Patent Publication Number: US-2013241208-A1

Title: Sail system for generating energy from a fluid flow

Description:
PRIORITY CLAIM 
     The instant application claims priority to European Patent Application No. EP12158441.1, filed on Mar. 7, 2012, which application claims priority to Italian Patent Application No. MI2011A002333 filed on Dec. 21, 2011. All of the foregoing applications are incorporated herein by reference in their entireties. 
     TECHNICAL FIELD 
     A solution according to one or more embodiments relates to the field of the generation of energy. More specifically, this solution relates to the generation of energy from a fluid flow. 
     SUMMARY 
     A number of energy generating systems or simply generators that produce useful energy (e.g., of electrical type) by converting another form of energy are known in the art; in particular, in recent years there has been a great development of the generators based on renewable energy sources. 
     In more detail, various types of generators have been developed that are able to convert kinetic energy of a fluid flow (e.g., wind and sea currents). Considering for exemplary purpose wind-power generators, there are two main generator types. A first type consists of big turbines installed on pillars; the fluid flow imparts a rotary motion to the blades of a turbine from which electric energy is generated. A second type of wind-power generators includes generators based on elements that oppose resistance to the kinetic force of the fluid, such as parachutes. 
     In general, the parachute generators may be switched between a closed condition and an open condition (during which they are moved by the fluid flow). Moreover, an aerostatic balloon may be provided for maintaining the parachute at a working altitude. The parachute and the aerostatic balloon are coupled by a cable to an energy converter placed on the ground. The parachute is opened and closed alternately. When opened, the parachute is carried away from the energy converter by the force of the fluid flow (thereby allowing the energy converter to generate energy). The parachute is then closed and returned close to the energy converter (by using a fraction of the generated energy), so as to be opened and closed again cyclically. 
     There are also parachute generators with double section (each one including a parachute and an aerostatic balloon), wherein the sections are coupled by a single cable that passes through an alternating-type converter. In this case, the parachutes are opened and closed alternately. When a first parachute is opened, it is carried away from the energy converter by the force of the fluid flow (so as to generate useful energy); at the same time, a second parachute is closed and is brought close to the energy converter by the first parachute (without using the useful energy produced by the energy converter). 
     The above-mentioned generators have a very low environmental impact, are simple, reliable, and require very low maintenance; these qualities make the generators very cheap, but at the same time with excellent energy efficiency. In the case of parachute wind-power generators, indeed, it is possible to exploit high-altitude winds. As it is known, the wind speed increases moving away from the ground; for example, at 800 m above ground level, the wind has an average speed of 6-10 m/s for about 5000-7000 hours per year (compared to 4-5 m/s for 2000-4000 hours per year at an altitude of 80 m above ground level, i.e., the working altitude of the turbine wind-power generators). 
     However, such parachute wind-power generators are affected by a problem relating to the switching between opening and closing of the parachute itself. In fact, this requires a non-negligible amount of energy since it has to be performed in opposition to the wind flow. 
     In the prior art, this problem was solved by controlling guide wires for opening and closing of the parachute directly from the ground. This solution has the disadvantage of being complex and prone to failures even in the simple case of twisting of the guide wires. When such twisting occurs (e.g., due to strong turbulences) the parachute should be brought to the ground and the guide wires should be released (for preventing damages to the parachutes and to the respective switching means), which task is complex and time consuming. 
     It is also possible to equip the parachute with a motor, but this should be able to generate a high-intensity torque in order to open/close the parachute in opposition to the wind strength, and therefore it should be with a heavy weight (thereby reducing the conversion efficiency) and with a power supply provided from the ground, for example, by a power wire, which would make the system very dangerous in the event of a breaking thereof. 
