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Title:
METHODS AND SYSTEMS FOR ENERGY CONVERSION
Document Type and Number:
WIPO Patent Application WO/2011/161203
Kind Code:
A2
Abstract:
A system and method for conversion of first energy. The conversion system comprises a housing for interacting with the first energy, e.g. a floating housing floating on sea waves. The system furthermore comprises an internal rotational mass configured to undergo threedimensional relative rotational movement in the housing induced by actuation of the housing by the first energy. The internal rotational mass has a center of mass coinciding with the center of rotation of the threedimensional rotational movement of the internal rotational mass in the housing. The system thereby is an energy conversion means adapted for converting first energy into a relative rotational movement between the housing and the internal rotational mass induced by inertial properties of the housing and the internal rotational mass.

Inventors:
DEGRIECK, Joris (Stationsstraat 104, Aalter, B-9880, BE)
VAN PAEPEGEM, Wim (Bonte Mantelstraat 6, Zwijnaarde, B-9052, BE)
VANTORRE, Marc (Drakenhoflaan 61, Antwerpen, B-2100, BE)
Application Number:
EP2011/060528
Publication Date:
December 29, 2011
Filing Date:
June 22, 2011
Export Citation:
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Assignee:
UNIVERSITEIT GENT (Sint-Pietersnieuwstraat 25, Gent, B-9000, BE)
DEGRIECK, Joris (Stationsstraat 104, Aalter, B-9880, BE)
VAN PAEPEGEM, Wim (Bonte Mantelstraat 6, Zwijnaarde, B-9052, BE)
VANTORRE, Marc (Drakenhoflaan 61, Antwerpen, B-2100, BE)
International Classes:
F03B13/20
Domestic Patent References:
2008-04-10
2010-09-23
Foreign References:
GB2409898A2005-07-13
US20090167033A12009-07-02
US4492875A1985-01-08
US4352023A1982-09-28
GB2462663A2010-02-17
US20090008942A12009-01-08
Attorney, Agent or Firm:
WAUTERS, Davy et al. (DenK iP bvba, Pastoor Ceulemansstraat 3, Schiplaken, B-3191, BE)
Download PDF:
Claims:
Claims

1. A system (100) for converting first energy, the system (100) comprising

a housing (110) for interacting with the first energy,

an internal rotational mass (120) configured to undergo a relative three dimensional rotational movement in the housing (110) induced by actuation of the housing (110) by the first energy, the internal rotational mass (120) having a center of mass coinciding with the center of rotation of the relative three dimensional rotational movement in the housing (110), the system (100) being

an energy conversion means adapted for converting the first energy into a relative three dimensional rotational movement between the housing (110) and the internal rotational mass (120) induced by inertial properties of the housing (110) and the internal rotational mass (120).

2. A system (100) according to claim 1, wherein the system (100) is adapted for converting the first energy into a relative three dimensional rotational movement between the housing (110) and the internal rotational mass (120), induced by the inertial properties of the housing (110) and the internal rotational mass (120).

3. A system (100) according to any of the previous claims, wherein the internal rotational mass (120) is moveably in contact with the housing by bearings.

4. A system (100) according to any of the previous claims, wherein the internal rotational mass (120) is a spherical mass and the housing comprises a hollow spherical shell so that the the spherical mass can perform relative three dimensional rotational movements in the housing (110)

5. A system (100) according to any of the previous claims, wherein the internal rotational mass (120) is suspended via a rotatable axle forming a first rotation axis for the internal rotational mass (120), whereby the rotatable axle and the energy generation means are configured for allowing energy conversion from rotational movement of the internal rotational mass, the internal rotational mass and the rotatable axle being suspended via a first pivot suspension inducing a second rotation axis for the internal rotational mass and the first pivot suspension itself acts as a mounting frame that itself is pivotely suspended via a second pivot suspension, inducing a third rotation axis for the internal rotational mass, so that energy generation from the three dimensional rotational movement of the internal rotational mass is obtained.

6. A system (100) according to claim 5, wherein the system (100) comprises a controller

(170) for avoiding that two or more of the rotation axes become parallel during use.

7. A system (100) according to any of the previous claims, wherein the system is adapted for adjusting the relative rotational movement of the housing (110) and/or the internal rotational mass (120) so as to optimize the energy conversion.

8. A system (100) according to claim 7, wherein said adjusting the relative rotational movement comprises adjusting the inertial properties of the internal rotational mass (120).

9. A system (100) according to claim 8, wherein, for adjusting the inertial properties of the internal rotational mass (120), the internal rotational mass (120) comprises a set of shiftable masses, shiftable towards and away from the centre of mass.

10. A system (100) according to any of claims 7 to 9, wherein said adjusting the relative rotational movement comprises adjusting the inertial properties of the system (100).

11. A system (100) according to claim 10, wherein, for adjusting the inertial properties of the system, the system comprises a means for altering the metacentric height of the system.

12. A system (100) according to claim 11, wherein the system comprises an altering means for shifting the center of mass of the system.

13. A system (100) according to any of claims 7 to 12, wherein the system being adapted for adjusting is a system being adapted for tuning during use the relative rotational movement of the housing (110) and/or the internal rotational mass (120) so as to optimize the energy conversion.

14. A system (100) according to any of the previous claims, the system comprising a controller (170) adapted for controlling at least the energy conversion of first energy into a relative three dimensional rotational movement between the housing (110) and the internal rotational mass (120) induced by inertial properties of the housing (110) and the internal rotational mass (120).

15. A system (100) according to any of the previous claims, wherein the rotational mass (120) operates as rotor and wherein the housing (110) operates as stator, the stator and rotor being configured to operate as energy generation means (130).

16. A system 100) according to claim 15, wherein at least one of the stator or rotor comprises a plurality of permanent magnets on its surface.

