| CLAIMS: 1 A power generation system including waterwheel having a rim adapted for rotation about a central axis when the waterwheel is submerged in a water current, wherein: hydrofoils are mounted at spaced intervals around the rim so as to project laterally from each side thereof wing-like and in cantilever fashion, each hydrofoil has a leading edge and a trailing edge and a chord-line that passes through said leading and trailing edges, each hydrofoil is rotatable relative to the rim about a pivot axis that is substantially parallel with the central axis, control means are provided to pivot each hydrofoil about its respective pivot axis so as to influence the angle of attack of said chord line of the respective hydrofoil relative to apparent direction of water flow in the vicinity thereof so as to thereby regulate hydrodynamic lift generated by the respective hydrofoil as the waterwheel rotates, and the system includes power generation means adapted to be driven by rotation of the waterwheel. 2 A power generation system according to claim 1 , wherein: ' said control means includes a servocontrol carried by each hydrofoil and adapted to maintain a predetermined angle of attack of the respective hydrofoil during limited rotation of the respective hydrofoil with the rim about the central axis, said servocontrol comprising: sensor means adapted to sense said apparent direction of water flow relative to the chord line and, thus, the actual angle of attack of the respective hydrofoil, and said sensor means being adapted to generate a local output signal indicative said actual angle of attack, a moveable control flap attached to the trailing edge of the respective hydrofoil and adapted for angular movement to hydrodynamically influence the angle of attack of the respective hydrofoil, and actuator means connected to receive said local signal together with an offset signal indicative of said predetermined angle of attack and adapted to control said angular movement of said control flap to adjust the actual angle of attack of the respective hydrofoil to better approximate said pretermined angle of attack indicated by said offset signal. 3 A power generation system according to claim 2, wherein: said sensor means comprises a sensor flap mounted near the leading edge of the respective hydrofoil for free angular movement about a sensor axis that is substantially parallel with the pivot axis of said hydrofoil, whereby said sensor flap is adapted to align with said apparent direction of water flow in the vicinity of the leading edge of the respective hydrofoil, and said sensor means includes rotary transducer means operably connected to the sensor flap and adapted to generate said local output signal indicative of the actual angle of attack of the respective hydrofoil. 4 A power generation system according to claim 2 or 3, wherein: said control means includes a central controller connected to the servocontrol system of each hydrofoil, and said central controller is adapted to generate and transmit a separate offset signal to the servocontrol of each respective hydrofoil. 5 A power generation system according to claim 4, wherein: said central controller is adapted to determine said offset signal having regard to at least one of the following parameters: desired power output of said power generation means, actual power output of the power generation means, speed of the water current, rotational speed of the waterwheel, rotational angle of the waterwheel, angular position of a selected hydrofoil. 6 A power generation system according to claim 4 or 5, wherein: said central controller is adapted to predetermine different angles of attack for different hydrofoils and to direct respectively different offset signals indicative of said different predetermined angles of attack to respective ones of said servocontrol systems as the waterwheel rotates in the current. 7 A power generation system according to any preceding claim, wherein: a plurality of hydrofoil assemblies are arranged at spaced intervals around the rim, each hydrofoil assembly includes a pair of said hydrofoils that have a common pivot axis, the hydrofoils of said pair being arranged on opposite sides of the rim, the hydrofoils of said pair are mounted on a common boom that extends through the rim and is rotatably mounted therein so that, when pivoted by said control means about said common pivot axis, both hydrofoils of the pair move in unison. 8 A power generation system according to claim 7, wherein: locking means are provided to selectively lock each boom against rotation with respect to the rim so that the angle of the pair of hydrofoils supported by the respective boom is fixed relative to the rim of the waterwheel while said boom is locked, and said locking means is operably connected to said central controller whereby the controller is enabled to unlock the respective boom to enable its rotation about the common pivot axis and with respect to the rim. 9 A power generation system according to claim 10, wherein: said locking means locks automatically upon alignment of co-acting stops on the rim and the respective boom as the boom rotates relative to the rim when the waterwheel is turning. 10 A method of generating power from a submerged waterwheel that has a central axis, a peripheral rim and plurality of hydrofoils mounted at spaced intervals around the rim, each hydrofoil being rotatable about a pivot axis that is substantially parallel with the central axis, the method comprising the step of: employing a servocontrol system associated with each hydrofoil to maintain a predetermined angle of attack relative to the apparent direction of water flow in the vicinity of the respective hydrofoil during at least part of the rotation of said hydrofoil about the central axis, so as to thereby regulate hydrodynamic lift generated by each hydrofoil as it rotates with the waterwheel. 