TECHNICAL FIELD

The present invention relates generally to efficient utilization of natural energy sources. Especially, the invention relates to an energy transducer adapted to convert fluid movements, for ins tance in water into electrical energy and/or useful mechanical energy.

BACKGROUND

Vortex-induced movements can be used to extract energy from various physical phenomena, for example represented by flows of water or wind. The energy contained in such movements may be turned into electricity by means of a converter assembly trans forming the mechanical energy in the moving fluid into electrical energy.

The electric power to offshore installations, such as fish farm daily feeding systems and sensor buoys, is often supplied via sub merged power cables. Here, a fluid transducer in the water flow surrounding the offshore installation constitutes a cost-effective alternative to a cable connection to an onshore power station. This is especially true if the fluid transducer is supplemented by a chargeable battery for buffering the produced electrical energy.

Below, follows examples of different designs for energy extraction based on flow-induced oscillations.

Malefaki and Konstantinidis, “Assessment of Hydrokinetic Energy Converter Based on Vortex-Induced Angular Oscillations of a Cylinder”, Energies 2020, 13, 717; doi:10.3390/en13030717 shows a novel converter where a cylinder undergoes angular os cillations with respect to a pivot point, in contrast to most previous configurations, where the cylinder undergoes flow-induced oscillations transversely to the incident free stream. A theoretical model is formulated to deal with the coupling of the hydrody namics and the structural dynamics, and the resulting nonlinear equations of cylinder motion are numerically solved in order to assess the performance of the energy converter. The hydrody- namical model utilizes an approach where the fluid forces acting on the oscillating cylinder are split into components acting along and normal to the instantaneous relative velocity between the moving cylinder and the free stream. It is found that peak effi- ciencies of approximately 20% can be attained by optimal selec tion of the main design parameters.

US 2008/0295509 discloses a converter for producing useable energy from fluid motion of a fluid medium. The converter inclu des a support structure, at least one movable element immersed in the fluid medium and is supported externally on the support structure such that the movable element can move relatively to the structure in response to the fluid motion by vortex induced motion, galloping or combination thereof, and at least one power device supported on the support structure and coupled to the mo- vable element. The power device converts motion of the movable element to useable energy.

US 9,010,197 describes a system that includes a body disposed in a flow field and a flow disturbance device configured to induce tuned and controlled flow fluctuations in the flow field that are coupled into and amplified by a boundary layer of the body and the flow field. The flow disturbance device is located on, within, or separated from the body. The body may be a bluff body or an airfoil and may be cylindrical in shape. The flow field is a fluid or plasma having a sub-critical flow rate. The flow disturbance device may be stationary or vibrating. The flow fluctuations are tuned to a frequency within an instability frequency band of the boundary layer. The frequency band may be a frequency band that naturally amplifies the flow fluctuations and alters the body's downstream vortex shedding pattern such that vortex-induced vibration characteristics experienced by the body are increased. As is apparent from the above, it is known to use a flow-distur bance element to cause vortex-induced vibrations of a body as a means to extract energy from a flow of fluid. However, the effi ciency of the known designs is relatively low, both with respect to the mechanical exchange and with respect to the conversion from body vibrations/oscillations into electricity or useful mechanical energy.

SUMMARY

The object of the present invention is therefore to offer a solution that mitigates the above problem and provides a reliable and efficient means of extracting energy from a fluid flow.

According to the invention, the object is achieved by an energy transducer containing a bluff body, a flow disturbance element and at least one energy conversion device. The bluff body is elongated along a first axis and is configured to be arranged in the fluid flow such that the fluid flow has a direction transverse to the first axis. The flow disturbance element is elongated along a second axis that is parallel to the first axis. The flow disturbance element is configured to disrupt the fluid flow to cause vortex shedding downstream of the bluff body relative to the flow direction. The flow disturbance element may be positioned at various places relative to the bluff body. However, the flow disturbance element has a tapered cross-section and is arranged with a widest cross- section end upstream of a narrowest cross-section end relative to the flow direction. The at least one energy conversion device is configured to transform movements of the bluff body into at least one of electrical energy and mechanical energy.

