GB2601721A

GB2601721A - Fluid turbine

Info

Publication number: GB2601721A
Application number: GB2010698.5A
Authority: GB
Inventors: Lee Crowder Jeffrey
Original assignee: CDR SYSTEMS Ltd
Current assignee: CDR SYSTEMS Ltd
Priority date: 2020-07-10
Filing date: 2020-07-10
Publication date: 2022-06-15
Also published as: GB202010698D0

Abstract

An axial fluid turbine system 110 comprising a rotor string 112, including a drive member 120 with at least two rotors 118 rotationally coupled to an axially spaced along it, and a generator 114 coupled to the drive member, for generating electric power. The drive member may comprise a flexible or rigid member. It may comprise a main link extending between a first rotor and the generator and one or more intermediate drive links extending between adjacent rotors, said drive links being connected by a flexible member. The blades (26, fig 1) of neighbouring rotors may be angularly offset in use. The rotor may comprise a hub and at least one blade connected to the hub so that it can move with respect to the hub between a stowed position and a deployed position in which the blades engage with the fluid flow. The blades may move into the second position automatically when immersed in a fluid flow. The system may comprise several parallel rotor strings connected to a single generator by a drive mechanism 132.

Description

Fluid Turbine

FIELD OF THE INVENTION

The present invention relates to fluid turbines, and in particular to fluid turbines that are arranged to generate electrical power. The present invention relates to a reaction turbine comprising a plurality of rotors.

BACKGROUND TO THE INVENTION

Water wheels, and later water turbines, have been used for hundreds of years to convert the energy from flowing water to useable kinetic energy, for example in a water mill, or to electrical energy, by means of an electrical power generator.

In a water turbine the flowing water acts on blades of a turbine runner or rotor, creating a force on the blades to spin or rotate the runner. The runner is connected to a drive shaft which may be connected to another machine or to a suitable electrical power generator.

There are two common types of water turbine, a reaction turbine and an impulse turbine. Reaction turbines are configured such that water acting on the turbine changes pressure and imparts energy to the turbine. Reaction turbines must, therefore, be fully encased or fully submerged in water. Impulse turbines are driven by a change in velocity of a jet of water. In particular, the change in momentum of the water flow as the water strikes the blades of the turbine rotates the turbine. In an impulse turbine it is the kinetic energy of the flowing water, rather than the pressure of the flowing water that changes.

Currently, there are a number of hydroelectric power stations around the world generating electrical power on different scales. A small hydroelectric power plant, serving a small town or industrial area, for example, may generate up to about 20 -2 -megawatts of electrical power. Larger hydroelectric power stations may generate several thousands of megawatts of electrical power. These hydroelectric power stations require significant infrastructure. Typically the larger power stations will be located in large dams, which in turn require the creation of reservoirs. This further limits the available locations for these power stations.

It is generally desirable to increase the amount of electrical energy that is generated from renewable sources, such as wind and water. It is, therefore, known to construct smaller hydroelectric power plants that provide electrical power to a local area, or single dwelling. For example, micro hydro power plants may be used to supply power to small communities or businesses, and typically generate between about 5 and 100 kilowatts of electrical power. In remote communities requiring only small amounts of electricity, it is known to provide a pico hydroelectric power plant. These pico-power plants will typically generate less than 5 kilowatts of electrical power.

These smaller-scale hydroelectric power plants, however, often require a structure (such as system of canals and/or pipes) to be created in order to divert a flow of water down a gradient and through the turbine. This adds to the cost and complexity of these systems, for a relatively small amount of electrical power generation.

One problem associated with some hydroelectric power plants is that in some instances, for example where water depth or other dimensions are restrictive, turbines with larger diameter rotors may be unsuitable or prohibitively expensive, but the available water pressure (head) and/or flow rate may be insufficient to drive a turbine of suitable dimensions with sufficient force.

There are several known systems that utilise a single propeller-type rotor attached to a drive shaft to generate electricity. In one known system, for example, the propeller is mounted to a sailing vessel and is arranged to be submerged below the surface of the water. When the vessel is sailing under the power of the wind, the propeller turns due to the flow of water and is used to generate electrical power for the vessel. This known system produces up to about 600 Watts of power. The -3 -electrical power may be used to run lighting and navigational equipment on board the vessel.

It will be appreciated that it is desirable to provide a cost-effective hydroelectric 5 power plant that may be used to generate electrical power in areas in which electrical generation currently relies on the use of fossil fuels.

