Abstract:
There is provided a free standing tidal power plant without a dam structure set to take up kinetic energy from a water flow. The tidal power plant includes a propeller-shaped water turbine in horizontal rotor configuration with rotor blades, the rotor blades having bidirectional profiles, wherein the rotor blades are fastened to a revolving unit, which defines a rotational plane, and an electric generator at least indirectly driven by the water turbine. Each rotor blade is associated with a first swivel axis and a second swivel axis, the first axis and the second swivel axis extending substantially along a longitudinal axes of a first coupling element and a second coupling element, respectively. The revolving unit includes a planar guide region having a sliding apparatus, a first guide groove and a second guide groove, the sliding apparatus connected to the rotor blade to transmit rotor blade forces.

Description:
This is a U.S. national phase application which is based on, and claims priority from foreign application Serial No. DE 102008051370.9-15, filed on Oct. 15, 2008 in Germany. 
     FIELD OF THE INVENTION 
     The invention relates to an underwater power plant with passive power control, especially a tidal power plant, having a propeller-shaped water turbine in horizontal rotor configuration. 
     BACKGROUND 
     Underwater power plants which absorb kinetic energy in a free-standing manner without any dam structure from a water flow are especially suitable for power generation from an ocean current, preferably a tidal current. An advantageous embodiment for underwater power plants comprises propeller-shaped water turbines with a plurality of rotor blades-which are fixed to a revolving unit. 
     Such a water turbine can be fastened to a support structure via a machine gondola in which an electric generator is typically accommodated, which support structure is arranged to be fixed to foundations in the floor of the water body or to be floating. For the economic design of generic underwater power plants, especially for power generation from a tidal current, the mean flow speeds that occur over the course of the year are typically used as a basis. However, in the case of a location of the plant in the sea, inflow speeds may occur in certain cases at least over short periods of time which lie above the chosen plant configuration. Such extreme situations occur especially under stormy conditions, for which the wind and waves run in the direction of the tidal current. The mechanical power absorbed by the water turbine from the flow needs to be limited in such cases of stress. 
     A known form of power limitation provides the use of an active blade angle adjusting device. This allows choosing the angle of attack of the rotor blades of the driving flow in such a way that a desired power curve is obtained. In extreme situations, the rotor blades are guided in the direction of the feathered pitch position and the power intake is thus limited. Accordingly, the components downstream of the water turbine are protected in the drive train against overloading. The disadvantageous aspect in this approach is the constructional effort required for arranging such a blade angle adjusting device. Moreover, additional movable components are necessary in the revolving unit which lead to an increased failure risk and consequently require regular maintenance. 
     In order to achieve a sturdy configuration of the system that requires as little maintenance as possible, speed guidance by means of the supporting effect through the electric generator in the drive train can be caused as an alternative measure for power limitation. The generator and possibly further braking apparatuses will reduce the speed of the rotor in the case of an overload in order to guide the same away from power optimum. This reduction in speed can go so far that a stall occurs at the profile of the rotor blades, so that power intake is reduced efficiently. The disadvantageous aspect in this approach is that mechanical loads occurring under the conditions of a stall are high as a result of the occurring blade excitations and require a respective configuration of the structural stiffness of the rotor blades. A high constructional effort is the result of this requirement placed on strength and makes the water turbine heavier. 
     When the above speed guidance of the water turbine for power limitation is arranged in such a way that in the case of overload an increase of speed to a rotational speed above the best point is allowed, it is possible to prevent stalling. However, high forces will act on the water turbine in the range of high rotational speeds. The increasing centrifugal moments need to be taken into account for the design of the plant especially in the range of high speeds. 
     SUMMARY OF THE INVENTION 
     The invention is based on the object of providing a power limitation in the case of overloading which is simple from a constructional standpoint and for which it is possible to omit an associated open-loop and closed-loop control device. Moreover, the power limitation shall be arranged in such a way that the impact of excessively high loads on the rotor can be excluded as a result of the construction. 
     The inventors have recognized for the solution of the above object that a sturdy and efficient power limitation is provided by means of self-adjustment of the rotor blades for a water turbine in horizontal-rotor design. For this purpose, a water turbine arranged in the manner of a propeller with a plurality of rotor blades fastened to a revolving unit is assumed. Each rotor blade is associated with a swivel axis which extends substantially along the rotor blades. The swivel axis is displaced towards the inflow profile nose in relation to the hydrodynamic center. 
     The respective rotor blade can rotate about the swivel axis either at the base point, which means the fastening point to the revolving unit, and/or the rotor blade performs a torsion about the swivel axis as a result of the load caused by the hydrodynamic forces. The torsion can extend over the entire blade or be limited to a portion of the longitudinal extension, which means over a section of the swivel axis. 
     The embodiment alternatives of a rotatably arranged fastening on the one hand and an elastic arrangement of the rotor blade in relation to a torsion on the other hand lead to an upwardly pivoting moment in the case of a respective choice of the swivel axis relative to the threading line of the hydrodynamic centers of the profile sections with the swivel axis as the surface normal. This shall be understood as a moment that twists the rotor blade in the direction of feathered pitch position or causes a torsion facing in this direction. 
     The centrifugal forces occurring during the revolving of the rotor blade act against the movement towards the feathered pitch position, so that in every operational situation there is a balance in moments between the centrifugal moments which generate a moment guiding the blade back and the upwardly pivoting, hydrodynamic moment. Additional elastic restoring forces must be considered in the case of an elastic deformation of the rotor blades or components attached thereto. They can arise during a torsion in the blade per se or a device for generating a restoring force is included in addition in the fastening of the rotor blade in the revolving unit. 
     In the event that a device for generating a restoring force is integrated in the fastening of the rotor blade in the revolving unit which elastically counteracts a rotation of the rotor blade about the swivel axis in the direction of the feathered pitch position, the rotor blades are automatically returned to the rotor plane after the fading of the overload case. The same applies when the upwardly swiveling movement in the direction of the feathered pitch position is based substantially on torsion of the rotor blades. 
     If such elastic restoring forces are missing for an arrangement with rotor blades which are fastened rotatably on the revolving unit, the return from the feathered pitch position must be caused by the effect of the centrifugal moments. For this purpose, the water turbine is accelerated either by means of motive operation of the electric generator of the underwater power plant to such an extent until return swiveling occurs or there is also a minimum driving moment in the feathered pitch position at inflow which is sufficient to operate the plant, so that automatic run-up to a speed threshold occurs for which the centrifugal moments exceed the hydrodynamic moments with upwardly swiveling effect and a return to the position for normal operation occurs. 
     According to a further development of the invention, the passive power control in accordance with the invention is used for an underwater power plant with bidirectional inflow. Profiles with bidirectional inflow for the rotor blades are advantageous especially for tidal power plants because in such a construction a cyclic change of the inflow direction does not require any rotation of the rotor blades about their longitudinal axis or a rotation of the entire plant about the vertical axis. For this purpose, it is possible to use hydrodynamic symmetrically arranged profiles of a lens-shaped configuration or point-symmetrical profiles with an S-twist. When the passive rotor blade adjustment is combined with inflow capability on both sides, a plant design is obtained which is especially simplified in a constructional respect concerning open-loop and closed-loop control: The requirements placed on the electronic control system which is necessary for monitoring the plant are accordingly reduced, so that an underwater power plant of high sturdiness is obtained. The invention is explained below in closer detail by reference to embodiments in conjunction with the drawings, which show in detail: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a rotor blade adjustment device in accordance with the invention; 
         FIG. 2  explains the inflow conditions and the resulting force action for a unidirectional rotor blade profile; 
         FIG. 3  illustrates the upward swiveling of a rotor blade in the direction of the feathered pitch position; 
         FIG. 4  shows the elastic torsion of a rotor blade in the direction of the feathered pitch position; 
         FIG. 5  shows a further development of the embodiment according to  FIG. 1  with additional weights and a device generating a restoring force; 
         FIG. 6  shows the embodiment according to  FIG. 5  in a cross-sectional view; 
         FIGS. 7 ,  8  show the hydrodynamic forces for inflow from different directions for a bidirectional profile; 
         FIG. 9  shows a cross-sectional view of the guide grooves of a rotor blade adjustment device in accordance with the invention for a bidirectional rotor blade profile; 
         FIG. 10  shows the sectional view B-B of  FIG. 9 ; 
         FIG. 11  shows the normal operating state for an inflow from a first direction for the sectional view of  FIG. 9  with a superimposed rotor blade profile; 
         FIG. 12  shows the overload state with a rotor blade upwardly swiveled to the feathered pitch position for the sectional view of  FIG. 9  with a superimposed rotor blade profile; 
         FIGS. 13 and 14  show the change of the swivel axis as a consequence of a reversal of the inflow direction for the sectional view of  FIG. 9  with a superimposed rotor blade profile. 
         FIG. 15  shows a free standing tidal power plant without a dam structure with a revolving unit  2  comprising a rotor blade  1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a longitudinal sectional view of a partial section of a rotor blade  1  with passive rotor blade adjustment. The rotor blade  1  is linked in a rotatable manner about a swivel axis  3  on a revolving unit  2  which is arranged in a hub-like manner in this case. An axial pin  5  is provided for this purpose which is inserted in a receptacle  6  in the revolving unit  2 . The receptacle  6  is associated with a bearing  7  which can be arranged as a water-lubricated slide bearing and which allows the axial pin  5  to perform a rotational movement. The bearing  7  must absorb the forces introduced by the rotor blade into the revolving unit  2 . In addition, a radial securing means  8  is provided on the axial pin  5  which grasps behind a thrust bearing on the revolving unit  2  in order to secure the rotor blade  1  in the radial direction. 
     As a result of the rotatable linkage of rotor blade  1  as shown in  FIG. 1  to the revolving unit  2 , the blade angle of the rotor blade  1  relative to the rotational plane  39  will set during the operation of the plant according to the balance of the acting moments. One of the acting moments is obtained by the hydrodynamic forces acting upon the rotor blade  1 . This is shown in  FIG. 2 , with a unidirectional profile of a rotor blade  1  being outlined according to the section A-A of  FIG. 1 . The illustrated profile is schematically simplified, with profiles similar to those of hydrofoils being generally used for underwater power plants of this kind. 
       FIG. 2  shows the driving flow c and the negative circumferential speed u of the rotor blade, which add up as vectors to the effective inflow w. It meets the profile nose  10  and generates a force effect on the profile which acts upon the hydrodynamic center  11 . It is composed of lift F a  and the drag F d  which result in the hydrodynamic force F r  when added up as vectors. In the present case, the hydrodynamic center  11  lies approximately at a quarter of the length of the profile chord  9 . The resulting hydrodynamic force F r  which acts upon the hydrodynamic center  11  can be broken down into two components. This is the tangential force F t  on the one hand which extends parallel to the profile chord  9 . On the other hand, a transversal force F q  is obtained with an orientation perpendicular to the profile chord  9 . 
     The swivel axis  3  is chosen in such a way according to the invention that the hydrodynamic forces generate a moment which is directed in the direction of the feathered pitch position. For this purpose, the swivel axis  3  is arranged upstream in a displaced manner, spaced from the hydrodynamic center. As a result of the thus resulting lever, the transversal force F q  generates a moment which is directed in a counter-clockwise direction for the present illustration. This moment acts in an upwardly swiveling manner and tries to twist the profile to the feathered pitch position, which means that the angle between profile chord  9  and the rotational plane  39  is increased in order to reduce the angle of attack of profile  40  in relation to the effective inflow w. A profile  40  in the initial position and, in broken lines, for the upwardly swiveled position  12  in the direction of the feathered pitch position is outlined for a predetermined driving flow c in  FIG. 3 . 
     For an alternative embodiment of the invention, the rotor blade  1  is linked in a non-rotatable way to the revolving unit  2 . Instead, the upward swiveling is caused by an elastic behavior of the rotor blade  1 . This is shown in  FIG. 4 . The position of a reinforcement  4  for a rotor blade  1  which is arranged as a hollow profile is chosen in such a way that the region of increased structure strength is disposed upstream in relation to the hydrodynamic center  11 . As a result of this measure, the transversal force F q  leads to a twisting about a torsion axis  13  which will be designated below, in accordance with the terminology as already chosen above, as swivel axis  3 , with such axis extending along the reinforcement  4  for the illustrated embodiment. A limited twisting of the rotor blade  1  about the swivel axis  3  is obtained for the profile  40  associated with the section A-A, as indicated in  FIG. 4 , so that the profile  40  is guided only from a specific radial distance by the revolving unit  2  to the region of the feathered pitch position. There is a possibility to adjust the characteristics for upward swiveling and the moment necessary for this purpose by adjusting the reinforcement  4  and the resulting local determination of the elastic counter-forces against torsion. 
     Furthermore, the two measures as described above concerning the twisting about the swivel axis  3  by a rotatable linkage of the rotor blade  1  to the revolving unit  2  and torsion about a swivel axis  3  arranged upstream in relation to the hydrodynamic center  11  can be combined with each other. 
     In contrast to the upwardly swiveling moments as a result of the hydrodynamic force action on the profile  40  of the rotor blades  1  as described above, the centrifugal forces originating during the rotation act upon the rotor blade  1 . They tend to guide the rotor blade  1  back to the rotational plane  39 , so that a balance of moments is obtained for a predetermined inflow speed on the rotor blade  1 . Such a configuration can be chosen by profile configuration and determination of position of the swivel axis  3  in conjunction with speed guidance caused by the directly coupled electric generator that the equilibrium position lies in the range of the feathered pitch position only in the case of an extreme load the rotor blade  1  assumes for normal operation an angle which is beneficial for propulsion in relation to effective inflow w. 
     The resulting equilibrium position can be influenced by further measures. Additional weights  14 . 1 ,  14 . 2  which are arranged in pairs are provided for this purpose in the further development of the embodiment according to  FIG. 1  which is shown in  FIG. 5 . The cross-sectional view of  FIG. 6  shows that the additional weights  14 . 1 ,  14 . 2  are spaced from the swivel axis  3  parallel to the rotational plane  39 . This increases the surface centrifugal moment of rotor blade  1 , so that the returning moments which guide the rotor blade  1  to the position of attack against the feathered pitch position are increased during the rotation. A device for generating a restoring force  15  can be provided in addition or alternatively. In  FIG. 5 , an apparatus is shown which is integrated for this purpose in the revolving unit  2  and which acts upon an extension about the axial pin  5 . Passive systems are especially preferred which generate a counter-force during a rotational movement of the rotor blade  1  in the direction of the feathered pitch position. Spring arrangements are suitable for this purpose. 
     Furthermore, the measures as shown in  FIGS. 5 and 6  improve the start-up of an underwater power plant with passive rotor blade adjustment in accordance with the invention. It is assumed that the rotor blades  1  are in the feathered pitch position and there is no overloading with respect to inflow. In the case that no device for generating a restoring force  15  has been provided, it is necessary to bring the water turbine at first to a specific revolving speed. This is achieved by a motive operation of the electric generator. From a specific revolving speed, the centrifugal forces will guide the rotor blade  1  to the rotational plane  39 , so that for normal operation the propulsion during generator operation of the electric generator as produced by the individual rotor blades  1  will make the water turbine revolve with such a speed that the centrifugal forces will hold the rotor blades against the upwardly swiveling hydrodynamic moments in the position of attack up to a predetermined speed of the driving flow c. 
       FIGS. 7 to 14  concern an embodiment of a passive rotor blade adjustment for a generic underwater power plant with a profile  16  with bidirectional inflow. Such a one is shown in a schematically simplified way in  FIGS. 7 and 8 , with a lens-shaped profile being assumed which is arranged symmetrically in relation to profile chord  9  and the central line  34 . The profile can perform an S-twist alternatively. In this case, the skeleton line is symmetric to point and follows an S-shaped contour. Such a profile arrangement is not shown in detail in the drawings. 
       FIG. 7  further shows in an exemplary profile sectional view the driving flow c from a first direction, the negative revolving speed u and the effective inflow w, with the resulting hydrodynamic force F r  being generated which acts upon the first hydrodynamic center  19 . The resulting hydrodynamic force F r ′ is obtained for a second opposite direction of inflow with the driving flow c′ which is shown in  FIG. 8 , which force acts upon the symmetrically disposed, second hydrodynamic center  20 . In order to realize a passive rotor blade adjustment, the swivel axis must be arranged upstream of the hydrodynamic center in order to provide a lever arm for generating an upwardly swiveling moment in relation to the transversal force F q , F q ′. There is a necessity, depending on the inflow direction, of performing a change from a first swivel axis  3 . 1  to a second swivel axis  3 . 2 . Active systems can be used for this purpose, but this would lead to an undesirable effort in regard to control systems. It is therefore preferable to also perform the transition from the first swivel axis  3 . 1  to the second swivel axis  3 . 2  in a passive way depending on the inflow direction, so that the change can be caused exclusively by the flow forces themselves. 
     The breakdown of the resulting hydrodynamic force F r , F r ′ into a transversal force F q , F q ′ which is perpendicular to the profile chord  9  and a tangential force F t , F t ′ which is parallel to the profile chord  9  as chosen in  FIGS. 7 and 8  leads to a tangential force F t , F t ′ for the illustrated embodiment which faces from the profile nose on the inflow side to the profile nose on the outflow side. This direction can be set by choosing the profile and its installation angle for a predetermined inflow speed range by determining the ratio between the buoyancy and the flow resistance in the profile. The following embodiment of a passive rotor blade adjustment for a profile  16  with bidirectional inflow assumes such a chosen direction for the tangential force F t , F t ′. However, the illustrated principle can also be applied to an oppositely directed tangential force F t , F t ′. 
       FIGS. 9 and 10  show a possible embodiment of a passive rotor blade adjustment for a rotor blade  1  with a bidirectional profile. A planar guide region  29  is provided for this purpose in the revolving unit  2 , on which a sliding apparatus  30  which is rigidly connected with the rotor blade  1  performs a guided sliding motion and simultaneously transmits rotor blade forces. For the present arrangement, the sliding apparatus  30  comprises an upper support plate  27  and a bottom support plate  28  which are applied in a plane-parallel manner and are chosen with respect to their distance in such a way that sliding on the upper and lower side of the same occurs during a relative motion to the planar guide region  29 . The acting running surfaces of these components are advantageously covered with a sliding material such as PTFE for example or are arranged as slide hearings. In this case, one component is preferably made of a hard material, typically special steel, and the counter-running surface is made of a soft material such as an elastomer, especially Orkot®. 
     The connection between the upper support plate  27  and the bottom support plate  28  occurs by a first coupling element  23  and a second coupling element  24  with a preferably cylindrical shape. The coupling elements  23 ,  24  are arranged with respect to the bidirectional profile of the rotor blade  1  in such a way that their longitudinal axes define the first and second swivel axis  3 . 1 ,  3 . 2 . The first coupling element  23  moves in a first guide groove  21  which reaches through the planar guide region  29 . Accordingly, a second guide groove  22  is associated with the second coupling element  24 . 
     The effectiveness of one each of the two swivel axes  3 . 1 ,  3 . 2  is determined by the moving capabilities of the coupling elements  23 ,  24  in the associated-guide grooves  21 ,  22 . This is shown from the sectional view C-C as shown in  FIG. 9 . The two guide grooves  21 ,  22  are arranged in a mirror-like manner with respect to one another and each comprise a short leg which extends at least in the end regions parallel to the profile chord  9  of the rotor blade  1  in the normal operating position. The short legs shall be designated below as tangential guides  25 . 1 ,  25 . 2 . They can be associated with a coinciding longitudinal axis of the tangential guides  33 . Furthermore, upward swiveling regions  26 . 1  and  26 . 2  are provided for the guide grooves  21 ,  22 , which regions are adjacent to the tangential guides  25 . 1 ,  25 . 2 . Preferably, the sliding surface of an upward swiveling region  26 . 1 ,  26 . 2  for the respectively associated coupling segment  23 ,  24  substantially follows the arc of a circle whose central point lies in the end region of the tangential guide  25 . 1 ,  25 . 2  of the opposite guide groove  21 ,  22  for the non-associated coupling element  23 ,  24 . The radius of the arc of the circle corresponds substantially to the distance of the central points of the two coupling elements  23 ,  24 . 
     The function of the guide grooves  21 ,  22  is shown in the  FIGS. 11 to 14 . The drawings show the projection of the profile  31  with bidirectional inflow to the sectional view of  FIG. 9  for different operating situations.  FIG. 11  shows the revolving speed v for normal operation for a driving flow. The first coupling element  23  is positioned in the end region of the first tangential guide  25 . 1  and the second coupling element  24  is disposed in the second guide groove  22  in such a way that an upwardly swiveling motion along the second upward swiveling region  26 . 2  is possible. Consequently, the first swivel axis  3 . 1  is determined by the first coupling element  23 . Furthermore, the hydrodynamic forces are taken up through the walls of the first guide groove  21 . In addition, the preferably planar arrangement of the upper and bottom support plates  27 ,  28  act in a load-absorbing way. Based on this normal operating state, the upward swiveling of the rotor blade  1  occurs in the case of overloading. This is shown in  FIG. 12 . 
     When there is a change of the direction of the driving flow c′, the change as shown in  FIGS. 13 and 14  occurs from the first swivel axis  3 . 1  to the second swivel axis  3 . 2 . The flow pressure on the rotor blade  1  and the gradually building tangential force F t  lead to a sliding motion of the coupling elements  23 ,  24  in the tangential guides  25 . 1 ,  25 . 2 . In the further course of this movement, the first coupling element  23  follows the curved sliding path  32  on the inside of the first guide groove  21  and the second coupling element  24  is moved up to the end region of the second tangential guide  25 . 2 . As a result, the second swivel axis  3 . 2  is determined by the second coupling element  24 . As a result of a subsequent increase in the revolving speed v, there is an equilibrium of moments which leads to a parallel alignment of the profile chord  9  relative to the axis of the tangential guides  33 . This position which is mirror-like to  FIG. 9  is not shown in detail in the drawings. 
     Further embodiments of the invention are possible. It is especially possible that the swivel axis has a curved progression, especially in the case of sickle-shaped rotor blades. The same applies to the case of a swivel axis  3  with a twisting of the rotor blade as a result of hydrodynamic forces when the reinforcing components which determine the swivel axis  3  do not extend in a straight line. A swivel axis which does not extend in a straight line represents a possible embodiment of the invention. Furthermore, an underwater power plant which is arranged in accordance with the invention can have a growth protection system, especially in the region of the rotor blade adjustment device. Such a system may comprise devices for heating parts of the plant in order to remove growth and to especially keep the guide paths and the running elements of the rotor blade adjustment in a functional state. Further embodiments of the invention are obtained from the scope of the following claims. 
     LIST OF REFERENCE NUMERALS 
     
         
           1  Rotor blade 
           2  Revolving unit 
           3  Swivel axis 
           3 . 1  First swivel axis 
           3 . 2  Second swivel axis 
           4  Reinforcement 
           5  Axial pin 
           6  Receptacle 
           7  Bearing 
           8  Radial securing means 
           9  Profile chord 
           10  Profile nose 
           11  Hydrodynamic center 
           11 . 1  Threading line of hydrodynamic centers 
           12  Upwardly swiveled position 
           13  Torsion axis 
           14  Additional weight 
           15  Device generating restoring force 
           16  Profile with bidirectional inflow 
           17  First profile nose 
           18  Second profile nose 
           19  First hydrodynamic center 
           20  Second hydrodynamic center 
           21  First guide groove 
           22  Second guide groove 
           23  First coupling element 
           24  Second coupling element 
           25 . 1  First tangential guide 
           25 . 2  Second tangential guide 
           26 . 1  First upward swiveling region 
           26 . 2  Second upward swiveling region 
           27  Upper support plate 
           28  Bottom support plate 
           29  Planar guide region 
           30  Sliding apparatus 
           31  Projection of the bidirection inflow profile 
           32  Curved sliding path 
           33  Longitudinal axes of tangential guides 
           34  Central line 
           39  Rotational plane 
           40  Profile 
         c, c′ Driving flow 
         d Angle of attack 
         u, u′ Negative revolving speed 
         v, v′ Revolving speed 
         w, w′ Effective inflow 
         F a , F a ′ Buoyancy 
         F d , F d ′ Flow resistance 
         F r , R r ′ Resulting hydrodynamic force 
         F q , F q ′ Transversal force 
         F t , F t ′ Tangential force