ARRANGEMENT IN A WAVE ENERGY RECOVERY APPARATUS AND METHOD FOR OPERATING A WAVE ENERGY RECOVERY APPARATUS

Abstract

This invention relates to an arrangement in a wave energy recovery apparatus and to a method for operating the wave energy recovery apparatus. The apparatus comprises at least a base (1), on which a reciprocating panel (2) is installed, a pivot shaft (7) for the reciprocating panel (2), a control system, and one or more power-take-off (PTO) units (3) to convert kinetic energy of waves or tidal currents to another type of energy. The arrangement comprises adjustment means (5, 5, 5) to set the top of the panel (2) in its vertical position to approximately correspond to the altitude of the surface (8) of the water.

Claims

1. Arrangement in a wave energy recovery apparatus comprising at least a base (1), on which a reciprocating panel (2) is installed, a pivot shaft (7) for the reciprocating panel (2), a control system, and one or more power-take-off (PTO) units (3) to convert kinetic energy of waves or tidal currents to another type of energy, and the arrangement also comprises adjustments means (5, 5, 5) to set the top of the panel (2) in its vertical position to approximately correspond to the altitude of the surface (8) of the water, characterized in that the first end of each PTO unit (3) is connected pivotally (5d) to the first adjustment means (5) and the second end of each PTO unit (3) is connected to an extendible element (5b) of the second adjustment means (5), and that the axial length of the second adjustment means (5) is adjustable by the extendible element (5b).

2. Arrangement in a wave energy recovery apparatus according to claim 1, characterized in that the arrangement comprises monitoring means to watch the altitude of the surface (8) of the water, which the monitoring means are connected at least to the adjustment means (5) to make the top of the panel (2) in its vertical position to follow the altitude of the surface (8) of the water when the altitude varies between the low and high tide.

3. Arrangement in a wave energy recovery apparatus according to claim 1, characterized in that the panel (2) together with each PTO unit (3) is supported on a base (1) by the adjustment means (5 and 5).

4. Arrangement in a wave energy recovery apparatus according to claim 1, characterized in that the height position of the panel (2) is arranged to be adjusted by moving the panel (2) and one end of each PTO unit (3) substantially simultaneously upwards and downwards.

5. (canceled)

6. Arrangement in a wave energy recovery apparatus according to claim 1, characterized in that the dimensioning of the arrangement is arranged so that the distance between the surface (8) of the water and the first adjustment means (5) on the base (1) is such that the first end (3a) of the PTO unit (3) or the corresponding point of the connection element (4, 4a) reaches to the fastening and hinge point (5d) of the first adjustment means (5) at the same time when the panel (2) still floats on the surface (8) of the water.

7. Arrangement in a wave energy recovery apparatus according to claim 6, characterized in that the distance from the centerline of the pivot shaft (7) to the connection point (3c) in the apparatus is equal or greater than the distance from the surface (8) of the water to the hinge point (5d) on the base (1).

8. Arrangement in a wave energy recovery apparatus according to claim 1, characterized in that the panel (2) is connected to each PTO unit (3) with a separate connection shaft (11) between the pivot shaft (7) and the input shaft (10) of the PTO unit (3).

9. Arrangement in a wave energy recovery apparatus according to claim 8, characterized in that the connection shaft (11) comprises a conical part (17) that is arranged to match with a conical counterpart at the end of the pivot shaft (7) and that is arranged to be tightened into the pivot shaft (7) with tightening element (12).

10. Arrangement in a wave energy recovery apparatus according to claim 8, characterized in that the connection shaft (11) and the input shaft (10) of the PTO unit (3) are connected together with a connection mechanism (6) that comprises, for instance two adjacent connection flanges (10a, 11a) that are fastened together.

11. Arrangement in a wave energy recovery apparatus according to any of the claims above, characterized in that the main bearings (13) of the pivot shaft (7) are inside each PTO unit (3) or between each PTO unit (3) and each end of the pivot shaft (7).

12. Arrangement in a wave energy recovery apparatus according to claim 1, characterized in that the base (1) comprises a channel (20) that is guided to run underneath the panel (2) so that the first end of the channel (20) is in the front side of the panel (2) and the second end of the channel (20) is in the back side of the panel (2), and that both the ends of the channel (20) are equipped with a hinged hatch (19) that is be openable and closable.

