Abstract:
A de-icing arrangement of a wind turbine rotor blade is provided. The de-icing arrangement includes an electrically conductive mat, an electrically conductive band for distributing an electric current along a first edge of the mat, and a current supply connector for connecting the band to a current supply, wherein at least the electrically conductive mat is embedded in the body of the rotor blade. A wind turbine including a number of rotor blades including such a de-icing arrangement according, and a current supply for connecting to the de-icing arrangements of a rotor blades. A method of incorporating a de-icing arrangement in a wind turbine rotor blade is also provided.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to European Patent Office application No. 13151143.8 EP filed Jan. 14, 2013, the entire content of which is hereby incorporated herein by reference. 
       FIELD OF INVENTION 
       [0002]    The invention describes a de-icing arrangement of a wind turbine rotor blade, a wind turbine, and a method of incorporating a de-icing arrangement in a wind turbine rotor blade. 
       BACKGROUND OF INVENTION 
       [0003]    During cold conditions, ice can build up on the rotor blades of a wind turbine. Such ice deposits or layers can have a detrimental effect on the performance of the wind turbine, since they alter the aerodynamic properties of the blade. The accumulated weight of the ice on the rotor blades can also result in unwanted loading of the wind turbine. Therefore, some effort is invested in avoiding the build-up of ice, or to melt ice deposits that have already formed on the rotor blades. In one approach, a mat of electrically conductive fibres (e.g. carbon fibres) is applied to the exterior of a previously manufactured and finished rotor blade. When the poles of a current supply are connected to opposite ends of the mat, the current flowing through the mat heats the fibres and prevents ice from building up, or causes an existing ice layer to melt and slide off. 
         [0004]    A problem associated with the known approach is that it is difficult to form a reliable electrical connection between the fibre mat and the poles of the current supply. Known connecting means may fail sooner or later as a result of unavoidable dynamic influences such as blade vibration, extreme temperature differences, material deterioration owing to sunlight, corrosion due to high humidity, etc., since the de-icing mat is mounted onto the exterior of the blade and is therefore exposed to all weather conditions. Furthermore, for a mat arranged on the outer surface of the blade, any metal components used to connect it to the current supply may act as attractors, and a flashover may occur from a lightning protection means to such a metal component during a lightning strike, or the metal component itself may be struck by lightning. As a result, the known approaches are also vulnerable to damage during a lightning strike. 
       SUMMARY OF INVENTION 
       [0005]    It is therefore an object of the invention to provide a more reliable way of dealing with ice deposits on a wind turbine rotor blade. 
         [0006]    This object is achieved by the de-icing arrangement of the claims; by the wind turbine of the claims, and by the method of the claims of incorporating a de-icing arrangement in a wind turbine rotor blade. 
         [0007]    According to the invention, a de-icing arrangement of a wind turbine rotor blade comprises an electrically conductive mat; an electrically conductive band for distributing an electric current along a first edge of the mat; and a current supply connector for connecting the band to a current supply; wherein at least the electrically conductive mat is embedded in the body of the rotor blade. 
         [0008]    A wind turbine rotor blade is usually made of fibreglass, and is usually moulded by arranging a “layup” of fibreglass mats about a core, spindle or other inner form, enclosing the entire layup in a mould, and performing vacuum extraction to draw a liquid resin through the fibreglass layers. Numerous variations to this process are known, and need not be elaborated upon here. Therefore, without restricting the invention in any way, it may be assumed herein that the rotor blade is made of fibreglass and moulded using an appropriate technique. The resin that is used to permeate the fibreglass also ensures a smooth outer surface. In the de-icing arrangement according to the invention, therefore, at least the electrically conductive mat is embedded in the body of the blade so that it is covered by resin. Of course, the de-icing arrangement can be incorporated in the body of the rotor blade so that it is also covered by a fibreglass layer in addition to a resin covering layer. An advantage of the de-icing arrangement according to the invention is that the vulnerable conductive mat is well protected from detrimental exterior influences such as humidity, extreme temperatures, ultraviolet radiation, etc., but is still highly effective. Furthermore, since resin is an effective electrical insulator, the likelihood of damage to the de-icing arrangement during a lightning storm is significantly reduced, as is the likelihood of a flashover from an attractor of a lightning protection system. 
         [0009]    According to the invention, a wind turbine comprises a number of such rotor blades (usually three), and a current supply for connecting to the de-icing arrangements of the rotor blades. 
         [0010]    An advantage of the wind turbine according to the invention is that with relatively little effort, the build-up of ice on the rotor blades can be avoided, so that the energy output of the wind turbine can be maintained even during very cold conditions. The lifetime of the rotor blades can also be extended, since the likelihood of cracking or other damage owing to ice deposits can be minimized. 
