Patent Application
20120090313
The present invention pertains to a device for a floating wave power plant. More precisely, the invention pertains to a submerged frame structure with a ballast element being an element in a floating wave power plant. The frame structure and thereby the wave power plant is preferably anchored to the seabed.
The wave power plant is presumed to absorb energy from the sea waves by means of a number of buoyancy elements. Each buoyancy element is connected to a pumping cylinder, and when the waves make the buoyancy element rise and descend, the buoyancy element's mechanical work is transformed into pressure energy in the flowing medium delivered by the pumping cylinder to a collecting pipe system. Each buoyancy element is connected to the frame structure, preferably at the cylinder's lower end, via a bearing which allows the cylinder's longitudinal axis to rotate freely to adjust to the buoyancy element's direction of movement when the direction of the buoyancy element's lifting force deviates from the vertical direction. The pressure energy of the flowing medium can be transformed into electrical energy in a power aggregate comprising a turbine and an electrical generator.
A pumping cylinder with a first and a second end, which at one end is connected to a buoyancy element for the purpose of producing work, as herein described, must be restrained by a reaction force in the cylinder's longitudinal direction at the second end, the reaction force being equal to the force from the buoyancy element, corrected for buoyancy and mass forces of the buoyancy element itself. Defined as work per unit time, the power absorbed by the pumping cylinder is at any time equal to the force in the piston rod multiplied with the velocity of the piston relative to the cylinder. The power exerted by the pumping cylinder is reduced if the cylinder's connection point in the frame structure yields to the force from the buoyancy element, because the piston's velocity relative to the cylinder is then reduced. If the cylinder's connection point yields to the buoyancy element's force, it will also take some time before the cylinder is back in its original position, and if the connection point is not back in its original position at the start of the next wave cycle, the cylinder's ability to exert work will be further reduced in that wave cycle. In order for the pumping cylinder to be able to produce as much work as possible, its connection point to the frame structure should ideally be at rest, and in practical terms that means that the movement of the cylinder's connection point should be small compared to the movement of the buoyancy element.
I addition to creating a virtually stationary restraint against the forces from the buoyancy elements, the challenge is also to make a robust structure without being too expensive to build, and at the same time being able to convert enough energy from the waves to give the power plant an acceptable economic situation.
From prior art pertaining to buoyancy elements connected to frames reference is made to WO/0196738 showing a frame kept floating by floats which do not exert power themselves. A separate power producing float is connected to a pumping cylinder on the frame structure and is thereby able to exert pump work.
Further reference is made to a solution for arranging floats on a floating structure utilized on Fred Olsen's floating test rig “Buldra”, as described in WO9004718 and WO2004113718.
Further reference is made to U.S. Pat. No. 7,444,811 where the buoyancy elements' movement converts mechanical energy directly to electrical energy by means of linear generators.
A solution where the frame structure is carried by the working floats alone is described in U.S. Pat. No. 4,742,241.
Furthermore a solution for providing a restraint for a singular buoyancy element is described in WO2006126887, showing a structure resembling an umbrella turned upside down. This body will exert a significant resistance to the movement in the water due to of its large exposed surface to the mass of water, but it will yield to some extent depending of its size relative to the size of the attached buoyancy element.
A further solution is to let the pumping cylinders have fixed connections to the seabed. This will be a viable solution where the water depth is limited. It may however also be interesting to extract energy from sea waves where the water depth is greater.
A solution where the buoyancy elements are not held down by pumping cylinders but rather are linked together is used in the wave power plant Pelamis. Several oblong floating elements are interconnected by special link joints where hydraulic pumps are placed. The pumps are driven by the forces and the movements that occur when this “sea snake” is moving in the waves. A similar principle is employed in NO326156 where the absorption of energy in the links is transformed directly into electrical energy as each joint is connected to a rotor that moves relative to a stator.
With the abovementioned challenges and known solutions in mind, the present invention brings forward a device for a floating wave power plant comprising a number of pumping cylinders connected to a number of buoyancy elements via connecting elements which buoyancy elements move the pistons of the pumping cylinders in full or partial pump strokes under the influence of waves. The invention is characterized by the feature that the pumping cylinders are connected to a submerged frame structure, and that the frame structure is connected to one or more ballast bodies via a number of connecting elements.
