CA3028064C

CA3028064C - Steerable underwater frame for submerged turbines

Info

Publication number: CA3028064C
Application number: CA3028064A
Authority: CA
Inventors: John Richard Carew Armstrong; Michael Torr Todman
Original assignee: TidalStream Ltd
Current assignee: TidalStream Ltd
Priority date: 2016-06-23
Filing date: 2017-06-23
Publication date: 2021-02-16
Anticipated expiration: 2037-06-23
Also published as: GB201610990D0; GB2566176A; GB201818815D0; WO2017221029A1; GB2566176B; CA3028064A1

Abstract

An underwater frame supporting a plurality of tidal turbines. The frame is tethered to a seabed fixing such that the frame and tidal turbines are positioned downstream of tidal flow. The frame is steerable so as to increase energy capture from said turbines by exposing them to undisturbed tidal flow.

Description

Steerable Underwater Frame for Submerged Turbines This invention relates to an underwater turbine support that can be steered while submerged to increase energy generated from the turbines supported thereupon.
Many systems have been proposed for the mounting of turbines for the generation of power from water currents associated with river, estuarine, thermal or tidal flows.
The more practical of these incorporate the means for installation and maintenance access as well as for positioning the turbines in the best part of the current. This is usually near the surface where the flow tends to be stronger, but deep enough that the rotors do not break the water surface in even the strongest sea states.
Whilst many proposed supports use tethering fore and aft to keep the turbines aligned in the same direction, others allow free swinging of the support around a single seabed anchorage so that the turbines can be self-aligning with the current flow. This maximises energy capture and reduces loads from off-axis operation. In the case of generation from tidal flows, this single-anchor approach means that the support will swing through a half-circle at each tide change to align first with the flood flow and then with the ebb.
Some systems, for instance that described in G82348249, propose that no special steering is necessary for turbines operating downstream of a single anchorage: the turbines on their support frame will naturally find their own alignment with the water current.
"Aligned"
underwater turbines, therefore, only change their orientation if the water current changes direction. Another example of an "aligned" underwater turbine is disclosed in GB 2 450 624.
In this example, thrust of the turbines is varied in order to maintain the turbines aligned with the prevailing water current flow. In other words, the variation of thrust maintains the orientation of the turbines in alignment with the flow.
In all the above-described systems, an essential feature is that the turbines remain "aligned"
with the flow at all times, so that energy is not lost (and loads do not rise)

2 because of misalignment of the turbine rotors. However, it is also the case that a rotor fixed so as to be stationary in a water current slows the water down as the flow spreads out to go around the rotor rather than through it: this is the inevitable consequence of the diffusing effect of the turbine operating in a fluid flow.
This limits the theoretical energy of an "aligned" turbine to the so-called Betz limit, namely 59%
of the energy available in the flow. In practice the highest efficiency of "aligned"
turbines is lower than this limit because of unavoidable hydrodynamic and mechanical losses in the rotor and drive train.
One way to boost the energy capture of a turbine in a flow is to mount it on a tethered wing, the wing being constrained to travel across the flow in a circular or closed path pattern, with energy being gathered from the turbine moving along the direction of travel of the wing rather than normal to the flow. Such a system is described in US
2015/0316931. In effect, the wing acts as a means of scanning across the water current flow, the wing being driven by the current flow, much like a kite in the wind.. As such, the winged underwater turbine moves rapidly through the flow, driving the turbines. As a result of the speed at which the winged frame travels through the water the turbine thereby gathers many times more energy. However the wing itself constitutes a large structure attracting considerable loadings to itself, and by moving rapidly through the flow - at several times the velocity of the flow - does constitute both a potential hazard and a vulnerable additional component.
The purpose of the present invention is to achieve the benefits of increased energy capture in a similar way but without the complication of the additional feature of the wing described above.
According to a first aspect of the present invention there is provided an underwater frame supporting a plurality of tidal turbines, the frame being tethered to a seabed fixing such that the frame and tidal turbines are positioned downstream of tidal flow, .. wherein the frame is steerable under the influence of differential thrust from the turbines, wherein the underwater frame is configured to be driven continually into fresh flow, so as to increase energy capture from said turbines by exposing them to undisturbed tidal flow.

