For example, it includes at least the degree of error associated with the measurement of the particular quantity. When used in the context of a range, the term "about" should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the range "from about 2 to about 4" also discloses the range "from 2 to 4."

[0049] A Mixer-Ejector Wind/Water Turbine (MEWT) provides an improved means of generating power from fluid currents. A primary shroud contains a rotor which extracts power from a primary fluid stream. A mixer-ejector pump is included that ingests flow from the primary fluid stream and secondary flow, and promotes turbulent mixing of the two fluid streams. This enhances the power system by increasing the amount of fluid flow through the system, increasing the unit mass air-flow at the rotor for more power availability, and reducing back pressure on turbine blades. The fluid dynamic principles of a Mixer-Ejector Turbine are not restricted to air and apply to any fluid, defined as any liquid (e.g., water) or gas (e.g., air)In other words, the aerodynamic principles of a mixer ejector wind turbine apply to hydrodynamic principles in a mixer ejector water turbine.

[0050] The term "airfoil" is used in the description and the claims as a generic term to refer to a foil used with a moving fluid and includes both airfoils used with flowing gas (e.g. air) and hydrofoils used with flowing liquid (e.g., water).

Generally speaking, a cambered airfoil has a low pressure/high fluid flow velocity surface, which may be called the suction surface (e.g., the upper surface of a subsonic aircraft wing), and a high pressure/low fluid flow velocity surface, which may be called the pressure surface (e.g., the lower surface of a subsonic aircraft wing).

[0051] The term "rotor" is used herein to refer to any assembly in which blades are attached to a shaft and able to rotate, allowing for the generation of power or energy from wind rotating the blades. Exemplary rotors include a propeller-like rotor or a rotor/stator assembly. Any type of rotor may be used with turbine shroud in the wind turbine of the present disclosure.

[0052] The leading edge of a turbine shroud may be considered the front of the wind turbine, and the trailing edge of an ejector shroud may be considered the rear of the wind turbine. A first component of the wind turbine located closer to the front of the turbine may be considered "upstream" of a second component located closer to the rear of the turbine. Put another way, the second component is "downstream" of the first component.

[0053] Some embodiments of the present disclosure relate to a wind turbine including a rotor and a turbine shroud in the form of a ringed airfoil with a plurality of pivotable airfoil segments, in which each pivotable airfoil segment has a low pressure surface in fluid communication with the rotor. Each pivotable airfoil segment may be rotated about an axis that changes a pitch of the pivotable airfoil segment. The turbine shroud may include mixing elements, some or all of which may be incorporated into the pivotable airfoil segments.
In some embodiments, the fluid turbine may include an ejector shroud in fluid communication with the exit of the turbine shroud. The ejector shroud may include a second ringed airfoil with a second plurality of pivotable airfoil segments.

[0054]
Generally speaking, the pivotable airfoil segments, which may be described as articulated shroud segments or variable pitch airfoil segments, provide a means of controlling the rotational speed of the rotor and therefore the torque on the generator and electrical generation components. The pivotable airfoil segments may provide a means of mitigating tower stress caused by excessive fluid speeds (e.g., wind speeds or current speeds) and oscillations resulting from fluid shear (e.g., wind shear or hydroshear). The pivotable airfoil segments may be used in, and may serve these functions in, fluid turbines incorporating a fixed-blade rotor as well as fluid turbines incorporating a variable pitch rotor.

[0055] Although some exemplary embodiments are described below as wind turbines including ringed airfoils and pivotable airfoil segments, the description also applies to fluid turbines generally, (e.g., ringed hydrofoils and pivotable hydrofoil segments). The term "airfoil" as used in the specification and the claims, includes, but is not limited to, airfoils for use with air and other gases, and hydrofoils for use with water or other liquids. Further, the description below referring to wind-related phenomena (e.g., wind shear) also applies to fluid-related phenomena generally (e.g. hydroshear).

[0056] Figure 1 is a perspective view of an exemplary embodiment of a shrouded fluid turbine of the present disclosure. Figure 2 is a perspective, exploded view of the shrouded fluid turbine of Figure 1. Referring to Figure 1, the shrouded fluid turbine 100 comprises a first ringed airfoil, which may be referred to herein as a turbine shroud 110, a nacelle body 150, and a rotor 140.
In some embodiments, the fluid turbine 100 further includes a second ringed airfoil, which may be referred to herein as an ejector shroud 120. The turbine shroud 110 includes a front end 112, also known as an inlet end or a leading edge. The turbine shroud 110 also includes a rear end, also known as an exhaust end or trailing portion 116. The ejector shroud 120, which may also be referred to herein as the ejector, includes a front end, inlet end or leading edge 122, and a rear end, exhaust end, or trailing edge 124.

[0057] The rotor 140 surrounds the nacelle body 150. The rotor 140 comprises a central hub 141 at the proximal end of the rotor blades. The central hub 141 is rotationally engaged with the nacelle body 150. The nacelle body 150 and the turbine shroud 110 are supported by a tower 102. The rotor 140, turbine shroud 110, and ejector shroud 120 are coaxial with each other, (i.e., they share a common central axis 105).