     A solution that solves at least partly the above-mentioned problems is disclosed in the international patent application WO 2010/015720, which is incorporated by reference, wherein a system for generating energy from a fluid flow is described, where a pair of parachute means, each one independently switchable between an active condition and a passive condition, is able to slide along coupling means. In operation, the parachute means of the pair switch between their active condition and passive condition in phase opposition with each other; the parachute means in the active condition causes the parachute means in the passive condition to slide along the coupling means up to a stable position. 
     In general terms, a solution according to one or more embodiments is based on the idea of using opposite winding and unwinding of a first and a second cable structure for actuating an automatic opening and closing of an element of resistance to the fluid flow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A solution according to one or more embodiments, as well as its features and its advantages, will be better understood with reference to the following detailed description of one or more embodiments thereof, given purely by way of a non-restrictive indication and without limitation, to be read in conjunction with the attached figures (wherein corresponding elements are denoted with equal or similar references and their explanation is not repeated for the sake of brevity). In this respect, it is expressly understood that the figures are not necessarily drawn to scale (with some details that may be exaggerated and/or simplified) and that, unless otherwise specified, they are simply intended to conceptually illustrate the structures and procedures described herein. In particular: 
         FIG. 1  illustrates a system for generating energy according to an embodiment in an initial condition; 
         FIG. 2  illustrates the system for generating energy of  FIG. 1  in an opening condition of sail means according to an embodiment; 
         FIG. 3  illustrates the system for generating energy of  FIG. 1  in a carrying away condition of an aerostatic module according to an embodiment; and 
         FIG. 4  illustrates the system for generating energy of  FIG. 1  in a retrieval condition of the aerostatic module according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , there is illustrated a system  100  for generating energy according to an embodiment, in an initial condition. 
     In detail, the system for generating energy, or simply generator  100 , is adapted to generate a form of useful energy—for example, electricity—from a kinetic energy associated with a fluid flow, such as, for example, a flow of air (wind) as shown pictorially in the figure and indicated by the reference  105 . The generator  100  is bound to the ground and includes an energy conversion element, for example, a converter  110  able to generate electricity from a rotation (in a first rotation direction) of a transmission shaft thereof, or simply shaft  115 . In addition, the converter  110  includes a retrieval element, for example, a winding electric motor (integrated in the converter and not shown) adapted to use, preferably, although not limitatively, a portion of the energy produced by the converter  110  to rotate the shaft  115  (in a second rotation direction, opposite the first one). 
     A first cable structure, such as a first coupling cable  120  (e.g., formed by a metallic and/or synthetic wire) is secured by a first end to a first winding element, such as a generator pulley  125 , preferably, but not limitatively, coupled with the shaft  115  of the converter  110  (i.e., fixed so as to have a central axis thereof corresponding to a central axis of the shaft  115 ). In this way, the coupling cable  120  may be unwound from/wound onto the generator pulley  125  according to a rotation in the first/second direction of the shaft  115 . A second end (opposite to the first one) of the coupling cable  120  is bound to an aerostatic module  130 . In particular, the coupling cable  120  is fixed to a second winding element, such as a first pulley  135  of the aerostatic module  130  (having a radius r1) coupled with a central body, or sleeve  140 —which, preferably but not limitatively, has a substantially cylindrical shape—of a telescopic structure  145 , included in the aerostatic module  130 . The first pulley  135  may be integral with the sleeve  140  of the telescopic structure  145 . In addition, the first pulley  135  includes a blocking element (of the mechanical and/or electromechanical type, such as a vice or a friction brake, not shown in the figures) provided in order to selectively block an unwinding/winding of the coupling cable  120  attached to the first pulley  135  (i.e., the blocking element is adapted to inhibit a rotation of the first pulley  135 ). 