11 A method according to claim 10, including the steps of: determining said apparent direction of water flow relative to each respective hydrofoil as it rotates around the central axis, generating a local signal indicative of said apparent direction of water flow, and directing said local signal to actuator means so as to control the angle of a trailing flap on the respective hydrofoil so as to maintain said predetermined angle of attack of the hydrofoil relative to said apparent direction of water flow as the hydrofoil rotates about the central axis. 13 A method according to claim 11 or 12, including the steps of: centrally generating an offset signal indicative of said predetermined angle of attack for each respective hydrofoil as said hydrofoil rotates around said axis, transmitting said offset signal to the servocontrol system of each respective hydrofoil so as to maintain said predetermined angle of attack of said hydrofoil as it rotates around said axis with the waterwheel. 14 A method according to claim 13, including the steps of: centrally generating an offset signal indicative of said predetermined angle of attack for each respective hydrofoil as said hydrofoil rotates around said axis, transmitting said offset signal to the servocontrol system of each respective hydrofoil so as to augment or override said local signal and to thereby maintain or vary said predetermined angle of attack of said hydrofoil as it rotates around said axis with the waterwheel. 15 A method according to any preceding claim, including the steps of: locking each hydrofoil against rotation about its pivot axis during at least part of a segment of its rotation about the central axis of the waterwheel, the segment being that in which the hydrofoil moves in substantially the same direction as the prevailing water current. 16 A method according to claim 15, wherein: each hydrofoil that is locked during its passage through said segment is oriented in a manner to substantially maximise its hydrodynamic drag. 17 A method according to claim 15 or 16, wherein: each hydrofoil is locked against rotation about its pivot axis by engagement of detent means operable between the hydrofoil and the rim of the waterwheel, and each hydrofoil is released by disengagement of said detent means under the control of central control means to release the hydrofoil for rotation about its pivot axis. |
This invention is concerned with the recovery of useful power from water currents of modest velocity. It is particularly - though not exclusively - suited to the construction, use and control of submerged waterwheels for generating power from ocean currents.
BACKGROUND TO THE INVENTION
Though ocean currents may be steady in direction and flow rate over relatively long periods of time, they generally do not have a speed of more than a few knots. They are thus unsuited to submerged axial flow turbines of practical size. However, waterwheels, which are well suited for generating power from slow moving surface water currents, are fundamentally problematic if submerged to take advantage of oceanic currents. A conventional undershot water wheel has radially extending vanes spaced around its periphery and is mounted vertically above the water level of a flowing stream so that its axis is horizontal and orthogonal to the water flow and so that only the lowermost vanes dip below the surface of the water. The push of the water current on the lowermost vanes effects rotation of the wheel, which is not impeded by the vanes that are out of the water even though they may be moving in the opposite direction to the water flow. However, when such a water wheel is fully immersed or submerged in a deep river or in an ocean current it may not even turn because the current presses equally on vanes on opposite sides of the wheel, whatever the orientation of the axis of rotation of the waterwheel.. Various ways of addressing this problem have been proposed in the prior art. As illustrated by US patents 4, 174,923 (Williamson, 1979), 4,203,702, (Williamson 1980) and 4,960,363 (Bergstein, 1990) a submerged wheel may be mounted in a fixed inlet structure that guides river or tidal current toward one side of the wheel and shields the opposite side so that it is not exposed to the current. Even so, the vanes on the upstream side will have a churning action in the water which surrounds them that will cause substantial drag. To address this problem, US patent 5,009,568 (Bell, 1991 ) which is directed to wave-driven radial-flow turbines, discloses an elaborate fixed structure that serves as a 'wave splitter' by directing incoming water in one direction onto one side of the turbine rotor and in the opposite direction on the other side. On the other hand, US patent 4,335,319 (Mettersheimer, 1982) discloses a submerged turbine fitted with a moveable inlet structure that can be moved to always face a current of water that varies in direction.
The disadvantage of large and costly inlet structures and the churning losses associated with shielded submerged waterwheels or turbine rotors has been addressed in the art by the use of feathering vanes on the side of the wheel that moves upstream. For example, US patents 1,017,409 (Granger, 1912) and 4,134,710 (Atherton, 1979) disclose the use of vanes that are pivotally attached to the wheel so that, as they move downstream (ie., with the water current) they swing against stops that hold them radial to drive the wheel but, as they move across the current and upstream (against the current), they are free to feather and align with the current so as to minimize drag. As the impact of the vanes striking their stops when they flip from feathered to driving position can be damaging, the vanes may be subdivided axially (as disclosed in US patent 4,134,71) or radially (as disclosed by US patent 5,266,006 in respect of windmills) into a plurality of smaller sub-vanes, each being pivoted to the wheel structure and each having its own stop. However, such submerged
waterwheels tend to be inefficient because (as in the conventional undershot wheel) only the vanes moving downstream help drive the wheel and the rest impede the wheel's rotation by churning or drag. Also, the structure of the wheel needed to position and support the vanes contributes substantial drag as it turns.