[0058] The turbine shroud 110 has the cross-sectional shape of an airfoil with a suction side 111 (i.e., low-pressure side or low pressure surface) on the interior of the turbine shroud and a high-pressure side or high pressure surface on the exterior of the turbine shroud 113. In some embodiments, the trailing portion 116 of the turbine shroud has mixing elements that extend downstream beyond the rotor blades. The mixing elements include inwardly directed mixing elements 117 extending inward toward the central axis 105 of the turbine shroud and outwardly directed mixing elements 115 extending outward away from the central axis 105. In some embodiments the trailing portion 116 of the turbine shroud is shaped to form the mixing elements.

[0059] In some embodiments, a mixer-ejector pump is formed by the ejector shroud 120 in fluid communication with the ring of inwardly directed mixing elements 117 and outwardly directed mixing elements 115 of the turbine shroud 110. The mixing elements extend downstream of the rotor 140 and, in some embodiments, may extend into the inlet end 122 of the ejector shroud 120. One skilled in the art will recognize that the mixer may not extend into the inlet end 122 of the ejector shroud 120 in all embodiments.

[0060] The mixer-ejector pump provides the means for turbulent mixing of fluid (e.g., air) that passes through the rotor 140 with fluid that bypasses the rotor 140. The fluid stream is divided into a low pressure - high velocity stream on the side of the turbine shroud 110, or first ring airfoil, that is proximal to the rotor plane, which may be referred to as a primary flow or primary stream, and a high pressure ¨ lower velocity stream on the exterior of the turbine shroud, or second ring airfoil 100, which may be referred to as a bypass flow or bypass stream. Mixing elements, such as inwardly directed mixing elements 117 and outwardly directed mixing elements 115, cause the primary fluid stream and the bypass fluid stream to intersect downstream of the rotor plane. Mixing elements include but are not limited to: mixing lobes, mixing slots, vortex generators or other ringed airfoil aerodynamic modifications that promote mixing. The mixing elements may be disposed at a variety of regions such as, but not limited to, the trailing portion 116 of the ringed airfoil.

[0061] Power extraction at the rotor 140 is coupled to, or defined by, energy exchange at the wake, which is downstream of the rotor plane. A pressure drop occurs in the wake of the rotor as a result of the energy taken out by the rotor 140. Mixing elements, such as inwardly directed mixing elements 117 and outwardly directed mixing elements 115, in combination with the ejector shroud 120 provide turbulent mixing of the primary and bypass streams such that the air pressure in the wake of the turbine rapidly returns to ambient pressure. With energized wake from mixing elements, it is possible to extract more energy from a shrouded rotor than from an open rotor of similar size. Although fluid turbine 100 of Figures 1 through 5 incorporates an ejector shroud, some embodiments obtain enhanced mixing and a resulting increase in energy extraction using a turbine including a turbine shroud having mixing elements without an ejector shroud.

[0062] Figures 2 and 3 are partially-exploded, partial section views illustrating various structural elements of the fluid turbine 100. In some embodiments, the turbine shroud 110 includes a polygonal or circular frame 130 that encircles the central axis 105. As illustrated in Figures 2 and 3, the outwardly directed mixing elements 115, which may also be referred to herein as outwardly curving airfoil segments, may be pivotally engaged with straight portions of the frame 130. Rotation of a pivotable airfoil segment, such as an outwardly directed mixing element 115, relative to an axis defined by a corresponding straight portion of the frame 130 changes the pitch of the pivotable airfoil segment relative to the central axis 105. In turbine shroud of Figures 1 to 5, inwardly directed mixing elements 117, which may also be referred to herein as inwardly curving airfoil segments, maintain a fixed orientation with respect to the frame 130. However, in some embodiments, inwardly directed mixing elements may pivot with respect to the frame. In some embodiments, only some of the outwardly directed mixing elements may be pivotally engaged with the frame.

[0063] Some embodiments include active or passive pitch control mechanisms that alter the pitch of one or more of the pivotable airfoil segments with respect to the central axis. For example, in fluid turbine 100, outwardly directed mixing elements 115 are pivotally engaged with arms 132 that are, in turn, laterally engaged with the nacelle body 150 as shown by Figures 2 through 5. Movements of the arms 134 provide actuation of the outwardly directed mixing elements 115 in a manner that changes the pitch of the airfoil segment with reference to the central axis 105. Figure 2 shows the turbine shroud 110 with the outward directed mixing elements configured with the leading edge 112 pitched toward the central axis and the trailing portion 116 pitched away from the central axis 105. In contrast, Figure 3 shows the outward directed mixing elements 115 configured with the leading edge 112 pitched away from the central axis 105 and the trailing portion 116 pitched toward the central axis 105.

[0064] In some embodiments, the pitch of each airfoil segment may be individually adjustable. In other embodiments, the pitch of the plurality of pivotable airfoil segments is adjusted as a group. For example, the pitch of the outwardly directed mixing elements 115 may be adjusted simultaneously or individually.

