Abstract:
The present invention generally relates to runner unit of a tidal power plant, and more particular to a blade of the runner unit. The blade according to the invention provides a maximised efficiency of energy production of the tidal power plant during functioning of both direct and reverse modes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to European Patent Convention No. 15290005.6 filed Jan. 13, 2015, the contents of which are hereby incorporated in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present invention generally relates to runner unit of a tidal power plant, and more particular to 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 fixed 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. 
       SUMMARY 
       [0006]    It is an object of the present invention to solve the aforementioned technical problem by providing a blade for a runner unit as substantially defined according to independent claim  1 . 
         [0007]    It is a further object of the present invention to provide a runner unit as substantially defined in dependent claim  13 . According to an aspect of the invention, this object is obtained by a blade of a runner unit for a tidal plant, the runner unit including a hub body, the blade ( 1 ) comprising an airfoil rotatably connected to the hub body and comprising reversible locking means adapted to block/release the blade in correspondence of at least two angular operative positions. 
         [0008]    According to another aspect of the invention, the blade is connected to the hub body through a rotative servomotor, adapted to cooperate with the reversible locking means such to block the blade in at least two angular operative positions. 
         [0009]    According to another aspect of the invention, the servomotor comprises a cylinder integral to the hub body and a rotative piston hosted therein and connected to the airfoil. 
         [0010]    According to another aspect of the invention, the rotative piston comprises a protrusion abutting against an inner wall of the cylinder and such to define first and second separated chambers between the piston and the cylinder. 
         [0011]    According to another aspect of the invention, the servomotor further comprises a fluid feeding means. 
         [0012]    According to another aspect of the invention, the fluid feeding means comprises a first pipe adapted to feed/release pressurised fluid to the first chamber and a second pipe adapted to feed/release pressurised fluid to the second chamber such to move the piston between the two angular positions. 
         [0013]    According to another aspect of the invention, the rotative servomotor comprises a lever element adapted to engage/disengage the reversible locking means. 
         [0014]    According to another aspect of the invention, the lever element is connected to the piston by bolting. Alternatively, the piston and lever element may be formed as one-piece, for example by forging. 
         [0015]    According to another aspect of the invention, the servomotor is connected to a trunnion portion of the blade. 
         [0016]    According to another aspect of the invention, the reversible locking means comprises a retractable locking pin mounted on the hub. 
         [0017]    According to another aspect of the invention, the locking means comprises a locking mechanism associated to the locking pin, the locking mechanism configured to move the locking pin from a locking position to a releasing position and vice versa. 
         [0018]    According to another aspect of the invention, the locking pin is configured to engage/disengage the lever element when the blade is in the two angular operative positions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The objects, advantages and other features of the present invention will become more apparent upon reading of the following non-restrictive description of 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: 
           [0020]      FIG. 1  shows a perspective view of a runner unit including a plurality of blades according to the present invention; 
           [0021]      FIG. 2  shows a top schematic view of a blade according to the present invention; 
           [0022]      FIG. 3  shows a particular of  FIG. 2 ; and 
           [0023]      FIGS. 4 and 5  show an operational sequence of the blade according to the present invention. 
       
    
    
       [0024]    An exemplary preferred embodiment will be now described with reference to the aforementioned drawings. 
       DETAILED DESCRIPTION 
       [0025]    With reference to  FIG. 1 , it is shown a runner unit, generally indicated with reference numeral  2 . The runner unit  2  is located within a tidal power plant which may be positioned between the sea and a lagoon basin (arrangement not shown). Runner unit  2  includes a hub body  3  to which a plurality of blades  1  are fitted. Blade  1  comprises an airfoil  100  and is mounted on the hub body  3  and 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. 
         [0026]    The following description is directed to a single blade  1 , but it will be appreciated that the same may apply to all blades of the runner unit  2 . 
         [0027]    Making now reference to following  FIG. 2 , it is shown a top schematic view of a blade  1  of  FIG. 1 . Blade  1  is, as it will be clear from the following detailed description of an exemplary and non-limiting embodiment, rotatably connected to the hub body  3  and comprises reversible locking means  4  adapted to block/release the blade  1  in correspondence of at least two angular operative positions. In the application herewith shown, the blade  1  operates in two different flow conditions, that is water flowing in direct mode and water flowing in reverse mode, the latter having an opposite verse with respect to the direct mode. For this reason, blade  1  is arranged such to present two distinct angular positions in which it is blocked by locking means  4 . In this exemplary embodiment, blade  1  is rotatably connected to hub body  3  through a rotative servomotor  5 , which is adapted to cooperate with locking means  4  such to displace and block the blade  1  in correspondence of the two angular operative positions. More in particular, servomotor  5  includes a cylinder  51  integral to the hub body  3  and a rotative piston  52  which is hosted in the cylinder  51  and integral to the airfoil  100 . As visible in the figure, the rotative piston  52  comprises a protrusion  521  which abuts against an inner wall  512  of the cylinder  51 . The presence of the protrusion  521  establishes a first chamber  7  and a second chamber  8 , fluidly separated from the first chamber  7 , both defined between the rotative piston  52  and the cylinder  51 . The servomotor  5  further comprises a pressurised fluid feeding means which includes a first pipe, schematically depicted and indicated with reference numeral  91 , adapted to feed/release pressurised fluid, such as oil, to the first chamber  7  and a second pipe  92  which is adapted to feed/release pressurised fluid to the second chamber  8 . The feeding of pressurised fluid in chamber  7  together with the release of pressurised fluid from chamber  8  determines the rotation of the piston  52  in clockwise direction. Conversely, the release of pressurised fluid in chamber  7  together with the feeding of pressurised fluid from chamber  8  determines the rotation of the piston  52  in counter clockwise direction. 