The above energy transducer is advantageous because the pro posed streamlined cross-section of the flow disturbance element causes the velocity of the fluid flow to increase locally. As a re sult, the induced vortex shedding is amplified in relation to what is accomplishable by a flow disturbance element with a symmetric profile, e.g. having a circular cross-section. The bluff body, however, preferably has a circular cross-section.

According to one embodiment of the invention, the flow disturban ce element is arranged to provide an opening for the fluid flow between the bluff body and a first surface of the flow disturbance element. The flow disturbance element is further oriented so that a gap between the bluff body and the first surface converges along the flow direction. This geometry causes an increased velocity of the fluid flow in the gap, which, in turn, generates a jet flow. The jet flow further breaks the symmetry of the fluid flow around the bluff body and thus improves the bluff body’s ability to undergo a galloping response from which a comparatively large amount of energy can be extracted.

According to another embodiment of the invention, the flow dis turbance element is arranged so that a main part of its volume is positioned upstream of a central line of the bluff body. Namely, this has proven to produce bluff body oscillations of especially large magnitudes.

According to other embodiments of the invention, the cross-sec tion of the bluff body is uniform over a first central main section along the extension of the bluff body. Analogously, the cross- section of the flow disturbance element is uniform over a second central main section of the flow disturbance element. Thereby, the chances improve that well balanced oscillations of the bluff body can be obtained. It is further advantageous if the second central main section overlaps at least a part of the first main section along the extension of the bluff body because then the flow disturbance element will disrupt the fluid flow around the bluff body in a relatively uniform and efficient manner. Namely, any parts of the flow disturbance element that do not overlap the bluff body transverse the flow direction will be less involved in producing useful vortex shedding downstream of the bluff body.

According to still another embodiment of the invention, the energy transducer contains at least one splitter plate, which is arranged on the bluff body. The at least one splitter plate has opposing main surfaces extending in parallel with the flow direction, and an outer edge of the at least one splitter plate is located further away from the central line of the bluff body than all parts of the flow disturbance element. In other words, the at least one splitter plate also effectively adjoins the flow disturbance element. The at least one splitter plate thereby increases a degree of vortex shedding correlation along the span of the bluff body being caused to oscillate by the vortex shedding. This effect is due to that the at least one splitter plate enforces highly synchronized vortex shed ding occurring in spanwise subsections into which the bluff body is divided by the at least one splitter plate. A higher degree of vortex shedding correlation is directly linked to larger amplitude of lift force by the oscillating bluff body, and thus increased vortex- induced motion displacements, which are generally desirable.

Preferably, the at least one splitter plate extends a threshold dis tance outside of an outermost part of the flow disturbance element relative to the central line of the bluff body, say the threshold distance may be equivalent to two to ten times the cross-section thickness of the flow disturbance element. Namely, this has proven to result in a constructive increase of said degree of vortex shed ding correlation.

Inter alia from a robustness point-of-view, it is advantageous if the bluff body exclusively has one degree of freedom in terms of movement, for example along a plane being perpendicular to the flow direction. Therefore, according to one embodiment of the in vention, the bluff body is connected to a support structure via a first set of resilient members, the bluff body is further connected to the at least one energy conversion device via a second set of resilient members, the first and second sets of resilient members are configured to restrict the movements of the bluff body to occur along a single dimension relative to a support structure, and each of the at least one energy conversion device is configured to trans form the movements of the bluff body along the single dimension into at least one of electrical energy and mechanical energy. It may also be advantageous if the at least one energy conversion device contains a suspension assembly configured to allow the bluff body to move with two degrees of freedom in the fluid flow relative to a support structure. Thus, the at least one energy con- version device may be connected to the bluff body via one or more resilient members. Preferably, a stiffness of at least one of the at least one resilient members is adjustable to allow adjustment of a natural frequency vibration of the bluff body in at least one of the flow direction and a direction transverse to the flow direction. Thereby, the energy transducer can be tuned to the fluid flow in which the bluff body is placed.

According to a further embodiment of the invention, at least one of the at least one energy conversion device contains a rotating drive generator anchored to the support structure and configured to transform two-dimensional movements of the bluff body into electrical energy. Thus, electrical energy may be produced at low losses based on the vortex-induced motions of the bluff body.

