The present invention relates to Electro-Thermal Heating (ETH) elements for wind turbine blades and, in particular, to preventing hotspots in ETH elements.

Background

Wind turbines generate electrical power from wind energy and can be situated on land or offshore. Wind turbines situated in cold climates can suffer from icing events where ice may be formed on the surface of the wind turbine blades due to freezing water on the cold surface. The accumulation of ice on the surface of a blade can result in undesirable consequences. For example, a change in the profile of the wind turbine blades due to the accumulation of ice may reduce the speed of rotation of the wind turbine. As a result, the wind turbine may operate below optimal speed and efficiency, which degrades the performance of the wind turbine. Also, the additional weight of the accumulating ice on the wind turbine blades may cause fatigue and stress failures of the blades.

Therefore, there is a need to be able to prevent or reduce the effects of icing on the blades of a wind turbine in order to prevent damage to the blades and also to increase the performance of a wind turbine.

Various systems and methods have been described to either, or both, to de-ice (e.g. remove ice accumulated) a wind turbine or to provide anti-icing (e.g. prevent ice from accumulating) for a wind turbine.

For example, it is known to attach heating mats to the wind turbine blades which, when supplied with electrical power, generate heat to increase the surface temperature of the surface of the blade. Such heating mats may be used for either or both of anti-icing or de-icing of the wind turbine blade.

The heating mats may cover a significant proportion of the blade and/or be located where anti-icing/de-icing is required. The heating mat may become damaged such that a hole occurs in the heating mat (e.g. during manufacture, installation and/or operation). The heating mat may also suffer from a short circuit in the electrical paths of the heating mat (e.g. during manufacture, installation and/or operation). In both circumstances, the damage or short circuit could cause hotspots and/or cold sections in the heating mat that can subsequently cause damage to the heating mat and/or the blade structure and/or degrade the performance of the anti-icing/de-icing.

The present invention seeks to address, at least in part, the problems and disadvantages described hereinabove and to seek to prevent or reduce hotspots and/or cold areas in the heating mat caused by damage or short circuits in the heating mat.

Summary of the Invention

According to a first aspect of the present invention there is provided an electro- thermal heating element comprising: at least two busbars, wherein a first busbar is positioned along a first edge of the electro-thermal heating element and a second busbar is positioned along a second edge of the electro-thermal heating mat, the second edge being opposite to the first edge; a first resistance along a first axis of the electro-thermal heating mat; wherein the first axis of the electro-thermal heating element is in a direction between the first busbar and the second busbar; a second resistance along a second axis of the electro-thermal heating mat; wherein the second axis is perpendicular to the first axis; and the second resistance is greater than the first resistance such that an electrical current flow between the first busbar and the second busbar flows substantially only along the first axis direction.

As such, the electro-thermal heating element comprises a different predetermined resistance along two axis of the electro-thermal heating element where the resistance along the second axis is greater than the resistance along the first axis. This advantageously substantially prevents electrical current from flowing in the direction of the second axis whilst allowing electrical current to flow in the direction of the first axis. The electrical current flowing in the direction of the first axis provides the necessary heating required for the electro-thermal heating element. Further, by having a greater resistance along the second axis the electro-thermal heating element substantially reduces the effects of any damage or short circuit within the electro-thermal heating element.

The electro-thermal heating element may further comprise a plurality of

unidirectional conductive fibres in the first axis direction of the electro-thermal heating mat, wherein the plurality of unidirectional conductive fibres provide the first resistance.

The plurality of unidirectional conductive fibres may comprise single fibres, weaved fibres and/or chopped fibres. The plurality of unidirectional conductive fibres may be one or more of carbon fibres, carbon coated fibres, or any other conductive material to form the unidirectional conductive fibre in the first axis direction.

The electro-thermal heating element may further comprise a high resistance material in the second axis direction, wherein the high resistance material provides the second resistance.

The high resistance material may be meshed with the plurality of unidirectional conductive fibres.

The plurality of unidirectional conductive fibres may be separated by the high resistance material.

