The present application relates to a power generation system and, more particularly, to a power generation system generating electrical energy from the movement of a liquid.

The present invention relates to a hydropower generation system using a

tethered/supported flapping foil to extract energy from a moving flow of liquid.

Research has indicated that flapping foil hydropower generation systems can extract significantly more power from a given flow than rotary, propeller based, systems. Moreover, flapping foil arrangements can operate in lower flow regimes than are required for underwater turbines, and have less of an environmental impact.

Flapping foil hydropower systems are at a very early stage in technological development and the current state of the art involves complex lab-based systems which require active controls or multi-linkage control arms to control the flapping foil. Moreover, prior art arrangements require substantial infrastructure to anchor the flapping device and align it to the liquid flow.

The present invention seeks to provide an alternative arrangement.

Accordingly, the present invention provides a power generation system comprising: at least one hydrofoil for arrangement in a moving liquid and configured to impart a force in a direction other than that of the direction of flow of the liquid; a force transfer arrangement operatively connected at or near the

hydrodynamic centre of the at least one hydrofoil; a power generation arrangement operatively connected to the force transfer arrangement to convert the force imparted by the at least one hydrofoil into electrical energy; and an anchor for securing the system.

Preferably, the force transfer arrangement is configured to allow movement of the at least one hydrofoil along an axis which is not parallel to the direction of flow of the liquid.

Preferably, said axis is substantially perpendicular to the direction of flow of the liquid.

Preferably, the force transfer arrangement is configured to allow movement of the at least one hydrofoil along an arcuate path, at least the centre of the arcuate path not being parallel to the direction of flow of the liquid.

Preferably, the at least one hydrofoil is configured to travel between a first and second location along said axis or arcuate path.

Preferably, the at least one hydrofoil is arranged to move substantially sinusoidally between the first and second locations in use.

Preferably, the at least one hydrofoil is configured to move along said axis or arcuate path at a velocity which is lower than the velocity of the liquid flow.

Preferably, the at least one hydrofoil is configured to move along the axis or arcuate path at a velocity around 20 - 40% of the velocity of the liquid flow. Preferably, the at least one hydrofoil is configured to operate with a Strouhal number of between 0.2 and 0.4.

Preferably, the power generation system further comprises a buoyant support arranged above the anchor in use.

Preferably, the support is configured to float on or near the surface of the liquid in use.

Preferably, the distance between the anchor and the support is adjustable.

Preferably, the anchor and support both include a pulley and the force transfer arrangement comprises a continuous belt arranged around the pulleys and

extending therebetween, wherein said force imparted by the hydrofoil causes the belt to travel through the pulleys, the power generation arrangement configured to convert the rotation of the pulleys into electrical energy.

Preferably, the power generation arrangement is associated with either the anchor or the support.

Preferably, the power generation arrangement is associated with both the anchor and the support.

Preferably, a line is secured between the anchor and the support, and the force transfer arrangement and power generation arrangement are configured to move along the line with the hydrofoil. Preferably, the force transfer arrangement is connected at one end to either the support or the buoy and at the other end to the at least one hydrofoil.

Preferably, the power generation system is configured to convert rotational, arcuate, osciallating or linear motion into electrical energy.

Preferably, the power generation system comprises a pitch control system to control the pitch of the at least one hydrofoil relative to the direction of flow in use.

Preferably, the pitch control system comprises providing wing twist on the at least one hydrofoil.

Preferably, the pitch control system comprises a wing-warping arrangement and/or at least one controllable flap provided on the trailing edge of the at least one hydrofoil.

Preferably, the at least one hydrofoil comprises a swept wing with washout.

Preferably, the pitch control means are configured to reverse the angle of pitch at each of the first and second locations.

Preferably, the power generation system is configured to be self-aligning with the flow direction of the liquid, such that at least one hydrofoil presents a leading edge to the liquid flow. Preferably, the power generation system comprises two hydrofoils arranged substantially parallel to one another.

Preferably, the at least one hydrofoil is substantially pitch stable.

Preferably, there is provided a power generation system comprising: at least one hydrofoil for arrangement in a moving liquid and configured to impart a force in a direction other than that of the direction of flow of the liquid; the at least one hydrofoil connected to an anchor using a resilient mounting, behind the hydrodynamic centre of the at least one hydrofoil; and a power generation arrangement operatively connected between the at least one hydrofoil and the anchor to convert the force imparted by the at least one hydrofoil into electrical energy.