17. A system (100) according to any of claims 15 or 16, wherein the rotational mass (110) is a spherical mass with permanent magnets at its surface acting as inertia rotor and configured to provide a relative rotation with respect to a spherical hollow floating house (120) acting as stator.

18. A system (100) according to claim 17, wherein the rotor and stator are connected to each other by hydrostatic bearings.

19. A system (100) according to any of the previous claims, the system (100) comprising an energy storage means (160) for storing energy converted from the motion.

20. A system (100) according to any of the previous claims, wherein the three dimensional rotational movement of the rotational mass (120) is not limited by end points in a direction of movement.

21. A system (100) according to any of the previous claims, wherein the system further comprises an energy generation means (130) for generating electrical energy from the relative three dimensional rotational movement.

22. A system according to claim 21, the energy generation means comprising one or more alternators mounted directly on a rotational axe of the system.

23. A system (100) according to any of claims 21 to 22, the system comprising a controller (170) for controlling the energy generation means (130).

24. A system (100) according to any of the previous claims, wherein the first energy is wave energy and the housing (110) is adapted for interacting with sea waves.

25. A method for converting first energy, the method comprising

- providing interaction between first energy sources and a housing (110), thus inducing relative three dimensional rotational movement between an internal rotational mass (120) and the housing (110), the internal rotational mass (120) having a center of mass coinciding with the center of rotation of the relative three dimensional rotational movement, and - converting first energy into the relative three dimensional rotational movement between the housing (110) and the internal rotational mass (120) induced by inertial properties of the housing (110) and the internal rotational mass (120).

Description:
Methods and systems for energy conversion

Field of the invention

The present invention relates to the field of energy conversion. More particularly, the present invention relates to methods and systems for converting energy such as sea wave energy.

Background of the invention

Extraction of energy contained in sea waves is nowadays a hot topic, like all other forms of "green" energy. It has the potential of production of electical energy from a renewable energy source, without emission of greenhouse gasses. There exist a number of types of wave energy converters, such as terminator devices, point absorbers, attenuators and overtopping devices. To our knowledge, currently existing point absorbers make use of variations in underwater pressure or of pure buoyancy. Buoyancy based absorbers are absorbers based on the upward acting force on a system in fluid and energy is extracted from the upward/downward movement of the absorber.

Energy conversion makes use of mechanical, hydraulic, pneumatic and/or electrical systems, and typically needs an external "reference", like a floating platform or an anchorage to the sea bed; therefore such energy conversion systems need external mechanical components to link the system with this reference, which are subject to the harse and stringent environmental conditions of seawater and therefore reduce the reliability of the overall system. In the past efforts have been made to avoid such an external reference.

One example of such a known system is a buoyancy system based on a gyroscope and described in International patent application WO2008/040822 Al. An installation is described for harnessing wave energy consisting of a floating structure and a gyroscopic device comprising a flywheel that can spin under the actuation of a motor. The spinning flywheel is subjected to a pitching torque caused by the waves and thereby powering the generator. The motor actuating the flywheel and the generator are controlled by a control unit taking into account information of the waves. The waves add to the controlled motion of the flywheel an additional motion in another direction, used for generating the electrical energy.

From US2009/0008942, a buoyancy system for converting wave energy into electric power is known, comprising a closed floating element and a mass forming element. The mass forming element is arranged mobile relative to the floating element so that it is set in relative motion under the action of waves on the floating element. The relative movement is created by the pendulum effect of the mass forming element. The system furthermore describes a locking means for mechanically locking and releasing the mass forming element as function of the dynamics of the system due to successive waves.

WO2010/107330 A2 describes an electrical spherical generator based on magnetic induction, designed to absorb kinetic energy from any movement such as for example wave energy. The floating system is based on a pendulum for interacting with gravity.

Existing wave energy converting systems typically can be optimally used in wave climates where large wave height occur, and often linked therewith a relatively small frequency. Amongst others due to their size constraints and their working principle, such systems typically are less suitable for energy gain in wave climates having a larger wave frequency. There is still need for a reliable energy converting system.

Summary of the invention

It is an object of embodiments of the present invention to provide good methods and systems for converting wave energy, including converting wave energy in wave climates characterized by high wave frequencies (f), by relatively small wave heights (H), or by an appropriate combination of a wave frequency and a wave height, e.g. by a good or high frequency to wave height ratio, i.e. f/H.

It is an advantage of embodiments according to the present invention that energy conversion systems and methods can be provided that are simple in nature, and thus are cost efficient and reliable.

It is an advantage of embodiments according to the present invention that the wave energy convertor has an energy conversion based on relative rotational movement of inertial mass and absorber. No translation from linear movement to rotational movement is required, which results in a system that can be conceived simple and which can be made using conventional components such as for example existing, reliable bearings and rotational machines. The latter is advantageous over linear movement conversion systems, for which more complex bearings and less efficient linear generators are required.

It is an advantage of embodiments of the present invention that efficient energy generation can be obtained with a relatively low mass. In the system, all energy conversion and control mechanisms may be located inside. It is an advantage of embodiments according to the present invention that the energy conversion systems and methods can be completely watertight and/or airtight. This is a significant advantage in maritime environments.

It is an advantage of embodiments according to the present invention that systems and methods can be provided operating without external reference. Thus no floating platform or anchorage to the sea bed is required for the extraction of energy. The methods and systems according to embodiments of the present invention therefore may operate at any proper waterdepth for which they are designed (in view of shape, dimension and inertia).

