OA18231A

OA18231A - Tower structure for vertical-axis wind turbine rotor.

Info

Publication number: OA18231A
Application number: OA1201700079
Authority: OA
Inventors: Raphaël MENARD
Original assignee: Wind-It
Priority date: 2014-09-05
Filing date: 2015-09-03
Publication date: 2018-09-04

Abstract

A tower structure comprising with a plurality of superimposed stage levels (NE) allows supporting one or more vertical-axis wind turbine rotors (32), with an enlarged area swept by each rotor, in order to increase the mechanical power recovered by the wind turbine. In addition, the tower structure is resistant to the mechanical stresses generated by the rotation of rotors stacked over a significant height.

Description

TOWER STRUCTURE FOR VERTICAL-AXIS WIND TURBINE ROTOR

The présent invention relates to a tower structure for supporting one or more verticalaxis wind turbine rotors.

Tower structures hâve already been proposed which hâve several levels of superimposed stages for supporting vertical-axis wind turbine rotors, with a rotor located in each stage level. For example, document FR 2,945,325 describes a hollow and perforated tower structure which is designed to contain vertical-axis rotors regardless of their rotational position. However, a tower structure as described in that document may affect the wind flow around the rotor blades, causing a réduction in the airflow efficiency of each rotor. Moreover, the hollow structure which is described does not necessarily maximize performance in the wind power production, both in terms of the geometrical configuration of the tower and the constitution of its structural éléments.

From this situation, the aim of the présent invention is to improve at least some of the following criteria in comparison to existing tower structures:

- the tower structure interfères the operation of the wind turbine as little as possible. In particular, the structure has very little effect on wind flow around the blades of the wind turbine rotors, to improve the airflow efficiency of each rotor which is supported by the tower structure;

- the tower structure has high rigidity and high résistance to the mechanical forces which can be generated by the rotation of the wind turbine rotors, including when the rotational speed varies;

- the tower structure enables each wind turbine rotor to sweep a larger area during its rotation, compared to existing structures, increasing the mechanical power which is recovered by the rotor;

- the tower structure is not subject to significant vibrations, such vibrations possibly generated by wind flow through the structure or by rotation of the rotors. This aim is achieved in particular through significant rigidity of the tower structure and through the lightness of its structural components;

- the tower structure itself, separately from the wind turbine rotors that it supports, causes reduced wind pressure exposure;

- the tower structure can easily be installed at its site of operation, stably on the ground or on floats in the case of offshore installation;

- the total mass ofthe tower structure is reduced, to limit the cost of raw materials and to facilitate transport of the component éléments of the structure. In particular, a modular geometry

-2is desired for the tower structure, where each of the structural components can easily be transported; and

- assembly of the tower structure can be carried out easily and quickly at the site of operation.

To achieve at least one of these or other aims, a first aspect of the invention provides a novel tower structure with a plurality of superimposed stage levels for supporting at least one vertical-axis wind turbine rotor, the structure having a vertical axis of symmetry and remaining identical to itself in rotations of ±120° (degrees) about this axis of symmetry. The structure comprises an assembly of linear and rigid segments, which includes at least:

- three uprights each extending between a base and a top ofthe structure;

- at least two sets of stage segments; and

- support segments.

In a tower structure according to the invention, the stage segments of one and same set are assembled at a stage level of the structure, to form a horizontal polygon at that stage level. Each stage segment forms one side of the polygon, and connections between the respective ends of two stage segments which are successive in the polygon form a vertex of the polygon. Three ofthe vertices of each polygon, called edge vertices, are respectively connected to the three uprights, and any two of the edge vertices of each polygon are separated by at least one other vertex of this polygon, called the intermediate vertex of the pair of two edge vertices. In addition, for each pair of edge vertices of any of the polygons, at least one intermediate vertex is further connected obliquely to at least one of the uprights to which one of the edge vertices of the pair is connected, by two support segments respectively towards the top and towards the bottom of the tower. For each polygon, each intermediate vertex is located outside the triangle formed by the edge vertices of that polygon.

The structure is further adapted for receiving a wind turbine rotor or a plurality of superimposed wind turbine rotors, each rotor being contained between two successive stage levels, and the axis of each rotor being coïncident with the axis of symmetry of the structure.

