SUBMERSIBLE PLANT COMPRISING BUOYANT TETHER

Abstract

The invention relates to a submersible power plan. The submersible power plant is submerged in a fluid. The power plant includes a structure and a vehicle where the vehicle has at least one wing. The vehicle is arranged to be secured to the structure by at least one tether. The vehicle is arranged to move in a predetermined trajectory by a fluid stream passing the vehicle. The tether includes an upper tether part and a lower tether part. The upper tether part has an average density higher than the fluid, has a hydrodynamic cross section and is arranged to be connected to the vehicle. The lower tether part has an average density lower than the fluid, has a non-hydrodynamic cross section and is arranged to be connected to the structure.

Claims

1. A submersible power plant, wherein the submersible power plant is submerged in a fluid, the power plant comprising: a structure and a vehicle having at least one wing, the vehicle being arranged to be secured to the structure by at least one tether, and being arranged to move in a predetermined trajectory by a fluid stream passing the vehicle, wherein: the tether comprises an upper tether part and a lower tether part, wherein the upper tether part has an average density higher than the fluid, has a hydrodynamic cross section and is arranged to be connected to the vehicle, and wherein the lower tether part has an average density lower than the fluid, has a non-hydrodynamic cross section and is arranged to be connected to the structure.

2. The submersible power plant according to claim 1, wherein the upper tether part comprises 30-70% of the length of the tether and the lower tether part comprises 70-30% of the length of the tether.

3. The submersible power plant according to claim 1, wherein the tether comprises an intermediate part having an average density lower than the fluid and a hydrodynamic cross section and is arranged in between the upper tether part and the lower tether part.

4. The submersible power plant according to claim 3, wherein the length of the upper tether part is between 20-40% of the length of the tether, the length of the intermediate tether part is between 20-60% and the length of the lower tether part is between 10-20% of the length of the tether.

5. The submersible power plant according to claim, 1, wherein the vehicle of the power plant has an average density lower than the fluid.

6. The submersible power plant according to claim 1, wherein the fluid is water and the average density of the lower tether part is between 700-900 kg/m3, specifically between 750-850 kg/m3, more specifically 800 kg/m3 and the average density of the upper tether part is between 1050-1250 kg/m3, specifically between 1100-1200 kg/m3, more specifically 1160 kg/m3.

7. The submersible power plant according to claim 3, wherein the fluid is water and the average density of the intermediate tether part is between 700-900 kg/m3.

8. The submersible power plant according to claim 1, wherein the tether comprises a shell member which forms the outer shape of the tether.

9. The submersible power plant according to claim 8, wherein the shell member comprises at least one of an elastomeric material, a thermoplastic material, a thermoset material, a carbon fibre laminate, a glass fibre laminate, a composite material, a material comprising polyurethane, a polyurethane elastomer material, steel and/or combinations thereof.

10. The submersible power plant according to claim 8, wherein the shell member comprises an outer layer of fibre, or composite or laminates, wherein an inner region is filled with filler material.

11. The submersible power plant according to claim 10, wherein the density of the lower part is adjusted by adding gas filled containers to the inner region of the lower tether part.

12. The submersible power plant according to claim 1, wherein the density of the lower tether part is adjusted by attaching elements with a density lower than the surrounding fluid to the outside of the tether.

13. The submersible power plant according to claim 1, wherein the vehicle comprises: a nacelle comprising a turbine connected to a generator, the turbine being driven by the movement of the vehicle; and front struts and a rear strut arranged to attach the vehicle to the tether.

14. The submersible power plant according to claim 13, wherein the upper tether part connects to the vehicle by a top joint.

15. The submersible power plant according to claim 1, wherein the lower tether part connects to the structure by a bottom joint.

16. The submersible power plant according to claim 1, wherein the tether is flexible.

17. Method for control of a submersible power plant, wherein the method comprises: arranging a tether connecting a submersible power plant with a structure, wherein the tether comprises an upper tether part and a lower tether part; arranging the upper tether part to have an average density higher than the surrounding fluid; arranging the upper tether part to have a hydrodynamic cross section; arranging the upper tether part to be connected to the vehicle; arranging the lower tether part to have an average density lower than the surrounding fluid; arranging the lower tether part to have a non-hydrodynamic cross section; and arranging the lower tether part to be connected to the structure, wherein when the submersible power plant moves in a predetermined trajectory, the tether of the submersible power plant experiences a reduction in tether vibrations induced by whiplash, and wherein when the submersible plant does not move in a predetermined trajectory, the tether of the submersible power plant forms an S-shape due to the difference in average density between a vehicle of the power plant, the upper tether part and the lower tether part.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] FIG. 1 schematically shows a power plant according to example embodiments of the application,

[0041] FIGS. 2a and 2b schematically shows two alternative embodiments of a tether,

[0042] FIG. 3 schematically shows a cross sectional view of an upper tether part of a tether,

[0043] FIG. 4 schematically shows the power plant during slack water.

