GB2499700A

GB2499700A - System for reducing hydrodynamic loads on turbine blades in flowing water

Info

Publication number: GB2499700A
Application number: GB1222892.0A
Authority: GB
Inventors: Graeme Mackie
Original assignee: Ocean Flow Energy Ltd
Current assignee: Ocean Flow Energy Ltd
Priority date: 2011-12-20
Filing date: 2012-12-19
Publication date: 2013-08-28
Anticipated expiration: 2032-12-19
Also published as: GB201121892D0; WO2013093452A1; GB2497763A; GB201222892D0; GB2499700B

Abstract

A turbine blade 1 is adapted to reduce the hydrodynamic loads thereon. The blade includes at least one orifice 2 on the suction face S of the blade and an air passageway 4 in fluid communication with the at least one orifice 2. Branch pipes 5, 6 may carry the air to both the pressure side P and suction side S. A turbine including at least one turbine blade is also provided, along with a method of reducing hydrodynamic loads on a turbine blade in marine currents by discharging air around a section of the blade in operation. A compressor 8 may feed air via a non-return valve 9 and a coupling 10 to a bore 12 in the turbine shaft 11. Flow of air to more than one set of orifices 2 may be controlled by valves 16 which may be operated remotely of may open automatically in response to a pre-set air supply pressure.

Description

System for Reducing Hydrodynamic Loads on Turbine Blades in Flowing Water Field of the Invention

The present invention relates to a system and method for reducing hydrodynamic loads on turbine blades in flowing water such as tidal streams, ocean currents and rivers, and in particular to reducing the effectiveness of the turbine under flow conditions that exceed the rated flow speed by altering the hydrodynamic forces generated by an element of the blade.

Background of the Invention

The present invention relates to horizontal axis turbines.

Horizontal axis turbines operating in tidal stream, river and ocean currents have a turbine diameter selected to deliver a certain shaft power at a particular speed of water flow,

often referred to as the rated speed of flow for the turbine. The shaft power delivered by the turbine is used to overcome the resistive torque of some form of power take-off device such as an electricity generator. If the turbine experiences flow speeds greater than the rated flow speed then the power generated by the turbine can overwhelm the power take-off device by delivering too great a shaft torque that if not absorbed by the power take-off device can lead to overspeed of the turbine and the power take-off device that is directly coupled to it.

One solution to this problem is to reduce the torque delivered by the turbine at the higher flow speeds by changing the turbine blade angle of attack to the flow such that the hydrodynamic lift force and therefore shaft torque generated by the turbine is reduced. This change in angle of attack is generally referred to as altering the pitch angle of the blades and turbines that allow such adjustment are referred to as controllable pitch turbines. Controlling the pitch requires some actuation mechanism such as gears, servo motors or hydraulic actuators to adjust the pitch angle and such mechanisms can be subject to failure. The consequences of failure of the pitch control mechanism can be an increase in the hydrodynamic forces generated through the blade being incorrectly angled to the direction of flow. This results in increased

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driving torque leading to overspeed of the power take-off device and also increased blade loading potentially leading to structural failure of the blade.

Another solution for reducing the torque delivered by the blade under increased flow conditions is to induce stall in the flow around the blade by increasing the induced angle of attack by slowing down the speed of rotation of the turbine. This can be achieved by increasing the resistive torque of the power take-off device. Blade stall will lead to a reduction in the driving torque and axial drag but it is difficult to maintain the turbine in stall mode while delivering a steady level of torque to the power take-off device. In addition the fluctuating hydrodynamic forces when operating in the stall mode can lead to unacceptable levels of vibration.

It would therefore be desirable to provide a system for reducing hydrodynamic loads on turbine blades in marine currents that does not involve adjustment to the blade pitch setting or inducing blade stall.

Summary of the Invention

According to a first aspect of the invention there is provided a turbine blade as specified in Claim 1.

Preferred features of the turbine blade are set out in the claims dependent on Claim 1 and/or the description.

According to a second aspect of the invention there is provided a turbine as specified in Claim 10.

Preferred features of the turbine are set out in the claims dependent on Claim 10 and/or the description.

According to a third aspect of the invention there is provided a method of reducing the hydrodynamic load on a turbine blade in marine currents as specified in Claim 20.

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Preferred features of the turbine blade are set out in the claims dependent on Claim 20 and/or the description.

A horizontal axis turbine preferably has two or more blades where the outer portion of the blade towards the blade tip has orifices towards the leading edge on the suction face of the blade or on both the pressure and suction faces of the blade.

Preferably, a conduit running up from the root of the blade supplies the orifices with compressed air.