[0065] The pitch control mechanism may incorporate one or more actuators for providing force to adjust the pitch of the pivotable airfoil segments. The one or more actuators may include, but are not limited to: mechanical actuators, hydraulic actuators, pneumatic actuators, electrical actuators, piezoelectric actuators, magnetic actuators and any combination of the aforementioned. One skilled in the art will readily recognize that the illustrated pivot and arm actuation mechanism is only one suitable embodiment and is not intended to be limiting in scope.

[0066] The cross-sectional views in Figures 4 and 5 illustrate the fluid flow (e.g., airflow) over the mixer shroud 110. In Figure 4, the outwardly directed mixing elements 115 are in a configuration that generates a maximum, or a relatively increased, amount of energy at the rotor 140. In Figure 5, the outwardly directed mixing elements 115 are in a configuration that generates a minimum, or a relatively reduced, amount of energy at the rotor 140. An incoming fluid stream (e.g., free stream air) is indicated generally by arrows 166.
A primary fluid stream 164 enters the turbine shroud 164 and passes through the rotor plane at the rotor 140, where energy is extracted and a pressure drop occurs in the portion of the primary fluid stream 164 that continues along the interior 111 of the turbine shroud 110 and along the interior surface of outwardly directed mixing elements 115. Fluid flowing over the exterior 113 of the turbine shroud, indicated by arrows 162, bypasses the turbine shroud 110 and the rotor 140 and therefore does not experience the pressure drop after the rotor plane.

As shown, the inwardly directed mixing elements 117 direct a portion of the relatively higher pressure bypass fluid stream 162 inward toward the central axis 105 and the relatively lower pressure primary fluid stream. Similarly, outwardly directed mixing elements 115 direct a portion 164 of the relatively lower pressure primary fluid stream exiting downstream from the rotor 140 to be directed away from the central axis 105 and toward the relatively higher pressure bypass fluid stream. The interaction of bypass fluid stream portions 162 from the inwardly directed mixing elements 117 and the primary stream portions 164 from the outwardly directed mixing elements 115 creates a plurality of mixing vortices that mix the relatively higher pressure bypass fluid stream with the relatively lower pressure primary fluid stream. This mixing may be referred to as turbulent mixing.

[0067] For the bypass stream that enters the ejector shroud 120, the camber of the ejector shroud creates a relatively lower pressure on the inner surface of the ejector shroud near the leading edge of the ejector, in comparison to the relatively higher pressure on the exterior surface 123 of the ejector shroud.
The lower pressure stream 160 on the interior of the ejector serves to draw in additional fluid flow that is further mixed with the inwardly directed bypass stream 162 and outwardly directed primary stream 164. An increase in pressure occurs on the interior of the ejector shroud as the flow moves from the upstream end of the ejector to the downstream end of the ejector 120. Airflow returns to ambient pressure upon exiting the ejector 120.

[0068] Referring to Figure 5, a cross-section depicts an exemplary outwardly directed mixing element 115 of the turbine shroud 110 rotated resulting in a different pitch of the pivotable airfoil segment. When the pitch of the pivotable airfoil segment (e.g., outwardly directed mixing element 115) is changed in this manner, turbulent mixing is reduced or eliminated. Without the substantial turbulent mixing of the primary 164 and bypass 160, 162 fluid streams, the pressure of the combined fluid stream does not approach ambient pressure as it exits the ejector shroud 120, which restricts the flow over the rotor 140 in a phenomenon known as diffuser stall.

[0069] In excessive fluid flow conditions (e.g., under high wind conditions) it is often desirable to slow the speed of the rotor to prevent damage to the electrical generation equipment. By controlling the pitch of some or all of the airfoils, the speed of the rotor 140, and thus the amount of energy transferred to the electrical generation equipment, is controlled without the alteration of the pitch of the rotor blades.

[0070] Figures 6 and 7 depict another embodiment including a fluid turbine in which both the turbine shroud and the ejector shroud have pivotable airfoil segments for pitch control. Figures 6 and 7 are perspective views of a fluid turbine 200 with multiple pivotable airfoil segments omitted to show a frame.
The turbine shroud, in the form of a first ringed airfoil, includes a first plurality of pivotable airfoil members, outwardly directed mixing elements 215. The ejector shroud, in the form of a second ringed airfoil, includes a second plurality of pivotable airfoil members, pivotable ejector segments 220. The frame includes turbine shroud frame members 230 and ejector frame members 232. Outwardly directed turbine mixing elements 215 are pivotally engaged with the turbine shroud frame members 230. Frame members 230 comprise a polygon or a faceted ring that encircles a central axis 105 of the fluid turbine. The outwardly directed mixing elements 215 are also pivotally engaged with arms 234 that are in turn engaged with the nacelle body 250. Movements of the arms 234 provide actuation of the outwardly directed mixing elements 215 in a manner that changes the pitch of the airfoil segment with reference to the central axis 205.
The outwardly directed mixing elements 215 may be actuated simultaneously or individually.