The high resistance material may be a non-conductive material and/or an insulating material. The high resistance material may be one or more of glass, glass fibres, fibres coated in an insulating sheath, air, or any other high resistance material. The second resistance may be at least five hundred times greater than the first resistance. The second resistance may be at least one thousand times greater than the first resistance.

The electro-thermal heating element may substantially prevent electrons from flowing in the direction of the second axis thereby substantially reducing the effects of any damage and/or a short circuit in the electro-thermal heating mat.

According to a second aspect of the present invention there is provided a wind turbine blade comprising one or more electro-thermal heating mats according to any of the features of the electro-thermal heating element in relation to the first aspect.

According to a third aspect of the present invention there is provided a wind turbine comprising one or more wind turbine blades according to the second aspect.

According to a fourth aspect of the present invention there is provided a method of forming an electro-thermal heating element comprising: forming the electro- thermal heating element with a first resistance along a first axis of the electro- thermal heating element and a second resistance along a second axis of the electro-thermal heating mat; wherein the second axis is perpendicular to the first axis and wherein the second resistance is greater than the first resistance such that an electrical current flow between the first busbar and the second busbar flows substantially only along the first axis direction; and attaching at least two busbars to the electro-thermal heating mat, wherein a first busbar is positioned along a first edge of the electro-thermal heating element and a second busbar is positioned along a second edge of the electro-thermal heating mat, the second edge being opposite to the first edge.

The method of forming the electro-thermal heating element may further comprise forming a plurality of unidirectional conductive fibres in the first axis direction of the electro-thermal heating mat, wherein the plurality of unidirectional conductive fibres provide the first resistance.

The method of forming the electro-thermal heating element may further comprise providing a high resistance material in the second axis direction, wherein the high resistance material provides the second resistance.

The high resistance material may be meshed with the plurality of unidirectional conductive fibres. The plurality of unidirectional conductive fibres may be separated by the high resistance material.

The second resistance may be formed to be at least five hundred times greater than the first resistance. The second resistance may be formed to be at least one thousand times greater than the first resistance.

Drawings

Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 shows a schematic of a wind turbine according to one or more

embodiments of the present invention.

Figure 2 shows a schematic drawing of a heating mat, according to one or more embodiments of the present invention.

Figure 3 shows a schematic drawing of damage to a heating mat, according to one or more embodiments of the present invention.

Figure 4 shows a schematic of a short circuit in a heating element according to one or more embodiments of the present invention. Figure 5 shows a schematic drawing of a heating element that reduces the effect of damage to a heating mat, according to one or more embodiments of the present invention.

Figure 6 shows a schematic drawing of a heating element that reduces the effect of a short circuit in the heating element, according to one or more embodiments of the present invention.

Embodiments

Figure 1 shows a schematic of a typical wind turbine 10, which includes

embodiments of wind turbine blades 19 according to the present invention. The wind turbine 10 is mounted on a base 12, which may be onshore foundations or offshore platforms or foundations. The wind turbine includes a tower 14 having a number of tower sections. A nacelle 16 is located and attached to the top of tower 14. A wind turbine rotor, connected to the nacelle 16, includes a hub 18 and at least one wind turbine blade 19, where in Figure 1 three wind turbine blades are shown although any number of wind turbine blades 19 may be present depending on the design and implementation of the wind turbine 10. The wind turbine blades 19 are connected to the hub 18 that in turn is connected to the nacelle 16 through a low speed shaft that extends out of the front of the nacelle 16.

With reference to Figure 2, in order to substantially prevent, or reduce, ice accretion on wind turbine blades, the blades may be fitted with Electro-Thermal Fleating (ETH) elements 201 , e.g. a heating mat, which can generate heat in order to heat the surface of the wind turbine blade. The ETH elements 201 are supplied with electrical power from a power supply 202 connected to busbars 203 on the ETH element 201 such that a current travels along conductive paths 204 in the X- axis direction of the ETH element 201 , as shown in Figure 2. The X-axis direction in this example is the direction between two adjacent busbars 203 that are formed on opposite edges of the ETH element 201. The Y-axis direction in this example is perpendicular to the X-axis and is in the direction parallel to the busbars 203 formed on the ETH element 201. Based on the electrical power supplied along with the resistance of the ETH element 201 it generates the required level of heat, which is used to heat the surface of the blade to prevent or reduce ice accretion on the blade. The ETH elements 201 are typically fabricated from a lightweight resistive material that has specific dimensions and resistive properties that enable the ETH element 201 to produce the required levels of heat for the location and requirements of the heating system for the blades.