Various proposals have been advanced to enable more of the vanes of a submerged waterwheel to contribute thrust rather than drag without the need for a large inlet structure to guide the current appropriately around the wheel. An example is provided by US patent 3,978,345 (Bailey, 1976) wherein it is proposed to stretch long flexible hydrofoil strips across a river between submerged wheels mounted on floating barges that are tethered across the river. The foil strips pass through the wheels and impart a rotary motion to the wheels. Their angle of attack [AOA] is controlled using trailing vanes in a manner disclosed by Bailey in US patent 3,407,770 relating to hydrofoil-supported watercraft. Another approach is to employ a 'linear wheel', such as that disclosed by US patent 4,163,905 (Davison, 1979) where the vanes are mounted between a pair of elongate chain loops that circulate around spaced end sprockets. The assembly is arranged across the water current and a mechanism is provided to change the angle of the vanes as they move from one direction or run to the other. US patent 4,163,905 also discloses the use of an auxiliary flap attached to each vane that is operable under remote control to effect fine adjustment of the angle of the vane as it moves along one run. Again, however, the structures needed to support the chain loops will be costly and likely to significantly impede or disturb the water flow over the vanes.
It should be noted that the cyclic adjustment of vane angle for the purpose of optimising power generation or transmission is functionally the opposite of the automatic feathering of vanes in a wind or water driven wheel for the purpose of shedding load during unusually high wind or water currents. Examples of the latter, which are superficially similar to the former in construction, are disclosed in US patents 1,582,361 (Welsch, 1926) and
4,052,134 (Rumsey, 1977). The waterwheel systems of the present invention normally cyclically adjust vane angle to optimise power generation but can also do so to either prevent over-speeding or to bring the waterwheel to a stop for servicing or inspection.
Finally, another known approach to the submerged waterwheel problem is to employ a vertical-axis rotor that can be located above the surface of the water and supports dagger- like blades, which extend into the water and have their pitch or AOA changed in a cyclic manner as they rotate with the rotor. Examples are disclosed in US patent 4,368,392 (Drees, 1983) and US patent application 2003/0231951 (Kaare, 2003), both of which disclose the use of computer-controlled systems for effecting the cyclic pitch change. An inverted blade wheel of much larger size has been proposed as a wind power generator, again with computer-controlled systems to effect cyclic pitch control of its many upwardly pointing blades. [See, US patent 4,546,264 (Detentson, 1985)] In earlier times the use of blade wheels for the propulsion of surface craft was proposed wherein the pitch angle was cyclically adjusted in a purely mechanical manner. Examples are US patents 2,015,514 (Ehrhart, 1935) and 2,250,772 (Mueller et al, 1941).
OBJECTIVES OF THE INVENTION
The objective of the present invention is to provide a power generation system having improved submersible waterwheel means suitable for sub-sea and other use that will be free of one or more of the disadvantages of the prior art.
OUTLINE OF INVENTION
From one aspect, the invention comprises a power generation system that has a
submergible waterwheel with a plurality of hydrofoils spaced around its rim so as to project laterally therefrom in cantilever fashion like the wing or wings of an aircraft. Each hydrofoil is pivotable relative to the rim about a pivot axis that is substantially parallel with the central axis of the waterwheel so that its AOA is thereby adjustable. The system includes control means to pivot each hydrofoil about its pivot axis as the waterwheel rotates so as to vary its AOA relative to the apparent direction of the water flow in its vicinity and thereby regulate the torque that it contributes to the driving the waterwheel.
The control means preferably includes an on-board servocontrol including sensor means for sensing and signalling the AOA of the foil to actuator means that moves a control flap attached to the trailing edge of the foil to adjust the AOA of the foil toward a desired or predetermined setting. The sensor means may include a sensor flap mounted at or near the leading edge of the foil, the flap being free to rotate about an axis that is parallel with the foil's pivot axis so that it aligns with the apparent current immediately ahead of the hydrofoil. The sensor flap preferably operates a rotary position transducer that generates a local control signal indicative of the instant AOA relative to the apparent current direction.
This signal can be compared with a reference or offset signal indicative of the desired AOA and the difference then fed to the actuator means to adjust the AOA accordingly. Thus, the use of such an on-board servocontrol for each hydrofoil assembly allows the AOA of every hydrofoil to be simultaneously and dynamically controlled as the waterwheel rotates about its central axis and as each foil simultaneously rotates about its respective pivot axis.
It will be appreciated that the apparent direction of water flow imdetentging on the leading edge of a hydrofoil is the vector sum of the prevailing ocean current and the rotational motion of the hydrofoil, both around the axis of the waterwheel and around its own pivot axis as waterwheel turns. For convenience this apparent direction of water flow will be referred to as the 'apparent current' or AC. [It is a well known concept that can explain how the surface speed of a sailing craft can significantly exceed the speed of the prevailing wind.] Therefore, the AOA of concern is the angle which the chord line of a hydrofoil makes with the direction of the apparent current, not the direction of the prevailing ocean current if the waterwheel is turning. The chord line is an imaginary line or plane passing through the centres of the leading and trailing edges of a hydrofoil. In the present case, the the pivot axis of a hydrofoil will usually intersect the chord line of that hydrofoil.
The control means may also include a central controller that is connected to the servocontrol of each foil, the central controller being adapted to generate the reference or offset signal with which the local signal from the sensor means is compared. The central controller thus, from time to time, establishes the desired or predetermined AOA of a foil to be maintained by the onboard servocontrol during waterwheel rotation. In this way the central controller is able to regulate the power output of the power generation system, including bringing the waterwheel to a stop.
The central controller may compute the instantaneous desired AOA of each hydrofoil assembly from a variety of inputs, including a flow sensor that measures the speed of the prevailing current, a rotational speed sensor that measures the rotational speed of the waterwheel and an angle sensor that tracks the changing angle of a reference point on the rim as it moves around the central axis, along with the desired power output of the power generation system. From these inputs the controller can continuously or periodically compute the direction of the apparent current faced by any foil in any position and effect the adjustment of its AOA via the onboard servocontrol actuator.
In addition to the onboard servocontrols and the central controller, the control system preferably includes locking means for locking a hydrofoil at a fixed angle about its pivot , axis during a predetermined segment of its rotation about the central wheel axis. Normally, the predetermined segment will be that in which the hydrofoils are hydrodynamically stalled while moving downstream with the current, the purpose of the locking being to maximise the drag of the hydrofoil which, in that segment, contributes to the driving torque of the wheel. However, the predetermined segment may be much larger - even a full 360 degrees - in the event that the rotation of the wheel is to be stopped quickly. Where maximisation of the power generated is desired, the locking means can be effected by the servocontrol system and/or the central controller, the latter being normal because it will be through the controller that the slowing down or stopdetentg of the . wheel is effected.
However, the locking means may comprise a supplementary control system wherein the entry and departure of a hydrofoil to and from the segment effects the locking and unlocking respectively.
In one embodiment, the locking means may simply comprise a locking detent that engages the boom of a hydrofoil assembly and stops it from turning relative to the rim, the operation of the detent being controlled by the central controller.
The hydrofoils are preferably arranged in hydrofoil assemblies in which a pair of hydrofoils is fixedly mounted on a common boom that is, in turn, rotatably mounted in a bearing in the rim so that it is coaxial with the pivot axis of each hydrofoil, one hydrofoil is on each side of the rim and both hydrofoils rotate with the boom. Thus both hydrofoils . of such a hydrofoil assembly will have the same AOA at all times. However, it is also envisaged that the individual hydrofoils of an assembly may be mounted on respective booms secured to the rim of the waterwheel and projecting laterally and coaxially therefrom. Each boom of such an assembly may be mounted so that it is rotatable in the rim and a hydrofoil may be secured to the boom to rotate with it about the pivot axis, or each boom may be fixed to the rim and the hydrofoil may be rotatably mounted on the respective boom. Each hydrofoil of an assembly may conveniently incorporate a longitudinal tubular spar that slips over its boom and is either fixed thereto or rotates thereon, both the spar and the boom being coaxial with the pivot axis of that hydrofoil. Finally, a hydrofoil assembly may include its on-board servocontrol system and, optionally, its locking means. From another aspect, the invention comprises a method of generating power from a submerged waterwheel of the general type disclosed above, the method including the step of employing an on-board servocontrol system carried by each hydrofoil or hydrofoil assembly to control its AOA as it simultaneously rotates around the central axis and its pivot axis.
The method may also include: employing the servocontrol system to generate a local input signal indicative of the instantaneous AOA of a hydrofoil and directing the local signal, together with a reference or offset signal indicative of a target or desired AOA to an actuator to adjust a trailing flap on the respective hydrofoil so as to thereby adjust the AOA toward said target AOA during at least a part of each rotation of the waterwheel.
Preferably, the method also includes centrally generating the reference or offset signal for each hydrofoil and transmitting it to the servocontrol system of the respective hydrofoil. Generation of the offset signal may take account of inputs derived from water current sensors, the speed of rotation of the waterwheel and/or the rotational position of the respective hydrofoil about the central axis and the like.
The method may also include the step of locking each hydrofoil or hydrofoil assembly against rotation about its pivot axis relative to the rim of the waterwheel during a segment of its rotation about the central axis; preferably, the segment during which the hydrofoil moves in substantially the same direction as the prevailing water current. The step of releasing each hydrofoil for normal pivotal movement about its pivot axis upon exiting the aforementioned segment is, of course, also envisaged. The locking may be effected by employing a detent that is automatically engaged at a predetermined angle and the unlocking may be effected by withdrawal of the detent by an actuator upon a signal from the central controller, the onboard servocontrol and/or by a dedicated locking/unlocking control. DESCRIPTION OF EXAMPLES
Having broadly portrayed the nature of the submerged waterwheel power-generation system of the present invention, one particular example will now be described in some detail with reference to the accompanying drawings, in which:
Figure 1 is a plan view of the oceanic power generation system of the chosen example.
Figure 1 A is an enlarged version of Figure 1.
Figure 2 is a front elevation of the system of Figure 1.
Figure 3 is a sectional side elevation of the system of Figure 1 taken on section plane
A-A of Figure 1 showing the waterwheel lowered in the operating position.
Figure 3A is an enlarged version of Figure 3.
Figure 4 is a side elevation of the system of Figure 1 showing the waterwheel assembly raised in the service position.
Figure 5 is a perspective view of the tip portion of a hydrofoil showing portion of the
' on-board servocontrol system.
Figure 6 is an enlarged detail of part of Figure 5.
Figure 7 is a similar view to that of Figure 5 but shows the hydrofoil of Figure 5 at a +15° angle of attack.
Figure 8 is a perspective view of the hydrofoil of Figure 5 in which portion of skin of the hydrofoil has been removed to show some internal detail.
Figure 9 is a plan view of the hydrofoil assembly of Figure 5.
Figure 10 is a sectional end elevation of the hydrofoil of Figure 9 taken on section line B - B of Figure 9.
Figure 11 shows, in views (a) - (h), the manner in which the waterwheel of the chosen example is fabricated:
View (a) is a side elevation of the assembled waterwheel,
View (b) is a front elevation of the waterwheel of view (a),
View (c) is a side elevation of one of the spokes of the waterwheel,
View (d) is a front elevation of the spoke of view (c),
View (e) is a front elevation of a boom that supports a pair of hydrofoils,
View (f) is a front elevation of a pair of hydrofoils in relation to the boom of view
(e),
View (g) is a side elevation of a rim-tie of the waterwheel,
View (h) is a plan view of the rim-tie of view (g),
Figure 12 is a side elevation of a bearing housing for a hydrofoil boom,
Figure 13 is a sectional front elevation of the bearing housing of Figure 12 taken on section line C - C of Figure 12, and
Figure 14 is a block diagram depicting the control system of the power generation system of the chosen example.
With general reference to Figures 1, 2, 3, 3 A, and 4 the oceanic power generation system 10 of the chosen example basically comprises a platform 12 supported by legs 13 that rest on or are anchored to the sea bed (not shown) and a waterwheel 14 supported by a pair of pivoting girder-like arms 16 that can be raised and lowered by a pair of winches 17 which extend laterally from a powerhouse 18 built on platform 12. System 10 is located off-shore in a steady ocean current indicated by arrows OC, waterwheel 14 being on the downstream side of platform 12. The proximal ends of arms 16 are supported by a pair of combined bearing and gearbox structures 20 one of which depends from each side of powerhouse 18, the distal ends of arms 16 having similar combined bearing and gearboxes 22 in which the ends of the axle 24 of waterwheel 14 are mounted. In this example, waterwheel 14 is vertical so that its central axis of rotation CA is horizontal.
As will be more clearly seen from the enlargements of Figures 1 A and 3 A a drive shaft 26 extends along the length of each arm 16 to mechanically connect the respective pair of bearing/gearboxes 20 and 22. Proximal bearing-gearboxes 20 connect drive shafts 26 to horizontal shafts (not shown) supported in structures 20 and connected through further gearing (not shown) to generators (not shown) in powerhouse 18. In this way axle 24 turns with waterwheel 14 about central axis CA to drive a power generator within powerhouse 18.
Figures 1, 2, 1 A, 3 and 3 A show waterwheel 14 lowered by arms 1 to its submerged operating position, while Figure 4 shows it raised by arms 16 above water-level WL in position for servicing from platform 12. It will be seen that, in this example, waterwheel 14 is a narrow fabricated structure having a rim 30 supported by twelve spokes 32 that are bifurcated at their inner ends to form braces 32a which are in turn secured by gusset plates 78 to large flanges 80 centrally located on axle 24 [see Figure 1 1 View 9b)]. The outer end of each spoke 32 carries a bearing housing 34 within which a horizontal boom 36 [see
Figure 11 View (e)] is rotatably mounted so as to extend outward from each side of rim 30 in cantilever fashion. The periphery of rim 30 of waterwheel 14 is formed by twelve rim ties 40 that link the outer ends of spokes 32 together. In this specification, the term 'rim' is used to designate the peripheral part of waterwheel 18 comprising both rim ties 40 and bearing housings 34. The structure and construction of waterwheel 14 is described in more detail below with reference to Figure 1 1 and its various views.
In this example, each boom 36 supports two hydrofoils 38, one on each side of rim 30 so that they have the appearance of a pair of wings as they rotate (with boom 36) about a common pivot axis PA (Figures 9 & 13). It is therefore convenient to collectively refer to a boom 36, its pair of hydrofoils 38 and associated on-board mechanisms (to be described) as a hydrofoil assembly 38A. Figure 3A show waterwheel 14 lowered to be driven by the prevailing ocean current OC, the waterwheel rotating anticlockwise as indicated by arrow R The operation of hydrofoil assemblies 38A may be described with reference to four sectors (or quarters) of a non-rotating analogue clock-face superimposed on waterwheel 14.
Thus, hydrofoil assemblies 38A in the IV, III and II o'clock positions have a positive AOA and therefore generate positive lifts which contribute significant torque to wheel 14; those in the I, XII and XI o'clock positions are essentially feathered and contribute little torque or and contribute little torque or drag on the wheel as they are more or less moving directly against current OC; those in the X, IX and VIII o'clock positions are 'flying' with a negative AOA and, thus, are being pressed downwards so they too contribute useful torque to rotation of wheel 14; and, finally, those in the VII, VI and V o'clock positions have such a large negative AOA that they are hydrodyamically stalled and create a large drag that usefully contributes torque to the rotation of wheel 1 by acting like the radial vanes of a conventional undershot wheel. The latter group of hydrofoil assemblies 38A are locked in position to prevent them from turning around to face the current and feather. The manner in which such locking is achieved will be described below. However, it may be noted that presence of such stalled hydrofoils indicates that they are travelling downstream at substantially less than the speed of ocean current OC, which is to be expected if waterwheel 14 is driving a substantial power generation load.
For the purpose of the above general description it could be assumed that the AOA of each hydrofoil 38 was the angle between its chord line (see below) and the horizontal direction of ocean current OC. However, the relevant AOA should be referenced to the direction of the apparent water current AC immediately ahead of the hydrofoil 38 (see Figures 5 and 7); that is the current relative to the leading edge of the hydrofoil. This is the vector sum of the direction and speed of the ocean current OC and the direction and speed of movement of the hydrofoil around both the wheel axis CA and its pivot axis PA. When the waterwheel is turning fast under light loads there will be a significant difference in direction between the apparent current and the ocean current, and it is important that the AOA of hydrofoils 38 be adjusted with respect to the former, not the latter. Referring now to Figures 5 - 10, it will be seen that hydrofoils 38 are constructed in a similar manner to aeroplane wings because they are required to generate hydrodynamic lift in a similar manner. Accordingly, hydrofoils 38 have smooth streamlined skins 50 with blunt and rounded leading edges 52 and finely tapered trailing edges 54, along with upstanding flow-control fences 56 around their surfaces at intervals there-along to inhibit lateral water flow toward their tips 58.
Each hydrofoil assembly 38A of this example includes an on-board servocontrol SC (See Figure 14) that adjusts the AOA of the respective hydrofoils 38 as they travel around central axis CA of waterwheel 14. Servocontrol SC includes a sensor vane 60 mounted on the tip 58 of each foil 38 near its leading edge 52 so that it is free to rotate about a sensor axis SA (Figures 5 and 7) which is parallel with central axis CA and pivot axis PA. (In this case, all are horizontal). Sensor axis SA is located near the leading edge 52 of vane 60 so the vane tends to align itself with the direction of the apparent current AC immediately ahead of foil 38. In this example, vane 60 is mounted on a rotary position transducer 62 that generates a local output signal indicative of the angle which vane 60 makes with the chord line CL; that is, it is indicative of the instantaneous AOA of foil 38 relative to the apparent current AC. This output is fed (via line 100, see also Figure 14) to a plurality of actuators 63 located within hydrofoil 38, each actuator 63 being mechanically coupled via a control rod 64 and a bell-crank 66 to control the angular position of a respective elongate trailing flap 68 hinged to the trailing edge 54 of hydrofoil 38. The number of actuators and flaps employed depends upon on the span of hydrofoil 38 and the forces thereon. Thus, sensor vane 60, transducer 62, actuators 63, their linkages 64 and 66 and associated trailing flaps 68 comprise servocontrol system SC which continuously aligns trailing flap 68 relative to the sensed AOA, subject to an offset signal angle that is output on line 102 from a central controller CC (to be described) and that is indicative of a desired AOA determined by controller CC. As depicted in Figure 5, the apparent current AC, sensor vane 60 and chord line CL of hydrofoil 38 are all aligned so that the AOA of hydrofoil is zero. That is, the hydrofoil of Figure 5 is feathered and does not generate lift or contribute to the driving torque to waterwheel 14. This might correspond to the configuration of the hydrofoil at the XII o'clock position in Figure 3A where it can be assumed that the direction of apparent current AC approximates that of the ocean current OC. In that case, the offset signal on_ line 102 would normally be zero, so the onboard servocontrol SC adjusts trailing flaps 68 to maintain the zero AOA as far as possible. In the configuration of Figure 6, however, it can be assumed that the offset signal on line 102 from central controller CC is indicative of an AOA of 15°, so servocontrol SC operates to set control flaps 68 so that the output of transducer 60 corresponds to an AOA of +15°, sensor vane 60 again being aligned with apparent current AC. The configuration of Figure 7 might correspond to that of the hydrofoil 38 at the ΙΠ o'clock position in Figure 3A where, say, an AOA of 15° ensures maximum lift; that is, a maximum contribution of torque. However, at this position the direction of the apparent current AC for that hydrofoil is substantially different to that of the ocean current OC.
Figures 8 - 10 further illustrate the construction of the hydrofoils 38. As is conventional, the foil has a plurality of vertical bulkhead-like ribs 70 spaced longitudinally along a central tubular spar 72 (shown in broken lines) which provides a convenient way of mounting hydrofoil 38 on its shaft-like supporting boom 36, to which it is fixed by cross- pins 74. As already noted, the whole hydrofoil assembly 38A, comprising boom 36, spar 72 the pair of hydrofoils 38 and the onboard control system SC, are free to rotate as a unit about pivot axis PA because boom 36 is rotatably housed in a bearing hub 34 on the distal end of the respective spoke 32 of waterwheel 14. Actuators 63 are conveniently mounted on selected ribs 70 as shown.
Figure 1 1 shows, in its various views, the way in which the rotor of waterwheel 14 can be constructed. Referring to View (a), wheel 14 is built up from twelve spokes 32 radiating from central shaft 24. Each spoke carries a bearing hub 34 within which a respective boom 36 [View (e)] can be rotatably housed. Rim 30 of waterwheel 18 is formed by rim ties 40 that join the outer extremities of spokes 32 together and by bearing hubs 34 on the outer ends of spokes 32. Braces 76 may also be fitted diagonally between spokes 32 for additional strength [View (a)]. As best seen in views (b), (c) and (d), each spoke 32 is bifurcated at its inner end to form two bracing prongs 32a, each of which is fitted with a gusset plate 78 by which it- is bolted to one of two large disc-like flanges 80 formed in the centre of wheel shaft 24. A journal 81 is formed on each end of shaft 24 for support by combined bearing and gearbox 22 and a bevel gear 82 is fitted on each end of shaft 24 inward of journal 81 to mesh with a similar gear (not shown) on the end of drive shaft 26 that is also mounted in a bearing within gearbox 22. It will be appreciated that shaft 24 needs to be more than twice as long as each hydrofoil 38 [View (f)] so waterwheel 14 can turn between arms 16 (Figl A). Hence, it may be preferable to construct shaft 24 as a large diameter hollow tube large enough to accommodate electrical generators and provide a passage for maintenance personnel.
The way in which boom 36 is supported by housing 34 to enable the boom to be locked against rotation will now be described with reference to Figures 12 and 13. Housing 34 comprises a hollow body 84 that forms the head of spoke 32, a pair of bearing caps 85 secured to body 84 by bolts 86 so as to support and enclose a pair of roller bearings 87 which rotatably receive boom 36. Boom 36 has a central boss 88 of enlarged diameter against which the inner races (not shown) of the bearings are clamped by means of lock nuts 89, the outer races (not shown) being clamped in place by caps 85 against body 84. A solenoid 90 having an armature-driven spring-loaded detent 91 extends into body 84 so that detent 91 rides on the surface of boss 88, solenoid 90 being secured in place by an end- plate 92 and bolts 93. A hole 94 is formed part way through boss 88 that forms a socket adapted to receive detent 91 so that, as boom 36 turns about pivot axis PA as housing 34 rotates with waterwheel 14, spring-loaded detent 91 will snap into hole 94 as soon as hole 94 becomes aligned with detent 91. This locks boom 36 against rotation about its pivot axis PA. When solenoid 90 is energised, detent 91 is withdrawn from hole 94 and boom 36 is again free to rotate about pivot axis PA.
Solenoid 90 is electrically connected via wires within its hydrofoil and via slip rings carried by boom 36 and coacting brushes within housing 34. These elements are not illustrated as they are common knowledge.
Finally, the control system CS for waterwheel 14 of oceanic power generation system 10 will now be described with reference to the diagram of Figure 14. Control system CS comprises both a central controller CC and the on-board servocontrol SC of each hydrofoil assembly 38A. Central controller CC is located within power station 18 (Figures 1 - 3) and is conveniently an electronic computer capable of receiving and monitoring a variety of inputs, processing these inputs and generating command and/or alarm signals. Controllers of this general type and programs for their operation are well known in the art. As noted above, the output of rotary transducer 62 of onboard servocontrol system SC is connected via line 100 to all actuators 63 in the respective hydrofoil and an overriding or offset signal is fed to signal line 100 via an output line 102 from central controller CC. Power is fed to actuators 63 and, if needed, to transducer 62 on power line 103. Signal and power lines 100, 102 and 103 can be arranged anywhere convenient inside hydrofoil 38, with signal line 102 and power line 103 being connected via sliprings in spar hub 34 (not shown in Figures 12 and 13). Controller CC has a variety of inputs: input 104 is the measured speed of the ocean current OC derived from a flow transducer 105 located at a suitable position in the current; inputs 106, 107 and 108 are derived from waterwheel 14 and comprise the rotational speed (106) and instantaneous angle (107) of waterwheel 14 as well as various fault alarms and monitoring inputs (108); input 110 is the power demand signalled by a remote command centre (not shown) on the electricity supply grid (not shown); input 112 is a measure of the actual power output from the submerged waterwheel generation system 10. In addition to these input signals there will be a variety of system monitoring and alarm signal inputs (not shown), as well as manual inputs which may include a manual emergency shut-down signal 1 14.
As described above, when a hydrofoil assembly reaches clock position VIII its negative AOA is so large that it is close to stalling, so it is at this point that boom 36 is locked to bearing hub 34 by the engagement of spring-loaded locking detent 91 in socket 94 formed in the central boss 88 of boom 36 within bearing hub 34. This locks the respective hydrofoil assembly 38A at its current angle relative to its pivot axis PA (and relative to its spoke 32) for the next three clock positions (VII, VI and V), after which detent 91 is withdrawn by energization of driving solenoid 90 to permit the hydrofoil to once again
'fly' - ie, generate lift by rotating about its pivot axis under the control of its servocontrol system SC. As illustrated by Figure 14, solenoid 90 is actuated via line 109 from central controller CC. Since locking detent 91 is spring-loaded to bear on boom 36, there is no need to actuate solenoid 90 to engage detent 91 with socket 94.
An important function of the computer-based central controller CC is to compute the appropriate offset and locking commands to be sent to each hydrofoil assembly 38A as it moves around central axis CA of wheel 18 through clock position after clock position. Assume, as shown in Figure 14, that the stationary clock positions are as indentified by Roman numerals around rotating wheel 1-8 and that each hydrofoil system 38A is identified by consecutive letters a to 1. Then, as wheel 18 rotates, central controller CC can readily identify the 'clock angle' of each hydrofoil assembly 38A and signal that hydrofoil assembly with an offset (and/or locking) signal corresponding to the appropriate AOA setting (or lock status) required of the respective hydrofoil assembly 38A.
The appropriate AOA for any given hydrofoil assembly 38A at any angular position may be computed by central controller CC taking into account the rotational speed of wheel 18 on input line 106, the speed of ocean current OC on line 104 and the difference between the desired and actual power outputs signalled on inputs 110 and 1 12 respectively. If the objective is to simply maintain maximum power output for the present measured speed of the ocean current OC, it will be necessary to employ central controller CC optimise wheel speed and AOA variation (and locked interval) as a particular hydrofoil assembly turns around central axis CA through 360°. To do this controller CC may compute the speed and direction of the apparent current AC faced by a hydrofoil assembly as it moves around central axis CC and to generate the appropriate offset signals. If less than the maximum power available from a given ocean current is needed, a new optimum variation of AOA and lock positions can be computed and signalled to the hydrofoil assemblies accordingly and more local control can be relegated to the onboard servocontrol SC. Similarly, the AOA and locking conditions appropriate for the braking or idling of the wheel can be computed and signalled to each hydrofoil assembly 38A in turn as it approaches the bottom sector of the waterwheel. As already mentioned, once the desired AOA for a particular clock position has been determined, it is signalled to a selected hydrofoil assembly 38A as an offset angle between the sensor flap 60 and the chord line CL of that assembly. The onboard servocontrol SC then operates to hold the AOA until a further offset command is received. While one example of an oceanic power generation system has been described and illustrated in detail, it will be appreciated that many variations and additions can be made without departing from scope br intent of the invention as defined by the following claims.
For example, in steady state conditions where the ocean current is steady and the power output optimised, there will be little or no variation in the appropriate AOA or lock condition for each angle or segment of wheel rotation. A constant stream of commands from the central controller to each hydrofoil assembly would then not be needed if the servocontrol system of each assembly included (i) a data recorder store the AOA and lock condition for each angular segment of wheel rotation and (ii) an angle sensor to indentify the instantaneous clock angle of the respective assembly.
Many different ways of mounting and raising the waterwheel known in the art can also be employed; for example, lifting and lowering the wheel assembly vertically between two legs of the platform. Two counter-rotating waterwheels can be mounted in spaced relation on the same shaft so that no net torque is transferred to the support structure. The waterwheel may be arranged horizontally on a rotating vertical shaft that depends from the structure and terminates in the generator room. Alternatively, the waterwheel or waterwheels can be mounted on a submerged structure that is built on the sea floor.
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