GB2531596A

GB2531596A - Tidal turbine system

Info

Publication number: GB2531596A
Application number: GB1418910.4A
Authority: GB
Inventors: Ayre Richard; Murphey Martin
Original assignee: Tidal Energy Ltd
Current assignee: Tidal Energy Ltd
Priority date: 2014-10-24
Filing date: 2014-10-24
Publication date: 2016-04-27
Also published as: GB201418910D0

Abstract

A tidal flow turbine system comprises a turbine having blades 123 which have blade tips 124 curved towards the direction of fluid flow. At excessive flow speeds, the coning angle of the blades is increased by actuator 29. The blade tip 124 operates at a reduced coning angle compared to a straight blade, which may reduce drag whilst maintaining power. The blades may be straight for the inner two-thirds of their length 123. The curved portion 124 may have a parabolic e.g. cubic curvature.

Description

TIDAL TURBINE SYSTEM

[0001] The present invention relates to a tidal turbine system, particularly to such a system for energy generation.

Background Art

[0002] Tidal turbine systems for energy efficient tidal flow power generation are proposed and disclosed in for example WO 2010/007342 A (TIDAL ENERGY LIMITED) 21.01.2010 and WO 2011/077128 A (TIDAL ENERGY LIMITED) 30.06.2011. Both these documents disclose utilisation of thrust control at high flow speeds to prevent undue thrust loading on the turbine.

[0003] It has recently been proposed to provide a coning turbine for tidal power generation. A coning turbine is one in which the angle of the turbine blades to the axis of the blades is decreased to prevent the turbine being structurally compromised when exposed to high flow rates. In such a turbine the coning angle is defined as the angle between a line extending from the tip of a turbine blade to its root and a plane orthogonal to the axis of the turbine blade.

Summary of invention

[0004] In the present invention a coning tidal flow turbine for use in a power generation system comprising a plurality of turbine blades distributed around a central axis in which the coning angle of the blades increases as the tide speed exceeds a predetermined maximum characterised in that towards their tips the turbine blades are curved away towards from a straight line extending from the roots of the blades initially along the length of the blades, the curve of each blade directed away from the axis.

[0005] The invention will now be further described by way of example only, and with reference to the accompanying drawings, in which:

Brief description of drawings

[0006] Figures 1 to 9 described a recently designed coning tidal turbine.

[0007] Figure 1 is a schematic representation of a coning tidal flow turbine system; [0008] Figure 2 is a schematic view of a coning tidal turbine having the turbine blades in a maximum contracted configuration (the coning angle is at its maximum); [0009] Figure 3 is a schematic view of the turbine of figure 2, but this time with the turbine blades in a maximum expanded configuration (the coning angle is at its minimum); [0010] Figure 4 is a schematic facing view of the turbine to show possible variation in swept area for the turbine shown in figures 2 and 3; [0011] Figure 5 is a diagrammatic cross section through the actuator control system of the tidal flow turbine system of figure 1 to 4; [0012] Figure 6 shows more detail of the control valve of figure 5 with the control valve in a position with the generator operating between its minimum and maximum power outputs; [0013] Figure 7 is similar to figure 6 but showing the control valve in a different position responsive to a different operating condition; [0014] Figure 8 is similar to figures 6 and 7 but showing the control valve in a different position responsive to a further different operating condition; [0015] Figure 9 shows an alternative arrangement for controlling and adjusting pressure in the actuator of figure 5.

[0016] Figure 10 and 11 illustrate the present invention.

[0017] Figure 10 is a schematic side view of a turbine according to the present invention; and [0018] Figure 11 shows a schematic view of a turbine as described with reference to figures 1 to 9 with blades as shown ion figure 10 comparing the turbine blades in a maximum expanded with the retracted position configuration. Description of embodiments [0019] In figure 1 there is shown a tidal flow energy generation arrangement 1. The tidal flow energy generation arrangement 1 is required to he operated in extreme conditions.