Such situations and areas are, for example: (i) semi-portable or fixed systems to generate electrical power in remote locations, 10 which may be used by land owners, the Armed Forces or emergency services, for example; (ii) larger fixed installations suitable for providing power to remote communities where strong offshore tidal flows or rivers are available; (iii) larger commercial units installed as stand-alone or multiple assemblies, in rivers, estuaries, offshore areas with strong tidal flow, and which may be used to supply electrical power to local communities or to the national grid, for example (these larger commercial units may be linked to existing offshore wind farms); (iv) systems configured to provide electrical power from sub-surface watercourses, drains, sewers and the like; and (v) systems connected to offshore installations and vessels at anchor, which may currently have a reliance on diesel electrical generators for example.

Against this background it is desirable to provide a cost-effective fluid turbine system for generating electrical power.

SUMMARY OF THE INVENTION

An aspect of the present invention provides an axial fluid turbine system comprising: a rotor string including a drive member and at least two rotors rotationally 30 coupled to the drive member and axially spaced along the drive member; and a generator for generating electrical power coupled to the drive member. -4 -

Preferably an axial spacing between adjacent rotors is no less than a radius of a smallest diameter rotor coupled to the drive member. Preferably an axial spacing between adjacent rotors is no less than a diameter of the smallest diameter rotor coupled to the drive member.

Preferably the at least two rotors are coaxial.

The rotor string is preferably not enclosed.

The drive member may comprise a flexible member. Alternatively the drive member may comprise a rigid member.

In preferred embodiments the drive member comprises a main drive link extending between a first rotor coupled to the drive member and the generator, and at least one intermediate drive link extending between adjacent rotors coupled to the drive member. At least two of said drive links are preferably connected by a flexible connector. At least one of the main drive link and the or each intermediate drive link may comprise a flexible member.

The rotors are preferably arranged to rotate in the same direction.

Preferably the spacing between adjacent rotors is uniform along the rotor string.

Each rotor preferably comprises at least two blades. Blades of neighbouring rotors are preferably angularly offset from one another in use.

In preferred embodiments each rotor comprises a hub and at least one blade connected to the hub, and the or each blade is arranged to move with respect to the hub between a first stowed position and a second deployed position in which the or each blade is arranged to engage with a fluid flow so as to drive rotation of the rotor.

Preferably the or each blade is arranged to move into the second position automatically when the rotor string is immersed in a fluid flow. -5 -

In preferred embodiments the axial fluid turbine system comprises a plurality of rotor strings. The axial fluid turbine system preferably further comprises a drive mechanism arranged to transmit drive from each of the plurality of rotor strings to a single generator.

Each of the rotor strings may extend parallel to each other.

In preferred embodiments the axial fluid turbine system further comprises a mounting system for mounting the turbine system to a supporting structure, the mounting system comprising a pivot permitting rotational movement between the or each rotor string and the supporting structure.

The or each rotor string preferably has neutral buoyancy.

In preferred embodiments said fluid is a liquid, and more preferably said liquid is water. The or each rotor string is preferably fully submerged in the liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described by way of example only, and with reference to the accompanying drawings, in which: Figure 1 illustrates a first embodiment of a fluid turbine system in accordance with the present invention; and Figure 2 illustrates a second embodiment of a fluid turbine system in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a fluid turbine system for generating electrical power. -6 -

The turbine system is arranged to be submerged in a fluid such that fluid flow relative to the turbine system may be used to generate electrical power. The turbine system is particularly suited to operation in a liquid, for example a body of water.

As can be seen in Figure 1, a fluid turbine system 10 according to a first embodiment of the invention comprises a rotor string 12, a generator 14 and a mounting system 16.

In this first embodiment, the generator 14 comprises an electrical power generator of a kind well known in the art. The generator 14 may comprise an alternator. The electrical generator 14 is not central to the present invention and will not be described in detail. The electrical generator 14 comprises an input shaft (not shown) which is arranged to be turned by an external torque or drive in order to drive the generator 14 and produce electrical power. In the present embodiment, torque is transmitted to the input shaft by the rotor string 12.

As shown in Figure 1, the rotor string 12 comprises a plurality of rotors or runners 18 and a drive member 20. The rotors 18 are spaced apart along a length of the rotor string 12. In use, when submerged in a fluid flow, the rotor string 12 is arranged to turn so as to drive the generator 14.