[0071] The pivotable ejector segments 220 are pivotally engaged with the ejector frame members 232. The pivotable ejector segments 220 are also pivotally engaged with arms 236 that are, in turn, laterally engaged with the nacelle body 250. Movement of the arms 236 provides actuation of the pivotable ejector segments 220 in a manner that changes the pitch of the pivotable ejector segments 220 with reference to the central axis 205. The pivotable ejector segments 220 may be actuated simultaneously or individually.

[0072] In some embodiments, all of the pivotable turbine shroud segments and/or all of the pivotable ejector shroud segments may adjust together to change an overall pitch of the turbine shroud and/or of the ejector shroud. In some embodiments, the pivotable turbine shroud segments and the pivotable ejector shroud segments may be adjustable such that a portion of the turbine shroud, or of the ejector shroud, has a different pitch than another portion of the turbine shroud, or of the ejector shroud.

[0073] Figures 8 and 9 illustrate the airflow over and through the fluid turbine with the outwardly directed mixing elements 215 and pivotable ejector segments 220 in different configurations. In Figure 8, the outwardly directed mixing elements 215 and pivotable ejector segments 220 are configured for generating the maximum, or a relatively increased, amount of energy at the rotor 240, which in turn is transferred to electrical generation equipment (not shown). In Figure 9, the outwardly directed mixing elements 215 and pivotable ejector segments 220 are configured for generating a minimum, or a relatively decreased, amount of energy at the rotor 240.

[0074] In Figures 8 and 9, an incoming fluid flow (e.g., free stream air) is indicated generally by arrows 266. Fluid entering the turbine shroud 264 passes through the rotor plane at rotor 240 where energy is extracted and a pressure drop occurs in the following stream that continues along an interior surface of the turbine shroud and a portion continues along the interior surface of outwardly directed mixing element 215. Fluid flowing over the exterior of the turbine shroud, indicated by arrow 262, bypasses the turbine shroud 210 and is directed inward by the inwardly directed mixing element 217. The outwardly directed mixing elements 215 cause the relatively lower pressure air exiting downstream from the rotor 240 to be mixed with the relatively higher pressure air 262.

[0075] The ejector shroud camber creates a relatively lower pressure on the inner surface of the ejector 221, near the leading edge, in comparison to the relatively higher pressure on the exterior surface 223 of the ejector. The relatively lower pressure stream 260 on the interior of the ejector 220 serves to draw in additional airflow that is further mixed with the inwardly directed fluid stream 262 and outwardly directed fluid stream 264. An increase in pressure occurs on the interior of the ejector 220 as the flow moves from the upstream end of the ejector 220 to the downstream end of the ejector 200. Upon exiting the ejector 220, the fluid flow returns to ambient pressure.

[0076] Referring to Figure 9, a cross section depicts the outwardly directed mixing element 215 of the turbine shroud rotated, with its pitch changed relative to a central axis 205. The pivotable ejector segment 220 is also rotated, changing its pitch relative to the central axis 205. When the pitch of the outwardly directed mixing elements 215 and the pivotable ejector segments 220 are changed in this manner, turbulent mixing of the primary fluid stream 264 and the secondary fluid stream 262 is reduced, and the additional fluid stream 260 through the ejector is not sufficient to provide turbulent mixing. Without the mixing of the primary fluid stream 264 and the bypass fluid stream 262, and without the injection of the fluid stream 260 at the ejector, the pressure of the combined fluid stream does not approach ambient pressure as it exits the ejector shroud, which restricts the flow of high speed, low pressure air over the rotor 240 causing diffuser stall. By controlling the pitch of some or all of the turbine shroud and ejector shroud airfoil segments, the speed of the rotor 240, and thus the amount of energy transferred to the electrical generation equipment may be controlled even without the alteration of the pitch of the rotor blades.

[0077]
Turbine shroud interior surface 211, turbine shroud exterior surface 213, ejector shroud interior surface 221 and turbine shroud exterior surface may be described as fluid contact surfaces. Pivoting the fluid contact surfaces changes a unit mass flow rate through at least a portion of the rotor plane associated with the pivoted fluid contact surfaces. The change in the unit mass flow rate changes the amount of energy extracted from the rotor, and the amount of energy transferred to associated electrical generation equipment (e.g., a generator).

[0078]
Figures 10 and 11 illustrate the basic principle of variable pitch ringed airfoils in fluid communication with a rotor blade, for the purpose of controlling the amount of energy directed to the rotor. The variable pitch turbine shroud airfoil and variable pitch ejector shroud airfoil may reduce or eliminate the need to pitch the rotor blades for control of energy extracted by the rotor from the fluid stream. A free stream fluid (e.g., free-stream air or wind) represented by arrows 366 enters the fluid turbine as a primary fluid stream 364, and bypass fluid streams 362 and 360. In the configuration depicted in Figure 10, the outwardly directed mixing elements 315 and ejector airfoil segments 320 have relatively little pitch, al, and a2 respectively, for maximum, or relatively increased, power extraction at the rotor 340. In the configuration depicted in Figure 11, the outwardly-directed mixing elements 315 and ejector airfoil segments 320 have relatively more pitch, a3, and a4 respectively, for decreased mixing, and less pressure differential across the ejector shroud 320, and consequently minimum, or relatively decreased, power extraction at the rotor 340.

