Abstract:
A fluid turbine lightning protection system includes at least one air termination device, formed at least in part of an electrically conductive material, that can be positioned on a shroud of a fluid turbine and placed in electrical communication with a down conduction system. The down conduction system is in electrical communication with an earth-termination system configured to dissipate electricity transferred thereto to the ground. The at least one air termination device is configured to intercept a lightning strike and direct it through the down conduction system, the earth-termination system, and into the ground. The at least one air termination device may be positioned on a turbine shroud based on a “Rolling Sphere” derivation wherein the “Rolling Sphere” derivation is derived from the equation r=10·I 0.65 , where I is the peak current in kiloamperes and r is the rolling sphere radius in meters.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional patent application No. 61/534,467 filed on Sep. 14, 2011, the contents of which is hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to the field of wind turbines and more particularly to the protection of shrouded turbines from lightning strikes. Utility scale wind turbines used for power generation may have one to five open blades comprising a rotor. The rotor transforms wind energy into a rotational torque that drives at least one generator rotationally coupled to the rotor, either directly or through a transmission assembly, to convert mechanical energy to electrical energy. Such turbines typically have long blades that are the most susceptible component of the wind turbine to lightning strikes. Wind turbines are required to be equipped with lightning protection systems in order to conduct large currents from lightning strikes to the ground without damaging the components of the turbine. Lightning strikes pose a threat to the blades, metallic rotational equipment, and electronic components. Increased density in wind farms poses additional threats from lightning strikes. 
         [0003]    Blades comprised of weakly conductive material such as carbon fiber or other fiber reinforced polymers, experience high currents and therefore excessive heat when struck by lightning. External protective conductors, such as lightning rods, are impractical on a fast moving aerodynamic structure such as a blade. Alternatively, a protective mesh allows for a conductive layer without external conductors, however, the point of contact of a lightning strike will often damage the composite surface and provide a stress point where a crack can form. 
         [0004]    Conventional wind turbine blades are typically engaged with bearing systems between the blades and a hub, a shaft and a nacelle, and between the nacelle and a tower. The blades are typically engaged with a bearing system at the root, and rotate about their long axis to alter the chord angle with respect to the wind direction for control of the rotor rotational speed with respect to the wind speed. In addition to this bearing system, the set of blades are further engaged with a hub that is connected to a shaft engaged with a bearing system within the generator. The shaft is rotationally engaged with the bearing system within the generator and drives the generator. The nacelle rotates about the support structure to yaw the turbine with respect to the wind direction and, for this reason, is engaged with a yaw bearing system between the nacelle and the tower. The bearing systems of the wind turbine as described above may provide a spark gap and can be damaged by high current passage. In this described example wind turbine, lightning striking the tip of a blade needs to be conducted through three bearing systems before reaching a direct connection to the ground. 
         [0005]    Additionally, wind turbines may be equipped with meteorological equipment and sensitive electronic equipment that can be damaged by minor lightning strikes. 
         [0006]    As the density of turbines in a wind farm increases, the potential for a single lightning strike to damage more than one turbine increases. Electrical installations, such as overhead lines, may provide protective conductors arranged around or above the installation. However, horizontal axis wind turbines having open blades present an obstacle to such protective conductors. 
       SUMMARY 
       [0007]    The present disclosure relates to a shrouded fluid turbine lightning protection system, a shrouded fluid turbine system comprising a lightning protection system, and a method of protecting a shrouded fluid turbine from a lightning strike. 