With reference to Figure 3, ETH elements 301 that are attached to, or embedded within, a wind turbine blade may suffer from damage, e.g. a hole 302. The damage 302 may occur during manufacture, during installation or during operation of the ETH element attached to, or embedded within, a wind turbine blade.

Once the ETH element 301 is damaged, for example, in the form of a hole 302 in the ETH element, the damage 302 prevent the electrons from flowing uniformly across the ETH element 301 causing the electrons to flow around the damaged section 302. This causes a greater concentration of the flow of electrons at specific locations around the damage 302, which in turn causes hotspots 303 as shown in Figure 3. The hotspots 303 caused by the damage 302 can cause further damage to the ETH element 301 and/or the blade surface which can substantially affect the effectiveness of the ETH element 301 as well as potentially cause a fire or damage to the blade structure and surface. Additionally, the damage may further cause cold areas to occur around the damage 302.

With reference to Figure 4, ETH elements 401 that are attached to, or embedded within, a wind turbine blade may suffer from a short circuit 402. The short circuit 402 may be caused by a structural defect in the ETH element 401 that occurs during manufacture, installation or operation (e.g. caused by impact damage, lightning damage, and so on). The effect of the short circuit 402 may be to cause a Lightning Protection System, which is earthed, to become connected to the ETH element 401. In this case, the electrons flowing across the ETH element 401 are drawn to the short circuit 402, which can cause a significant hotspot 403 in the area of the short circuit 402. The significant hotspot 403 caused by the short circuit 402 may cause significant damage to the ETH element 401 and to the blade structure to which the ETH element 401 is attached or embedded within, and may even cause a fire in the blade.

Additionally, as the current is drawn to the short circuit 402 rather than flow uniformly across the ETH element 401 then this further causes a significant cold area 404 from the location of the short circuit 402 up to the busbar at the edge of the ETH element 401 in the x-axis direction. The cold area 404 would therefore not enable the ETH element 401 to heat the blade in this section of the ETH element meaning that ice can accrue on the blade in this section, thereby degrading the performance of the blade.

In order to prevent or significantly reduce the effects of damage or a short circuit in the ETH element, the ETH element is advantageously formed with a first predetermined resistance along a first axis (e.g. in the X-axis direction), where the first predetermined resistance in the X-axis direction between the two adjacent busbars is set at the appropriate resistance value that will provide the appropriate level of heating based on the electrical power that is to be supplied to the ETH element. The appropriate level of heating is dependent on the temperature required and the location of the ETH element on/in the blade.

Along the second axis (e.g. in the Y-axis direction) of the ETH element, the ETH element is formed with a second predetermined resistance, where the second predetermined resistance is significantly higher than the first predetermined resistance in the X-axis. Ideally, the second predetermined resistance would be an infinite resistance in order to completely prevent electrons from flowing in the Y- axis direction. However, in reality it would be difficult to form an ETH element with an infinite resistance in the Y-axis direction. Therefore, the second predetermined resistance in the Y-axis direction may be at least 500 times greater than the first predetermined resistance in the X-axis and may be at least 1000 times greater than the first predetermined resistance in the X-axis direction.

By forming the ETH element with a significantly higher predetermined resistance in the Y-axis direction compared to the X-axis direction, the invention advantageously prevents, or substantially prevents, electrons (i.e. electrical current) from flowing in the direction of the y-axis thereby preventing or

substantially reducing the effects of the damage or short circuit in the ETH element, as will be described in more detail below.

