CA3120551A1 - A free stream turbine and system - Google Patents

Abstract

A runner or rotor for a free stream turbine is provided, the runner (1) having a rotational axis (R) and at least one runner cell, each runner cell defined by a pair of end plates (5, 7) with a plurality of runner blades (3) extending laterally between the end plates, wherein the runner blades are circumferentially arranged about a central void (13) through which the rotational axis extends. Turbines and energy systems which incorporate the runner are also provided.

Description

A FREE STREAM TURBINE AND SYSTEM
Field of the Invention The present invention relates to the field of renewable energy, and specifically free stream turbines such as those used in the field of tidal energy. The present invention is an improved free stream turbine which converts tidal and other free stream energy into electricity.
Background of the Invention Tidal turbines and other types of free stream turbines are known. Distinct from barrage systems, such turbines are submerged in a body of water where tidal currents or other naturally-occurring water movements are present. The turbines may be axial-flow, similar to a wind turbine, or else they may be cross-flow.
To date, many tidal turbine proposals in particular have cost too much and/or have been relatively inefficient. The high costs can result from relatively complicated installation, including the forming of substantial subsea or floating structures upon which the turbines must be installed, as well as the separate power generation systems needed to convert the movement of the turbines into electricity. In addition, known tidal turbines are susceptible to fouling from debris in the tidal stream.
Additionally, existing tidal turbines are designed to be large scale units which cannot be readily reduced in scale and still operate relatively efficiently as portable individual units, for example.
It is an aim of the present invention to obviate or mitigate one or more of these disadvantages with existing free stream turbines, including tidal, wave power and open river turbines.
Summary of the Invention According to a first aspect of the invention there is provided a runner for a free stream turbine, the runner having a rotational axis and at least one runner cell, each runner cell defined by a pair of end plates with a plurality of runner blades extending laterally between the end plates, wherein the runner blades are circumferentially arranged about a central void through which the rotational axis extends.

2 The runner may further comprise a plurality of runner cells arranged co-axially. The runner blades of each runner cell may be rotationally offset from the runner blades of an adjacent runner cell.
Preferably, each blade has a distal edge and a proximal edge and is curved so as to define an inner concave surface and an outer convex surface, and wherein the proximal edge of each blade is located radially outwards from a rotational axis of the runner such that the plurality of proximal edges define an outer boundary of the central void of a respective runner cell.
Each blade may have a degree of curvature of 150-170 . Alternatively, each blade may have a hydrofoil profile.
The runner may be formed from a plastic. The runner may further comprise a metal reinforcement member located about the end of each blade where the blade end attaches to a respective end plate.
Each blade may have a metal core with a plastic body moulded about the metal core.
Preferably, at least one of the end plates of the or each runner cell comprises a buoyancy tank.
According to a second aspect of the invention there is provided a turbine comprising a runner according to the first aspect of the invention.
According to a third aspect of the invention there is provided a turbine, comprising:
a runner comprising a plurality of runner blades;
a first tank non-rotatably attached to a first end of the runner;
a second tank non-rotatably attached to a second end of the runner;
first and second support axles upon which the first and second tanks are rotatably mounted such that the runner and first and second tanks rotate about an axis of rotation defined by the support axles; and

3 a generator apparatus sealed within the second tank and non-rotatably attached to the second support axle such that the apparatus remains stationary as the tanks and runner rotate about the axle.
The runner may comprise a runner according to the first aspect of the invention.
Preferably, the second tank has a pair of tank end walls, and wherein the generator apparatus comprises:
a ring gear fixed to a tank end wall of the second tank; and a plurality of generator modules arranged circumferentially about the axis of rotation, each module comprising a drive gear engaged with the ring gear, whereby rotation of the ring gear transfers a drive torque to the drive gear of each module to drive each generator.
Preferably, the ring gear is formed from roller chain, with the drive gear of each module having a plurality of teeth which engage the roller chain.
Preferably, the second support axle is hollow and contains at least one umbilical connectable to the generator apparatus.
The turbine may further comprise a buoyant support spar to which a free end of the second support axle is attached, and one or more anchors which anchor the spar and first support axle to the seabed by way of a plurality of anchor cables.
The support spar may include a plurality of interconnected ballast tanks, at least one of the ballast tanks being adapted to receive and discharge one or more fluids in order to adjust the buoyancy of the turbine.
The turbine may further comprise at least one spar winch housed within the support spar, and an axle winch housed within the first support axle, wherein each winch has a winch cable attached to the one or more anchors, whereby operation of the winches allows the turbine to adjust its depth within a body of water.
The turbine may further comprise a stator arranged about the runner, the stator comprising:

4 a first flow passage having a distal end and a proximal end, the first flow passage configured to direct fluid flow to and from the runner: and a second flow passage having a distal end and a proximal end, the second flow passage configured to direct fluid flow to and from the runner;
wherein the first and second flow passages are diametrically opposed to one another about the runner, and wherein the distal ends of the first and second flow passages have a larger surface area than the proximal ends of the respective flow passages.
Preferably, the stator further comprises third and fourth flow passages configured to direct fluid flow to and from the runner, wherein the third and fourth passages are diametrically opposed to one another about the runner and are substantially perpendicular to the first and second flow passages. The cross sectional area of the third and fourth flow passages may be substantially constant.
Alternatively, the turbine may further comprise a stator arranged about the runner, the stator comprising:
a first pivotable vane located on a first side of the runner and adapted to selectively deflect fluid flow towards the runner when upstream of the runner, the first vane having a pivot axis substantially parallel to the rotational axis of the runner; and a second pivotable vane located on a second, opposite side of the runner and adapted to selectively deflect fluid flow towards the runner when upstream of the runner, the second vane having a pivot axis substantially parallel to the rotational axis of the runner.
Preferably, the first and second vanes each have biasing means which bias the respective vanes towards an inactive position when downstream of the runner.
Preferably, at least one of the first and second tanks includes a ballast chamber adapted to receive and discharge one or more fluids in order to adjust the buoyancy of the turbine.
According to a fourth aspect of the invention there is provided an energy system comprising:
a pair of turbines according to the second or third aspect of the invention;

a pair of inner support axles to which respective inner ends of both of the turbines are rotatably mounted such that the turbines are substantially co-axial;
a pair of outer support axles to which outer ends of both turbines are rotatably mounted;

5 wherein one of the pair of turbines is configured to rotate about its respective support axles in a first direction and the other of the turbines is configured to rotate about its respective support axles in a second, opposite, direction.
Preferably, the turbines are arranged such that the second tanks of the turbines are proximate the respective inner support axles, and the first tanks of the turbines are proximate the respective outer support axles.
The system may further comprise a buoyant support spar to which the inner support axles are attached by respective universal joints, and one or more anchors which anchor the spar to the seabed by way of one or more anchor cables.
According to a fifth aspect of the present invention there is provided an energy system comprising:
a plurality of turbines according to the second or third aspect of the invention;
a pair of buoyant support spars to which respective ends of each of the turbines are rotatably mounted;
a plurality of mooring chains attaching the buoyant spars to the seabed or a dedicated foundation; and a pair of tether lines for each support spar, where each tether line of the pair has one end connected to a respective support spar and one end connected to the seabed or foundation, such that a distance between the seabed ends of the pair of tethers is less than a distance between the support spar ends of the pair of tethers;
wherein each support spar includes a fin attached thereto, the fin adapted to selectively adjust the angle of the support spar and turbines under the action of fluid flow over the fin.
Preferably, the or each support spar includes a plurality of interconnected ballast tanks, at least one of the ballast tanks being adapted to receive and discharge one or more fluids in order to adjust the buoyancy of the system.