Alternatively, or additionally, at least one of the at least one en ergy conversion device contains a linear electric generator, which is configured to transform movements of the bluff body in at least one direction into electrical energy, for example transverse and parallel to the directions of the flow. Thereby, electrical energy may be produced efficiently based on the vortex-induced motions of the bluff body. The above suspension assembly may, in turn, include a directional gearbox, a coil spring and a belt reel, where the directional gearbox is mechanically connected to the rotating drive generator and is configured to convert alternating forward and backward rotations of the bluff body into a unidirectional rotation movement. The coil spring is mechanically connected to the directional gearbox and is configured to apply a constant torsion force to the unidirectional rotation movement. The belt reel mechanically interconnects the coil spring and the bluff body. The belt reel is further configured to forward the two-dimensional movements of the bluff body to the directional gearbox.

According to yet another embodiment of the invention, at least one of the at least one energy conversion device contains a double-action pump unit, which is configured to transform move- ments of the bluff body into mechanical energy embodied by an elevated gas pressure. Hence, the fluid flow may create useful mechanical energy without take a detour via electricity.

Further advantages, beneficial features and applications of the present invention will be apparent from the following description and the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now to be explained more closely by means of preferred embodiments, which are disclosed as examples, and with reference to the attached drawings. Figures 1a-c schematically illustrate a bluff body and a flow dis turbance element of an energy transducer ac cording to one embodiment of the invention;

Figures 2a, b show a support structure and a set of generic energy conversion devices according to one embodiment of the invention;

Figure 3 shows an energy conversion device according to one embodiment of the invention, which energy conversion device contains a linear electric ge nerator;

Figure 4 shows an energy conversion device according to one embodiment of the invention, which energy conversion device contains a double-action pump unit;

Figure 5 shows an energy conversion device according to one embodiment of the invention, which energy conversion device contains a coil-spring based de- sign; and

Figures 6a, b show a support structure and a set of generic energy conversion devices according to one embodiment of the invention, wherein the bluff body is restricted to move exclusively in a single dimension.

DETAILED DESCRIPTION

In Figure 1a, we see a side view of a bluff body 110 and a flow disturbance element 120, which form parts an energy transducer according to one embodiment of the invention. Figure 1 b shows a perspective view of the energy transducer in Figure 1a.

The bluff body 110 is elongated along a first axis A1 and is prefer ably uniform along its entire extension. The bluff body 110 may thus be represented by a cylinder having a circular or elliptical cross- section. In any case, the bluff body 110 is configured to be arranged in a fluid flow F having a direction DF transverse to the first axis A1 .

The flow disturbance element 120 is elongated along a second axis A2 that is parallel to the first axis A1. The flow disturbance element 120 is configured to disrupt the fluid flow F to cause vortex shedding downstream of the bluff body 110 relative to the flow direction DF. TO this aim, the flow disturbance element 120 may be arranged so that a main part of its volume is positioned upstream of a central line of the bluff body 110.

Figure 1c shows a detailed side view of the flow disturbance ele- ment 120 and how it may be arranged relative to the bluff body 110. According to the invention, the flow disturbance element 120 has a tapered cross-section, i.e. a general wedge shaped profile, and is arranged with a widest cross-section end 121 upstream of a narrowest cross-section end 122 relative to the flow direction DF. Preferably, the flow disturbance element 120 has a streamlined cross-section similar to that of an aircraft wing, and the flow disturbance element 120 is preferably arranged with a high-pressure side facing towards the bluff body 110 and a low- pressure side facing away from the bluff body 110.

Moreover, it is advantageous if the flow disturbance element 120 is arranged to provide an opening 130 for the fluid flow F between the bluff body 110 and a first surface 131 of the flow disturbance element 120 as illustrated in Figure 1c. It is further desirable that the flow disturbance element 120 is oriented so that a gap bet ween the bluff body 110 and the first surface 131 converges along the flow direction DF. Namely, this geometry causes the fluid flow F to attain a higher velocity in the gap than on a second side 132 of the flow disturbance element 120, which second side 132 is opposite to the first side 131. Consequently, a jet flow of fluid is generated. The jet flow, in turn, further breaks a symmetry of the fluid flow F in comparison to what would have been the case had the flow disturbance element 120 had a symmetric cross-section, for instance shaped as a circular cylinder. The further broken symmetry of the fluid flow F improves the bluff body’s 110 ability to undergo oscillating movements with a galloping response.