13. Arrangement in a wave energy recovery apparatus according to claim 12, characterized in that the arrangement comprises spring mechanisms that are arranged to keep the hatches (19) closed in normal weather conditions and to let the hatches (19) open when the pressure of water outdoes the spring force.

14. Arrangement in a wave energy recovery apparatus according to claim 1, characterized in that the arrangement comprises the main bearings (13) of the pivot shaft (7) between each PTO unit (3) and each end of the pivot shaft (7), and an intermediate structure (1b) onto which the main bearings (13) are supported, and which intermediate structure (1b) is supported between the base (1) and each PTO unit (3) by the adjustment means (5 and/or 5).

15. Method for operating a wave energy recovery apparatus comprising at least a base (1), on which a reciprocating panel (2) is installed, a pivot shaft (7) for the reciprocating panel (2), a control system, and one or more power-take-off (PTO) units (3) to convert kinetic energy of waves or tidal currents to another type of energy, and the top of the panel (2) in its vertical position is set to approximately correspond to the altitude of the surface (8) of the water by adjusting the altitude of the panel (2) with adjustment (5, 5, 5), characterized in that during the installation phase the panel (2) with each PTO unit (3) are let to float on the surface (8) of the water approximately above the pre-installed base (1), and the method comprises at least the steps as follows: while the panel (2) and each PTO unit (3) are floating on the surface (8) of the water the first end of the wave recovery apparatus is descended to its production site and connected pivotally to hinge points (5d) of the first adjustment means (5) thereafter the second end of the wave recovery apparatus with the panel (2) are descended to their production site and the second end of the wave recovery apparatus is connected to extendible elements (5b) of the second adjustment means (5) so that the second end of the wave recovery apparatus is movable towards the surface (8) of the water and back towards the bottom (9) of the sea the installation is completed.

16. Method for operating a wave energy recovery apparatus according to claim 15, characterized in that the altitude of the surface (8) of the water is monitored with monitoring means, and with the help of the data received from the monitoring means the altitude of the top of the panel (2) in its vertical position is placed to approximately correspond to the moving altitude of the surface (8) of the water by ascending the panel (2) with the adjustment means (5) when the surface (8) of the water is ascending and descending the panel (2) with the adjustment means (5) when the surface (8) of the water is descending.

17. Method for operating a wave energy recovery apparatus according to claim 15, characterized in that the height position of the panel (2) is adjusted by moving the panel (2) and one end of each PTO unit (3) substantially simultaneously upwards and downwards continuously and substantially in the same rhythm and into the same direction with the alteration of the surface (8) of the water.

18. (canceled)

19. Method for operating a wave energy recovery apparatus according to claim 15, characterized in that the when taking the wave recovery apparatus into servicing or maintenance the second end of the wave recovery apparatus is unfastened from the second adjustment means (5) and the second end of the wave recovery apparatus with the panel (2) are ascended to the surface (8) of the water the first end of the wave recovery apparatus keeping connected to the hinge points (5d) of the first adjustment means (5).

20. Method for operating a wave energy recovery apparatus according to claim 15, characterized in that the when taking the wave recovery apparatus into servicing or maintenance the second end of the wave recovery apparatus is unfastened from the second adjustment means (5) and the second end of the wave recovery apparatus with the panel (2) are ascended to the surface (8) of the water, and thereafter the first end of the wave recovery apparatus is unfastened from the first adjustment means (5) and ascended to the surface (8) of the water.

Description

[0007] In the following, the invention will be described in detail by the aid of examples by referring to the attached simplified and diagrammatic drawings, wherein

[0008] FIG. 1 presents a simplified oblique top view of a wave energy recovery arrangement according to the arrangement of the invention, where the apparatus has been installed on a base onto the sea bottom,

[0009] FIG. 2 presents in a top view and in a simplified and diagrammatic way a wave energy recovery arrangement according to FIG. 1, equipped with two power-take-off units, and the panel being in a horizontal position,

[0010] FIG. 3 presents in a top view and in a simplified and diagrammatic way another wave energy recovery arrangement according to the invention, equipped with one power-take-off unit, and the panel being in a horizontal position,

[0011] FIG. 4 presents in a side view and in a simplified and diagrammatic way the arrangement according to the invention, where the heavy gravitational base has been anchored onto the surface of the sea bottom, and the wave recovery apparatus is floating above the base before the final installation,