The ETH element may be formed by, for example, unidirectional conductive fibre or strands in the X-axis direction meshed, or weakly meshed, with a high resistance material in the Y-axis direction. The terms meshed or weakly meshed also includes the unidirectional conductive fibre or strands in the X-axis direction being woven with the high resistance material in the Y-axis direction. The high resistance material may be one or more of glass fibres, fibres coated in an insulating sheath, or any other high resistance and insulating material in the Y-axis direction.

Additionally, or alternatively, the unidirectional conductive fibres or strands in the X-axis direction may be separated in the Y-axis direction by a high resistance material such as an insulating material, non-conductive material or air. The high resistance material used to separate the unidirectional may be, for example, a resin, glass, air, or any other suitable material

In both examples, the amount of material in the Y-axis direction may be the minimum amount of material required to hold or maintain the ETH element together and/or in a useable form to attach to or embed within a wind turbine blade. In other words, the plurality of unidirectional conductive fibres in the first axis direction of the electro-thermal heating element are held together (e.g.

stabalised) by the high resistance material in the second axis direction of the electro-thermal heating mat.

The unidirectional conductive fibre or strands in the X-axis direction may be formed of single conductive fibres, a weave of conductive fibres, or from chopped fibres and may be one or more of carbon fibres, carbon coated fibres, e.g. carbon coated glass fibres, or any other conductive material to form the unidirectional conductive fibre or strands in the X-axis direction. With reference to Figure 5, the advantageous ETH element 501 of the present invention reduces the effects of damage 502 to the ETH element 501. In this example, the damage 502 is a hole in the ETH element 501 , similar to that shown in Figure 3. However, in contrast to Figure 3, as the ETH element of Figure 5 is formed of a predetermined resistance in the X-axis direction that is required for the level of heating needed from the ETH element 501 and formed with a significantly high resistance in the Y-axis direction, then the ETH element 501 structure effectively prevents the electrons from flowing around the damaged section 502.

By preventing the electrons from flowing around the damage 502 hotspots around the damage 502 are effectively reduced which prevents any further damage occurring to the blade structure or the ETH element from the excessive heat generated by any hotspot. Essentially, the trade-off for preventing the potentially damaging hotspots is that a cold stripe 503 will occur along the length of the ETH element 501. The size of the cold stripe 503 will substantially match the diameter of the hole 502.

With reference to Figure 6, the advantageous ETH element 601 of the present invention reduces the effects of a short circuit 602 in the ETH element 601. In this example, the short circuit 602 occurs towards the centre of ETH element 601 , similar to that shown in Figure 4. However, in contrast to Figure 4, as the ETH element of Figure 6 is formed of a predetermined resistance in the X-axis direction that is required for the level of heating needed from the ETH element 601 and formed with a significantly high resistance in the Y-axis direction, then the ETH element 601 structure effectively prevents the electrons from flowing to the short circuit 602 from conductive paths that are not in-line with the short circuit 602.

By preventing the electrons from flowing towards the short circuit 602 from conductive paths that are not in-line with the short circuit 602 then the significant hotspot centered around the short circuit 602 is effectively reduced which prevents any further damage occurring to the blade structure or the ETH element from the excessive heat generated by any hotspot. A significantly reduced hot stripe may still occur between the busbar and the short circuit equivalent to the size of the short circuit, but the hot stripe should be significantly reduced so that it does not cause any further damage to the ETH element or the blade structure. Further, the ETH element 601 of the present invention also reduces the cold area as the current still flows along conductive paths of the ETH element 601 that are not in-line with the short circuit towards the second busbar enabling the ETH element 601 to provide more efficient heating to the blade structure. Accordingly, the ETH element of the embodiments advantageously reduces the effects of damage and/or short circuits in the ETH element enabling a more efficient and safe heating of the blade structure should any damage or short circuit occur in the ETH element. While embodiments of the invention have been shown and described, it will be understood that such embodiments are described by way of example only and it will be appreciated that features of different embodiments may be combined with one another. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the scope of the present invention as defined by the appended claims. Accordingly, it is intended that the following claims cover all such variations or equivalents as fall within the spirit and the scope of the invention.