According to the invention, said oscillating movements of the bluff body 110 form a basis for extracting energy from the fluid flow F.

In order to extract energy from the fluid flow F, the energy trans ducer contains at least one energy conversion device. Figures 2a and 2b show side and perspective views respectively of a support structure 250 according to one embodiment of the invention. Here, a set of generic energy conversion devices is illustrated as 211 , 212, 213 and 214 respectively. The set of energy conversion devices 211 , 212, 213 and 214 is configured to transform move ments of the bluff body 110 into at electrical energy and/or mechanical energy as will be described below. Figures 2a and 2b illustrate one embodiment of the invention, where a respective suspension assembly is connected to side plates 140 and 143 of the bluff body 110. The suspension assem blies are configured to allow the bluff body 110 to move with two degrees of freedom in the fluid flow F relative to a support structure 250 onto which the suspension assemblies are mounted.

According to embodiments of the invention, the at least one en ergy conversion device 211 , 212, 213 and 214 is connected to the bluff body 110 via at least one resilient member, which in Figures 2a and 2b is illustrated as 221 , 222, 231 , 232, 233, and 234 respectively. It is preferable if a stiffness of at least one of the resilient members 221 , 222, 231 , 232, 233 and/or 234 is adjust able so as to vary the bluff body’s 110 stiffness in a direction in- line with and/or traverse the flow direction DF. Thereby, it is possible to adjust a natural frequency vibration of the bluff body 110 in the flow direction DF and/or a direction transverse to the flow direction DF.

As mentioned above, the cross-section of the bluff body 110 may be uniform along its entire extension. According to one embodi ment of the invention, the cross-section of the bluff body 110 is at least uniform over a first central main section M1 along the extension of the bluff body 110.

To make good use of the flow disturbance element 120, it is de- sirable that the cross-section of the flow disturbance element 120 is uniform over a second central main section M2 of the flow dis turbance element 120, and that the second central main section M2 overlaps the first main section M1 as much as possible. Ac cording to one embodiment of the invention, the second central main section M2 overlaps at least a part of the first main section M1 along the extension of the bluff body 110.

To increase the degree of vortex shedding correlation along the span of the oscillating part of the energy transducer, i.e. the bluff body 110, it is advantageous if at least one splitter plate 141 and 142 respectively is arranged on the bluff body 110. Each of the at least one splitter plate 141 and 142 has opposing main surfaces extending in parallel with the flow direction DF. An outer edge Eo of the at least one splitter plate 141 and 142 is located further away from the central line of the bluff body 110 than all parts of the flow disturbance element 120. Additionally, the bluff body 110 may contain side plates 140 and 143 respectively, which are arranged at each end of the bluff body 110 and preferably have the same outline as the at least one splitter plate 141 and 142, for example as illustrated in Figure 1b. The at least one splitter plate 141 and 142 enforces a highly synchronized vortex shedding in the spanwise subsections that are created between the at least one splitter plate 141 and 142 and the side plates 140 and 143, if included in the design. A high degree of vortex shedding correla tion is directly linked to a larger amplitude of lift force generated by the oscillating part, i.e. the bluff body 110, and thus increased vortex-induced motion displacements.

The at least one splitter plate 141 and 142, as well as any side plates 140 and 143, preferably extends a threshold distance d-m outside of an outermost part of the flow disturbance element 120 relative to the central line of the bluff body 110. It is generally advantageous if the threshold distance d-m is equivalent to two to ten times a maximum thickness of the cross-section of the flow dis- turbance element 120, so that the at least one splitter plate 141 and 142 reaches well outside the flow disturbance element 120. Namely, this has proven to result in a constructive increase of the degree of vortex shedding correlation.