3 In this specification, the terms "fresh flow" and "undisturbed tidal flow", refer to underwater current flow that has not yet been slowed down by the diffusing effect of a turbine operating in the flow. It will be understood that undisturbed tidal flow is only obtainable for a very limited amount of time, until the flow starts being diffused by the presence of the underwater turbines. As such, the invention suggests to continuously steer the frame and turbines slightly out of alignment and into fresh, that is, non-diffused flow. This differs from the disclosure of US 2015/0316931 in that these common winged frames expose the wings to fresh flow, not the turbines.
The frame may be steerable laterally and movable in a reciprocating side to side motion.
The differential thrust may be achieved by varying the speed of the turbine rotors. In particular, the speed of the turbine rotors is typically characterised by the so-called tip-speed ratio. The tip-speed ratio (or TSR) for turbines is the ratio between the tangential speed of the tip of a blade and the speed of the tidal stream at the plane of the rotor. It is known that the thrust created by a turbine is dependent on the tip-speed ratio of the turbine and generally increases as the turbine blades rotate faster. As such, varying the tip-speed ratio on at least one of the turbines will inevitably change the thrust of said turbine and therefore introduce a differential thrust that can be used to steer the frame with respect to the tidal stream. Introducing a differential thrust by varying the tip-speed ratio of at least one turbine can be achieved by either increasing or reducing the speed of said turbine.
The tip-speed ratio of the turbines is further directly linked to its power coefficient, which determines its output power. The maximum power output, occurring at the power coefficient peak value of any turbine, is dependent on several parameters but is typically achieved at tip-speed ratios of between 4 and 6. As will be described in more detail below, the optimum output power is achieved at the peak of a bell curve, which decreases if the tip-speed ratio is above or below the tip-speed ratio that provides the maximum power output.

4 In an embodiment of the present invention, the differential thrust is achieved by varying the tip-speed ratio of at least one of the turbines so that the power coefficient of said turbine falls below its peak value. The skilled person will appreciate that, depending on various factors, such as the size and form of the turbines, the shape of the frame, etc., every turbine has a peak power coefficient that will be obtained at a specific tip-speed ratio. The peak value is described in more detail herein below and can be determined by calculation or experiment. . The tip-speed ratio may be varied in such a way that the power coefficient of said at least one turbine remains above 90%, preferably above 95%, of the peak value. This embodiment is based on the fact that, for most turbines, the thrust coefficient of a turbine changes monotonically and more significantly as a function of the tip-speed ratio around its maximum power coefficient value than does the power output coefficient. As such, it was found that it is generally sufficient to sacrifice no more than 10% of the power coefficient peak value of the at least one turbine in order to achieve the desired differential thrust.
Since the thrust of a turbine reaches a level of saturation at high tip-speed ratios, the change in thrust may be more significant when the tip-speed ratio of the respective turbine is decreased. In other words, the change of thrust per incremental change in tip-speed ratio may be higher at lower tip-speed ratios, resulting in more substantial differential thrust at lower tip-speeds. However, it is of course also feasible to increase the tip-speed ratio of the respective turbine. In another embodiment, the tip-speed ratio of at least one turbine might be decreased, while the tip-speed ration of at least one other turbine might be increased in order to maximise the differential thrust.
In particular, turbines on opposite sides of the underwater frame may be adjusted in opposite ways such that one or more turbines on one side of the frame are set at an increased tip-speed ratio, whereas one or more turbines on the opposite side of the frame are set at a decreased tip-speed ratio. The term "opposite sides" refers to opposite sides of a vertical or horizontal axis that intersects a centre of gravity of the underwater frame.
The preferred method of variation of the tip-speed ratio of at least one turbine is by electronic variation of the electrical loading on the turbine. It may also be achieved in other ways, for instance by application of a brake or by adjusting the blade pitch of the rotor blades.
In yet an alternative embodiment the frame may be provided with lateral thrusters and

