GB2445413A

GB2445413A - Fluid turbine with secondary turbine driven by induced flow

Info

Publication number: GB2445413A
Application number: GB0700105A
Authority: GB
Inventors: Uwe Bernhard Pascal Stein; Lars Johanning
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-01-04
Filing date: 2007-01-04
Publication date: 2008-07-09
Also published as: GB0700105D0

Abstract

A fluid turbine for use in a tidal stream 8 has turbine blades 4 comprising conduits 5. A secondary flow is induced as fluid is drawn into a low pressure region at the surface of the blade 4, and also due to centrifugal force. The secondary flow drives a second turbine 12 at a relatively high speed, and so reduces the torque applied to the gearbox 15. This system addresses the problem of very high shaft torques from the primary turbine 4.

Description

2445413

Description

Background to the invention

The kinetic energy present in open flowing masses of water, primarily tidal and ocean 5 currents or river flows, offers a large resource of energy. The fluid velocity of these flows is, in most areas, too low for economical extraction. However, in areas around islands and in rivers, these velocities can reach many meters per second, offering high enough power densities for potentially economical extraction.

10 Many different absorber means have been presented in the past. Most of these are based on absorber mechanisms, typical for wind energy converters, such as horizontal or cross flow turbines. In order to achieve good turbine efficiencies and to avoid blade cavitation, the ratio between the blade tip speeds and the water flow velocity has to be limited. Especially in large systems, this results in low rotational speeds of the is turbines, resulting in extremely high torques on the drive shafts, which requires large and expensive gearboxes. In order to justify the very high installation cost attached to tidal current turbines, a natural trend towards larger machines can be expected, increasing the problem of gearbox torques. This patent describes a method of addressing the problem of high turbine shaft torques, offering a solution for this 20 fundamental limitation and allowing the design of large tidal current turbines.

Brief summary of invention

A method and apparatus intended to extract energy from slow flowing fluids with particular reference to open flowing masses of water, primarily tidal or ocean currents 25 or river flows. More specifically, this invention relates to an apparatus intended to concentrate the energy in such flows in order to reduce the size, and to increase the rotational speed of the final fluid power to shaft power converter.

This is accomplished, by having a first absorber profile arranged as part of an 30 absorber turbine, placed in the primary fluid flow. Rather than using the forward force of the blade to create torque on the turbine shaft, this absorber turbine is primarily used to create a secondary fluid flow of significant larger pressure differential and smaller flow rate. This secondary flow is created by allowing fluid to be drawn into the low-pressure region of the absorber turbine blades. A smaller fluid driveable 35 engine, for example a turbine of Francis or Kaplan type, is placed into this flow in order to convert fluid power into exportable shaft power. This shaft power is most likely to be used to operate a generator producing exportable electricity.

After Bernoulli, the pressure energy of the free flowing fluid equals the kinetic 40 energy, and is therefore limited to: 0.5* fluid-density* fluid-velocityHowever, with the correct angle of attack, the flow over a profile will create a low-pressure region on one side of the profile, which can have a pressure differential many times larger than the limit described by Bernoulli. This effect combined with the potentially much higher velocity of the blade, if arranged as part of a turbine, can create pressure 45 differences across the chore line of many atmospheres.

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This principle could be applied to both cross flow or horizontal type turbines as used for wind energy converters. The turbine itself would preferably be driven by the fluid flow, but could also be operated by means of a separate mechanical drive mechanism or be linked with the drive of the fluid driveable engine.

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If fluid is allowed to be drawn into this region it will, ultimately reduce the available pressure potential, but if the flow is kept within limits, this set-up offers a way to produce a secondary fluid flow of large pressure differential. This secondary fluid flow can than be used to operate a compact and fast spinning fluid driveable engine 10 for example of Francis or Kaplan turbine type, requiring a much smaller, no or gear box to operate a generator.

A further effect of the absorber turbine type arrangement is that the fluid would be subject to a centrifugal force, on its way towards the high velocity region of the blade, l s and thereby acts as a centrifugal pump. A further object of the present invention is the concept of combining the effect from the low-pressure region of the blade, with the centrifugal effect from the cavity inside the blade, in order to further increase the pressure differential across the fluid driveable engine. The pressure potential created by the centrifugal force would naturally be limited by maximum blade tip speed, zo which in most cases has to be limited in order to avoid cavitation.