         [0011]    According to the invention, the method of incorporating a de-icing arrangement in a wind turbine rotor blade comprises the steps of arranging an electrically conductive mat on a rotor blade layup; arranging an electrically conductive band along a first edge of the mat; arranging a current supply connector in the rotor blade layup, which current supply connector is realised to connect the band to a current supply; and embedding at least the electrically conductive mat in the body of the rotor blade. 
         [0012]    An advantage of the method according to the invention is that the cost of manufacture of the rotor blade can be kept relatively low, since the de-icing arrangement can simply be incorporated with relatively little effort in the usual manufacturing procedure. 
         [0013]    Particularly advantageous embodiments and features of the invention are given by the dependent claims, as revealed in the following description. Features of different claim categories may be combined as appropriate to give further embodiments not described herein. 
         [0014]    The electrically conductive mat can be realised in any suitable manner, using any arrangement of electrically conductive fibres or wires that can dissipate heat when an electrical current passes through. For example, a mat can be formed by spot-welding thin and narrow strips of metal to give a lattice structure. However, a preferred material may be carbon fibre, since carbon fibre mats are readily available and can be made as a favourably thin layer suitable for embedding in a rotor blade in a vacuum extraction moulding procedure. Therefore, without restricting the invention in any way, it may be assumed in the following, that the electrically conductive mat essentially comprises a carbon fibre mat. It may also be assumed that the rotor blade is formed in a vacuum extraction moulding procedure, although other methods of construction are not to be ruled out. 
         [0015]    As mentioned above, in the de-icing arrangement according to the invention, the conductive mat is entirely embedded underneath the outer surface of the rotor blade, for example under a resin outer “skin” or even underneath a thin layer of fibreglass. In a further preferred embodiment of the invention, the electrically conductive band and the current supply connector are also embedded in the material of the rotor blade. In this way, the likelihood of a bolt of lightning being attracted to any component of the de-icing arrangement is minimized. In another preferred embodiment of the invention, any components of the de-icing arrangement in the interior of the blade (e.g. conductive leads, ends of fasteners, etc.) can also be covered with an insulating material. In this way, a flashover from a lightning conductor (arranged in the interior of the blade) to any component of the de-icing arrangement can effectively be prevented. 
         [0016]    During operation of the wind turbine, the leading edges of the rotor blades pass through very cold, moisture-laden air. Therefore, in a preferred embodiment of the invention, the conductive mat is arranged on both sides of a leading edge of the rotor blade. The effectiveness of the de-icing arrangement is greatest at that part of the blade, since ice tends to form primarily on the leading edge. 
         [0017]    Ice is more likely to build up at regions further out along the rotor blade. Therefore, in a preferred embodiment of the invention, the conductive mat extends over at least 90% of the length of the rotor blade, so that the de-icing arrangement effectively extends to the tip of the blade 
         [0018]    When current flows through the material of the conductive mat, heat is dissipated, since the fibres of the mat present a relatively high electrical resistance. In a particularly preferred embodiment of the invention, therefore, the conductive mat is embedded to a depth of at most 5.0 mm beneath an outer surface of the rotor blade, so that the conductive mat is favourably close to the surface of the rotor blade. In this way, a very effective heat transfer to the blade outer surface is ensured. 
         [0019]    The current supply connector can be connected in any suitable way to the current supply. In a preferred embodiment of the invention, the current supply connector comprises an exterior connector part and an interior connector part, and these are preferably realised to be securely fastened together such that an electrical connection is made between the band, the mat and an electrical lead to the current supply. The interior connector part can be arranged in the interior of the blade so that it can be connected to an electrical lead on the interior of the rotor blade. These elements could be welded together. However, in a particularly preferred embodiment of the invention, the current supply connector and the interior connector part form a pressure connection with the electrical lead. This type of connection is robust and does not require the use of metals that can be welded. 
         [0020]    Preferably, the interior connector part comprises an aluminium body, or a body formed of an aluminium alloy. Such a material has a favourable conductivity, but is malleable enough to form a favourable pressure connection with an electrical lead. A bushing can be formed in the interior connector part, and a threaded bolt (of steel or aluminium) can be screwed into the bushing. The malleable material ensures a tight threaded connection that is unlikely to become loose over time. 
         [0021]    The current source can be a battery or generator for supplying a suitable level of current. The current source can supply direct or alternating current, as appropriate. 
         [0022]    The electrical resistance of a mat made of a fibre such as carbon fibre is quite high, and an electrical current will always seek the path of least resistance (i.e. the most direct path to the opposite electrical pole). Therefore, in a particularly preferred embodiment of the invention, a conductive band is arranged along one edge of the conductive mat, and a pressure connection is formed between the conductive band and the conductive mat. For example, threaded fasteners can be screwed from the outside through the band, mat and rotor blade body and into a suitable counterpart, so that the band and mat are pressed together. In this way, any current flowing through the band will be essentially evenly spread along the band, and therefore the electrical potential in the mat will be essentially the same for all points along that edge of the mat. 
