Abstract:
Turbine systems for extracting energy from water traveling relative to the turbine system can include a rotor assembly for extracting the energy, a turbine shroud having a turbine shroud inner volume within which at least a portion of the rotor assembly is disposed, and an ejector shroud having a ejector shroud inner volume within which at least a portion of the turbine shroud is disposed. The turbine shroud and the ejector shroud can each have a terminus comprising a plurality of turbine shroud mixer elements or ejector shroud mixer elements, respectively. One or more of the mixer elements and ejector shrouds comprise a mixer/ejector pump which increases the energy extraction potential of the turbine system. One or more of the turbine shroud mixer elements, ejector shroud mixer elements, and ejector shroud and turbine shroud inlets can be asymmetric about a plane passing through the axis of rotation of the rotor assembly. Methods, systems, apparatus and articles of manufacture relating to these features and others are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. provisional patent application Ser. No. 61/124,397, filed on Apr. 16, 2008 and entitled “Water Turbines with Mixers and Ejectors,” This application is also a continuation-in-part of co-pending application for U.S. patent Ser. No. 12/053,695, filed on Mar. 24, 2008 and entitled “Propeller Propulsion Systems Using Mixer Ejectors” which claims the benefit of and incorporates by reference U.S. provisional patent application Ser. No. 60/919,588, filed on Mar. 23, 2007 and entitled, “Mixer-Ejector Power/Propulsion System.” The disclosures of the applications from which the current applications derives a priority right are incorporated by reference herein in their entireties. 
     
    
     FIELD 
       [0002]    The instant subject matter relates to axial flow turbine and ejector systems, such as for example, those used to extract energy when immersed in a flowing water stream, such as that of an ocean-current, tidal-current, river/stream current and other fluid currents. 
       BACKGROUND 
       [0003]    Axial flow water turbine systems that extract energy from flowing water are referred to herein as “current turbines.” Current turbines usually contain a propeller-like device or “rotor,” that is directed to receive a moving stream of water. As depicted in  FIG. 1 , a rotor can be either unshrouded or contained in a shroud. As the current hits the rotor, the current produces a force on the rotor in such a manner as to cause the rotor to rotate about its center. The rotor can be connected to either an electric generator or mechanical device through linkages such as gears, belts, chains or other means. Such turbines can be used for generating electricity and/or to drive rotating pumps or moving machine parts. They may also be used in large electricity generating “current turbine farms” (also termed “current turbine arrays”) containing multiple such turbines in a geometric pattern designed to allow maximum power extraction with minimal impact of each such turbine on one another and/or the surrounding environment. 
         [0004]    The ability of an unshrouded rotor to convert fluid power to rotating power, when placed in a stream of width and depth larger than its diameter, is limited by the well documented theoretical value of 59.3% of the oncoming stream&#39;s power, known as the “Betz” limit which was documented by A. Betz in 1926. This productivity limit applies especially to the traditional multi-bladed axial current and tidal turbines shown in  FIG. 1A . Attempts have been made to try to increase current turbine performance potential beyond the “Betz” limit. Properly designed shrouds can cause the oncoming flow to speed up as it approaches the rotor compared to what is experienced by an unshrouded rotor. The oncoming flow is thereby concentrated into the center of the duct. In general, for a properly designed rotor, this increased flow speed over that of an unshrouded rotor causes more force on the rotor and subsequently higher levels of power extraction than the same size unshrouded rotor. Previous shrouded current turbines such as those shown in  FIG. 1B  have employed entrance concentrators and exit diffusers to increase the flow velocities at the turbine rotor. Diffusers, which typically include a pipe-like structure with openings along the axial length to allow slow, diffusive mixing of water inside the pipe with that outside the pipe, generally require long lengths for good performance, and tend to be very sensitive to oncoming flow variations. Such long, flow sensitive diffusers are impractical in many installations. Short diffusers can stall and thereby reduce the energy conversion efficiency of the system. 