6 Preferably, the system further comprises at least one spar winch housed within the support spar, and first and second axle winches housed within the outer support axles, wherein each winch has a winch cable attached to the one or more anchors, whereby operation of the winches allows the system to adjust its depth within a body of water.
According to a sixth aspect of the invention there is provided an energy system comprising:
a support frame;
a plurality of turbines according to the second or third aspect of the invention, the support axles of each turbine being non-rotatably fixed to the frame;
wherein the support frame is attached to a buoyant member floating on the surface of a body of water such that the support frame and turbines are submerged below the buoyant member.
Preferably, the turbines are arranged on the frame such that their axes of rotation lie in a plane which is substantially perpendicular to a longitudinal axis of the buoyant member.
Preferably, the frame is pivotably attached to the buoyant member such that the turbines can be moved between positions upon and below the surface of the body of water.
The turbines may be arranged on the frame such that their axes of rotation lie in a plane which is at an oblique angle to a longitudinal axis of the buoyant member.
Preferably, the buoyant member includes a plurality of interconnected ballast tanks, at least one of the ballast tanks being adapted to receive and discharge one or more fluids in order to adjust the rotational position of the buoyant member and frame relative to the surface of the body of water.
According to a seventh aspect of the invention there is provided an energy system comprising:
a support frame having a base member fixable to the bottom of a body of water;

7 a plurality of turbines according to the second or third aspect of the invention, the support axles of each turbine being non-rotatably fixed to the frame;
wherein the turbines are arranged on the frame such that their axes of rotation lie in a plane which is at an oblique angle to a longitudinal axis of the base member.
According to a eighth aspect of the invention there is provided an energy system comprising:
first and second turbines, each turbine comprising a turbine according to the second or third aspect of the invention;
a support frame to which both turbines are rotatably fixed, such that the axes of rotation of the turbines are substantially parallel; and a support pillar having a first end fixable to the bottom of a body of water, and a second end to which the support frame is attached, wherein a longitudinal axis of the pillar and the axes of rotation are substantially parallel.
Brief Description of the Drawings Preferred embodiments of the present invention will now be described, by way of example only, with reference to the following drawings:
Figure 1 shows a turbine runner for an improved free stream turbine;
Figure 2 is a section view of an end portion of the turbine runner shown in Figure 1;
Figure 3 is a perspective view of a free stream turbine incorporating the runner of Figures 1 and 2;
Figure 4 is an end view of a generator apparatus used in the turbine of Figure 3;
Figure 5 is a perspective view of the free stream turbine when installed in a tidal flow area;
Figure 6 is a perspective view of a first embodiment of an energy system;
Figure 7 is a perspective view of a second embodiment of an energy system;
Figure 8 is an end view of an alternative generator apparatus;
Figure 9 is a side view of a third embodiment of an energy system;
Figure 10 is a side view of a fourth embodiment of an energy system;
Figure 11 is a side view of a fifth embodiment of an energy system;
Figure 12 is a side view of a sixth embodiment of an energy system;

8 Figure 13 is a side view of a seventh embodiment of an energy system;
Figure 14 is an alternative runner for the free stream turbine;
Figure 15 is a side view of an eighth embodiment of an energy system;
Figure 16 is a perspective view of a ninth embodiment of an energy system;
Figure 17 is a side view of a tenth embodiment of an energy system;
Figure 18 is a modified runner for the free stream turbine;
Figure 19 is a cross sectional view through a stator and runner of a free steam turbine;
Figure 20 is a perspective view of a further embodiment of a free stream turbine;
Figure 21 is an end view of a further embodiment of an energy system;
Figure 22 is a side view of a runner reinforcement member; and Figure 23 is an exploded view showing how adjacent runner cells may be attached to one another.
Detailed Description of the Drawings Figure 1 shows a rotor, also known as a runner, for use in a free stream turbine. The runner 1 comprises a plurality of curved blades 3 which extend laterally from a first end plate 5 to a second end plate 7.
Figure 2 is a perspective view of a vertical section taken through the runner adjacent a first end thereof, the first end of the runner including the first end plate 5.
Each blade 3 has a distal edge 9 and a proximal edge 11, and is curved so as to define an inner concave surface 10 and an outer convex surface 12. Each blade has a degree of curvature C, which is preferably 150-160 and is most preferably substantially 160 . The proximal edge 11 of each blade 3 is located radially outwards from a rotational axis R of the runner such that the blades are arranged circumferentially about the axis R, and the plurality of proximal edges 11 define a circumferential outer boundary of a central void 13 inside of the plurality of runner blades 3. At least one of the blades may be hollow and contain an umbilical for servicing an adjacent generator apparatus.
The runner forms part of a free stream turbine, that is an open (i.e. uncased) cross-flow reaction water turbine, which in use will preferably have its axis R
horizontal below the water surface and at right angles to the tidal stream. In one example the

9 runner may be around 8m in diameter and 32m long, and have 10 blades directing flow into and across the void 13, which may be 5m in diameter. The end discs and, where present, intermediate discs support the blades, so the runner does not need a central shaft. Where intermediate discs are present the cells created by those discs may be 8-12m long.
When installed as part of a free stream turbine the runner 1 is unidirectional, rotating in the same direction regardless of whether the tidal stream is ebbing or flowing.
Any debris wrapping round a blade will be washed off when the runner has rotated through 180 . The blades and end discs may be formed from a plastics material.
Preferably, that plastics material is High Performance Polyethylene (HPPE), High Density Polyethylene (HDPE) or Unplasticised Polyvinyl Chloride (uPVC). The blades themselves may be formed by longitudinally slicing 1400mm diameter HPPE