It is an advantage of embodiments according to the present invention that the behaviour of the energy conversion system, more particularly the absorber thereof, can be tuned to the actual environment conditions, such as for example the actual sea state. It is an advantage of embodiments according to the present invention that tuning of the energy conversion system may be performed in combination with a predetermined control strategy of power take-off. It is an advantage of embodiments according to the present invention that, this control strategy can, e.g. continuously, adapt the moment of inertia of an internal mass, i.e. use the internal mass as the tuning parameter. It is an advantage of embodiments according to the present invention that different tuning techniques can be applied.

It is an advantage of embodiments according to the present invention that this control strategy can influence the relative movement of the internal mass and the housing in order to maximize the energy conversion. It is an advantage of embodiments according to the present invention that no end stops are required, as the system can work in any positional configuration. The latter can be obtained because the internal mass can rotate as required and each angular position of the system is an allowable and operational position. Using an adequate design, the absorber may go overtop and continues to work appropriately. In other words, no end points in the possible motion of the system are present, allowing that the system continuously can be in an operational position. It is an advantage of embodiments according to the present invention that no actuation different from actuation by the environment, e.g. waves, is required. In this way no additional actuation means is required for making the system operable.

It is an advantage of embodiments according to the present invention that energy can be extracted from movement in more than one direction. The latter results in an increased energy extraction.

The above objective is accomplished by a method and device according to the present invention.

The invention relates to a system for converting first energy, the system comprising a housing for interacting with the first energy, an internal rotational mass configured to undergo a three dimensional relative rotational movement in the housing induced by actuation of the housing by the first energy, the internal rotational mass having a center of mass coinciding with the center of rotation of the three dimensional relative rotational movement in the housing, the system being an energy conversion means adapted for converting the first energy into a three dimensional relative rotational movement, between the housing and the internal rotational mass induced by inertial properties of the housing and the internal rotational mass, e.g. inertia of the internal rotational mass with respect to the housing. It is an advantage of embodiments of the present invention that the system for wave energy conversion does not comprise end of movement states for which no further movement in the same direction is possible anymore. It is an advantage of embodiments of the present invention that the systems can be relatively small and light. The system may be adapted for converting the first energy into a three dimensional relative rotational movement between the housing and the internal rotational mass, induced by the inertial properties of the housing and the internal rotational mass.

It is an advantage of embodiments according to the present invention that energy extraction from movement in all directions can be obtained.

The internal rotational mass may be moveably in contact with the housing by bearings.

The internal rotational mass may be a spherical mass and the housing may comprise a hollow spherical shell so that the spherical mass can perform relative three dimensional rotational movements in the housing. The internal rotational mass may be suspended via a rotatable axle forming a first rotation axis for the internal rotational mass, whereby the rotatable axle and the energy generation means are configured for allowing energy conversion from rotational movement of the internal rotational mass. The internal rotational mass and the rotatable axle may be suspended via a first pivot suspension inducing a second rotation axis for the internal rotational mass and the first pivot suspension itself may act as a mounting frame that itself is pivotely suspended via a second pivot suspension, inducing a second rotation axis for the internal rotational mass, so that energy generation from rotational movement of the internal rotational mass in different, non-parallel, directions is obtained. It is an advantage of embodiments according to the present invention that energy extraction can be performed based on both rolling movement and pitching movement, resulting in a highly efficient system.

The system furthermore may comprise a controller for avoiding that the plane formed by the two axes of rotation becomes perpendicular to the vector of angular velocity of the housing. The system may be adapted for adjusting the relative rotational movement of the housing and/or the internal rotational mass so as to optimize the energy conversion. It is an advantage of embodiments according to the present invention that the systems and methods can be adapted to the wave state it is operated in. The latter results in an efficient system according to the present invention, even for different wave states.

Adjusting the relative rotational movement may comprise adjusting the moment of inertia or the inertial properties of the internal rotational mass. For adjusting the inertial properties of the internal rotational mass, the internal rotational mass may comprises a set of shiftable masses, shiftable towards and away from the centre of mass.

Adjusting the relative rotational movement may comprise adjusting the inertial properties of the system.

For adjusting the inertial properties of the system, the system may comprise a means for altering the metacentric height of the system. The system may comprise an altering means for shifting the center of mass of the system. For example, the altering means may comprise a means for vertically shifting the position of the internal rotational mass and/or for introducing liquid in the system. Shifting the center of mass of the system may be performed with or without change in the total mass of the system. The system being adapted for adjusting may be a system being adapted for tuning during use the relative rotational movement of the housing and/or the internal rotational mass so as to optimize the energy conversion.

Adjusting and/or tuning may be performed taking into account the wave frequency f, the wave height H or an appropriate combination of the wave frequency f and the wave height H such as the ratio f/H. It is an advantage of embodiments according to the present invention that the systems and methods can be adapted to changes in the sea state, resulting in an efficient system according to the present invention.

The system may comprise a controller adapted for controlling at least the energy conversion of first energy into a relative three dimensional rotational movement between the housing and the internal rotational mass induced by inertial properties of the housing and the internal rotational mass. Such controlling may comprise adjusting or tuning during use. The controller may be adapted for controlling the relative rotational movement of the housing and/or the internal rotational mass as to optimize the energy conversion. It is an advantage of embodiments according to the present invention that, this control strategy can influence the relative movement of the internal mass in order to maximize the energy extraction. The controller may be adapted for controlling tuning of the moment of inertia of the internal rotational mass or the system as function of characteristics of the first energy.

The system further may comprise an energy generation means for generating electrical energy from the three dimensional relative rotational movement.

The first energy may be wave energy and the housing may be adapted for interacting with sea waves. It is an advantage of embodiments of the present invention that energy can be extracted from sea waves in a plurality of wave climates, including e.g. in a moderate wave climate typically characterized by high characteristics frequencies, high wave heights or an appropriate combination of a frequency and wave height, e.g. characterized by high f/H ratio's. Where reference is made to a moderate wave state, reference may be made to a wave state with an energy content of about 20kW to 30kW per meter wave crest or less, such as for example occurring at the North Sea, embodiments of the present invention not being limited thereby.