It would further be conceivable that the absorber turbine blades would change their blade profile along their length towards a geometry of little or no lift at the blade tip in order to avoid cavitation and to allow for much higher blade tip speeds. This 25 arrangement would not allow for an increase pressure differentia] since this is limited to the ambient fluid pressure by cavitation, but could be used to increase the centrifugal force acting on the fluid in the conduit inside the blade. In this arrangement the pressure differential created by means of centrifugal force could dominate against that created by the flow over the absorber blade.

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It would further be conceivable, that one fluid driveable engine would be connected to a plurality of absorber turbine units, the absorber turbine units placed in close proximity inside the primary fluid flow, connected through pipes linking the different units with the fluid driveable engine. With this arrangement it would be possible to 35 limit the size and the forces on the absorber turbine, while increasing the size of the apparatus, in order to improve its economics.

It would further be conceivable that more than one fluid driveable engine would be connected to one or more absorber turbines. This arrangement might offer advantages 40 during part load operation since only some of the fluid driveable engines would have to operate, while others could be stopped in order to reduce losses and wear.

It would further be conceivable that a variable pitch turbine would be used for the 45 absorber turbine. The fluid driveable engine, if based on a turbine of Kaplan of Francis type, might also employ variable pitch rotor blades and guide vanes in order to adjust to the ratio between flow, power and rotational speed.

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The speed of the fluid driveable engine might be constant or variable depending on the generator and gearbox type used. If an active drive is used in order to control the rotational speed of the absorber turbine, this could be operated independently, or with a fixed gear ratio to the secondary turbine.

5

It would further be conceivable that the system were arranged in a way, whereby a nacelle containing the fluid driveable engine or parts of it, and possibly a generator unit, is arranged in a way, allowing for it to be detached from the main turbine and its sub-frame. This arrangement would allow for the nacelle to be brought up to the 10 surface for inspection or repair, without the need to raise the main turbine and its sub-frame.

It would further be conceivable that the system is arranged in a way, whereby the output of the fluid driveable engine is coupled into a driveshaft of substantial length in 15 order to locate the generator above or close to the surface of the primary fluid flow.

Embodiment of the Invention

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which:

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• Figure 1 shows the principle of the absorber blade

• Figure 2 shows one possible embodiment of this principle in conjunction with a horizontal axis flow turbine, operating with variable pitch blades, a yaw bearings in order to adjust for different directions of water flow, and a Francis turbine as

25 the fluid driveable engine.

• Figure 3 shows a further possible embodiment of this principle, in conjunction with a fixed zero pitched turbine, of so called Wells type, with no slew bearing, having valves in the blades in order to adjust for the different direction of flow

• Figure 4 shows the arrangement of figure 3 with a detachable generating nacelle 30 • Figure S shows a possible blade arrangement in order to adjust for different angles of attack

Other arrangements based on the same principles are not ruled out.

35 Figure 1 shows the basic principle of the absorber blade with a section of the absorber blade (1), the secondary fluid flow (2), and the fluid driveable engine (3), here shown as Francis type turbine. In its most simple form the proposed energy converter system could be formed from one or multiple main absorber blades initialled in the fluid flow and supported by a substructure to the seabed. The blade or blades would have a 40 single or multiple openings on the low-pressure side of the profile, which are connected to the fluid driveable engine. The main absorber blade would be, either fixed or allowed to change its orientation by rotation around one or more of its axis's, in order to adjust for the present direction of primary fluid flow. The lift crated by the absorber blade could further be used to create a down force against the seabed 45 potentially increasing the stability of the support structure and therefore reducing the size and cost of its foundation. In arrangements, consisting of multiple absorber blades, it would further be possible to counterbalance the lift created by the blades against each other.

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In order to increase the pressure differential and to reduce the flow-rate of the secondary fluid flow, it would be of advantage to increase the flow velocity experienced by the absorber blade or blades. This could be accomplished by acceleration of the fluid, possibly with the means of a flow duct.

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Figure 2 shows one possible embodiment of this principle in conjunction with a horizontal axis flow turbine, operating with variable pitch blades, a yaw bearings in order to adjust for different directions of water flow, and a Francis turbine as the fluid driveable engine.

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In the apparatus illustrated, the absorber blade (4) forms part of the horizontal axis turbine. This blade has a conduit (S) along its length, connected on one end to openings (6) along the downstream side of the blade, on the other to the manifold (11) at the centre of the absorber turbine. The absorber turbine is supported through the is bearing element (9) onto a central static shaft (10). A pitch control device (20) might be installed, in order to change the pitch angle of the blades.