[0079]
Figures 12 and 13 schematically depict fluid flow in another embodiment of a fluid turbine including a turbine shroud with mixer elements, but no ejector shroud. As shown, the turbine shroud includes outwardly curving airfoil segments 415 that are pivotable. In Figure 12, the outwardly curving airfoil segments 415 have a relatively small pitch a5 relative to a central axis 405 of the fluid turbine, resulting in increased mixing of a primary fluid stream that flows along the outwardly curving airfoil segment 415 and a bypass fluid stream 462 that flows along an inwardly curving airfoil segment 417. Even without an ejector, mixing elements of the turbine shroud (e.g., outwardly curving airfoil segment 415 and inwardly curving airfoil segment 417) produce a plurality of mixing vortices downstream of the rotor 440. In comparison, in Figure 13, outwardly curving airfoil segments 415 have a relatively larger pitch a6, similar to that of the inwardly curving airfoil segments 417, which greatly reduces mixing between the primary fluid stream 464 and the bypass fluid stream 462, and, consequently, decreases power extraction from the rotor 440.

[0080] In some embodiments, a nacelle body of a fluid turbine includes a generator. For example, Figures 14 and 15 depict embodiments of a fluid turbine 510 including a turbine shroud with a plurality of pivotable airfoil segments, outwardly curving mixing elements 515, and a plurality of fixed airfoil segments, inwardly curving mixing elements 517. The pivotable airfoil segments are actuated using arms 532. Fluid turbine 510 also includes an ejector with ejector airfoil segments 520 that are actuated using arms 536. In the embodiment of Figure 14, a nacelle body 550 includes a generator 543 that is in direct communication with a rotor 540, or more specifically, in direct communication with a central body 541 of the rotor. In the embodiment of Figure 15, a nacelle body 551 includes a generator 544 that is in communication with the rotor 540, or more specifically, in communication with a central body 541 of the rotor through via a gearbox assembly 545. One of ordinary skill in the art will recognize that, in various embodiments, a generator may communicate with a rotor via many different structures or mechanisms.

[0081] Figure 15 depicts a wind park or wind farm including an array 600 of individual wind turbines 602 . . . 620 that supply power for a utility grid 630, in accordance with some embodiments. The individual wind turbines 602 . . . 620 each include one or more ringed airfoils (e.g., turbine shroud, or turbine shroud and ejector shroud) that have pivotable airfoil segments for varying a pitch of at least a portion of the ringed airfoil as described above.

[0082] Some embodiments include methods for operating a shrouded fluid turbine. For example, in method 700 of Figure 17, a shrouded fluid turbine is provided that includes a rotor 140 and a ringed airfoil (e.g., turbine shroud 110) including a low pressure surface 111 in fluid communication with the rotor 140 (step 710). A pitch of at least a portion of the ringed airfoil (e.g., turbine shroud 110) is altered (step 720).

[0083] In some embodiments, the ringed airfoil includes a plurality of pivotable airfoil segments (e.g., outwardly curving segments 115) and altering a pitch of at least a portion of the ringed airfoil including changing a pitch of at least one of the plurality of pivotable airfoil segments. In some embodiments, the pitch is altered to reduce a unit mass flow rate through the rotor plane.

[0084] In some embodiments, a pitch of a first portion of the ringed airfoil is altered to be different than a pitch of a second portion of the ringed airfoil. In some embodiments, altering a pitch of the first portion of the ringed airfoil to be different than a pitch of a second portion of the ringed airfoil reduces fluid shear forces (e.g., wind shear forces) on the shrouded fluid turbine.

[0085] In some embodiments, the pitch of at least a portion of the ringed airfoil is altered at least once while the rotor is rotating about a central axis of the shrouded fluid turbine (e.g., while in use). For example, during use under excessively windy conditions, the pitch may change to reduce the unit mass fluid flow through the wind turbine. In some embodiments, the pitch of at least a portion of the ringed airfoil is continuously altered over a period of time during operation of the shrouded fluid turbine (e.g., to continuously respond to wind shear or support structure oscillations).

[0086] In method 800 of Figure 18 at least one variable associated operation of a shrouded fluid turbine is measured (step 810). A pitch of at least a portion of a shroud of the shrouded fluid turbine is altered based on the measured at least one variable (step 820) .

[0087] In some embodiments, altering a pitch of at least a portion of the shroud based on the measured at least one variable at least partially compensates for fluid shear (e.g., wind shear) forces on the shrouded fluid turbine. In some embodiments, the measured at least one variable includes a load variable. Examples of load variables include, but are not limited to:
blade load, blade bending, blade tip acceleration, nacelle tilt loading, and load as a function of azimuthal rotor position. In some embodiments, the measured at least one variable includes a first fluid velocity measured at a first portion of a rotor plane and a second fluid velocity measured at a second portion of the rotor plane.