       SUMMARY 
       [0005]    In a first implementation, a turbine system for extracting energy from water traveling relative to the turbine system in an incoming current flow direction has an inlet end adapted to be directed into the incoming current flow direction and an outlet end opposite the inlet end. The water has a non-uniform flow velocity distribution across the inlet end of the turbine system. The turbine system includes a rotor assembly, a turbine shroud having a turbine shroud inner volume within which at least a portion of the rotor assembly is disposed, and an ejector shroud having a ejector shroud inner volume within which at least a portion of the turbine shroud is disposed. The rotor assembly is axially symmetric about an axis of rotation and has an upstream rotor face oriented toward the inlet end. The turbine shroud includes a turbine shroud inlet disposed nearer the inlet end than the rotor face and a turbine shroud terminus disposed nearer the outlet end than the rotor assembly. The turbine shroud terminus includes a plurality of turbine shroud mixer elements. The turbine shroud inlet is adapted to direct a first volume of water moving in the incoming current flow direction to the rotor assembly such that the first volume causes the rotor assembly to spin and to extract energy from the first volume of water before the first volume of water at a lower energy is discharged from the turbine shroud into the ejector shroud via the turbine shroud terminus. The ejector shroud includes an ejector shroud inlet and an ejector shroud terminus. The ejector shroud inlet is asymmetric about the plane passing through the axis of rotation such that it has greater cross-sectional area on a lower velocity side of a plane passing through the axis of rotation than on a higher velocity side of the plane passing through the axis of rotation. The ejector shroud terminus extends in the current flow direction beyond the turbine shroud mixer elements. 
         [0006]    In a second interrelated implementation, a turbine system includes a rotor assembly that is axially symmetric about an axis of rotation and that has an upstream rotor face oriented toward the inlet end, a turbine shroud having a turbine shroud inner volume within which at least a portion of the rotor assembly is disposed, and an ejector shroud having a ejector shroud inner volume within which at least a portion of the turbine shroud is disposed. The turbine shroud includes a turbine shroud inlet disposed nearer the inlet end than the rotor face and a turbine shroud terminus disposed nearer the outlet end than the rotor assembly. The turbine shroud terminus includes a plurality of turbine shroud mixer elements that are asymmetric about a plane passing through the axis of rotation such that at least one of the turbine shroud mixer elements on a lower velocity side of the plane passing through the axis of rotation is larger than at least one of the turbine shroud mixer elements on a higher velocity side of the plane passing through the axis of rotation. The turbine shroud inlet is adapted to direct a first volume of water moving in the incoming current flow direction to the rotor assembly such that the first volume causes the rotor assembly to spin and to extract energy from the first volume of water before the first volume of water at a lower energy is discharged from the turbine shroud via the turbine shroud terminus. The ejector shroud includes an ejector shroud inlet and an ejector shroud terminus extending in the current flow direction beyond the turbine shroud mixer elements. 
         [0007]    In a third interrelated implementation, a method of extracting energy from water traveling relative to a turbine system in a current flow direction includes capturing a first volume of the water into a turbine shroud having a turbine shroud inner volume within which at least a portion of a rotor assembly is disposed, directing the first volume of water through the rotor assembly such that the rotor assembly extracts energy from the first volume of water before the first volume of water at a lower energy is discharged from the turbine shroud via the turbine shroud terminus, capturing a second volume of the water into an ejector shroud having a ejector shroud inner volume within which at least a portion of the turbine shroud is disposed, and mixing the first and the second volumes into a mixed volume before discharge of the mixed volume from the ejector shroud terminus. The turbine shroud includes a turbine shroud inlet disposed nearer the inlet end than the rotor assembly and a turbine shroud terminus disposed nearer the outlet end than the rotor assembly. The turbine shroud terminus includes a plurality of turbine shroud mixer elements. The ejector shroud includes an ejector shroud inlet and an ejector shroud terminus. The ejector shroud terminus extends in the current flow direction beyond the turbine shroud mixer elements. 