or HDPE extruded pipe.
The runner torque is derived firstly from the reaction of the runner blades diverting the direction of the tidal flow entering the runner. This is supplemented by a Coanda effect from the convex back side of the blades. Secondly, as the tidal flow exits through the other side of the rotating runner it is reoriented to the original direction by the blades. This particular configuration needs no static guide vane stator, although this may be added for the purpose of directing flow to the runner if the environment dictates that this is desirable. An internal vane, fixed to a static central shaft from the non-drive end, may be incorporated to guide the incoming flow in the runner's central void to the orientation of the exit opposite.
Figure 3 is a perspective view of a free stream turbine 15 incorporating the runner shown in Figures 1 and 2. For illustrative clarity the runner 3 is shown in Figure 3 as a cylinder but it should be understood that it would in reality be an open runner of the type described above. The turbine 15 has a first cylindrical tank 20 non-rotatably attached to the first end of the runner 3, whereby the first tank and runner rotate together. A second cylindrical tank 22 is non-rotatably attached to the opposite, second end of the runner 3, whereby the first and second tanks 20,22 and runner 3 all rotate together. The first tank 20 at the non-drive end incorporates a first stub axle, which turns in a fixed journal bearing (neither are shown in figure 3).
The second tank 22 at the generator drive end turns on a fixed second support axle such that the axis of rotation R of the runner 3, and hence the tanks 20,22 is defined by the stub and support axles. A generator apparatus 24 is sealed within the second tank 22 and non-rotatably attached to the second support axle 23 such that the apparatus 24 remains stationary as the tanks 20,22 and runner 3 rotate about the 5 axles.
The tanks 20,22 may be the same diameter as the runner 3. Hence, in a preferred embodiment the tanks 20,22 are 8m in diameter. The first tank 20 may be lm wide, whilst the second tank 22 may be 2m wide. Both tanks 20,22 provide additional

10 buoyancy to the turbine 15, where the runner 3 may be fabricated from the aforementioned plastics material and therefore have a degree of buoyancy in its own right. The larger second tank 22 provides a dry machinery space for the generator apparatus 24. The end walls of the second tank 22 are each located by bearings that are mounted on the second axle 23. The bearings incorporate thrust races to transfer to each other the load on the second tank's end discs arising from the tank being submerged in the sea by around 15m, for example. An inner face of the outboard end wall of the second tank 22 has attached to it a ring gear 26, which may be formed from maintenance-free roller chain. The ring gear may be around 6m in diameter. The outboard end wall of the second tank 22 is removed in Figure 3 for illustrative purposes.
As will be described in more detail below the ring gear 26 drives individual generators with step up trains, which form individual generator modules and collectively form the generator apparatus 24. These generator modules are mounted upon a sub-frame which does not rotate relative to the second axle 23. The second axle 23 may also be hollow and/or ported to carry umbilicals such as cables and tubes, and protrudes from the outboard end wall of the second tank 22 through an end tightened cable gland. The remote end of the second 23 axle is locked into a horizontal spar, the ends of which are attached by anchor chain to the seabed.
These latter features are not shown in Figure 3 but will be described in more detail below.
Figure 4 shows the generator apparatus 24 in more detail, again with the outboard end wall of the second tank 22 removed. In the preferred embodiment illustrated there are twelve separate generator trains or modules 28, which minimise the tooth

11 load at the slow rotating ring gear 26 rotating with the runner 3. The number of generator trains may be increased or reduced where appropriate. For example, a six train configuration may be used instead. However, the greater the number of trains the lesser the tooth load from the gear trains at the ring gear 26.
Each generator train is mounted on the generator sub-frame 29, which takes the form of a circular disc fixed to the second axle 23. Each train has a primary drive gear sprocket 30 which is engaged with the ring gear 26 via a sprocket 31. A
chain or belt drive 32 connects the primary drive gear 30 to a second drive gear 34 having fewer teeth than the primary gear 30. The secondary drive gears 34 each connect to and drive individual generators 36 of a known type, where the driving of the generators by the secondary drive gears 34 generates electricity. These generator trains incorporate resilient couplings or preloaded friction clutches to equalise loading and thus prevent excessive load on a train where the primary and secondary drives are out of phase.
Figure 4 also shows the second support axle 23 and a journal bearing 38 mounted on the axle which permits the second tank 22 to rotate upon the axle 23.
As discussed above, the second tank 22 and its ring gear 26 rotate with the runner 3, whilst the sub-frame disc 29, to which the generator modules are attached, is stationary. However, the sub-frame 29 may be selectively unlocked and rotated manually on the second axle 23 to facilitate the maintenance of the machinery components accessed by a waterproof hatch (not shown) on the outboard end wall or the circumferential shell of the second tank 22.
At least one umbilical passes through the second support axle 23 and connects the generator apparatus to an onshore base. The umbilical may comprise one or more of: an export DC power cable, a service AC power cable, signal fibre cable and an air line. An extension of the air and control umbilical may run along the runner inside one of the runner blades, which is hollow. The machinery space in the second tank 22 is kept dry and pressurised to match the water pressure by the air line.
Spent air is discharged through the second support axle 23.

12 The turbine power is delivered as voltage-stabilised DC current and cabled ashore for inversion to AC. A
battery, supercapacitor bank or a kinetic device can be incorporated ashore to cover power loss at slack water.
Figure 5 shows a preferred arrangement for subsea installation of the turbine 15. In Figure 5, the first tank 20 and associated first stub axle 21 are facing the viewer, with the second tank 22 facing away from the viewer and the runner 3 inbetween the two.
As with Figure 3, the runner is shown schematically in cylindrical form but in reality takes the open form shown in Figures 1 and 2.
A buoyant support spar 40 floats mid-water and has a free end of the second support axle attached thereto. At least two anchor cables 42,44 are suspended from cradles holding both ends of the spar 40 and are attached to respective anchors or piles 46,48 lying on the seabed 50. The support spar may include a plurality of interconnected ballast tanks, adapted to receive and discharge one or more fluids in order to adjust the buoyancy of the turbine 15. The spar 40 also has a pair of spar winches 52,54 housed at either end thereof, and a third winch 56 is housed within the journal bearing locating the first stub axle 21. Each of these winches has a winch cable 58 connected to a cradle, to which one or more of the anchors 46,48 and/or additional seabed infrastructure are attached. Thus, when the two ends of the spar and the first stub axle journal are unlocked from the cradles, operation of the winches 52-56 allows the turbine 15 to be raised in a controlled manner from the operational position to the surface.
The at least two anchor cables 42,44 face in opposite directions to the tidal stream.
These cables 42,44 hang in a catenary which rises and falls at the seabed in response to tension, of which the primary source is the buoyancy of the whole turbine assembly. In addition there may be vertical tethers 45 and also at right angles to the tidal stream a third anchor (not shown). This optional side anchor can provide additional stability in the lateral direction.
The first stub axle 21 is attached to the first tank 20 with a water-lubricated journal bearing, which may be located in the mooring cradle. At least one vertical anchor cable 45 secures the first tank (non-drive) end of the turbine 15, along with optional lateral anchor cables.