     Similarly, a third winding element, such as a second pulley  150  of the aerostatic means  130  (having a radius r2, preferably but not limitatively, such that r2 &lt;r1, preferably, but not limitatively, 1.5·r2≦r1≦2.5·r2, for example, r1≧2·r2) may also be coupled with the sleeve  140  of the telescopic structure  145 . A first end of a second cable structure, such as a second coupling cable  155 , is fixed to such second pulley  150 . Again, the second pulley  150  is integral with the sleeve  140  of the telescopic structure  145 . In addition, the second pulley  150  includes a further blocking element (not shown in the figures, and similar to that included in the first pulley  135 ) adapted to selectively block an unwinding/winding of the second coupling cable  155  attached to the second pulley  150  (i.e., the block element is adapted to inhibit rotation of the second pulley  150 ). 
     The second coupling cable  155  is fixed through a second end thereof to an aerostatic element, such as a balloon  160 , which is inflated with an element or a mixture of elements having a density lower than the one of the fluid that surrounds the generator  100  (for example, in the case of air, helium He in gas phase). The balloon  160  is designed to support the aerostatic module  130  immersed in the fluid at a predetermined distance from the converter  110  (in the example at issue, floating in air). In addition, the second coupling cable  155  is bound (for example, through a middle portion thereof) to an element of resistance to the fluid flow, such as a sail  165  (shown in  FIG. 1  being furled). A pair of sets of ropes, such as a first rope  170 L and a second rope  170 R, are each fixed to the sail  165  at opposite edges thereof and at a free end of a first screw  175 L and of a second screw  175 R, respectively, of the telescopic structure  145 . Preferably but not limitatively, the free ends of the two screws  175 L and  175 R are provided with suitable fastening elements (not visible in the figure) adapted to gyroscopically fasten the ropes  170 L and  170 R. In other words, the fastening between the free ends of the two screws  175 L and  175 R and the ropes  170 L and  170 R (as well as any other cable fastened thereto) is designed in such a way to prevent entanglements of the ropes  170 L and  170 R during rotations of the two screws  175 L and  175 R. 
     The two screws  175 L and  175 R are coupled with the sleeve  140  of the telescopic structure  145  at two opposite ends thereof. For example, a threaded portion (not shown in  FIG. 1 ) of each one of the screws  175 L and  175 R engages corresponding threaded holes  180 L and  180 R formed at the ends of the sleeve  140 . The threads of the screw  175 L and of the threaded hole  180 L on the left are formed with a direction opposite the threads of the screw  175 R and of the threaded hole  180 R on the right (e.g., clockwise and counter-clockwise, respectively). In this way, a rotation of the sleeve  140  in a first direction unscrews both screws  175 L and  175 R that extend from the respective threaded holes  180 L and  180 R; on the contrary, a rotation in a second direction tightens both the screws  175 L and  175 R that shorten going back into the respective threaded holes  180 L and  180 R. 
     Preferably, although not limitatively, the aerostatic module  130  includes a stabilizing element  185 , for maintaining the aerostatic module  130  in a functional disposition with respect to the fluid flow (e.g., horizontal with respect to the ground and transversal with respect to the direction of the fluid flow). The stabilizing element  185  is coupled to the telescopic structure  145 , for example, at the free ends of the screws  175 L and  175 R via coupling elements, such as third coupling cables  190  (in a similar manner as described with respect to the ropes  170 L and  170 R). Thanks to the stabilizing element  185 , it is possible to avoid any tangling of the coupling cables  120 ,  155  and  190  and/or the ropes  170 L and  170 R—or at least substantially reduce the possibility of such tangling—and, consequently, a resulting malfunction of the generator  100 . 
     Now, after describing the generator  100  per above, a complete cycle of operation of the generator is described, referring to  FIGS. 1-5 . 
     Initially (as shown in  FIG. 1 ), both the pulleys  135  and  150 , provided on the telescopic structure  145 , are maintained blocked by the respective blocking means, while the converter  110  allows a free rotation of the generator pulley  125  coupled with the shaft  115  of the converter  110 . Therefore, (as shown in  FIG. 2 ) the aerostatic module  130  rises to a first distance from the converter  110 , such as an operative altitude (e.g., 800 m), driven by the balloon  160 . Such operative altitude is determined by a ratio between the density of the element (or mixture of elements) within the balloon  160  and a density of the fluid in which the aerostatic module  130  is immersed and, in inverse manner, by a mass (i.e., by the weight) of the aerostatic module  130 . 
