CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention is related to wind energy plant with a rotor hub housing, which has a pitch bearing. 
     The pitch bearing is a rolling bearing with an inner and an outer ring, one of the rings being connected to the rotor hub housing and the other ring being connected to the rotor blade. Via a pitch system, which consists preferably of a pitch drive, a pitch control and for instance an auxiliary power supply, the rotor blade is rotated around its longitudinal axis and thus its angle of attack is adjusted towards the wind. 
     Known constructions for the rotor hub housing consist of a cast body with a flange for mounting the pitch bearing. Erich Hau describes in “Windkraftanlagen, Grundlagen, Technik, Einsatz, Wirtschaftlichkeit, 3th edition, Springer Verlag Berlin” in chapter 7.5, the entire contents of which is incorporated herein by reference, that the quality and the weight of the rotor blades is essentially determined by the construction of the blade joining to the rotor hub, besides to the proper execution of the rotor blade. 
     From DE 10 2004 023 773 B3, the entire contents of which is incorporated herein by reference, it is known to realise the pitch bearing with an inner ring, which is fixedly connected to the rotor hub. The outer ring connected to the rotor blade has an outer toothing which is in meshing engagement with a drive pinion of the pitch system for adjusting the rotor blade. 
     From DE 10 2004 017 323 A1, the entire contents of which is incorporated herein by reference, a blade joining for a rotor blade is known, in which an arresting device is provided, which keeps the rotor blade free from backlash with a predetermined angle position. 
     From WO 2004/090326 A1, the entire contents of which is incorporated herein by reference, a rotor hub body is known, which has a three-arm stiffening, formed in one piece, in the region of the rotor blade joining. 
     From DE 20 2004 003 521 U1, the entire contents of which is incorporated herein by reference, a rotor hub body for a wind energy plant is known, which is realised with ring flanges for fastening a pitch bearing. Inside the ring flanges, one stiffening bulkhead is provided at a time, which is realised either as a cast piece in one piece with the rotor hub housing, or which can be welded or screwed together with the rotor hub as an autonomous component part. The stiffening bulkhead is disposed in the rotor hub housing at a side facing the rotor shaft. 
     The present invention is based on the objective to provide a stiffening element for a rotor hub housing which simplifies the installation of the wind energy plant. 
     BRIEF SUMMARY OF THE INVENTION 
     The wind energy plant according to the present invention has a rotor hub casing, on which at least one pitch bearing is attachable via joining means. A rotor blade is fixed on the pitch bearing with its blade root; preferably it is screwed together with the same. 
     According to the present invention, a stiffening element is provided, which is arranged between the pitch bearing and the rotor hub housing and fastened on the same via joining means. The stiffening element according to the present invention is formed as a separate construction part and it has the particular advantage that it can be mounted on the rotor hub housing together with the pitch beating. In this, the installation takes place preferably via the same joining means by which the pitch bearing is attached on the rotor hub housing, the stiffening element being preferably screwed to the rotor hub housing together with the pitch bearing in. A further advantage of the stiffening element according to the present invention is that in contrast to a rotor hub housing cast in one piece, the basic body of the rotor hub housing and the stiffening elements can be produced much more simply with respect to the casting technique. 
     Practically, the stiffening element has circularly arranged through-bores, which are provided for the accommodation of screws. 
     In one preferred embodiment, the stiffening element serves as a carrier element for components of the pitch system at the same time. The components can be independently mounted onto the stiffening element. Also, before installation to the rotor hub housing, the stiffening element can be tested independently. 
     Preferably, the stiffening element is formed in the shape of a ring. Also, it can be formed in the shape of a disc, plate or vat, wherein one or more openings are then provided in the stiffening element. 
     In a preferred embodiment, pitch bearing and rotor blade in the region of the blade root on the one hand, and rotor hub housing as well as stiffening element on the other hand are realised such that their stiffnesses correspond to each other. In order to achieve the same stiffness at the rotor blade side and at the rotor hub side, an additional stiffening element can be provided at the rotor blade side between the pitch beating and the rotor blade root. The pitch bearing is preferably realised as a rolling bearing with an inner ring and an outer ring, the outer ring being fixed on the rotor hub and the inner ring being connected to the rotor blade, and as the case may be to a stiffening element at the rotor blade side, for instance. The pitch beating experiences an approximately equal stiffness for both rings. Through this, it is avoided that unequal forces attack on inner ring and outer ring, which load the pitch bearing unfavourably. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       A preferred example of realisation is explained in more detail in the following by means of the drawings. 
         FIG. 1  shows the stiffening element according to the present invention, which is realised as a vat, 
         FIG. 2  shows the installation of the stiffening element according to the present invention on the rotor hub housing, and 
         FIG. 3  shows the stiffening element according to the present invention with components of the pitch system, the pitch bearing and a second stiffening element at the rotor blade side. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While this invention may be embodied in many different forms, there are described in detail herein a specific preferred embodiment of the invention. This description is an exemplification of the principles of the invention and is not intended to limit the invention to the particular embodiment illustrated 
       FIG. 1  shows a stiffening element  10 , which is preferably made from steel or a cast material. As the cast material, cast iron with nodular graphite is preferably used, for instance corresponding to DIN EN 1563 (EN-GJS-400-18, for instance). In a circulating edge  12 , the stiffening element  10  is formed significantly thicker than in its central region  14 . The central region  14  has a way-through  16 , which leads into the rotor blade. The circulating ring  12  is provided with openings  18  in the form of elongated holes, which are needed for the installation of the rotor blade screws. In a region situated at the edge, the circulating ring  12  is provided with a joining piece  20  for attachment of a pitch drive  38 . In this region, the circulating ring  12  is broader in the radial direction than in the remaining region of the stiffening element. At its flat sides, the stiffening element  10  is provided on its entire perimeter with flange planes  22 ,  24  situated on the exterior. In  FIG. 1 , the flange plane  22  is represented, which points towards the rotor hub housing. In  FIG. 2 , the opposing flange plane  24  can be recognised, which points off from the rotor hub housing. The stiffening element  10  has circularly arranged through-bores  26 , through which fastening bolts (not shown) are guided in the installation of the stiffening element. 