         [0008]    An example embodiment of a shrouded fluid turbine lightning protection system includes at least one air termination device that can be positioned on a shroud of a fluid turbine. The at least one air termination device is formed at least in part of an electrically conductive material and in electrical communication with a down conduction system. The down conduction system is in electrical communication with an earth-termination system that is configured to dissipate electricity transferred thereto to the ground. The at least one air termination device is configured to intercept a lightning strike and direct it through the down conduction system, the earth-termination system, and into the ground. The at least one air termination device may be positioned on a turbine shroud based on a “Rolling Sphere” method wherein the “Rolling Sphere” method is derived from the equation r=10·I 0.65 , where I is the peak current in kiloamperes and r is the rolling sphere radius in meters. The shrouded fluid turbine may include a turbine shroud, and the at least one air termination device may also be positionable on the turbine shroud. The shrouded fluid turbine may include an ejector shroud, and the at least one air termination device may also be positionable on the ejector shroud. An electrically conductive material may be integrated with the turbine shroud and/or the ejector shroud and electrically engaged with the down conductive system. The electrically conductive material may be positioned on a leading or trailing edge of the turbine shroud, a leading or trailing edge of the ejector shroud, or integrated with the surface of either the turbine shroud or the ejector shroud. 
         [0009]    An example embodiment relates in general, to a shrouded fluid turbine comprising a ringed turbine shroud that surrounds a rotor, and at least one air termination device positioned on the shroud of the fluid turbine. The at least one air termination device is formed at least in part of an electrically conductive material and in electrical communication with a down conduction system. The down conduction system is in electrical communication with an earth-termination system that is configured to dissipate electricity transferred thereto to the ground. The at least one air termination device is configured to intercept a lightning strike and direct it through the down conduction system, the earth-termination system, and into the ground. The at least one air termination device may be positioned on a turbine shroud based on a “Rolling Sphere” method wherein the “Rolling Sphere” method is derived from the equation r=10·I 0.65 , where I is the peak current in kiloamperes and r is the rolling sphere radius in meters. This embodiment may further comprise an ejector shroud that surrounds the exit of the turbine shroud. 
         [0010]    In one embodiment, the turbine shroud may comprise a set of mixing lobes along the trailing edge. 
         [0011]    In one embodiment, the set of mixing lobes along the trailing edge are in fluid communication with the inlet of the ejector shroud. Together, the mixer lobes and the ejector shroud form a mixer-ejector pump that provides increased fluid velocity near the inlet of the turbine shroud, at the cross sectional area of the rotor plane. The mixer-ejector pump further provides a means of energizing the wake behind the rotor plane. The combination of the effects of the mixing lobes and the energized wake provide a rapidly-mixed, short wake when compared to non-shrouded horizontal axis wind turbines. The at least one air termination device may also be positionable on the ejector shroud. An electrically conductive material may be integrated with the turbine shroud and/or the ejector shroud and electrically engaged with the down conductive system. 
         [0012]    The electrically conductive material may be positioned on a leading or trailing edge of the turbine shroud, a leading or trailing edge of the ejector shroud, or integrated with the surface of either the turbine shroud or the ejector shroud. 
         [0013]    An example embodiment relates to a method of protecting a shrouded fluid turbine from a lightning strike. In the example method, a shrouded fluid turbine is provided and a peak lightning strike current is determined for the shrouded fluid turbine. A “Rolling Sphere” circumference is calculated based on the equation r=10·I 0.65 , where I is the peak current in kiloamperes and r is the rolling sphere radius in meters. One or more air termination devices are positioned on the shroud such that the calculated “Rolling Sphere” will contact the one or more air termination devices before contacting the shroud when the “Rolling Sphere” is rolled along an exterior of the shroud. 
         [0014]    The present embodiment discloses a Primary Lightning Protection system (LPS) and a Secondary Lightning Protection System (LPS2). The LPS is intended to intercept and conduct lightning strikes of a range from approximately ≧25 kA to ≦200 kA, safely from the air-termination system through the down conduction system to the earth-termination system. The corresponding rolling sphere system includes a range of radii from ≧81 m to ≦313 m. A secondary lightning protection system (LPS2) is comprised of materials integral to the shroud surfaces in combination with a down conduction system to the earth-termination system. The LPS2 is intended to intercept and conduct lightning strikes of a range from approximately ≧3 kA to ≦10 kA usually in the form of static electricity. These charges correspond to a rolling sphere radius of 20 m. 