The drive member 20 comprises an elongate member connected at a first end to the generator and extending away from the generator 14 to a second end along an axis A of the turbine system 10. The first end of the drive member 20 defines a first end of the rotor string 12. The second end of the drive member 20 defines a second end of the rotor string 12. The first end of the drive member 20 is connected to the input shaft of the generator 14. The drive member 20 comprises drive links 22, 24. The drive member 20 comprises a main drive link 22 and at least one intermediate drive link 24. In this embodiment, a plurality of intermediate drive links 24 are provided. The main drive link 22 extends between a first end of the drive member 20 and a first rotor 18 nearest the generator 14. The intermediate drive links 24 extend between neighbouring rotors 18 along the rotor string 12. In this -7 -embodiment, the drive member 20 is provided by a single member, such that the main drive link 22 and intermediate drive links 24 are contiguous. In this embodiment the drive member 20 comprises a flexible member in the form of a flexible shaft or cable. The main drive link 22 may be longer than the or each intermediate drive link 24 Each of the plurality of rotors 18 comprises a propeller-type rotor having a central hub and a plurality of angularly spaced, pitched blades 26 which extend radially from the hub. In this embodiment, the turbine system 10 comprises five rotors 18, and each rotor has three blades 26. Each rotor 18 is arranged to rotate with the drive member 20. Each rotor 18 is arranged to rotate about an axis substantially parallel to the axis A and in a plane substantially perpendicular to the axis A. The rotors 18 are axially spaced apart along the drive member 20, with the first rotor 18 nearest the first end of the drive member 20 and, in this embodiment, a fifth rotor 18 nearest the second end of the drive member 20. The intermediate drive links 24 comprise portions of the drive member 20 between adjacent rotors 18. Accordingly, four intermediate drive links 24 are provided, to extend between neighbouring ones of each of the five rotors 18. The rotors 18 are arranged in an inline configuration along the drive member 20. In some embodiments the rotors 18 are substantially coaxial or coaxially arranged.

The rotors 18 are rotationally coupled to the drive member 20. Each rotor 18 is rotationally coupled to at least one intermediate drive link 24. The intermediate drive links 24 and main drive link 22 are rotationally coupled to one another so as to transmit torque from each rotor 18 to the drive member 20 as a whole. In this way, when the rotors 18 are placed in a fluid flow, the rotors 18 are arranged to turn the drive member 20 in order to drive the generator 14. In some embodiments, each rotor 18 is mounted on a respective intermediate drive link 24. In other embodiments, each rotor 18 comprises a shaft which may be connected to intermediate drive links 24. -8 -

In use, the turbine system 10 is arranged to be mounted such that the drive member 20 extends substantially parallel to the direction of a fluid flow. In this arrangement, fluid flow passes in a direction substantially perpendicular to a plane of rotation of the rotor blades 26, such that the fluid flow causes rotation of the rotors 18. The rotors 18 are therefore axial flow rotors. With this arrangement, the turbine system comprises an axial turbine or axial flow turbine system. In this way, each of the rotors 18 simultaneously drives rotation of the drive member 20.

The turbine system 10 is arranged to be mounted such that at least part of the turbine system 10 is submerged. In some embodiments the turbine system 10 is fully submerged. The turbine system 10 may be mounted to a variety of submerged surfaces or objects, such as a seabed or riverbed, an anchored vessel or offshore rig, or an inner surface of a subsurface water course.

In this embodiment, the turbine system 10 is mounted in a submerged position by the mounting system 16 which is connected to the generator 14. The mounting system 16 comprises a mounting post 28 and a support member 30. The mounting post 28 extends between a supporting surface or structure (not shown) to which the turbine system 10 is mounted, and the support member 30. The support member 30 is connected to an end of the mounting post 28 and is arranged to support the generator 14. In this embodiment the mounting system 16 includes a pivot (not shown) to allow the turbine system 10 to pivot with respect to the structure or surface to which it is mounted.

In this first embodiment, the second end of the rotor string 12 is not mounted or tethered in position. Since the first end of the rotor string 12 is connected to the generator 14, which is held in position by the mounting system 16, the free second end of the rotor string 12 may be urged in a direction downstream of the first end by the fluid flow. Together with the pivot provided by the mounting system 16, this allows the rotor string 12 to be aligned by and with the fluid flow, to extend substantially parallel to the fluid flow. Advantageously, in this way, the turbine system 10 may be self-correcting or self-aligning with respect to the direction of fluid -9 -flow, in a manner analogous to a wind vane. Alignment of the rotor string 12 with the fluid flow orients the rotors 18 to face the fluid flow, thereby improving the interception of the fluid flow by the rotor blades 26 and increasing the torque supplied to the drive member 20 (and thus generator 14) for a given flow rate.

