WO2018116276A1

WO2018116276A1 - Wave motion generator based on a dielectric elastomer with stiffness compensation

Info

Publication number: WO2018116276A1
Application number: PCT/IB2017/058362
Authority: WO
Inventors: Marco Fontana; Luca DANIELE; Giacomo MORETTI; Francesco DAMIANI; Michele RIGHI; Rocco Vertechy; Boris TEILLANT; Miguel VICENTE; Antonio SARMENTO; David Forehand; David Ingram
Original assignee: Scuola Superiore Sant'anna; Alma Mater Studiorum Universtita' Di Bologna; Wave Energy Centre – Centro De Energia Das Ondas
Priority date: 2016-12-23
Filing date: 2017-12-22
Publication date: 2018-06-28
Also published as: IT201600130691A1

Abstract

The device (1) for generating electric current by means of the sea waves, comprises a rigid box- shaped structure (3) immersed in the sea (M), which defines an insulated chamber (4) closed at the top by the elastic membrane (2) of a dielectric elastomeric generator, whose upper side (2A) is subjected to the wave action. In the insulated chamber (4) it its present a pressurized compressible fluid (6), whose pressure can be regulated during operation by means of dynamic response variator means (5), comprising members (51, 52) for the inflow and the outflow. The particular geometry of the device results in the combination of a negative hydrostatic stiffness effect (the hydrostatic forces tend the more to deform the membrane (2) downwards, the more the membrane (2) itself is deformed) which is opposed to the intrinsic positive elastic stiffness of the elastic membrane (2), allowing the natural frequency of the device to be tuned to the typical frequency of the sea waves through an appropriate selection of the design parameters. By increasing or reducing the pressure of the compressible fluid (6) in the insulated chamber (4), moreover, they are increased or decreased the stiffness of the elastic membrane (2) and its deformability level under the action of said wave motion, thus optimizing the energy efficiency of the device in the different sea conditions and ensuring its safety in extreme conditions.

Description

DESCRIPTION OF THE INVENTION

TITLE

WAVE MOTION GENERATOR BASED ON A DIELECTRIC ELASTOMER WITH STIFFNESS

COMPENSATION

TECHNICAL FIELD

The present invention falls within the technical field concerning electric power generators that use non-polluting renewable energies, such as the wave motion of the seas.

STATE OF THE ART

The idea, in absolute, is not unpublished and different technical solutions have been proposed and tested, based on a number of concepts that differ from each other in the choice of the system for converting the energy released by the wave motion into mechanical energy to be used for electricity production.

Among said technical solutions there are those which exploit the properties of electromechanical membrane transducers, capable of generating electric current when subjected to elongation and/or deformation stresses impressed by an external mechanical energy source (or vice versa).

The transducers which, in general, fall into the aforementioned typology already find applications in other fields; one of the most common is represented by the microphones, in which the sound waves generated by the voice or by other sounds are switched into electrical impulses to be sent to a recorder or to an amplifier. As is known, the specular application are the loudspeakers, in which the electrical signal determines the vibration of the membrane translator, which generates sound waves reproducing sounds and voices.

A specific type of electromechanical transducer is that of dielectric elastomer generators. The latter are deformable systems, based on polymeric materials, able to convert mechanical energy into electrical energy in a direct way. In functional terms, a dielectric elastomer generator is a deformable electrical capacitor, consisting of one or more dielectric elastomer membranes (capable of undergoing deformations over 500% of its initial length) coated with deformable electrodes. The mechanism of energy conversion is based on the principles of electrostatics (in fact there is no significant or functional contribution from electromagnetic or electrodynamic phenomena). In its simplest formulation, an operating cycle provides that from an initial configuration the elastic membrane is deformed by the mechanical work of an external source up to a dilated configuration, after which the generator is activated by imposing a voltage to the electrodes. Subsequently the membrane is left free to return to the initial configuration. During the transition between the dilated configuration and the initial configuration there is a reduction in capacity which causes an increase in the potential energy of the charges present on the electrodes (part of the mechanical work previously done on the system is converted into electrical energy). Finally, the capacitor is completely discharged, thus recovering an amount of electrical energy greater than that spent to activate it.

Recently it has been observed that the application of dielectric elastomer generators to the conversion of energy from wave motion is potentially promising.

The most simple type of said transducer is essentially constituted by a sandwich of three layers, obtained by two externals deformable conductive layers (electrodes) made for example of thin metal films (obtained by sputtering), or conductive elastomeric materials, which are separated by a dielectric elastomeric material.

The above devices are structured and sized according to certain operating conditions, variable in a predictable interval, within which the operating response can be considered repeatable. Outside this range the transducer might be affected by malfunctions with possible breakages, for example if the mechanical stresses exceed the maximum bearable values. In the prior patent US 9419545 B2 granted following application US20155214862, a system for generating electric current associated with the keel of a boat and driven by the wave motion is defended.

The transformation into electric current takes place, preferably, by means of an electromechanical generator with an elastic membrane.

The energy produced can be used directly for the propulsion of the boat or to feed onboard services or stored in batteries for other uses.

In the earlier document WO2013072123 it is claimed a converter which obtains electrical current from the wave motion by means of an electromechanical transducer. The converter is placed above the water surface and the transducer receives the thrust of the waves from bottom to top.

In the earlier document WO2008133774 it is defended another system to obtain electric current from the sea waves, by electromechanical transducers in which electroactive polymers are present. The system includes a sort of vertical bellows, in which the lower plate is anchored at a fixed height from the seabed while the upper one moves vertically by the waves action; inside the bellows there are a certain number of elastically extendable rods which constitute said electromechanical transducers, connected above and below the aforementioned plates and which are induced to follow the variation in distance which continuously intervenes between them, thereby producing electrical current.

In the earlier document CN101459390 it is described a generator formed by an elastically extensible electromechanical transducer which can take the form of a membrane or an elongated body, capable of generating electric current.

In the earlier document CN 104901584 it is described a generator which exploits a channeled air flow acting on an electromechanical transducer; the resonance phenomenon is provoked and exploited to obtain a load of alternating high frequency verse that generates electrical energy. This, evidently, is a considerable limitation for the application of interest to the present invention, since the wave motion can have extremely variable intensities, and assume storm connotations, in which case the forces involved become so high that they can easily destroy the structures realized according to the teachings of the solutions previously listed.

German patent application DE 10 2009 053393 describes a system for extracting wave energy in which a dielectric elastomer generator is associated with a buoy. In a described embodiment, the device is constituted by a rigid box-shaped structure defining at least a chamber insulated from the outside closed by an elastic membrane of said dielectric elastomeric generator, with the aforesaid rigid box-shaped structure designed to be partially immersed in the sea in a way that an outer side of the elastic membrane is subjected to the action of the wave motion. The device is anchored to the seabed and floating thanks to the air inside the insulated chamber.

The proposed solution is extremely interesting and yet the energy that can be extracted from the wave motion is very limited due to the extreme variability of the conditions of the wave motion. In fact, in order to be efficient, the wave energy converter must be dynamically tuned (eg resonant) with the typical frequencies of the sea waves, to ensure efficient operation and high wave energy capture factor. To do this, it is necessary to properly calibrate the stiffness of the device to match the dynamic response with the wave frequency. It is necessary to tune the device with different sea conditions, for example by adjusting the stiffness when sea conditions change.

In general, the technical solutions reported in the previous patents are limited by the fact that the high mechanical stiffness due to the elasticity of the dielectric elastomeric material influences the dynamic response of the wave energy conversion device, preventing or making complex the achievement of a tuning of the natural frequency of the device with the frequencies of the incident waves, thus limiting the maximum energy convertible from the system.

SUMMARY OF THE INVENTION Therefore, the object of the present invention is to propose a device for generating electric current by means of marine waves, which for this purpose exploits the properties of a dielectric elastomer generator, but incorporates technical solutions suitable for making the same device usable in the actual use conditions, maximizing the extracted energy by tuning the natural frequency of the device with that of the incident waves and being able to withstand even extreme stresses in the event of a storm.

In particular, a specific object of the invention is to propose a device for generating electric power by means of marine waves through which it is possible to realize a tuning between the frequency of the waves and the natural frequency of the device by means of a negative hydrostatic stiffness effect in order to compensate for the elastic stiffness of the dielectric elastomer.

A further object of the invention is to propose a device for generating electric current by means of marine waves, which can be used in a simple and effective way to adjust the dynamic response (stiffness) of the system according to the sea conditions in order to both tuning the natural frequency with that of the waves and limiting the deformations of said dielectric elastomeric generator within pre- established limits.

A further object of the invention is to provide a device of simple design, devoid of moving rigid bodies that can easily deteriorates in the marine environment, therefore reliable over time and not very expensive in terms of periodic maintenance.

Another object of the invention is to provide a compact device, which can also be installed in relatively low seabeds, therefore not too far from the shore, with evident benefits in installation costs and with advantageous containment of the length of the cables intended to transport the electrical energy produced on the mainland.

These and other objects are fully achieved by means of a device for generating electric current by means of marine waves, of the type comprising at least one dielectric elastomeric generator designed to be deformed and/or elongated by the thrust action received from the marine wave motion, with the aforesaid device characterized in that it comprises:

- a rigid box-shaped structure defining at least one chamber insulated from the outside, closed at the top by an elastic membrane of said dielectric elastomeric generator, with the aforementioned rigid box-shaped structure designed to be immersed in the sea so that the top side of the aforementioned elastic membrane is subjected to the action of the wave motion; wherein geometrical, physical and/or installation parameters of said device are defined such that the combination of negative hydrostatic stiffness and positive elastic stiffness of the elastic membrane gives the device a natural frequency contained in the frequency range typical of wave motions, that is, in a range from 0.05 Hz to 0.3 Hz.

The peculiar definition of the geometrical, physical and installation parameters of the device allows to exploit the effect of negative stiffness to match the natural frequency of the device with that of the wave motion without introducing additional components, taking advantage of the particular geometry of the device. This allows to realize devices with high performances of energy capture from wave motion while limiting the dimensions of the device itself.

Advantageously, the device includes:

- elastic response variators means, comprising a compressible fluid introduced by pressure into said insulated chamber, affecting the inner side of said elastic membrane;

- members for the inflow and outflow of the aforesaid compressible fluid towards and from said insulated chamber adapted to modify the amount of compressible fluid present inside the insulated chamber and consequently the dynamic response of the system comprising said elastic membrane and its deformability level under the action of said wave motion.

Thanks to the possibility of varying the elastic response of the device, it is possible to adapt the latter to the wave motion changes in such a way as to always maintain substantially tuned the natural frequency of the device with that of the wave motion. BRIEF DESCRIPTION OF THE DRAWINGS

The characteristics of the invention will be made clear in the following description of preferred embodiments of the device for generating electric current by means of sea waves in question, in accordance with what included in the claims and with the aid of the attached drawings, in which :

- Fig. 1 shows a schematic side view, in section, of the device in question, positioned on a seabed, in a not working condition;

- Fig. 2 shows a plan view of the device of Fig. 1 ;

- Figs. 3 and 4 show views similar to Fig. 1 with the device working.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the aforementioned figures, it has been referenced as 1 the device for generating electric current by means of marine waves, object of the present invention, as a whole.

The device 1 is of the type comprising at least one dielectric elastomeric generator comprising an elastic membrane 2, capable of generating electrical current when subjected to elongation and/or deformation stresses impressed by an external mechanical energy source.

According to an already known technique, the aforementioned dielectric elastomeric generator is for example made of several layers of elastomeric material interposed between deformable laminar electrodes, so as to form a set of capacitors associated mechanically in series but electrically in parallel.

With reference to the invention, in the device 1 a rigid box-shaped structure 3 is provided defining at least a chamber 4 insulated from the outside, closed at the top by the elastic membrane 2 of said dielectric elastomeric generator. In the preferred embodiment shown, the insulated chamber 4 has a cylindrical shape and the elastic membrane 2 has a circular shape.

The rigid box-shaped structure 3 is designed to be immersed in the sea M so that the upper side 2A of the elastic membrane 2 is subjected to the action of the overlying wave motion. The rigid box-shaped structure 3 is held steady with respect to the seabed, FM, in proximity and resting on it, as illustrated in the figures, or maintained at a constant height, however in such a way as to cancel the hydrodynamic stresses insisting on the rigid box-shaped structure 3 itself.

What has just been said can be simply obtained by the weight of the rigid box-shaped structure 3 or by anchoring means or hydrodynamic brakes not illustrated in detail.

In the preferred embodiment of the figures, there is provided a flow collector 30, of tubular shape open at the ends, made integral with the rigid box-shaped structure 3, arranged above the elastic membrane 2, centered with respect to the same.

In accordance with the shape of the insulated chamber 4, the illustrated flow collector 30 has also a cylindrical section.

The flow collector 30 is designed to convey the hydrodynamic motion of the aforementioned marine waves to the underlying electromechanical elastic membrane generator, according to the differential pressure principle.

For the flow collector 30, variants of different lengths are obviously possible, ie non-perfectly cylindrical shapes, for example troncated-conical, flared or similar, designed to optimize the flow of the water column inside it.

The device 1 also comprises dynamic response variator means 5, arranged for example outside the insulated chamber 4 and provided for introducing in pressure into the latter a compressible fluid 6 (air or gas), intended to affect the inner side 2B of the mentioned elastic membrane 2.

The dynamic response variator means 5 are constituted, for example by at least one tank 50 for said compressible fluid 6 and by known members 51 , 52 (pumps, pipes, solenoid valves, pressure gauges and other components, not shown in detail) for the inflow and the outflow of the same compressible fluid 6 towards and from said insulated chamber 4.

It is important to highlight the fact that the device, unlike other known solutions, is shaped so as to exploit the wave motion according to the differential pressure principle. In particular, the aforesaid device achieves a negative hydrostatic stiffness effect because to a deformation of the dielectric elastomeric membrane 2 downwards corresponds an increase in the hydrostatic pressure which tends to further deform the membrane 2 itself downwards (and vice versa, to a deformation upward corresponds a decrease of the hydrostatic pressure on the membrane). This negative hydrostatic stiffness has clearly opposite contribution with respect to the elastic stiffness of the dielectric elastomeric membrane, whose stiffness is positive because to an increase in deformation (expansion of the membrane downwards) corresponds an increase in the elastic return forces on said membrane. In the present invention, the abovesaid effect is exploited by suitably defining geometrical, physical and installation parameters of the device in such a way that the combination of negative hydrostatic stiffness and positive elastic stiffness of the elastic membrane 2 give the device 1 a natural frequency contained in the typical frequencies range of the wave motions, that is to say, in a range between 0.05 Hz and 0.3 Hz. This peculiar and innovative definition of the geometrical, physical and installation parameters of the device that make it possible to exploit the negative hydrostatic stiffness of the elastic membrane 2 leads to the considerable advantage of being able to realize devices 1 having high performances for capturing energy from wave motion while limiting the dimensions of the device itself.

Moreover, by increasing or decreasing the pressure of the compressible fluid 6 in the insulated chamber 4, it is increased or reduced the stiffness of the elastic membrane 2 and its deformability level under the action of the wave motion, and the dynamic response of the system comprising the elastic membrane 2 is overall changed. As intuitively understandable, this makes it possible to adapt the device to the sea conditions so that the device 1 is dynamically tuned (for example, resonant) with the typical frequencies of the sea waves, thus optimizing the energy efficiency of the device under operating conditions, and avoiding that the dielectric elastomeric generator can undergo extreme stresses and therefore break itself when the wave motion becomes too intense. The dynamic response variator means 5 can easily be set up to be remotely controlled with suitable organs, and/or connected to a weather station with appropriate interface means for being able to adapt in advance, and automatically, to the forecasts relating to the sea conditions.

In the accompanying drawings and in the present description the constructional details placed downstream of the dielectric elastomeric generator and which concern the transport of electrical energy to the mainland are omitted, since they are of known art and irrelevant for the purposes of understanding the described device.

From the above it is clear how the device represents a concrete and realistic technical solution for the generation of electric current by means of marine waves, exploiting the properties of elastomeric dielectric generators.

Thanks to the aforementioned effect of negative hydrostatic stiffness, it is indeed possible to counteract the effect of positive mechanical stiffness of the dielectric elastomeric membrane. This allows, through an appropriate selection of the design parameters, in particular geometrical, physical and of installation, to intrinsically obtain a natural frequency of the device comparable to that of the sea waves, while limiting the dimensions. More precisely, the diameter, the thickness and the preload of the elastic membrane 2, as well as the installation depth below the sea level of the device 1 , are defined and combined so as to give the device 1 a natural frequency contained in the typical frequency range of the wave motions, that is between 0.05Hz and 0.3Hz.

Moreover, thanks to the dynamic response variator means, the device can be even more precisely and efficiently tuned with the wave motion in the real use conditions and in the presence of different wave motion conditions and can be able to withstand even extreme stresses in the event of a storm.

The possibility of further varying the behavior of the dielectric elastomeric generator allows to always have the optimum efficiency in all working conditions. An advantage of the invention consists in realizing the device in such a way that its main components are particularly resistant to deterioration in the marine environment, making the device itself reliable over time and not very expensive in terms of periodic maintenance.

Thanks to the simple solution of the flow collector, the device can also be installed in relatively low seabeds, therefore not too far from the shore, still guaranteeing high efficiency; with this, evident benefits are obtained in the installation costs (and maintenance) and in the containment of the lenght of the cables intended to transport the electric energy produced on the mainland.

The advantages shown above remain unchanged even in the presence of variations or modifications to what above described by way of example.

For example, the tank 50 which in the example shown is located outside the rigid box-shaped structure 3 could be housed inside it, for example at the bottom, or in a gap between an outer wall and an inner wall.

It is understood however that what above said has exampling and not limiting value, therefore any modifications of detail that may be necessary to be made for technical and/or functional reasons, are considered from now as remaining within the same protective scope defined by the claims below.

Claims

1. Device for generating electric current by means of marine waves, of the type comprising at least one dielectric elastomeric generator designed to be deformed and/or elongated by the thrust action received from the marine wave motion, with the aforesaid device (1) characterized in that it provides:

- a rigid box-shaped structure (3) defining at least one chamber (4) insulated from the outside closed at the top by an elastic membrane (2) of said dielectric elastomeric generator, with the aforementioned rigid box-shaped structure (3) designed to be immersed in the sea (M) so that the upper side (2A) of said elastic membrane (2) is subjected to the wave motion action;

wherein geometrical, physical and/or installation parameters of said device (1) are defined such that the combination of negative hydrostatic stiffness and positive elastic stiffness of said elastic membrane (2) gives said device (1) a natural frequency contained in the typical frequency range of wave motions.

2. Device according to claim 1 , characterized in that the natural frequency of said device (1) is contained in a range between 0.05 Hz and 0.3 Hz.

3. Device according to claim 1 , characterized in that said insulated chamber (4) has a cylindrical shape and said elastic membrane (2) has a circular shape, said geometrical, physical and/or installation parameters of said device (1) comprising the diameter, the thickness and the preload of said elastic membrane (2), as well as the depth of installation below the sea level of said device (1).

4. Device according to claim 1 , characterized in that said device (1) comprises: - elastic response variator means (5), comprising a compressible fluid (6) introduced by pressure into said insulated chamber (4), affecting an inner side (2B) of said elastic membrane (2);

- members (51 , 52) for the inflow and outflow of said compressible fluid (6) towards and from said insulated chamber (4) suitable for modifying the quantity and pressure of said compressible fluid (6) present inside said insulated chamber (4) itself and consequently the dynamic response of the system comprising said elastic membrane (2).

5. Device according to claim 1 , characterized in that said device (1) is suitable for being anchored near the seabed.

6. Device according to claim 1 , characterized in that it provides a flow collector (30), of tubular shape open at the ends, made integral with the rigid box-shaped structure (3), arranged in correspondence of said elastic membrane (2), centered with respect to the same, the aforesaid flow collector (30) being designed to convey the hydrodynamic motion of the aforementioned marine waves towards said elastic membrane (2).

7. Device according to claim 1 , characterized in that said compressible fluid (6) consists of air.

8. Device according to claim 1 , characterized in that said compressible fluid (6) consists of gas.

9. Device according to any one of the preceding claims, characterized in that said rigid box-shaped structure (3) is held steady with respect to the seabed (FM) by means adapted to cancel the hydrodynamic stresses insisting on it.

10. Device according to claim 4, characterized in that said dynamic response variator means (5) comprise at least one tank (50) for said compressible fluid (6), separated from the said insulated chamber (4) and connected thereto by said members (51 , 52) for the inflow and outflow.

11. Device according to claim 4, characterized in that there are provided organs for remote control of said dynamic response variator means (5).

12. Device according to claim 4, characterized in that said dynamic response variator means (5) are interlocked to a weather detection station by interface means, adapted to act in advance and automatically on the same said dynamic response variator means (5) according to the forecast related to the detected sea conditions.