       FIG. 2  shows a rotor hub housing  28 , which is equipped with one stiffening element  10  at a time on each of its three lugs for the rotor blades. For attachment of the stiffening element and the pitch bearing, respectively, the lugs of the rotor hub housing  28  each have a mounting flange  30 , from which project fastening bolts  33 . 
     Further,  FIG. 2  shows the pitch bearing  32 , which consists of a rotatable bearing with an inner ring  34  and an outer ring  36 . Inner ring  34  and outer ring  36  are each at a time provided with circularly arranged through-bores. The through-bores of the outer rim correspond to the through-bores  26  of the stiffening element. The fastening bolts  33  on the mounting flange  30  mentioned above pass through the stiffening element  10  and through the outer ring  36  of the pitch bearing  32 . The stiffening element  10  and the pitch bearing  32  are secured via nuts  31  on the fastening bolts. The inner ring  34  of the pitch bearing  32  is also provided with circularly arranged through-bores. The inner ring  34  has a toothing  35  on its inner side, via which the inner ring  34  can be rotated with respect to the outer ring  36  with the aid of a pitch drive, and thus the angular position of the rotor blade can be adjusted. In this, the stiffening element  10  and its circulating ring  12  in particular are formed such that they do not come into contact with the inner ring. 
       FIG. 3  shows the stiffening element  10  in its function as a carrier element for components  38 ,  42  of the pitch system. The pitch drive  38  of the pitch system is fastened on the attachment lug  20  and passes through the stiffening element  10 , so that it meshes with a pinion  47  in the toothing  35  of the inner ring of the pitch bearing  32 . Switching cabinets  42  for a power supply and control, for a converter and for an accumulator for instance, are only schematically represented in  FIG. 3 . The pitch drive as well as the switching cabinets  42  are pre-mounted on the stiffening element  10  and can be tested independently from the rotor hub housing. 
     In  FIG. 3 , a second stiffening element  44  is represented in addition. With its through-bores  46 , the stiffening element  44  corresponds to the through-bores in the inner ring of the pitch bearing. The stiffening element  44  is screwed between pitch bearing and blade root of the rotor blade (not shown). 
     The stiffening element  10  according to the present invention has a series of advantages. For instance, the technical requirements of casting when the rotor hub housing  28  is produced are significantly reduced, because the stiffening elements  10  and the rotor hub housing  28  are cast separately from each other. Through this, the manufacturing cost is reduced and the casting quality is improved. By mounting components of the pitch system on the stiffening element  10 , additional carrier constructions are avoided, which had to be provided for mounting the components up to now. Even the mounting itself is simplified, because the components of the pitch system are also mounted directly when the stiffening element  10  is mounted. 
     The rotor side stiffening element  44  is formed such that together with the rotor blade root (not shown), it has a stiffness approximately equal to that of the rotor hub housing  28  and of the stiffening element  10  at the rotor hub housing side, through which similar stiffness conditions result on the junctions of inner and outer ring. Through this co-operation of the stiffening elements, the loads acting on the pitch bearing  32  are minimised and its lifespan is increased. 
     The above disclosure is intended to be illustrative and not exhaustive. This description will suggest many variations and alternatives to one of ordinary skill in this art. All these alternatives and variations are intended to be included within the scope of the claims where the term “comprising” means “including, but not limited to”. Those familiar with the art may recognize other equivalents to the specific embodiments described herein which equivalents are also intended to be encompassed by the claims. 
     Further, the particular features presented in the dependent claims can be combined with each other in other manners within the scope of the invention such that the invention should be recognized as also specifically directed to other embodiments having any other possible combination of the features of the dependent claims. For instance, for purposes of claim publication, any dependent claim which follows should be taken as alternatively written in a multiple dependent form from all prior claims which possess all antecedents referenced in such dependent claim if such multiple dependent format is an accepted format within the jurisdiction (e.g. each claim depending directly from claim  1  should be alternatively taken as depending from all previous claims). In jurisdictions where multiple dependent claim formats are restricted, the following dependent claims should each be also taken as alternatively written in each singly dependent claim format which creates a dependency from a prior antecedent-possessing claim other than the specific claim listed in such dependent claim below. 
     This completes the description of the preferred and alternate embodiments of the invention. Those skilled in the art may recognize other equivalents to the specific embodiment described herein which equivalents are intended to be encompassed by the claims attached hereto.
A wind energy plant with a rotor hub housing, which is connectable to at least one pitch bearing via joining means, and with at least one rotor blade, the blade root of which is connected to the pitch bearing, wherein a stiffening element is provided, which is fastened between the pitch bearing and the rotor hub housing.