     At the operative altitude, the blocking means unblocks the cables  120  and  155  coupled to the respective pulleys  135  and  150  of the aerostatic module  130 . 
     The blocking means may be actuated for unblocking the respective pulleys  135  and  150  either through an electro-mechanical element—for example, controlled via radio-frequency signals—or mechanically—for example, through a mechanism that unblocks the pulleys in response to changes in the strain of the first coupling cable  120  and/or the second coupling cable  155  at the reaching of the operative altitude. 
     The balloon  160  may thus ascend further in altitude by a predetermined height h (being not limited by the weight of the aerostatic module  130  once the second pulley  150  has been unblocked), thus unwinding a portion of the coupling cable  155  wound on the second pulley  150 , thereby causing a rotation about its longitudinal axis of the sleeve  140  (integral with the second pulley  150 ). At the same time, the rotation of the sleeve  140  causes the winding of a portion of the first coupling cable  120  about the first pulley  135  (which is also integral with the sleeve  140 )—leading to a re-approaching between the aerostatic module  130  and the converter  110 . 
     The difference between the radius r1 of the first pulley  135  with respect to the radius r2 of the second pulley  150  (preferably but not limitatively, with r1&gt;r2) allows the balloon  160  to rise to the height h, while maintaining the telescopic structure  145  substantially at the operative altitude, thereby causing the rotation of the sleeve  140 . 
     Moreover, the rotation of the sleeve  140  causes the extension of both the screws  175 L and  175 R that come out, unscrewing themselves, from the threaded holes  180 L and  180 R, respectively, formed in the sleeve  140 , thereby extending the telescopic structure  145  (which reaches an elongated extension). Both the screws  175 L and  175 R come out by a distance x determined by the number of rotations performed by the sleeve  140 , in turn depending on a length y of the portion of coupling cable  155  unwound by the balloon  160  for reaching the height h, and by the radius r2 of the second pulley  150 . 
     It is important to note that the upward movement by the height h of the balloon  160  unfolds the sail  165  along a direction substantially perpendicular to the ground (where the converter  110  is positioned). At the same time, the extension of the screws  175 L and  175 R unfolds the sail  165  along a substantially horizontal direction (with respect to the ground). 
     Thanks to the upward movement of the balloon  160  combined with the extension of the screws  175 L and  175 R, the sail  165  is brought to a fully unfolded condition. Therefore, the sail  165  increases its resistance to the fluid flow  105  from a minimum value (corresponding to a furled condition, illustrated in  FIG. 1 ) up to a maximum value proportional to its sail surface A s  (in the fully unfolded or hoisted condition, shown in  FIG. 2 ). The sail  165  is designed so that the resistance opposed to the fluid flow  105  makes the entire aerostatic module  130  drift along the direction of the fluid flow  105  (by absorbing part of the kinetic energy thereof). In other words, when the sail  165  is unfolded, the aerostatic module  130  is carried away from the converter  110  being driven by a portion of the kinetic energy of the fluid flow  105 , which reaches the unfolded sail  165  (as shown in  FIG. 3 ). 
     When the sail  165  is in the fully unfolded condition, the blocking elements of both the pulleys  135  and  150  are actuated again for blocking a rotation thereof. In this way, further rotation of the sleeve  140  is prevented. 
     The fluid flow  105  performs a work by transferring part of its kinetic energy to the sail  165  and, therefore, to the aerostatic module  130 , which is moved. The carrying away of the aerostatic module  130  from the converter  110  causes an unwinding of the first coupling cable  120  from the generator pulley  125  and, thus, a rotation of the shaft  115  with which the same is integrally coupled. The rotation of the shaft  115  has the effect of transferring (at least in part) the kinetic energy associated with the movement of the aerostatic module  130  to the converter  110 . The converter  110  is configured for converting the kinetic energy of the rotating shaft  115  (at least in part) into a form of useful energy (e.g., electric energy). 