The conduit may be supplied with a compressed gaseous fluid, such as compressed air. A gaseous fluid has a much lower density and viscosity that the surrounding water in which the turbine is operating. Where the gaseous fluid is air, the system includes an air inlet pipe that has an opening above the waterline which is connected to a compressor that pumps the compressed air through a coupling, and preferably into a hollow bore on the centreline of the turbine drive shaft from where the compressed air is distributed to the various blade air supply conduits.

Advantageously, gaseous fluid is distributed to the gaseous fluid supply conduits of the blade through the blade hub.

The invention provides a system for reducing the effectiveness of the turbine under flow conditions that exceed the rated flow speed by altering the hydrodynamic forces generated by an element of the blade. This is achieved by discharging air around an element of the blade so that the hydrodynamic lift and drag forces on the section of blade local to the discharge of air are significantly altered.

The discharge of air under pressure around the blade, in particular on the suction face of the blade alters the skin friction and pressure field around the blade such that the hydrodynamic forces are altered to the benefit of reducing the driving torque and axial drag force generated by the blade. However, unlike operation in the stall regime the reduction in driving torque and

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turbine drag with air injection does not lead to torque control instability and is analogous to operating a turbine of smaller diameter outwith the stall region of operation.

Other inventions (Somerville GB2186033, Barbu et al US2008/0317598, Pesetsky et al US2011/0103950) involve pumping or sucking the same fluid as the operating medium in which the turbine is working in order to achieve active flow modification in order to alter the aerodynamics of the blade, generally to increasing the lift generated by the foil at larger angles of attack that would without fluid injection exhibit flow separation. This is the opposite effect to that obtained with this invention in that in this invention the emission of a fluid of lower density, that is air, from the surface of a turbine blade operating in water results in a reduction of turbine efficiency, not an increase.

Another invention (Vigars GB2486699) uses the centrifugal force generated by a rotating turbine to pump water from the tip of the turbine blade which effectively reduces the shaft power transmitted by the turbine and so reduce the overspeed of the turbine. This invention again emits from the blade the same fluid (water) as the medium in which it is operating.

Another invention (Bova nd Grabau US 2010/0014970) discharges pressurised air from the trailing edge of a wind turbine in order to destroy lift and reduce the efficiency of the blade. While the reducing of blade efficiency is the same objective as in this patent, the mechanism of injecting air, the same fluid medium in which the blade is operating, from the trailing edge rather than the leading edge would not have the desired effect of reducing the viscous drag of the moving blade as it would not be surrounded by a gas bubble screen of lower density than the surrounding fluid proposed in this invention.

Brief Description of the Drawings

In the drawings, which illustrate preferred embodiments of a turbine blade and turbine according to the invention:

Figure 1 is a schematic representation of a typical turbine blade;

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Figure 2 is a schematic cross-sectional plan view of a turbine blade according to the invention; and

Figure 3 is a schematic representation of a turbine according to the invention.

Detailed Description of the Preferred Embodiments

Figure 1 shows a typical blade 1 fitted with multiple orifices 2. Optionally a plate 3 can be attached to the blade 1 to provide an aerated section of the blade and a non-aerated section of the blade. The plate extends outwards from the outer surface of the blade 1. Without the plate 3, when the blade is at the bottom of its rotational path due to the buoyancy of the air bubbles the air bubbles would migrate to the non-aerated section of the blade along the outer surface of the blade.

Figure 2 shows the blade 1 in cross-sectional plan view. In use one side of the blade 1 is subject to a positive pressure and the other to a negative pressure. A centre air supply pipe 4 and branch pipes 5, 6 carry air to orifices 2 on the pressure side P and the suction side S of the blade 1 respectively.

Air may be supplied to the orifices in a number of different ways. Figure 3 shows one possible arrangement for supplying air to the blades which consists of a supply pipe 7 piercing the water surface. Air from this supply pipe is compressed by a compressor 8 and fed via a nonreturn valve 9 to a coupling in the form of a swivel 10 that connects the compressed air supply to the turbine shaft 11 which has a centreline bore hole 12, the bore hole 12 having an inlet 12' in the side wall of the turbine shaft 11. The swivel 10 provides an air passageway that is in fluid communication with the inlet 12' of bore 12. The swivel 10 is fitted with seals 13 to prevent the air escaping. The turbine shaft 11 is coupled to some power take-off device 14 such as an electrical generator.

There is a back pressure due to the head of water which varies depending upon whether the blade is at the top or bottom or some intermediate position of its swept rotation. Flow

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regulating valves 15 can be fitted to ensure that the flow is distributed correctly. The regulating valve 15 is a non-return valve which prevents ingress of water via the orifices 2.