With this arrangement, the turbine system 10 is arranged to pivot so as to maintain alignment of the rotor string 12 with the direction of fluid flow in the event that the direction of fluid flow changes. The turbine system 10 may therefore pivot to adjust or compensate for smaller perturbations in fluid flow direction, and/or may pivot such that the rotor string 12 extends in a generally opposite direction, for instance when installed in a tidal stream, where the fluid flow direction may reverse.

A turbine system 110 according to a second embodiment of the invention is shown in Figure 2. The turbine system 110 is similar to the turbine system 10 of the first embodiment, and only differences will be described in detail.

In this second embodiment, the turbine system 110 comprises a plurality of rotor strings 112. The rotor strings 112 extend substantially parallel to one another and to an axis A' of the turbine system 110. The first ends of the rotor strings 112 are aligned with one another and the rotor strings 112 are laterally spaced apart such that they extend in a substantially coplanar arrangement. In this way, rotors 118 of the rotor strings 112 are arranged in an array, as can be seen in Figure 2. Four rotor strings 112 are provided in this embodiment.

The first end of a drive member 120 of each rotor string 112 is connected to a drive mechanism 132. The drive mechanism 132 is disposed between the rotor strings 112 and a generator 114, and is connected to an input shaft of the generator 114. The drive mechanism 132 is arranged to couple rotation of the drive members 120 to rotation of the generator input shaft in order to transmit drive from the drive members 120 to the generator 114. In this embodiment, the drive mechanism 132 comprises a generator sprocket wheel 134, a plurality of drive sprocket wheels 136 and a chain 138 The generator sprocket wheel 134 is mounted to rotate with the -10 -input shaft. Each drive sprocket wheel 136 is mounted to rotate with a respective drive member 120, proximate the first end of the drive member 120. The generator sprocket wheel 134 and drive sprocket wheels 136 are laterally spaced apart and aligned with one another so as to rotate in a plane substantially perpendicular to the axis A'. In this embodiment the generator sprocket wheel 134 is disposed between two pairs of drive sprocket wheels 136. The chain 138 engages with and entrains the drive sprocket wheels 136 and generator sprocket wheel 134 so as to transmit drive from the drive members 120 to the generator 114. Each drive member 120 and its respective sprocket wheel 136 are mounted for rotation to a chassis (not shown) connected to a mounting system 116. It will be appreciated that, in addition to a suitable chassis arranged to support the drive mechanism 132, the drive mechanism 132 may be partially or fully surrounded by a suitable housing.

The mounting system 116 preferably comprises a pivot such that the turbine system 110 may be aligned with a fluid flow. In this second embodiment however, each drive member 120 comprises a generally rigid member, and each drive member 120 is mounted to rotate on a fixed axis with respect to the turbine system 110. In this way, the rotor strings 112 are held parallel to one another, such that rotors 118 on neighbouring rotor strings 112 may be prevented from colliding with one another or with other parts of the turbine system 110.

In the embodiments described above, the second end of the or each rotor string is not tethered, such that the turbine system may pivot to align with a fluid flow. In some embodiments, the second end of each rotor string may be connected to a chassis, and the chassis may move or pivot to align the rotor strings with the fluid flow. In this case, the drive members may be rigid or flexible members. It will be appreciated however, that in other embodiments the second end of the or each rotor string may be mounted or tethered such that an axis of rotation of the rotor string is fixed. This may be advantageous where the turbine system is installed in a space where it is not desirable for the system to pivot freely, and/or where the direction of fluid flow remains generally constant parallel to the axis of rotation.

In some embodiments in which the second end of the or each rotor string is tethered and the turbine system is installed a reversible flow, such as a tidal stream, the rotor string may rotate in opposite directions dependent upon the direction of fluid flow. A suitable coupling to the generator may be provided so that generator output may be maintained even when the direction of fluid flow changes, such as during incoming and outgoing tides. Tethering of the or each rotor string at both ends may also reduce the risk of damage to the turbine system when installed in a turbulent flow. In such embodiments, the or each drive member may comprise a flexible member or a rigid member.

In some embodiments, the turbine system may include more than one generator. In particular, the turbine system may include two generators spaced apart, and the or each rotor string may extend between the generators. The generators may tether opposite ends of the rotor string(s). In this arrangement, both generators may be driven simultaneously. In other embodiments, the turbine system may be arranged such that a first generator is driven when fluid flow is in a first direction, and a second generator is driven when fluid flow is in a second direction.

