The invention relates to the field of hydrogenerators, and more specifically to a hydrogenerator having a stator defining an inner stator opening, and an annular rotor adapted to be rotatably mounted in the stator opening and defining an inner rotor opening in which inwardly projecting propeller blades of the rotor are mounted. The rotor may be rotatably driven by water generally axially flowing through the rotor opening, whereby the

hydrogenerator, submerged into water, through appropriate electromagnetical components in the stator and rotor, generates electrical energy. BACKGROUND OF THE INVENTION

EP 1 739 007 discloses a shaftless propeller which may be made to cooperate with a stator having a circular opening and a rotor mounted in said opening. The rotor comprises an annular rotor body with a circular rotor opening having a multiplicity of inwardly projecting propeller blades. Bearings between the rotor and the stator comprise water lubricated tiltable pad bearings, both for axial support and for radial support of the rotor with respect to the stator.

A propeller of this kind may be used to generate energy from a flow of liquid, in particular water. The stator may have electrical stator windings provided around the stator opening. The rotor may be provided with permanent magnets at its outer circumference facing the stator windings. When the propeller is driven by the liquid flowing through the rotor opening and engaging the propeller blades to rotate the rotor with respect to the stator, electric power can be generated.

A problem in the known propeller, when used as a generator, is that the axial tiltable pad bearings, which need to provide a relatively large contact surface area due to the axial forces to be expected, prevent the rotor from starting to rotate from standstill, even if liquid flows through the rotor opening and engages the propeller blades. In standstill, the pads of the tiltable pad bearings on the stator are in contact with an opposite journal surface of the rotor, such that high friction forces in the axial bearing occur which cannot be overcome by the water driving the propeller blades to start up rotation of the rotor. In fact, the water engaging the propeller blades cause the pads to be pressed on the journal surface even stronger, thus increasing the friction force. Since the rotor cannot be made to rotate by the water engaging the propeller blades, no water film may form between the pads and the journal surface. To remedy such situation, and in order to bring the rotor into rotation for forming said water film, the propeller must at least temporarily be electrically energized to operate as a motor, whereby the rotor is set into motion at least to such extent that a water film indeed is formed, to thereby drastically reduce the friction between rotor and stator and allow the rotor to continue to rotate without supplying electrical energy to the propeller, and harvest electrical energy instead.

The above problem is increased in that also the radial support of the rotor is provided by tiltable pad bearings which cause similar problems of friction at standstill as the axial tiltable pad bearings.

Accordingly, the known propeller requires a substantial supply of electrical energy from standstill to be set into a generator state afterwards. Such electrical supply may not be available from a public or private electricity network, or from electrical energy storage means. The know propeller further requires a sophisticated control of the propeller to reach the generator state from standstill through a motor state.

Based on the above, there is a need for a hydrogenerator which does not need initial supply of electrical energy in order to be able to function as a generator.

SUMMARY OF THE INVENTION

It would be desirable to provide a hydrogenerator having an axial support of the rotor that does not impede rotor rotation, in particular from standstill. It would also be desirable to improve not only the axial support of the rotor, but also the radial support.

To better address one or more of these concerns, in a first aspect of the invention a hydrogenerator for converting water flow energy into electrical energy in a submerged state is provided. The hydrogenerator comprises: a stator defining an inner stator opening, the stator comprising electrical stator windings; an annular rotor rotatably mounted in the stator opening and defining an inner rotor opening in which propeller blades of the rotor are mounted to be engaged by water flowing axially through the rotor opening in a first direction, the rotor comprising a magnetic structure configured to interact with the stator windings to generate electrical energy when the rotor rotates relative to the stator; a radial bearing assembly for radially positioning the rotor relative to the stator, the radial bearing assembly comprising water lubricated tiltable pads supporting a journal surface; and an axial bearing assembly for axially positioning the rotor relative to the stator under a thrust force exerted by the flowing water. The axial bearing assembly comprises repulsive permanent magnet units configured to act between stator and rotor to axially force the rotor in a second direction relative to the stator opposite said first direction, and an axial stop assembly configured to limit axial movement of the rotor in said second direction.

In standstill, the repulsive permanent magnet system will force the rotor in the second direction to reach an axial position in which the axial stop assembly limits or stops a further movement of the rotor. Thus, in said axial position, the rotor has an axial surface (i.e. a surface extending at right angles to an axis of rotation of the hydrogenerator) resting against an axial surface of the stator under preload of the repulsive magnet force. Friction between the rotor surface and the stator surface is generated, which may be sufficient to keep the rotor in one angular position relative to the stator when the hydrogenerator as a whole is moved, e.g. during transport or installation thereof.