Figure 3 also illustrates an alternative arrangement of air supply pipes 4 in the lower blade of the turbine, where more than one air distribution pipe 4 is provided in the blade 1. It will be understood by one skilled in the art that each of the blades 1 of the turbine may be a blade of the lower type shown in Figure 3. The air distribution pipes 4 are controlled by valves 16 such that the air can be supplied selectively to more than one set of orifices 2. The valves 16 can be actuated remotely or set to selectively open automatically in response to a pre-set air supply pressure thus the extent of the blade surface area that receives the discharge of air can be altered by adjusting the supply pressure.

The direction of water flow in this arrangement is shown as 17.

In the preferred embodiment compressed air is the gaseous phase fluid passed released from the blades. There is an abundant and free source of air, which is directed to the blades via the surface piercing pipe 7. However, other gaseous phase fluids may also reduce the hydrodynamic load on the turbine blades. It would be clear to one skilled in the art that the environmental consequences of releasing any gas into the ocean would need to be assessed before deployment.

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Claims

1. A turbine blade adapted for reducing hydrodynamic loads thereon, the blade have a pressure face and a suction face, wherein the blade includes at least one orifice on the suction face of the blade and a gaseous phase fluid passageway in fluid communication with the at least one orifice.

2. A turbine blade according to Claim 1 wherein the at least one orifice is provided on each of the pressure and suction faces and a gaseous phase fluid passageway is in fluid communication with the at least one orifice on each of the suction and pressure faces.

3. A turbine blade according to Claims 1 or 2, wherein the blade has a tip and wherein the or each orifice is situated towards the tip of the blade.

4. A turbine blade according to any preceding claim, wherein the blade has a leading edge and wherein the at least one orifice is situated towards the leading edge of the blade.

5. A turbine blade according to any preceding claim, wherein the outer surface of the blade comprises a first section and a second section, wherein the or each orifice is in fluid communication with the first section and wherein the first and second sections are separated by a barrier extending from the outer surface of the blade.

6. A turbine blade according to any preceding claim, wherein the gasesous phase fluid passageway in fluid communication with the at least one orifice is a conduit.

7. A turbine blade according to Claim 6, wherein the gaseous phase fluid passageway in communication with at least one orifice comprises a plurality of conduits, each conduit in fluid communication with at least one orifice.

8. A turbine blade according to any preceding claim, further including flow control means, the flow control means configured to control the flow of gaseous phase fluid to the at least one orifice.

9. A turbine blade according to Claim 8, wherein the flow control means includes one or more valves.

10. A turbine including at least one turbine blade according to any of Claims 1 to 9, the turbine further including a shaft mounting the at least one blade, the shaft including a

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shaft fluid passageway, the shaft fluid passageway having an inlet and an outlet, the outlet being in fluid communication with the fluid passageway of the blade.

11. A turbine according to Claim 10, wherein the inlet of the shaft fluid passageway is situated in a side wall of the shaft.

12. A turbine according to Claim 10 or 11, further including a coupling, the coupling being mounted on the shaft for relative rotation between the coupling and the shaft, the coupling having an inlet and an outlet, the outlet of the coupling being in fluid communication with the inlet of the shaft fluid passageway.

13. A turbine according to Claim 12, wherein the coupling surrounds the shaft.

14. A turbine according to Claim 13 when dependent on Claim 11, wherein the outlet of the coupling is in fluid communication with the inlet of the shaft fluid passageway.

15. A turbine according to any of Claims 10 to 14, further including a power take off device.

16. A turbine according to any of Claims 12 to 15, including a pump or compressor having a gaseous fluid inlet and a gasoues fluid outlet, wherein the fluid outlet is attached to inlet of said coupling.

17. A turbine according to Claim 16, further including a non-return flow control situated between the compressor outlet and the inlet of the said coupling.

18. A turbine according to Claim 16 or 17, wherein the gaseous fluid inlet is in communication with the atmosphere.

19. A turbine according to any of Claims 10 to 18, wherein the said turbine is a horizontal axis turbine.

20. A method of reducing hydrodynamic load on a turbine blade in marine currents comprising the step of:

i) mounting a turbine according to any of Claims 10 to 19 in a body of flowing water;

ii) attaching the turbine to a source of compressed gaseous fluid;

iii) passing the compressed gaseous fluid through orifices in the turbine blade.

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21. A method of reducing hydrodynamic load according to Claim 20, comprising the further step of controlling the flow and/or the pressure of the compressed gaseous fluid through the orifices in the turbine blade to degree of hydrodynamic load reduction.

22. A turbine blade substantially as shown in, and as described with reference to, the drawings.

23. A turbine substantially as shown in, and as described with reference to, the drawings.

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