[0088] In some embodiments, altering a pitch of at least a portion of the shroud based on the measured at least one variable dampens oscillations in a support structure for the shrouded fluid turbine. In some embodiments, the measured at least one variable includes a tower base movement variable and altering the pitch of at least a portion of the shroud based on the measured at least one variable reduces movement of the tower base. In some embodiments, the tower base movement variable is any of tower-top acceleration, tower tilt and rotor-power output.

[0089] When the fluid velocity at a given area (e.g., the lower portion) of the turbine rotor plane is of a different velocity than that a different area (e.g., the upper portion) of the turbine rotor plane, fluid shear (e.g., wind shear) and resultant oscillations can occur adversely affecting the rotor blades, tower and electrical generating equipment. By controlling the pitch of individual shroud segments, the effects of fluid shear can be mitigated.

[0090] In some embodiments, the shrouded wind turbine supplies power for a utility grid, such as one of shrouded fluid turbines 602 . . . 620 that supplies power to utility grid 630. The measured at least one variable may include a control variable and the pitch may be altered to augment or reduce a power output of the shrouded wind turbine. The control variable may be any of, but is not restricted to: a rotor speed, a rotor-power output, a rotor-shaft torque, and an ambient wind speed.

[0091] Figure 19, schematically illustrates a method 900 of controlling a power output of an array 600 of shrouded wind turbines 602 . . . 620. One or more of the shrouded wind turbines may include a ringed airfoil with pivotable shroud segments for changing a pitch of the pivotable shroud segments. An active power of the array 600 is measured (step 910). A pitch of at least a portion of the shroud of at least one of the array 600 is altered based on the measured reactive power to augment or reduce the active power of the array 600 (step 920). In some embodiments, the method 800 controls power during a low-voltage ride-through.

[0092] In fluid turbine arrays, such as wind farms, generally speaking, upwind or leading turbines (e.g., shrouded fluid turbines 602, 604, 606) encounter faster incoming wind than downwind turbines (e.g., shrouded fluid turbines 610, 612, 616, 618), and accordingly are able to extract more energy than downwind turbines. In some circumstances, it may be desirable to reduce the amount energy extracted by the upwind or leading turbines by changing the fluid flow through the turbine. As explained above, increasing a pitch of outwardly extending mixing elements in a turbine shroud and increasing a pitch of ejector segments reduces an overall fluid flow through the fluid turbine. In some embodiments, a pitch may be changed on only a portion of a ringed airfoil of one or more selected fluid turbine(s) (e.g., fluid turbines 604, 605, 606) in the array causing the selected fluid turbines 604, 605, 606 to yaw out of the wind. The selected fluid turbines 604, 605, 606, which are rotated out of the wind, do not extract as much power from the incoming wind creating a lower wind reduction for trailing fluid turbines (e.g., fluid turbines 610, 612). Further, if the selected fluid turbines 604, 605, 606 yaw far out of the wind, profiles of the selected fluid turbines 604, 605, 606 may be reduced, which results in less wind reduction for trailing fluid turbines 610, 612. In some embodiments the yawing deflects wind from the selected fluid turbines 604, 605, 606 to a second set of turbines 610, 612 to equalized power output from each turbine in the array.

[0093] The active power production of a group of mixer-ejector turbines can be controlled based on grid frequency or deviation from a grid frequency target, or may be controlled based on maximum KWh supplied to the grid. Further, articulated or pivotable shroud segments can be configured to deliver less than the maximum power output so that a reserve of available power is available as required.

[0094] By controlling the power output of each turbine individually the reactive power of the group of mixer-ejector turbines is controlled. Fluid turbines controlled in this manner can respond appropriately based on grid voltage or an external target power production.

[0095]
Embodiments may be utilized in conjunction a variety of forms of decentralized energy resources. One skilled in the art will recognize that the fluid turbine arrangements in embodiments may be utilized in the generation of power in conjunction with overall power production in large-scale power grids. To ensure stable and controllable power production, some embodiments may be interfaced with the power grid in a variety of suitable ways. One suitable approach for controlling and monitoring the output of some embodiments is a Supervisory Control And Data Acquisition (SCADA) system. A SCADA system for use with embodiments typically includes inpuVoutput signal hardware and controllers at the various location(s) to be monitored and/or controlled; a SCADA
hub for monitoring and controlling the location(s); a communication link(s) from the location(s) to the SCADA hub; and one or more supervisory stations at location(s) remote from the SCADA hub and in communication with the SCADA
hub.

