Abstract:
The present invention generally relates to a runner unit of a tidal power plant, and more particular to a device for reversing a blade of the runner unit. The device according to the invention is lighter and more efficient with respect to known solutions which involve articulated mechanisms as it is based on an auxiliary servomotor including a reciprocating linear rack which acts on the blade to be reversed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to European Patent Application No. 15290145.0 filed Jun. 3, 2015, the contents of which are hereby incorporated in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present invention generally relates to a runner unit of a tidal power plant, and more particular to a device for reversing a blade of the runner unit. 
       BACKGROUND 
       [0003]    As well known, tidal power plants are arranged to convert into electricity the energy of tides. To this purpose, in case of a tidal lagoon, turbine housing may be arranged between the sea and the lagoon basin. The turbine housing may include a bulb runner unit comprising a plurality of blades fit thereon which are moved by the flow of water. The bulb runner is integral to a rotating shaft which cooperates with an electricity generator. 
         [0004]    Depending on the tidal level, when the water level (also known as “head”) of the sea rises with respect to the level of the lagoon, water can start flowing through the turbine to produce energy. Similarly, as the sea level starts to fall, a tidal head can be created by holding water back in the lagoon until a sufficient head is formed. Thus the process can be reversed and the water flows in the opposite direction from the lagoon to the sea through the turbine. In this way the generation of electricity is maximised, as it occurs with the flow of water in both senses. 
         [0005]    However, the blades fit to the bulb unit usually have a fixed direction with respect to the flow of water. The consequence of such arrangement usually ensures an acceptable efficiency when the runner is operating in the direct mode, which is when the water flows from the lagoon to the sea, but at the same time a significant decrease of efficiency is experienced when operating in the reverse mode, since the same inclination of the blades is maintained in both operating modes, or at least the blades present a profile optimised for a flow in the opposite direction. 
         [0006]    Known mechanisms installed in the runner unit usually allow an angle of rotation generally limited to values which are less than 40 degrees and in any case much less than 180 degrees because of the high friction occurring around the dead centres of the control mechanisms. A complete inversion of the blade would correspond to a rotation angle of more than 180 degrees, for example on the order of 220 degrees. 
         [0007]    An attempt to solve the aforementioned technical problem is described in patent publication U.S. Pat. No. 2,951,380, which will be now discussed. 
         [0008]    With reference to  FIG. 1  and the sequence illustrated in  FIGS. 2 a  to 2 f   , the control mechanism generally comprises a main servomotor  12  including a piston  11  which controls the position of the blade through a rod  10 . Connected to the rod  10  is a cross-head  8 . Each blade includes a journal  3  supported by bearings and a lever  6  is keyed to the journal between the bearings. A connecting-rod  7  is articulated at one end to the lever  6  and to the other end to the cross-head  8 . Such crank gear has a dead centre, and for this reason the control mechanism comprises an auxiliary servomotor. In particular, the latter comprises a tooth sector  13  secured to the lever  6 , situated on the same side of the lever  6  and is symmetrical to the axial plane of the crank. Furthermore, an additional crank  14  is secured to toothed sector  15  and rotates on a pin  16  mounted on the hub. A connecting-rod  17  is articulated at the end of the crank  14  and is driven by an auxiliary servomotor  18 . As it is clearly indicated in the sequence of  FIGS. 2 a   - 2   f,  toothed sectors  13  and  15  interact solely when the main servomotor  12  drives the lever  6  in its dead position ( FIGS. 2 b -2 e   ). Then the auxiliary mechanism is driven such that toothed sector  15  meshes with sector  13  and the dead centre is passed. With the cooperation of the two mechanisms a complete inversion of the blade, with an angle greater than 180 degrees, is achieved. 
         [0009]    However, the disclosed mechanism has technical disadvantages. In fact, the auxiliary mechanism is based on a rotative gear, which is the toothed sector  15 , in order to enable the further rotation of the lever  6  and hence pass the dead zone. For such reason, the auxiliary servomotor, which comprises the servomotor  18  acting on the connecting-rod  17 , must include the crank  14  articulated thereto. 
         [0010]    It will be appreciated that such pivot in the mechanism inevitably involves the presence of two elements moving, that is the articulated rod  17  and the crank  14  on which the meshing gear is provided, which makes the mechanism heavier and may cause wear at the interface of the coupling rod-crank. Furthermore, the hub must be designed to also support the pin  16  acting as pivot of the crank  14 , and the pin  20  acting as pivot of the servomotor  22  as they are both fixed thereto. 
       SUMMARY OF THE INVENTION 
       [0011]    It is an object of the present invention to solve the aforementioned technical problems by providing a device for reversing a blade for a runner unit as substantially defined according to independent claim  1 . 