The connecting elements may be different forms of cable, wire, chain or similar, which mainly may be flexible with respect to bending but which should have low elasticity with respect to tension forces.
In the preferred embodiment of the invention, the mass of the ballast body and the and physical dimension of the frame structure are made to suit the number of buoyancy elements and the buoyancy of the buoyancy elements such that the frame structure obtains an inertia with respect to linear acceleration and angular acceleration. Furthermore the average value of the lifting force of the combined buoyancy elements will vary less over time than the average value of the lifting force of the individual buoyancy elements over time. As a result the frame structure provides a virtually stationary underwater downhaul for the pumping cylinders.
The wave power plant absorbs energy from the waves by means of a greater number of buoyancy elements. Each buoyancy element is connected to a pumping cylinder, and when the buoyancy element is lifted or lowered by the waves, the buoyancy elements' mechanical work is transformed into pressure energy in a liquid, delivered by the pumping cylinder to a collecting pipe system.
In different embodiments the lower end of each cylinder may be connected to the frame structure in different ways, e.g. by a fixed connection or by a bearing that allows the cylinder's longitudinal axis to pivot when the buoyancy element's force deviates in direction from the vertical.
The pressure energy of the flowing medium is transformed into electrical energy in a power aggregate comprising a turbine and an electrical generator. If the power aggregate is placed on the frame structure, it may be placed in a generator housing being virtually naturally buoyant, and may be connected to the frame structure in such a way that it may be easily disconnected and brought to shore or to a floating vessel for overhaul.
The frame structure keeps the holding brackets for the cylinders in place relative to each other in the horizontal plane. In addition, the frame structure keeps the said holding brackets virtually at rest in the vertical direction in spite of the variation in force acting on the frame structure from each buoyancy element. The virtually stationary restraint for the buoyancy elements' pumping cylinders is achieved through having several buoyancy elements being in different phase of the work cycle connected to via the frame structure to a common ballast body with a great mass that gives the system a large inertia.
The invention seeks to utilize the fact that a floating construction having sufficiently great mass, stiffness and many buoyancy elements that are spread with a large spacing between them compared to the lengths of passing waves will be virtually at rest and will to only a small degree be influenced by the variable buoyancy force that the waves exert on each individual buoyancy element.
Furthermore, this solution seeks to minimize material weight in the elements exposed to structural loads through letting the vertical force from each buoyancy element be transferred as directly as possible into tension in the respective connecting element like an oblique cable going down to the ballast body. The purpose of the ballast body is to give the whole construction the desired inertia and also to give the buoyancy elements the desired downward acting load so that the buoyancy elements' pumping power will be optimal.
Preferably the buoyancy elements are arranged with a mutual horizontal spacing whereby the buoyancy elements due to their mutual horizontal spacing will at any time be in different phases of the force cycles created by the passing waves. The total buoyancy force from the buoyancy elements will thus be smoothed out and will be more equal to the average buoyancy the higher the number of buoyancy elements is and the greater horizontal length is over which the buoyancy elements are spread out by means of the frame structure. From this follows that the centre of gravity for the whole construction's mass has the ability to be virtually at rest with respect to vertical translational movement, heave, an ability which increases by the number of buoyancy elements and the size of the frame structure.
The size of the frame structure, seen in the propagating direction of the waves, should with regard to heave be at least one wavelength.
In order for the construction to be virtually at rest in spite of a sea state with waves, it should in addition to having small heave movement also have small angular movements like pitching and rolling. The solution is to give the frame structure a sufficiently large horizontal dimension so that the moments of the buoyancy elements about the centre of gravity more or less balance each other at any time. This is achieved to an acceptable degree when the frame structure's size, seen in the propagating direction of the waves, is at least two wavelengths. If the horizontal size is increased, this will increase the construction's ability to stay virtually at rest.