[0020] To be commercially competitive with other forms of power production areas of the seabed of high tidal flow energy concentration need to be utilised. These areas are difficult and dangerous to work in and the structure and its installation and retrieval need to take into account significant environmental hazards. The current flow, for example, is fast, typically upward of 4 Knots. Areas are often in deep water, which may be deeper than those in which a piling rig can operate. Storm conditions can cause costly delays and postponement. Tidal reversal is four times a day in 2 tidal cycles and the time for tidal reversal may be very short (for example between 15 and 90 minutes). Additionally, in such high tidal flow areas, the seabed is often scoured of sediment and other light material revealing an uneven rock seabed, which makes anchorage difficult. In the situations described it may be impossible for divers or remote operated vehicles to operate on the structure when positioned on the seabed. Installation, recovery and service are therefore most conveniently carried out from the surface. To be environmentally acceptable, all parts of the structure and any equipment used in deployment or recovery must be shown to be recoverable.

[0021] The arrangement 1 comprises a freestanding structural frame assembly comprising steel tubes 2 (circa 1.5 m diameter). The frame assembly comprises welded tubular steel corner modules 3. The corner units are interconnected by lengths of the steel tubes 2. The structure as shown in the drawings is triangular in footprint and this may for certain deployment scenarios be preferred however other shape footprints (such as rectangular) are also envisaged in such arrangements the angular configuration of the corner modules 3 will of course be different to that shown and described in relation to the drawings.

[0022] The corner modules 3 comprise first and second angled limbs 7, 8 extending at an angle of 60 degrees to one another. The angled tube limb 7 is welded onto the outer cylindrical wall of limb 8. Angled tube limbs 7 and 8 are fixed to a respective nacelle tower 9. The corner module 3 and interconnecting tubes 2 include respective flanges 4 for bolting to one another. The tube limb 8 of the corner modules include a flap valve comprising a hinged flap closing an aperture in a baffle plate welded internally of the end of tube limb 8. Water can flood into and flow out of the tube limb 8 (and therefore into the tubes 2) via the flap valve. Once flooded and in position on the seabed, the flap valve tends to close the end of the tube limb 8 preventing silting up internally of the tubular structure.

[0023] The corner modules 3 also include a structural steel plate (not shown) welded between the angled tubular limbs 7, 8. A lifting eye structure is welded to the steel plate. An end of a respective chain 14 of a chain lifting bridle arrangement is fixed to the lifting eye. A respective lifting chain 14 is attached at each node module 3, the distal ends meeting at a bridle top link. In use a crane hook engages with the top link for lifting. Self-levelling feet 15 may be provided for each of the corner modules 3. This ensures a level positioning of the structure on uneven scoured seabed and transfer of vertical loadings directly to the seabed.

[0024] The structure is held in position by its own mass and lack of buoyancy due to flooding of the tubes 2 and end modules 3. The tubes 2 are positioned in the boundary layer close to the seabed and the structure has a large base area relative to height. This minimises potential overturning moment. Horizontal drag is minimised due to using a single large diameter tubes 2 as the main interconnecting support for the frame.

[0025] The structure forms a mounting base for the turbine generators 19 mounted at each corner module 3, the support shaft 20 of a respective turbine generator 19 being received within the respective mounting tube 3 such that the turbine generators can rotate about the longitudinal axis of the respective support shaft 20. Power is transmitted from the corner mounted turbine generators 19 to onshore by means of appropriate cable as is well known in the marine renewables industry.

[0026] The turbine generators are shown most clearly in figures 2 and 3. The turbines 19 are rotatably mounted self-orientating down current mounted turbines. This means that the turbines 19 self-orientate rotatably (for example on mounting pintle 22) depending upon the tidal flow direction (direction A in the figures) such that the three turbine blades 23 are positioned downstream of the supports 20. The turbine blades 23 rotate about their axis and when rotating define, by their tips, an effective swept area shown by the dashed line circles X, X1 as shown in figures 1 and 4. In accordance with the present invention the turbine system is provided with the facility to have a variable or adjustable swept area X, X1 dependent upon operational considerations. The swept volume is conical by virtue of the blades being of common length and angled backwardly in the direction of tidal flow. The invention may correspondingly be defined in terms of the facility to control the conical swept volume between the blades. The blades define the conical swept volume, which is variable and adjustable. The inventive concept could therefore be described as controlled or tailored variable coning' of the turbine blades 23. The blades are of conventional section as used in underwater generator applications, twisting from their roots to tips (25 and 24 respectively in figures 2 and 3).