[0012] FIG. 5 presents in a side view and in a simplified and diagrammatic way the arrangement according to FIG. 4 in the phase where the end of the wave recovery apparatus is descended onto the bottom of the sea,

[0013] FIG. 6 presents in a side view and in a simplified and diagrammatic way the arrangement according to FIG. 4 in the situation where the wave recovery apparatus is ready to be used in its lowermost position,

[0014] FIG. 7 presents in a side view and in a simplified and diagrammatic way the arrangement according to FIG. 4 in the situation where the wave recovery apparatus is ready to be used in its uppermost position,

[0015] FIG. 8 presents in a side view and in a simplified and diagrammatic way another arrangement according to the invention in the situation where the wave recovery apparatus is ready to be used in its lowermost position,

[0016] FIG. 9 presents in a side view and in a simplified and diagrammatic way another arrangement according to the invention in the situation where the wave recovery apparatus is ready to be used in its uppermost position,

[0017] FIG. 10 presents in a front view and in a simplified and diagrammatic way an arrangement according to the invention with one power-take-off unit at both sides of the panel, and the panel equipped with a continuous hollow shaft in its upper edge,

[0018] FIG. 11 presents in a side view, partially cross-sectioned and in a simplified and diagrammatic way a type of connection according to the invention, which connection is used to connect the panel with the power-take-off unit,

[0019] FIG. 11a presents in a side view, partially cross-sectioned and in a simplified and diagrammatic way another type of connection according to the invention, which connection is also used to connect the panel with the power-take-off unit,

[0020] FIG. 12 presents in a side view, partially cross-sectioned and in a simplified and diagrammatic way another type of connection according to the invention, which connection is used to connect the panel with the power-take-off unit,

[0021] FIG. 13 presents in a simplified oblique top view an auxiliary shaft to connect the panel with the power-take-off unit,

[0022] FIG. 14 presents a simplified oblique top view of another wave energy recovery arrangement according to the arrangement of the invention, where the apparatus has been installed on a base onto the sea bottom and equipped with an arrangement to guide a part of the water flow to pass the panel,

[0023] FIG. 15 presents in a side view, partially cross-sectioned and in a simplified and diagrammatic way a wave energy recovery arrangement according to FIG. 14,

[0024] FIG. 16 presents in a side view and in a simplified and diagrammatic way another arrangement according to the invention in the situation where the wave recovery apparatus is ready to be used in its lowermost position, and

[0025] FIG. 17 presents in a side view and in a simplified and diagrammatic way the arrangement according to FIG. 16 in the situation where the wave recovery apparatus is ready to be used in its uppermost position.

[0026] FIGS. 1 and 2 present a wave energy recovery apparatus according to the arrangement of the invention. FIG. 1 presents the wave energy recovery apparatus in its operating position on the base 1 which is installed onto the bottom of the sea, and FIG. 2 presents the wave energy recovery apparatus from above in its transporting position floating on the surface of the sea.

[0027] The wave energy recovery apparatus according to the example comprises a panel 2 reciprocating about its pivot shaft 7 installed on bearings supported by the two power-take-off (PTO) units 3 having a connection mechanism 6 to connect the PTO units 3 to the pivot shaft 7 of the reciprocating panel 2 at both ends of the panel 2. The pivot shaft 7 is at the lower edge of the panel 2 when the panel 2 is in its vertical position. Preferably the pivot shaft 7 is at least as long as the panel 2 is wide. In this way it helps to support the panel 2 and to align the bearings in the same line. The first ends 3a of the PTO units 3 are connected to each other by a connection element 4 that can be for instance a hollow beam. The connection element 4 could be also repeated at the mid length of the PTO units 3 for instance in case that the panel 2 is wider.

[0028] The first ends 3a of the PTO units 3 are supported on the base 1 by first adjustment means 5 which are, for instance, support elements whose length is preferably fixed but adjusted to be suitable exactly to the intended production site. The second ends 3b of the PTO units 3 are not connected to each other but supported on the base 1 by second adjustment means 5 which are, for instance, support elements whose length in the axial or vertical direction is arranged to be adjustable. The wave energy recovery apparatus comprises also a supply means to provide the wave energy captured to further use. The supply means may comprise for example electric cables, water pipes, etc.