Such a tower structure is composed of a limited number of segments whose outer diameters can be structurally optimized, so that its wind pressure exposure is particularly low. For the same reason, the structure has reduced interférence with the wind flow around the blades of the wind turbine rotors. In certain wind directions, tests hâve shown that the structure could also hâve a favorable effect on performance in energy production. In other words, the tower structure has very little impact on the airflow efficiency of each blade. The mechanical power that is recovered by each rotor from the wind is thus increased.

-3Furthermore, such a tower structure is mainly composed of triangular shapes, so that there is little or no geometrical deformation. Each segment is subjected mainly to tensile or compressive stresses, with almost no shearing stresses, while bending instabilities are reduced. The structure also has high stiffness and high résistance to the mechanical forces which can be generated by the rotation and/or accélération of the rotors. These triangular shapes also reduce the propagation of vibrations in the tower structure. In other words, the tower structure provides an optimized compromise between high stiffness and reduced total weight.

With these good mechanical properties, the tower structure may further be used to support additional equipment, such as cellular téléphoné antennas.

Due to the polygons in each stage level, with intermediate vertices outside the triangles of the edge vertices, each wind turbine rotor can sweep an area that extends beyond the triangle of the edge vertices. The areas swept by the rotors can thus be larger, while remaining contained within the structure. In the context of the présent invention, the term area swept by a rotor is understood to mean the latéral surface area of the cône or cylinder which is traced by each blade of the rotor.

In addition, the small number of segments constituting the tower structure also helps to reduce its on-site manufacturing and installation costs.

Finally, the three-fold symmetry around a vertical central axis, with three feet, gives high stability to the tower structure when it is installed on the ground or on floats in the case of offshore operation.

In preferred embodiments of the invention, the following improvements may be used, separately or with several of them combined:

- each upright may be parallel to a respective meridian plane which contains the axis of symmetry of the structure;

- each set of stage segments may comprise six stage segments of identical lengths, such that each polygon is a regular hexagon. Then, for each hexagon and for each pair of edge vertices of this hexagon, the single intermediate vertex of the pair is connected obliquely to the two uprights of the edge vertices of the pair by four support segments, one towards the top and one towards the bottom of the tower for each of the two uprights. In addition, for each hexagon, the six support segments connecting the three intermediate vertices to the uprights towards the bottom hâve a first common length, and the six support segments connecting the same three intermediate vertices to the uprights towards the top hâve a second common length;

- between two successive stage levels in the structure, ail support segments connecting intermediate vertices of the highest or of the lowest of these two successive stage

-4levels may be connected to the uprights at one and same height ofthe structure;

- the tower structure may further comprise, for each stage level, radial segments which extend respectively from at least some of the vertices of the polygon of this stage level, in the direction of the axis of symmetry of the structure, to form a wind turbine rotor support. Possibly, such radial segments may be associated only with the intermediate vertices;

- for at least one of the stage levels and for at least some of the vertices of the polygon of this stage level, two radial segments may extend from each of the vertices of the polygon, respectively towards two points of convergence which are located on the axis of symmetry of the structure, the two points of convergence being spaced apart along the axis of symmetry and being common to the vertices of the polygon of the stage level, from which the radial segments extend;

- each upright may be rectilinear and may approach the axis of symmetry of the structure in the upward direction towards the top of the tower, at least between the lowest stage level and the highest stage level. Between these two stage levels, the uprights then form the edges of a cône with an équilatéral triangular base;

- at least some of the uprights, stage segments, and support segments may each hâve an outer boundary of rounded cross-section along at least a portion of the length of said upright, stage segment, or support segment. An even better compromise can thus be obtained between résistance of the upright or segment to mechanical stresses and minimal interférence with wind flow; and

- thanks to its modular composition, the tower structure may comprise n+1 stage levels for receiving n wind turbine rotors, where n is a non-zero integer that is less than twenty, preferably equal to three, four, five, or six.

A second aspect of the invention proposes a tower which comprises:

- a tower structure in accordance with the first aspect of the invention; and

- at least one vertical-axis wind turbine rotor, each rotor being contained between two successive stage levels of the structure, and the axis of each rotor being coïncident with the axis of symmetry of the structure.

Advantageously, in order to increase the area which is swept by at least one of the wind turbine rotors, the path traced by the upper radial end of a rotor during one rotation of that rotor may extend radially beyond, in a projection onto a horizontal plane, the triangle of the edge vertices of the polygon of the stage level which is located just above the rotor. The same characteristic may also apply for the lower radial end of a rotor with respect to the stage level located just below that rotor.

-5To avoid significant imbalances and to reduce the vibrations likely to be generated by the rotation of each wind turbine rotor, each of the rotors may comprise three wind turbine blades.

Finally, the tower may further comprise antennas, in particular cellular téléphoné antennas, fixed to the structure above the stage level that cornes after the highest wind turbine rotor in the upward direction ofthe tower.

Other features and advantages of the invention will become apparent in the following description of some non-limiting exemplary embodiments, with reference to the accompanying drawings, in which:

- Figure 1a is a perspective view of a tower structure according to the invention, and Figure 1b is a view of the tower equipped with wind turbine rotors;

- Figures 2a and 2b are two élévation views of the tower structure of Figure 1a, and Figure 2c is a corresponding plan view;

- Figure 3a is a perspective view of a segment of the tower structure of Figures 1a and 2a-2c;

- Figure 3b corresponds to Figure 3a, with a wind turbine rotor in an operational position;

- Figure 3c is a perspective view of a stage level according to one particular embodiment of the invention;

Figure 4 is a plan view corresponding to Figure 3b, showing two paths traced by ends of the wind turbine rotor; and

- Figure 5 is a table of characteristics calculated for several stage level shapes.

For the sake of clarity, the dimensions of the éléments represented in these figures do not correspond to actual dimensions nor to actual size ratios. In addition, identical référencés indicated in different figures designate identical éléments or éléments which hâve identical functions.

The référencés hâve the following meanings:

Δ axis of symmetry of the tower structure

1, 2, 3 uprights of the tower structure, also called ribs

NE stage levels stage segments edge vertices intermediate vertices support segments, also called diagonal braces connections between support segments and uprights radial segments bearing unit of a wind turbine rotor wind turbine rotor wind turbine rotor blade wind turbine rotor arm wind turbine rotor shaft

SS additional segments of the tower structure cellular téléphoné antennas

Axis Δ is vertical above the ground, and the tower structure assembly has three-fold symmetry around axis Δ. The uprights 1 to 3 are identical, and each extend from the base ofthe structure to its top. The lower end of each upright thus constitutes one of the three feet of the tower structure. The portion of the tower structure that is below the lowest stage level is commonly referred to as the base or underframe, the portion which is between the lowest and highest stage level is commonly referred to as the running height, and a structural part which may optionally be added above the highest stage level is called the crown.

In the embodiment which is described first, each upright is parallel to a meridian which contains axis Δ, and the distance between the uprights decreases towards the top of the structure, until the highest stage level.

In any horizontal cross-sectional plane between the base and the top of the structure, the uprights 1-3 define the vertices of an équilatéral triangle.

Each stage level NE is horizontal and comprises six stage segments 10 which are connected together to form a hexagon. Preferably, the six stage segments 10 in each stage level NE separately hâve the same common length, such that the hexagon is regular. This length of the stage segments 10 can vary from one stage level NE to another. Each hexagon has alternating edge vertices 11 and intermediate vertices 12. The edge vertices 11 are connected to the uprights 1 to 3, in the corresponding stage level NE. In contrast, the intermediate vertices 12 are connected to the uprights 1 to 3 outside the stage level NE, by support segments 20. Each intermediate vertex 12 is thus connected to the same two uprights

-7as the neighboring edge vertices 11 in the hexagon. Thus, each intermediate vertex 12 is connected towards the top to two ofthe uprights 1 to 3, and towards the bottom to the same two uprights. The connections 21 of the support segments 20 to the uprights 1 to 3 are preferably ail located at the same height between two successive stage levels NE, both for the support uprights 20 extending from the lower of these two stage levels and for those which extend from the higher of the same two stage levels. Altematively, the support uprights 20 extending from the lower of the two successive stage levels NE may be connected to the uprights 1 to 3 below or above the support uprights 20 which corne from the higher of these two stage levels NE.

Each stage level NE may further comprise radial segments 30 (Figure 3a) for supporting at least one bearing unit 31 of the wind turbine rotors located just above or just below this stage level NE. For example, the radial segments 30 extend from intermediate vertices 12 in the direction of axis Δ, substantially horizontally or with a small inclination, to the bearing unit 31.

Each rotor 32 (Figure 3b) may comprise a central shaft 35, which is superimposed on axis Δ, several blades 33 which are radially offset, preferably three blades 33, and substantially horizontal arms 34 connecting the blades 33 to the shaft 35. The blades 33 may be inclined with respect to the vertical direction, for example so as to be substantially parallel to the uprights 1 to 3. Such blades 33 may hâve a small cross-section, be lightweight, and be inexpensive. Optionally, two wind turbine rotors 32 which are successive in the tower structure, one just above the other with only one stage level NE between them, may be twinned together so as to be intégral. In this case, the bearing unit 31 may be common to the twinned two rotors 32. This bearing unit 31 may then be located at the stage level NE which is located between the twinned two rotors 32.

In the variant of Figure 3c, two radial segments 30_A and 30_B extend from each of the three intermediate vertices 12 of a same stage level NE, towards points A and B referred to as points of convergence and located on the axis of symmetry Δ of the tower structure. Three radial segments 30_A thus converge towards point A, and three radial segments 30_B converge towards point B. Points A and B are spaced apart along axis Δ such that a bearing unit 31 can be placed on the axis of symmetry Δ, between the central ends of radial segments 30_A of the stage level NE on the one hand and those of radial segments 30_B ofthe same stage level NE on the other hand. Depending on the composition that is adopted for each wind turbine, a bearing unit 31 placed in this manner may be associated with a stator and a brake also located between points A and B.

Référencés C-j and C₂ in Figure 4 dénoté the two circles which are respectively traced by the lower and upper ends of the blades 33, for the wind turbine rotor 32 located between the

-8two stage levels NE represented, during a rotation of this rotor about axis Δ. Tj and T₂ dénoté the two équilatéral triangles which are formed by the edge vertices 11 of each of the two stage levels NE, for the lower level and T₂ for the upper level. As is shown in Figure 4, circle C-i extends beyond triangle Ti while remaining within the hexagon ofthe stage segments 10 ofthe lower stage level. Similarly, circle C₂ extends beyond triangle T₂ while remaining within the hexagon of the stage segments 10 of the upper stage level. Thus, thanks to the polygonal shape of the stage levels with intermediate vertices outside the triangles formed by the edge vertices, the wind turbine rotors can hâve larger diameters while remaining contained within the tower structure. The mechanical power recovered by the tower rotors is thus greater, without increasing the distance between the uprights.

The table in Figure 5 gathers the values calculated for four shapes of stage levels NE, respectively équilatéral triangular, square, regular hexagonal, and regular octagonal. For each shape, the outer circle circumscribes the polygon of the stage level, and its radius is denoted R-ι. The polygon ofthe stage level in turn circumscribes an inner circle whose radius is denoted R₂. R₂ is therefore the maximum radius of a wind turbine rotor end which is located at the relevant stage level, without taking into account a safety margin at the blade tip to ensure that the blade does not strike the support segments. The table shows the quotient values R₂/R-i for each stage level shape, as well as the quotient value of the area S which is swept by the blade for this stage level shape, over the area swept S_Hexa for the case of the regular hexagonal shape. For this calculation, it is assumed that the length of the blades is the same for each stage level shape. The swept area increases with the number of sides of the stage level polygon. The regular hexagonal shape corresponds to the embodiment of the invention described above. It constitutes an optimal compromise between the largest area swept by each blade and the number of uprights which remains equal to three to limit the weight of the tower structure, the uprights representing a significant contribution to the weight ofthe structure.

ln the embodiment represented in Figures 1 and 2, the tower structure comprises seven stage levels NE. Each tower structure segment that is comprised between two successive stage levels NE contains a wind turbine rotor, such that the tower represented is designed for six superimposed rotors. One or more additional structural segments SS may be provided above the highest stage level NE, for uses of the tower structure beyond the function of supporting wind turbine rotors. The uprights 1 to 3 may be parallel to the axis of symmetry Δ in these additional segments SS. For example, three additional segments SS may be arranged above the last stage level NE, to support cellular téléphoné antennas 40. Additionally or alternatively, the additional segments SS may be used to support meteorological measurement instruments, radio antennas, satellite communication antennas, fire surveillance caméras, solar or photovoltaic panels, electric cable support arms, etc. Equipment which is thus supported by

-9the tower structure may be supplied with electrical energy by the wind turbine(s) whose rotors are also supported by the same tower structure. Installations can thus be implemented which are energy-autonomous and which are advantageous for some sites of operation, in particular sites that are isolated and/or offshore, where connection to a power grid is difficult.

The uprights 1 to 3, the stage segments 10, the support segments 20, and the radial segments 30 may ail be métal beams, assembled together in a manner known to a person skilled in the art of métal constructions. However, it is preferred to create such uprights or segments in the form of métal tubes of circular cross-section in order to reduce the turbulence which may be caused by wind flow around each upright or segment.

For example, the différence in height between two successive stage levels NE may be about 5.8 m (meters), the first stage level NE may be 8 m above the ground, and the additional structural segments SS above the top rotor may hâve a cumulative height of 10.5 m. For six superimposed rotors, the described tower structure then has a total height of 51.79 m. The distance of each of the uprights 1 to 3 from axis Δ, measured horizontally, may vary for example between 6.58 m at ground level, 5.03 m for the lowest stage level NE, and 3.04 m for the highest stage level NE.

Measurements carried out on a 10/33^rd scale model of a tower as described above hâve shown that the loss in energy efficiency for each wind turbine, caused by the tower structure, is less than 15%.

A maintenance access ladder as well as electrical cables (not shown) may be arranged along one ofthe uprights 1 to 3. This upright may be selected such that any airflow interférence caused by the ladder and the electrical cables is reduced as much as possible. For example, these éléments may be arranged along the upright that is primarily leeward of the dominant wind at the site of operation. In addition, some of the electric cables may connect the wind turbine stators by being arranged along some of the radial segments to reach the stator concerned.

It is understood that numerous variants or modifications may be introduced to the embodiment of the invention just detailed, while retaining at least some of the mentioned advantages. Among these variants and modifications, we list the following in a non-limiting manner:

- the number of stage levels may be modified, to adapt the tower structure to variable numbers of wind turbine rotors to be supported. Persons skilled in the art will understand that the modular design of the tower structure facilitâtes changing the number of rotors;

- when the tower structure has more than three stage levels NE, some of them may be

-10without a wind turbine rotor in the final tower;

- the uprights may hâve variable shapes in the meridian planes: rectilinear for their entire length or in portions of their length, having progressive curvature, inclined towards the axis of symmetry Δ in the direction of the top of the tower, or conversely inclined outwards, or having a distance from the axis of symmetry Δ which is minimal at an intermediate height between the lowest stage level NE in the tower and the highest stage level; etc.;

- the tower structure may spiral about the axis of symmetry Δ, with an angular shift of the uprights about axis Δ between two successive stage levels. Such a spiraling structure may be adapted to withstand greater mechanical forces caused by rotation ofthe rotors; and

-the polygons of the stage levels may hâve nine sides, composed of as many stage segments. In this case, two support segments may be used to connect each intermediate vertex to the upright ofthe nearest edge vertex, towards the top and towards the bottom.

Claims

1. A tower structure with a plurality of superimposed stage levels (NE) for supporting at least one vertical-axis wind turbine rotor, the structure having a vertical axis of symmetry (Δ) and remaining identical to itself in rotations of ±120° about said axis of symmetry, the structure comprising an assembly of linear and rigid segments which includes at least:

- three uprights (1, 2, 3) each extending between a base and a top ofthe structure;