Advantageously, in the present invention, the use of a plurality of rotors in an inline arrangement allows for a greater torque to be generated for a given swept area of the rotors. In this way, smaller-diameter rotors may be used, such that the turbine system may be less obtrusive and may be installed, for example, in shallower water or a narrow watercourse. In some embodiments though, fewer, larger-diameter rotors may be used.

The rotor string of the present invention is not housed in a casing or tube, but is arranged to operate in an open fluid flow such as a tidal stream. Accordingly, fluid flow is not constrained or channelled adjacent the rotors. Furthermore, the rotors are spaced apart along the drive member to reduce the effects of turbulence or slowing of fluid flow from upstream rotors on downstream rotors. In this way, energy may be captured from a larger volume of water, meaning that the turbine system may be used in fluid flows having a lower pressure and/or flow rate. In other words, -12 -a rate of fluid flow and/or an amount of kinetic energy driving the rotors may be replenished or maintained along the length of the rotor string by the fluid flow outside the swept area of the rotors. In contrast, in turbine systems in which a rotor is housed or contained, the volume of water outside the swept area of the rotor (i.e. outside the casing) may not contribute to driving the rotor.

It will be appreciated that more or fewer rotors may be provided on the or each rotor string and that the length of the or each rotor string can be altered. Conveniently, the turbine system of the present invention may be scaled for use in different applications or conditions. The turbine system may be scaled by varying the number, dimensions and arrangement of rotors, drive links and/or drive members.

Simply, additional drive links and rotors may be added to the turbine system. In this way the length of the rotor string(s) may be extended. The inline arrangement of the rotors allows the turbine system to be scaled without changing the diameter of the rotors or a cross sectional area of the turbine system.

In some applications though, larger or smaller diameter rotors may be used. In some embodiments, the diameter of the rotors may vary along the rotor string. For example, rotor diameter may increase towards the second end of the rotor string.

It has been found that an increase in diameter of all of the rotors in the rotor string increases electrical output from the generator. This is primarily due to the increase in the swept area of the rotors.

Furthermore, it has been found that an increase in performance of the turbine system, i.e. an increase in the torque generated by the rotor string, occurs when the spacing between adjacent or neighbouring rotors is increased. In preferred embodiments a spacing between adjacent rotors along the drive member, i.e. a length of each of the intermediate drive links, is equal to or greater than a diameter of the rotors in the rotor string. In some embodiments, the spacing between adjacent rotors may be equal to or greater than a radius of the rotors. Where rotor diameter -13 -varies along the rotor string, the spacing between adjacent rotors may be equal to or greater than a radius (or in other embodiments, a diameter) of either of the adjacent rotors.

The torque supplied to the generator for a given fluid flow rate may be varied by changing the number of rotors. In this way, the generating capacity or output of the turbine system can be modified. The generator output may be a function of the number of rotors. The generator output may be proportional to the number of rotors.

The rotors of the or each rotor string may have any number of blades. Each rotor may have at least two blades.

Furthermore, each rotor along the drive member may be oriented with respect to neighbouring rotors such that the blades are spaced around the drive member. In other words, blades of one rotor may be angularly offset from blades of another rotor. In other embodiments blades of each of the rotors may be aligned with each other. In some embodiments, each rotor may be arranged to rotate to a limited extent on and with respect to the drive member (i.e. in substantially the same plane of rotation as in use). In this way, the blades of the rotors may be angularly aligned with one another when the turbine system is to be stored or transported, but when immersed in a fluid flow, each rotor may turn until it reaches a stop, such that the blades of the rotors may be angularly offset from one another in use. Partial rotation of each rotor in this way may be limited by a stop or locking device, such as peg, pin or similar.

Where the turbine system is to be installed in a larger area, with a lower flow rate (such as on the seabed in a tidal stream) the or each rotor string may be longer and may be provided with a larger number of rotors. In a setting with a high flow rate, such as a fast flowing river, fewer rotors may be used. In some embodiments, the generator may include a gearing mechanism such that a desired operating speed of the generator may be maintained. For example, a longer rotor string having a larger number of rotors may be used in a fluid flow having a slow flow rate to provide -14 -a higher torque but at a lower rate of rotation. In this case, the gearing mechanism may provide a suitable gear ratio to maintain a sufficiently high speed of operation (e.g. revolution rate) of the generator. In some embodiments, the generator may comprise a low-revolution, high output generator.