5 is then steerable by the forces generated by said thrusters. In a further alternative embodiment the frame may be provided with controllable lift surfaces and is steerable by manipulation of said lift surfaces.
In addition to the aforementioned lateral motion, in the frame may additionally be steerable vertically and movable in a reciprocating up and down movement. The lateral and vertical movement components may be combined such that the frame follows a circular, elliptical or figure of eight path.
In order to facilitate the aforementioned movement, the frame is preferably tethered to a single point on the seabed fixing. The tether may be either a flexible or a rigid tether. Where the tether is rigid, it will be understood that appropriate articulation means are provided at opposing ends of the tether between, respectively, the frame and tether, and tether and sea-bed fixing.
The torque of the turbine rotors may also controllable so as to provide roll stabilisation and/or underwater positioning of the frame, in use. Alternatively, or in addition to, such torque control, the buoyancy of the frame may be controlled so as to provide roll stabilisation and/or underwater positioning of the frame. The frame may be provided with one or more chambers to which water can be admitted to, or removed from, in order to alter the buoyancy of the frame.
The frame is preferably movable between a substantially vertical operating position and a substantially horizontal maintenance position. In the maintenance position the frame lies at or near the water surface and thereby allows access to the turbines.
Preferably a portion of the frame extends above the water surface when the frame is in the vertical operating position. This provides a variable buoyancy reaction to oppose horizontal thrust forces, and also serves to indicate the position of the frame to vessels

6 in the vicinity. In the absence of a part of the frame above the water surface, ballast water may be pumped in or out of one or more chambers to provide reaction to horizontal thrust forces and stabilise the vertical position of the frame in the water.
According to a further aspect of the present invention there is provided a method of operating an underwater frame supporting a plurality of tidal turbines, the frame being tethered to a seabed fixing such that the frame and tidal turbines are positioned downstream of tidal flow, wherein the frame is continuously steered into fresh flow, so as to increase energy capture from said turbines by exposing them to undisturbed tidal flow.
The purpose of the present invention is to arrive at the same goal of enhanced energy capture described in the prior art by amplifying the area of flow intercepted, but by a different method. Rather than using differential thrust to "align" multiple turbines on a tethered frame with the flow, in one embodiment, the present invention proposes to scan the turbines in an arc across the flow first in one direction and then in the other, thereby continuously steering the turbines into undisturbed flow. A further beneficial side-effect of this side-to-side motion of the frame and turbines is that the speed of frame and turbines through the water is increased, thereby enhancing energy capture from the turbines.
If water depth allows, it may also be possible to allow an up-and-down oscillation of the frame, such that the turbines move in an ellipse rather than a straight line, further enhancing the intersected area. It will be appreciated that vertical and horizontal movement components may be utilised in order for the frame to follow different path configurations. In addition to an elliptical path, the frame may, for example, follow a circular or figure of eight path. It will be appreciated that other path shapes are possible. From the point of view of the turbines, rather than being subject to flow velocities reduced by diffusion, they are instead subject to fresh or partially fresh water flows, less affected by diffusion, with higher velocities giving rise to greater energy capture. Since the diffusion loss in velocity - known as 'induction' -can be typically one third of the upstream velocity, and since energy is proportional to the cube of velocity, the energy capture from totally fresh flow would be (3/2)^3 or 3.4 times that from flow with an induction factor of one third. Of course, such

7 enhancements could not be practically obtained, since some degree of induction would still accompany laterally moving turbines, but nevertheless a useful degree of enhancement could in theory be obtained.
This invention is quite distinct from that described in US 2015/0316931. In the first place, no 'wing' is involved; just turbines supported on a tethered frame.
Secondly, it is not the speed of movement through the water that leads to the enhancement of energy;
rather it is the displacement of the turbines to a zone of fresh flow that produces the enhancement. The speed of movement of the frame does not in itself contribute to the enhancement, however if the speed is too slow the induction velocity loss associated with diffusion will have time to build up and will to that extent limit the enhancement.
Too fast a motion of the frame will on the other hand lead to drag losses that offset the energy enhancement, so that the speed of displacement is in fact critical, but for different reasons from the wing speed of US 2015/0316931. It is likely that different frame speeds will be required to maximise the enhancement at different levels of flow velocity.
Motions of the frame can be induced by a number of means, for instance by differential thrust induced by varying the rotor speed or blade pitch of the turbines individually or in groups. Alternatively; sideways-acting thrusters can physically propel the frame in its arc of motion, or lift surfaces can be exposed or have their angle of attack altered to generate sideways force to provide the motion. With sufficient turbines, motions can also be induced in a vertical direction or combined to give an elliptical or circular path of motion. It may also be desirable to include in the control system means for also varying the differential torque between turbines such that the frame remains stable in roll in the desired configuration.
Other features of the invention will be apparent from the following description of a preferred embodiment shown by way of example only in the accompanying drawings in which:
Figure 1 shows a perspective view of an underwater frame supporting a plurality of turbines;