The rotational mass may operate as rotor and the housing may operate as stator, the stator and rotor being configured to operate as energy generation means. In one exemplary embodiment, at least one of the stator or rotor may comprise a plurality of permanent magnets on its surface, embodiments of the present invention not being limited thereto. The rotational mass may be a spherical mass with permanent magnets at its surface acting as inertia rotor and configured to provide a relative rotation with respect to a spherical hollow floating house acting as stator.

The rotor and stator may be connected to each other by hydrostatic bearings.

The system may comprise an energy generating means for generating electrical energy from the relative three dimensional rotational movement. The energy generation means may comprise one or more alternators mounted directly on a rotational axe of the system.

The system may comprise an energy storage means for storing energy converted from the motion.

The movement of the rotational mass may not be limited by end points in a direction of movement. It is an advantage of embodiments according to the present invention that no end points of motion are present in the system. Such end points typically result in vulnerable configurations for the system and damage of the system when the first energy is very high. The present invention also relates to a method for converting first energy, the method comprising providing interaction with first energy sources and a housing, thus inducing three dimensional relative rotational movement between an internal rotational mass and the housing, the internal rotational mass having a center of mass coinciding with the center of rotation, and converting first energy into the relative rotational movement between the housing and the internal rotational mass induced by inertia of the internal rotational mass. Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

Brief description of the drawings

FIG. 1 is a schematic representation of an energy converter according to an embodiment of the present invention. FIG. 2A and FIG. 2B illustrate two settings of a tunable component of an energy converter according to an embodiment of the present invention.

FIG. 3 is a schematic representation of a cardan mounting of a component of an energy converter, as can be used in an embodiment of the present invention.

FIG. 4 is a schematic representation of a spherical rotor/stator configuration in an energy converter, as can be used in an embodiment of the present invention.

FIG. 5 illustrates a setup for tuning an energy conversion system by changing the metacentric height, according to an embodiment of the present invention.

FIG. 6 illustrates different parameters determining a state of an energy conversion system, as can be used in an embodiment of the present invention.

FIG. 7 illustrates the effect of the position of the center of mass of the energy conversion system on the k-parameter, as can be used in embodiments of the present invention.

FIG. 8 to FIG. 12 illustrates absorbed power as function of the k-parameter and the generator constant for different seastates for an energy conversion system, illustrating features and advantages of embodiments according to the present invention.

The drawings are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Any reference signs in the claims shall not be construed as limiting the scope. In the different drawings, the same reference signs refer to the same or analogous elements.

Detailed description of illustrative embodiments

Where in the present invention reference is made to a "sea state" or "wave state" , reference is made to the temporal distribution of the wave height, wave period and wave direction. Embodiments of the present invention are not limited to one certain type of wave climate. On the other hand embodiments of the present invention can be designed or adapted as to operate preferably in certain wave climates. In general, embodiments of the present invention depend on the rolling and pitching behavior of the absorber as induced mainly by short and fast waves. The sea state also may be characterized by a combination of the wave frequency and wave height, e.g. by the ratio of the wave frequency to the wave height and adjusting or tuning may be performed taking into account the wave frequency, the wave height or a combination thereof such as for example the ratio frequency to wave height. Where in the present invention reference is made to buoyancy, reference is made to the principle that objects are kept afloat due to upward force.

Where in embodiments of the present invention, reference is made to metacentric height, reference is made to the distance between the centre of gravity and the metacentre, whereby the metacentre is the point at which a vertical line through the heeled centre of buoyancy crosses the line through the vertical centre of buoyancy when the object is not heeled.

In a first aspect, the present invention relates to a system for converting first energy. The first energy may be environmental energy, such as wind or wave energy. The first energy may be induced energy, induced by human or environment. The first energy can be e.g. converted into electrical energy. The converter described in embodiments of this and further aspects may be operated using any suitable energy source provided it allows inducing roll or pitch movement of an outer housing of the system. Two examples thereof, embodiments of the present invention not being limited thereto, are wind energy or wave energy such as sea wave energy. It is advantageous to use embodiments of the present invention with energy sources in configurations whereby the housing undergoes fast roll or pitch movements. One set of advantageous embodiments comprises a converter for converting wave energy such as sea wave energy and converting it into in first instance usable mechanical energy which can be used either directly (e.g. to drive a stone cruncher) or further converted into e.g. electrical energy (AC or DC to be injected into the grid), chemical energy (e.g. hydrogen generation, battery storage) or hydraulic energy. In most cases the target will be electrical energy. By way of illustration, embodiments of the present invention not being limited thereto, embodiments and examples will be illustrated for a wave energy converter extracting energy from the waves and converting this energy into electrical energy. Where other energy sources are used and/or where the energy is converted into other forms of energy, the same principles apply.

A wave energy converter as envisaged in some embodiments of the present invention typically is based on a floating converter having a floating housing. The floating housing may be positioned just below the surface of the water, so that the wave motion is still felt, but visual pollution will be less and weather and other possibly negative effects occurring for a housing partly above the water surface can be avoided.