         [0023]    To ensure a favourable pressure connection, a preferred embodiment of the invention comprises the step of arranging a counterpart to the electrically conductive band so that a fastener such as a wood screw can be simply screwed through the various layers. For example, a strip of plywood can be used as a counterpart. Plywood can easily be bent into shape, so that such a pressure connection can be made with relatively little effort. 
         [0024]    As indicated above, the mat is preferably embedded in the rotor blade body, close to the outer surface, so that heat can be effectively transferred to the rotor blade surface. In a further preferred embodiment of the invention, therefore, to increase the effectiveness of the heat transfer, the mat is realised to comprise an arrangement of raised “bumps”. For example, a slub fibre could be used to weave the mat. Alternatively, low “bumps” could be manually applied to the mat using a thermally and electrically conductive material such as solder. Such slubs or bumps can increase the effectiveness of the heat transfer to the surface at such points, since any fibreglass layer and/or resin coating will be thinnest above those raised elements. The height of the slubs or bumps is preferably chosen so that the rotor blade can still be formed to have a smooth outer surface, i.e. a slub or bump does not result in a noticeable irregularity on the rotor blade surface. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention. 
           [0026]      FIG. 1  is a schematic diagram of a rotor blade with a de-icing arrangement according to an embodiment of the invention; 
           [0027]      FIG. 2  shows a detail of the de-icing arrangement of  FIG. 1 ; 
           [0028]      FIG. 3  shows a first embodiment of an electrical connection between the conductive mat, the conductive band and the current supply connector of the de-icing arrangement of  FIG. 2 ; 
           [0029]      FIG. 4  shows a second embodiment of an electrical connection between the conductive mat, the conductive band and the current supply connector of the de-icing arrangement of  FIG. 2 ; 
           [0030]      FIG. 5  shows a cut-away view of a rotor blade with an embodiment of a de-icing arrangement; 
           [0031]      FIG. 6  shows a cross-section through the rotor blade of  FIG. 5 ; 
           [0032]      FIG. 7  shows a schematic rendering of an embodiment of a wind turbine according to the invention. 
       
    
    
       [0033]    In the diagrams, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale. 
       DETAILED DESCRIPTION OF INVENTION 
       [0034]      FIG. 1  is a schematic diagram of a rotor blade  2  with a de-icing arrangement  1  according to an embodiment of the invention. The rotor blade  2  has a rounded leading edge  20  and a flat, relatively thin trailing edge  21 . The diagram shows an electrically conductive mat  10  embedded in the blade and covered by an outer resin “skin”. The entire de-icing arrangement is embedded in this way, so that none of its components is exposed to the elements. The diagram also shows a current supply  13 , represented by a battery whose two poles are connected to opposite ends of the mat  10  by means of leads  130 ,  131  running inside the body of the blade  2 , as indicated by the broken lines. The de-icing arrangement  1  is embedded in the blade  2  so that the mat  10  lies along the leading edge  20 . 
         [0035]      FIG. 2  is a cut-away diagram showing a detail of the de-icing arrangement  1  of the previous diagram. A connector  12  passes through a conductive band  11 , the conductive mat  10 , and fibreglass layer  200  of the rotor blade  2 . As will be explained in more detail below, the connector  12  makes an electrical connection between the current supply lead  130  (in the interior of the blade  2 , indicated by the broken line) and the conductive mat  10 . The connector  12  effectively presses the conductive band  11  to the mat  10  at a point within the connector  12 . The conductive band  11  is, in turn, pressed firmly to the mat  10  by a series of fasteners  110  at several other points along the conductive band  11 , so that any current passing into the conductive band  11  (via the current supply lead  130  and the connector  12 ) is distributed essentially evenly along the outer edge of the conductive mat  10 . At least the mat  10  and band  11  will be covered by a resin layer  210  after the blade manufacturing process is complete. Of course, the connector  12  and fasteners  110  may also be covered by the resin layer  210 . Their dimensions are exaggerated in the diagram only to clearly indicate their positions. 
         [0036]      FIG. 3  shows a first embodiment of an electrical connection between the conductive mat  10 , the conductive band  11  and the current supply lead  130  of the de-icing arrangement of  FIG. 2 . Here, the connector comprises a bolt  120  with a shaft that passes through the conductive band  11 , the conductive mat  10 , and the fibreglass body  200  of the blade and is secured in a threaded nut  122  in the interior of the blade. The current supply lead  130  is pressed between the blade body  200  and the nut  122 , so that an electrical path for the current is given through the nut  122  and bolt  120  and into the mat  10  and band  11 . A washer  121  between band  11  and mat  10  further improves the current distribution. A tight connection between nut  122  and bolt  120  ensures a reliable electrical connection between the components. The bolt  120 , washer  121 , band  11 , and nut  122  can all be made of a suitable electrically conductive material that is also resistant to corrosion, such as aluminium or an aluminium alloy. 
         [0037]      FIG. 4  shows a second embodiment of an electrical connection between the conductive mat  10 , the conductive band  11  and the current supply lead  130  of the de-icing arrangement  1  of  FIG. 2 . Here, the connector comprises an intermediate component  123  shaped to provide a hard and conductive body against which the current supply lead  130  can be pressed. Here also, a bolt shaft passes through the conductive band  11  and the conductive mat  10 , and is secured in a threaded nut  122 ′. The current supply lead  130  is therefore pressed between the intermediate component  123  and the nut  122 , so that a better path is provided for electrical current, since the intermediate component  123  can withstand deformation better than the fibreglass  200  of the rotor blade. The intermediate component  123  can also be made of aluminium, an aluminium alloy, or any other suitable conductive metal. 
         [0038]      FIG. 5  shows a cut-away view of part of a rotor blade, showing parts of the de-icing arrangement embedded in the rotor blade  2  underneath an outer resin layer  210 . The diagram shows the conductive mat  10  arranged to lie over the leading edge of the rotor blade, and indicates the two current supply leads  130 ,  131  that are arranged in the interior of the blade. These can be flat bands  130 ,  131  of a conductive material, for example a woven copper strip. Preferably, the current supply leads  130 ,  131  are covered by an insulating layer to avoid any flashover from a component of a lightning protection system. For example, the current supply leads  130 ,  131  could also be embedded underneath a resin layer during the moulding process, or can be enclosed in a suitable plastic or other insulating layer. The conductive band  11  is arranged along the edge of the mat  10  that is to be connected to the positive pole of the voltage source by means of a first current supply lead  130 . A second connector can make an electrical connection between the opposite end of the mat  10  and the other current supply lead  131 , which is connected to the negative pole of the voltage source. A second conductive band  11  could be arranged along that opposite end of the mat  10 , but this is not strictly necessary. 
         [0039]      FIG. 6  shows a cross-section through the rotor blade  2 , showing how the conductive band  11  might be pressed to the mat  10  at several points. In this embodiment, a flexible piece of wood such as a plywood strip  111  is arranged on the interior of the rotor blade  2  to correspond with the position of the conductive band  11  on the exterior. Threaded wood screws  110  are passed through the conductive band  11  and mat  10  and into the wood counterpart  111 , and are tightened sufficiently to press the band  11  and mat  10  together, giving an effective current path. 
         [0040]    Instead of using a nut  122  and plywood strip  111  as shown here to secure the bolt  120  and fasteners  110  in place, a custom-made wedge or horseshoe-shaped conductive element could be formed. This might be shaped to fit into the interior of the rotor blade in the region of the leading edge and extending to correspond to the length of the band  11 . Such a conductive element might have a bushing so that the bolt  120  can be screwed into it, and further bushings for bolts (instead of wood screws). Equally, the material of the conductive element could be soft enough (e.g. aluminium) so that wood screws  110  can be screwed in directly. 
         [0041]    The preparatory steps of arranging the mat  10  on the fibreglass layup  200 , securing the band  11  in place using woodscrews  111 , arranging the supply leads on the interior of the layup  200 , and electrically connecting the supply leads  130  to the mat  10  can all be carried out prior to a vacuum extraction process. Once all these steps have been performed, vacuum extraction can be carried out, resulting in the components  10 ,  11 ,  12 ,  110  of the de-icing arrangement being covered by a coating of resin  210 . Alternatively, the de-icing arrangement could be applied to a fibreglass body of a partially finished blade, and a final resin coating could be manually applied to cover the mat  10  and any connectors  12 , fasteners  110  and bands  11 . 
         [0042]      FIG. 7  shows a schematic rendering of an embodiment of a wind turbine  3  according to the invention, with three rotor blades  2 , each with a de-icing arrangement  1  connected to a current supply  13 . The current supply  13  can be realised in any suitable manner in the wind turbine  3 , for example as a battery previously charged using superfluous energy. During very cold conditions, the heat dissipated in the mats  10  of the de-icing arrangement  1  can prevent the build up of ice on the rotor blades  2 , particularly along the leading edges  20  of the blades  2 , and/or can thaw any ice deposits that may have formed. 
         [0043]    Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. For example, instead of using a single wide mat arranged about the leading edge and extending onto opposite sides of the rotor blade, three or more mats could be arranged in a parallel manner, separated by a certain distance and connected electrically using the conductive band. The heat dissipated by each mat can be sufficient to ensure that no ice can build up on the rotor blade in the region between mats. In this way, a more economical de-icing arrangement can be realised. 
         [0044]    For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.