         [0008]    One or more additional optional variations and features can be included in a given implementation of the instant subject matter. The ejector shroud mixer elements and turbine shroud mixer elements can be specifically designed to form a mixer/ejector pump which increases the energy extraction potential of the system by both increasing the flowrate through the turbine rotor and by mixing the low energy turbine shroud exit flow with bypass flow that enters the ejector shroud inlet without passing through the turbine rotor. The ejector shroud inlet can be adapted to direct a second volume of water moving in the current flow direction into the ejector shroud inner volume, and the ejector shroud interior volume can include a plurality of ejector shroud mixer elements that cause the first volume of water to mix with the second volume of water before exiting through the ejector shroud terminus. The turbine shroud and the ejector shroud shapes can minimize a velocity gradient presented to the rotor face, maximize the first volume of water, and maximize mixing of the first and the second volumes before discharge from the ejector shroud terminus. The velocity gradient is measured along the rotor face. 
         [0009]    A center body about which the rotor assembly rotates can be included. The turbine shroud can include a stator assembly that includes stator vanes arrayed axially about the center body. The stator vanes can be rotatable to adjust the first volume by increasing or decreasing the open flow area presented to the incoming current flow direction. The turbine shroud inlet can include one or more movable door elements that are operable to increase or reduce the first volume flowing through the rotor assembly. A deflector shaped to inertially separate suspended debris and/or aquatic debris from the first volume prior to the first volume encountering the rotor face can be positioned ahead of the center body. The center body can include a downstream end projecting from the center body toward the turbine shroud terminus and into the ejector shroud. The center body can include a central hollow cavity adapted to allow suspended aquatic debris and/or aquatic life to pass through the center body toward the turbine shroud terminus without encountering the rotor blades. The central hollow cavity, which can optionally include mixer elements at its trailing edge, can also pass high energy bypass flow to the ejector shroud to enhance mixing performance in the ejector shroud. The downstream end can include one or more center body mixer elements. Flow through the hollow center body with downstream mixer elements can enhance the performance of the mixer/ejector pump operation. 
         [0010]    The turbine shroud inlet can have a non-circular cross-section that has greater cross-sectional area on the lower velocity side of the plane passing through the axis of rotation than on the higher velocity side of the plane passing through the axis of rotation. The turbine shroud mixer elements can include one or more of mixer lobes and mixer slots. The rotor assembly can include a rotor hub, an outer rotor ring, and a first plurality of radially oriented rotor blades disposed between the hub. The ejector shroud terminus region can include a second plurality of ejector shroud mixer elements that can include one or more of mixer lobes and mixer slots. 
         [0011]    The plurality of ejector shroud mixer elements can be asymmetrical about the plane passing through the axis of rotation. For example, one or more of the ejector shroud mixer elements on the lower velocity side of the plane passing through the axis of rotation can be larger than one or more of the ejector shroud mixer elements on the higher velocity side of the plane passing through the axis of rotation. Similarly, the plurality of turbine shroud mixer elements can be asymmetrical about the plane passing through the axis of rotation, one or more of the turbine shroud mixer elements on the lower velocity side of the plane passing through the axis of rotation being larger than one or more of the turbine shroud mixer elements on the higher velocity side of the plane passing through the axis of rotation. 
         [0012]    A second ejector shroud having a second ejector shroud inner volume within which at least a portion of the ejector shroud is disposed can be included. The second ejector shroud can include a second ejector shroud inlet and a second ejector shroud terminus region. The second ejector shroud inlet can be asymmetrical about the plane passing through the axis of rotation such that it has greater cross-sectional area on a lower velocity side of the plane passing through the axis of rotation than on the higher velocity side of the plane passing through the axis of rotation, the second ejector shroud terminus extending in the current flow direction beyond the ejector shroud mixer elements. 
         [0013]    The instant subject matter can provide many advantages. For example current turbines are conceptually similar to wind turbines but differ in detail in order to mitigate water-borne complications, such as: forces approximately 900 times greater than those encountered by wind turbines, significant buoyancy induced vertical forces, damaging asymmetrical/unsteady loads due to significant vertical variation in the incoming velocity field caused by the close proximity of a fixed surface such as a basin floor or wall or the hull of a ship, barge, or other waterborne craft to which the current turbine is secured. Sediment scouring can also occur fore and aft of the current turbine due to flow velocity profile disruptions caused by lower energy water exiting the turbine and re-mixing with current flow that bypasses the turbine inlet or inlets. Aquatic life safety, water corrosion and fouling avoidance systems, and floating debris management can also preset important challenges for the efficient use of a current turbine. These demands typically require use of stronger, heavier, and water resistant materials, different supporting mechanisms and internal structure, different aero/hydrodynamic shaping and careful management of the water flow fore and aft of the current turbine. All of these factors can add significantly to expenses incurred per generated energy unit. 