Embodiments of the present invention will now be described, by way of non-limiting example only, with reference to the figures in which:

Figure 1 schematically illustrates a power generation system according to a first embodiment of the present invention;

Figure 2 schematically illustrates a power generation system according to a second embodiment of the present invention.

Figure 3 schematically illustrates a power generation system according to a third embodiment of the present invention.

Figure 4 illustrates a hydrofoil which may be used with the present invention; and Figure 5 illustrates another hydrofoil which may be used with the present invention. Figure 1 illustrates a power generation system 1 , comprising a hydrofoil 2 which is arranged in a moving liquid 3. The liquid is preferably water. The system may be placed in a river or in the sea, or in any body of water experiencing a flow. As the liquid 3 passes over the hydrofoil 2, it causes the hydrofoil 2 to impart a force in a direction 4 other than that of the direction of flow 5 of the liquid 3.

The force in the direction 4 is caused by the pressure imbalance over the two surfaces of the hydrofoil 2 caused by the flow 3 of liquid passing over the hydrofoil 2. The force generated by the hydrofoil 2 may be in a different direction to that indicated by arrow 4. However, there will at least be a component of the force in the direction 4.

The power generation system 1 further comprises a force transfer arrangement 6 operatively connected at or near the hydrodynamic centre 7 of the hydrofoil 2.

In the arrangement shown, the power generation arrangement comprises a belt 6. Although a belt 6 is provided in the embodiment described, the belt 6 could alternatively be a chain, line, rope etc.

The power generation system 1 shown in figure 1 further comprises an anchor 10, which may be secured to or placed on the bed of the channel in which the liquid 3 is flowing. The mass of anchor 10 may be such that the anchor 10 does not need to be secured to the bed.

The power generation system 1 further comprises a buoyant support (buoy) 1 1 arranged above the anchor 10 in use. The buoyant support 1 1 is configured to float at or near the surface of the liquid 3 in use. Each of the anchor 10 and buoy 1 1 comprises a pulley 12. The belt 6 is arranged around the pulleys 12 and extends therebetween, such that it creates a continuous loop around the pulleys 12.

Accordingly, it will be appreciated from figure 1 that the force in direction 4 causes the belt 6 to travel through the pulleys 12 (in an anti-clockwise direction with reference to figure 1 ). In use, assuming that the belt 6 is substantially taut between the respective pulleys 12 of the anchor 10 and buoyant support 1 1 , the hydrofoil 2 moves substantially along a linear path (axis), between a first location, adjacent to the buoyant support 1 1 , and a second location, adjacent the anchor 10. The axis of movement of the hydrofoil is not parallel to the direction of flow of the liquid. Preferably, the axis may be substantially perpendicular to the direction of flow of the liquid. The axis of movement is preferably configured so as to extract the most energy from the flow of liquid 3. In practice, as illustrated in figure 1 , the belt 6 may not be entirely taut and may have some slack. Accordingly, the belt 6 at the point of attachment to the hydrofoil 2 may not be perpendicular to the direction of fluid flow. Nevertheless, a significant component of the force 4 exerted by the hydrofoil 2 will still be transferred into the belt 6.

Preferably, the hydrofoil 2 is configured to move along the axis at a velocity which is lower than the velocity of the flow 5 of the liquid 3. In one embodiment, the velocity of the hydrofoil is around 20% to 40% of the velocity of the liquid flow.

The power generation system 1 further comprises a power generation arrangement, which may be associated with or housed within one or both of the anchor 10 and the buoyant support 1 1 . In the embodiment shown in figure 1 , the power generation arrangement preferably comprises an electrical generator which converts rotary motion of the pulley(ies) into electrical energy. It will be appreciated that by connecting the rotary electrical generator with either or both of the pulleys 12, and otherwise securing the rotary generator relative to the anchor 10 or buoyant support 1 1 , the rotary motion caused by the movement of the hydrofoil 2 along the direction 4 will cause electrical energy to be generated.