13 The whole assembly will be restrained providing say 10m clear passage for vessels below the surface of the sea so it will not interfere with navigation. The underside may be around 2m above the seabed and its station maintained by the restraining anchor cables. Heavier chains may be employed and or a supplementary tank at the end of the spar with greater unidirectional torque. Torque is otherwise absorbed by ballast tanks and vertical tethers at each end of the spar. A siamesed pair of turbines can be arranged to balance torque, as will be discussed in reference to Figure below.
Piles are the preferred anchors for the system, whilst buoyancy tensioning cables to the seabed are used in preference to a rigid support structure.
With structural support the turbine can also be configured in several different ways both horizontally and vertically, and with practical provision for maintenance incorporated.
The turbine assembly can be brought to the surface by its own buoyancy, which as necessary can be adjusted by the integral ballast tanks in the spar and/or first and second tanks 20,22. Control is maintained thanks to the cables on the drums of the winches inside each end of the spar and the stub axle journal, providing a link between the turbine assembly and the cradles located by the anchors and tethers into which cradles the ends of the spar and the stub axle 21 journal are normally locked.
To raise the turbine to the surface the cradle releases the journal at the non-drive end and the two spar mooring and tether cradles at the generator end. The winches unwind the ropes (e.g. buoyant polypropylene plaited rope) on the winch drums, allowing the buoyant turbine to rise vertically. Trim may be maintained by reference to water depth pressure sensors in the spar and/or first and second tanks. The manoeuvre can be automated. Once the turbine is afloat on the surface the winches will slack the mooring cables and the tethers of the generator end and the winch cables will be disconnected and buoyed off.
The turbine is now afloat on the surface, and is still attached to the single point mooring at the first stub axle end. It can therefore now swing freely, such that the turbine may lie at a right angle to the tidal stream or wind. The runner is locked

14 against rotation. The spar ends may be folded in to reduce its length down to substantially the same as the width of the turbine for ease of towing or mooring.
The trim can be adjusted by the ballast tanks with air from the umbilical or a cylinder.
The initial raising part of transfer process is preferably carried out at slack water.
The runner is located on the inboard end of the second tank by a central spigot and its oversize end disc is bolted to a mating protruding annular flange on the periphery of the runner end disc. In a major overhaul the generator capsule formed by the second tanks can be detached from the runner and floated in a substantially vertical orientation. This allows the watertight hatch on the outboard end wall of the second tank to be removed for access. The first tank at the other end of the runner is attached in an identical flange arrangement as that for the second tank, allowing the runner and its rotation to be reversed if desired.
Reinstalling is done by reversing the procedure. The turbine can be towed to a service base, or serviced on site. The computerised control system may be based ashore via the fibre of the umbilical, but may be operated remotely from the service vessel.
A first embodiment of an energy system is shown in Figure 6, the system using turbines of the kind already described above. This system comprises a pair of identical turbines 15,15' arranged such that they are substantially co-axial, with the second tanks 22,22' of the turbines adjacent one another. The second tanks 22,22' are each connected to a fixed station inner support axle 62 upon which the turbines may rotate whilst their respective generator sub-frames are connected to the inner support axle 62 in the same manner as described above. Only one of the inner support axles is visible in Figure 6. A pair of outer support axles 64 rotatably support the first tanks 20,22' of the turbines 15,15'. The first turbine 15 is configured to rotate about the support axles 62,64 in a first direction and the second turbine 15', which is substantially identical to the first turbine, is configured to rotate about the axles 62,64 in a second, opposite, direction. The system further comprises a buoyant support spar 80 to which the inner support axles are attached, and one or more anchors (not shown), which anchor the spar to the seabed by way of one or more anchor cables (not shown). The inner support axles 62 are attached to the support spar 80 by respective universal joints (not shown), which do not allow the inner axles 62 to rotate but do allow relative movement between the spar and the axles. The support spar 80 may be of the type described above with reference to Figure 5, including ballast tanks and/or winches. This arrangement balances torque from the two turbines, and allows the sharing of umbilicals and minimises the length 5 required for the spars.
Figure 7 shows a second embodiment of an energy system. This system comprises first and second turbines of the type shown in Figures 3 and 4. The first tank 20,20' of each turbine 15,15' is rotatably mounted upon a support (not shown) fixed to the 10 seabed 50 such that the rotational axes of the turbines are substantially parallel and in a substantially vertical orientation. The second tanks 22,22' are rotatably attached to a support frame 90, the support frame 90 being attached to the top end of a support pile or pillar 100 which is also fixed to the seabed 50. A
longitudinal axis of the pillar 100 is substantially parallel with the rotational axes of the turbines. The

15 pillar 100 may have a teardrop profile, with a distal edge 102 which tapers outwardly towards a proximal edge (not shown) of the pillar.
This second embodiment may have the first tanks 20,20' rotatably mounted substantially vertically upon a rectangular base frame (not shown) piled to the seabed, but which may be raised to allow for the passage of boulders along the seabed. Rotation of the turbines relative to the frame may be achieved using first stub axles (not shown), upon which are mounted respective water-lubricated bushes formed from a laminated plastic or else a rubber compound. Second support axles 23,23' connect the turbines into the support frame 90, which may be a spectacle crosstree bracket. The bracket 90 may be free to lift and rotate 90 to allow the turbines to float to the surface for service. The teardrop profile of the pillar 100 may be created by attaching stiffening fairing pieces fore and aft of a cylindrical central pillar. One fairing may double as a diving bell, entered through a door in its side. An umbilical from the shore comprising export power, digital data and control fibre cables and an airline may terminate in a watertight junction box within the bell space of the fairing.
Figure 8 shows an alternative embodiment of generator apparatus which may be used with the turbines of the present invention, again with the outboard end wall of the second tank 122 removed. In the alternative embodiment illustrated there are

16 twelve separate primary drive gears 130, but these are connected to a single generator. This is made possible by replacing the individual chain or belt drives with two drive belts or chains 140, 142 and associated idler tensioners, where the drive chains 140,142 divide the primary drive gears 130 into two groups of six. Each of the two groups has one linking chain 140,142 that also drives a bevel step up gear 144,146. Each train is protected from overload due to loss of its phasing with the ring gear by a sprung coupling at the primary drive and preloaded slip clutches at both ends of the generator shaft 148. The same principals may be used where two to six generators and their associated drive trains are employed.
Figures 9 and 10 show third and fourth embodiments of an energy system, both of which incorporate open runner turbines of the type described above. Each of these systems has a support frame 150,160 with a base member 152,162 fixable to the seabed 50 by way of a plurality of piles 154,164. The first and second support axles of each turbine 15 are non-rotatably fixed to the frame 150,160. The frames 150,160 have a generally triangular, or A-shaped, profile where the frame member 151,161 to which the turbines 15 are fixed lies in a plane which is at an oblique angle to a longitudinal axis of the respective base members 152,162. Hence, the rotational axes R of the turbines 15 lie in that same oblique plane.
In these embodiments flow is concentrated by stator guides 156,166. The third embodiment has guides 156 at the top and bottom of the frame 150, whilst the fourth embodiment has a guide 166 at the bottom only. In each arrangement the oblique positioning of the turbines 115 means that there is a degree of overlap of the runners 3 in the direction of flow. In the third embodiment the frame 150 is hinged to allow the turbines 115 to rise to the surface for service. Alternatively, both embodiments may employ winches in the turbines to control the rise of the turbines to the surface in the same manner as with the single turbine described above.
In the deployed state the oblique angle of the plane which the rotational axes R lie is preferably around 45 to the vertical, which gives inherent deflection of the current from the runner blades. The frames carry journal bearings and sockets for the stub axles at the outboard end of the turbines. Individual turbines may disconnected and floated out for repair.

