Patent Publication Number: US-2019195203-A1

Title: Wind turbine blade for a wind turbine

Description:
TECHNICAL FIELD 
     The invention relates to a wind turbine blade for a wind turbine, wherein the wind turbine blade comprises a lightning protection system. 
     BACKGROUND ART 
     Wind turbines are highly exposed to lightning strikes owing to their configuration, height and location. In particular, the wind turbine blades as the component of the wind turbine comprising weakly conductive material and representing the most distant point from the ground&#39;s surface have the highest risk of being struck by a lightning. In the case of a lightning strike to the wind turbine blade a very high current propagates through the wind turbine causing severe and very costly damages, especially concerning the wind turbines blades. 
     For the above reasons, protective measures in the form of reliable and high performance lightning protection systems are needed for wind turbines. Generally, in a conventional system, a lightning protection system comprises several point-like lightning receptors located at the surface of the blade and a further down conduction system to the root of the blade, usually in the form of down conducting cables. 
     EP 2 122 162 A2 discloses a lightning protection system for a wind turbine blade comprising a down conduction system in the form of a metallic mesh, a very small receptor and an electrical conductor running inside of the blade. The purpose of the electrical conductor is to separate the metallic mesh and the receptor so that the receptor is electrically isolated. 
     Even though it has been known in the above described prior art to provide wind turbines with relatively small receptors and to spread the current to several down conductors in the form of a metallic mesh the above described lightning protection system does not guarantee a sufficiently safe and reliable protection against lightning since it is possible that the lightning does not strike the relatively small receptive area for lightning strikes and since the components of the lightning protection system do not provide a broad and highly conductive cross section of conduction. 
     SUMMARY OF INVENTION 
     Technical Problem 
     It is the object of the present invention to provide an improved wind turbine blade with a lightning protection system. A further object of the invention is to provide a wind turbine blade with a lightning protection system which provides an improved protection and an enhanced conduction. Another object of the invention is to reduce the occurrence of lightning strikes which do not strike into receptors of the lightning protection system. 
     According to the present invention the wind turbine blade has a tip end, a tip end area and a root end area. The tip end is the most distant point of the blade from a hub of a nacelle at which the blade is typically mounted. The tip end area is hence an area at the tip end of the blade whose dimensions are small compared to the complete length of the blade, whereas the root end area is an area at the opposite side of the blade. 
     To prevent the blade from being damaged by lightning strikes, the wind turbine blade has a lightning protection system comprising at least one metal foil, wherein a first end of the metal foil is disposed in the tip end area of the blade and wherein the metal foil extends from said first end towards the root end area of the blade. 
     The term “metal foil” refers to a piece of metal whose thickness is significantly smaller than its longitudinal and transverse dimensions. The metal foil according to the present invention consists of one integral piece of metal. 
     According to the invention, the metal foil has a first end which is disposed in the tip end area of the blade. The metal foil extends from this first end towards the root end of the blade. The metal foil can extend all the way from the first end to the root end of the blade or only for a part of the blade length. Preferably, the metal foil is connected to a further down conduction system of the lightning protection system in the root end area. Due to this arrangement of the metal foil, the metal foil can function as a down conductor. 
     In addition, the metal foil is located in proximity, preferably in close proximity, to the outer surface of the blade so that it can directly function as a lightning receptor. In particular, the metal foil is preferably located in the radial outer 10% of the blade wall with respect to the blade wall thickness. 
     According to the invention, the lightning protection system further comprises a first metallic lightning receptor which is disposed between the first end of the metal foil and the tip end of the blade. The first metallic lightning receptor can be arranged anywhere between the first end of the metal foil and the tip end of the blade. This also includes the case in which the first metallic lightning receptor is only arranged at one end of the specified distance (for example the tip end of the blade). The first metallic lightning receptor is also considered to be arranged between the first end of the metal foil and the tip end of the blade if the first metallic lightning receptor is only partly arranged between the first end of the metal foil and the tip end of the blade while another part of the first metallic lightning receptor is arranged outside of the specified distance. 
     Furthermore, the first metallic lightning receptor is in direct electrical contact with the metal foil. This direct electrical contact can be achieved by an adjoining arrangement of only the first end of the metal foil and an end of the first metallic lightning receptor. In order to provide a larger contact area, the first metallic lightning receptor and the metal foil preferably overlap. Preferably, the blade comprises a plurality of foils. In this case, the first metallic lightning receptor is considered to be in direct electrical contact with a metal foil, even if another metal foil is disposed between the first metallic lightning receptor and the metal foil. 