         [0015]    The turbine shroud and/or the ejector shroud provide a platform for an integrated lightning protection system resulting in a system with reduced complexity when compared to the lightning protection systems of horizontal axis wind turbines. The turbine shroud and/or the ejector shroud include few to no electrical components or mechanical moving parts, thus further reducing the risk of critical damage to the shrouded turbine. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    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. 
           [0017]      FIG. 1  is a front, right, perspective view of an example embodiment of the present disclosure; 
           [0018]      FIG. 2  is a front, orthographic view of the example embodiment of  FIG. 1 ; 
           [0019]      FIG. 3  is a side, orthographic view of the example embodiment of  FIG. 1 ; 
           [0020]      FIG. 4  is a front, orthographic view of an example embodiment of the present disclosure; 
           [0021]      FIG. 5  is a side, orthographic view of the example embodiment of  FIG. 4 ; 
           [0022]      FIG. 6  is a front, orthographic view of an example embodiment of the present disclosure; 
           [0023]      FIG. 7  is a side, orthographic view of the example embodiment of  FIG. 6 ; 
           [0024]      FIG. 8  is a front, right, perspective, detail view of an example embodiment of the present disclosure; 
           [0025]      FIG. 9  is a right, perspective, detail view of the example embodiment of  FIG. 8 ; 
           [0026]      FIG. 10  is a front, right, perspective view of an example embodiment of the present disclosure; 
           [0027]      FIG. 11  is a front, orthographic view of the example embodiment of  FIG. 10 ; and 
           [0028]      FIG. 12  is a side, orthographic view of the example embodiment of  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    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. 
         [0030]    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. 
         [0031]    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.” 
         [0032]    A Mixer-Ejector Turbine (MET) provides an improved means of generating power from fluid currents. The Mixer-Ejector Turbine includes tandem cambered shrouds and a mixer/ejector pump. The primary shroud contains a rotor, which extracts power from a primary fluid stream. The tandem cambered shrouds and ejector bring more flow through the rotor allowing more energy extraction due to higher flow rates. The mixer/ejector pump transfers energy from the bypass flow to the rotor wake flow allowing higher energy per unit mass flow rate through the rotor. These two effects enhance the overall power production of the turbine system. 
         [0033]    A shrouded turbine provides an improved means of generating power from fluid currents. The shrouded turbine includes only a single shroud and does not include a mixer/ejector pump. The sole shroud contains a rotor, which extracts power from a primary fluid stream. 
         [0034]    The term “rotor” is used herein to refer to any assembly in which one or more blades are attached to a shaft and able to rotate, allowing for the extraction of power or energy from fluid (wind or water) rotating the blades. Exemplary rotors include a propeller-like rotor or a rotor/stator assembly. Any type of rotor may be enclosed within the turbine shroud in the shrouded turbine of the present disclosure. 
         [0035]    The leading edge of a turbine shroud may be considered the front of the fluid turbine, and the trailing edge the turbine shroud may be considered the rear of the fluid turbine. In embodiments with an ejector shroud, the trailing edge of the ejector shroud may be considered the rear of the fluid turbine. A first component of the fluid 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. 
         [0036]    In one embodiment, the present disclosure relates to a shrouded fluid turbine that includes a turbine shroud that surrounds a rotor and an integrated lightning protection system that employs the structure and surfaces of the shroud. In some embodiments, the present disclosure relates to a shrouded fluid turbine that includes a turbine shroud that surrounds a rotor, an ejector shroud that surrounds the exit of the turbine shroud and an integrated lightning protection system that employs the structure and surfaces of the shrouds. 
         [0037]    Various standards for lightning protection of wind turbines exist, and, as such, lightning protection systems typically employ a multi-faceted approach to the reduction of risk. An example external lightning protection system may consist of an air-termination system, a down conduction system, and an earth termination system. An air-termination system also known as a lightning rod, is a component of an external lightning protection system intended to intercept lightning flashes. A down conduction system is a conductor that is intended to conduct the lightning current from the air-termination system to the earth-termination system. An earth-termination system is a network of electrically interconnected rods, plates, mats, piping, grids, or other conductive components, installed below grade to establish a low resistance contact with the earth. 