As will be described later, the power generation system 1 further comprises a pitch control system to control the pitch of the hydrofoil 2 relative to the direction of the flow 5 in use. Accordingly, when the hydrofoil reaches the extent of its motion in either direction (either to the first or second position), the pitch control system serves to reverse the pitch of the hydrofoil 2, such that it then imposes a force in a direction opposite to that previously (opposite to arrow 4 shown in figure 1 ). Consequently, the hydrofoil 2 is caused to move in the opposite direction. The hydrofoil 2 may, in use, effectively oscillate between the first and second positions, with appropriate pitch controls. Preferably, the pitch control system is operable to reverse or otherwise alter the pitch of the hydrofoil when it is at or near the extent of motion along the axis (i.e. at or near the first or second position). Position and/or proximity sensors may be provided to determine the position of the hydrofoil relative to the first and second positions. The power generation arrangement(s) is/are preferably configured so as to convert rotary motion in either direction into electrical energy, either directly or through the use of gearing.

A benefit of the arrangement illustrated in figure 1 is that it will preferably "self-align" with the flow 5 of a liquid 3. Preferably, the arrangement can rotate about a substantially vertical axis, so that the leading edge of the hydrofoil 2 is always presented to the flow 5 of liquid 3. Consequently, installation of the power generation system 1 shown in figure 1 may be straightforward. The system may be launched from a vessel and the anchor allowed to sink and settle upon, or to be secured to, the bed. It will then self-align with the flow, without needing any additional installation or configuration. The length of the belt 6 may be configured according to a known or anticipated depth of the liquid at the installation site.

Preferably, the total length of the belt 6 is less than twice that of the lowest anticipated height of the liquid, so as to ensure that the belt 6 is substantially taut at all times. In some embodiments, the buoyant support 1 1 may generally be arranged under the surface of the liquid, which will provide additional tension in the belt 6 in use.

In an alternative arrangement, not shown, a fixed line is secured between the anchor 10 and the buoyant support 1 1 , and the force transfer arrangement and power generation arrangement are configured to move along the line with the hydrofoil 20. Accordingly, movement of the hydrofoil 20 along the fixed line may cause the rotation of a wheel engaged with the fixed line, from which electrical energy may be extracted using a rotary power generation system.

Preferably, in any of the embodiments described, the distance between the anchor 10 and buoyant support 1 1 may be adjustable. Preferably, the power generation system 1 operates with a Strouhal number in a range of 0.2 to 0.4, wherein

Strouhal number = flapping frequency X flapping amplitude

flow velocity

A consequence of operating the hydrofoil at such relatively low velocities

advantageously makes the power generation system embodying the present invention environmentally friendly. Specifically, fish and other marine mammals are able to outswim and avoid the moving hydrofoil(s). Operating in the Strouhal number range of 0.2 to 0.4 allows the hydrofoil to operate at high force coefficients.

Accordingly, although the vertical flapping velocity is low, the gearing may be high, and therefore the hydrofoil can extract substantial energy from the flow of liquid.

Figure 2 shows another power generation system 50A embodying the present invention. As with the arrangement shown in figure 1 , the power generation system 50A comprises an anchor 10 and a buoyant support 1 1 . A line 30 (which may comprise a belt, chain etc.) is secured between the anchor 10 and the buoyant support 1 1 , so as to substantially secure the buoyant support 1 1 in a required location. An arm 31 is pivotably connected at one end to the buoyant support 1 1 and pivotably connected at the other end to the hydrofoil 20. Accordingly, the arm 31 (effectively providing a force transfer arrangement) is configured to allow movement of the hydrofoil 20 along an arcuate path. At the extremes (first and second positions) of movement of the hydrofoil 20 along the arcuate path, the pitch control system serves to reverse the pitch, so that the hydrofoil 20 effectively oscillates along the arcuate path. The movement of the hydrofoil 20 along the arcuate path causes the arm 31 to rotate relative to the buoyant support 1 1 , in which a power generation system is provided, converting the rotational (at least arcuate) movement into electrical energy.

Figure 3 shows an alternative power generation system 50B, in which the arm 31 is pivotably connected to the anchor 10, without requiring an additional buoyant support 1 1 or line 30. The power generation system 50B otherwise operates in a similar manner to that illustrated in figure 2. In an alternative (not shown) to the arrangements shown in figures 2 and 3, the end of the arm 31 remote from the hydrofoil 20 may be connected to line 30 at a point between the anchor 10 and the buoyant support 1 1 . The connection point may be adjustable. Alternatively, either the anchor 10 or the buoyant support 1 1 may comprise a projection which projects into the liquid (upwards from the anchor 10 or downwards from the buoyant support 1 1 ) to which the arm 31 is connected.

Such an arrangement may advantageously allow the efficient extraction of energy from the flow, as a result of the centre point of the arcuate path preferably being perpendicular to the direction of flow.

Figure 4 illustrates the hydrofoil 20 shown in figures 1 to 3. The hydrofoil 20 comprises a plurality of flaps 21 , which are preferably independently adjustable.

The wing flaps 21 are on the trailing edge of the hydrofoil 20 and provide wing twist which may be used to control pitch stability.

As illustrated in figure 5, as an alternative to using flaps 21 , a hydrofoil 120 may be provided with a wing-warping system.

Preferably, hydrofoil 20, 120 is provided with wing sweep, preferably with wash out, which aids stability of the hydrofoil and advantageously helps to shed any weed or other foreign matter which may otherwise become entangled with the arrangement in use. The swept profile conveniently provides yaw stability, which promotes 'self- alignment' of the hydrofoil 2 with the direction of flow.

When adopting the hydrofoil 20 with the arrangements shown in any of figures 1 to 3, during the upstroke of the operation of the hydrofoil, the flaps 21 are adjusted to control the pitch stability of the hydrofoil such that it adopts a stable positive angle of attack ("nose up" relative to the flow). This requires that the flaps 21 towards to the wing tips of the hydrofoil are set in their upwards position (as shown in figure 4b) and the flaps 21 towards the wing centre are set in their downwards position (shown in figure 4b). For the downward stroke, the flaps are adjusted so as to adjust to control the pitch stability so that the hydrofoil adopts a stable negative angle of attack (nose down relative to the flow). This requires that the flaps towards the wing tips of the hydrofoil are set in their downwards position and the flaps 21 towards the wing centre are set in their upwards position (the opposite to that shown in figure 4b).

The hydrofoil 120 of Figure 5 may be configured in a similar manner as that 20 of figure 4, to adjust the angle of attack. Specifically, during the upstroke, the hydrofoil 120 is configured such that the trailing edge of the wing tips are raised, and the trailing edge of the central section of the hydrofoil 120 is lowered. The opposite configuration is adopted for the downstroke.

The pitch control system serves to adjust, preferably to reverse, the pitch at or near the extremes of movement of the hydrofoil 2. In one embodiment, the pitch control system may be mechanical. The belt 6 and/or arm 31 may be provided with at least one position sensor, which detect(s) the extremes of movement/positon. When it is detected that the belt 6 and/or arm 31 (and thus the hydrofoil 2) is at or near one extreme of movement, the pitch control system is operable to adjust/reverse the pitch accordingly. Alternatively or additionally, the pitch control system can be configured to detect when the hydrofoil 2 is at or near one extreme of motion. The proximity of the hydrofoil 2 to an extreme of motion could be determined by pressure sensors mounted on or near the hydrofoil 2, which are configured to determine when the hydrofoil is at or near the surface or sea/river bed.

Preferably, the hydrofoil is attached to the force transfer arrangement at/or substantially near its hydrodynamic centre of lift, through a pivotal connection free to rotate in all three axes.

Although power generation systems illustrated in the figures comprise only one hydrofoil, it is envisaged that two or more hydrofoils may also be adopted.

Preferably, the at least one hydrofoil is substantially pitch stable. In situations where the flow conditions are well characterized and substantially constant, a passive aeroelastic flapping motion may be adopted, eliminating the need for any active pitch control system. In such an arrangement, the at least one hydrofoil is connected to an anchor using a resilient mounting, mounted behind the hydrodynamic centre of the at least one hydrofoil. A power generation arrangement is then operatively connected between the at least one hydrofoil and the anchor to convert the force imparted by the at least one hydrofoil into electrical energy. With this arrangement, with appropriate tuning of the stiffness of the fixing and its resiliency, and the relative position of the hydrodynamic centre of the hydrofoil, a self-reinforcing oscillation is promoted, where the foil reverses direction at the end of the strokes. The relative position of the hydrofoil and the anchor, (lever) can be used to generate electrical energy.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.