[0029] FIG. 3 presents in a top view and in a simplified and diagrammatic way another wave energy recovery arrangement according to the invention. In this case the wave energy recovery apparatus is equipped with one PTO unit 3 only. In order to achieve a similar support structure as in the apparatus of FIG. 2 the first end 3a of the PTO unit 3 is connected to a rectangular connection element 4a that forms the same kind of a frame as in the structure of FIG. 2, where the first end of the connection element 4a connected with the first end 3a of the PTO unit 3 and the second end of the connection element 4a connected with the pivot shaft 7 or its extension. Also this type of the wave energy recovery apparatus comprises a connection mechanism 6 to connect the pivot shaft 7 of the reciprocating panel 2 at both ends of the panel 2 with both the PTO unit 3 and the second end of the connection element 4a. In this wave energy recovery apparatus the connection element 4a can also be a hollow beam.

[0030] FIGS. 4-9 present the wave energy apparatus according to the arrangement of the invention in four different situations, from the beginning of the installation to the final working situation when the tide is at its highest point. All the main parts, such as the panel 2, PTO units 3, connection elements 4, 4a and the pivot shaft 7 of the wave recovery apparatus can be hollow or equipped with pontoons so that the whole wave recovery apparatus can float on the surface 8 of the sea.

[0031] FIG. 4 presents the situation where the wave recovery apparatus has brought to its production site and is floating on the surface 8 of the sea above the base 1 that is pre-installed onto the sea bottom 9. The first end of the base 1 is equipped with the first adjustment means 5 which comprise, for instance, first support elements 5c whose length in the axial or vertical direction is fixed, and which first support elements 5c are equipped with hinge elements 5d which are designed to be pivotally connected to the connection point 3c at the first end of each PTO unit 3.

[0032] Correspondingly the second end of the base 1 is equipped with the second adjustment means 5 which comprise, for instance, second support elements 5a whose length in the axial or vertical direction is adjustable. For that purpose the second support elements 5a are equipped with extendible elements 5b, such as piston rods, rack and pinion, or rotating threaded bar, which are designed to be connected to the second ends of the PTO units 3.

[0033] FIG. 5 presents the situation where the first end of the wave recovery apparatus is descended to its production site onto the bottom 9 of the sea. The dimensioning of the arrangement is arranged so that the distance between the surface 8 of the water and the adjustment means 5 and/or 5 of the base 1 is such that the first end 3a of the PTO unit 3 or the corresponding point of the connection element 4, 4a reaches to the fastening and hinge point 5d of the first support elements 5c at the same time when the panel 2 still floats on the surface 8 of the water. It can also be mentioned that the distance from the centerline of the pivot shaft 7 to the connection point 3c is equal or greater than the distance from the surface 8 of the water to the hinge point 5d. And it can also be noticed that as far as a realistic depth the deeper the water the longer the support elements 5a and 5c can be.

[0034] However, if the depth of the water is so great that the length of the support elements 5a and 5c cannot be realistically increased the base 1 cannot be installed directly onto the bottom 9 of the sea but the base 1 has to be installed on separate adjustment means 5 that comprise, for instance, support legs 1a that are fastened to the bottom 9 of the sea. The length of the support legs 1a can be varied so that the base 1 can be installed always at the correct distance from the surface 8 of the water regardless of the depth of the water at the production site. FIGS. 8 and 9 present that kind of a situation. In that case the heavy gravitational base 1 is replaced with lighter concrete or steel frame base 1. Also other solutions to install the base 1 at a certain height can be used.

[0035] The correct dimensioning makes the servicing or maintenance also easier and more cost effective. For the maintenance only the second end of the apparatus must be unfastened from the second adjustment means 5 and the PTO units 3 can be turned upwards into about a vertical position around the hinge points 5d. After that the panel 2 floats on the surface 8 of the water as shown, for instance in FIG. 5. If that is not enough for the maintenance the first end of the PTO units 3 can also be unfastened from the first adjustment means 5 and the first end of the PTO units 3 can be ascended to the surface 8 of the water. The situation then corresponds what is shown in FIG. 4.

[0036] As mentioned above in the situation of FIG. 5 the first end of the wave energy apparatus has been descended down to the water and connected pivotally to its fastening points with the first support elements 5c and the hinge elements 5d. The panel 2 is still floating on the surface 8 of the water and is supporting the second ends 3b of the PTO units 3 or the connection element 4a about on the surface 8 of the water. After the first end of the apparatus is fastened to the first support elements 5c the second end of the apparatus is also descended down to water and fastened with the second support elements 5a through the extendible elements 5b. At the same time the pivot shaft 7 and the panel 2 descent also into the water.