     The first metallic lightning receptor is arranged in proximity to the outer surface of the blade, preferably in close proximity, so that the first metallic lightning receptor can directly function as a lightning receptor. In particular, the first metallic lightning receptor is preferably located in the radial outer 10% of the blade with respect to the blade wall thickness. 
     By providing a lightning protection system according to the present invention a higher acquisition rate of lightning strikes without consequential damages to the wind turbine blade can be achieved by means of providing a larger receptive area. Apart from an enhanced receptive function of the lightning protection system, the invention results in better down conducting properties. 
     In a preferred embodiment, the metal foil comprises a plurality of apertures, which in particular all have the same aperture size. The size of the apertures is preferably sufficiently small so that the possibility of a lightning strike to the blade through an aperture instead of a strike to the metal foil can be ruled out. Preferably, the size of the apertures amounts between 0.5 mm and 3 mm, especially preferred between 1 mm and 2 mm, so that foils with a fine net structure can be used. The aperture size is defined as the largest possible distance between two opposing aperture sides. In a particular preferred embodiment, the size of the apertures amounts to less than 10 mm, preferably less than 5 mm and particularly preferably less than 2 mm. 
     Further preferred, the apertures are arranged within the metal foil in such a way that a net structure of the metal foil is defined. The metal foil therefore comprises webs of continuous metal foil running in two preferential directions. In contrast to a lightning protection mesh as it is known in prior art, the metal foil does not consist of separate conductors which are woven in order to form a meshwork. Preferably, the apertures are arranged in such a way that a regular net structure of the metal foil is formed. 
     In a further embodiment, the net structure of the metal foil is oriented in a diagonal way to the longitudinal direction of the blade. The term diagonal is to be understood that both preferential directions of the net structure enclose an angle with the longitudinal direction of the blade which is between 0° and 90°, preferably between 20° and 80° and especially preferably between 50° and 70°. In this way, the foil does not experience the full strain of the blade which is mostly stressed along its longitudinal direction. Therefore, the stress and the correlated fatigue load acting on the net structure of the metal foil will be much lower using the above described diagonal orientation. 
     The metal foil can, for instance, be produced by deep drawing or rolling of one piece of metal. Preferably, the metal foil is produced by the steps of slotting, drawing and rolling of the metal foil. Alternatively, it is also possible to punch a plurality of holes to an already rolled metal foil. In both ways, the metal foil consists of one continuous piece of metal. In particular, the metal foil is non-woven and not a woven mesh of single wires of fibers. The advantage of using a non-woven metal foil is an improved conductivity and less cost compared to woven metal meshes, since no weaving process is necessary. 
     Preferably, the metal foil is formed as a strip which is arranged substantially parallel to the longitudinal direction of the blade. In a further preferred embodiment of the invention, the metal foil has a constant width. 
     Preferably, the lightning protection system of the wind turbine blade comprises a plurality of metal foils. The lightning protection system preferably consists of one metal foil located along the suction side of the blade and one metal foil located along the pressure side of the blade. In a particularly preferred embodiment, the lightning protection system comprises two metal foils on the suction side and the pressure side of the blade respectively. 
     A certain percentage of the area of the apertures compared to the total area of the metal foil should not be exceeded as the down conduction requires a minimal cross section of conducting material. 
     Preferably, the wind turbine blade comprises at least one spar cap extending from the tip end area of the blade to the root end area of the blade, said at least one spar cap preferably extending substantially parallel to the longitudinal direction of the blade. The spar cap preferably comprises carbon fibers which as a conductive material are prone to a lightning strike. In order to protect the spar cap from a direct lightning strike, the metal foil is disposed outside from the spar cap and in radial direction behind the spar cap. Preferably, the metal foil is disposed outside from the spar cap and in radial direction behind the spar cap along the entire length of the spar cap so that a lightning strike to the spar cap can successfully be prevented. By radial direction “thickness direction” is meant, which corresponds to a transverse direction or cross direction of a section of the blade wall wherein the opposite blade wall section is not included. In particular, the thickness direction is substantially perpendicular to a center line of the blade wall section. 
     According to another embodiment of the invention, the wind turbine blade comprises a plurality of metal foils which are electrically connected amongst each other to avoid a potential difference and therefore an arc-over between the metal foils, preferably by means of metal foil sections connecting the plurality of metal foils with each other. 