8 Figure 2 shows a plan schematic view of the lateral motion of an underwater frame according to an aspect of the present invention; and Figure 3 shows a perspective view of an underwater frame supporting a plurality of turbines and thrusters.
Figure 4 shows a schematic diagram of the output power coefficient and thrust coefficient as a function of the tip-speed ratio of a typical turbine.
Referring to figure 1 there is shown a frame 10 having a plurality of tidal turbines 12.
In the embodiment shown, the frame 10 includes a body 14 having a pair of legs extending downwardly therefrom. Each leg 16 is provided with a respective further body 18 towards its distal end 20. The frame 10 further includes a plurality of lateral extensions 22 to which the turbines 12 are mounted. The bodies 14, 18 may be provided with internal compartments to which sea water may be admitted to and removed from. Admission and/or removal of sea water from the compartments .. permits the buoyancy of the frame and turbine combination to be altered.
Alteration of the buoyancy in this manner may be used to, for example, trim the structure while submerged and move the structure between a submerged operating position and a maintenance position at or near the water surface. Additionally or alternatively, floodable compartments may be provided in the legs 16. The form of the frame 10 is .. provided for the purpose of illustration and is not intended to be limiting.
The frame 10 is tethered to a seabed fixing 24 by a flexible tether 26. A
rigid tether with appropriate articulations at opposing ends may also be used.
.. Referring now to Figure 2, the thrust of the turbines 12 is varied differentially (by rotor speed or blade pitch) so as to provide rotation of the frame 10 anti-clockwise from position 1 to position 2. The net rotor thrust will then cause the whole frame 10 with its tether 26 to rotate anti-clockwise to position 3 and then to position 4.
Once at the extreme position of position 4 is reached, the thrust differential is reversed so that the frame 10 rotates the other way, and hence pulls its tether 26 clockwise up to position 8. In this way the turbines 12 are driven continually into fresh flow and as

9 PCT/GB2017/051849 a result capture more energy. The motion of the frame 10 is reciprocated in a side to side motion.
Motion of the frame 10 in the manner described above may be achieved by methods other than turbine thrust variation. For example, movement of the frame 10 may be achieved by the provision of lateral thrusters on the frame 10 and/or the provision of controllable lift surfaces.
In Figure 3, two of the turbines at the extremities of the frame have been replaced by thrusters 27 which may be powered- in either direction - differentially to generate the moment required to displace the frame and followed the desired oscillatory or cyclic motion.
Figure 4 shows a schematic diagram of the power output coefficient Cp and thrust coefficient CT as functions of the tip-speed ratio TSR for an exemplary turbine. The behaviour of the output coefficient Cp as a function of the tip-speed ratio TSR is shown by line 30: a generally bell curved shape. As discussed hereinbefore, the peak value or maximum power output coefficient Cpmax will be reached at medium tip-speed ratios, typically between 4 and 6. The peak value of the bell curve described by line 30 is labelled in Figure 4 as point b. In a normal operating condition, all of the turbines of the present invention are typically set to a tip-speed ratio (TSR1) that results in the maximum power output coefficient Cp., at point b. As will be understood, this tip-speed ratio TSR1 may change dependent on several characteristics of the turbine, however it will generally be easy for the skilled practitioner to determine TSR1 via calculation or experiment.
As can be derived from Figure 4, if the tip-speed ratio is reduced, the power output coefficient Cp decreases. For example, if the tip-speed ratio is set to value TSR2, which is lower than TSR1, the power output coefficient Cp decreases to value Cpmm which will be described in more detail below. Similarly, if the tip speed ratio is higher than TSR1, such as the tip-speed ratio TSR3 shown in Figure 4, the power output coefficient Cp will also decrease. For example, if the tip-speed ratio is set to value TSR3, the power output coefficient yet again drops to Cp min.