         [0014]    Various features of the current turbines according to the instant subject matter can advantageously address many of these challenges. For example, an ejector shroud can be provided that encompasses a turbine shroud housing the rotor assembly. A second water volume flowing into the ejector shroud bypasses the turbine shroud and therefore does not have energy extracted. This second water volume is actively mixed with a first water volume after the first volume has passed through the rotor assembly and had energy extracted. The mixing occurs within the ejector shroud and prior to discharge out the terminus of the ejector shroud. 
         [0015]    First-principles-based theoretical analysis of the current turbines as described herein indicate: that they are capable of producing three or more time the power of currently available un-shrouded turbines for the same rotor frontal area. The instantly disclosed current turbines can increase the productivity of current and tidal farms by a factor of two or more. 
         [0016]    The details of one or more variations of the instant subject matter are set forth in the accompanying drawings and the description below. Other features and advantages of the instant subject matter will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0017]    The accompanying drawings, which are incorporated in and constitute a part of this specification, show certain aspects of the instant subject matter and, together with the description, help explain some of the principles associated with the disclosed embodiments and implementations. In the drawings, 
           [0018]      FIGS. 1A ,  1 B, and  1 C are schematic diagrams illustrating examples of current turbine systems; 
           [0019]      FIGS. 2A ,  2 B,  2 C, and  2 D are schematic diagrams illustrating multiple views of an implementation of a current turbine system; 
           [0020]      FIG. 3A  and  FIG. 3B  are schematic diagrams illustrating a front perspective view of a current turbine system having one six-bladed rotor; 
           [0021]      FIGS. 4A and 4B  are schematic diagrams illustrating front perspective views of a current turbine system with a stator-rotor turbine and with portions broken away to show interior structure, such as a power takeoff in the form of a wheel-like structure attached to the outer rim of the rotor and power take-off to a ring generator at the rotor inner ring; 
           [0022]      FIGS. 5A ,  5 B,  5 C, and  5 D are schematic diagrams illustrating optional installations of current turbine systems; 
           [0023]      FIG. 6  is a schematic diagram illustrating an alternate implementation of a current turbine system with a mixer/ejector pump having mixer lobes that vary in shape and size around the circumference on the terminus regions of the turbine shroud and ejector shroud; 
           [0024]      FIGS. 7A ,  7 B,  7 C, and  7 D are schematic diagrams illustrating alternate implementation of a current turbine system with two optional pivoting rudders and wing for current flow alignment and movement, flow blockage/control doors and stators that can rotate into or out of a plane passing through the door or stator and the center body of the current turbine system; 
           [0025]      FIGS. 8A ,  8 B, and  8 C are schematic diagrams illustrating alternate implementations of a current turbine system with an open passageway center body having lobe mixers and ejectors with slot mixers; 
           [0026]      FIGS. 9A ,  9 B,  9 C, and  9 D are schematic diagrams illustrating alternate implementations of a current turbine system with an inlet debris blocker system; 
           [0027]      FIGS. 10A and 10B  are schematic diagrams illustrating an alternate implementations of a water turbine system of a current turbine system with a two stage mixer/ejector system; and 
           [0028]      FIG. 11  is a process flow chart illustrating a method according to an implementation of the instant subject matter. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    Gas turbine concepts and technology have yet to be applied commercially to axial flow current turbines. Most existing current turbines use a single multibladed rotor based on propulsive propeller concepts to extract the current energy. As a result, a significant amount of the flow passing through the current turbine blades converts some of the flow energy into swirling flow about the axis. This swirl component absorbs energy that cannot be delivered to the generator plus it induces flow rotation in the wake of the system that can induce current bed scouring, sediment stirring and aquatic life disorientation. These effects can be mitigated and even eliminated using mature gas turbine stator/rotor turbine aero/hydrodynamic flow considerations. Gas turbine rotor/stator design approaches can be applied to current turbines to essentially eliminate the detrimental effects of exit-flow swirl on the environment aft of the turbine. 