[0037] FIG. 6 presents the situation where the wave energy recovery apparatus has been installed onto its production site and is functioning in a normal way in the situation of a low tide. The second adjustment means 5 are in their lowermost positions and the top of the upper part of the panel 2 reaches in its vertical position to just about the surface 8 of the water.

[0038] FIG. 7 presents the situation where the wave energy recovery apparatus is functioning in a normal way in the situation of a high tide. The second adjustment means 5 are in their uppermost positions and the top of the upper part of the panel 2 reaches again in its vertical position to just about the surface 8 of the water. The length adjustment of the second adjustment means 5 can be made in different ways. For instance, the arrangement comprises monitoring means or follow-up means to follow the height position or altitude of the surface 8 of the water substantially continuously and when the height of the water increases the monitoring means instructs, through the control system of the apparatus, the second adjustment means 5 to extend the length of the second support elements 5a by pushing the extendible elements 5b outwards from the cylinder part of the support elements 5a, which causes the panel 2 to ascend towards the surface 8 of the water preferably the same travel as the water has ascended, and vice versa when the height of the water decreases. Then the monitoring means instructs, through the control system of the apparatus, the second adjustment means 5 to pull the extendible elements 5b inwards to the cylinder part of the support elements 5a, which causes the panel 2 to descend towards the bottom of the sea.

[0039] The solution according to FIGS. 8 and 9 is otherwise the same as the solution according to FIGS. 4-7 but the base 1 has been installed at a certain height between the surface 8 of the water and the bottom 9 of the sea. The correct height position of the base 1 can be achieved in different ways. Here it has been done with separate support legs la that act as fixed adjustment means 5, but the base 1 could also be floating and anchored to the bottom 9 of the sea at a wanted altitude level between the surface 8 of the water and the bottom 9 of the sea.

[0040] FIGS. 10-13 present different connections between the pivot shaft 7 of the panel 2 and the PTO units 3. These connections are special connections which are arranged to make it possible to use the second adjustment means 5 to set or move the top of the panel 2 in its vertical position to approximately correspond the altitude of the surface 8 of the water in all situations. It is preferable that related to the adjustment of the height position of the panel 2 each PTO unit 3 moves upwards and downwards along with the panel 2. That is why the proper connection with the pivot shaft 7 of the panel 2 and each PTO unit 3 is significant.

[0041] One advantageous way to connect the pivot shaft 7 of the panel 2 to the PTO unit 3 is to make the pivot shaft 7 hollow and substantially as long as the width of the panel 2, and to fasten the pivot shaft 7 at a lower edge of the panel 2. That kind of solution maintains the stiffness of the pivot shaft 7 and its bearings. In order to make the apparatus easy to install and easy to take for maintenance it is preferable to place a separate connection shaft 11 between the pivot shaft 7 and the input shaft 10 of the PTO unit 3. In the solutions according to FIGS. 10, 11 and 11a the main bearings 13a of the panel 2 are inside the PTO 3 on the input shaft 10, whereas in the solution of FIG. 12 the main bearing 13 is between the pivot shaft 7 and the PTO 3 on the connection shaft 11. Regardless of the cylindrical part 15 for the main bearing 13 the connection shaft 11 can be about similar in all the solutions. The connection shaft 11 and the input shaft 10 of the PTO unit 3 are connected together with the connection mechanism 6 that comprises, for instance two adjacent connection flanges 10a, 11a that are bolted together. The first connection flange 10a is at the end of the input shaft 10 of the PTO unit 3 outside the PTO unit 3, and the second connection flange 11a is at the first end of the connection shaft 11 outside the first end of the pivot shaft 7.

[0042] In the solution of FIG. 1 la the connection shaft 11 is pushed through the PTO unit 3 into the pivot shaft 7 so that the connection mechanism 6 is arranged at the opposite side of the PTO unit 3 than the panel side. In that case the input shaft 10 of the PTO unit 3 is hollow and the connection shaft 11 is installed through the input shaft 10. The solution of the hollow input shaft 10 makes the easier installation possible. In that case the panel 2 and the PTO unit 3 can be first placed side by side without the connection shaft 11, and after that the connection shaft 11 can be pushed into its location through the input shaft 10 of the PTO unit 3. That kind of the structure also improves fatigue durability.