         [0038]    One method of determining the risk of direct lightning attachment is known as the “Rolling Sphere” method, in which an imaginary sphere is rolled about the surfaces of a 3D digital model of the wind turbine. The radius of the sphere is defined as: 
         [0000]        r= 10 ·I   0.65    
         [0039]    where r is the rolling sphere radius in meters [m] and I is the Peak Current in kiloamperes [kA]. For given rolling sphere radius r, it can be assumed that all lightning strikes with peak values higher than the corresponding current will be intercepted. As the imaginary sphere is rolled about the surfaces of the 3D digital model of the wind turbine, it is determined if the sphere is able to come in contact with the wind turbine prior to contacting an air termination system. There is risk of direct lightning attachment to the turbine where the sphere is able to come in contact with the wind turbine prior to contacting an air termination system. 
         [0040]      FIGS. 1 through 9  depict example embodiments of a shrouded fluid turbine having two shrouds.  FIG. 1  is a perspective view of an exemplary embodiment of a shrouded fluid turbine of the present disclosure.  FIG. 2  is a front view of the fluid turbine of  FIG. 1 .  FIG. 3  is a side view of the fluid turbine of  FIG. 1 . Referring to  FIG. 1  through  FIG. 3 , the shrouded fluid turbine  100  comprises a turbine shroud  110 , an ejector shroud  120 , a rotor  140 , and a nacelle body  150 . The turbine shroud  110  includes a front end  112 , also known as an inlet end or a leading edge, and a rear end or trailing edge  116 , also known as an exhaust end. The trailing edge  116  includes high energy lobes  117  and low energy lobes  115 . The depiction of the recited high energy lobes  117  and low energy lobes  115  is solely for illustrative purposes. One of ordinary skill in the art will readily recognize that the shape and orientation of the lobes may take numerous forms and the illustrated embodiment is not intended to be limiting in scope. The ejector shroud  120  includes a front end  122 , also known as an inlet end or leading edge, and a rear end or trailing edge  124 , also known as an exhaust end. Support members  106  connect the turbine shroud  110  to the ejector shroud  120 . 
         [0041]    The rotor  140  surrounds the nacelle body  150  and 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 rotor  140 , turbine shroud  110 , and ejector shroud  120  are coaxial with each other, i.e., they share a common central axis  105 . 
         [0042]    At least one air termination device  161 , between 1/80 to 1/20 the diameter of the ejector in length, is engaged with the turbine shroud  110 . Additionally, at least one air termination device  164  between 1 m and 2 m in length is also engaged with the ejector shroud  120 . 
         [0043]    A Primary Lightning Protection system (LPS) is illustrated in  FIGS. 1  through  FIG. 5 .  FIG. 4  is a front view of an example embodiment, and  FIG. 5  is a side view of the example embodiment of  FIG. 4 . 
         [0044]      FIG. 2  and  FIG. 3  depict a rolling sphere radius in relation to the outer surface of the fluid turbine  100 . The rolling sphere circumference is illustrated by arc  170  and the radius is illustrated by arrow  172 . The rolling sphere arcs  170  have an approximate diameter of 313 meters, which corresponds to a lightning current of up to 200 kA. It can be seen in  FIGS. 2  and  3  that the arcs  170  do not come in contact with the body of the turbine shroud  110  or the ejector shroud  120  before coming in contact with at least one of the air termination devices  161 / 164 . It can be further seen in  FIGS. 2 and 3  that the arcs  170  do not come in contact with the rotor  140 , the hub  141 , or the nacelle  150  before coming in contact with at least one of the air termination devices  161 / 164 . The air termination devices  161 / 164  are in electrical communication with a down conduction system  176  by way of electrically coupled first and second conductors  174 / 175 . The down conduction system  176  transfers electricity to an earth-termination system  178  for dispersion of the electricity. 