DETAILED DESCRIPTION

[0044] FIG. 1 schematically shows a submersible power plant 1 according to example embodiments of the application. The submersible power plant 1 is submerged in a fluid and comprises a structure 2 and a vehicle 3 comprising at least one wing 4. The vehicle 3 is arranged to be secured to the structure 2 by means of at least one tether 5. The vehicle 3 is arranged to move in a predetermined trajectory 6 by means of a fluid stream passing the vehicle 3. The predetermined trajectory may be a figure eight, a circle, an oval or another suitable closed trajectory. In FIG. 1 the direction of the fluid stream is pointing essentially into the figure. The fluid stream can for instance be an ocean current, a tidal stream or a river stream.

[0045] The vehicle 3 further comprises front struts 7 and a rear strut 8. The vehicle 3 may comprise a nacelle 9 which is attached to the wing 4. The nacelle 9 may be positioned below or above the wing 4 and is attached to the wing 4 for instance by means of a pylon. The vehicle 3 may further comprise control surfaces, for instance in the form of a vertical rudder 10. The front struts 7 are attached to the wing 4 and the rear strut 8 is in one example embodiment attached to the nacelle 9. The vehicle 3 is steered along the predetermined trajectory 6 by means of a control system that may control one or more control surfaces or other steering means. The control system can be implemented for instance by means of one or more on-board CPUs or control circuit boards or by signals sent from a remote control centre.

[0046] The nacelle 9 comprises a turbine 11 rotatably connected to a generator 12. The movement of the vehicle 3 through the fluid causes the turbine 11, and thereby the generator 12, to rotate. In this way electrical power is generated. The submersible plant comprises a power take off system feeding the electrical power through electrical cables in the tether 5 to an electricity supply network, which in turn transfers the power to a power grid.

[0047] The tether 5 comprises an upper tether part 5a and a lower tether part 5b. The upper tether part 5a has a hydrodynamic profile or cross section and has an average density higher than the fluid in the fluid stream. The lower tether part 5b has a non-hydrodynamic profile or cross section and has an average density lower than the fluid in the fluid stream. The upper tether part 5a connects to the vehicle 3 by means of a top joint 13 to which the struts are attached. The lower tether part 5b connects to the structure 2 by means of a bottom joint 14.

[0048] FIGS. 2a and 2b schematically shows two alternative embodiments of a tether 5. In FIG. 2a the transition between the upper tether part 5a and the lower tether part 5b is distinct meaning that there is no transition part between the upper tether part 5a and the lower tether part 5b. The hydrodynamic profile of the upper tether part 5a ends at a transition point 15 between the upper tether part 5a and the lower tether part 5b where the non-hydrodynamic profile of the lower tether part 5b continues. In FIG. 2b the upper tether part 5a and the lower tether part 5b transitions from the hydrodynamic shape of the upper tether part 5a to the non-hydrodynamic shape of the lower tether part 5b by means of a transition part 5c. The transition part 5c can take any suitable intermediate shape.

[0049] The upper tether part 5a and the lower tether part 5b can be connected in a number of ways as long as the mechanical connection between the upper tether part 5a and lower tether part 5b is made strong enough to meet the force requirements of the respective upper tether part 5a and the lower tether part 5b.

[0050] FIG. 3 schematically shows a cross sectional view of an upper tether part 5a of a tether 5 according to one example embodiment. The cross section of the upper tether part 5a is hydrodynamic and can have any suitable airfoil or hydrofoil shape. Hence, the outer shape may have/form a wing-shaped, or drop-shaped, cross-sectional profile, or a wing-like structure. Hence, according to an exemplifying embodiment, the cross-sectional profile of the upper tether part 5a corresponds to a wing profile, which provides reduced drag in relation to a non-wing profiled cross-section having the same effective thickness in relation to the relative flow direction of the liquid. Furthermore, with a wing profile, the effective thickness in relation to the relative flow direction of the liquid may be reduced while maintaining the same cross-sectional area of a tensile force bearing portion, which may further reduce the drag.

[0051] The lower tether part 5b can have any suitable non-hydrodynamic cross section, for example axisymmetrical shapes such as elliptical, circular or oval. The length of the tether 5 may be between 1 and 500 meters, specifically between 20 and 300 meters, more specifically between 30 and 200 meters.