17 A fifth embodiment of an energy system is shown in Figure 11, where again the system incorporates open runner turbines 15 of the type described above.
Similar to the third and fourth embodiments the system here comprises a support frame 170 but here instead of being fixed to the seabed the frame is pivotable relative to a buoyant member 172 floating on the surface. The turbines 15 are non-rotatably fixed to the frame. When the frame pivots to the deployed position shown in Figure lithe axes of rotation R of the turbines 15 lie in a plane which is at an oblique angle to a longitudinal axis of the buoyant member 172. As with the third embodiment, guides 176 are provided at either end of the frame 170 so as to direct flow towards the turbines 15. The buoyant member 172 may be a semi-submersible vessel tethered on station and aligned to the tidal stream. The array of turbines 15 can be raised to surface on the pivoting frame 170 using their own buoyancy.
The sixth embodiment of an energy system shown in Figure 12 combines elements of the second (Figure 7) and fifth (Figure 11) embodiments. Here there are a pair of vertically deployed turbines 15, although only one is visible in the side view of Figure 12. The pair of turbines 15 can be pivoted between retracted (horizontal) and deployed (vertical) positions using the buoyancy of the tanks 20,22 and runner 3 of each turbine 15. A retractable bowsprit 183 extends from the buoyant member from which the turbines 15 are suspended, and tethers 184 can be used fore and aft to secure the turbines 15 in the deployed position whilst in use.
A seventh embodiment of an energy system is shown in Figure 13. This embodiment comprises a support frame 190 to which a plurality of turbines 15 are rotatably attached by fixed support axles. The support frame 190 is attached to a buoyant member 192 floating on the surface such that the support frame 190 and turbines 15 are submerged below the buoyant member 192. The axes of rotation R

of the turbines 15 lie in a plane which is substantially perpendicular to a longitudinal axis of the buoyant member 192.
The frame 190 may be V-shaped with a number of separate buoyant members 192 attached to the portions of the frame closest to the surface. The frame 190 may have two side members 194 and a pair of cross members 196 which extend between the side members 194, where the side members are attached to the buoyant members 192. This frame 190 may include a number of ballast tanks enabling it to

18 be orientated in a deployed state as shown in Figure 13 or raised with the turbines 15 disposed horizontally on the surface for service. Moorings are by chain cable 198 from the middle cross member 196, which will be the centre of pressure.
Figure 14 shows an alternative reaction prime mover for the turbines, which would be used instead of the runner. This prime mover 200 is formed from a continuous belt 202 which may be formed of moulded plastic. The belt 202 includes a plurality of closely spaced narrow blades 204 that are driven by the tidal flow at 45 so as to drive at least one generator assembly 206 located at one end of the belt 202.
The prime mover may also be used to trap and recover plastic waste from the water.
An eighth embodiment of an energy system is shown in figure 15, which is surface supported by a catamaran or trimaran with buoyant members 212 in the form of long cylindrical hulls. A frame 210 supporting one or two sets of turbines 15 is rigidly attached to the buoyant members 212 at around 45 . This embodiment can ballasted to cant 45 to bring the turbines to the surface for servicing.
As seen in figure 15 the illustrated embodiment comprises two pairs of turbines 15, with the frame 210 being an inverted A-shape. The rotational axes R of the turbines 15 lie in a plane which is at substantially 45 to the buoyant members 212 on the surface. The uppermost pair of turbines rotate in the opposite direction to that of the lower pair of turbines. Mooring, at the centre of pressure, is from an extension of the main cross beam 216. There is also a second cross beam 218 at the bottom end of the frame 210. The main beams 214 of the frame 210 and the buoyant members 212 are arranged with ballast tank partitions. The main and the cross beams 214,216 together with the turbines 15, are kept close to neutral buoyancy when operational. For servicing they are blown and float to the surface lifting the rigidly attached buoyant members 212 largely clear of the water. The turbines 15 can then be floated off and towed away for service or changed out. This variant may have a capacity of the order of 20mW.
A ninth embodiment of an energy system is shown in figure 16. This embodiment has vertical open runners 303 and comprises upper and lower triangular frames 305,307 attached to one another. The three corners of the lower frame 307 are attached to three seabed piles (not shown). Three vertical support spars 309 links

19 the respective corners of the frames 305,307. The upper frame 305 may be detached to allow the turbines 15 to be floated to the surface for service under the control of a winch and cable, or else the entire frame may be detached from the piles and raised using the buoyancy of the turbines. The ninth embodiment comprises two pairs of turbines 15, where one of each pair of turbines rotates in the opposite direction to the other of that pair, thereby balancing the torque in the system.
A tenth embodiment of an energy system is shown in Figure 17. This seabed tethered array variant may comprise a single row or two or more rows of turbines 15 in a frame 410 resting at 45 to the seabed 50. The frame 410 rests on a piled cradle 500 and is attached to floats 412 at its top end. The floats 412 are tethered at their ends vertically to sea bed piles 413 such that the cradle 500 takes all horizontal forces and the tether all vertical forces. The whole assembly is attached by means of ropes 502 from the ends of the floats 412 nearest to the rotor frame 410 to a buoyant mooring 504 held mid-water at the working depth of the topside of the floats 412.
When the turbines 15 are operational the floats 412 are de-ballasted. To raise the whole assembly to the surface the floats 412 are ballasted by the admission of water to compartments within the floats. Once the buoyancy has become minimal the cradle 500 is unlocked as are the ends of the tethers at the end of the floats 412.
The cradle 500 has attached to it a guide chain that runs inside a leg to the float 412 above, with a tail length held in a locker open to the sea. The tether ends are already attached to chain cables in a separate locker open to the sea. With a centre of buoyancy well below the middle of the floats 412 the assembly can be brought to the surface by control of the float buoyancy in the manner of a submarine.
The assembly is held in position by the near slack water tidal stream and the mooring junk rope as well as the chain tails of the tethers. Once the floats 412 are on the surface the chain tails and the umbilical are buoyed off. The buoys are then linked with a hose rope to enable them to be sunk by ballasting and recovered when required again by the admission of air from the umbilical air line via the hose rope.
The assembly is then free to swing on the mooring. The floats 412 can then be re-ballasted to cant them through 45 such that the turbines 15 are on the surface, part-submerged allowing them to be serviced.