         [0045]      FIG. 4  and  FIG. 5  depict a rolling sphere radius of approximately 80 meters, which corresponds to a lightning current in the range of approximately ≧25 kA to ≦50 kA. The rolling sphere circumference is depicted by arcs  270  and the radius is depicted by arrow  272 . It can be seen in  FIG. 4  and  FIG. 5  that the rolling sphere has minimal contact with the turbine shroud  210  and the ejector shroud  220 . Simultaneous contact of the rolling sphere with either of the turbine shroud  210  or the ejector shroud  220  and at least one of the air termination devices  261 / 264  is possible. In such a situation, e.g., in the even of simultaneous contact, current is safely conducted to an earth-termination  278  system through the air termination devices  261 / 264  that are coupled with a down conduction system  276  by way of electrically coupled first and second conductors  274 / 275 . This provides for the conduction of the current without significant damage to the turbine shroud  210  or the ejector shroud  220 , due to the current range. It can be further seen in  FIG. 4  and  FIG. 5  that the arcs  270  do not come in contact with the rotor  240 , the hub  241 , or the nacelle  250  before coming in contact with at least one of the air termination devices  161 / 164 . 
         [0046]    A secondary lightning protection system (LPS2) is illustrated in  FIG. 6  and  FIG. 7 . The LPS2 comprises electrically conductive materials integrated with the surfaces of a turbine shroud  310  and an ejector shroud  320 . The electrically conductive materials are connected with a down conduction system  376  by way of electrically coupled first and second conductors  374 / 375 , the down conduction system  376  is connected to an earth-termination system  378 .  FIG. 6  and  FIG. 7  depict a rolling sphere radius of approximately 20 meters, which corresponds to a lightning current in the range of approximately ≧3 kA to ≦10 kA. The rolling sphere circumference is depicted by arcs  370  and the radius is depicted by arrow  372 . The LPS2 is intended to intercept and conduct lightning strikes of a range from approximately ≧3 kA to ≦10 kA usually in the form of static electricity. Exposed hardware, for example, various metal fasteners (not shown) on the surface of the turbine shroud  310  and the ejector shroud  320  provide a sufficient means of dissipating a static charge prior to contact with the rotor  340 , the hub  341 , or the nacelle  350 . 
         [0047]    In addition to the protection provided by the air termination devices, shroud surfaces with embedded or integrated materials provide additional lightning protection to rotating and electrical generation components. 
         [0048]      FIG. 8  and  FIG. 9  illustrate an air-termination system integrated into the shroud surfaces. Referring to  FIG. 8 , a mixer ejector turbine  400  comprises a turbine shroud  410  surrounding a rotor  440 , engaged with a hub  441  that is further engaged with a nacelle  450 . An ejector shroud  420  has an inner diameter greater that the outer diameter of the trailing edge of the turbine shroud  410  and the injector shroud  420  and the turbine shroud  410  are concentric to one another. In some embodiments the ejector shroud  420  surrounds the trailing edge of the turbine shroud  410 . In some embodiments the ejector shroud  420  is located downstream from the trailing edge of the turbine shroud  410 . A first electrically conductive material  470  is comprised of metalized polymers, fiber reinforced composites with electrically conductive fibers woven into the reinforcement, or composites with metallic characteristics such as those provided by nanoparticles made from graphite. The first electrically conductive material  470  is engaged with the leading edge of the turbine shroud  410  and further conductively engaged with an internal structure  472  of the turbine shroud  410  that is both electrically conductive and insulated. The internal structure  472  is further conductively engaged, through an electrical wiper system, as described below, with a down-conductive system  476  that is conductively engaged with an earth-termination system  478 . A second electrically conductive material  475  is engaged with the trailing edge of the ejector shroud  420  and is further conductively engaged with an internal structure  474  of the ejector shroud  420  that is further conductively engaged with the down-conductive system  476  that is conductively engaged with the earth-termination system  478 . 
         [0049]    Electrical wiper systems, also known as slip rings, are commonly used to transfer electricity between stationary and rotating components and include conductive arms engaged with rotating disks. The conductive arms are often formed of metal such as brass or copper with a combination carbon and metallic substance at the distal ends. The distal ends engage with the rotating disks and provide rotational electrical connectivity. 