[0052] The upper tether part 5a comprises at least one shell member 15 which forms the outer shape of the upper tether part 5a. The shell member 15 comprises at least one of an elastomeric material, a thermoplastic material, a thermoset material, a carbon fibre laminate, a glass fibre laminate, a composite material, a material comprising polyurethane, a polyurethane elastomer material, or other suitable materials, and/or combinations thereof. Alternatively, the shell member 15 may comprise an outer layer(s) of fibre, or composite, laminates, wherein an inner region may be filled with filler material. As can be seen from FIG. 3, various cables run through the tether 5. Examples of cables running through the tether 5 are power and data communication cables. Additionally a tensile force bearing member runs through the tether 5 to provide an elastic tether 5 and to allow for a flexible and thus robust and logistically beneficial tether 5, e.g. allowing for coiling or winding. For example, the tensile force bearing portion comprises UHMWPE (Ultra-high-molecular-weight polyethylene), for example Dyneema or similar high performance fibres. Furthermore, a steel wire rope, or steel wire ropes, may be utilized as tensile force bearing portion, or as tensile members. Preferably, the entire tether 5 is elastic.

[0053] The lower tether part 5b comprises at least one shell member which forms the outer shape of the lower tether part 5b. The shell member comprises at least one of an elastomeric material, a thermoplastic material, a thermoset material, a carbon fibre laminate, a glass fibre laminate, a composite material, a material comprising polyurethane, a polyurethane elastomer material, or other suitable materials, and/or combinations thereof. Alternatively, the shell member may comprise an outer layer(s) of fibre, or composite, laminates, wherein an inner region may be filled with filler material. As with the upper tether part 5a, cables run through the lower tether part 5b. Examples of cables running through the tether 5 are power and data communication cables. Additionally a tensile force bearing member runs through the tether 5 to provide an elastic tether 5 and to allow for a flexible and thus robust and logistically beneficial tether 5, e.g. allowing for coiling or winding. For example, the tensile force bearing portion comprises UHMWPE (Ultra-high-molecular-weight polyethylene), for example Dyneema or similar high performance fibres. Furthermore, a steel wire rope, or steel wire ropes, may be utilized as tensile force bearing portion, or as tensile members.

[0054] FIG. 4 schematically shows the submersible power plant 1 during slack water. According to example embodiments of the invention the submersible power plant 1 comprises a tether 5 that is capable of handling the conditions of both movement along a predetermined trajectory 6 as well as keeping a good position in slack water. A tether 5 comprising an upper tether part 5a having an average density higher than the fluid, has a hydrodynamic cross section and is arranged to be connected to the vehicle 3 and a lower tether part 5b having an average density lower than the fluid, has a non-hydrodynamic cross section and is arranged to be connected to the structure 2 allows for the submersible power plant 1 to handle the conditions of slack water well.

[0055] In FIG. 4 it can be seen that the submersible plant 1 comprises three power plant sections with different buoyancy. The first power plant section is the vehicle 3 itself which has positive buoyancy and will strive to reach the surface as indicated by the arrow next to the vehicle. The buoyancy of the vehicle 3 can be adjusted by implementing one or more known buoyancy techniques, for instance in the wing 4. The second section is the upper tether part 5a which has negative buoyancy. The negative buoyance is achieved for instance by adjusting the amount of material used to form the upper tether part 5a or by using materials with various densities. This part thus sinks which is indicated by the arrow next to the upper tether part 5a. The third power plant section is the lower tether part 5b which has positive buoyancy. The positive buoyancy is achieved for instance by having a shell member comprising an outer layer of fibre, or composite or laminates, wherein an inner region may be filled with filler material. The density of the lower part is thus controlled by adding gas filled containers to the inner region of the lower tether part 5b. Alternatively, the density of the lower tether part 5b is controlled by attaching elements to the outside of the tether 5 having a density lower than the surrounding fluid. The lower tether part 5b will strive to reach the surface as indicated by the arrow.

[0056] The effect of the varying densities of the three power plant sections is that the tether 5 during slack water forms a non-linear shape, preferably a figure S-shape due to that the average density of the vehicle 3 of the power plant 1, the upper tether part 5a and the lower tether part 5b are different as described above. Another effect is that it is possible to control the position of the vehicle 3 either in relation to the surface of the body of fluid in which the power plant 1 is submerged, indicated by depth d1, or in relation to a bottom surface over which the vehicle 3 moves, indicated by depth d2, or both.