The flow channels from the different blades are connected through the manifold (11) and applied to the fluid driveable engine, shown as of Francis type construction with 20 the housing and guide vanes (13) located around the turbine element (12). The torque from the fluid driveable engine is transferred onto the shaft (14) and coupled into the gear box (15). The output from the gearbox (15) is coupled against the generator (16). Depending on the type of generator and rotational speed of the secondary turbine, the gearbox (15) might not be required, and the generator (16) could be coupled directly 25 to the shaft (14). The generator (16) might be of AC or DC nature, and might be operated at a fixed rotational speed or at different speeds, its electricity output exported through the power cable (17). The gearbox (15) and the generator (16) are enclosed in the watertight nacelle (18). The nacelle (18) is supported on the sub-frame (19) and anchored against the seabed (7). A yaw control mechanism (21) is installed 30 between the nacelle (18) and the sub-frame (19) in order to adjust the turbine direction for different directions of primary fluid flow. A brake element (22) might be used to stop the main turbine. The apparatus further employs a bypass valve (23), installed aiong the length of the connecting conduit between the main absorber unit and the fluid driveable engine, linking the primary fluid flow and the secondary fluid flow, 35 here shown at the base of turbine blade (4). This valve is used to control the pressure difference between the two fluid flows, in order to limit the rotational speed and drag force of the absorber turbine and to limit the flow experienced by the fluid driveable engine. The switching of the bypass valve (23) could be passively controlled from the pressure differential across it, the primary fluid flow velocity, the turbine speed, or 40 through an actuator operated from a control system.

Figure 3 shows a further possible embodiment of this principle, in conjunction with a fixed zero pitched turbine, of so called Wells type

45 For bi-directional flows, typical for tidal currents, a so-called Wells turbine would offer the advantage, in that it removes the need to change the nacelle direction or pitch of the blades. This type of turbine operates with zero pitch, relative to the rotational direction of the blades.

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The vector, between the velocity of the blade and the fluid velocity, form the angle of attack. With the low-pressure region, typically present towards the forward end of the blade, a forward force is created. This type of turbine can only produce positive torque over a limited ratio between the blade velocity and the primary fluid velocity.

5 It typically has a lower efficiency compared to the asymmetrical pitched design. This however would not be so significant in the arrangement proposed apparatus, since only neutral balance, between the blade drag force, and the blade forward force, at the target rotational speed would be required.

10 In order for this arrangement to work in both directions, an arrangement to change the connection from the secondary turbine to the present low-pressure side of the blade is required. This could simply be realised with one way valves, only allowing only flow from the secondary fluid flow into the primary fluid flow. These valves could be installed in openings on both sides of the blade. During operation, the pressure in the is secondary fluid flow (24), would fall below that of the pressure on the current high-pressure side (25) of the blade (26), resulting in the closure of the valve (27). The fluid flow from the secondary turbine (28) would open the valve (29) on the current low-pressure side of the blade allowing fluid to exit the blade. For flows in reverse direction, valve (27) will open and valve (29) will close.

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The concept of the Wells type turbine could also be combine with a pitched type turbine, whereby some fraction of the blade or a separate blade of variable pitch nature would be used in order help the starting and to stabilize the speed of the absorber turbine. This could simply be achieved, by having a small fraction of the 25 blade or a separate blade, which can swivel or bend to some degree with the flow to produce a pitch angle.

Figures 4a and 4b

Figures 4a and 4b illustrate a possible secondary fluid flow exit valve arrangement, 30 suitable for the absorber turbine arrangement shown in figure 3. Said vale is formed by the movable section (30) along the length or part of the absorber blade (31), operating by moving towards the present low pressure side of the blade, and thereby covering the exits on the present high pressure side (32) and in this position allowing the fluid from the secondary fluid flow (33) to exit from the openings on the present 35 low pressure side (34) of the blade. As well as acting as a valve such an arrangement could be used to create an asymmetrical blade profile, adjusting its shape depending on the direction of the primary fluid flow.

40 Figure 5

Figure 5 illustrates an arrangement with provision to detach the nacelle (35), from the sub-frame (36) be means of a locking mechanism (37). The illustration shows a fluid driveable engine of Kaplan type with its main turbine (38) and guide vanes (39), a generator unit (40) and a power cable (41). This arrangement would allow for 45 nacelle, with its fluid driveable engine and its generator unit, to be brought up to the surface of the primary fluid flow, for inspection or repair without the need to raise the main turbine and its sub-frame.

