Abstract:
A wind turbine blade having a Radar Absorbing Material (RAM) and a lightning protection system arranged for guaranteeing the performance of the lightning protection system and the integrity of the RAM. The lightning protection system comprises lightning receptors located at a tip region and one or two down-conductors disposed inside of the wind turbine blade for driving lightning current to ground. The RAM covers the entire wind turbine blade except the tip region and comprises at least a functional layer and a reflector layer connected to the one or two down conductors by way of auxiliary cables.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The inclusion of Radar Absorbing Material (RAM) into composite blades of wind turbines to reduce its radar reflectivity so that they do not interfere with radar systems such as air traffic control systems, weather or marine radar systems is known. 
         [0002]    WO 2010/122350 A1 describes a combined prepreg material comprising first and second layers impregnated with a matrix material such as a resin to be used particularly in wind turbine blade. The first layer is a functional layer of RAM and the second layer is a keying layer that comprises a keying medium to facilitate bonding of the combined prepreg material to a gel coat. 
         [0003]    U.S. 2012/0207612 A1 discloses a wind turbine blade that includes a multilayer composite structure including a first reflective layer and a second layer with a plurality of resistive circuit analogue (CA) elements. The CA elements are tuned so as to interact with said first layer to provide absorption of electromagnetic (EM) energy over a desired frequency range. The parameters of the CA elements can be varied to provide for frequency tuning and to maintain absorption at a specific frequency range despite varying layer separation, while at the same time ensuring that the mechanical properties of the CA layer are compatible with integration into the turbine blade. 
         [0004]    WO 2015/061670 A1 discloses a suitable composite laminate for wind turbine blades for the attenuation of the reflection of electromagnetic radiation in the S or X bands up to a peak of −20 dB. The composite laminate comprises particularly one or more functional layers having a printed circuit for absorbing the electromagnetic radiation incident on the composite laminate and a conducting layer. 
         [0005]    None of said patent applications mentions any relation between RAM and lightning protection. 
         [0006]    U.S. 2014/0118177 A1 discloses a wind turbine blade wherein the RAM includes a ground plane having an electrical conductivity and/or a dielectric constant that is higher in the presence of an electric field having a frequency of 1 GHz and above than in the presence of an electric field having a frequency of 10 MHz and below. The ground plane can be tuned to be highly reflective at radar frequencies and benign at lightning discharge frequencies. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0007]    The invention provides a wind turbine blade having RAM and a lightning protection system that are arranged for guaranteeing the performance of the lightning protection system and the integrity of the RAM. The lightning protection system comprises lightning receptors located at a tip region located between the end of the blade and a cross section at a radius R having a length ranging from 80-90% of the length of the blade and one or two down-conductors disposed inside of the wind turbine blade for driving lightning current to ground. The RAM covers the entire wind turbine blade except the tip region and comprises at least a functional layer and a reflector layer in the laminate of each shell of the wind turbine blade. The reflector layers are connected to the one or two down conductors in at least two cross sections at radiuses R 1 , R 2  having a length ranging, respectively, from 0-20% and 80-90% of the length of the blade by means of auxiliary cables. 
         [0008]    In an embodiment the section of the down-conductor between two pairs of auxiliary cables is removed. 
         [0009]    In an embodiment the wind turbine blade further comprise caps made of carbon fiber composite material whether in inner areas of shells or in blade beams that also connected to the auxiliary cables. If wind turbine blade would comprise caps made of fiber glass composite material would not be connected to the auxiliary cables. 
         [0010]    Advantageously, the reflector layers are metallic meshes made of one of the following materials: copper, brass, aluminum, steel, stainless steel. 
         [0011]    Advantageously the reflector layers and the caps made of carbon fiber composite material comprise first and second terminals (such as metallic brackets) embedded into the composite laminate of the shells of the wind turbine blade to facilitate its connection to the auxiliary cables. 
         [0012]    In a first embodiment the external layer of the laminate of the shells is a functional layer. 
         [0013]    In a second embodiment, the laminate of the shells comprises at least a functional layer and a reflector layer embedded into layers of a glass fiber composite material. 
         [0014]    In a third embodiment, the laminate of the shells comprises two functional layers and a reflector layer embedded into layers of a glass fiber composite material. 
         [0015]    Other characteristics and advantages of the present invention will be clear from the following detailed description of embodiments illustrative of its object in relation to the attached figures. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]      FIGS. 1 a , 1 b  and 1 c    are schematic cross sectional views of three embodiments of a composite laminate of a wind turbine blade shell with Radar Absorbing Material. 
           [0017]      FIGS. 2 a  and 2 b    are a schematic plan and a cross sectional view of a wind turbine blade having shells of a composite material including Radar Absorbing Material and a lightning protection system according to the invention. 
           [0018]      FIGS. 3 a , 3 b , 3 c  and 3 d    are schematic diagrams illustrating the lightning protection of the Radar Absorbing Material of the wind turbine blade in four embodiments of the invention. 
           [0019]      FIGS. 4 a -4 b  and 5 a -5 b    are a schematic diagram and a cross sectional view of a wind turbine blade that includes caps of carbon fiber composite material illustrating two embodiment of the lightning protection of the Radar Absorbing Material. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    As noted in the Background, the laminate of the shells of a wind turbine blade formed by layers of composite material shall incorporate as Radar Absorbing Material (RAM) one or more functional layers with specific characteristics of resistivity and a conductive reflector layer to avoid that they reflect incident electromagnetic emissions from radar systems. 
         [0021]    Composite layers are made up of resin materials and fibers with high mechanical properties that form hard sheets attached to each other after curing providing the required mechanical strength (hardness, tensile strength, etc.). The composite layers may comprise glass fiber or carbon fiber cloths and epoxy or polymeric resin. They may also comprise other fibers such as aramids, basaltic fibers or boron fibers as well as polymeric resins such as polyesters or vinyl esters. 
         [0022]    The patent documents mentioned in the Background describe various alternatives for the functional layers and the reflector layer. For example WO 2015/061670 discloses a functional layer made up of glass fibers and conducting ink (carbon based) and a reflector layer that can be a sheet, mesh or foil made up of aluminum, copper or carbon. 
         [0023]    Regarding the arrangement of the mentioned layers in the laminate of wind turbine blade shells according to the present invention are particularly included laminates with a functional layer as the outer layer  31  of the laminate and a reflector layer  33  embedded between layers  30  of a composite material of, preferably, glass fiber (see  FIG. 1 a   ), laminates with a functional layer  31  and a reflector layer  33  embedded between layers  30  of a composite material of, preferably, glass fiber (see  FIG. 1 b   ) and laminates with two functional layers  31 ,  31 ′ and a reflector layer  33  embedded between layers  30  of a composite material of, preferably, glass fiber (see  FIG. 1 c   ). 
         [0024]    The laminate of the wind turbine shells may also include a coating (not shown in the Figures) of a suitable material to protect it from erosion and other damage caused by atmospheric agents such as wind and rain. 
         [0025]    In order to avoid that RAM may be damaged by lightning, the invention proposes, firstly, separating clearly a first part  11  of the wind turbine blade  10  that inorporates RAM from a second part  12  that includes the lightning receptors  21 ,  21 ′,  21 ″ in a tip region extended from a radius R having a length comprised between the 80-90% of the length of the wind turbine blade until the end of the blade and, secondly, connecting the reflector layers  33  (embedded in the composite laminates of shells  13 ,  15 ) to a down-conductor  23  (or two down-conductors  23 ,  23 ′ joined to the spars  14 ,  16  of the blade) of the lightning protection system of wind turbine blade  10  that drives to ground the currents received by lightning receptors  21 ,  21 ′,  21 ″ by means of auxiliary cables  29 ,  29 ′ that equipotentialize the reflector layers  33  of shells  13 ,  15  and the down-conductor  23  (see  FIGS. 2 a , 2 b   ). 
         [0026]    The functional layers  31 ,  31 ′ of shells  13 ,  15  are not connected to the down-conductor  23  or down-conductors  23 ,  23 ′ because they have a very low portion of metallic material and consequently the risk of being damaged by any lightning current flow is very low. In many embodiments the functional layers  31 ,  31 ′ comprise metallic elements not connected between them so that they are not conductive layers. 
         [0027]    The invention is applicable to wind turbine blades  10  with reflector layers  33  made of any conducting material and particularly applicable to wind turbine blades  10  having a metallic mesh made of copper, brass, aluminum, steel or stainless steel as reflector layers  33 . The distance between a reflector layer  33  and a functional layer  31  or  31 ′ may be between 0.3-40 mm. 
         [0028]    In the the embodiment illustrated in  FIG. 3 a    the reflector layers  33  of shells  13  and  15  are connected through two pairs of auxiliary cables  29 ,  29 ′ in cross-sections at radiuses R 1 , R 2  having respectively lengths comprised between 0-20% and 80-90% of the length of the blade to a first section  24 ′ of the down conductor  23  in the tip region and to a second section  24 ″ from the second pair of auxiliary cables  29 ,  29 ′ to his ground connection. 
         [0029]    In the embodiment illustrated in  FIG. 3 b    the reflector layers  33  of shells  13  and  15  are equipotentialized with a down-conductor  23  through two pairs of auxiliary cables  29 ,  29 ′ in cross sections at radiuses R 1 , R 2  having, respectively lengths comprised between 0-20% and 80-90% of the length of the blade. 
         [0030]    In the embodiment illustrated in  FIG. 3 c    the reflector layers  33  of shells  13  and  15  are equipotentialized with a down-conductor  23  through two pairs of auxiliary cables  29 ,  29 ′ in cross-sections at radiuses R 1 , R 2  having, respectively lengths comprised between 0-20% and 80-90% of the length of the blade and through two additional pairs of auxiliary cables  29 ,  29 ′ in cross sections at intermediate radiuses between R 1  and R 2 . 
         [0031]    In the embodiment illustrated in  FIG. 3 d    the reflector layers  33  of shells  13  and  15  are equipotentialized with two down-conductors  23 ,  23 ′ through two pairs of auxiliary cables  29 ,  29 ′ in cross sections at radiuses R 1 , R 2  having, respectively lengths comprised between 0-20% and 80-90% of the length of the blade. 
         [0032]    In the embodiment illustrated in  FIGS. 4 a  and 4 b    the shells  13  and  15  also comprise caps  19  of carbon fiber composite material in their inner areas that, as the reflector layers  33 , are equipotentialized with a down-conductor  23  through two pairs of auxiliary cables  29 ,  29 ′ in cross sections at radiuses R 1 , R 2  having, respectively lengths comprised between 0-20% and 80-90% of the length of the blade. 
         [0033]    In the embodiment illustrated in  FIGS. 5 a -5 b    the shells  13  and  15  also comprise caps  19  of carbon fiber composite material in their inner areas that, as the reflector layers  33 , are equipotentialized with the down-conductors  23 ,  23 ′ through two pairs of auxiliary cables  29 ,  29 ′ in cross sections at radiuses R 1 , R 2  having, respectively lengths comprised between 0-20% and 80-90% of the length of the blade. 
         [0034]    As illustrated in  FIGS. 3 a -3 c , 4 a  and 5 a    the reflector layers  33  are provided with terminals  37  (typically metallic brackets) that remain embedded in the composite laminate of shells  13 ,  15  to facilitate its connection with the auxiliary cables  29 ,  29 ′. Similarly the caps  19  illustrated in  FIGS. 4 a , 5 a    are provided with suitable terminals  39 . 
         [0035]    The main advantage of the invention is that guarantee the performance of the lightning protection system of the wind turbine blade  10  and the integrity of the Radar Absorbing Material after a lightning strike allowing therefore the installation of wind turbines in sites close to airports, weather radars and other radar emitting locations. 
         [0036]    Although the present invention has been described in connection with various embodiments, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made, and are within the scope of the invention.