Abstract:
A heating assembly for a wind turbine: generator, the assembly comprising: a heat reservoir mounted within a blade of the wind turbine generator; a heat source for supplying heat to the heat reservoir; a plurality of thermal conductors projecting front said heat reservoir to a surface of said blade.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to wind turbine generators (WTG&#39;s) used in the generation of electricity. In particular, the invention relates to means of removing ice from a rotor blade of a wind turbine generator. 
       BACKGROUND 
       [0002]    Blade de-icing is critical in WTG because there is a 20% to 50% increase in the loss production factor. Ice accretion on wind turbine blades causes:
       Change in aerodynamic shape resulting in significantly reduced power production   Increased root loads and reduced blade life   Unbalanced mass on the rotor plane   Danger of ice throw from the blades       
 
         [0007]    In the case of melting ice, the principal characteristic is the surface-ice interface temperature which has to be above freezing. When melting occurs at the blade surface-ice interface, chunks of ice fall off as a result of wind and gravity forces. 
         [0008]    The amount of heat and the time required to melt the ice depends on numerous factors. These include the thickness of the ice layer, the loss of heat from the external surfaces of the blade, the external ambient temperature, and most importantly, the efficiency of the method fro transferring the heat from the source to the frozen areas. 
       SUMMARY OF INVENTION  
       [0009]    In a first aspect the invention provides a heating assembly for a wind turbine generator, the assembly comprising: a heat reservoir mounted within a blade of the wind turbine generator; a heat source for supplying heat to the heat reservoir; a plurality of thermal conductors projecting from said heat reservoir to a surface of said blade. 
         [0010]    Accordingly, the delivery of heat through conduction from the reservoir to, or adjacent to, the surface of a rotor blade, then the elevation of temperature to the surface will consequently create a liquid/solid phase change allowing the ice to break up and fall from the blade. 
         [0011]    In one embodiment the heat source may include an insulated duct for delivering hot air from a hot air source to the heat reservoir. Further, the heat reservoir may be substantially hollow or void into which the hot air is directed. Further, such a void may include an array of heat transfer fins within the void of the heat reservoir such that hot air delivered from the duct into the heat reservoir heats the heat transfer fins delivering heat to a thermal mass of said heat reservoir. 
         [0012]    In one embodiment the heat reservoir may be mounted to a structural support, or spar, of the blade. Alternatively, the heat reservoir include a portion of said spar. For instance, the heat reservoir may have a portion for receiving heat such as a cavity for receiving hot air with a portion of the spar acting as a thermal mass for receiving heat such that conductors project from said thermal mass. In a further embodiment the heat conductors may project from the heat reservoir to a surface layer of the blade. In a further embodiment the surface layer may be a heat conductive material such as aluminum nitride or boron nitride. Still further, said conductive layer may be a single layer covering the blade. Alternatively, there may be to plurality of heat conductive layers located on said blade. 
         [0013]    The thermal conductors, or conductive rods, may cover the final third of the blade span. Accordingly, the weight of the heating assembly may be reduced by concentrating the application of heat to the most critical region of the blade. 
         [0014]    The thermal conductors may project from the heat reservoir and terminate at a point adjacent to the surface of the blade. A tip of the thermal conductors may be sandwiched in between the material of the leading edge, and so allow the heat to be conducted to the leading edge and spread uniformly along the length of the leading edge. In a still further embodiment, heat may be applied to the blade adjacent to both the leading edge and the trailing edge. Ice that is removed from the leading edge may migrate around the blade and re-freeze on the trailing edge. By providing heat to the trailing edge, this migrating ice may be prevented from re-freezing and so prevented from re-forming. 
         [0015]    Alternatively, the thermal conductors may terminate so as to be flush with a surface of the blade. In a further alternative, the thermal conductors may terminate at, or adjacent to, a leading edge of the blade. In a still further embodiment, the thermal conductors may terminate at a thermal layer applied to the surface, or leading edge of the blade. 
         [0016]    An advantage of the present invention may include, the speed at which the blade surface-ice interfacial layer reaches above freezing point is increased. 
         [0017]    The present invention may operate when the blades are either stationary or when they are rotating. 
         [0018]    The use of thermally conductive Aluminium or Boron Nitride may be advantageous as both materials have good dielectric properties (dielectric constant values are similar to that of E-glass, which is used in the construction of the blades). Such materials also have good thermal conductivity. The use of these materials will not result in additional susceptibility to lightning strikes on the blades. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]    It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention. Other arrangements of the invention are possible and consequently, the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention. 
           [0020]      FIG. 1  is a cross-sectional view of a wind turbine generator blade having a heat assembly tab according to the embodiment of the present invention; 
           [0021]      FIG. 2  is a detailed view of an end of a wind turbine generator blade showing a portion of the heating assembly according to a further embodiment of the present invention; 
           [0022]      FIG. 3  is a cross-sectional view of a heat reservoir according to a further embodiment of the present invention. 
           [0023]      FIG. 4  is an elevation view of a wind turbine generator for receiving a heat assembly according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIG. 1  shows a cross-section of a rotor blade  10  for a wind turbine generator. The blade  10  has suffered an accretion of ice  35  on a leading edge. As a means of removing the ice and/or preventing its formation, a heating assembly  5  has been mounted within the blade  10  which provides heat to the leading edge  25  so as to melt the contact interface of the ice with the blade and so allowing the ice to fall off. 
         [0025]    The heating assembly  5  comprises a heat reservoir  12  mounted within the blade  10 . The blade may be mounted directly to the structural spar  30  of the blade. Alternatively, the heat reservoir may be formed as part of the spar itself. 
         [0026]    The heat reservoir  12  receives heat from a heat source through a heat transfer conduit  15  which may be a conventional duct depending upon the delivery of heat. For instance, in the case of hot air being pumped to the heat reservoir  12 , the duct  15  may be an insulated hot air duct. 
         [0027]    Projecting from the heat reservoir  12  is a plurality of thermal conductors  20  projecting to the leading edge  25  or alternatively adjacent to the leading edge. Accordingly, the conductors may penetrate the blade so as to be flush with a surface of the blade or alternatively applying heat to the surface in order to achieve heating of the ice  35 . 
         [0028]      FIG. 2  shows one embodiment of the present invention whereby the conductor  45  projects to the leading edge  50  so as to be in contact with a thermal layer  40  placed, or applied, about the leading edge. The conductor transmits heat  47  from the heat reservoir (not shown) to the layer  40  so as to transfer heat around the leading edge so as to either remove ice or prevent its formation. Such a thermal layer/skin may be of a similar material to the conductor. For instance, both the conductor and skin may be of material such as aluminum nitride or boron nitride. These materials are effective thermal conductors, and avoid the use of metals within the blade which may represent a lightning hazard to the overall structure. 
         [0029]    The layer  40  may be of the order of 150 to 200 microns subject to the material. Thus, the layer may be a spray-on layer which is consistent with such thickness. 
         [0030]    It is not the intention to make the leading edge, or the thermal layer, a thermal mass for retaining heat, but merely to elevate the temperature of the leading edge sufficiently so as to remove or prevent ice build-up. The heat reservoir which is more easily insulated therefore provides a thermal mass to maintain the communication of heat to the leading edge. Accordingly, the heat reservoir may be of sufficient thermal mass to allow for intermittent transfer of heat from the heat source and so avoid the need for a continuous flow of heat. Alternatively, such a continuous flow of heat, such as a continuous flow of hot air, may be used in order to transfer sufficient heat to the leading edge. 
         [0031]      FIG. 3  shows one possible arrangement of the heat reservoir  65 . Here, a hollow container  70  having sufficiently thick walls to provide a thermal mass and defining a void/cavity therein. Within the cavity is an array of heat transfer fins  75  arranged to receive heat from a heat source. In this embodiment the heat source is hot air delivered to the heat reservoir  70  through an insulated hot air duct  80 . 
         [0032]    The heat reservoir  65  is mounted to a spar  85  acting as a structural element within a blade  60 . The heat reservoir  65  is located within a 1st third of the blade  60  with the heat conductors (not shown for clarity) having as short a path as possible from the heat reservoir  65  to the leading edge of the blade. 
         [0033]      FIG. 4  shows a wind turbine generator  90  into which the heat assembly may be mounted. A heat generator (not shown) may be mounted in the nacelle  94  or the tower  92  supporting the nacelle, subject to the form of the heat generator. This may include a heating coil through which hot air is passed, or a hot water interface heated by solar thermal energy. The particular form the heat generator does not limit the invention, and many such generators of heat may be used to provide sufficient heat to operate the heat assembly. 
         [0034]    The blades  95  into which the heat assembly is mounted include a leading edge  100 , about which the ice forms. The blade further includes a first third  105  which, by virtue of the distance from the nacelle will have the greatest influence on the torque of the blade, and the final third  107 , allowing the most efficient application of heat to the blade.