The system for first energy conversion into electrical energy, also referred to as the first energy converter, according to embodiments of the present invention comprises a housing for interacting with the first energy, e.g. floating on sea waves. The system furthermore comprises an internal rotational mass configured to undergo three dimensional rotational movement in the housing induced by actuation of the housing by the first energy. The internal rotational mass has a center of mass coinciding with the center of rotation of the three dimensional rotational movement. The relative three dimensional rotational movement of the internal rotational mass is induced by the difference in relative rotational movement between the internal rotational mass and the housing. According to embodiments of the present invention, the system thus is an energy conversion means adapted for converting the first energy into a relative three dimensional rotational movement between the housing and the internal rotational mass induced by inertial properties of the housing and the internal rotational mass, e.g. induced by inertia of the rotational mass with respect to the housing. The system furthermore typically comprises an energy generating means, e.g. a generator being coupled to the internal rotational mass or a generator comprising the internal rotational mass as component, for generating electrical energy by further conversion of the energy induced by the relative movement between the housing and the internal rotational mass induced by inertia of the internal rotational mass. By way of illustration, the present invention not being limited thereby, standard and optional features of an exemplary schematically drawn energy converter will be discussed in more detail, with reference to FIG. 1.

The converter 100 for converting first energy into electrical energy comprises a housing 110. The housing 110 advantageously may be a floating point absorber. The housing 110 may have adequate geometry so that it maximizes roll and pitch movements when it is actuated by the first energy source. More particularly, when a wave converter is envisaged, the housing 110 may have adequate geometry for increasing or optimizing roll and pitch movements of the floating point absorber. The relative movement of the internal rotational mass may occur in any direction, advantageously resulting in less influence of the orientation of the housing with respect to the energy source. The shape can be a box beam, a cuboid, an elliptical cylinder, ... The housing 110 advantageously may be made of metal, natural materials, polymers, composite materials, ...

The system 100 furthermore comprises an internal rotational mass element 120 placed inside the housing 110. The internal rotational mass element 120 is configured so that it can perform relative three dimensional rotational movement with respect to the housing 110. Such relative three dimensional rotational movement according to embodiments of the present invention is based on a moment of inertia of the internal rotational mass element 120, whereby due to the inertia of the housing and of the internal rotational mass element, the internal rotational mass element 120 does not immediately follow the movements of the housing induced by the first energy sources. This relative three dimensional movement of the mass element 120 comprises mechanical energy that can either be used directly and/or it allows generation of electrical energy.

The internal rotational mass element 120 typically is rotatably connected to the housing, such that the center of mass of the internal rotational mass element 120 coincides with the center of rotation of the internal rotational mass element 120 for the relative three dimensional rotational movements of the mass element 120 in the housing. The internal rotational mass element 120 may be made of a metal such as for example steel, concrete, composite materials ... . The shape of the internal rotational mass element may vary. In some examples, the shape of the internal rotational mass element may for example be cylindrical, whereby the center of rotation then typically may be a rotation axis. The internal rotational mass may be disc-shaped, whereby the disc advantageously has a certain thickness. The shape of the internal rotational mass element advantageously is a substantially three dimensional object. Advantageously the shape of the internal rotational mass is spherical in some examples. A typical mass and moment of inertia of the internal rotational mass element 120 can be selected as function of the characteristics of the first energy. As will be indicated later, two examples of possible setups may be a spherical mass in a hollow spherical housing, such that the spherical mass can rotate e.g. on bearings in the spherical shaped inner volume of the housing on the one hand, or a mass suspended in a multi pivot arrangement allowing rotation in all directions on the other hand. The latter will be discussed in more detail below. In some embodiments according to the present invention, the moment of inertia or the inertial properties of the internal rotational mass element 120 may be controllable or it may even be tuneable during use. The latter may be advantageous as it allows adjusting the energy converter to the conditions of the first sources, e.g. to a wave state, or even to variations of the conditions of the first sources during use. A plurality of possibilities exist for tuning the system. In one example, shifting mass sub-elements closer to or further away from the center of rotation of the internal rotational mass element 120 may be provided. This shifting of mass sub-elements closer to or further away from the center of rotation directly influences the moment of inertia of the internal mass of rotation. An example of an internal rotational mass element 120 that is controllable or tuneable is shown in FIG. 2A and FIG. 2B based on a disc-shaped internal rotational mass element which for example could be suspended in a multi pivoting arrangement so that rotation in all directions is possible. FIG. 2A and FIG. 2B thus illustrate two settings of such a tuneable internal rotational mass element.

As indicated above, the internal rotational mass element 120 typically is rotatably connected to the housing 110. In some examples, e.g. examples wherein a cylindrical or disc-shaped mass element 120 is used, the rotatable connection may be via a mechanical axis fixedly connected to the internal rotational mass element 120 and connected through multiple, at least two, pivotable connections so that three dimensional rotation can occur. In some examples, e.g. examples wherein a spherical mass element 120 is used, the rotatable connection may for example be performed directly by a bearing between the housing 110 and the internal rotational mass element 120.

The system thus is an energy conversion means adapted for converting the first energy, e.g. wave energy, into a relative rotational movement between the housing and the internal rotational mass induced by inertial properties of the housing and the internal rotational mass. The converter 100 furthermore may comprise an energy generator 130. Such an energy generator may be adapted for generating energy by conversion, i.e. it may be adapted for converting the energy present in the relative rotational movement of the internal rotational mass element and the housing, into another type of energy such as electrical energy, chemical energy or hydraulic energy. In one embodiment, the energy converter 130 is an electrical energy generator 130 positioned between the internal rotational mass element 120 and the housing 110. This generator can convert the mechanical energy comprised in the relative motion between the housing 110 and the internal rotational mass element 120 into electrical energy. A number of different techniques for generating electrical energy could be used. For transferring the relative motion and the comprised mechanical energy in this movement, the generator 130 may comprise in one example a gearbox 200, a set of belts running over a drum, a set of pistons, etc. In some embodiments, the internal rotational mass element itself may be adapted for operating as a rotor enclosed by a stator fixedly connected to the housing or being part of the housing. The latter may be advantageous, as it allows avoiding of the need for gearing, especially when the internal rotational mass element has a significantly large diameter allowing a relatively high rotation speed at the outer surface of the internal rotational mass. A more detailed description thereof is provided in a particular embodiment below. It should further be noted that the term generator is used for a component of which its main function is converting mechanical energy into electrical energy. However, it will be clear to those skilled in the art that occasionally during operation the generator can be steered as such that it converts electrical energy into mechanical energy, whereby the generator then effectively works as a motor. Hence component 130 could be denoted as generator/motor.