In some embodiments, the rotor string may comprise a modular assembly, such that rotors may be easily added or removed. For example, a module may comprise a drive link, a rotor and a connector for reversibly or releasably connecting the drive link to another drive link in the rotor string. In other embodiments, a module may comprise a plurality of rotors and drive links. Conveniently, a modular arrangement allows the turbine system to be scaled on-site, for instance where the turbine system is to be used in a remote location where the properties of available fluid flow may be determined on arrival and/or in the event of changing conditions.

In the second embodiment described above, four rotor strings are provided. In other embodiments, more or fewer rotor strings may be provided. Also, other configurations of the rotor strings may be contemplated. In some embodiments, the rotor strings may be angularly spaced around an axis of the turbine system. For example, six rotor strings may be provided, and the first ends of the rotor strings may be disposed in a generally hexagonal grouping, i.e. with an angle of approximately 60° between adjacent rotor strings. In such embodiments, a drive system may comprise a plurality of chain or pulley drives to transmit drive from each rotor string to the input shaft of the generator. In some embodiments having more than one rotor string, each rotor string may comprise a single rotor.

The drive member may be provided by a single rigid or flexible member, such that each drive link comprises a portion of the member. In other embodiments, the drive links may be connected by connectors or joints. In some embodiments, the drive links may be connected by flexible connectors or joints such that the drive links may pivot with respect to one another. In this case, the drive links may be rigid or flexible members. The flexible joints may comprise universal joints. The drive member may comprise a jointed member or a torque tube. In the same rotor string, some of the -15 -drive links may be flexible members whilst others may be rigid members. The drive links may be connected to one another and/or to the generator by shackles.

In some embodiments, a plurality of adjacent drive links may be provided by a single rigid member. For instance, a plurality of rotors may be mounted on a common shaft.

Such a member may be flexibly connected to another rigid member comprising a plurality of drive links. In this way, the rotors and drive links may be grouped into sets. For example, the turbine system may comprise one or more sets connected in an inline arrangement, where the drive links of each set comprise a single rigid member and adjacent sets are coupled by a flexible connector. This arrangement may be more suitable for a smaller-scale installation in which the direction of fluid flow is generally constant.

In larger installations which may generate more power, and which may have larger diameter rotors, each drive link may comprise a rigid member and a flexible connector (e.g. a universal joint) may be provided between adjacent drive links such that a flexible connector is provided between neighbouring rotors. In this case, each set comprises a single rotor and drive link. With this arrangement, the turbine system may be less susceptible to damage due to misalignment and/or from changes of direction of fluid flow (e.g. tides).

In some embodiments, the drive member may be hollow. In such embodiments, the hollow interior of the drive member may house electrical cabling and/or a mechanical actuator for effecting adjustment of a pitch angle of the rotor blades, which in some embodiments may be adjustable. In this way, an angle of attack of the rotor blades may be adjusted in use. The hollow drive member may be provided by a plurality of concentrically arranged tubular members.

In some embodiments of the present invention, the rotor string may be arranged to have substantially neutral buoyancy. In this way, the drive member and rotors may be maintained at a suitable height or depth with respect to the generator. In embodiments where the drive member is rigid and/or is tethered at the second end, -16 -neutral buoyancy of the drive member and rotors may help to distribute loads on parts of the turbine system more evenly, thereby reducing wear. In other embodiments, the rotor string may have positive or negative buoyancy. The buoyancy of the rotor string may be determined by the materials from which the drive member and rotors are manufactured, and/or by the use of flotation or ballast devices.

In further embodiments of the invention, the drive member or the or each drive link may comprise a wire, cable, rope (e.g. steel cable or wire rope), tubing (e.g. of a plastics material), chain, flexible shaft or similar. Multi-strand stainless steel or galvanised wire rope, or nylon rope may be used, which may be fitted with shackle terminations. The drive member may be arranged to flex in order to compensate for misalignment or changes in fluid flow, whilst providing sufficient torsional stiffness to transmit torque from the rotors to the generator. The mounting system may comprise legs, a clamp, cradle, bracket, anchor, tether, or other suitable means for mounting the turbine system. In some embodiments the mounting system may comprise a pivot, swivel, turntable or flexible tether, such that the orientation of the turbine system may be varied, as described above.