[0057] Another advantage of the non-linear shape is that the vehicle 3 and tether 5 strives to approach each other. The principle behind this is that when a flexible body having two ends, e.g. a tether, experiences a force on the middle of the body, the two ends will strive to move towards each other while the body forms an arc. The first tether part is attached to the vehicle 3 and the lower tether part 5b. When the upper tether part 5a sinks due to having a higher density than the fluid a first end part 16 and a second end part 17 of the upper tether part 5a strives to move towards each other as the upper tether part 5a forms an arc. A third end part 18 and a fourth end part 19 of the lower tether part 5b displays the same behaviour as they are in turn attached to the upper tether part 5a and the structure 2. Arrows 16a, 17a, 18a, 19a next to the end parts 16, 17, 18, 19 aim to illustrate the forces acting on the respective end part. As the fourth end part 19 is fixed to the structure 2 and cannot move sideways this results in that the vehicle 3 as well as the upper tether part 5a moves sideways towards the structure 2. The resulting forces on the different parts of the tether 5 and vehicle 3 makes the tether 5 and vehicle 3 move towards the structure 2 as indicated by arrow 20. The lower tether part 5b, with its positive buoyancy strives to right itself in an upright position. All these effects aim towards reducing or completely removing the risk of the tether 5 tangling, twisting or otherwise damaging the tether 5. The non-linear shape and the movement of the vehicle 3 towards the structure 2 also improves the handling of the power plant 1 when the direction of the fluid stream changes direction, for instance for a tidal stream.

[0058] FIG. 5 schematically shows a submersible power plant 1 according to a second example embodiment. The submersible power plant 1 is submerged in a fluid and comprises a structure 2 and a vehicle 3 comprising at least one wing 4. The vehicle 3 is arranged to be secured to the structure 2 by means of at least one tether 5. The vehicle 3 is arranged to move in a predetermined trajectory 6 by means of a fluid stream passing the vehicle 3. In FIG. 1 the direction of the fluid stream is pointing essentially into the figure. The fluid stream can for instance be an ocean current, a tidal stream or a river stream.

[0059] The vehicle 3 further comprises front struts 7 and a rear strut 8. The vehicle 3 may comprise a nacelle 9 which is attached to the wing 4. The nacelle 9 may be positioned below or above the wing 4 and is attached to the wing 4 for instance by means of a pylon. The vehicle 3 may further comprise control surfaces, for instance in the form of a vertical rudder 10. The front struts 7 are attached to the wing 4 and the rear strut 8 is in one example embodiment attached to the nacelle 9. The vehicle 3 is steered along the predetermined trajectory 6 by means of a control system that may control one or more control surfaces or other steering means. The control system can be implemented for instance by means of one or more on-board CPUs or control circuit boards or by signals sent from a remote control centre.

[0060] The nacelle 9 comprises a turbine 11 rotatably connected to a generator 12. The movement of the vehicle 3 through the fluid causes the turbine 11, and thereby the generator 12, to rotate. In this way electrical power is generated. The submersible plant comprises a power take off system feeding the electrical power through electrical cables in the tether 5 to an electricity supply network, which in turn transfers the power to a power grid.

[0061] The tether 5 comprises an upper tether part 5a, a lower tether part 5b and an intermediate tether part 5d. The upper tether part 5a has a hydrodynamic profile or cross section and has an average density higher than the fluid in the fluid stream. The lower tether part 5b has a non-hydrodynamic profile or cross section and has an average density lower than the fluid in the fluid stream. The intermediate tether part 5d has a hydrodynamic profile or cross section and has an average density lower than the fluid in the fluid stream. The upper tether part 5a connects to the vehicle 3 by means of a top joint 13 to which the struts are attached. The lower tether part 5b connects to the structure 2 by means of a bottom joint 14.

[0062] The upper tether part 5a and the intermediate tether part 5d can be connected in a number of ways as long as the mechanical connection between the upper tether part 5a and intermediate tether part 5d is made strong enough to meet the force requirements of the respective upper tether part 5a and the intermediate tether part 5d. The intermediate tether part 5d and the lower tether part 5b can be connected in a number of ways as long as the mechanical connection between the intermediate tether part 5d and lower tether part 5b is made strong enough to meet the force requirements of the respective intermediate tether part 5d and the lower tether part 5d. See also the figure description of FIGS. 2a and 2b for example connections/transitions between tether parts.

[0063] The intermediate tether part 5d is made as the upper tether part 5a, differing in density.

[0064] Reference signs mentioned in the claims should not be seen as limiting the extent of the matter protected by the claims, and their sole function is to make claims easier to understand.

[0065] As will be realised, the invention is capable of modification in various obvious respects, all without departing from the scope of the appended claims. Accordingly, the drawings and the description are to be regarded as illustrative in nature, and not restrictive.