However, when there is a water flow through the rotor opening in the first direction, the propeller blades experience a driving force having both a tangential and an axial component in the first direction. The axial component of the driving force counteracts the repulsive magnet force and lowers the preload on said axial surfaces of the stator and the rotor to zero. Accordingly, the friction between said surfaces vanishes, and the axial bearing assembly operates frictionless, allowing the hydrogenerator having a radial bearing assembly comprising tiltable pads to autonomously start rotating from standstill without supplying electrical energy to the hydrogenerator.

In a preferred embodiment, the repulsive permanent magnet units comprise a first ring-shaped axial stator face provided with permanent stator magnets and a first ring-shaped axial rotor face provided with permanent rotor magnets, the first stator face facing the first rotor face, and wherein the permanent stator magnets at a side facing the first rotor face all have a same first magnetic pole, and the permanent rotor magnets at a side facing the first stator face all have the same first magnetic pole. An advantage of such an embodiment is that the permanent magnet units may be easily manufactured, e.g. by mounting them on a support ring, or on a plurality of support ring segments, and next the permanent magnet units may be mounted on the axial surfaces of the stator and rotor, respectively. Only when assembling the stator and rotor, the repulsive force between the stator and the rotor need be overcome by application of an external force on the rotor relative to the stator to bring the rotor in a position in which the axial stop assembly can be mounted. Once the axial stop assembly has been mounted, the external force can be removed.

In a preferred embodiment, the first stator face is a first stator end face, and the first rotor face is formed by a first collar at a first rotor end, the first collar comprising a first collar opening in line with the rotor opening. Advantages of such an embodiment are its simplicity of construction and its accessibility at the stator and rotor ends, while also providing for a relatively simple method of assembly of the hydrogenerator by avoiding an axial support of the rotor relative to the stator located in a region of a radial gap between the stator and the rotor.

Preferably, the collar comprises a flow guide component to optimize the inflow of water into the rotor opening. In an embodiment, the flow guide component provides a gradually tapering surface towards the rotor opening, thus lowering turbulence of water near the rotor opening and thereby increasing the power output of the hydrogenerator.

In a preferred embodiment of the hydrogenerator, the axial stop assembly comprises a second ring-shaped axial stator face and a second ring-shaped axial rotor face, the second stator face facing the second rotor face. The axial stop assembly provides a mechanical stop to limit an axial movement of the rotor caused by the magnetic repulsive force operative between the stator and the rotor.

Preferably, the second stator face is a second stator end face, and the second rotor face is formed by a second collar at a second rotor end, the second collar comprising a second collar opening in line with the rotor opening. Advantages of such an embodiment are its simplicity of construction and its accessibility at the stator and rotor ends, while also providing for a relatively simple method of assembly of the hydrogenerator by avoiding an axial support of the rotor relative to the stator located in a region of a radial gap between the stator and the rotor.

In a preferred embodiment, the second rotor face is provided with a ring-shaped axial stop element, or a plurality of ring sector shaped axial stop elements. The ring-shaped axial stop element may provide a stop effect which is evenly distributed near the circumference of the axial stop assembly, thereby avoiding a tilting of the rotor and the stator relative to each other.

In a further preferred embodiment, the pad is supported on the stator and is tiltable around an off-center axis, whereby a first part of the pad tangentially extending from the off- center axis is longer than a second part of the pad tangentially extending from the off-center axis, and wherein the tiltable pad bearing system further comprises a cylindrical journal surface on the rotor, the journal surface facing the pads, and wherein the rotor is configured to rotate the cylindrical journal surface in a direction from the first part of the pad to the second part of the pad. In operation of the hydrogenerator, with the rotor rotating relative to the stator, a water film is intended to be formed between the pads and the journal surface. Such water film exerts a pressure on the pad and on the journal surface. The pressure force on the first part of the pad is larger than the pressure force on the second part of the pad due to the greater surface area of the first part of the pad compared to the surface area of the second part of the pad. As a result, the pad will slightly tilt such that the distance between the first part of the pad and the rotor journal surface increases. This promotes water to enter the gap between the pad and the journal surface, and a water film to form. In a hydrogenerator in operation, comprising a radial bearing assembly with a plurality of tiltable pads which support a journal surface of the rotor through a water film, and comprising an axial bearing assembly comprising repulsive magnet units, there is no mechanical contact between the stator and the rotor. Thus, the friction between the stator and the rotor is extremely low, leading to low friction losses. At the same time, wear of the stator and rotor is prevented, which leads to an extremely long maintenance-free service life of the hydrogenerator.