In some embodiments, the turbine system may be mounted such that the generator is positioned out of or above the fluid flow, and the or each rotor string is at least partially submerged in the fluid flow. For example, the turbine system may be mounted to a gantry, deployed arm, post, bridge, bank, platform, barge, boat or similar structure. In such embodiments, at least part of the rotor string may extend from the generator into the fluid flow. In this case, the rotor string may comprise at least one flexible drive link and/or flexible connector between drive links. For example, the main drive link may comprise a flexible shaft and the or each intermediate drive link may comprise a rigid shaft. In this way, the or each intermediate drive link may extend generally parallel to the direction of fluid flow, so as to align the rotors to intercept the fluid flow, whilst the main drive link may extend at an angle to the or each intermediate drive link, so as to transmit drive to a generator disposed above the fluid flow. A flexible connector may be provided -17 -between any of the drive links, and/or between the main drive link and the generator input shaft.

Conveniently, the turbine system may be portable for use in a wide variety of applications, such as shorter-term use in temporary or semi-permanent applications. For instance, the use of a flexible drive member and/or flexible connectors between drive links may allow the rotor string to be folded away for storage and transportation, and easily unfolded for deployment at a suitable location, without complex assembly. Flexible drive links may be coiled for storage.

Connectors (e.g. shackles) between drive links may allow disassembly of the turbine system. In some embodiments, a diameter of the rotors may be about 300 mm. A length of the main drive link may be between about 3 to about 5 metres.

Furthermore, each rotor may comprise folding or pivoting parts such that the rotor may be switched between a first, stowed configuration, and a second, deployed configuration in which the blades are arranged to extend radially from the hub in an angularly spaced arrangement so as to provide a propeller-type rotor. The blades may be moved between configurations manually, by an electrical or mechanical mechanism, or automatically when acted upon by the fluid flow.

Each rotor may comprise folding or pivoting blades. In some embodiments, each rotor blade is pivotally mounted to the rotor hub such that each blade may pivot about an axis substantially parallel to an axis of rotation of the rotor in use. In this arrangement, in the stowed configuration, the blades may be pivoted so as to reduce a diameter of the rotor. Pivoting of the blades with respect to the hub may be limited by contact with a stop such as part of a drive link, a peg, latch, cam, or similar. In such embodiments, the blades may be arranged to pivot into a deployed configuration when immersed in the fluid flow. Where the rotor has two opposed blades, in the stowed configuration, the blades may extend in substantially the same direction away from the hub.

In further embodiments, the blades may be arranged to pivot about a radial axis (i.e. -18 -an axis substantially perpendicular to and intersecting the axis of rotation of the rotor). In this way, the blades may be pivoted so as to extend in a plane generally parallel to the drive member. In this way, where the or each rotor has two opposed blades, the blades may extend in a plane generally parallel to the drive member, such that the rotor string can be stored or transported in a generally flat arrangement. In other embodiments, the blades may be arranged to pivot or fold radially inwards towards the hub such that a length of each blade extends generally parallel to the drive member. In this way, the blades may be stowed generally parallel to one another and to the drive member. With this arrangement, the blades may be automatically pivoted radially outwards into a deployed configuration when immersed in the fluid flow.

In yet further embodiments, the or each rotor may be pivotally mounted to the drive member. The hub may be pivotally mounted to the drive member such that the rotor can pivot about an axis substantially perpendicular to the drive member. With this arrangement, the rotor may be pivoted such that the blades extend in a plane generally parallel to the drive member. In this case, in a stowed configuration, the rotor blades extend in a plane generally parallel to a respective drive link and/or the drive member.

With these arrangements, the turbine system may be arranged to be transported in a more compact, stowed arrangement, and deployed and retrieved with minimal assembly, disassembly or configuration. This may be particularly advantageous where the turbine system is to be used in a remote location. For example, the turbine system could be deployed by mounting the generator to a bridge over a fast-flowing river in a remote community, and lowering the rotor string into the river flow. In some portable embodiments, the drive member may simply comprise a length of rope. Each rotor blade may be releasably secured to its respective hub, for example by a suitable fastener, clip, or similar. In this way, individual blades may be replaced without the need to disassemble other parts of the rotor string. This may be advantageous, for example, if blades become damaged, or to change the type of blade, for instance to suit different conditions.

-19 -In some embodiments of the invention, the or each hub may have a streamlined shape. The hub may be shaped to reduce drag or turbulence in the fluid flow and/or to stabilise the rotor in the fluid flow. The hub may comprise a generally conical or frustoconical hub body.