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Claims

Claims

1. An apparatus for extracting energy from a primary fluid flow, the apparatus, compromising; a absorber profile arranged as a blade of a absorber turbine; a fluid s driveable engine; a conduit linking exits on said absorber profile and said fluid driveable engine, a secondary fluid flow through the fluid driveable engine into the low-pressure region of said absorber profile, said turbine installed in the primary fluid flow, said absorber profile being shaped and arranged to form at least one low pressure region over its surface, the exits on said absorber profile,

10 being arranged to allow said secondary fluid flow to be drawn into said low pressure region, said conduit being arranged to apply a centrifugal force on said secondary fluid flow through the circular motion of the absorber profile, said secondary fluid flow being drawn from a region of higher pressure in the primary fluid flow, with a fluid driveable engine installed in the secondary fluid flow,

15 converting fluid power into shaft power.

2. Apparatus according to claim one, in which the motion of the absorber turbine is achieved through forces created by the primary fluid flow, acting either on the absorber profile itself, or on a further profile linked to absorber profile.

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3. Apparatus according to claim one, in which the rotation of the absorber turbine is mechanically linked to a drive mechanism, possibly that of the fluid driveable engine.

25 4. Apparatus according to any one of the preceding claims, in which the absorber turbine is of horizontal axis turbine nature, with one or a plurality of blades, these being blades of fixed or variable pitch nature.

5. Apparatus according to any one of the preceding claims, in which the absorber

30 turbine is of cross flow turbine nature with one or a plurality of blades, these being blades of fixed or variable pitch nature.

6. Apparatus according to any one of the preceding claims, in which the fluid, used for the secondary fluid flow, differs from that in the primary fluid flow in being

35 sourced from a different location or it being a different type of fluid altogether.

7. Apparatus according to any one of the proceeding claims, in which the connecting conduit between the fluid driveable engine and the absorber profile has multiple exits points out of the absorber profile into the primary fluid flow.

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8. Apparatus according to claim 7, in which fluid control valves are installed into the conduits, serving the different exit points, in order to control the flow ratio through the different exits, the valve being of either passive or active nature.

45 9. Apparatus according to claim 8, in which the valves are of one way valve type, preventing flow from the primary fluid flow into the conduit but allowing flow from the conduit into the primary fluid flow.

10. Apparatus according to claims 8, and 9, in which the valve is formed by one or

50 many movable section, along the length of the absorber blade, operating by

— (p —

moving towards the current low pressure side of the blade, thereby covering the exits on the high pressure side and allowing fluid to exit from the openings on the low pressure side as well as changing the blade profile.

5 11. Apparatus according to any one of the preceding claims, in which the absorber profile is of Wells turbine type, operating with fixed zero pitch profiled blades maintaining the same direction of rotation in reversing primary fluid flows.

12. Apparatus according to claim 11, in which only a fraction of the turbine is of

10 Wells type, having at least one further profile or section of a profile with operates with variable pitch.

13. Apparatus according to any one of the preceding claims, in which one fluid dividable engine is connected to multiple absorber turbine.

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14. Apparatus according to any one of the preceding claims, in which one absorber turbine is connected to multiple fluid dividable engines.

15. Apparatus according to any one of the preceding claims, in which the fluid

20 driveable engine is coupled to a electricity generating apparatus, by means of a drivetrain, the drivetrain maintaining the rotational speed of the fluid turbine or creating a gear ratio, the electrical generator being located closely to the fluid driveable engine or remotely linked by an extended driveshaft, the location of the electricity generating apparatus not necessarily being inside the primary fluid

25 flow.

16. Apparatus according to any one of the preceding claims, in which the fluid driveable engine is located remotely from the absorber turbine, linked through the conduit, the location not necessarily being inside the primary fluid flow.

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17. Apparatus according to any one of the preceding claims, in which the fluid driveable engine installed onto the support structure of the absorber turbine in a detachable way allowing for it to be removed without the need to move the support structure of the main turbine.

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18. Apparatus according to any one of the preceding claims, in which the fluid driveable engine is installed into a central support pylon, supported on the lower end by the seabed, the pylon supporting the absorber turbine, the pylon on its upper end raising above the surface of the sea, the fluid powered engine arranged

40 to allow access from the upper end of said pylon for service and repair.

19. Apparatus according to any one of the preceding claims, in which the absorber turbine is supported from both sides of its central axis in order to increase the stability of the turbine and supporting structure.

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20. Apparatus according to any one of the preceding claims, in which a bypass valve is initialled, in the conduit, between the absorber turbine and the fluid driveable engine, said bypass valve linking the primary fluid flow and the secondary fluid flow, in order to control the pressure difference between the two fluid flows, said so bypass valve could be passively controlled, for example by linking its position to

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the pressure differential across it, the tidal current or primary fluid flow velocity, the turbine speed, or said valve could be controlled by means of an actuator, operated from a control system.