     The system  100  is thus able to generate useful energy from the energy associated with the fluid flow  105  (in which it is immersed) unless unavoidable losses due to energy conversion and to non-ideal components are included in the system  100 . 
     The aerostatic module  130  is driven by the fluid flow  105  up to reach a maximum distance from the converter  110 , wherein the portion of the coupling cable  120  being wound on the pulley  125  has been completely unwound. Consequently, a recovery of the aerostatic module  130  begins (shown in  FIG. 4 ) in order to return the generator  100  to the initial condition, for example, for starting a new cycle of energy production. 
     The blocking elements unblock the coupling cables  120  and  155  wound on the respective pulleys  135  and  150  of the aerostatic module  130  (e.g., in response to a radio-frequency signal or a strain variation of the first coupling cable  120 ). The first portion of the coupling cable  120  (previously wound on the second pulley  135  during the rise to the height h of the balloon  160 ) unwinds, thereby causing a revolution of the sleeve  140  of the telescopic structure  145  having an opposite direction with respect to the revolution described above—moreover, the aerostatic module  130  moves away from the converter  110  by a distance equal to the approaching made during the unfolding of the sail described above. Such rotation of the sleeve  140  causes a winding on the second pulley  150  of a portion of the second connecting cable  155  initially wound thereon. As a result, the balloon  160  is pulled back towards the telescopic structure  145 , dropping from the height h substantially to the altitude of the telescopic structure  145 . The difference in extension of the radius r1 of the first pulley  135  with respect to the radius r2 of the second pulley  150  (preferably but not limitatively, with r1&gt;r2) allows the balloon  160  to be pulled back by the height h, while maintaining the telescopic structure  145  substantially at the operational altitude, thereby causing the rotation of the sleeve  140 . 
     At the same time, the rotation of the sleeve  140  causes the retraction of both the screws  175 L and  175 R that are screwed into the threaded holes formed in the sleeve  140  of the telescopic structure  145 , which shortens. Preferably but not limitatively, during the screwing, the screws  175 L and  175 R substantially retract by the distance x described above. 
     It is important to note that the balloon  160 , by retreating, furls the sail  165  along a direction substantially perpendicular to the ground (i.e., where the converter  110  is placed). At the same time, the screwing of the screws  175 L and  175 R furls the sail  165  along a direction substantially horizontal to the ground. Therefore, the sail  165  is completely furled and a minimum resistance is opposed to the fluid flow  105  by it (i.e., by exposing only a fraction of its surface A s  to the fluid flow, as shown in  FIG. 4 ). 
     At the end of the rotation of the sleeve  140  of the telescopic structure  145 , the pulleys  135  and  150  are blocked again (e.g., in a manner complementary to that previously described for their unblocking). 
     At this point, the aerostatic module  130  may be returned to its initial state (shown in  FIG. 1 ) to start a new cycle of energy production. 
     The converter  110  activates the recovery element, which uses a part of the electricity produced by the converter  110  to rotate the shaft  115  in the second rotation direction, opposite the first one. In this way, the coupling cable  120  is wound back onto the pulley  125  by dragging the aerostatic module  130  to the initial state in opposition to the fluid flow. 
     Thanks to the minimum resistance that the sail  165  opposes to the fluid flow, the aerostatic module  130  may be brought back to the initial condition with low energy consumption (since the frictional forces that develop between the fluid flow  105  and the sail  165  are minimal, as the latter is furled). 
     Afterwards, a new complete cycle may be started, which takes place in the same way as described above. 
     The generator  100  according to an embodiment allows generating useful energy in a reliable manner and with a switching between the opening (i.e., unfolding) and closing (i.e., furling) of the element of resistance to the fluid flow that requires extremely reduced consumption of the produced energy with respect to one or more prior-art solutions. 
     Naturally, in order to satisfy local and specific requirements, a person skilled in the art may apply to the embodiments described above many logical and/or physical modifications and alterations. More specifically, although these embodiments have been described with a certain degree of particularity, it should be understood that various omissions, substitutions, and changes in the form and details as well as other embodiments are possible. Particularly, different embodiments may even be practiced without the specific details (such as the numerical examples) set forth in the preceding description to provide a more thorough understanding thereof; conversely, well-known features may have been omitted or simplified in order not to obscure the description with unnecessary particulars. Moreover, it is expressly intended that specific elements and/or method steps described in connection with any embodiment of the disclosed solution may be incorporated in any other embodiment as a matter of general design choice. In any case, the terms “comprising,” “consisting,” “having,” “including,” and “containing” (and any of their forms) should be understood with an open and non-exhaustive meaning (i.e., not limited to the recited elements), the terms “based on,” “dependent on,” “according to,” “function of” (and any of their forms) should be understood as a non-exclusive relationship (i.e., with possible further variables involved), and the term “a” should be understood as one or more elements (unless expressly otherwise stated). 
     For example, an embodiment proposes a system for generating energy from a flow of a fluid (of any type). The system includes energy conversion means (of any type), sail means (of any type, shape or size) switchable between an active condition to be carried away from energy conversion means by a fluid flow and a passive condition in which such carrying away is minimized. The system also includes coupling means (in any number, of any type, shape or size) for coupling the sail means with the energy-conversion means, the energy-conversion means converting part of the kinetic energy provided to the sail means by the fluid flow into said energy when the sail means in the active condition are carried away from the energy-conversion means. The system also includes recovering means (in any number, of any type, shape or size) for bringing back the sail means in the passive condition towards the energy-conversion means, and switching means (in any number, of any type, shape or size) for switching the sail means from the passive condition to the active condition and from the active condition to the passive condition at the reaching of a first distance and of a second distance, respectively, of the sail means from the energy-conversion means, the second distance being higher than the first distance. In an embodiment, the switching means includes a telescopic structure (of any type, shape or size) and the coupling means includes a first cable structure (of any type, shape or size) coupled between the telescopic structure and the energy conversion means and a second cable structure (of any type, shape or size) coupled between the telescopic structure and the sail means, the first cable structure and the second cable structure alternately winding onto the telescopic structure and unwinding from the telescopic structure during each switching of the sail means between the passive condition and the active condition for varying an extension of the telescopic means between a shortened extension in the passive condition in which an unfolding of the sail means is reduced and an elongated extension in the active condition in which the unfolding of the sail means is increased. 
     However, similar considerations apply if the system has a different structure or includes equivalent components (for example, of different materials), or it has other operative characteristics. In any case, every component thereof may be separated into more elements, or two or more components may be combined together into a single element; moreover, each component may be replicated to support the execution of the corresponding operations in parallel. It is also pointed out that (unless specified otherwise) any interaction between different components generally does not need to be continuous, and it may be either direct or indirect through one or more intermediaries. 
     However, intermediate functional extensions are not excluded. 
     In an embodiment, the sail means includes a sail (of any type, shape or size) having a first edge and a second edge opposite to each other; the switching means includes a first ropes set (in any number, of any type, shape or size) coupled between the first edge of the sail and a first end of the telescopic means, and a second ropes set (in any number, of any type, shape or size) coupled between the second edge of the sail and a second end of the telescopic means. 
     However, the ropes sets may be coupled to positions other than the edges of the sail and the ends on the telescopic means. 
     In an embodiment, the telescopic structure includes a sleeve (of any type, shape or size, see below) for the winding of the first cable structure and the second cable structure, the sleeve having a first axial hole threaded in a first direction and a second axial hole threaded in a second direction, a first screw screwed in the first threaded hole and a second screw screwed in the second threaded hole, the first screw and the second screw having a free end that defines the first end and the second end, respectively, of the telescopic structure. 
     However, the first end and the second end of the telescopic structure may be provided in other positions other than the free ends of the telescopic structure. 
     In an embodiment, the telescopic structure includes a first pulley and a second pulley (both of any type, see below) integral with the sleeve for the winding of the first and second cable structure, respectively. 
     However, the telescopic structure may include winding/unwinding means other than the first and second pulleys. 
     In an embodiment, the first pulley has a first radius and the second pulley has a second radius lower than the first radius. 
     However, different relations between the first and the second radii are not excluded. 
     In an embodiment, the second radius is shorter than a half of the first radius. 
     However, different ratios between the first and the second radii are not excluded. 
     In an embodiment, the switching means includes blocking means (in any number, of any type, shape or size) for blocking the first cable structure and the second cable structure with respect to the telescopic structure to prevent their winding and unwinding, and actuator means (in any number, of any type, shape or size) for disabling the blocking means at the reaching of the first distance, the first cable structure winding onto the telescopic structure and the second cable structure unwinding from the telescopic structure at the first distance, for activating the blocking means at the reaching of a predetermined unwinding of the second cable structure, for disabling the blocking means at the reaching of the second distance, the second cable structure winding onto the telescopic structure and the first cable structure unwinding from the telescopic structure at the second distance, and for activating the blocking means at the reaching of a predetermined unwinding of the first cable structure. 
     In an embodiment, the system further includes aerostatic means (in any number, of any type, shape or size) coupled with the sail means, the aerostatic means facilitating the winding of the first cable structure onto the telescopic structure and the unwinding of the second cable structure from the telescopic structure at the first distance. 
     However, other means is not excluded for facilitating the winding of the first cable structure and the unwinding of the second cable structure. 
     In an embodiment, the predetermined unwinding of the second cable structure and the predetermined unwinding of the first cable structure correspond to a complete unwinding of the second cable structure and a complete unwinding of the first cable structure, respectively, from the telescopic structure. 
     However, nothing prevents having the predetermined unwinding of the second cable structure and the predetermined unwinding of the first cable structure correspond to a partial unwinding of the second cable structure and of the first cable structure, respectively. 
     A different embodiment proposes a method for generating energy from a fluid flow. The method includes the following steps. Sail means, coupled through coupling means to energy conversion means, is switched between an active condition to be carried away from the energy conversion means by the fluid flow and a passive condition in which such carrying away is minimized. Part of a kinetic energy provided to the sail means by the fluid flow is converted, by means of the energy-conversion means, into said energy when the sail means in the active condition is carried away from the energy-conversion means. The sail means in the passive condition is recovered towards the energy-conversion means. The sail means is switched from the passive condition to the active condition and from the active condition to the passive condition at the reaching of a first distance and of a second distance, respectively, of the sail means from the energy-conversion means (with the second distance that is higher than the first distance). In a solution according to an embodiment, the coupling means includes a first cable structure coupled between the telescopic structure and the energy-conversion means, and a second cable structure coupled between the telescopic structure and the sail means. 
     The step of switching includes alternatively winding onto a telescopic structure and unwinding from the telescopic structure the first cable structure and the second cable structure during each switching of the sail means between the passive condition and the active condition for varying an extension of the telescopic structure between a shortened extension in the passive condition for reducing an unfolding of the sail means and an elongated extension in the active condition for increasing the unfolding of the sail means. 
     However, similar considerations apply if the same solution is implemented with an equivalent method (by using similar steps with the same functions of more steps or portions thereof, removing some steps being non-essential, or adding further optional steps); moreover, the steps may be performed in a different order, concurrently or in an interleaved way (at least in part). 
     From the foregoing it will be appreciated that, although specific embodiments have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the disclosure. Furthermore, where an alternative is disclosed for a particular embodiment, this alternative may also apply to other embodiments even if not specifically stated.