[0043] The flanges 10a, 11a can be complemented or replaced with different kinds of connection mechanisms, for example with axial grooves and counter grooves or axial teeth and counter teeth on the periphery or outer circle of the shafts 10 and 11, or with other suitable solutions. In case of the solution of FIG. 11a the axial connection grooves and/or teeth of the input shaft 10 are on the inner circle of the input shaft 10 and the counter grooves or teeth are on the periphery or outer circle of the connection shaft 11.

[0044] The second end of the connection shaft 11 comprises threads 18 for a tightening element 12, such as a nut, by which the connection shaft 11 is tightened at its place in the both ends of the hollow pivot shaft 7. In addition the connection shaft 11 comprises a conical part 17 that matches with a conical counterpart at the end of the pivot shaft 7. The conical part 17 makes the sufficient tightening possible when the tightening element 12 is used. Further the connection shaft 11 comprises a first cylindrical part 16 and a second cylindrical part 15 when the bearing solution according to FIG. 12 is used. Finally the first end of the connection shaft 11 comprises threaded holes 14 for fastening the second flange 11a to the end of the connection shaft 11. The form and parts of the connection shaft 11 can vary depending on the mechanical structures between the end of the pivot shaft 7 and the PTO 3.

[0045] The hollow pivot shaft 7 also works as a flooding tank. Any leaks in the upper buoyancy tank of the panel 2 are lead down to this lower tank and thus loads due to possible accidental flooding can be reduced. The arrangement can also comprise a water alarm system to give an early warning for leakages in the buoyancy tank. This arrangement gives time to plan a recovery during a safe weather window.

[0046] FIGS. 14 and 15 present another wave energy recovery arrangement according to the arrangement of the invention. In this case the wave energy recovery apparatus has been installed on a thicker base 1 onto the sea bottom and equipped with an arrangement to guide a part of the water flow to pass the panel 2. The base 1 comprises a channel 20 that is guided to run underneath the panel 2 so that the first end of the channel 20 is in the front side of the panel 2 and the second end of the channel 20 is in the back side of the panel 2. Both the ends of the channel are equipped with a hinged hatch 19 that can be opened and closed. Preferably the hatches 19 are closed with spring mechanisms that are not shown in the figures, and the hatches 19 are arranged to open by the pressure of water against the hatches 19. In normal situations the spring mechanisms keep the hatches 19 closed but in the stormy conditions when the pressure of water is high the pressure outdoes the spring force and the first hatch 19 that is in front of the panel 2 in the direction of water flow turns down to open the channel 20 as is shown in FIG. 15. Then the water pressure in the channel 20 pushes the second hatch 19 that is behind the panel 2 in the direction of water flow open and the excess water flow can pass the panel 2. The same works also in the opposite direction, if needed, but usually the water flow from the direction of the shore is not so strong that the passing of water flow would be needed. The arrangement can also comprise actuators by which the hatches 19 are opened instead of the opening that takes place by the pressure of water.

[0047] FIGS. 16 and 17 present another arrangement according to the invention in the situations where the wave recovery apparatus is in its lowermost position and its uppermost position. These situations correspond to situations explained in connection with FIGS. 6 and 7 but now the structure of the arrangement is different. In this solution the main bearings 13 of the pivot shaft 7 are between the PTO units 3 and the pivot shaft 7 as shown in FIG. 12. In that case the arrangement needs an intermediate structure 1b onto which the main bearings 13 are supported. The intermediate structure 1b can be a solid plate-like base or it may comprise, for instance, two separate support beams that are supported by the adjustment means 5 and 5 and which base or support beams themselves support the main bearings 13. This solution makes it possible to use main bearings 13 outside the PTO units 3, and at the same time the follow-up of the surface of the water is possible during the variations of the tide.

[0048] It is common to all of the embodiments of the invention that the panel 2 is installed on the base 1 so that the variation of the height or altitude of the water surface, for example during the low and high tide can be followed and the vertical position of the panel 2 can adjusted according the height of the water surface so that the recovery capacity is all the time as efficient as possible.

[0049] It is obvious to the person skilled in the art that the invention is not restricted to the examples described above but that it may be varied within the scope of the claims presented below. Thus, for example, the structure of the arrangement can be different from what is presented.