In further embodiments, each rotor may be mounted to a drive member using a fail-safe device such as a shear pin or similar, such that a rotor may be rotationally decoupled from the drive member in the event that the rotor is blocked from rotating (e.g. by debris). Similarly, a fail-safe or disconnect may be provided between each rotor string and the generator in order to prevent damage to the generator from excess force (e.g. in flood conditions). In this way, the turbine system may continue to operate if rotation of one or more of the rotors is blocked and damage to the turbine system may be prevented. Additionally or alternatively, each rotor may comprise a freewheel or clutch mechanism such that rotation of the rotor is coupled to rotation of the drive member only in one direction of rotation. With this arrangement, perturbations in the speed of rotation of a rotor may not affect the rate of rotation of the drive member.

Any parts of the turbine system may comprise a corrosion-resistant material. In some embodiments, the rotors may comprise a plastics material, which may reduce the weight and cost of the turbine system.

It will be appreciated that the turbine system of the present invention need not be used only to drive an electrical generator, but could be used to drive or power another output such as a pump, or other apparatus or machinery.

Further modifications and variations not explicitly described above are also possible without departing from the scope of the invention as defined in the appended claims.

Claims

-20 -CLAIMS1. An axial fluid turbine system comprising: a rotor string including a drive member and at least two rotors rotationally 5 coupled to the drive member and axially spaced along the drive member; and a generator for generating electrical power coupled to the drive member.
2. An axial fluid turbine system according to Claim 1, wherein an axial spacing between adjacent rotors is no less than a radius of a smallest diameter rotor coupled to the drive member.
3. An axial fluid turbine system according to Claim 1, wherein an axial spacing between adjacent rotors is no less than a diameter of the smallest diameter rotor coupled to the drive member.
4. An axial fluid turbine system according to any preceding claim, wherein the at least two rotors are coaxial.
5. An axial fluid turbine system according to any preceding claim, wherein the rotor string is not enclosed.
6. An axial fluid turbine system according to any preceding claim, wherein the drive member comprises a flexible member.
7. An axial fluid turbine system according to any one of Claims 1 to 5, wherein the drive member comprises a rigid member.
8. An axial fluid turbine system according to any preceding claim, wherein the drive member comprises a main drive link extending between a first rotor coupled to the drive member and the generator, and at least one intermediate drive link extending between adjacent rotors coupled to the drive member.
9. An axial fluid turbine system according to Claim 8, wherein at least two of said drive links are connected by a flexible connector.
10. An axial fluid turbine system according to Claim 8 or Claim 9, wherein at least one of the main drive link and the or each intermediate drive link comprises a flexible member.
11. An axial fluid turbine system according to any preceding claim, wherein the rotors are arranged to rotate in the same direction.
12. An axial fluid turbine system according to any preceding claim, wherein spacings between adjacent rotors is uniform along the rotor string.
13. An axial fluid turbine system according to any preceding claim, wherein each rotor comprises at least two blades.
14. An axial fluid turbine system according to any preceding claim, wherein blades of neighbouring rotors are angularly offset from one another in use.
15. An axial fluid turbine system according to any preceding claim, wherein each rotor comprises a hub and at least one blade connected to the hub, and the or each blade is arranged to move with respect to the hub between a first stowed position and a second deployed position in which the or each blade is arranged to engage with a fluid flow so as to drive rotation of the rotor.
16. An axial fluid turbine system according to Claim 15, wherein the or each blade is arranged to move into the second position automatically when the rotor string is immersed in a fluid flow.
17. An axial fluid turbine system according to any preceding claim comprising a plurality of rotor strings.
18. An axial fluid turbine system according to Claim 17, further comprising a drive mechanism arranged to transmit drive from each of the plurality of rotor strings to a single generator.
19. An axial fluid turbine system according to Claim 17 or Claim 18, wherein each of the rotor strings extend parallel to each other.
20. An axial fluid turbine system according to any preceding claim further comprising a mounting system for mounting the turbine system to a supporting structure, the mounting system comprising a pivot permitting rotational movement between the or each rotor string and the supporting structure.
21. An axial fluid turbine system according to any preceding claim, wherein the or each rotor string has neutral buoyancy.
22. An axial fluid turbine system according to any preceding claim, wherein said fluid is a liquid.
23. An axial fluid turbine system according to Claim 22, wherein said liquid is water.
24. An axial fluid turbine system according to Claim 22 or Claim 23, wherein the or each rotor string is fully submerged in the liquid.