Preferably, the pad, at the side facing the journal surface, is concave having a radius which is larger than a radius of the journal surface. Advantageously, in this embodiment, a gap between an end of the first part of the pad, on the one hand, and the journal surface, on the other hand, is widened, making it even easier for water to enter this gap to form a water film.

In a preferred embodiment , the pad is movable in a radial direction. A radially movable pad can cope with unroundness or other irregularities in the radial bearing assembly, thus providing lower demands on tolerances in the hydrogenerator construction.

Preferably, the pad which is movable in the radial direction is preloaded in a direction towards the journal surface by a preloading member. Thus, during operation of the

hydrogenerator a constant water-lubricated contact between the pad and the corresponding journal surface is maintained.

Preferably, the preloading member comprises a spring element or a rubber element.

In particular in the case of a rubber or similar resilient energy-absorbing element, a stable radial bearing is ensured.

In a preferred embodiment, the pad is tiltable around an axis, whereby a first part of the pad tangentially extends from the axis in one direction, and a second part of the pad tangentially extending from the axis in a direction opposite to said one direction, and wherein each one of the first and the second part of the pad is preloaded. In such embodiment, an excessive tilting of the pad, with a concomitant loss of the water lubrication, can be avoided.

In a preferred embodiment, a bearing surface area provided by the pads in a lower part of the stator opening is greater than a bearing surface area provided by the pads in an upper part of the stator opening. Most of the load exerted by the rotor on the stator originates from the weight of the rotor, which is a gravity effect in a downward direction. Thus, it suffices to have most pads in a lower part of the stator opening to provide a bearing surface area designed to support the rotor. In an upper part of the stator opening, relatively few pads are necessary, providing a relatively small bearing surface area which is sufficient since the rotor load to be supported in this upper part of the stator opening is low.

These and other aspects of the invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings in which like reference symbols designate like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts a perspective view of an embodiment of a hydrogenerator according to the present invention, as seen from a water inlet side.

Figure 2 depicts another perspective view of the hydrogenerator of Figure 1 , as seen from a water outlet side.

Figure 3 depicts a schematic axial cross-section of the hydrogenerator of Figures 1 and 2.

Figure 4 depicts a perspective view of an axial cross-section of a part of the hydrogenerator of Figures 1 and 2.

Figure 5 depicts a perspective view of a stator of the hydrogenerator of Figures 1 and

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 depicts a hydrogenerator (sometimes also referred to as a hydro-powered generator) 1 for energy harvesting at locations with low water height difference (as from about 2.5 m).

The hydrogenerator 1 comprises a generally annular (outer) stator 2 and a generally annular (inner) rotor 3 which is rotatably supported in a stator opening. The rotor 3 has a rotor opening 4 having an inner circumference defining a water passage. At the inner

circumference of the rotor 3, a plurality of (in the non-limiting embodiment shown: six) blades 5, supported on cylinder sector shaped blade supports 6, are mounted such that the blades 5 project inwardly. The radial length of the blades 6 is selected to allow a center region of the rotor opening 4 to be free of obstacles, as seen in an axial direction 7 of the hydrogenerator 1. The free center region allows creatures living in the water and other objects to pass the hydrogenerator 1 without engaging the hydrogenerator 1 , thus preventing damage to any of the creature, object or hydrogenerator 1. It is noted that other types of propeller blades may be used. The rotor 3 is provided with a first collar 8 comprising a ring configured for guiding water into the rotor opening 4 in a first direction 9, to drive the rotor 3 for rotation in tangential direction 10 relative to the stator 2. Thus, the ring of the first collar 8 is at an inlet opening of the hydrogenerator 1. The first collar 8 extends radially from the rotor opening 4 over a first axial end face of the stator 2 (which first axial end face is not visible in Figure 1). Referring to Figure 2, a second collar 1 1 of the rotor 3, comprising three second collar parts 11 a, 11 b and 1 1c is shown. The second collar 11 (or the second collar parts 11 a, 11 b, 1 1c) extends/extend radially from the rotor opening 4 over an second axial end face 12 of the stator 2.

Referring to Figures 2 and 3, the stator 2 has a radially extending flange 13 near the inlet opening of the hydrogenerator 1. As indicated in Figure 3, the flange 13 may be provided with a plurality of holes 14 to allow the stator to be mounted in a supporting structure (not shown). The stator 2 further has a radially extending flange 15 comprising the second axial end face 12 of the stator 2.