         [0012]    It is a further object of the present invention to provide a runner unit as substantially defined in dependent claim  10 . 
         [0013]    According to an aspect of the invention, this object is obtained by a device for reversing a blade of a runner unit, comprising an annular lever fixed to a trunnion portion of the blade and rotatably mounted on the runner unit; a main servomotor comprising a piston adapted to reciprocate along a shaft of the runner unit and coupled to the annular level such to cause a first rotation of the blade until the piston reaches a dead zone position; an auxiliary servomotor arranged to cause a further rotation of the blade; wherein the auxiliary servomotor comprises a reciprocating linear rack configured to cooperate with a toothed sector provided on the annular level when the piston is at or in proximity of said dead zone position. 
         [0014]    According to a preferred aspect of the invention, the linear rack comprises a rod having a rack element fixed at its end. 
         [0015]    According to a preferred aspect of the invention, the auxiliary servomotor comprises a servomotor body hosting a reciprocating piston integral to the linear rack. 
         [0016]    According to a preferred aspect of the invention, the linear rack is articulated by means of a first pivot on the servomotor body. Alternatively, the linear rack may be mounted cantilever on the servomotor body. 
         [0017]    According to a preferred aspect of the invention, the servomotor body is articulated on the inner wall of the runner unit by means of a second pivot. Alternatively, the servomotor body may be secured to the inner wall of the runner unit. 
         [0018]    According to a preferred aspect of the invention, the auxiliary servomotor is operated by oil. 
         [0019]    According to a preferred aspect of the invention, the annular lever comprises a pin eccentric with respect to an axis of rotation of the annular lever, the pin being hosted in a groove formed in said piston. 
         [0020]    According to a preferred aspect of the invention, the device further comprising a nut arranged around the pin and within the groove. 
         [0021]    According to a preferred aspect of the invention, the main servomotor is operated by oil. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The objects, advantages and other features of the present invention will become more apparent upon reading of the following non-restrictive description of preferred embodiments thereof, given for the purpose of exemplification only, with reference to the accompany drawing, through which similar reference numerals may be used to refer to similar elements, and in which: 
           [0023]      FIG. 1  shows a control mechanism for reversing a blade according to the prior art; 
           [0024]      FIGS. 2 a -2 f    show a sequence of the various configurations of the control mechanism of  FIG. 1  during operation; 
           [0025]      FIG. 3  shows a perspective view of a runner unit including a plurality of blades according to the present invention; 
           [0026]      FIGS. 4 and 5  depict a cross and plant sectional views of the runner unit of  FIG. 3 , showing a device for reversing a blade according to the invention; 
           [0027]      FIG. 6-11  show a sequence of the various configurations of the device for reversing a blade during operation. 
       
    
    
       [0028]    An exemplary preferred embodiment will be now described with reference to the aforementioned drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    With reference to  FIG. 3 , it is shown a runner unit, generally indicated with numeral reference  3 . The runner unit  3  is located within a tidal power plant which may be positioned between the sea and a lagoon basin (arrangement not shown). Runner unit  3  includes a hub body  33  to which a plurality of blades  2  are fitted. Each blade  2  comprises an airfoil  20  mounted on the hub body  33  and it is moved by a flow of water which may occur in a direct mode, that is from the lagoon to the sea along verse indicated by arrow A, and in a reverse mode, that is from the sea to the lagoon along verse indicated by arrow B. The flow of water generates a rotation of hub body  33  about an axis R that is transferred to a shaft (not visible) which is in turn connected to an electric generator (not shown) for the generating of electricity. 
         [0030]    The following description is directed to a device for reversing a blade  2  around a rotational axis S arranged perpendicular to the hub body  33 . It will be appreciated that a device according to the invention will be provided for each blade  2  of the runner unit  3 . 
         [0031]    Making now reference to following  FIG. 4 , it is shown a section view of runner unit  3  along a plane which contains both axis rotational axis R and S. Airfoil  20  is rotatably connected to hub body  33  by means of a trunnion portion  21  of the blade which is secured to an annular level  4 , rotatable about the axis S on the runner unit  3 . Annular lever  4  is coupled to a main servomotor which comprises a piston  51  adapted to reciprocate along a shaft  31  of the runner unit  3 . The coupling is arranged such that the position of the piston  51  along the shaft  31  determines the angular position of the blade with respect to rotational axis S, as it will be clear in the following detailed description of a preferred embodiment. 