To recommission, the process is reversed using the cradle's chain to locate the whole while held by the mooring. Remote sensing and automation may substitute for some of the elements of rigging required to allow transfer from operational mode 5 to surface service mode.
As shown in figure 18, the runner 1 may include one or more intermediate plates 4 located intermediate the two end plates 5,7 where the intermediate plates 4 divide the runner into two or more cells. The blades 3 may pass through the intermediate 10 plates 4, or else separate blades may be installed in each cell either side of the intermediate plate(s).
Instead of intermediate plates, the end plates of a runner may define a runner cell.
The runner may have a pair of runner cells, where an end plate of one runner cell is 15 non-rotatably attached to an end plate of the other runner cell such that the pair of runner cells are co-axial. The runner may further comprise one or more supplementary runner cells, where an end plate of each supplementary cell is non-rotatably attached to an end plate of the pair of runner cells or an adjacent supplementary runner cell, such that the or each supplementary runner cell is co-

20 axial with the pair of runner cells. The runner blades of each runner cell may be rotationally offset from the runner blades of an adjacent runner cell, as will be described in more detail below. In place of adjacent end plates being attached together to form the runner one may alternatively use a single plate to define the end of two adjacent cells, with the blades of the respective cells being attached to either side of the single plate.
Figure 19 shows a cross sectional view through a stator which may be used in conjunction with the open runners of the present invention. As can be seen in the figure, the runner 1 is located centrally with the stator 600 arranged around the runner so as to direct flow tangentially to the arc of the runner blades. The stator 600 has a first flow passage 602 having a distal end 601 and a proximal end 603.
The first flow passage 602 is configured to direct sea water or other fluid flow to and from the runner 1. The stator 600 also has a second flow passage 606 having a distal end 605 and a proximal end 607. Again, the second flow passage 606 is configured to direct fluid flow to and from the runner 1. The first and second flow passages

21 602,606 may be diametrically opposed to one another about the runner 1. The flow passages 602, 606 may taper towards the runner 1, with curved inner walls which direct the fluid flow into the runner. In other words, the distal ends 601,605 of the flow passages 602,606 may have a larger surface area than the proximal ends 603,607 of the respective flow passages.
The stator may further comprise third and fourth flow passages 608,610 configured to direct fluid flow to and from the runner 1. These third and fourth passages 608,610 may also be diametrically opposed to one another about the runner 1, as well as being substantially perpendicular to the first and second flow passages 602,606. The cross sectional area of the third and fourth flow passages 608,610 may be substantially constant. The third and fourth flow passages 608,610 induce a low pressure area for the wake, being at right angles to the fluid flow. The stator 600 separates the inward fluid flow to the runner from both the outflow and the wake.
The stator 600 has semi-symmetrical profiles orientated for both ebb and flow, but also constructed to provide the stiffness to withstand the force of the tidal stream.
The stator 600 may be formed from the same plastics material as the runner.
The stator 600 may be attached to torque tubes and/or support spars of the systems described above.
Figure 20 shows a further embodiment of a free stream turbine, which is shown in a vertical configuration but may also be employed in a horizontal, cross-flow configuration if desired. In this instance the runner is formed from a plurality of runner cells all arranged co-axially as described above. A single end plate may define the ends of adjacent cells. This particular runner 700 is made up of four cells 700A-700D defined between five end plates, but a number of the end plates and blades have been removed for illustrative purposes only. Each cell 700A-700D
includes three blades 702, and each blade may have a hydrofoil profile. The blades 702 of each adjacent cell may be rotationally offset from one another.
A stator 720 is arranged about the runner 700. The stator includes a first pivotable vane 722 located on a first side of the runner 700 which is adapted to selectively deflect fluid flow towards the runner when upstream of the runner. The first vane 722 has a pivot axis 724 substantially parallel to the rotational axis R of the runner

22 700. A second pivotable vane 726 is located on a second, opposite side of the runner 700 and is also adapted to selectively deflect fluid flow towards the runner when upstream of the runner. The second vane 726 also has a pivot axis 728 substantially parallel to the rotational axis R of the runner. When a vane 722,726 is upstream of the runner is obviously dictated by the direction of flow of the tidal stream or other fluid within which the turbine is submerged.
The first and second vanes 722,726 may each have a biasing means 730 which biases the respective vanes towards an inactive position, neutral position when downstream of the runner 700. The biasing means 730 may be a torsion spring or a compression spring, for example. In figure 20 the first vane 722 is shown in an active, flow-deflecting position where its biasing means is compressed and the vane 722 lies against a stop 723 at approximately 30 to the direction of flow D.
The second vane 726 has the same arrangement but in Figure 20 it is shown in the neutral, inactive position where the biasing means 730 is holding the vane 726 substantially parallel to the flow direction D. The first, upstream vane 722 is therefore diverting flow into the active sector of the uni-directional runner and away from impeding the blades travelling in the opposite direction to the flow as they complete their rotation back to their starting point.
The stator 720 may be employed with other runners and turbines described herein, and is therefore not limited to use with the specific runner shown in Figure 20. The vanes are to divert the flow away from the path for blades rotating against the stream and thereby supplementing the flow to the blades developing torque.
The runner shown in Figure 20 uses a Darrieus-type arrangement, and preferably has three blades in each cell of the runner. If the blades have a hydrofoil profile for about 100 of the runner's rotation the current of water flowing towards and over the foil induces a powerful lift and thus the turning moment on the runner.
Whereas a three blade arrangement favours maximum efficiency, torque is confined to a limited arc of the rotation. As described above, the blades of each cell may be offset rotationally from those in the adjoining cell. For a four cell runner the offset is preferably 30 , which is one quarter of the 120 spacing between each blade in the three blade set. If this offset is employed the resulting torque will now have 12 peaks

23 instead of just three. This will result in a relatively steady load on the generator, and also avoids the possibility of the dynamic stalling of the runner.
The blades used in the runners described herein may have a metal core, such as steel or aluminium, for example, with the blade profile then moulded over the core.
The profile may be over-moulded using a resin and internal cavities are filled with closed cell polyurethane foam, for example.
The cell end plates of any of the runners described herein can be made buoyant to enable the whole or part runner to be floated to the surface. One arrangement is constructed with a pair of central circular tanks half the width (side to side) of the disc. These interlock male to female with a central spigot. Wedge-shaped full width tank spokes fit to one or other of the central hub tanks. Bolts secure the components together to achieve a strong beam structure that can withstand the tidal stream.
Figure 21 illustrates a further embodiment of an energy system incorporated open runner turbines 15. The system includes a buoyant support spar 802 at either end of the turbines 15 with the turbines rotatably supported by the spar in the same manner as described above. Only one spar is shown in Figure 21 as it is an end view of the system. Each spar 802 has a pair of spurs 804,806 which extend from the respective upper and lower sides of the spar at an acute angle, which may be substantially 45 . The turbine support axles are rotatably supported at the free end of each spur 804,806. The ends of each spar 802 are secured to the seabed or a dedicated foundation 800 by an inclined tether line 808 to limit the upward movement of the system caused by the buoyancy of the turbines and/or support spars.
Each pair of tethers 808 is arranged so as to form a trapezoid along with the seabed/foundation 800 and the support spar 802. A distance D1 between each pair of tethers 808 where they connect to the seabed/foundation is shorter than a distance D2 between those tethers 808 where they connect to the support spar 802.
Longitudinal and lateral movement of the system is restrained by way of mooring chains 810, which are attached to the centre of the spar 802 as well as by a foundation 800 or piles on the seabed. The mooring arrangement is preferably a star mooring, where the chains are splayed out from each support spar to four radial locations so that they take the longitudinal and lateral loads applied to the system.

24 Each spar 802 also has a fin, or paravane, 812 attached to the outer side of the spar, where the fin is substantially parallel to the spar. The fins 812 are adapted to selectively adjust the angle of the support spars and turbines under the action of fluid flow over the fin. The fins thus induce an upward thrust force from the fluid flow to counter downward drag on the system from the mooring cables and/or tethers.
With the lift force of the passive fin/paravane the angle of attack of the system is adjusted automatically by the force of the fluid stream on the system structure. This is achieved by an isosceles trapezoid tethering arrangement effected by the aforementioned tethers and mooring chains, which causes the tethered system to tilt and thus whichever is the leading edge of the fin to tilt upwards with the stream inducing an upward force. This is sufficient force to compensate for the downward component of the horizontal mooring cables and in addition maintain the tension in the vertical tethers. The force generated by the fluid stream broadly equates to the upward force required to ensure the stability of the structure suspended across the stream.
Figure 22 shows a metal reinforcement member which in use will reinforce the joint where the runner blades are attached to the runner end plates. Figure 23 shows the reinforcement member in situ in the runner, with the blades foreshortened and the other reinforcement members removed for illustrative clarity only.
Referring initially to Figure 22, the reinforcement member 900 has a body 902 and a curved slot 904 which effectively divides the body into a pair of prongs 903,905. The slot 904 is shaped so as to closely match the curvature of the blades. Each prong 903,905 includes a number of fixing apertures 908 for receiving fixing bolts or the like.
Referring now to Figure 23, when the runner is being assembled the ends of the blades 3 are already attached to a respective end plate 5, preferably by welding although mechanical fixtures may be used instead. A reinforcement member 900 is provided for the end of every blade, so there will be two reinforcement members for each blade. A reinforcement member 900 is slid over the end of each blade 3, with the open end of the slot 904 being presented to the distal edge 9 of the blade and then slid over the blade until the open end of the slot is adjacent the proximal edge 11 of the blade. Once in this position and flush with inner surface of the end plate 5, the reinforcement member is bolted or otherwise mechanically fixed to the end plate.
As shown in Figure 23, in the instances where a pair of adjacent runner cells are 5 made up of respective pairs of end plates 5,5', rather than the adjoining cells sharing a single end plate between them, a friction plate or gasket 910 may be placed between the two end plates 5,5' prior to them being mechanically attached to one another. In addition, in the instances where the blades of one runner cell are not rotationally offset from those of the adjoining cell, the bolts or other fixtures which 10 attach the reinforcement members 900 of one runner cell may pass through the end plate(s) into the adjoining cell in order to fix the reinforcement members of the blades 3' of that adjoining cell too.
The reinforcement members 900 spread load from the bolts or other mechanical 15 fixtures which connect adjoining runner cells via their end plates, or to matching cylindrical tanks. The reinforcement members and bolts clamp the adjoining end plates together, and also prevent bending of the blade relative to the weld where the blade joins to the end plate. The friction plate or gasket may be metal, providing additional stiffness.
The reinforcement members enable the predominantly plastic structure of the runner to be endowed with sufficient longitudinal stiffness to withstand the load of the tidal stream. The blades themselves may be stiffened further if required by intermediate plates positioned between the two end plates in a runner cell. The blading would pass through slots in those intermediate plates.
The present invention is designed to harvest energy from a high volume but very low grade source such as a tidal stream. It requires relatively little subsea infrastructure in comparison to existing free stream turbine systems, primarily thanks to the structural integration and the buoyancy of the runners and tanks used for the turbine.
The runner of each turbine is unidirectional, rotating in the same direction regardless of the direction of tidal flow, thus reducing the complexity and associated expense of the turbine. The design and arrangement of the runner blades means that the runners are self-cleaning, releasing any entrained debris as they rotate. The blade material is resistant to cavitation damage commonly experienced with reaction runners.
The runner design is self-supporting without the need for a central shaft or axle extending across the centre of the runner. Thus, there is no impediment to flow with this runner.
Having the generator assembly in a second tank again lessens complexity by avoiding the need for a separate unit and drive transmission. It also allows the main bearing to be in a dry, watertight space with an outboard gland sealing the second tank on the outside.
The runner design means that a variety of turbine systems can be created using the same basic design: a buoyant turbine tethered in midwater, a seabed system which can have multiple runners, and for deeper waters a vertical version as well as surface-supported versions.
A horizontal runner variant may be moored and tethered just submerged below the surface such that the reciprocating flow of waves and swell imparts energy to the turbine. The tether may be adjusted in length by a mechanism controlled by rise and fall of tide height.
The runner, turbine and associated systems may also be scaled down into small, portable units for use in small-scale hydro generation, where the turbine would be deployed in a stream or river. This would be beneficial in remote locations where there is no power source for lighting, the load from which has been greatly reduced by the introduction of LEDs.
Although the preferred embodiments of the invention are shown in tidal energy applications, the invention is not limited to such applications. The runners, turbines and systems of the present invention may be employed in any free stream application. That is to say, the invention is equally suitable for stream, river and canal environments, for example, as it is tidal applications at sea.

It should also be understood that the turbines and energy systems of the present invention are not limited to the use of the specific generator arrangements described herein. The generator arrangement used may be a set which is known in the art.
Modifications and improvements may be incorporated without departing from the scope of the invention as defined by the appended claims.

Claims (38)

CLAIMS:

1. A runner for a free stream turbine, the runner having a rotational axis and at least one runner cell, each runner cell defined by a pair of end plates with a plurality of runner blades extending laterally between the end plates, wherein the runner blades are circumferentially arranged about a central void through which the rotational axis extends.

2. The runner of claim 1, further comprising a plurality of runner cells arranged co-axially.

3. The runner of claim 2, wherein the runner blades of each runner cell are rotationally offset from the runner blades of an adjacent runner cell.

4. The runner of any preceding claim wherein each blade has a distal edge and a proximal edge and is curved so as to define an inner concave surface and an outer convex surface, and wherein the proximal edge of each blade is located radially outwards from a rotational axis of the runner such that the plurality of proximal edges define an outer boundary of the central void of a respective runner cell.

5. The runner of claim 4, wherein each blade has a degree of curvature of 150-170 .

6. The runner of any of claims 1 to 3 wherein each blade has a hydrofoil profile.

7. The runner of any preceding claim, wherein the runner is formed from a plastic.

8. The runner of claim 7, further comprising a metal reinforcement member located about the end of each blade where the blade end attaches to a respective end plate.

9. The runner of claim 7 or claim 8 wherein each blade has a metal core with a plastic body moulded about the metal core.

10. The runner of any preceding claim, wherein at least one of the end plates of the or each runner cell comprises a buoyancy tank.

11. A turbine comprising a runner according to any preceding claim.

12. A turbine, comprising:
a runner comprising a plurality of runner blades;
a first tank non-rotatably attached to a first end of the runner;
a second tank non-rotatably attached to a second end of the runner;
first and second support axles upon which the first and second tanks are rotatably mounted such that the runner and first and second tanks rotate about an axis of rotation defined by the support axles; and a generator apparatus sealed within the second tank and non-rotatably attached to the second support axle such that the apparatus remains stationary as the tanks and runner rotate about the axle.

13. The turbine of claim 12, wherein the runner comprises a runner according to any of claims 1-10.

14. The turbine of claim 12 or claim 13, wherein the second tank has a pair of tank end walls, and wherein the generator apparatus comprises:
a ring gear fixed to a tank end wall of the second tank; and a plurality of generator modules arranged circumferentially about the axis of rotation, each module comprising a drive gear engaged with the ring gear, whereby rotation of the ring gear transfers a drive torque to the drive gear of each module to drive each generator.

15. The turbine of claim 14, wherein the ring gear is formed from roller chain, with the drive gear of each module having a plurality of teeth which engage the roller chain.

16. The turbine of any of claims 12 to 15, wherein the second support axle is hollow and contains at least one umbilical connectable to the generator apparatus.

17. The turbine of any of claims 12 to 16, further comprising a buoyant support spar to which a free end of the second support axle is attached, and one or more anchors which anchor the spar and first support axle to the seabed by 5 way of a plurality of anchor cables.

18. The turbine of claim 17, wherein the support spar includes a plurality of interconnected ballast tanks, at least one of the ballast tanks being adapted to receive and discharge one or more fluids in order to adjust the buoyancy of 10 the turbine.

19. The turbine of claim 17 or claim 18, further comprising at least one spar winch housed within the support spar, and an axle winch housed within the first support axle, wherein each winch has a winch cable attached to the one 15 or more anchors, whereby operation of the winches allows the turbine to adjust its depth within a body of water.

20. The turbine of any of claims 11 to 19, further comprising a stator arranged about the runner, the stator comprising:
20 a first flow passage having a distal end and a proximal end, the first flow passage configured to direct fluid flow to and from the runner: and a second flow passage having a distal end and a proximal end, the second flow passage configured to direct fluid flow to and from the runner;
wherein the first and second flow passages are diametrically opposed 25 to one another about the runner, and wherein the distal ends of the first and second flow passages have a larger surface area than the proximal ends of the respective flow passages.

21. The turbine of claim 20, wherein the stator further comprises third and fourth 30 flow passages configured to direct fluid flow to and from the runner, wherein the third and fourth passages are diametrically opposed to one another about the runner and are substantially perpendicular to the first and second flow passages.

22. The turbine of claim 21, wherein the cross sectional area of the third and fourth flow passages is substantially constant.

23. The turbine of any of claims 11 to 19, further comprising a stator arranged about the runner, the stator comprising:
a first pivotable vane located on a first side of the runner and adapted to selectively deflect fluid flow towards the runner when upstream of the runner, the first vane having a pivot axis substantially parallel to the rotational axis of the runner; and a second pivotable vane located on a second, opposite side of the runner and adapted to selectively deflect fluid flow towards the runner when upstream of the runner, the second vane having a pivot axis substantially parallel to the rotational axis of the runner.

24. The turbine of claim 23, wherein the first and second vanes each have biasing means which bias the respective vanes towards an inactive position when downstream of the runner.

25. The turbine of any of claims 11 to 24, wherein at least one of the first and second tanks includes a ballast chamber adapted to receive and discharge one or more fluids in order to adjust the buoyancy of the turbine.

26. An energy system comprising:
a pair of turbines according to any of claims 11 to 25;
a pair of inner support axles to which respective inner ends of both of the turbines are rotatably mounted such that the turbines are substantially co-axial;
a pair of outer support axles to which outer ends of both turbines are rotatably mounted;
wherein one of the pair of turbines is configured to rotate about its respective support axles in a first direction and the other of the turbines is configured to rotate about its respective support axles in a second, opposite, direction.

27. The system of claim 26, wherein the turbines are arranged such that the second tanks of the turbines are proximate the respective inner support axles, and the first tanks of the turbines are proximate the respective outer support axles.

28. The system of claim 26 or claim 27, further comprising a buoyant support spar to which the inner support axles are attached by respective universal joints, and one or more anchors which anchor the spar to the seabed by way of one or more anchor cables.

29. An energy system comprising:
a plurality of turbines according to any of claims 11 to 25;
a pair of buoyant support spars to which respective ends of each of the turbines are rotatably mounted;
a plurality of mooring chains attaching the buoyant spars to the seabed or a dedicated foundation; and a pair of tether lines for each support spar, where each tether line of the pair has one end connected to a respective support spar and one end connected to the seabed or foundation, such that a distance between the seabed ends of the pair of tethers is less than a distance between the support spar ends of the pair of tethers;
wherein each support spar includes a fin attached thereto, the fin adapted to selectively adjust the angle of the support spar and turbines under the action of fluid flow over the fin.

30. The system of claim 28 or claim 29, wherein the or each support spar includes a plurality of interconnected ballast tanks, at least one of the ballast tanks being adapted to receive and discharge one or more fluids in order to adjust the buoyancy of the system.

31. The system of any of claims 28 to 30, further comprising at least one spar winch housed within the support spar, and first and second axle winches housed within the outer support axles, wherein each winch has a winch cable attached to the one or more anchors, whereby operation of the winches allows the system to adjust its depth within a body of water.

32. An energy system comprising:
a support frame;
a plurality of turbines according to any of claims 11 to 25, the support axles of each turbine being non-rotatably fixed to the frame;
wherein the support frame is attached to a buoyant member floating on the surface of a body of water such that the support frame and turbines are submerged below the buoyant member.

33. The system of claim 32, wherein the turbines are arranged on the frame such that their axes of rotation lie in a plane which is substantially perpendicular to a longitudinal axis of the buoyant member.

34. The system of claim 32, wherein the frame is pivotably attached to the buoyant member such that the turbines can be moved between positions upon and below the surface of the body of water.

35. The system of claim 34, wherein the turbines are arranged on the frame such that their axes of rotation lie in a plane which is at an oblique angle to a longitudinal axis of the buoyant member.

36. The system of claim 32, wherein the buoyant member includes a plurality of interconnected ballast tanks, at least one of the ballast tanks being adapted to receive and discharge one or more fluids in order to adjust the rotational position of the buoyant member and frame relative to the surface of the body of water.

37. An energy system comprising:
a support frame having a base member fixable to the bottom of a body of water;
a plurality of turbines according to any of claims 11 to 25, the support axles of each turbine being non-rotatably fixed to the frame;
wherein the turbines are arranged on the frame such that their axes of rotation lie in a plane which is at an oblique angle to a longitudinal axis of the base member.

38. An energy system comprising:
first and second turbines, each turbine comprising a turbine according to any of claims 11 to 25;
a support frame to which both turbines are rotatably fixed, such that the axes of rotation of the turbines are substantially parallel; and a support pillar having a first end fixable to the bottom of a body of water, and a second end to which the support frame is attached, wherein a longitudinal axis of the pillar and the axes of rotation are substantially parallel.