[0027] The turbine blades 23 are mounted at pivot mountings 26 to a boss 28. An actuator arrangement comprising a ram rod 29 is provided to drive the blades from a collapsed or contracted configuration as shown in figure 2 to an expanded configuration as shown in figure 3. Rods 30 have pivotal mountings 31 at one end to the turbine blades and at the other end pivotal mountings 32 near the distal end of the ram rod 29. Movement of the ram rod 29 is actuated by a hydraulic ram 27 (seen in figures 5) operated by a control system (described in below in relation to figures 4 and 5) contained in a nacelle 33 such that the turbine blades 23 each take up a common angle with respect to the horizontal axis Z. When the distal end ram rod 29 moves further away from boss 28, the ram rod 29 pulls rods 30 closer to the axis Z and thus the cone angle formed by the blades is decreased. When the ram rod 29 retreats the rods 30 are splayed outwards and the cone angle formed by the turbine blades 23 is increased. This arrangement provides that the turbine blades of a specific turbine are caused to move in unison, to the same angular degree. This results in them adopting a common angular attitude at any given position.

[0028] Figure 3 shows the turbine 19 in an operational position in which the turbine blades 23 are expanded out to a maximum swept area configuration. This corresponds to the dashed line X in figure 4. In Figure 2 the blades are shown completely collapsed or contracted. In operational mode the most contracted configuration may be expanded somewhat from the configuration to give an effective swept area at a minimum of X1. The effective swept area may therefore be varied in operation over a range between X1 and X. [0029] In figures 2 and 3 the angle a is the mathematical cone angle formed at the tip of the cone swept by the rotating blades 23. In figure 2 with the blades retracted this angle is close to zero. The angle 13 is the coning angle, formed between a line extending from the tips 24 of the blades 23 to their roots 25 at their pivot mountings 26 and a plane orthogonal to the axis of the turbine blades. When the blades are retracted this coning angle is close to 90°, when the blades are fully extended it is about 7°.

[0030] The control system, described in more detail in relation to figures 4 and 5, varies the 'in operation' configuration of the blades 23 between the maximum expanded and maximum contracted configuration, dependent upon the tidal flow rate and/or power output requirement of the turbine. Typically, a more expanded blade configuration is adopted to accommodate relatively lower tidal flow speeds and a relatively more contracted configuration is adopted at relatively higher tidal flow speeds. A more expanded blade configuration provides higher power output for a given tidal flow speed and a relatively more contracted configuration provides lower power output for a given tidal flow speed. Importantly, a relatively more contracted configuration protects against damage to the turbine at times of high tidal flow speed.

[0031] In the embodiment described, the turbine generator is mounted on a support post or tower provided on a support structure which is gravity mounted on the sea bed. It should be appreciated that the turbine generator could be any form of horizontal axis turbine generator however mounted, for example on a gravity based structure, a piled tower or post, or on a floating device.

[0032] Figure 5 shows the actuator arrangements in more detail. Turbine blades 23, their pivot mountings 26 to boss 28, rods 30 and their pivot mountings 31 and 32, the latter to ram rod 29 are shown and are as previously described, as is nacelle 33 and pintle 22.

[0033] The ram rod 29 is mounted in a hydraulic ram 27 which is surrounded by a spring 34 bearing against a thrust plate 35 rigidly joined to the ram rod 29 at one end and to the rear 37 of boss 28 at its other end.

[0034] The proximal end of ram rod 29 is surrounded by a hydraulic slip ring 38 which can supply hydraulic fluid to a hydraulic cavity 40 inset into the ram rod 29 between the proximal end for the ram rod and a thickened body portion of the ram 27. It can be seen that if there is no hydraulic pressure in the hydraulic cavity 40, the spring 34 will bear on thrust plate 35, tending to push the ram rod 29 out of the hydraulic ram 27, and increasing the coning angle 13 of the turbine blades 23 in turn decreasing the swept area of the turbine blades. If hydraulic fluid is pumped into cavity 40 to bear against the distal end of the ram rod, it will push the ram rod to the right (as seen in figure 5) compressing spring 34, decreasing the coning angle (3 of the turbine blades 23.

[0035] High pressure hydraulic supply to the slip ring 38 is from a high pressure feed 42 through an automatic control valve 43 to a ram feed 44. Connected to boss 28 is a gear box 56 whose output shaft 57 drives the hydraulic pump 52. Hydraulic fluid is pumped from the pump though generator feed 51 to a generator 53 which is mounted in the base of the structure. Low pressure hydraulic fluid returns to the pump 52 from the generator through return duct 58.

[0036] In normal operation, when the turbine blades are turning and the generator is operating within its capacity, control valve 43 allows connection between high pressure hydraulic feed 42 and ram feed 44. With this connection in place, spring 34 will be compressed as the ram rod 29 moves to the right (as seen in figure 5) and the turbine blades will extend outwards to sweep area X in figures 1 and 4.

[0037] As the rotation of the turbine blades increases, the generator will reach its upper capacity, at this point the control valve 43 will tend to close off the high pressure hydraulic feed from the ram feed, and connect the ram feed instead to the hydraulic ram vent 54, reducing the pressure in ram feed 44 allowing the spring 34 to urge the ram rod 29 to the left (as seen in figure 5) and so reducing the cone angle of the turbine blades, so to bring the system into balance again.

[0038] The automatic control valve 43 is seen in greater detail in figures 6 to 8. The valve comprising a twin piston arrangement 46 with two double acting pistons 49 and 50 at each end of a connecting rod 48 in a cylinder 47. A pressure monitor duct 45 connects one end of cylinder 47 to a hydraulic generator feed 51 between hydraulic pump 52 and a generator 53 (the generator and its hydraulic motor are standard and are not shown). The other side of cylinder 47 is connected to a hydraulic ram vent duct 54 connected to the low pressure return duct 58 from generator 53. High pressure feed 42 and ram feed 44 are connected to cylinder 47 between the two piston heads 49 and 50, the ram feed is placed mid-way along the cylinder, with the high pressure feed 42 closer to the pressure monitor duct 45. When the twin piston 46 is at a mid-point in the cylinder 47 there is hydraulic connection between the high pressure hydraulic feed 42 and the ram feed 44. A spring 55 bears against piston head 49, tending to urge it to increase the volume of the cylinder 47 swept by piston head 49 and to decrease that swept by piston head 50.

[0039] When pressure in the monitor duct is normal, the twin piston 46 takes up a position shown in figure 6, with both the high pressure feed 42 and hydraulic ram vent 54 closed by pistons 50 and 49 respectively. In this position the pressure in the ram feed 44 and thus in the ram remains in a constant position as do the positions of the turbine blades. This should be seen as the normal operating position.

[0040] Responding to an increase in turbine speed, as the pressure in the pressure monitor duct 45 increases further with excess pressure in the hydraulic generator feed 51 to generator 53, piston 46 moves against tension in spring 55. The piston head 50 moves across the end of the high pressure hydraulic feed 42, still blocking it, but piston head 49 moves providing a direct hydraulic connection between the ram feed 44 and hydraulic ram vent 54, releasing pressure from the hydraulic ram 27. (This position is shown in figure 7). This is turn will increase the coning angle (3 of the blades 23 reducing the pressure generated by pump 52 eliminating the over-pressure in hydraulic generator feed 51. At which point the twin piston 46 will move back to the position shown in figure 6 maintaining the adjusted pressure in the ram feed and keeping the turbine blades in a constant position, and thus the swept area of the turbine blades is held constant in a new position.

[0041] On the other hand, in tidal conditions with little or no water flow (as at close to high tide or low tide), pressure in hydraulic generator feed 51 will fall, and the generator 53 will stop, (this situation is shown in figure 8). A spring loaded motorised valve 60 in the high pressure hydraulic feed 42 will close supply to the automatic valve 43, and spring 55 will urge twin piston 46 towards the top of cylinder 47, causing a direct link between ram feed 44 and hydraulic ram vent 58 via the motorised valve 60, and for the ram 27 to thus become depressurised. The turbine blade will retract to the position seen in figure 2 and the nacelle 33 will turn freely on pintle 22 about support shaft 20, so as to self-align for the next tidal flow. Once that flow commences, a signal will be sent from land to operate motorised valve 60, resuming supply through to the high pressure hydraulic feed to the automatic control valve 43, moving the twin piston 46 again to the midpoint in cylinder 46, reconnecting the high pressure hydraulic supply 42 to ram supply 44, and repressurising the ram 27. The ram rod 29 again moves to the right (as seen in figure 5) and the coning angle 13 of the blades decreases and the swept area of the turbine blades X-X1 (figure 4) increases again, increasing the pressure in hydraulic generator feed 51, in turn causing the generator 53 to resume power generation.

[0042] The high pressure hydraulic feed 42 is fed from a separate pump (not shown) at constant pressure.

[0043] In the arrangement shown, the automatic control valve 43 is shown in the nacelle. However, if generator 53 is mounted in the base of the structure, the automatic control valve 43 can be mounted with it, in a sealed generator area, with the ram feed leading into the nacelle 33 to slip ring 38. This leads to a relatively lower maintenance arrangement, as the automatic control valve and generator can all be easily reached is the structure is lifted from the water.

[0044] An alternative arrangement is shown on figure 9. Here the automatic control valve 43 is replaced by a spring loaded three position motorised valve 62 connecting directly between high pressure feed 42 and ram feed 44. The generator 53 is driven mechanically directly from the turbine through gearbox 56, rather than using a hydraulic pumping system. The valve 62 is controlled using signals from the generator 53. At rest the spring loaded valve closes the high pressure feed 42 and vents the ram feed to hydraulic ram vent 54. When the generator is working below its optimum power, the generator sends a signal to a relay in valve 62 to open it. This permits high pressure hydraulic fluid to access ram feed 44, causing the ram rod 2 to move to the right as before with the coning angle 13 decreasing and the swept area X-X1 in figure 4 increasing in turn increasing the power generation. Once the generator starts running above its rated power, it sends another signal to the valve 62 once more to close the connection from high pressure feed 42 and ram feed 44, and once more to open the connection between ram feed 44 and hydraulic ram vent 54, increasing the cone angle 13 and reducing again the swept area X-X1 as previously described. The cables taking the output from generator 53 are shown as 64.

[0045] When the generator is running between a minimum rated output and its maximum output, the generator sends a further signal to the valve 62 closing both the high pressure feed 42 and to hydraulic ram vent 54. In this position the pressure in the ram 27 remains constant as does the position of turbine blades 23. As the tidal current in the waterway in which the structure has been placed slows to a point where the generator can no longer generate power, the generator will signal the valve 62 once again to close the high pressure feed 42, and connect once more the ram feed 44 and hydraulic ram vent 54. The turbine blades 23 will return to the positions shown in figure 2, the nacelle 33 will be free to rotate about supports 20 until the next tidal current picks up.

[0046] Spring tide flows carry substantial quantities of kinetic energy, however by their nature they might only occur for 20% of a year. Ideally a turbine should be large enough to capture full power at neap flow rates, and yet be able to cope structurally with damaging energy levels at spring flow and storms.

[0047] At neap flows the tidal turbine might sweep an area 20 meters in diameter. However, as water speed doubles the power entrained increases eight times, increasing the coning angle enables the rotor effectively to diminish in diameter as flows increase. This in turn causes less excess stress to be transmitted to the device at high flow. However the rotor loses power production as well as generating damaging drag induced thrust as the coning angle of the blades progressively increased.

[0048] In a further development of the invention shown in figures 10 and 11 the blades 123 are curved outwards away from a straight line extending along the blades initially from their roots 125 and common axis Z towards their tips 124. This development enables the major part of the turbine blades effectively to have a greater coning angle than would be the case if the blade was flat along its length, this also is found to reduce drag induced thrust.

[0049] As the tips 124 of the curved blades 123 operates at a smaller coning angle than would be the case if the blade was flat along its length as in figures 2 and 3, the curves blades produce 123 both power and lift. In figure 11, shows the nacelle 33 of a tidal turbine 19 with these lift type blades 123 having a curve 126 towards their tips 124.

[0050] Ideally the blades 123 are straight for about two thirds of their length from their root 125 towards their tips 124 and then curve in a parabolic curve (when seen from the side) towards their tips 124. Ideally shape of this parabola follows a cube law where the displacement of the blade from a straight line is a function of the cube of the distance travelled from the start of the curve from the root 125 of the blade.

[0051] Using a curved blade design in combination with a retracting blade arrangement the power and thrust relationship can be maintained as the turbine cones.

Claims

Claims 1. A coning tidal flow turbine for use in a power generation system comprising a plurality of turbine blades distributed around a central axis in which the coning angle of the blades increases as the tide speed exceeds a predetermined maximum characterised in that towards their tips the turbine blades are curved away towards from a straight line extending from the roots of the blades initially along the length of the blades, the curve of each blade directed away from the axis.
2. A coning tidal flow turbine according to claim 1 in which the curve is parabolic.
3. A coning tidal flow power generation system according to claim 2 in which the displacement of the curve from straight line extending from the root of the blade initially along the length of the blade follows a cubic relationship with the distance from the start of the curve.
4. A coning tidal flow turbine according to any one of claims 1 to 3 additionally comprising a hydraulic pump, and a generator, the hydraulic pump to pump hydraulic fluid to drive the generator responsive to rotation of the blades, characterised in that the effective swept area of the turbine blades is adjustable by means of an actuator and a control valve, said control valve controlling hydraulic pressure in the actuator responsive to pressure of the hydraulic fluid pumped by the hydraulic pump.
5. A coning tidal flow turbine according to claim 4 characterised in that the actuator is a hydraulic ram.
6. A coning tidal flow turbine according to claim 4 or 5 wherein the actuator moves turbine blades in unison.
7. A coning tidal flow turbine according to anyone of claims 4 to 6 wherein, in operation, the actuator varies the configuration of the blades between a maximum expanded and maximum contracted configuration, dependent upon the tidal flow rate and/or power output requirement of the turbine.
8. A coning tidal flow turbine according to any one of claims 4 to 7 characterised in that it comprises a hydraulic ram, a high pressure hydraulic feed, a ram feed, and a hydraulic ram vent, the three positions of the valve being in a first position to close the high pressure hydraulic feed, and to join the ram feed and the hydraulic ram vent, in a second position to close the ram vent and to join the ram feed and the high pressure hydraulic feed and, in the third position to close both the hydraulic ram vent and the high pressure hydraulic feed.
9. A coning tidal flow turbine according to claim 8 characterised in that the control valve comprises twin pistons in a cylinder, the position of the twin pistons in the cylinder is responsive to the pressure of the hydraulic fluid.
10. A coning tidal flow turbine according to claim 9 characterised in that the control valve is connected to a high pressure hydraulic feed and to the hydraulic ram feed, and in which the pressure of hydraulic fluid in the hydraulic ram feed is determined by the position of the twin pistons in the cylinder.
11. A coning tidal flow turbine according to claim 10 characterised in that in additionally comprising a valve to close and open the high pressure hydraulic feed.
12. A coning tidal flow turbine according to claim 10 or 11 characterised in that the control valve additionally comprising a hydraulic ram vent, the position of the twin pistons may allow hydraulic fluid to vent from the hydraulic ram responsive to excess pressure in the hydraulic fluid pumped to the generator.
13. A coning tidal flow turbine according to any one of claims 9 to 12 characterised in that a pressure monitor duct is connected to a duct supplying the hydraulic fluid pumped by the hydraulic pump to the generator and that said pressure monitor duct is connected to the cylinder to one side of the twin pistons to apply pressure to that side, and a spring means urges the other side of the twin pistons against that pressure.
14. A coning tidal flow turbine according to claim 13 characterised in that excess pressure in the duct supplying the hydraulic fluid pumped by the hydraulic pump to the generator urges a piston of the twin pistons against the spring means to allow hydraulic fluid to vent from the hydraulic ram to the hydraulic ram vent.
15. A coning tidal flow turbine according to claim 13 or 14 characterised in that reduced pressure in the duct supplying the hydraulic fluid pumped by the hydraulic pump to the generator allows the spring to urge the double acting piston to allow hydraulic fluid to vent from the hydraulic ram to the hydraulic ram vent via a motorised valve.
16. A coning tidal flow turbine according to claim 13, 14 or 15 characterised in that when the generator is operating within it normal operating range, the pressure applied to one side of the double acting piston from the duct supplying the hydraulic fluid pumped by the hydraulic pump to the generator is balanced by the pressure applied to the other side of the double acting piston by the spring means and fluid from the high pressure hydraulic feed can reach the hydraulic ram feed via a motorised valve.
17. A coning tidal flow turbine according to claim 11 characterised in that the position of the valve is controlled by signals from the generator.
18. A coning tidal flow turbine according to claim 17 characterised in that the first position of the valve is adopted when the generator is operating below it minimum rated power output or not all, the second position is adopted when the generator is operating above its maximum rated power output, and the third position is adopted when the generator is operating between its minimum and maximum rated power outputs.
19. A tidal flow power generation system substantially as hereinbefore described with reference to the figure 10 and11 of the accompanying drawings.