It is further derivable from Figure 4 that changing the tip-speed ratio not only affects the power output coefficient Cp but also changes the thrust coefficient CT of the exemplary turbine. This behaviour is depicted by line 40, which shows the thrust 5 coefficient CT as a function of tip-speed ratio TSR. As will be appreciated, during normal use, that is when the exemplary turbine rotates at tip-speed ratio TSR1, the coefficient of thrust has a value of CT1, represented by point e along line 40. If the tip-speed ratio of the exemplary turbine is reduced from TSR1 to TSR2, the coefficient of thrust decreases from value CT1 to CT min along the path between points e and d of

10 line 40. If, on the other hand, the tip-speed ratio is increased from value TSR1 to value TSR3, the thrust coefficient increases from CT1 to CTmax between points e and f along line 40. This change of thrust in at least one turbine can be used to create a differential thrust within the frame that can be applied to steer the frame laterally and/or vertically. To this end, the underwater turbine of the present invention comprises a control unit (not shown) configured to continuously steer the frame into undisturbed flow by creating differential thrust, e.g. through variation of turbine speeds and/or use of thrusters and/or controllable lift surfaces. The tip-speed ratio of the turbine is preferably adjusted electronically by control of the electrical loading on the generators and hence of their power output. However, it is also feasible to adjust the pitch of the turbine blades or implement mechanical brakes.
It is preferred to vary the tip-speed ratio TSR within certain limits so that power loss is minimised. In particular, the tip-speed ratio TSR should not be changed so as to cause the power output to drop below Cpmin as this will result in unacceptable losses. In one embodiment of the present invention, this threshold is set to at least 90% of the peak value. i.e. CPmm = 0.9 x Cumax In other words, the tip-speed ratio of the exemplary turbine will only ever be varied between tip-speed ratios TSR2 and TSR3, such that the power output coefficient Cp never drops below Cpinin, which is at least 90% of C Pmax =
Figure 4 further shows that adjusting the tip-speed ratio TSR between values and TSR3 will cause a proportionally bigger change in the coefficient of thrust than the change in the power output. For example, it may be that changing the tip-speed

11 ratio such that the coefficient of power Cp reduces by about 10% will reduce the coefficient of thrust of that same turbine by about 20%. In other words, Cpim, will be around 90% of Cpmax, whereas CTmul will be around 80% of CT' As such, a small decrease in the coefficient of power will cause a proportionally higher change in .. thrust, which is sufficient to steer the frame in the desired direction.
The above is also true for an increase of the tip-speed ratio between TSR1 and TSR3.
That is, the increase of coefficient of thrust will again be proportionally higher than the decrease in the coefficient of power. However, as line 40 reaches saturation at high tip-speed ratios, the proportional increase in coefficient of thrust between TSR1 and TSR3 is smaller than the proportional difference in thrust between TSR2 and TSR1. In other words, CTmax may be around 115% of CT1 whereas CImm may be about 80% of CT1 Accordingly, it may be found that obtaining a differential thrust by reducing the tip-speed ratio may be more effective than by increasing the tip-speed .. ratio.

Claims

1. An underwater frame supporting a plurality of tidal turbines, the frame being tethered to a seabed fixing such that the frame and tidal turbines are positioned in a tidal flow, wherein the frame is steerable under the influence of differential thrust from the turbines, characterised in that the underwater frame is configured to be driven continually into undisturbed tidal flow, so as to increase energy capture from said turbines by exposing them to undisturbed tidal flow; wherein the differential thrust is achieved by reducing the tip-speed ratio of at least one of the turbines on a first side of the frame and increasing the tip-speed ratio of at least one of the turbines on an opposite, second side of the frame.

2. An underwater frame as claimed in claim 1 wherein the frame is steerable laterally and movable in a reciprocating lateral motion.

3. An underwater frame as claimed in claim 1 or 2 wherein the differential thrust is achieved by varying the tip-speed ratio of at least one of the turbines so that the power coefficient of said turbine falls below a peak value of the power coefficient of said turbine.

4. An underwater frame as claimed in claim 3 wherein the tip-speed ratio of the at least one turbine is varied such that the power coefficient of said at least one turbine remains above 90% of the peak value.