         [0030]    Additionally, traditional single rotor systems, such as depicted in  FIG. 1A , are delayed in rotational start-up and thus energy production until the local axial velocity level is high enough to induce positive aero/hydrodynamic lift and torque on the rotor airfoil. Properly designed inlet stator/rotor systems according to the instant subject matter do not have this limitation and are therefore capable of inducing torque on the rotor and producing power for all local velocities levels above zero. Also, previous shrouded current turbines have failed to account for aero/hydrodynamic efficiency of the flow around the exterior of the shroud, especially in the presence of a free surface, basin floor or sidewall, or the hull of a watercraft. Adaptation of current turbine inlets to manage debris and/or aquatic life approaching the inlet is also an optional feature of the instant subject matter. An aerodynamically or hydrodynamically tailored bulbous shape can be positioned in front of the inlet to first deflect the incoming water and any suspended debris contents outward. The water current stream has lower inertia than larger suspended debris and/or aquatic life and therefore can follow the contour of the bulbous shape to enter the turbine shroud or the ejector shroud. Higher inertia suspended objects, such as aquatic animals, debris, and the like deviate from the water stream lines and therefore fail to enter the turbine shroud or the ejector shroud. 
         [0031]    To achieve increased power and efficiency in currents, it is generally necessary to closely tailor the aero/hydrodynamic designs of the shroud and rotor to the vertically varying velocity profile approaching the turbine. Velocity profiles generally follow a 1/10 th  power-law dependency between minimum and maximum levels, which usually, but not always occur at the current bed and free surface respectively. While wind turbines encounter a similar vertical variation, it is not nearly as severe as the current turbine case because a wind turbine is of miniscule vertical scale compared to the earth&#39;s atmosphere height. Water is approximately 900 times more dense than air. Because the power generated depends on the density of the fluid and the cube of the local velocity while the axial force depends on the density and square of the velocity, this level of variation causes significant asymmetric power delivery and structural loads on the rotor as well as shroud system unless controlled by the aero/hydrodynamic design. Whereas wind turbines are generally symmetric about their center axis of rotation, shrouded current turbines provide the opportunity to employ asymmetric features to control and mitigate incoming velocity profile induced complications. In particular, although the interior surface of the shroud must necessarily be near-circular where it surrounds the rotor, this limitation does not apply to the remainder of the shroud geometry, either internally or externally. Thus, aero/hydrodynamic contour variation around the circumference of the shroud can be used to reduce the distortion of the incoming flow to an acceptable level by the time it reaches the rotor face. Further, such asymmetric or ovalated aero/hydrodynamic contouring can reduce the impact of the flow exiting the system on the surrounding environment by reducing scouring and sediment stirring of the current basin and walls. 
         [0032]    Ejectors draw flow into a system and thereby increase the flow rate through that system. By using ring airfoil concepts in the design of the multiple shrouds of an ejector, the rotor size required for a desired power output level can be reduced to as much as a half or less than that imposed on an unshrouded rotor. Shorter rotor blades are less costly and structurally more robust. Further, the axial forces imposed on the rotor by the current can also be reduced by a half or more while the remaining loads are shifted to the non-rotating elements of the shrouded system. Load carrying by static, non-rotating parts are vastly simpler and economical to design, manufacture and maintain. 
         [0033]    Mixer/ejectors are short compact versions of ejector jet pumps that are relatively insensitive to incoming flow distortions and have been used extensively in high speed jet propulsion applications involving flow velocities near or above the speed of sound. See, for example, U.S. Pat. No. 5,761,900 by one of the inventors, Dr. Walter M. Presz, Jr, which also uses a mixer downstream to increase thrust while reducing noise from the discharge. In all previous power generation applications of mixer/ejector technology, including those for wind turbines designed by the inventors, the multiple three dimensional surfaces that induce the flow mixing between two streams, herein after termed the mixing elements, are all the same size and arrayed in a repeating pattern around the circumference of a shroud. To accommodate the velocity distortion embedded in the flow approaching a current turbine and to work efficiently within the attendant ovalated shroud inlets, advanced mixing element design can be employed to affect maximum mixing and pumping for each circumferential sector of the system. 