Stator 2 comprises a hermetically sealed annular stator space 20 formed by flanges

13 and 15, a cylindrical outer casing 16 and a cylindrical inner casing 17. The annular stator space 20 contains electrical windings 22 wound around an iron core 24. The rotor 3 comprises a hermetically sealed annular rotor space 21 formed by a rotor body, flanges 26, and a cylindrical outer casing 28. The annular rotor space 21 contains a plurality of permanent magnets 30. Alternatively, the hydrogenerator may comprise a synchronous reluctance type electrical machine having a specific magnetic structure instead of permanent magnets. The electrical windings 22 of the stator 2 face the permanent magnets 30 creating a radial magnetic flux. In operation, when the rotor 3 rotates in direction 10 (Figures 1 , 2), the permanent magnets induce a generation of electrical power in the electrical windings 22. The power to be supplied to a load, and any sensor signal or other signal, may be output through terminals 32 (Figure 5).

Referring to Figures 3 and 4, a radial bearing assembly comprises two cylindrical journal surfaces 40 provided on the rotor 3. In the embodiment shown, the journal surfaces 40 are formed by strips 42 extending circumferentially around the rotor 3 near the respective ends thereof. The material of the journal surfaces 40 may be any metal alloy or plastic material, but preferably is a high density polyethylene, more preferably an ultra high molecular weight polyethylene, UHMWPE. The strips 42 are mounted on the rotor 3 with a dovetail connection.

On the stator 2, the radial bearing assembly comprises a plurality of pads 44

(sometimes also referred to as "Michell sliders") fixed in a holder 46. The pads 44 are facing the journal surfaces 40. Each holder 46, and thereby also the pad 44 fixed in the holder 46, is tiltable around an axis defined by a pin 48. This axis preferably is off-center relative to the holder 46 and pad 44, whereby a first part (indicated by 44a) of the pad 44 tangentially extending from the off-center axis is longer than a second part (indicated by 44b) of the pad 44 tangentially extending from the off-center axis. The pin 48 is fixed to the stator 2 by a support 50. The holder 46 is connected to the pin 48 through a hole 52 in an arm 54 of the holder 46, allowing the holder 46 not only to tilt relative to the pin 48, but also to move in a radial direction relative to the pin 48, i.e. to move at right angles to the longitudinal axis of the pin 48 and to the journal surface 40. The (arm 54 of the) holder 46 is fixed in a longitudinal direction of the pin 48 by rings 56. Blocks 58 made from a soft resilient and elastic material, such as rubber or a rubber compound, and placed on either tangential side of the pin 48, radially press the holder 46 in the direction of the rotor 3, and at the same time allow the aforementioned tilting and moving of the holder 46 (and, consequently, also of the pad 44). This soft suspension of the pad 44 ensures that, in case of unroundness or other irregularities between the stator 2 and the rotor 3, the influence thereof on the load per pad 44 is minimized. This will lead to a similar load for each pad 44. Instead of blocks 58, also spring elements could be used, preferably damped spring elements.

The pads 44, at their side facing the journal surface 40, preferably are concave having a radius which is greater than a radius of the cylindrical journal surface 40.

As shown in Figure 5, more pads 44 are mounted in a lower part of the stator 2 than in an upper part of the stator 2. Accordingly, a bearing surface provided by the pads 44 in a lower part of the stator opening is greater than a bearing surface provided by the pads 44 in an upper part of the stator opening. The pressure exerted by the (weight of the) rotor 3 on the lower part of the stator 2 can thus be received in an optimum way. This is not necessary in the upper part of the stator 2. Restricting the number of pads, while having sufficient pads 44 to center the rotor 3 relative to the stator 2, limits any friction generated between the pads 44 of the stator 2 and the journal surfaces 40 of the rotor 3.

A pad 44 preferably is made from a hard material, such as a hardened metal or a metal provided with a hardened surface, in particular a hardened surface facing the journal surface 40 of the rotor 3. The pads may further be made from a ceramic material.

Returning to Figure 3, an axial bearing assembly comprises repulsive permanent magnet units 60, 62 configured to act between stator 2 and rotor 3 to axially force the rotor in a direction 64 relative to the stator 2, opposite to direction 9. An axial stop assembly is formed by a ring or ring segment(s) 66 connected to the rotor 3 interacting with the second axial end face 12 of the stator 2 through second collar 11 (or second parts 1 1a, 11 b, 11 c) on which the ring or ring segment(s) 66 is/are mounted.