5. An underwater frame as claimed in claim 4 wherein the tip-speed ratio of the at least one turbine is varied such that the power coefficient of said at least one turbine remains above 95% of the peak value

6. An underwater frame as claimed in any one of claims 1 to 5 wherein the differential thrust is achieved by reducing the tip-speed ratio of at least one of the turbines or wherein the differential thrust is achieved by increasing the tip-speed ratio of at least one of the turbines.

7. Ari underwater frame as claimed in any one of claims 1 to 6 wherein the differential thrust is achieved by varying the blade pitch of turbine rotors on the at least one turbine.

8. An underwater frame as claimed in claim 2 wherein the frame is provided with lateral thrusters and is steerable under by the output of said thrusters.

9. An underwater frame as claimed in claim 2 wherein the frame is provided with controllable lift surfaces and is steerable by manipulation of said lift surfaces.

10. An underwater frame as claimed in any one of claims 2 to 9 wherein the frame is additionally steerable vertically and movable in a reciprocating up and down movement.

11. An underwater frame as claimed in claim 10 wherein the frame is steered both laterally and vertically such that the frame follows a circular, elliptical or figure of eight path.

12. An underwater frame as claimed in any one of claims 1 to 11 wherein the frame is tethered to a single point on the seabed fixing.

13. An underwater frame as claimed in claim 12 wherein the tether is a flexible tether.

14. An underwater frame as claimed in claim 12 wherein the tether is a rigid tether having pivotable connections at opposing ends.

15. An underwater frame as claimed in any one of claims 1 to 14 wherein the torque of the turbine rotors is controllable so as to provide roll stabilisation and/or underwater positioning of the frame.

16. An underwater frame as claimed in any one of claims 1 to 15 wherein the buoyancy of the frame is controllable so as to provide roll stabilisation and/or underwater positioning of the frame.

17. An underwater frame as claimed in claim 16 wherein the frame is provided with one or more chambers to which water is admittable to or removable from in order to alter the buoyancy of the frame.

18. An underwater frame as claimed in any one of claims 1 to 17 wherein the frame is movable between a substantially vertical operating position and a substantially horizontal maintenance position.

19. An underwater frame as claimed in claim 18 wherein a portion of the frame extends above the water surface when the frame is in the vertical operating position.

20. A method of operating an underwater frame supporting a plurality of tidal turbines, the frame being tethered to a seabed fixing such that the frame and tidal turbines are positioned in a tidal flow, wherein the frame is steered under the influence of differential thrust from the turbines, characterised in that the frame is driven continually into undisturbed tidal flow so as to increase energy capture from said turbines by exposing them to undisturbed tidal flow;
wherein the differential thrust is achieved by reducing the tip-speed ratio of at least one of the turbines or wherein the differential thrust is achieved by increasing the tip-speed ratio of at least one of the turbines.

21. A method as claimed in claim 20 wherein the frame is steered laterally in a reciprocating lateral manner.

22. A method as claimed in claim 20 or 21 wherein the differential thrust is achieved by varying a tip-speed ratio of at least one of the turbines so that the power coefficient of said turbine falls below a peak value of the power coefficient of said turbine.

23. A method as claimed in claim 22 wherein the differential thrust is achieved by varying the tip-speed ratio of the at least one turbine in such a way that the power coefficient of said at least one turbine remains above 90% of the peak value.

24. A method as claimed in claim 23 wherein the differential thrust is achieved by varying the tip-speed ratio of the at least one turbine in such a way that the power coefficient of said at least one turbine remains above 95% of the peak value.

25. A method as claimed in any one of claims 20 to 24 wherein the differential thrust is achieved by reducing the tip-speed ratio of at least one of the turbines on a first side of the frame and increasing the tip-speed ratio of at least one of the turbines on an opposite, second side of the frame.

26. A method as claimed in any one of claims 20 to 25 wherein the differential thrust is achieved by varying the blade pitch of turbine rotors on the at least one turbine.

27. A method as claimed in any one of claims 20 to 26 wherein the frame is steered by lateral thrusters provided on the frame

28. A method as claimed in any one of claims 20 to 27 wherein the frame is steered by controllable lift surfaces of the frame.

29. A method as claimed in any one of claims 20 to 28 wherein the frame is additionally steered vertically in a reciprocating up and down manner.

30. A method as claimed in claim 29 wherein the frame is steered both laterally and vertically such that the frame follows a circular, elliptical or figure of eight path.