         [0032]    Preferably, the main servomotor is of an oil type, and the displacement of the piston  51  along indicated directions C or D occurs by controlling, by a dedicated pressured fluid feeding system (not shown), the difference of pressure established within chambers  513  and  514 . More specifically, when the thrust created by the pressure in chamber  513  exceeds the thrust created by pressure of chamber  514  the piston moves along direction D, and vice versa. 
         [0033]    Preferably, in this non-limiting exemplary embodiment the annular lever  4  and the piston  51  are coupled by means of a pin  42 , integral to the annular lever  4  and eccentric versus axis S, that is the axis of rotation of the lever. Pin  42  is hosted into a groove  511  formed into the piston  51 . 
         [0034]    The device according to the invention further comprises an auxiliary servomotor  6  which will be better described with reference to next  FIG. 5 , depicting the runner unit  3  in a sectional view along a plane perpendicular to axis S. More in particular, auxiliary servomotor  6  comprises a reciprocating linear rack  61  which includes a rack element  612  articulated to a rod  611  by means of a first pivot arranged at an end  613  of the rod  611 . Alternatively, rack element  612  may also be fixed to the rod  611 . In this exemplary and non-limiting embodiment, auxiliary servomotor  6  is of a oil type, and comprises a servomotor body  62  hosting a reciprocating piston  621  integral to the linear rack  61 . The linear rack  61  is preferably mounted cantilever on the servomotor body  62 , which is articulated to an inner wall  32  by means of a second pivot. Alternatively, the servomotor body  62  may be secured to the inner wall  32  of the hub body  33 . 
         [0035]    Still with reference to  FIG. 5 , the rack element  612  is configured to cooperate with a toothed sector  41  provided on the annular lever  4  to cause a further rotation of the blade, additional to the rotation established by the main servomotor. 
         [0036]    With reference to the following sequence of  FIGS. 6-11  the mode of operation of the device for reversing the blade will be described in details. 
         [0037]      FIGS. 6-11  show on the right a schematic representation of the device for reversing the blade  2  according to the invention for subsequent angular positions of the blade during the inversion. On the left side it is shown the device within the runner unit in the two correspondent sectional views described in preceding  FIGS. 4 and 5 . 
         [0038]    With reference to  FIG. 6 , it is shown a normal functioning of the runner unit in a direct mode with the flow of water oriented along direction A. Blade  2  has a pitch with respect to water flow such to maximise the conversion of energy by enabling the rotation of the shaft of the runner around axis R. As it is clearly visible, in such configuration toothed sector  41  of annular lever  4  and the rack element  612  of auxiliary servo motor  6  do not mesh. When the flow of water inverts the direction, blade  2  has to be reversed in order to elaborate the inversed flow of water, for the energy conversion process, with the quite same efficiency as for the direct flow. As detailed above, the main servomotor (generally indicated in the sequence of  FIGS. 6-11  with numeral reference  5 ) determines the angular position of the lever  4 , and consequently of the blade  2 , by displacing the eccentric pin  42  which is hosted in groove  511  formed in piston  51 . More in particular, as pin  42  has a circular section and the groove a square one, a nut  512  is arranged around pin  42  and within the groove  511 . 
         [0039]      FIG. 7  shows the initial step of the inversion of the blade  2  during transition when the runner unit is stopped. Piston  51  is moved towards direction D by means of an over pressure established in chamber  513  with respect to chamber  514 . As a consequence, pin  42  is dragged by the groove  511  and thus the lever  4  is caused to achieve a first rotation about the axis S. 
         [0040]    With reference to  FIG. 8 , it is shown the piston  51  which has substantially reached the end of its path and thus it is at or at least in the proximity of a dead zone position. In such configuration, auxiliary servomotor  6  is arranged such that rack element  612  starts engaging with tooth sector  41 . At this point, piston  621  is driven by servomotor  6  to linearly slide rack  61  such to cause the lever  4 , by means of the coupling between toothed sector  41  and rack element  612 , to further rotate and pass the dead zone position. 
         [0041]      FIG. 9  shows the end position of lever  4  with toothed sector  41  and rack element  612  meshing. 
         [0042]    With reference to  FIG. 10 , having passed the dead zone, servomotor  5  may start acting on lever  4  again by moving piston  51  along opposite direction C, this time establishing an overpressure in chamber  514  versus chamber  513 . This determines the rotation of the lever  4  until the final pitch of the reverse mode is achieved, as shown in last  FIG. 11 , where the flow of water has been reversed and it is now directed along arrow B. 
         [0043]    It will be appreciated that, from the reversed mode depicted in  FIG. 11  it is possible to return to the pitch of the blade of the direct mode shown in  FIG. 6 , by reversing all the operations of the device above described. 
         [0044]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.