Figure 3 shows an energy conversion device according to one embodiment of the invention, where the energy conversion device contains a linear electric generator 300. The linear electric generator 300 is configured to transform kinetic energy represen ted by movements M of the bluff body 110 in at least one direction into electrical energy, for example in the above-mentioned direction in-line with and/or traverse the flow direction D _F . The linear electric generator 300 may contain a stator 310 with a coil 315, and a translator 320 carrying permanent magnets 325, which translator 320 is adapted to move linearly inside the stator 310. Thus, when the translator is caused to move in response to the movements M of the bluff body 110, an electric current is induced in the coil 315, which can be fed out as output energy from the energy conversion device.

Figure 4 shows an energy conversion device according to one embodiment of the invention in the form of a linear pneumatic ge- nerator. Here, the energy conversion device contains a double action pump unit 400. The double-action pump unit 400 is confi gured to transform kinetic energy represented by the movements M of the bluff body 110 into mechanical energy embodied by an elevated gas pressure G. The movements M of the bluff body 110 causes a piston 410 to compress a gas in a chamber 420. The chamber is connected to a valve 430, which is adapted to forward the elevated gas pressure G for use as mechanical energy.

Figure 5 shows an energy conversion device according to one embodiment of the invention, which energy conversion device contains a coil-spring based design. Analogous to the embodi ment discussed above with reference to Figure 3, kinetic energy represented by movements M of the bluff body 110 is here con verted into electric energy. In this embodiment, the energy trans ducer contains at least rotating drive generator, here exemplified by 511 and 521 being anchored to a support structure (not shown) and configured to transform two-dimensional movements M of the bluff body 110 into electrical energy P _E ., which movements M typically are represented by alternating forward and backward rotations of the bluff body 110. Preferably, the design includes at least one suspension assembly 510 and 520 respectively being configured to allow the bluff body 110 to move with two degrees of freedom in the fluid flow F relative to the support structure. Each of the at least one suspension assembly 510 and 520 respectively, may, in turn, contain a di- rectional gearbox 512/522, a coil spring 513/523 and a belt reel 514/524. The directional gearbox 512 and 522 respectively is me chanically connected to the rotating drive generator 511 and 521 respectively. The directional gearbox 512 and 522 respectively is configured to convert the alternating forward and backward rota- tions M of the bluff body 110 into a unidirectional rotation move ment. The coil spring 513 and 523 respectively is mechanically con nected to the directional gearbox 512 and 522 respectively and is configured to apply a constant torsion force to the unidirectional rotation movement. The belt reel 514 and 524 respectively mechanically interconnects the coil spring 513/523 and the bluff body 110. Each belt reel 514 and 524 is further configured to forward the two-dimensional movements M of the bluff body 110 to the directional gearbox 512 and 522 respectively. Referring to Figures 6a and 6b, we see a support structure 250 and a set of generic energy conversion devices 215 and 216 respectively according to one embodiment of the invention. Here, the bluff body 110 is restricted to move exclusively in a single dimension. This means that the bluff body 110 moves in one plane only, which preferably extends essentially perpendicular to the flow direction DF.

In this embodiment, the bluff body 110 is connected to a support structure 250 via a first set of resilient members 235 and 237 respectively. The bluff body 110 is also connected to the at least one energy conversion device, here designated by 215 and 216, via a second set of resilient members 236 and 238 respectively. The first set of resilient members 235 and 237 and the second set of resilient members 236 and 238 are configured to restrict the movements of the bluff body 110 so that the movements only occur in a single dimension relative to a support structure 250, for example towards and away from the support structure 250 along a plane being traverse to the flow direction DF.

Analogous to the above, each of the energy conversion devices 215 and 216 respectively is configured to transform the move- ments of the bluff body 110 along the single dimension into at electrical energy, mechanical energy, or both.

Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed in- vention, from a study of the drawings, the disclosure, and the ap pended claims.

The term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components. The term does not preclude the presence or addition of one or more additional elements, features, integers, steps or components or groups thereof. The indefinite article "a" or "an" does not exclude a plurality. In the claims, the word “or” is not to be interpreted as an exclusive or (sometimes referred to as “XOR”). On the contrary, expressions such as “A or B” covers all the cases “A and not B”, “B and not A” and “A and B”, unless otherwise indicated. 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.

It is also to be noted that features from the various embodiments described herein may freely be combined, unless it is explicitly stated that such a combination would be unsuitable. The invention is not restricted to the described embodiments in the figures, but may be varied freely within the scope of the claims.