         [0034]    Current turbines, like wind turbines, must be able to adjust the power output to be compatible with the rated power level of the generator. Traditional three bladed wind turbines can experience wind speeds from up to 10 times their average operating wind speed and must incorporate a complicated mechanical shut down system to avoid damage to the generator and/or the structure. Current turbines experience less extreme velocity variations and thus typically incorporate differently designed shut down systems. Multi-shrouded mixer/ejector current turbines employing stator/rotor systems offer three means of affecting shut down in addition to a standard breaking system. The stators can be articulated to essentially shut the inlet opening, blocker doors, built into the inner surfaces of the shrouds, can be swung into the flow field thereby obstructing flow passage, and/or an inlet debris blockage bulb can be moved into the inlet to reduce the flow rate. 
         [0035]    Anchoring systems for shrouded current turbines are very different than the tall towers used for wind turbines and, as such, they must be integrally designed to avoid compromising the aero/hydrodynamic efficiency of the tightly coupled system. Systems on poles or on platforms such as shown in  FIG. 1  will encounter different levels and sources of aero/hydrodynamic interference that must be reduced to assure efficient delivery of energy. 
         [0036]    Multi-shrouded mixer/ejector current turbines provide the opportunities for unique integration of the rotor and generator systems. Because current turbines are not required to change direction or, for the tidal case only do so on a twice a day schedule, the generator can be more conveniently placed for efficiency and/or easier maintenance servicing. Employing rotor tip shrouds, as frequently employed in gas turbines, allows use of a rim gear/drive system and placement of the generator in or on the shroud. Additionally, it allows the center body to be designed as an open conduit for aquatic to pass through. 
         [0037]      FIGS. 2-10  show a number of implementations that illustrate some of the features that are within the scope of the current subject matter. According to one implementation, a water turbine system includes an aerodynamically or hydrodynamically contoured turbine shroud  102  is noncircular at some points along its axial extent. An aerodynamically or hydrodynamically contoured center body  103  is enclosed within and attached to the turbine shroud  102  that has a turbine shroud inlet  105  through which a first volume of water is drawn. The center body  103  is axially symmetric about a rotor axis of rotation. A turbine stage  104  surrounds the center body  103  and includes a stator ring  106  of stator vanes  108   a  and an impeller or rotor  110  having impeller or rotor blades  112   a . The rotor  110  includes a rotor face formed by the leading edge of the rotor blades  112   a . The rotor  110  is positioned downstream from the stator vanes  108   a  such that the rotor face is substantially aligned with trailing edges of the stator vanes  108   a . The stator vanes  108   a  are mounted on the center body  103 , and the rotor blades  112   a  are attached and held together by inner and outer rings or hoops or alternatively by a hub  112   b  and an outer ring  112   c . The inner ring or hub encircles and is rotatable about the center body  103 . A terminus region of mixer element that includes a terminus region or end portion of the turbine shroud  102  includes a ring of mixer lobes  120   a  that extend downstream beyond the rotor blades  112   a  and vary in shape or size as needed to fill the gap between the turbine shroud  102  and ejector shroud  128  and deliver ingested water to the vicinity of the center body  103 . This is similar to the ejector lobes shown in U.S. Pat. No. 5,761,900, wherein the mixer lobes  120   a  extend downstream and into an inlet  129  of the ejector shroud  128 . The ejector  122  also includes a shroud  128  that can be noncircular for portions of its axial length and that surrounds the ring of mixer lobes  120   a  on the turbine shroud. The ejector shroud  128  can include mixer elements of varying sizes and shapes in its terminus region as shown in  FIG. 6 . 