Optimal power production from the buoyancy elements will depend on the degree of ballasting and among others the control of the pressure levels in the pumped medium and the control of the attached turbine. It is assumed that the maximal power production at a given sea state is achieved when each of the buoyancy elements in still water is loaded down with a tension force that is approximately 50% of the buoyancy element's maximal net lifting capacity. The maximal static net lifting capacity is equal to the maximum buoyancy force of a fully submerged buoyancy element minus the weight the buoyancy element. The total submerged weight of the frame structure and the ballast body should be chosen with the purpose of obtaining maximal power production, and from this the optimal weight of the ballast body can be calculated.
In further embodiments at least one of the ballast bodies can be connected to anchoring devices at the sea bottom via anchoring lines.
Furthermore, the sea bottom anchored ballast body can be provided with a connecting body which is further connected to the anchor lines, which connecting body is provided with a revolving mount such that the frame structure can revolve about a substantially vertical axis. By being able to revolve in the horizontal plane, the wave power-plant can optimize its power production as the waves change direction of propagation, and can give the wave power plant a great enough dimension in the direction of wave propagating to minimize angular accelerations of the construction. The longitudinal axis of the structure should preferably be at an angle to the propagating direction of the waves in order not to be too much in the lee of each other.
The frame structure may furthermore be directly connected to anchoring devices via one or more anchoring lines. Furthermore, one or more of the anchoring lines can be connected to a winch which by regulating the length of one or more of the anchoring lines can turn the frame structure in the horizontal plane.
The frame structure may furthermore or alternatively be provided with one or more powered devices exerting thrust in order to turn the frame structure in the horizontal plane.
The ballast body may furthermore be shaped as a container with an upward facing opening facilitating a filling of the container with a suitable ballast material from above. A suitable way of bringing the ballast material into place may be to dump it from a surface vessel when the frame structure is mounted hanging from the buoyancy elements and the empty ballast body is suspended from the frame structure via the connecting elements.
The frame structure may be constructed as a framework with truss members and nodes, where the nodes are formed as joints between the members.
An ideal framework has nodes that behave like joints, i.e. the nodes can take up small changes in angle between the truss members without exerting resistance against the angular deflections. The nodes according to the this embodiment thereby facilitate that bending moments will not be introduced into the truss members of the framework, in contrast to in a frame structure where one or more truss members run through a node or the truss members are rigidly connected in the nodes. By having joints that allow angular changes also out of the frame's plane, the great vertical forces from the buoyancy elements will not introduce bending stresses in the truss members of the framework, as the vertical forces from each pumping cylinder will be taken up directly by the vertical component of the force in the respective connecting element from the ballast body. A framework as described is assumed to be economically favorable compared to a frame structure with rigid nodes where the member stays have to be designed to withstand considerable bending moments in addition to the axial forces. To secure a linked node from yielding downward at the loss of lifting force from the attached buoyancy element, the framework can be provided with stoppers at the nodes. Also the frame structure may be provided with some minor net buoyancy so the node can be kept in place even without lifting force from the buoyancy element.
Furthermore, the pumping cylinders' connection to the frame structure may be rotatable about at least one mainly horizontal axis.
Different embodiments of the invention are described with reference to the enclosed figures, where
The pumping cylinders 4 are connected to the frame 3 and the cylinders' piston rods are each connected to its respective buoyancy element 5. Slack anchoring lines 6 keep the wave power plant in a sufficiently accurate horizontal position. The anchoring lines are connected to the connecting body 8 which is rotatable with respect to the ballast body 1, thus allowing the wave power plant to obtain a desired orientation relative to the direction of propagation of the waves. A virtually naturally buoyant generator house 10 containing an electric power aggregate may be placed as shown with suitable connection devices 11 for easy connection and disconnection. Electrical power cables 9 and potentially communication cables 12 going from the wave power plant are led down through a tube centrally placed through the ballast body 1 and further through a central opening in the connecting body 8 and down to the sea bottom as shown.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20091383 | Apr 2009 | NO | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/NO10/00125 | 3/31/2010 | WO | 00 | 12/23/2011 |