         [0050]    Referring to  FIG. 9 , a mixer ejector turbine  500  comprises a turbine shroud  510  that surrounds a rotor  540 , engaged with a hub  541  that is further engaged with a nacelle  550 . An ejector shroud  520  has an inner diameter greater than the outer diameter of the trailing edge of the turbine shroud  510 , and the ejector shroud  520  and the turbine shroud  510  are concentric with one another. In some embodiments the ejector shroud  520  surrounds the trailing edge of the turbine shroud  510 . In some embodiments the ejector shroud  520  is located downstream from the trailing edge of the turbine shroud  510 . A first electrically conductive material  580  is integrated into the surface of the turbine shroud  510  and further conductively engaged with an internal structure  572  of the turbine shroud that is conductively engaged  510  that is electrically conductive and insulated. The internal structure  527  is further conductively engaged with a down-conductive system  576  that is conductively engaged to an earth-termination system  578 . A second electrically conductive material  586  is integrated into the surface of the ejector shroud  520  and is further conductively engaged with an internal structure  574  of the ejector shroud  520  that is electrically conductive and insulated. The internal structure  574  is further conductively engaged with the down-conductive system  576  that is conductively engaged to the earth-termination system  578 . 
         [0051]      FIGS. 10 through 12  depict example embodiments of a single shroud fluid turbine.  FIG. 10  is a perspective view of an exemplary embodiment of a single shroud fluid turbine of the present disclosure.  FIG. 11  is a front view of the single shroud fluid turbine of  FIG. 10 .  FIG. 12  is a side view of the single shroud fluid turbine of  FIG. 10 . Referring to  FIG. 10  through  FIG. 12 , the single shrouded fluid turbine  600  comprises a turbine shroud  610 , a rotor  640 , and a nacelle body  650 . The turbine shroud  610  includes a front end  612 , also known as an inlet end or a leading edge, and a rear end or trailing edge  616 , also known as an exhaust end. The trailing edge  616  includes high energy lobes  617  and low energy lobes  615 . 
         [0052]    The rotor  640  surrounds the nacelle body  650  and includes a central hub  641  at the proximal end of the rotor blades. The central hub  641  is rotationally engaged with the nacelle body  650 . The rotor  640  and turbine shroud  610  are coaxial with each other, i.e., they share a common central axis  605 . 
         [0053]    At least one air termination device  661 , between 1/80 to 1/20 the diameter of the turbine shroud  610  in length, is engaged with the leading edge  612  of the turbine shroud  610 . Additionally, at least one air termination device  664  between 1 m and 2 m in length is also engaged with the trailing edge  616  of the turbine shroud  610 . 
         [0054]    A Primary Lightning Protection system (LPS) is illustrated in  FIG. 10  through  FIG. 12 .  FIG. 11  is a front view of an example embodiment, and  FIG. 12  is a side view of the example embodiment of  FIG. 11 . 
         [0055]      FIG. 11  and  FIG. 12  depict a rolling sphere radius in relation to the outer surface of the fluid turbine  600 . The rolling sphere circumference is illustrated by arc  670  and the radius is illustrated by arrow  672 . The rolling sphere arcs  670  have an approximate diameter of 313 meters, which corresponds to a lightning current of up to 200 kA. It can be seen in  FIGS. 11 and 12  that the arcs  670  do not come in contact with the body of the turbine shroud  610  before coming in contact with at least one of the air termination devices  661 / 664 . It can be further seen in  FIGS. 11 and 12  that the arcs  670  do not come in contact with the rotor  640 , the hub  641 , or the nacelle  650  before coming in contact with at least one of the air termination devices  661 / 664 . The air termination devices  661 / 664  are in electrical communication with a down conduction system  676  by way of an electrically coupled conductor  675 . The down conduction system  676  transfers electricity to an earth-termination system  678  for dispersion of the electricity. 
         [0056]    Although a few example embodiments have been 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 disclosure. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims.