GB2538278A

GB2538278A - Tidal generator

Info

Publication number: GB2538278A
Application number: GB1508192.0A
Authority: GB
Inventors: Lawrence Silverton Charles; Ewart Anderson David
Original assignee: RENEWABLE DEVICES MARINE Ltd
Current assignee: RENEWABLE DEVICES MARINE Ltd
Priority date: 2015-05-13
Filing date: 2015-05-13
Publication date: 2016-11-16
Also published as: GB201508192D0

Abstract

A submersible tidal turbine generator assembly comprises an electrical generator, a housing 20, 22 containing the generator, and a turbine 24, 26 connected to drive the generator. The assembly includes a ballast tank 14 connected to the housing, which can be filled with water to control the buoyancy of the generator assembly. The ballast tank 14 may include an end cone 30 with an axis of rotational symmetry misaligned such that in use the assembly experiences a hydrodynamic downwards force. The turbine blades may include winglets (34, 36, figure 4) to reduce the noise produced by the turbine 24, 26.

Description

Tidal Generator

Field of the invention

The present invention relates to submersible tidal generators that harness the energy of tidal currents to generate electricity and more specifically to several innovations relating to the design and installation method of generator.

Background of the invention

For countries having large coastal areas, recovering energy from tidal flow provides a convenient invisible source of renewable energy, but installation of existing tidal generators is significantly more expensive than other more conventional forms or energy generation, including other forms of renewable groan energy.

Much of the cost of deployment and maintenance of tidal stream turbines concerns the lifting and retrieval of the units for servicing and repair. All sub-sea systems that are physically attached to foundations or floating devices tethered by chains, rods or cables, are transported using large vessels capable of lifting and deploying generators weighing in excess of twenty five tonnes. These vessels are expensive and the lifting relies upon favourable weather conditions.

Summary of the invention

According to a first aspect of the present invention, there is provided a submersible generator assembly for generating electricity from tidal flows, the assembly comprising an electrical generator, a housing containing the generator, and a turbine connected to drive the generator and rotatable, when in use, by tidal current -2 -flowing past the generator assembly, wherein the assembly further includes a ballast tank connected to the housing, which tank is capable of being filled with water in order to control the buoyancy of the generator assembly, the assembly having fixings for enabling the assembly to be secured directly to a rigid underwater mounting structure.

A pre-constructed turbine generator assembly can currently be supported on the surface of the water by inflatable balloons, enabling it to be towed into position by boat. At the relevant location, the balloons would be deflated, allowing the generator assembly to be submerged in a controlled manner and then anchored to the sea bed. This, however, requires additional specialist equipment, that must be effectively and safely secured to the turbine.

By contrast, in the first aspect of the invention, the ballast tank forms part of the tidal generator assembly and, in an embodiment of the invention, the equipment required to lower and raise the tidal generator is also incorporated in the assembly, thereby considerably simplifying its deployment and its recovery from the seabed or servicing.

In some embodiments, the assembly may comprise two generators driven by respective turbines and each housed in a respective housing, the two generators being disposed on opposite sides of the ballast tank. With such a geometry, it is simpler to ensure that the assembly remains upright as it is towed to the installation site.

If the turbines of the two generators rotate in opposite sense, the torque reaction on the structure supporting the generator assembly is minimised.

The invention further provides, in accordance with a second aspect, a method of installing a generator assembly which comprises the steps of ensuring that the ballast tank is sufficiently empty to enable the assembly to float, towing the floating generator assembly to a desired location above the seabed, admitting water into the ballast tank until the assembly achieves a negative buoyancy in order to submerge the assembly, and securing the generator assembly to an underwater mounting structure after it has been submerged.

According to a third aspect of the invention, there is provided a submersible generator assembly for generating electricity from tidal flows, the assembly comprising at least one electrical generator, an elongate housing containing the generator, and a turbine connected to drive the generator and rotatable, when in use, by tidal current flowing past the generator assembly, wherein the housing is terminated at at least one of its axial ends by a cone, and wherein at least one of the cones at an end of the housing has an axis of rotational symmetry misaligned with the axis of the generator housing, such that tidal water flowing over the housing and the cone, during use, generates a hydrodynamic force that opposes the forces on the turbine caused by the turbine rotor.

Tidal generators are similar in design to aircraft fuselages and include at one end a turbine that is rotated by the flow of water past the generator. The rotation of the turbine turns a generator that is connected, via appropriate circuitry, to the national grid or an electrical load.

As stated previously, the tidal generator assembly may be essentially buoyant and secured to the sea bed by chains, rods or cables. The tension within these cables is affected by the drag on the body of the tidal generator from the water flow past it. With this in mind the nacelle or housing is shaped to be hydro-dynamically efficient. -4 -

By anchoring the tidal generator to the sea bed, the drag forces acting on the turbine plane and the body increase the forces on the tethering hardware and the foundations within the sea bed.

By a nose and/or tail cone on the nacelle of the generator the forces on the foundation are reduced to counter the reaction forces of the turbine and also maintain the desired attitude of the generator assembly relative to the seabed.

In designing a marine turbine, it is important to take into consideration not only factors affecting efficiency and reliability but also the volume of the generated noise, which has been found to be detrimental to marine wildlife.

In accordance with a fourth aspect of the invention, there is provided a turbine for a submersible generator, rotatable about an axis and having two or more blades that extend generally radially and are disposed symmetrically around the axis, each blade having, at least at its radially outer end, a generally elliptical cross-section with the major axis of the ellipse lying in a plane normal to the axis of rotation of the turbine, wherein the radially outer end of each turbine blade is formed with two winglets, one extending axially forwards and the other extending axially rearwards, with a central plane of each winglet lying at an angle of least 5° relative to the plane normal to the axis of rotation of the turbine, and wherein the winglets are shaped to generate a tip vortex to counteract the tip vortex created by rotation of the turbine blade, thereby enabling the turbine to operate without tip vortices at the radially outer ends of the blades.

A substantial proportion of the noise generated by a conventional marine turbine is caused by a vortex that -5 -occurs at the tip of each blade. By providing winglets to counteract and minimise such vortices, it is possible reduce the noise considerably and avoid or at least reduce the risk to marine wildlife.

Because of the danger to surrounding wildlife, the number of sites available for locating known tidal generators has been very limited. By addressing the noise issue created by rotation of the turbines, it is possible to increase the number of sites at which tidal generators may be used without damaging the surrounding environment.

In some embodiments of the invention, each of the winglets, when view in a direction parallel to the axis of the turbine, is not symmetrical about a radial plane and when view radially, the two winglets are rotationally symmetrical about a radial axis.

In some embodiments of the invention, the central planes of the winglets are equally inclined to the plane normal to the axis of rotation of the turbine.

A tidal generator is required to operate both when the tide is coming in and when it is going out. While one may be able to rotate the generator assembly so as always to point into the direction of current flow, this requires frequent intervention. In is therefore preferred to avoid the need to rotate the generator assembly by designing the turbine blades to operate efficiently in both directions of water flow.

To this end, in some embodiments of the invention, the blades, in each cross-section in a tangential plane, may be substantially rotationally symmetrical about a central point. -6 -

Conventional wisdom dictates that a turbine blade should optimally have a aerofoil cross-section and such a blade geometry does not operate efficiently when the direction of water flow is reversed. A Wells turbine has blades that are symmetrical about a central plane with the aim of enabling the turbine to rotate in the same direction regardless of the direction of fluid flow. Such a construction is however different from that contemplated herein, where reversal of the direction of fluid flow causes a reversal of the direction of rotation of the turbine. The cross-section of the blades in tangential planes, in some embodiments of the invention, have rotational symmetry about a central point as opposed to the mirror symmetry about a central plane of a Wells turbine.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a submersible generator assembly comprising two turbine driven generators, Figure 2 is a front view of the assembly shown in Figure 1, Figure 3 is a plan view from above of the assembly shown in Figures 1 and 2, Figure 4 is a perspective view of one of the blades of the turbines shown in Figures 1 to 3, Figure 5 is an exploded view of the blade shown in Figure 4, and Figure 6 is a view from one end of the turbine blade shown in Figures 4 and 5. -7 -

Detailed description of the drawings

The assembly 10 shown in Figures 1 to 3 comprises two central cylindrical bodies 12 and 14 arranged on above the other. The upper of the two cylindrical bodies 12 has two laterally extending wings 16 and 18 each connected at its distal end to the housing, or nacelle, or a respective one of two generators 20 and 22, driven by respective rotors 24 and 26 each having a plurality of radially extending turbine blades 32. The lower of the two cylindrical bodies 14 serves as a ballast tank and contains a plurality of bladders that may be filled selectively either with water or with a gas to adjust the buoyancy of the assembly 10.

The upper cylindrical body 12 contains the pumps and control systems needed to inflate the bladders in the ballast tank or to fill them with water. The body 12 also contains an electrical control system for converting the voltages produced by the two generators 16 and 18 into a voltage of the correct magnitude, frequency and phase to be connected to the national grid or to electrical loads deriving their power directly from the submersible assembly.

The upper cylindrical body 12 and the nacelles of the generators 16 and 18 are terminated with nose and/or tail cones 30 that are arranged with their axes pointing downwards towards the seabed. In other words, the nose and/or tail cones are not symmetrical but point towards the sea bed. The purpose of these cones is to generate a hydrodynamic force that opposes the forces on the turbine caused by the turbine rotors.

The turbines are designed with blades that will rotate with equal efficiency, albeit in the opposite direction, when the direction of water flow is reversed and the radially outer end of each turbine blade 32, as shown in -8 -Figures 4 to 6, is provided with winglets 34 and 36 that are designed to minimise tip vortices so as to reduce the noise produced by rotation of the turbines so as to reduce the adverse environmental impact of the generator assembly.

The configuration of the winglets 34 and 36 is such that the tip of each turbine blade is rotationally symmetrical about the radial axis of the blade 32, but as best seen from Figure 6, the tip is not mirror symmetrical. Instead the upper edges of the two winglets are offset to opposite sides of the central plane of the rotor that bisects the plane of bifurcation of the two winglets.

Each turbine blade 32, as shown in the exploded view of Figure 5, is formed of an outer moulding 32a supported by an inner frame 32b. The inner frame 32b is made up of three longitudinal ribs 40 that interlock with and are held in place relative to one another by four eye-shaped differently sized spacers 42. This construction reduces the mass of the blades and offers further advantages discussed in more detail below.

A bi-directional airfoil formed along the blades is described as a transposed airfoil shape with the high and low pressure sides using similar shapes so that the effect on the pressure gradient across the rotor is similar when both the direction of flow and the direction of rotation are reversed. This design, unlike existing bidirectional tidal turbines does not use a simple rotation of 180° about a point central to the chord, but includes an offset between the upper and lower surface angle of attack allowing for the differences between up stream and downstream flows.

The blade design may also be described as consisting of a upper aerofoil section and a lower airfoil section.

The lower section is formed by a transformation of 180+X degrees about the point of centre of chord. i.e.: -9 -cos 0 sin 0 -sin 0 cos 9 where 9 is 180+X This allows the rotor to be driven, or the rotor to drive, fluid with equal efficiency in both axial directions as the rotor reverses its direction.

This is to be contrasted with a Wells turbine which is symmetrical and drives fluid in the same direction regardless of the direction of rotation.

The twisted winglets 34 and 36 are wing tips which extend axially forwards and backwards, are offset radially and incorporate a twist. Traditional winglets extending from the wing tips and angled in either the high pressure or low pressure direction are designed to limit the radial flow that bleeds off the blade tips. Their modus operandi is to limit the radial flow by creating a high pressure zone at the tip on the upper or lower surfaces. This method reduces but does not eliminate the tip vortices by reducing the flow in the radial direction as the creation of the high pressure zone causes drag and inefficiencies at the blade tip which outweigh the benefits of radial flow inhibiting as the radial flow approaches zero. The twisted winglets are designed to create a counter rotating tip vortex which along with creating a high pressure zone at the blade eliminates the tip vortex in its entirety.

The twisted wingtips can be described as winglets on upper and lower surfaces angled between 5° and 175° relative to the central plane of the blade tip and shaped to create a counter-rotating vortex at the tip to effectively eliminate the tip vortex caused by the pressurised working of the blade. One embodiment may have one tip leaning rearwards and the other forwards. An -10 -alternative embodiment one of the winglets may toe while the other toes out. As a further possibility, an embodiment may use a combination of leaning and toe in or toe out to achieve the desired result.

By reducing noise, the illustrated tidal flow generator can eliminate the destructive influence of cavitation in marine turbines vastly increasing turbine life, reducing maintenance and therefore substantially reducing the cost of marine turbine generation. In particular, the design reduces the low frequency operational acoustics due to tip vortices which, in arrays, is harmful to marine mammals therefore allowing widespread deployment in areas previously deemed too sensitive.

Elimination of tip vortices also minimises disruption of the sea bed.

A still further advantage of the turbine design is that it minimises the shear forces and pressure gradients allowing significant reduction in fish mortality due to pressure shock.

The described rotor is able to operate more efficiently in areas of turbulent flow and in flows that have rapid accelerations and decelerations and area where the flow angles other than parallel with the axis of rotation.

Much of the cost of deployment and maintenance of tidal stream turbines concerns the lifting and retrieval of the units. All sub-sea systems that are physically attached to foundations (rather than floating devices tethered by chain, rod or cable) are transported using large vessels with the capacity to lift and deploy turbines in excess of 25 tonnes. These vessels are expensive and the lifting relies upon favourable weather conditions.

The built-in buoyancy system of the illustrate generator allows it to be towed to its operating site as a barge and then sunk and lifted using small and medium sized vessels. This reduces the cost of maintenance and eliminates the risks associated with lifting at sea.

The generator is designed with an integrated lower cylindrical body 14 which contains a plurality of bladders which can be filled with air or flooded from the surface.

When flooded the generator can be lowered using a small crane for directional stability onto its foundation. As the bladders are filled with air the turbine will become neutrally buoyant to allow the turbine to be raised using a small crane. At full buoyancy the turbine floats and can be serviced at the service or towed to dock for replacement.

In an embodiment of the invention, the main cylindrical body 12 houses a grid synchronisation system that operates remotely and does not need any sensors for speed, phase order or phase angle on the mechanical rotor or generator itself.

Induction generators are the cheapest, most reliable and longest lasting forms of large scale electrical generators available. Due to their need to be excited by the grid before they can generate they must be in phase and at the correct frequency before they can be connected (synchronised) with the grid. Though there are a number of autonomous grid synchronising relays available, they all rely on monitoring the physical rotational speed and direction of the generator.

In order to remove all electronics from the sub sea turbine the preferred grid synchronisation system injects a single pulse of low power synchronisation current to the stator via the three phase connection from the shore. It then monitors the output to identify speed and rotational -12 -direction which it can detect by the waveform returned. This information is sent to a phase order relay to adjust the connection whilst the rotor is spinning unloaded (not generating) then, once the waveform is returned at the correct frequency for synchronisation (50Hz) the synchronisation relay connects the generator to the grid.

The turbine blade is constructed using a steel frame within a Composite (Glass reinforced plastic or Carbon Io fibre or both) moulding. The steel frame is shaped to provide the structural integrity of the blade both radially and axially. This steel structure provides the full mechanical reaction to the hydrodynamic forces applied to the turbine blade and transfers the full force to the blade root. The voids between the steel structure and the outer form are filled with high density foam to transfer the forces effectively and inhibit moisture within the blade.

The outer composite moulding serves to create the hydrodynamic shape and to spread the loadings across the steel structure itself. Each blade operates on a similar principle to an air frame in an aircraft which typically use an aluminium or wooden load bearing structure of stringers and formers within a skin made of fabric, wood, aluminium or composite material. However this has not been applied to tidal turbines which currently either use composite structures thorough the whole blade, sometimes ending in a steel root, or fully metal blades.

In embodiment of the invention, the shape of the steel structure is optimised for loadings and the moulding is shaped for the hydrodynamics.

Existing turbine blades that use composite structures within the blades often contain a composite shear web. This provides axial strength by stopping the skins collapsing.

By contrast, though the steel frame of the illustrated -13 -turbine blade 32 provided this shear web property, it is not a shear web but the load bearing structure of the whole blade. The problem with the traditional approach is that composites are difficult to model and therefore it is difficult to understand the ultimate strength, fatigue and long term failure rates.

Claims

-14 -Claims 1. A submersible generator assembly for generating electricity from tidal flows, the assembly comprising an electrical generator, a housing containing the generator, and a turbine connected to drive the generator and rotatable, when in use, by tidal current flowing past the generator assembly, wherein the assembly further includes a ballast tank connected to the housing, which tank is capable of being filled with water in order to control the buoyancy of the generator assembly.
2. A submersible generator assembly as claimed in Claim 1, wherein the assembly comprises two generators driven by respective turbines and each housed in a respective housing, the two generators being disposed symmetrically on opposite sides of the ballast tank.
3. A submersible generator assembly as claimed in Claim 2, wherein the turbines of the two generators rotate in opposite sense.
4. A method of installing a generator assembly as claimed in any preceding claim, which comprises ensuring that the ballast tank is sufficiently empty to enable the assembly to float, towing the floating generator assembly to a desired location above the seabed, admitting water into the ballast tank until the assembly achieves a negative buoyancy in order to submerge the assembly, and se curing the generator assembly to the seabed after it has been submerged.

-15 -
5. A submersible generator assembly for generating electricity from tidal flows, the assembly comprising at least one electrical generator, an elongate housing containing the generator, and a turbine connected to drive the generator and rotatable, when in use, by tidal current flowing past the generator assembly, wherein the housing is terminated at at least one of its axial ends by a cone, and wherein at least one of the cones at an end of the housing has an axis of rotational symmetry misaligned with the axis of the generator housing, such that tidal water flowing over the housing and the cone, during use, generates a hydrodynamic force urging the generator assembly downwards towards the seabed.
6. A turbine for a submersible generator, rotatable about an axis and having two or more blades that extend generally radially and are disposed symmetrically around the axis, each blade having, at least at its radially outer end, a generally elliptical cross-section with the major axis of the ellipse lying in a plane normal to the axis of rotation of the turbine, wherein the radially outer end of each turbine blade is formed with two winglets, one extending axially forwards and the other extending axially rearwards, with a central plane of each winglet lying at an angle of least 5° relative to the plane normal to the axis of rotation of the turbine, and wherein the winglets are shaped to generate a tip vortex to counteract the tip vortex created by rotation of the turbine blade, thereby enabling the turbine to operate without tip vortices at the radially outer ends of the blades.
7. A turbine for a submersible generator as claimed in claim 6, wherein the central planes of the winglets are equally inclined to the plane normal to the axis of rotation of the turbine.

-16 -
8. A turbine as claimed in claim 7, the central planes of the winglets are inclined at angle of 5° to the plane normal to the axis of rotation of the turbine.
9. A turbine as claimed in any one of claims 6 to 8, wherein the blades, in each cross-section in a tangential plane, are substantially rotationally symmetrical about a central point such that the turbine operates in opposite directions with substantially equally efficiency upon reversal of the direction of flow of the tidal water.
10. A turbine as claimed in claims 6 to 8, wherein each the winglets, when view in a direction parallel to the axis of the turbine, is not symmetrical about a radial plane and when view radially, the two winglets are rotationally symmetrical about a radial axis.