         [0028]    Still with reference to  FIG. 2 , servomotor  5  comprises lever elements  53  and  63 , which, in this exemplary case, are positioned around the rotative piston  52  at two distinct angular locations. Lever elements  53 ,  63  may be connected to the piston  52  by bolting. 
         [0029]    Each lever element  53  and  63  is advantageously adapted to engage/disengage the reversible locking means  4 . Advantageously, locking means  4  comprises a retractable pin  41  mounted on the hub body  3 . 
         [0030]      FIG. 2  depicts the blade  1  when positioned in the direct operating mode. Lever  53  engages locking pin  41  and maintains the blade in a fixed orientation with respect to the hub body  3 . The blade efficiently operates in this configuration in the direct mode, that is when the flow of water goes from the lagoon to the sea. When the verse of the flow of water reverses, flowing in opposite direction (from the sea to the lagoon), the orientation of the blade with respect to the hub body  3  is advantageously changed to maximise efficiency. 
         [0031]    To this purpose, locking pin  41  is retracted (as it will be explained with reference to the following figure) this way disengaging lever element  53 , and at the same time fluid feeding means associated to servomotor  5  determines, by feeding pressurised fluid to chamber  7  and releasing pressurised fluid from chamber  8 , the rotation of the piston  52  and the airfoil  100  in a clockwise direction. The rotation of the airfoil  100  occurs until lever element  63  engages locking pin  41 , blocking the blade in the other operative reverse mode position. 
         [0032]    With now reference to following  FIG. 3  it is shown in more detail the reversible locking means  4  mounted on the hub body  3 . In particular, reversible locking means  4  comprises the retractable locking pin  41  and a locking mechanism, generally indicated with numeral  42 , associated to the locking pin  41  and configured to move locking pin  41  from a locking position, in which it engages the lever element ( FIG. 3  above), to a releasing position in which the lever element is disengaged and free to rotate ( FIG. 3  bottom). In this non-limiting example, locking pin  41  is slidable into a recess engraved in the hub body  3  and comprises a protuberance  424  providing a stepped region. Locking mechanism  42  includes a spring element  421  provided on the locking pin  41  exerting a resilient action, and a chamber  422  defined on the opposite side of the protrusion  424  between the pin  41  and the recess, the chamber  422  being fluidly to connected to a pipe  423  for feeding/releasing pressurised fluid, such as oil. Feeding pressurised oil in the chamber  422  determines the compression of the spring element  421  and consequently the retraction towards the hub body  3  of the locking pin  41  ( FIG. 3  bottom). In the same way, the release of pressurised fluid from chamber  422  disposes for the predominance of the resilient action of spring element  421  determining the outward movement of the pin  41  and the consequent engaging with lever element ( FIG. 3  top). 
         [0033]    As an alternative to the embodiment described, the locking pin  41  may be actuated by two fluid chambers disposed opposite to protrusion  424 . More specifically, spring element  421  would be replaced by a fluid chamber similar to the fluid chamber  422 ; spring reaction and consequent movement of the locking pin  41  would be actuated by feeding/release of pressurised fluid therein. 
         [0034]    With reference to  FIGS. 4 and 5 , it is shown an operational sequence of the blade  1  according to the invention. In particular,  FIG. 4  left side shows the blade  1  located in the angular position corresponding to the direct operational mode. Lever element  53  engages locking pin  41  and the blade is blocked in such angular position. The piston of the servomotor  5  is advantageously connected to a trunnion portion  11  of the blade  1 . 
         [0035]    When a change of blade orientation is needed, locking mechanism retracts locking pin  41  which disengages lever element  53  and servomotor  5  determines the rotation of the piston in a clockwise direction and of the airfoil  100  ( FIG. 4  right). 
         [0036]      FIG. 5 , left side, shows the lever element  63  approaching locking pin  41  during the rotation of the servomotor  5 . 
         [0037]    Finally,  FIG. 5  right side shows the lever element  63  engaging locking pin  41  and therefore the blade being rotated and blocked in the second operative position corresponding to the reverse operational mode. 
         [0038]    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 is 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.