Permanent magnet unit 60 comprises permanent magnets 70 forming a first ring- shaped axial stator face. Permanent magnet unit 62 comprises permanent magnets 72 forming a first ring-shaped axial rotor face. The first stator face faces the first rotor face. The permanent magnets 70 of permanent magnet unit 60 of the stator 2, at a side facing the first rotor face, all have the same first magnetic pole (the first magnetic pole being either a south pole or a north pole), and the permanent magnets 72 of permanent magnet unit 62 of the rotor 3, at a side facing the first stator face, all have the same first magnetic pole. Accordingly, similar poles of the stator permanent magnet unit 60 and the rotor permanent magnet unit 62 are directed towards each other, causing the permanent magnet units 60 and 62 to repel each other, where the repulsive force increases with decreasing distance between the permanent magnet units 60, 62.

Referring to Figures 1 and 3, once water starts to flow through the rotor opening 4 in direction 9, an axial thrust on the rotor 3 in direction 9 is generated by the water flow interacting with the propeller blades 5. The rotor 3 axially moves in direction 9 such that the ring or ring segment(s) 66 is free from second axial end face 12 of the stator 2, until the thrust force is balanced with the repulsive force generated by the repulsive permanent magnet units 60 and 62. Thus, the axial bearing between the stator 2 and the rotor 3 is completely contactless, and only has water between the repulsive permanent magnet units 60 and between the ring or ring segment(s) 66 and the second axial end face 12. Tangential drag forces on this water generate a very low amount of friction in the axial bearing assembly.

When the hydrogenerator 1 is out of operation, i.e. when the rotor is at standstill, or has a low speed of rotation, an axial thrust on the rotor in direction 9 is low or absent, at least lower than the repulsive force generated by the repulsive permanent magnet units 60 and 62 until the ring or ring segment(s) 66 abut(s) the second axial end face 12 of the stator 2. This situation differs from the operational situation shown in Figure 3, where there is a gap between the ring or ring segment(s) 66 and the second axial end face 12.

The radial bearing assembly comprising the pads 44 and the journal surfaces 40, allows for the axial movements of the rotor 3 relative to the stator 2, in particular during taking the hydrogenerator 1 into and out of operation by starting or stopping, respectively, the flow of water through the rotor opening 4. In particular during start-up of the hydrogenerator 1 , first the ring or ring segment(s) 66 come free from the second axial end face 12 when the rotor 3 tends to move axially in direction 9 by the thrust exerted on the propeller blades 5 by the flow of water in the rotor opening 4. Thus, the rotor 3 moves axially, and at the same time experiences a tangential force by the thrust exerted on the propeller blades 5 by the flow of water in the rotor opening 4. At that time, there is still little or no water between the pads 44 and the journal surfaces 40 of the rotor 3. However, the mechanical friction to be overcome is relatively low, taking into account that (a) only in a lower part of the stator opening a substantial amount of pads 44 are present, and (b) the pads 44 already move relative to the journal surfaces 40. Accordingly, the rotor 3 is quickly able to gain rotational speed, and water films between the pads 44 and the journal surfaces 40 will be established, thus canceling the mechanical friction, and leaving minimal friction in the water films.

The hydrogenerator 1 can be arranged in a river, a canal, or a pipeline. The hydrogenerator 1 can be installed in a river barrage without the need of large civil

adaptations. In use, the hydrogenerator 1 is fully submerged under water. The stator 2 does not mechanically contact the rotor 3 during generator operation. There is a very low friction between the stator 2 and the rotor 3, and start-up can be autonomous. The hydrogenerator 1 has a very long and maintenance free life time (estimated over 60 years).

As explained above, a submerged hydrogenerator converts water flow energy into electrical energy, and comprises: a stator defining an inner stator opening, the stator comprising electrical stator windings; an annular rotor rotatably mounted in the stator opening and defining an inner rotor opening in which propeller blades of the rotor are mounted to be engaged by water flowing axially through the rotor opening in a first direction, the rotor comprising a magnetic structure configured to interact with the stator windings to generate electrical energy when the rotor rotates relative to the stator; and a water lubricated radial tiltable pad bearing assembly. An axial bearing assembly comprises repulsive permanent magnet units to axially force the rotor in a second direction relative to the stator opposite said first direction, and an axial stop assembly to limit axial movement of the rotor in said second direction.

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting, but rather, to provide an understandable description of the invention.

The terms "a", "an", "first", "second" etc. as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. In addition, singular references do not exclude a plurality. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language).

Any reference signs in the claims should not be construed as limiting the scope of the claims or the invention.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically.