This invention relates to a turbine blade and method of manufacture, particularly for a submersible turbine blade assembly for power generation.

In water current turbine systems for power generation comprising a powertrain, turbine hub and turbine blades, the pressure field around the turbine blade changes during operation of the turbine. Typically, one side of the blade experiences a pressure rise while the other side experiences a pressure drop. If the pressure drops sufficiently, a vapour pocket (bubble) is formed. Formation of a vapour pocket arises when the local vapour pressure drops below the saturated vapour pressure. The surface of the blade typically acts as a nucleating agent. This bubble will move with the flow along the surface of the blade until it collapses, which occurs once the static pressure has increased to a high enough value. This typically occurs near the trailing edge of the blade. This collapse causes a local shock wave that can lead to a peak cyclic stress that can erode away the surface of the blade, causing damage and loss of performance over time.

Cavitation is a common phenomenon around control valves, pumps, propellers and impellers. The problem is often tackled by designing these devices such that cavitation is avoided. KR20120121209 describes a ship's propeller having a layer of aluminium foil, with a ceramic coating to allow a diamond coating layer to be adhered to protect the propeller surface. US 4847122 describes a polymer composition containing a rheo logical additive to protect a surface against cavitation.

WO2005075838 describes protecting a water-solid interface from cavitation by introducing bubbles on and/or along the surface of the solid exposed to the liquid. Selectively placed protrusions and other deformations to the surface are introduced to maintain the bubbles. DE4443440 describes depositing a hard metal layer over an elastic layer to protect a component from erosion due to cavitation.

However, cavitation cannot always be avoided by design and current surface protection, such as ceramic or metal coatings are not suitable for tidal turbine blades because of the type of composite material that the tidal turbine blade is made of.

In accordance with a first aspect of the present invention a method of manufacturing a turbine blade comprises providing a polyurethane coating in areas of a blade which require protection to form a protective layer and a plurality of layers of composite material in, or on, a mould to form a composite turbine blade component, such that the protective layer is flush with the surface of the blade component; curing the coating and composite material; and removing the mould.

The present invention provides protection from cavitation for tidal turbine blades in the aft region of the hydrofoil affected by cavitation. Used herein, the expression trailing edge refers to this aft region, which may be up to 50% of the aft chord if the cavitation is spread out widely.

Preferably, the coating and layers are applied to a female mould.

Preferably, the method further comprises forming the turbine blade by combining two or more moulded cured components.

The blade may be manufactured by combining different parts of the blade, such as the leading edge and main body being combined with a trailing edge panel.

Alternatively, when combining two components to form the turbine blade, the components comprise opposite halves of the blade.

Preferably, the step of combining the components comprises adhesive bonding. Preferably, the coating is applied to the trailing edge of the blade.

Preferably, the coating is applied in a region extending from the tip of the trailing edge by between one fifth and two fifths of the chord of the blade.

In accordance with a second aspect of the present invention, a water current turbine comprises a hub and a plurality of turbine blades, each blade comprising layers of composite material with an integral protective polyurethane coating manufactured according to the method of the first aspect.

Preferably, the composite material comprises glass or carbon reinforced plastic.

Preferably, the protective coating extends over at least one fifth of the chord of the blade from its trailing edge.

Preferably, the polyurethane coating is applied to at least one surface of the trailing edge of the blade.

Preferably, the polyurethane coating is applied to the outboard trailing edge surface and the tip of the trailing edge.

An example of a turbine blade according to the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 is an overview of an example of a typical tidal turbine blade;

Figure 2 is illustrates an example of a tidal turbine blade according to the present invention, including a protective layer along the trailing edge and tip; Figure 3 is a cross section of through a first example of a blade according to the present invention;

Figure 4 is a cross section of through a second example of a blade according to the present invention; and,

Figure 5 is a flow diagram of the construction process for forming a blade according to the present invention.

As mentioned above, a particular problem with tidal turbine blades is the damage done by erosion of a surface, typically the trailing edge where the blade structure if often most vulnerable, due to cavitation caused by a pressure reduction at that surface in use. Tidal turbine blades are typically manufactured from an epoxy based composite material, usually a fibre reinforced plastic, such as glass or carbon reinforced plastic. Metals, or ceramics, which have been used to provide cavitation protection, are difficult to integrate into composite manufacturing methods. The composites used in tidal blades cannot tolerate the high temperatures often required for depositing metal or ceramic coatings. Using adhesive to connect a pre-manufactured metal or ceramic plate to the composite blade is known not to be a reliable interface, especially when exposed to water.

Fig. 1 gives an overview of a typical submersible tidal turbine blade 1. On the leading edge 3, as the water flows past, a pressure rise is experienced. Subsequently, pressure drops, the greatest pressure drop generally being experienced between 20% to 50%) chord on the suction (low pressure) side surface of the hydrofoil and here bubbles start to be formed. The pressure typically recovers between 70%> to 100% chord, and here is where cavitation is initiated. In the example of a turbine blade of the present invention as illustrated in Fig.2, part of the trailing edge 4 along the outer span of the blade, including the tip 5, is protected by a coating 6 that reduces the eroding effect of cavitation on that surface. The turbine blade may be manufactured by providing a polyurethane coating and a plurality of layers of composite material in a mould to form a composite turbine blade component, as discussed in more detail below. The coating and composite material layers are cured and removed from the mould and two or more moulded cured components are combined to form the turbine blade.

The tip is the most susceptible to cavitation as the speed is highest here and it is only a small area to cover, but coating the trailing edge up to the tip would still provide useful protection. The region of the trailing edge of the blade which is susceptible to cavitation can be determined by calculation and is typically the outboard trailing edge and the tip.

Fig.3 shows a cross-section through the blade 1 where a coating has been applied as part of the manufacturing process using a female mould, such that the coating is flush with the surface of the blade. The leading edge 3 encounters the water first as water flows past in the direction of the arrows 9 and the trailing edge 4 is subjected to cavitation in the process as described above. Vapour pockets 7 form on the low pressure side of the blade and these pockets collapse 8 near the trailing edge 4 of the low pressure side. In this example, the trailing edge is protected from cavitation by means of the protective layer 6 over typically the last 15% of the blade length.

Fig.4 shows a cross section through a blade with a coating which has been applied after the blade has been manufactured, either as an additional step before installation, or as part of a repair to a blade which has been used and suffered the effects of cavitation. Although otherwise similar to the example of Fig.3, the effect of applying the coating after manufacture of the blade trailing edge is that the coating is not flush with the blade. For a blade skin thickness of between 5mm and 10mm, the additional coating may add another 1mm to 5mm, so it is preferable to form the protective coating as part of the manufacturing process to get it flush with the blade surface.

The present invention solves the problem of being unable to protect epoxy composite components from cavitation due the poor adhesion between the epoxy composite and its protective layer by applying during manufacture, or post- manufacture, a polyurethane (PU) protective layer to those surfaces which require protection, specifically the trailing edge 4 and tip 5 of the blade 1. The PU layer is elastic, rubbery and impact resistant. In cases where it has been determined that cavitation occurs on other surfaces, then these may also be provided with a protective coating, but the trailing edge and the tip are where cavitation most commonly occurs.

Polyurethane comprises a blend of liquid isocyanate and polymeric polyols as a resin at a specified stoichiometric ratio. The application of polyurethane to the surface of the blade that needs protection is an integral part of the manufacturing process, although in some cases, a protective layer may be added to already formed blades. There are a number of ways of manufacturing water current turbine blades. One of these involves using two part moulds, either male or female, layering the epoxy composite into the two parts of the mould, curing the composite and removing the cured parts from the moulds before combining the halves to form a finished part.

Using this method of manufacture, the blended polyurethane is poured into the mould to form one of the layers of the composite blade and cured together with the epoxy composite. The application of the protective layer as part of the manufacturing process results in an effective bond and shows good adhesion properties with the epoxy based composite materials, as well as the protective layer being flush with the surface of the blade. The PU layer is chosen to have good impact properties when used in a marine environment. Fig.5 illustrates the process, for using either male or female moulds. The type of mould to be used is chosen 10. If a female mould is used, the blended polyurethane is poured or brushed 11 into the mould to form the protective layer before the necessary layers of composite material, such as glass or carbon reinforced plastic, are applied 12 to form the body of the blade 1. When a male mould is used the layers of composite material are laid down first 13 and then the coating layer is poured or brushed on 14. The combination of polyurethane coating and composite material are then cured 15, the cured parts are removed from the moulds 16 and then combined 17 to form the turbine blade.

In some cases, for example for maintenance purposes, it may be necessary to provide protection to already formed turbine blades, in which case the blended polyurethane is applied directly onto the surface where required and left to cure. The surface is sanded before application of the polyurethane coating to ensure good adhesion and then cured at an appropriate temperature.

The tidal turbine blade is protected against erosion due to cavitation by means the PU coating. The coating provides a protective barrier for the blade structure, which may be adapted to the curvature of the blade and formed in a constant or variable thickness. This significantly reduces the requirement for blade repairs by preventing cavitation damage in the first place and further saving cost by reducing the frequency of inspection of the blades.