[0096] The SCADA system for use may be configured to collect a large amount of data from one or more shrouded fluid turbines to which it is connected, either directly or indirectly. Additionally, in accordance with some embodiments, the SCADA system may be configured to control one or more shrouded fluid turbines to which it is connected by means of control routines feeding control parameters and settings to fluid turbine assembly, so that a stable and controlled power supply can be ensured. As appreciated by one of skill in the art, ensuring a stable and controllable power generation from one or more shrouded fluid turbines may include the use of meteorological modeling to predict changes in power production from fluid turbine generators. In accordance with one embodiment, a SCADA system may use data derived from monitoring the power output from the fluid turbine generators of a turbine farm (e.g., wind farm or wind park), and the power-transmission line. In accordance with this embodiment, the power output may be predicted using system-modeling algorithms understood in the art, and the power generation may be stabilized by storing or releasing generated power in unstable periods. Such system-modeling algorithms may be based on meteorological predictions as well as a variety of suitable alternative modeling and prediction data.

[0097] In accordance with other aspects, the pitch of at least one shroud segment can be altered such that at least a portion of the rotor-swept area may be shaded while the fluid turbine is in operation. When employed in a wind turbine application, for example, such shading of at least a portion of the rotor-swept area may reduce the effect of sun shadowing, also known as shadow flicker, on the ground.

[0098] In accordance with some embodiments, one or more shroud segments may be actively or passively controlled to break up any negative coatings that may attach to the shroud segments. For example, one or more shroud segments may be actuated to break up ice accumulation.

[0099] Furthermore, the power output of the fluid turbine system may be controlled by changing the pitch of one or more shroud segments. In one embodiment, a control parameter representative of power output may be measured for use in the control of one or more shroud segments. Suitable control parameters, as understood in the art, may include rotor torque, generator current, or other suitable indicators of power output. In an embodiment, the control parameter may be compared with a fixed reference range to determine if the power output is within an acceptable range. If the power output is outside the acceptable range, one or more shroud segments may be articulated to adjust the power-output to a value within the acceptable range. Controlling power output in this manner allows for the set of shrouds to be configured in such a manner as to gradually increase speed during start-up, reduce speed during shutdown, during low-voltage-ride-through or to generate the minimum amount of power in excessive wind conditions, thus allowing for continued optimal power output during excessive wind conditions.

[00100] Some embodiments may be used to minimize or control stress from asymmetric loading to within an acceptable range. In one embodiment, a measurement of the tower base moment or indicators thereof including tower top acceleration, tower tilt or rotor power output may be obtained and the pitching shroud segment(s) of the current invention may be utilized in a manner such that tower stress is maintain constant and/or reduced.

[00101] Individual shroud segments can be pitched in order to mitigate asymmetric loading including wind-driven asymmetric loading, nacelle tilt or yaw loading, or blade loading caused by reverberation between the tower and the blade, known as tower shadow. Shroud segments are pitched in response to blade load, blade bending, tip acceleration or nacelle tilt loading; or by monitoring the load vs. rotor azimuth for each blade continuously, to reduce increased speed in a specific area of the rotor sweep.

[00102] Individual shroud segments can also be utilized to apply a yaw moment to yaw the turbine upwind or downwind accordingly to reduce the overall power output of the rotor/generator and/or to deflect wind to other turbines in a wind park so as to provide equal power output from each turbine in the park.

[00103] In view of the embodiments described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this invention.
Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. 112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words 'means for together with an associated function.

Claims

1. A shrouded fluid turbine comprising:
a rotor; and a ringed airfoil comprising a plurality of pivotable airfoil segments, each pivotable airfoil segment having a low pressure surface in fluid communication with the rotor.

2. The shrouded fluid turbine of claim 1, wherein each pivotable airfoil segment is rotatable about an axis to change a pitch of the pivotable airfoil segment.

3. The shrouded fluid turbine of claim 1, further comprising a pitch control mechanism that alters the pitch of at least a portion of the ringed airfoil.

4. The shrouded fluid turbine of claim 3, wherein the pitch control mechanism is configured to continuously change a pitch of at least a portion of the ringed airfoil while the shrouded fluid turbine is in use.

5. The shrouded fluid turbine of claim 1, wherein a pitch of each of the plurality of pivotable airfoil segments is individually adjustable.

6. The shrouded fluid turbine of claim 1, wherein the plurality of pivotable airfoil segments includes a plurality of outwardly curving airfoil segments, and wherein the ringed airfoil further comprises a plurality of inwardly curving airfoil segments.

7. The shrouded fluid turbine of claim 1, wherein the ringed airfoil further comprises a frame, and each of the plurality of pivotable airfoil segments is pivotably coupled to the frame.

8. The shrouded fluid turbine of claim 1, wherein the ringed airfoil further comprises a plurality of arms, each arm coupled to, and configured to adjust a pitch of, one or more of the plurality of pivotable airfoil segments.

9. The shrouded fluid turbine of claim 1, wherein the ringed airfoil comprises a plurality of mixing elements configured to create a plurality of mixing vortices downstream of the rotor.

10. The shrouded fluid turbine of claim 9, further comprising a second ringed airfoil downstream of the ringed airfoil having the plurality of mixing elements.