     In a further preferred embodiment of the invention, the first metallic lightning receptor is flat-shaped or bar-shaped. The term “flat-shaped” refers to the feature that the thickness of the first metallic lightning receptor is significantly smaller than its longitudinal and transverse dimensions. In the case of a “bar-shaped” first metallic lightning receptor, both, the thickness dimension as well as the transverse dimension, are significantly smaller than the longitudinal dimension of the first metallic lightning receptor and have preferably the same order of magnitude. 
     In another preferred embodiment of the invention, the blade comprises an outer blade layer representing the outermost layer of the blade. The outer blade layer can preferably be a glass laminate layer or only a thin protective layer such as a thin glass fleece layer. Preferably, the metal foil is only covered by the outer blade layer in the outer radial direction of the blade. Furthermore, the first metallic lightning receptor preferably is also covered only by the outer blade layer in the radial outer direction of the blade. By this arrangement, the first metallic lightning receptor can function as a direct receptor of a stroke of lightning. The metal foil and the first metallic lightning receptor are therefore protected from environmental influences, such a corrosion, physical damage, such as gas. At the same time, the protective layer is sufficiently thin so that the first metallic lightning receptor and the metal foil can still function as direct lightning receptors. 
     Preferably, the shape of the first metallic lightning receptor is adapted to the shape of the outer blade layer, especially preferably, the first metallic lightning receptor clings to the inside of the outer blade layer. In an especially preferable embodiment, the first metallic lightning receptor adjoins the inside of the outer blade layer along a substantial part of its length and width. 
     In a further embodiment, the first metallic lightning receptor comprises a metallic plate. The metallic plate is defined by a piece of material which is substantially defined by two opposite sides, wherein these sides run preferably parallel to each other and wherein the sides are extended in comparison to the distance between the sides. Preferably, the metallic plate has a constant thickness. 
     In a further embodiment of the invention, the first metallic lightning receptor tapers towards the tip end of the blade. In other words, the width of the first metallic lightning receptor in circumferential direction of the blade decreases in direction towards the tip end of the blade so that the width of the metallic lightning receptor at the first end of the metal foil is larger than the width of the metallic lightning receptor at a location towards the tip end of the blade. Preferably, the first metallic lightning receptor tapers continuously towards the tip end of the blade and especially preferably at a constant rate. 
     According to a further embodiment of the invention, the first metallic lightning receptor extends over substantially the entire blade surface in the tip end area. In particular, the first metallic lightning receptor extends over the entire blade surface in the blade tip end area. Preferably, the first metallic lightning receptor extends over the entire inner blade surface in the tip end area. Especially preferably, the blade comprises an outer blade layer whose inner side in the tip end area of the blade is completely covered by the first metallic lightning receptor wherein the metallic lightning receptor is arranged adjoining the inner surface of the outer blade surface. Due to this arrangement of the first metallic lightning receptor, a large receptive area is created as well as a large cross section of conduction. 
     In a further preferred embodiment, the first metallic lightning receptor comprises a solid metallic member. Preferably, the solid metallic member occupies a substantial part of the volume, preferably the entire volume, inside the blade in its tip end area. This inside volume is defined by the outer blade surface in the tip end area. 
     According to another embodiment of the invention, the first metallic lightning receptor extends continuously from the first end of the metal foil to the blade tip. Preferably, the first metallic lightning receptor has a first end from which it extends towards the blade tip. This first end of the first metallic lightning receptor can be disposed between the first end of the metal foil and the blade tip end including the case in which the first end of the first metallic lightning receptor is disposed directly at the first end of the metal foil. Alternatively, the first end of the first metallic lightning receptor can also be disposed beyond the first end of the metal foil towards the root end of the blade. Preferably, the first metallic lightning receptor extends continuously from its first end towards the blade tip, especially preferably all the way to the blade tip. “Continuously” signifies that there is no interruption of the first metallic lightning receptor from the first end towards its opposite end. In the case of the first metallic lightning receptor continuously extending all the way to the blade tip, the first metallic lightning receptor is disposed along the entire distance between its first end and the blade tip end. 
     According to a further embodiment of the invention, the first metallic lightning receptor is arranged overlapping with the metal foil. This is especially the case, when the first end of the first metallic lightning receptor is disposed beyond the first end of the metal foil so that between the first end of the metal foil and the first end of the metallic lightning receptor an overlap section is defined. Preferably, the first metallic lightning receptor and the metal foil are arranged adjoining in this overlap section so that a direct electrical contact is achieved. 
     In an alternative embodiment of the invention, the first metallic lightning receptor and the metal foil are not arranged overlapping. In this case, the direct electrical contact between the first metallic lightning receptor and the metal foil is only realized by an abutting arrangement of the first end of the first metallic lightning receptor and the first end of the metal foil. 
     According to another preferred embodiment, the first metallic lightning receptor and the metal foil are clamped between two metal plates in the overlap section. In another preferred embodiment, the lightning protection system comprises at least one disk receptor wherein the disk receptor is disposed in proximity to the first end of the metal foil. Preferably, said disk receptor is disposed in the overlap section of the first metallic lightning receptor and the metal foil. In this case, one of the metal plates between which the first metallic lightning receptor and the metal foil are clamped can preferably be a base plate of the disk receptor. The other metal plate could be a counter metal plate for achieving a clamping connection between the first metallic lightning receptor and the metal foil. 
     In a further embodiment, the first metallic lightning receptor and the metal foil are connected by means of clamping, caulking or by means of a bonding agent. “Connected” signifies that the first metallic lightning receptor and the metal foil are attached to each other by means of clamping, caulking or by means of a bonding agent. 
     In another embodiment of the invention, the first metallic lightning receptor, preferably the material of the first metallic lightning receptor, comprises copper. Preferably, the first metallic lightning receptor is entirely made of copper. Alternatively, other metals with a high conductivity can be used. 
     In a further preferred embodiment, the lightning protection system comprises a second metallic lightning receptor which is disposed at the tip end of the blade and which is in direct electrical contact with the first metallic lightning receptor. Disposed at the tip end signifies that the second metallic lightning receptor provides a receptive area at the tip end of the blade so that a lightning can directly strike into the tip end of the blade. Preferably, the second metallic lightning receptor is covered only by the outer blade layer, being especially preferably only a protective layer, in the radial outer direction of the blade. 
     The direct electrical contact between the first metallic lightning receptor and the second metallic lightning receptor is preferably achieved by an adjoining arrangement, preferably in an overlap section between the second metallic lightning receptor and the first metallic lightning receptor, providing a large area of direct electrical contact. Alternatively, the second metallic lightning receptor and the first metallic lightning receptor can abut at their respective ends resulting in a smaller direct electrical contact area. 
     In another preferred embodiment, the second metallic lightning receptor comprises a rod receptor. Preferably, the outer layer of the blade provides an opening for the tip of the rod receptor through which the tip of the rod receptor extends to the outside of the blade. Hence, a lightning can directly strike into the tip of the rod receptor without damaging the outer blade layer. In a preferred embodiment, the rod receptor comprises a base plate, wherein the base plate is located inside the blade. Furthermore, the second metallic lightning receptor can preferably comprise at least one disk receptor wherein the disk receptor is preferably mounted on the same base plate as the rod receptor. Preferably, the base plate is arranged between the first end of the metal foil and the tip end of the blade. The disk receptor being mounted on the base plate protrudes slightly to the outside of the blade. Preferably, two disk receptors are mounted on the base plate of the rod receptor, the disk receptors being electrically and mechanically connected by connection means, in particular by a metal bolt. 
     According to a further embodiment of the invention, the second metallic lightning receptor comprises a solid metallic blade tip. Preferably, the solid metallic blade tip comprises connection means for connecting to the first metallic lightning receptor. Preferably, the connection means are configured as connection webs, which are preferably metal plates extending towards the root end of the blade. By providing connection means, a larger contact area between the first metallic lightning receptor and the second metallic lightning receptor can be achieved. 
     In a further preferred embodiment, the first metallic lightning receptor is connected to the second metallic lightning receptor by means of clamping, caulking or by means of a bonding agent. 
     In a further embodiment of the invention, the second metallic lightning receptor or the metal foil comprise copper. Preferably, the second metallic lightning receptor or the metal foil are entirely made of copper. Instead of using copper, other metals with a high conductivity can also be used. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will be described below with reference to the following figures which show in schematic representation 
         FIG. 1  is a top view of a section of a wind turbine blade with a lightning protection system; 
         FIG. 2  is a cross sectional view of the section of the blade of  FIG. 1  in its tip end area; 
         FIG. 3  is a longitudinal sectional view of the section of the blade of  FIG. 1  in its tip end area; 
         FIG. 4  is a longitudinal sectional view of the section of the blade of  FIG. 1  along the cutting line A-A; 
         FIG. 5  is a top view of a section of a wind turbine blade with a lightning protection system; 
         FIG. 6  is a cross sectional view of the section of the blade of  FIG. 5  in its tip end area; 
         FIG. 7  is a longitudinal sectional view of the section of the blade of  FIG. 5  in its tip end area; 
         FIG. 8  is a longitudinal sectional view of the section of the blade of  FIG. 5  along the cutting line A-A; 
         FIG. 9  is a top view of a section of a wind turbine blade with a lightning protection system; 
         FIG. 10  is a cross sectional view of the section of the blade of  FIG. 9  in its tip end area; 
         FIG. 11  is a longitudinal sectional view of the section of the blade of  FIG. 9  in its tip end area; 
         FIG. 12  is a longitudinal sectional view of the section of the blade of  FIG. 9  along the cutting line A-A; 
         FIG. 13  is a top view of a section of a wind turbine blade with a lightning protection system; 
         FIG. 14  is a cross sectional view of the section of the blade of  FIG. 13  in its tip end area; 
         FIG. 15  is a longitudinal sectional view of the section of the blade of  FIG. 13  in its tip end area; 
         FIG. 16  is a longitudinal sectional view of the section of the blade of  FIG. 13  along the cutting line A-A; 
         FIG. 17  is a top view of a section of a wind turbine blade with a lightning protection system; 
         FIG. 18  is a cross sectional view of the section of the blade of  FIG. 17  in its tip end area; 
         FIG. 19  is a longitudinal sectional view of the section of the blade of  FIG. 17  in its tip end area; and 
         FIG. 20  is a longitudinal sectional view of the section of the blade of  FIG. 17  along the cutting line A-A. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows a top view of a section of a wind turbine blade  10  comprising a tip end area  11  and a tip end  12 , wherein the tip end  12  is disposed in the tip end area  11 . The wind turbine blade  10  further has a lightning protection system comprising two metal foils  13 ,  14  formed as strips out of copper which both have a first end  13   a ,  14   a  disposed in the tip end area  11  of the blade  10 . The metal foils  13 ,  14  extend from their first ends  13   a ,  14   a  along the longitudinal direction of the blade  10  towards its root end. 
     The blade  10  further comprises an outer blade layer  25  (see  FIGS. 2 and 3 ) of which in  FIG. 1  only the outline is shown. The metal foils  13 ,  14  are arranged in radial direction behind spar caps  26   a ,  26   b  which are located underneath the outer blade layer  25 . Since the spar caps  26   a ,  26   b  are disposed underneath the outer blade layer and the metal foils  13 ,  14 , the spar caps  26   a ,  26   b  are shown by broken lines. The spar caps  26   a ,  26   b  extend from the root end of the blade  10  towards the tip end  12  of the blade along a substantial part of the length of the blade. However, the spar caps  26   a ,  26   b  are not disposed at the tip end area  11  of the blade  10 . 
     The metal foils  13 ,  14  are arranged substantially parallel to the longitudinal direction of the blade  10 . For the section of the blade  10  shown in  FIG. 1  the metal foils  13 ,  14  overlap with each other. When extending towards the root end of the blade  10  the metal foils increasingly diverge from each other until they are not arranged overlapping anymore for the substantial part of the length of the blade  10 . 
     The lightning protection system further comprises a first metallic lightning receptor  17 . The first metallic lightning receptor  17  being a metallic plate out of copper is disposed between the first ends  13   a ,  14   a  of the metal foils  13 ,  14  and the tip end  12  of the blade  10 . The first metallic lightning receptor  17  extends continuously from a first end  17   a  to the blade tip end  12  of the blade  10 . The first end  17   a  of the first metallic lightning receptor  17  is arranged towards the root end of the blade  10  as seen from the first ends  13   a ,  14   a  of the metal foils  13 ,  14 . The first metallic lightning receptor  17  extends over the entire blade surface in the tip end area  11  of the blade  10 . 
     The lightning protection system further comprises two disk receptor pairs of which two disk receptors  18 ,  19  are shown in  FIG. 1 . The disk receptors  18 ,  19  are disposed in proximity to the first ends  13   a ,  14   a  of the metal foils  13 ,  14 . The disk receptors  18 ,  19  comprise a base plate  21   a  which is disposed in outer radial direction behind the first metallic lightning receptor  17 . The disk receptors  18 ,  19  are embedded inside the blade  10  and are mounted on the base plate  21   a . The disk receptors  18 ,  19  protrude out of the outer blade layer  25  to the outside of the blade  10 . The metal foils  13 ,  14  have approximately the same width as the base plate  21   a.    
       FIG. 2  shows a cross sectional view of the section of the blade of  FIG. 1  in its tip end area  11 . The outer blade layer  25  representing the outermost layer of the blade  10  is shown by a thick line. The outer blade layer  25  covering the first metallic lightning receptor is only a thin protective layer. The first metallic lightning receptor  17  extends over the entire blade surface in the blade tip end area  11  adapting to the shape of the blade surface. The first metallic lightning receptor  17  is arranged adjoining the inside of the outer blade layer  25  in the blade tip end area  11  except for overlap sections  23   a ,  23   b  (see  FIG. 4 ) with the metal foils  13 ,  14 . Since only the outer blade layer  25  separates the first metallic lightning receptor  17  from the outside of the blade  10  in radial outer direction, the receptive properties of the first metallic lightning receptor  17  are only slightly decreased and the first metallic lightning receptor  17  can function as a direct lightning receptor. 
     The adjoining arrangement can also be seen in  FIG. 3  which shows a longitudinal sectional view of the section of the blade  10  of  FIG. 1  in its tip end  12 . The first metallic lightning receptor  17  extends all the way to the tip end  12  of the blade  10 . 
       FIG. 4  shows a longitudinal sectional view of the section of the blade  10  of  FIG. 1  along the cutting line A-A. On the other side of the blade  10  which is not shown in  FIG. 1  two more metal foils  15 ,  16  are arranged so that the entire blade  10  comprises two metal foils on each side and therefore four metal foils  13 ,  14 ,  15 ,  16  are shown by broken lines in  FIG. 4 . Furthermore, the blade  10  comprises four spar caps of which two spar caps  26   a ,  26   c , one on each side of the blade  10 , are shown in  FIG. 4 . On both sides the metal foils  13 ,  14 ,  15 ,  16  extend from their first ends  13   a ,  14   a ,  15   a ,  16   a  disposed in the tip end area  11  of the blade  10  towards the root end area of the blade  10 . For the length of the spar caps  26   a ,  26   c  the metal foils  13 ,  14 ,  15 ,  16  are disposed in radial direction behind the spar caps  26   a ,  26   c  and therefore between the spar caps  26   a ,  26   c  and the radial outer blade layer  25  which can only be a thin protective layer. 
     In  FIG. 4  two disk receptors  18 ,  20  of the two disk receptor pairs are shown which are mounted on a base plate  21   a ,  21   b  respectively. The disk receptors  18 ,  20  being disposed on opposite sides of the blade  10  are connected by a bolt  22  serving as an attachment as well as an electrical connection. The two metal foils of each side  13 ,  14  and  15 ,  16  are arranged at the inside of one of the base plate  21   a ,  21   b  respectively. Therefore, the metal foils  13 ,  14  are arranged overlapping with the base plate  21   a , and the metal foils  15 ,  16  are arranged overlapping with base plate  21   b.    
     The first metallic lightning receptor  17  extending over the entire surface of the blade  10  in its tip end area  11  has a first end  17   a  at each side of the blade  10 . At the one side of the blade  10 , the first metallic lightning receptor  17  is arranged at the inside of the metal foils  13 ,  14  whereas at the opposing side the first metallic lightning receptor  17  is arranged at the inside of the metal foils  15 ,  16 . Even though between the metal foil  13 ,  16  and the first metallic lightning receptor  17  another metal foil  14 ,  15  is disposed, the first metallic lightning receptor and the metal foils  14 ,  15  are nonetheless considered to be in direct electrical contact. In this way, overlap sections  23   a ,  23   b  between the first metallic lightning receptor  17  and at least one of the metal foils  13 ,  14 ,  15 ,  16  are created. This results in a very large direct electrical contact area between the first metallic lightning receptor  17  and at least one of the metal foils  13 ,  14 ,  15 ,  16  on each side. For connection purposes there is another metal plate  24   a ,  24   b  on each side of the blade  10  which clamps the first metallic lightning receptor  17  and the metal foils  13 ,  14 ,  15 ,  16  in their overlap section  23   a ,  23   b . Therefore, the first metallic lightning receptor  17  is arranged between the base plates  21   a ,  21   b  and the metal plates  24   a ,  24   b  in the overlap section  23   a ,  23   b . Outside of the overlap section  23   a ,  23   b  the first metallic lightning receptor  17  is disposed at the inside of the outer blade layer  25 . The metal plates  23   a ,  23   b , the base plates  21   a ,  21   b , the metal foils  13 ,  14 ,  15 ,  16  and the first metallic lightning receptor  17  are made out of copper. 
       FIG. 5  shows a top view of a section of another wind turbine blade  10  with a tip end  12 , a tip end area  11  and a lightning protection system. The metal foils  13 ,  14 , the outer blade layer  25  and the spar caps  26   a ,  26   b  are arranged in the same way as in  FIG. 1 . 
     The first metallic lightning receptor  17  in  FIG. 5  is a metallic plate being disposed between the first ends  13   a ,  14   a  of the metal foils  13 ,  14  and the tip end  12  of the blade  10  and extending continuously from a first end  17   a  to the blade tip  12  of the blade  10 . Furthermore, the first metallic lightning receptor  17  extends over the entire blade surface in the tip end area  11 . The first end  17   a  of the first metallic lightning receptor  17  is arranged between the first end  13   a ,  14   a  of the metal foils  13 ,  14  and the root end of the blade  10  but in proximity to the first ends  13   a ,  14   a  of the metal foils  13 ,  14 . 
     In  FIG. 6  a cross sectional view of the section of the blade of  FIG. 5  in its tip end area  11  is shown. The first metallic lightning receptor  17  and the outer blade layer  25  are arranged in the same way as in  FIG. 2 . 
       FIG. 7  shows a longitudinal sectional view of the section of the blade  10  of  FIG. 5  in its tip end area  11 . The first metallic lightning receptor  17  and the outer blade layer  25  are arranged in the same way as in  FIG. 3 . 
       FIG. 8  shows a longitudinal sectional view of the section of the blade  10  of  FIG. 5  along the cutting line A-A. The arrangement of the outer blade layer  25  and the spar caps  26   a ,  26   b  is the same as in  FIG. 4 . The metal foils  13 ,  14 ,  15 ,  16  are disposed at the inside of the outer blade layer  25  along their complete length. The first metallic lightning receptor  17  extends from the blade tip  12  beyond the first ends  13   a ,  14   a ,  15   a ,  16   a  of the metal foils  13 ,  14 ,  15 ,  16  towards the root end of the blade  10  until its first end  17   a . The first metallic lightning receptor  17  is arranged at the inside of the outer blade layer  25  at the tip end area  11  of the blade except for overlap sections  23   a ,  23   b  at which the first metallic lightning receptor  17  is arranged adjoining the inside of at least one of the metal foils  13 ,  14 ,  15 ,  16  on each blade side. The first metallic lightning receptor  17  and the metal foils  13 ,  14 ,  15 ,  16  are connected by means of a bonding agent. 
       FIG. 9  shows a top view of a section of another wind turbine blade  10  with a tip end  12 , a tip end area  11  and a lightning protection system. The first metallic lightning receptor  17 , metal foils  13 ,  14 , the outer blade layer  25  and the spar caps  26   a ,  26   b  are arranged in the same way as in  FIG. 5 . The only difference is that in  FIG. 9  the first metallic lightning receptor  17  is a solid metallic member out of copper. Since the first metallic lightning receptor  17  is solid the entire volume inside of the blade  10  in its tip end area  11  is occupied with the first metallic lightning receptor  17 . 
     In  FIG. 10  a cross sectional view of the section of the blade of  FIG. 9  in its tip end area  11  is shown. The first metallic lightning receptor  17  adapts to the shape of the blade surface being arranged adjoining the inside of the outer blade layer  25  in the tip end area  11  of the blade  10  except for overlap sections  23   a ,  23   b  with the metal foils  13 ,  14 ,  15 ,  16  (see  FIG. 12 ). 
       FIG. 11  shows a longitudinal sectional view of the section of the blade  10  of  FIG. 9  in its tip end area  11 . The first metallic lightning receptor  17  extends to the tip end  12  of the blade  10  occupying the entire volume inside of the blade  10  in its tip end area  11 . 
       FIG. 12  shows a longitudinal sectional view of the section of the blade  10  of  FIG. 9  along the cutting line A-A. The arrangement of the outer blade layer  25  and the spar caps  26   a ,  26   b  is the same as in  FIG. 4 . The metal foils  13 ,  14 ,  15 ,  16  are disposed at the inside of the outer blade layer  25  along their complete length. The first metallic lightning receptor  17  extends from the blade tip  12  beyond the first ends  13   a ,  14   a ,  15   a ,  16   a  of the metal foils  13 ,  14 ,  15 ,  16  towards the root end of the blade  10  until its first end  17   a . The first metallic lightning receptor  17  is arranged at the inside of the outer blade layer  25  at the tip end area  11  of the blade except for overlap sections  23   a ,  23   b  at which the first metallic lightning receptor  17  is arranged adjoining the inside of at least one of the metal foils  13 ,  14 ,  15 ,  16  on each blade side. The first metallic lightning receptor  17  and the metal foils  13 ,  14 ,  15 ,  16  are connected by means of a bonding agent. 
     In  FIG. 13  a top view of a section of another wind turbine blade  10  with a tip end  12 , a tip end area  11  and a lightning protection system is shown. The metal foils  13 ,  14 , the outer blade layer  25  and the spar caps  26   a ,  26   b  are arranged in the same way as in  FIG. 1 . The first metallic lightning receptor  17  in  FIG. 13  is a metallic plate out of copper being disposed between the first ends  13   a ,  14   a  of the metal foils  13 ,  14  and the tip end  12  of the blade  10 . 
     The lightning protection system further comprises a second metallic lightning receptor  27  at the tip end  12  of the blade  10 . The second metallic lightning receptor  27  comprises a rod receptor  28  which is mounted on a base plate  29  inside of the blade  10  (see  FIGS. 14, 15 and 16 ). The first metallic lightning receptor  17  extends continuously from a first end  17   a  (see  FIG. 16 ) to the second metallic lightning receptor  27 . Starting from its first end  17   a  towards the second metallic lightning receptor  27  the width of the first metallic lightning receptor  17  tapers so that its width  17   b  at the second metallic lightning receptor  27  is smaller than its width  17   c  at its first end  17   a.    
     The second metallic lightning receptor  27  further comprises two disk receptors  30 ,  31  being mounted on the base plate  29  of the rod receptor  28  of which one disk receptor  30  protruding through the first metallic lightning receptor  17  and the outer blade layer  25  can be seen in  FIG. 13 . The first metallic lightning receptor  17  extends beyond the base plate  29  (see  FIGS. 14, 15 and 16 ) of the second metallic lightning receptor  27  but does not extend all the way to the tip end  12  of the blade  10 . 
     In  FIG. 14  a cross sectional view of the section of the blade  10  of  FIG. 13  in its tip end area  11  is shown. Apart from the rod receptor  28  the two disk receptors  30 ,  31  are mounted on the base plate  29  of the rod receptor  28 . The first metallic lightning receptor  17  adapts to the shape of the blade surface. The first metallic lightning receptor  17  is in direct electrical contact with the disk receptors  30 ,  31  and therefore in direct electrical contact with the second metallic lightning receptor  27 . 
       FIG. 15  shows a longitudinal sectional view of the section of the blade  10  of  FIG. 13  in its tip end area  11 . The first metallic lightning receptor  17  extends beyond the base plate  29  of the rod receptor  28  but not all the way to the tip end  12  of the blade  10 . The rod receptor  28  protrudes through the outer blade layer  25  at the tip end  12  of the blade  10 . 
       FIG. 16  shows a longitudinal sectional view of the section of the blade  10  of  FIG. 13  along the cutting line A-A. The arrangement of the first metallic lightning receptor  17 , the metal foils  13 ,  14 ,  15 ,  16 , the spar caps  26   a ,  26   b  and the outer blade layer  25  is the same as in  FIG. 4 . 
       FIG. 17  shows a top view of a section of another wind turbine blade  10  with a tip end  12 , a tip end area  11  and a lightning protection system. The metal foils  13 ,  14  and the spar caps  26   a ,  26   b  are arranged in the same way as in  FIG. 1 . 
     The first metallic lightning receptor  17  in the form of a metallic plate extends continuously and in a tapering way from its first end  17   a  to a second metallic lightning receptor  27 . The width  17   b  of the first metallic lightning receptor  17  at the second metallic lightning receptor  27  is smaller than its width  17   b  at its first end  17   a . The second metallic lightning receptor  27  comprises a solid metal blade tip  32 . At the radial outside of the solid metal blade tip  32 , there may be an outer blade layer  25  in form of a protective coating or no outer blade layer at all. 
     In  FIG. 18  a cross sectional view of the section of the blade of  FIG. 17  in its tip end area  11  is shown. The first metallic lightning receptor  17  adapts to the shape of the blade surface and is arranged at the inside of the outer blade layer  25  along its complete width. 
     In  FIG. 19  a longitudinal sectional view of the section of the blade  10  of  FIG. 17  in its tip end area  11  is shown. The first metallic lightning receptor  17  is in direct electrical contact to the second metallic lightning receptor  27  in the form of the solid metallic blade tip  32  by means of connections means in the form of connection webs  33   a ,  33   b.    
       FIG. 20  shows a longitudinal sectional view of the section of the blade  10  of  FIG. 17  along the cutting line A-A. The arrangement of the first metallic lightning receptor  17 , the metal foils  13 ,  14 ,  15 ,  16 , the spar caps  26   a ,  26   b  and the outer blade layer  25  is the same as in  FIG. 4 .