E.g. wave energy converters furthermore may be provided with a mooring element 140, so as to prevent drifting away. It nevertheless is an advantage of embodiments according to the present invention that for proper operation of the converter, there is no need for a fixed reference point and consequently for proper operation no anchorage system is required. The mooring element nevertheless may be used to retain the system of floating away. Additionally, the mooring element may be such that the converter is retracted to remain just below the water level, preventing the converter from being exposed to breaking waves or extreme sea states and meanwhile reducing the visual hinder of the converter.

Without embodiments of the present invention being bound by theory, the operation of the WEC can be by way of illustration explained in more detail by the following theoretical considerations. Treating the housing 110 of a WEC as being a rigid body, its movement with respect to a global Cartesian axis system (x,y,z) can be described in a unique way by means of six parameters, being for example the three components of the velocity vector v c = dr c /dt of its center of mass€, and the three components of the angular velocity vector co= ddldt of a locally attached axis system (ξ,η,ς) having its origin in c . Doing so, the corresponding components of motion are typically classified as: surge, the horizontal, longitudinal motion along the x-axis,

sway, the horizontal, transverse motion along the y-axis,

heave, the vertical motion along the z-axis,

roll, the angular motion around the x-axis,

pitch, the angular motion around the y-axis,

and yaw, the angular motion around the z-axis.

At each moment of time, the housing or shell has a linear momentum p = m.v c , with m being the shell mass, and an angular momentum L = I c .O) , with I c being the moment of inertia tensor. The mass and the moment of inertia tensor for a rigid body are easily obtained from: m = J]J fc )dV, T c = f c

f E 3 - f c ® f c )dV where p{r c ) is the material density distribution function, E 3 is the 3x3 identity matrix, and ^. and r c ® r c stand for the length of the location vector, respectively the outer vector product.

The equations of motion of the shell in the waves can then simply be expressed as:

y, F^ d and ' ^_ M^ h d are the resulting hydrodynamic forces and moments on the shell from the action of the waves, and ^ p mech&ei • ^ j^ mech&ei g rou p 3 otner mechanical, hydraulic and/or electro-magnetic forces acting on the shell, such as anchoring (actuated by 140) and power take-off forces (actuated by 130). In the above equations, resulting forces are reduced to the center of mass C, and the resulting moments are equally taken around the center of mass.

The interaction of the housing with the incident waves is associated with radiation and diffraction effects; the resulting hydrodynamic action ^ F^ d (and the associated moments) can be split up in a pure hydrodynamic excitation component ^ F x , a hydrodynamic restoring component ^ F™ s = k hyd r r , a hydrodynamic damping component p damp _ ^hyd ^ ^ a nc | a hydrodynarpjcaiiy added inertial component —m y a c . From the latter expressions, it is observed that there is a clear mutual interaction between the acting hydrodynamic forces and the shell movements in different directions.

Similar to the movement of the housing, the equations of motion of the inertial mass 120 can be expressed as:

where all parameters now apply to the corresponding elements of the rotor 120. The actual movement of the rotor is determined by the mechanical reaction forces (and associated moments) of the rotor suspension within the shell structure, and by the mechanical, hydraulic and/or electro-magnetic forces exerted by the power take-off devices. It is clear that for embodiments of the present invention, power can be converted through energy extraction from relative rotational movements between the buoy shell structure (housing 110) and the inertial mass (120):

'extr with M extr the mutually acting power extraction moments between shell and rotor. It is this extracted power that is available at the output of the generator 130.

The converter according to embodiments of the present invention may comprise an electricity transportation means 150 for transporting energy, e.g. directly to a land line or to an off-shore collector collecting the electrical energy from more than one converter before transporting it on-shore. The converter alternatively or in addition thereto may comprise an electrical energy storing element 160 for storing electrical energy on board of the system. Energy may also be stored in an intermediate form such as hydrogen or batteries.

According to some embodiments of the present invention, the converter furthermore may comprise a controller 170. This controller may have at least one but also a plurality of functions including one or more of the ones explicitly mentioned below. One functionality of the controller may be controlling a generator/motor 130 such that an active control of the movement can be realized. A second functionality may be controlling the moment of inertia or the inertial properties of a tuneable or controllable internal rotational mass element 120. An advantage of such a controller may be that it allows on the spot adjustment of the amount of power extracted from the waves by changing the moment of inertia of the controllable internal rotational mass element 120 or the movement of the internal mass

(properties m and I c in the equations above) , as such allowing quick adjustment to changing conditions of first energy sources, such as for example to a changing wave state.

Next to this, the controller can also be in charge of controlling the power electronics which are converting the mechanical energy into electrical energy as well as the power electronics included in the transportion means 150. The controller can also provide in a communication link to communicate the output of several on board sensors.

According to some embodiments of the present invention, the converter furthermore may comprise a locking system 190 allowing locking of the relative position of the housing and the internal rotational mass element 120, such that the locked positions can be released at a moment that an optimum energy extraction can be performed. The locking system 190 may for example comprise a braking system e.g. fixedly connected to the housing and operating on the internal rotational mass element 120 and a controller for controlling such braking system.

Further features and advantages are described with reference to a number of embodiments, embodiments of the present invention not being limited thereto.