         [0038]    The center body  103 , as shown in  FIG. 2 , can be connected to the turbine shroud  102  through the stator ring  106  (or other means) to eliminate the damaging, annoying and long distance propagating low-frequency pressure waves produced by traditional current and tidal turbines as the turbine&#39;s blade wakes strike the support tower. The aerodynamic profiles of the turbine shroud  102  and ejector shroud  128  preferably are aerodynamically cambered to increase flow through the turbine rotor in such a way as to reduce the vertical variation in the velocity at the rotor face induced by upstream distortions. 
         [0039]    Applicants have calculated that, for optimum efficiency in the preferred embodiment  100 , the area ratio of the ejector pump  122 , as defined by the cross sectional area of the ejector shroud terminus divided by the cross sectional area of the turbine shroud terminus will be between 1.5 and 4.0. The number of mixer lobes  120   a  would be between 6 and 14. Each lobe will have inner and outer trailing edge angles between 5 and 25 degrees. The primary lobe exit location will be at, or near, the entrance location or inlet  129  of the ejector shroud  128 . The height-to-width ratio of the lobe channels will be between 0.5 and 4.5. The mixer penetration will be between 30% and 80%. The center body  103  plug trailing edge angles will be thirty degrees or less. The length to diameter (L/D) of the overall system  100  will be between 0.5 and 1.25. 
         [0040]    In general, a current turbine energy conversion system includes an axial flow current turbine  100  that includes stator vanes  108   a  and impeller or rotor blades  112  and that is surrounded by an aerodynamically contoured turbine shroud  102  that incorporates mixing elements  120   a  in its terminus region or end portion and a separate ejector shroud  128  overlapping, but aft, of the turbine shroud  102 . The ejector shroud  128  can also incorporate advanced mixing elements, such as for example mixer lobes  119  or mixer slots, in its terminus region. A ring  118  of mixer features such as lobes or slots  119  located at the terminus  117  of the ejector shroud  128  can be thought of as a mixer/ejector pump that provides the means for consistently exceeding the Betz limit for operational efficiency of the current and tidal turbine system  100 . 
         [0041]      FIG. 2A  shows a turbine stage  104  that includes a rotor assembly  110  that is rotatably mounted on a center body  103 , surrounded by turbine shroud  102  with embedded mixer elements  120   a  having trailing edges inserted slightly in the entrance plane of ejector shroud  128 . The turbine stage  104  and ejector shroud  128  are structurally connected to the turbine shroud  102 , which itself is the principal load carrying member. 
         [0042]    The length of the turbine shroud  102  can in some implementations be equal to or less than the turbine shroud&#39;s  102  maximum outer diameter. The length of the ejector shroud  128  can in some implementations be equal to or less than the ejector shroud&#39;s maximum outer diameter. The exterior surface of the center body  103  can be aerodynamically or hydrodynamically contoured to minimize the effects of flow separation downstream of the current turbine system  100 . The center body  103  can be longer or shorter than the turbine shroud  102  or the ejector shroud  128 , or their combined lengths. 
         [0043]    The cross sectional area of the turbine shroud inlet  105  and the turbine shroud terminus  115  can be equal to or greater than that of the annulus occupied by the turbine stage  104 , but need not be circular in shape so as to allow better control of the flow source and impact of its wake. The internal flow path cross-sectional area formed by the annulus between the center body  103  and the interior surface of the turbine shroud  102  is aerodynamically shaped to have a minimum area at the plane of the rotor assembly  110  and to otherwise vary smoothly from their respective entrance planes to their exit planes. The turbine shroud  102  and the ejector shroud  128  external surfaces are aerodynamically or hydrodynamically shaped to assist guiding the flow into the turbine shroud inlet  105 , eliminating flow separation from their surfaces, and delivering smooth flow into the ejector shroud entrance  129 . The ejector  128  entrance area, which may be noncircular in shape, is larger than the cross sectional area of the turbine shroud terminus  115  including the mixer features  118  at the turbine shroud terminus. The cross-sectional area at the ejector shroud terminus  117  can also be noncircular in shape. 