11. The shrouded fluid turbine of claim 10, wherein the second ringed airfoil comprises a second plurality of pivotable airfoil segments.

12. The shrouded fluid turbine of claim 1, wherein the rotor is in direct communication with a generator.

13. The shrouded fluid turbine of claims 1, wherein the rotor is in communication with a generator via a gearbox assembly.

14. A shrouded fluid turbine comprising:
a rotor defining a rotor plane; and a ringed airfoil having a plurality of fluid contact surfaces pivotable to change a unit mass flow rate through at least a portion of the rotor plane.

15. A method of operating a shrouded fluid turbine, the method comprising:
providing a shrouded fluid turbine comprising:
a rotor; and a ringed airfoil including a low pressure surface in fluid communication with the rotor; and altering a pitch of at least a portion of the ringed airfoil.

16. The method of claim 15, wherein the ringed airfoil comprises a plurality of pivotable airfoil segments, and wherein altering the pitch of at least a portion of the ringed airfoil comprises changing a pitch of at least one of the plurality of pivotable airfoil segments.

17. The method of claim 15, wherein altering the pitch of at least a portion of the ringed airfoil comprises altering the pitch to reduce the unit mass fluid flow rate through the rotor plane.

18. The method of claim 15, wherein altering the pitch of at least a portion of the ringed airfoil comprises altering a pitch of a first portion of the ringed airfoil to be different than a pitch of a second portion of the ringed airfoil.

19. The method of claim 18, wherein altering the pitch of the first portion of the ringed airfoil to be different than the pitch of the second portion of the ringed airfoil reduces fluid shear forces on the shrouded fluid turbine.

20. The method of claim 15, wherein the pitch of at least a portion of the ringed airfoil is altered at least once while the rotor is rotating about a central axis of the shrouded fluid turbine.

21. The method of claim 15, wherein the pitch of at least a portion of the ringed airfoil is continuously altered during operation of the shrouded fluid turbine.

22. A method of operating a shrouded fluid turbine having a rotor and a shroud with a low pressure surface in fluid communication with the rotor, the method comprising:
measuring at least one variable associated with operation of the shrouded fluid turbine; and altering a pitch of at least a portion of the shroud based on the measured at least one variable.

23. The method of claim 22, wherein the shroud comprises a plurality of pivotable shroud segments and altering a pitch of at least a portion of the shroud comprises changing a pitch of at least one of the plurality of pivotable shroud segments.

24. The method of claim 22, wherein altering a pitch of at least a portion of the shroud based on the measured at least one variable at least partially compensates for fluid shear forces on the shrouded fluid turbine.

25. The method of claim 24, wherein the measured at least one variable comprises a load variable selected from a group consisting of: blade load, blade bending, blade tip acceleration, nacelle tilt loading, and load as a function of azimuthal rotor position.

26. The method of claim 24, wherein the measured at least one variable comprises a first fluid velocity measured at first portion of a rotor plane and a second fluid velocity measured at a second portion of the rotor plane.

27. The method of claim 24, wherein altering a pitch of at least a portion of the shroud based on the measured at least one variable dampens oscillations in a support structure for the shrouded fluid turbine.

28. The method of claim 24, wherein the measured at least one variable comprises a tower base moment variable, and wherein altering a pitch of at least a portion of the shroud based on the measured at least one variable reduces movement of the tower base.

29. The method of claim 28, wherein the tower base movement variable is selected from a group consisting of: tower-top acceleration, tower tilt and rotor-power output.

30. The method of claim 22, wherein the shrouded wind turbine supplies power for a utility grid, and the measured at least one variable comprises a control variable; and wherein a pitch of at least a portion of the shroud is altered to augment or reduce a power output of the shrouded wind turbine based on the measured at least one variable.

31. The method of claim 30, wherein the control variable is selected from a group consisting of: rotor speed, rotor-power output, rotor-shaft torque, and ambient wind speed.

32. A method of controlling a power output of an array of shrouded wind turbines, each shrouded wind turbine including a rotor and a shroud having a low pressure surface in fluid communication with the rotor, the method comprising:
measuring a reactive power of the array; and altering a pitch of at least a portion of the shroud of at least one of the array of shrouded wind turbines based on the measured reactive power to augment or reduce the reactive power of the array.

33. The method of claim 32, wherein the shroud of each of the array of shrouded wind turbines comprises a plurality of pivotable shroud segments, and wherein altering a pitch of at least a portion of the shroud of the at least one of the array of shrouded wind turbines comprises changing a pitch of at least one of the plurality of pivotable shroud segments.

34. The method of claim 32, wherein the power output is controlled during a low-voltage ride-through.

35. The method of claim 32, wherein altering a pitch of at least a portion of the shroud of the at least one of the array of shrouded wind turbines causes a first turbine or a first set of turbines to yaw.

36. The method of claim 32, wherein the yawing reduces the overall power output of the first turbine or first set of turbines.

37. The method of claim 35, wherein the yawing deflects wind from the first turbine or set of turbines toward a second turbine or set of turbines to equalize power output from each turbine in the array.