In one embodiment, the internal rotation mass itself is used as rotor and can perform three dimensional relative movements with respect to a stator of a generator surrounding the internal rotational mass. As the internal rotational mass has its rotation axis in the center of mass, the relative movements are induced by the moment of inertia of the internal rotational mass with respect to movements induced by the environment on the floating housing. Formulated differently, the embodiment has an axis whereon a rotor may be positioned whereby the rotor operates as internal rotational mass. It is an advantage of embodiments according to embodiments of the present invention that, by using the internal rotational mass itself as rotor an efficient system can be obtained. In view of having a significantly large moment of inertia, the size of the internal rotational mass typically may be quite large. By using the outer surface of the internal rotational mass as outer rotor surface, a relative high tangential speed of this rotor surface is obtained even at low rotational speeds. Such high tangential speeds in the magnetic air gap are required to obtain a sufficiently high efficiency of the converter and to this end quite often gearing is required, using this outer rotor surface may prevent such intermediate gearing. A particular energy conversion mechanism may e.g. then be obtained by applying a number of magnets to the outer surface of the internal rotational mass. In such examples, the stator may be formed by a part of a wall of the floating housing itself, although they are not limited thereto.

In one particular embodiment, the internal rotational mass is configured so that it can perform relative movements induced by its moment of inertia, in reply to both rolling and/or pitching of the floating house using a multi-pivot suspension system. The latter is possible if relative movements are not only possible in one plane, but also in other directions. Such a configuration may for example be obtained using a Cardan suspension, whereby the internal rotational mass and a rotational axis allowing rotation in one direction is suspended in a first pivoted support allowing rotation of the mass about a second axis, whereby the first pivoted support itself also is suspended via a second pivoted support allowing rotation about a second axis. In this way relative movement in all directions, and not only directions within one plane, can be induced by the moment of inertia. By way of example such a suspension is shown in FIG. 3. Whereas in principle such a Cardan suspension may be subject to singular states in which no further movement or only a restricted movement is possible if stand still is obtained, i.e. states wherein the plane formed by the two axes of rotation becomes perpendicular to the vector of angular velocity of the housing, the latter can be remedied by providing a particular driving of the generator, e.g. by providing a blocking mechanism preventing the Cardan suspension from reaching such singular states or by using the generator as a motor to move away from this/these singular state(s).

In another particular example, an energy converter as described above is disclosed, wherein the internal rotational mass is spherical equipped with permanent magnets on its outer surface. Hence this spherical rotational mass will act as a spherical rotor, which is in its turn surrounded by a hollow spherical stator, which advantageously is the floating housing. The spherical rotor thereby is an inertia rotor freely relatively moveable with respect to the stator, the movement being due to the difference in relative rotational movement of the housing and the rotor. The rotor comprises at its surface a set of permanent magnets. The spherical rotor may be in contact with the hollow spherical stator using hydrostatic bearings, such that the friction between stator and rotor can be kept as low as possible. The spherical rotor, the permanent magnets thereon, the different degrees of freedom for movement (rotational directions) the stator poles and hydrostatic bearings are illustrated in FIG. 4.

In one aspect, the present invention also relates to a controller for controlling an energy conversion system comprising an internal rotational mass element with tunable moment of inertia or rotor 120 to generator 130 transmission ratio. The controller may comprise an input port for receiving conditions regarding the first energy sources as input. The latter may be received through measurement using sensors, as data from an external source, etc. The controller furthermore may comprise a processor for adjusting, if required, the moment of inertia or the inertial properties of the internal rotational mass element. The controller may for example be adapted for shifting masses or for altering a metacentric height, ... thus altering the inertial properties of the internal rotational mass element. The controller may alternatively or in addition thereto be a controller for controlling an energy conversion system, the controller being adapted for controlling a braking system to lock and release the housing and the internal rotational mass element with respect to each other to allow relative movement between the two components when it is most appropriate. The controller may be implemented as software or as hardware. The controller may for example be programmed for controlling a generator 130 such that an active control of the movement can be realized, for controlling the moment of inertia of a tuneable or controllable internal rotational mass element 120, for tuning, for optimizing the extracted power, for controlling the power electronics which are converting the mechanical energy into electrical energy as well as the power electronics included in a transportation means 150. The controller can also provide in a communication link to communicate the output of several on board sensors. Further features and functionalities may be as described for the controller in the system according to the first aspect.

Whereas above some examples have been shown on how inertial properties of the internal rotatable mass 120 can be adjusted or controlled (i.e. the shiftable masses in FIG. 2A and FIG. 2B), embodiments of the present invention are not limited thereto. In one embodiment, tuning or adjusting the energy converting system may be performed by tuning the inertial properties of the internal rotatable mass 120, whereby the internal rotatable mass 120 has a substantially spherical outer surface, and whereby radially oriented bars with shiftable masses are provided, along which the masses can be shifted in radial direction.

In another embodiment, tuning or adjusting of the energy converting system may be performed by adjusting the metacentric height of the energy conversion system. Adjustment of the metacentric height may be performed in a plurality of ways, and the system may be adapted therefore. The metacentric height can be changed by altering the vertical position of the center of mass. By changing the height of the center of mass during operation, the eigenfrequency of the energy converting system is influenced. Such a change of the center of mass can for example be performed by adjusting the height positioning of a component of the energy converting system, such as for example adjusting the height of the internal power take-off unit, above also referred as the energy generating means. Nevertheless, also other components may be adjusted in height. The system therefore may be equipped with a height positioning means of a component of the system. In still another embodiment, tuning or adjusting of the energy converting system also is performed by virtually adjusting the metacentric height, e.g. by providing or removing internal free liquid in the system. The system therefore may comprise a system for filling or emptying a compartment of the energy converting system thus allowing to adjust the eigenfrequency of the energy converting system. The compartment may be one or more well positioned ballast tanks.