         [0044]    An example of a power take-off  130  as shown in  FIG. 4A  and  FIG. 4B  can take the form of a wheel-like structure mechanically linked at an outer or inner rim of the rotor assembly  110  to a power generator (not shown) below or above the rotor assembly  110 . A vertical support shaft  132  with a rotatable coupling at  134  as shown in  FIG. 4A  and  FIG. 5A  can rotatably support the current turbine system  100  and can be located forward of the center-of-pressure location experienced by the current turbine system  100  for self-aligning of the current turbine system when submerged in a flowing current. Self-moving vertical rudders  136  and generally horizontal wings  135  (see  FIG. 7 ), affixed to upper and lower surfaces of the turbine and/or the ejector shrouds  102  and  128  respectively, to stabilize alignment directions with different current and tidal streams and provide steering during vertical movements. 
         [0045]    A current turbine system  100 , can be structurally supported by other systems as shown for example in  FIG. 5A ,  FIG. 5B ,  FIG. 5C , and  FIG. 5D , such as for example a pole  133 , a fixed foundation  137 , tethers  138 , or a water borne craft  139  such as a barge or float. 
         [0046]    Variable mixer element geometries can be used and optimized to extract maximum energy from the bypass air flow as shown in  FIG. 6 . The mixer elements  140  can be asymmetric in relation to a plane passing through the rotor assembly  110  axis of rotation as  FIG. 6  shows. 
         [0047]      FIG. 7  show control rudders and wings  135  and  136 , and optional flow blockage doors  140   a ,  140   b . They can be rotated via linkage (not shown) into the flow stream to reduce or stop flow through the turbine  100  when damage, to the generator or other components, due to high flow velocity is possible.  FIG. 7D  presents another optional variation of a current turbine system  100 . The stator vanes&#39; exit-angle incidence can be mechanically varied  142 , in situ, for example by pivoting the stator vanes to accommodate variations in the fluid stream velocity so as to assure minimum residual swirl in the flow exiting the rotor. 
         [0048]    Additional alternative variations can include an open passageway center body  144  as shown in  FIG. 8A  and  FIG. 8B  that can include center body mixer elements  145 ; slot type mixers  146  as shown in  FIG. 8C ; a center body that includes a debris deflectors  147  as shown in  FIGS. 9A ,  9 B,  9 C and  9 D; and multiple ejector shrouds  148  as shown in  FIG. 10A  and  FIG. 10B . 
         [0049]      FIG. 11  is a process flow chart illustrating a method according to an implementation of the instant subject matter. At  1102  a first volume of water is captured into a turbine shroud having a turbine shroud inner volume within which at least a portion of a rotor assembly is disposed. The turbine shroud includes a turbine shroud inlet disposed nearer the inlet end than the rotor assembly and a turbine shroud terminus disposed nearer the outlet end than the rotor assembly. The turbine shroud terminus includes a plurality of turbine shroud mixer elements. At  1104 , the first volume of water is directed through the rotor assembly such that the rotor assembly rotates and extracts energy from the first volume of water before the first volume of water at a lower energy is discharged from the turbine shroud via the turbine shroud terminus. At  1106 , a second volume of the water is captured into an ejector shroud having a ejector shroud inner volume within which at least a portion of the turbine shroud is disposed. The ejector shroud includes an ejector shroud inlet and an ejector shroud terminus that extends in the current flow direction beyond the turbine shroud mixer elements. At  1110 , the first and the second volumes are combined or mixed into a mixed volume before discharge of the mixed volume from the ejector shroud terminus. Other structural features as described above can be included in the structure employed in methods according to the instant subject matter. 
         [0050]    The implementations set forth in the foregoing description do not represent all implementations consistent with the instant subject matter. Instead, they are merely some examples consistent with aspects related to the described subject matter. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. Although a few variations have been described in detail above, other modifications or additions are possible. In particular, further features and/or variations may be provided in addition to those set forth herein. For example, the implementations described above may be directed to various combinations and subcombinations of the disclosed features and/or combinations and subcombinations of several further features disclosed above. In addition, the logic flow depicted in the accompanying figures and/or described herein do not require the particular order shown, or sequential order, to achieve desirable results. Other embodiments or implementations may be within the scope of the following claims.