WIND TURBINE BLADE WITH ELECTRO-THERMAL HEATING ELEMENT

Abstract

A wind turbine blade comprising an electro-thermal heating element with a tapering width. The electro-thermal heating element comprises: electrically resistive sheet material; a first electrode which is in electrical contact with the sheet material and positioned at a first end of the element; and a second electrode which is in electrical contact with the sheet material and positioned at a second end of the sheet material. An electrically conductive strip extends across a width of the element. The sheet material has a first part on a first side of the strip and a second part on a second side of the strip. The strip is in electrical contact with the first and second parts of the sheet material. The first part of the sheet material has a first width, and the second part of the sheet material has a second width which is different to the first width.

Claims

1. A wind turbine blade comprising an electro-thermal heating element, the electro-thermal heating element comprising: electrically resistive sheet material; a first electrode which is in electrical contact with the sheet material and positioned at a first end of the resistive sheet material; a second electrode which is in electrical contact with the sheet material and positioned at a second end of the sheet material; and an electrically conductive strip which extends across a width of the element, wherein the sheet material has a first part on a first side of the strip and a second part on a second side of the strip, the strip is in electrical contact with the first and second parts of the sheet material, the first part of the sheet material has a first width, and the second part of the sheet material has a second width which is different to the first width.

2. A wind turbine blade according to claim 1 wherein the first part of the sheet material has a substantially constant width, and the second part of the sheet material has a substantially constant width.

3. A wind turbine blade according to claim 1 wherein the sheet material has a width with a step change at the conductive strip.

4. A wind turbine blade according to claim 1 wherein the first part of the sheet material and the second part of the sheet material are formed from a single mat of the sheet material with a pair of surfaces, and the strip is carried by one of the surfaces of the mat.

5. A wind turbine blade according to claim 1 wherein the blade has a root and a tip, and the second part of the sheet material is closer to the tip than the first part of the sheet material.

6. A wind turbine blade according to claim 1 wherein the heating element is positioned in a part of the blade where the blade tapers inwardly towards the tip.

7. A wind turbine blade according to claim 1 further comprising a system for driving the electro-thermal heating element by causing electrical currents to flow through the electrically resistive sheet material via the first and second electrodes.

8. A wind turbine blade according to claim 1 wherein the first part of the sheet material is configured to generate a first heat flux, and the second part of the sheet material is configured to generate a second heat flux which is greater than the first heat flux.

9. A wind turbine blade according to claim 1 wherein the first and second parts of the sheet material have substantially the same sheet resistance.

10. A wind turbine blade according to claim 1 wherein the sheet material comprises randomly oriented fibres.

11. A wind turbine blade according to claim 1 wherein the strip is in physical contact with at least the first part of the sheet material.

12. A wind turbine blade according to claim 1 wherein the strip extends across a full width of the first part of the sheet material.

13. A wind turbine blade according to claim 1 wherein the electro-thermal heating element further comprises a second electrically conductive strip which extends across the width of the element, the second part of the sheet material is on a first side of the second strip, the sheet material has a third part on a second side of the second strip, the second strip is in electrical contact with the second and third parts of the sheet material, and the third part of the sheet material has a third width which is different to the second width.

14. A wind turbine blade according to claim 13 wherein the third part of the sheet material has a substantially constant width.

15. A wind turbine blade according to claim 1 wherein each part of the sheet material is configured to generate a heat flux which is substantially uniform across its area.

16. A wind turbine, comprising: a tower; a nacelle disposed on the tower; a generator disposed in the nacelle; a rotor coupled to the generator and having a hub at a distal end; a plurality of blades disposed on the hub of the rotor; wherein at least one blade of the plurality of blades comprises an electro-thermal heating element, the electro-thermal heating element comprising: electrically resistive sheet material; a first electrode which is in electrical contact with the sheet material and positioned at a first end of the element; a second electrode which is in electrical contact with the sheet material and positioned at a second end of the sheet material; and an electrically conductive strip which extends across a width of the element; wherein: the sheet material has a first part on a first side of the strip and a second part on a second side of the strip, the strip is in electrical contact with the first and second parts of the sheet material, the first part of the sheet material has a first width, and the second part of the sheet material has a second width which is different to the first width.

17. A wind turbine according to claim 16 wherein the first part of the sheet material has a substantially constant width, and the second part of the sheet material has a substantially constant width.

18. A wind turbine according to claim 16 wherein the sheet material has a width with a step change at the conductive strip.

19. A wind turbine according to claim 16 wherein the first part of the sheet material and the second part of the sheet material are formed from a single mat of the sheet material with a pair of surfaces, and the strip is carried by one of the surfaces of the mat.

20. An electro-thermal heating element, comprising: an electrically resistive sheet material; a first electrode in electrical contact with the sheet material and positioned at a first end of the resistive sheet material; a second electrode which is in electrical contact with the sheet material and positioned at a second end of the sheet material; and an electrically conductive strip which extends across a width of the element, wherein the sheet material has a first part on a first side of the strip and a second part on a second side of the strip, the strip is in electrical contact with the first and second parts of the sheet material, the first part of the sheet material has a first width, and the second part of the sheet material has a second width which is different to the first width.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0032] FIG. 1 shows a wind turbine;

[0033] FIG. 2 shows a wind turbine blade with an electro-thermal heating element;

[0034] FIG. 3 shows a mat of electrically resistive material;

[0035] FIG. 4 shows an electro-thermal heating element incorporating the mat of FIG. 3;

[0036] FIG. 5 shows the element from one edge;

[0037] FIG. 6 shows a first drive system for the electro-thermal heating element;

[0038] FIG. 7 shows a second drive system for the electro-thermal heating element;

[0039] FIG. 8 shows how currents would flow across the electro-thermal heating element of FIG. 6 without the copper strips;

[0040] FIG. 9 shows three panels of electrically resistive sheet material;

[0041] FIG. 10 shows an electro-thermal heating element incorporating the panels of FIG. 9;

[0042] FIG. 11 shows the element of FIG. 10 from one edge;

[0043] FIG. 12 shows an electro-thermal heating element with heating mat panels connected by H-shaped electrical connectors;

[0044] FIG. 13 shows four panels of electrically resistive sheet material which are used to construct the element of FIG. 12;

[0045] FIG. 14 shows a tapered electro-thermal heating element;

[0046] FIG. 15 shows lines of current in the tapered mat of the element of FIG. 14 in the absence of copper strips;

[0047] FIG. 16 shows a stepped electro-thermal heating element with stepped copper strips;

[0048] FIG. 17 shows four panels of electrically resistive sheet material which are used to construct the element of FIG. 16;

[0049] FIG. 18 shows an electro-thermal heating element with overlapping panels;

[0050] FIG. 19 shows an electro-thermal heating element with parts of different lengths;

[0051] FIG. 20 shows a mat of electrically resistive material with a width which steps down and then up; and

[0052] FIG. 21 shows an electro-thermal heating element incorporating the mat of FIG. 20.

DETAILED DESCRIPTION OF EMBODIMENT(S)

[0053] FIG. 1 shows a wind turbine 1. The wind turbine 1 has a tower 2 and a nacelle 3 at the top of the tower 2. A wind turbine rotor 4 is connected to the nacelle 3 and arranged to rotate relative to the nacelle 3. The wind turbine rotor 4 comprises a wind turbine hub 5, and multiple wind turbines blades 6 extending from the hub 5. While a wind turbine rotor 4 having three blades 4 is shown, a different number of blades, such as two or four, may be used.

[0054] Each blade 6 has a row of electro-thermal heating elements embedded along its leading edge. The heating elements may be used for either or both of anti-icing (preventing ice accumulating) or de-icing (removing accumulated ice) of the blade 6. FIG. 2 shows a single one of such heating elements 10, the other heating elements being omitted from FIG. 2.

[0055] The heating element 10 comprises an electro-thermal heating mat 9 made of an electrically resistive sheet material such as randomly oriented fibres (for example a carbon fibre veil or a carbon/glass fibre veil) or a metallic mesh, The mat 9 is shown in FIG. 3 without any of the other components of the heating element 10.

[0056] The mat 9 is manufactured in a rectangular shape, then cut on one side to form a stepped shape which reduces in width from one end to the other. The mat 9 has four parts 11,12,13,14 with reducing widths w1, w2, w3, w4 respectively. The mat 9 has a leading edge 15 which runs along the leading edge of the blade as shown in FIG. 2, and a stepped trailing edge 16 which is cut with a step 11a, 12a, 13a between each adjacent part.

[0057] The leading edge 15 of the mat is shown as a straight line in the drawings for ease of illustration, but typically it is cut with a gentle curve to conform to the curved shape of the blade. Similarly, the trailing edge 16 of the mat is shown as a straight line between the steps 11a-13a for ease of illustration, but typically it is cut with a gentle curve which follows the curve of the leading edge 15. So, in this case the parts 11-14 are not strictly rectangular, although their widths w1-w4 between the curved edges of the mat are substantially constant.

[0058] Four parts 11-14 are shown in FIG. 3, but a smaller or larger number may be used in practice. For instance, there may be only two or three parts, or a much larger number of parts.

[0059] All of the parts may have different widths as shown in FIG. 3, or only some of them may have different widths.

[0060] As shown in FIG. 5, the four parts 11-14 of the mat 9 are formed from a single sheet of the electrically resistive sheet material with an upper surface 9a and a lower surface 9b.

[0061] As shown in FIG. 4, a first copper electrode 20 is attached to the upper surface 9a of the mat in electrical contact with the sheet material and positioned at a first end of the element 10. A second copper electrode 21 is attached to the upper surface 9a of the mat in electrical contact with the sheet material and positioned at a second end of the element 10.

[0062] In this example the electrodes 20, 21 are attached to the upper surface 9a of the mat, but in other examples the electrodes may be attached to the lower surface 9b or both surfaces 91, 9b.

[0063] The electrodes 20, 21 in this example are made of copper, but other electrically conductive materials (typically metals) may be used.

[0064] Copper strips 22, 23, 24 extend across the width of the element at intermediate points along the length of the element. Each strip is carried by, and in electrical contact with, the upper surface 9a of the mat. Each strip is positioned at the junction between an adjacent pair of parts of the mat, at the step change in width. Thus, each strip has a first part of the mat on one side and a second part of the mat on the other.

[0065] For example, the mat has a first part 11 on a first side of the strip 22 and a second part 12 on a second side of the strip 22. The strip 22 is in electrical contact with the first and second parts 11, 12 of the sheet material via the upper surface 9a of the mat. The first part 11 of the sheet material has a first width w1, and the second part 12 of the sheet material has a second width w2 which is less than the first width w1. This repeats along the length of the element 10, with the width of the sheet material stepping down at each successive copper strip.

[0066] The strips 22-24 in this example are made of copper, but other electrically conductive materials (typically metals) may be used,

[0067] As can be seen in FIG. 5, the electrodes 20, 21 and copper strips 22-24 have substantially rectangular cross-sections, although other cross-sectional shapes are possible.

[0068] The sheet material of the mat 9 has a sheet resistance chosen such that when a voltage is applied between the electrodes 20, 21, the mat 9 produces heat at a desired heat flux due to resistive or ohmic heating.

[0069] The electrodes and copper strips are spaced apart by lengths L1, L2, L3 and L4. These lengths may be the same, or different.

[0070] The heat flux H for each part of the mat is related to the width w by the equation:

[00001]$H\propto {\left(\frac{1}{w}\right)}^2$

[0071] So if, for example, the widths of the parts 11-14 change in the series 100%, 90%, 80%, 70%, then the heat fluxes change in the series 100%, 123%, 156%, 204%. So, in this case the heat flux at the narrow end of the element is over twice the heat flux at the wide end.

[0072] In another example, a four part stepped element of length 1.6 m and width reducing from 0.5 m to 0.35 m with a total resistance of 115 Ohms will deliver a heat flux varying from 1.45 KW/m.sup.2 to 3 KW/m.sup.2 if driven with a 400V source.

[0073] Another example is given in Table 1 below, which shows various parameters for a four part stepped element of length 2 m and width reducing from 0.5 m to 0.35 m with a total resistance of 80 Ohms, driven by a 577V source.

TABLE-US-00001 TABLE 1 Part Number 1 2 3 4 Total Length mm 500 500 500 500 2000 Width mm 500 450 400 350 Area m{circumflex over ( )}2 0.25 0.23 0.20 0.18 0.85 RS ohms/sq 20 20 20 20 Resistance ohms 20.0 22.2 25.0 28.6 95.8 Current amp 6.0 6.0 6.0 6.0 24.1 Voltage volts 120 134 151 172 577 Power W 726 806 907 1037 3475 Heat Flux kW/m{circumflex over ( )}2 2.90 3.58 4.54 5.92

[0074] As shown in FIG. 2, the blade 6 has a root 6a and a tip 6b. The heating element 10 is positioned in an outboard part of the blade where the blade tapers inwardly towards the tip 6b. This outboard part of the blade tends to accrete more ice than the inboard part, and the aerodynamic power of the blade generally increases towards the tip 6b, So, positioning the narrow end of the heating element closer to the tip 6b than the wide end (as shown in FIG. 2) gives the desired result that the blade is heated more intensely towards the tip. The electrodes 20, 21 and copper strips 22-24 are parallel with each other and with a chord of the blade.

[0075] The inwardly tapering profile of the heating element 10 also gives the benefit of enabling it to be matched to the inwardly tapering profile of the blade 6 (desirably the width of the heating element is about 20% or 25% of the chord of the blade).

[0076] FIG. 6 shows a first system for driving the electro-thermal heating element 10 by causing electrical currents to flow through the electrically resistive sheet material via the electrodes 20, 21. Each electrode 20, 21 has a respective connector 20a, 21b which is connected to a power source 30. The power source 30 applies an AC or DC voltage V between the electrodes 20, 21. If the resistance between the electrodes is R, then the total power output by the heating element is V.sup.2/R or I.sup.2R, where I is the current. The dashed arrows in FIG. 6 show the direction of current, which runs in straight lines and in one direction only. Because each part of the mat 9 has a substantially constant width and has an electrode or conductive strip at each end (with substantially zero resistance) the current density (and associated heat flux) within each part of the mat is uniform across the entire length and width of the part.

[0077] FIG. 7 shows a second drive system. In this case the power source 30 drives the electrodes 20, 21 at the same voltage and the central copper strip 23 is used as a neutral electrode so the current flows towards or away from the neutral electrode.

[0078] FIG. 8 shows the currents which would flow through the stepped mat of FIG. 3 without any of the copper strips 22-24. At each step change 11a-13a in width, the lines of current bunch together, creating an undesirable hot spot. There is also an intense hot spot at the trailing edge of the heating element next to the electrode 21 Such hot spots are reduced or removed entirely by the copper strips 22-24 which create lines of equipotential across the width of the mat.

[0079] The copper strips 22-24 can also be used as “dummy” busbars which enable repair to be achieved in a more effective manner, as described in WO2019/001657, the contents of which are incorporated herein by reference.

[0080] In the embodiment of FIGS. 3-5 the element comprises a single mat 9 of resistive sheet material which is cut to shape, FIGS. 9-11 shows an alternative element 100 in which the four parts are made from separate rectangular panels 111-114 of electrically resistive sheet material which are joined together by copper strips 122-124 and have electrodes 120, 121 at either end. The panels 111-14 are separated by gaps which are filled by the copper strips 122-124.

[0081] Each panel 111-114 has a substantially constant width w1-w4 to achieve uniform heat flux within the panel. The width of the sheet material has a step change 112a, 112a, 113a at each copper strip.

[0082] In the case of FIGS. 10 and 11, each strip 122-124 is rectangular in plan, and in physical contact with the edges of the panels on either side of the strip. FIG. 12 shows an alternative element 200 in which panels 211-214 (shown individually in FIG. 13) are electrically coupled by H-shaped connectors. Each connector comprises a pair of copper strips 220, 221 which extend across the width of the element, and a cross-bar 222 which may be a copper strip or wire for example. Each strip 220, 221 is carried by, and in electrical contact with, the upper surface of a respective one of the panels 211-214. The panels 211-14 are separated by gaps which are bridged by the conductive cross-bars 222.

[0083] In the embodiments above, each part of the heating mat has a substantially constant width w1-w4 to achieve uniform heat flux within the panel. However, this is not essential and in an alternative embodiment of the invention shown in FIG. 14, an element 300 is provided with a heating mat 309 which is cut so that each part 311-314 has a continuously tapering width with copper strips 322-324 arranged as shown.

[0084] FIG. 15 shows the currents which would flow through the tapered mat 309 of FIG. 14 without any of the copper strips 322-324. The lines of current in the lower part of the mat 309 are parallel, but in the tapering upper part they converge to generate an intense hot spot 350. The element of FIG. 14 has hot spots 330 at the corner of each part 311-314, but these are less intense than the hot spot 250 in the case of FIG. 15. The heat flux in the hot spot 250 can exceed design limits and result in damage to the area.

[0085] In the embodiment of FIG. 4 the copper strip 22 is carried by the first part 11 of the sheet material so that it is in physical contact with the first part 11, but not in physical contact with the second part 12 of the sheet material (although it is in electrical contact with the second part 12). In the alternative embodiment of FIG. 16 each copper strip has a stepped shape and spans across the junction so that it is in physical contact with both parts of the mat on either side of the strip. Each strip has a wide strip portion 22a, 23a, 24a carried by the wider part of the mat, and a narrow strip portion 22b, 23b, 24b carried by the narrower part of the mat.

[0086] In the embodiment of FIGS. 10 and 12, the separate panels do not overlap. In an alternative embodiment shown in FIGS. 17 and 18, heating mat panels 511-514 are cut to shape as shown in FIG. 17 then overlapped as shown in FIG. 18. Copper strips 522-524 are attached to the upper faces of the widest panels 511-513 to form the completed element 500.

[0087] FIG. 19 shows a heating element 600 with first and second parts 611, 612 which have different lengths as well as different widths.

[0088] FIG. 20 shows a heating mat 709 which is cut to form parts 711, 712, 713 with different widths. The width reduces at the first step between the parts 711, 712, then increases at the second step between the parts 712, 713. Copper strips 722, 723 and electrodes 720, 721 are then added as shown in FIG. 21 to form the heating element 700.

[0089] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.