Tuning according to the above described embodiments may be performed in an automated and or automatically way, as described above for example using a controller. Tuning may be performed for each wave, at regular moments in time, when the wave state is substantially changing, etc. An automated feedback system may be provided in the controller, in combination with control sensors or mechanisms. In some embodiments this may be done such that tuning can be done without human interference.

By way of illustration, embodiments of the present invention not being limited thereby, an example of tunable system is discussed. The example provided relates to a cubic housing with an internal rotational mass being a rotor. The inertia of the housing as well as the internal rotational mass is about 100000 kg.m 2 . The mass of the housing in the present example is about 6825 kg, whereas the mass of the internal rotational mass is 44390 kg, resulting in a mass of the total system of about 50 ton. Tuning of the wave energy conversion in the present example was performed by changing the metacentric height of the system, by changing the center of mass. In the present example this was performed by shifting the center of mass of the internal rotational mass, as indicated in FIG. 5 by the two positions indicated by G ro and G' ro - It is to be noticed that even a small displacement of the center of mass results in a large change in characteristics of the system, thus providing the possibility of tuning over a wide range. By way of illustration, FIG. 6 illustrates the typical parameters involved in the effect of the vertical position of the internal rotational mass on the k-parameter being the hydrostatic restoring coefficient. The hydrostatic restoring coefficient is representative for the couple acting on the heeled object for bringing the object in a non-heeled position. The pressure point B, the metacenter M and the center of gravity G are shown in a coordinate system Oxy fixed to the energy conversion system. FIG. 6 also indicates the parameter values for the example currently discussed, embodiments of the present invention not being limited thereby. The metacenter M can be determined as being at 1.04m above the line of water, i.e. at 2,04m from the bottom of the housing. The latter can be determined using the moment of inertia and the pressure point at lm of the bottom of the housing. FIG. 7 shows a graph indicating the k-parameter as function of the vertical position of the internal mass. The internal mass advantageously is chosen such that there is large inertia while on the other hand sufficient controllability is to be present.

It is an advantage of embodiments according to the present invention that systems are obtained that can be used for a variety of sea states. Table 1 provides an overview of different seastates, and their corresponding wave height H, frequency f, and period between two peaks is given below.

Tabel 1

By way of illustration, a set of simulation results is further provided, illustrating the surprisingly good power absorption for the energy conversion system according to an embodiment of the present invention. For a large set of combinations of the k-parameter and the generator constant b gen being representative for the external damping and being representative for the generator couple, the absorbed power was calculated. The k- parameter for the energy conversion system varies from 100 kNm to 350kNm in steps of lkNm b gen varies from OkNm s to 200 kNm s in steps of 0.5 kNm s. The physically applicable values for the k parameter for the wave energy converter are within the range 0 kNm to 260 kNm. Both the absorbed power P a bs, the amplitude of movement of the housing and the amplitude of the internal mass are calculated. The latter are indicated on FIG. 8 to FIG. 12 as contour lines. For analyzing the results, a 30° amplitude is considered a conservative value, while for a 60° amplitude it is considered that the linear behavior is not plausible anymore. The absorbed power is largest when the energy conversion system is in resonance with the waves. The frequency of the waves reduces for higher sea states, as can be seen in Table 1. FIG. 8 to FIG. 12 indicate the absorbed power P a bs as function of the k-parameter and the generator constant. In the drawings, the white arrows indicate the direction of increasing absorbed power. In the drawings, a shift of the resonance area towards smaller k values (SS2 : 250 kNm; SS3 : 200 kNm; SS4 : 150 kNm), or lower eigenfrequencies can be noticed. Furthermore, the resonance area is smaller for higher sea states, corresponding with the smaller bandwidth. The latter implies that for higher sea states the energy conversion system needs to be tuned more finely for obtaining high absorbed power. It is to be noticed that for higher sea states the resonance area corresponds with a non-linear situation.

The results shown in FIG. 8 to FIG. 12 also indicate that it may be advantageous for tuning the energy conversion system to the particular waves, which may for example be performed using a system according to some embodiments of the present invention, whereby wave characteristics may be sensed using sensors.

A saturation effect on the absorbed power can be noticed for higher sea states. In other words, the amount of power that is gained reduces for higher sea states.

FIG. 8 illustrates the obtained power as function of the k-parameter and the generator constant for the second sea state, for a system as described above. FIG. 9 illustrates the obtained power for the third sea state, FIG. 10 illustrates the obtained power for the fourth sea states and FIG. 11 illustrates the obtained power for the fifth sea state, but for a higher inertia value of the internal mass.

Surprisingly it was found that for relatively low sea states, the power obtainable with an energy conversion system according to embodiments of the present invention with a mass as low as 50 ton can be comparable with the power obtainable with a pendulum based energy conversion system with a mass of 900 ton. The latter indicates advantages of embodiments of the present invention whereby systems with a relatively low mass can be used for obtaining good power absorption. Embodiments of the present invention thus may obtain good power absorption, using lightweight systems.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims.

For example, whereas embodiments of the invention have been described with reference to systems, embodiments of the present invention also encompass methods for converting first energy into electrical energy. The method comprises providing interaction with first energy sources and a housing, thus inducing relative movement between an internal rotational mass and the housing, the internal rotational mass having a center of mass coinciding with the center of rotation, and generating energy from the relative rotational movement between the housing and the internal rotational mass induced by inertia of the internal rotational mass. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

The foregoing description details certain embodiments of the invention. It will be appreciated, however, that no matter how detailed the foregoing appears in text, the invention may be practiced in many ways, and is therefore not limited to the embodiments disclosed. It should be noted that the use of particular terminology when describing certain features or aspects of the invention should not be taken to imply that the terminology is being re-defined herein to be restricted to include any specific characteristics of the features or aspects of the invention with which that terminology is associated.