- at least two sets of stage segments (10); and

- support segments (20), wherein the stage segments (10) of one and same set are assembled at a stage level (NE) of the structure, to form a horizontal polygon at said stage level, each stage segment forming one side of the polygon and connections between respective edges of two successive stage segments in the polygon forming a vertex of said polygon, three of the vertices of each polygon, referred to as edge vertices (11), being respectively connected to the three uprights (1, 2, 3), and any two of the edge vertices of each polygon being separated by at least one other vertex of said polygon, referred to as the intermediate vertex (12) ofthe pair of two edge vertices, wherein, for each pair of edge vertices (11) of any one of the polygons, at least one intermediate vertex (12) is further connected obliquely to at least one of the uprights (1, 2, 3) to which one of the edge vertices of the pair is connected, by two support segments (20) respectively towards the top and towards the bottom of the tower, wherein, for each polygon, each intermediate vertex (12) is located outside the triangle formed by the edge vertices (11 ) of said polygon, and the structure is further adapted for receiving a wind turbine rotor (32) or a plurality of superimposed wind turbine rotors, each rotor being contained between two successive stage levels (NE), and the axis of each rotor being coïncident with the axis of symmetry (Δ) of the structure.

2. The tower structure according to claim 1, wherein each upright (1, 2, 3) is parallel to a respective meridian plane which contains the axis of symmetry (Δ) of the structure.

3. The tower structure according to claim 1 or 2, wherein each set of stage segments (10) comprises six stage segments of identical lengths, such that each polygon is a regular hexagon, wherein, for each hexagon and for each pair of edge vertices (11) of said hexagon, the single intermediate vertex (12) of the pair is connected obliquely to the two uprights of the edge vertices of said pair by four support segments (20), one towards the top and one towards the bottom of the tower for each of the two uprights, and wherein, for each hexagon, the six support segments (20) connecting the three intermediate vertices (12) to the uprights (1, 2, 3) towards the bottom hâve a first common length, and the six support segments connecting said three intermediate vertices to the uprights towards the top hâve a second common length.

4. The tower structure according to any one of the preceding claims, wherein, between two successive stage levels (NE) in the structure, ail support segments (20) connecting intermediate vertices (12) ofthe highest or ofthe lowest of said two successive stage levels are connected to the uprights (1, 2, 3) at one and same height ofthe structure.

5. The tower structure according to any one of the preceding claims, further comprising, for each stage level (NE), radial segments (30) which extend respectively from at least some of the vertices of the polygon of said stage level, in the direction of the axis of symmetry (Δ) ofthe structure, to form a wind turbine rotor support.

6. The tower structure according to claim 5, wherein, for at least one of the stage levels (NE) and for at least some ofthe vertices ofthe polygon of said stage level, two radial segments (30_a, 30_b) extend from each of said vertices of the polygon, respectively towards two points of convergence (A, B) which are located on the axis of symmetry (Δ) of the structure, the two points of convergence being spaced apart along said axis of symmetry and being common to the vertices of the polygon of said stage level, from which the radial segments extend.

7. The tower structure according to claim 5 or 6, wherein the polygon vertices from which the radial segments extend are intermediate vertices (12).

8. The tower structure according to any one of the preceding claims, wherein each upright (1, 2, 3) is rectilinear and approaches the axis of symmetry (Δ) of the structure in the upward direction towards the top of the tower, at least between the lowest stage level and the highest stage level.

9. The tower structure according to any one of the preceding claims, wherein at least some ofthe uprights (1, 2, 3), stage segments (10), and support segments (20) each hâve an outer boundary of rounded cross-section along at least a portion of a length of said upright, stage segment, or support segment.

10. The tower structure according to any one of the preceding claims, comprising n+1 stage levels (NE) for receiving n wind turbine rotors (32), n being an integer equal to three, four, five, or six.

11. The tower comprising:

- a tower structure according to any one of the preceding claims; and

- at least one vertical-axis wind turbine rotor (32), each rotor being contained between two successive stage levels (NE) of the structure, and the axis of each rotor being coïncidentwith the axis of symmetry (Δ) ofthe structure.

12. The tower according to claim 11, wherein, for at least one rotor (32), the path traced by the upper radial end of said rotor during one rotation of the rotor extends radially beyond, in a projection onto a horizontal plane, the triangle (T₂) of the edge vertices (11) of the polygon of the stage level (NE) which is located just above the rotor.

13. The tower according to claim 11 or 12, wherein, for at least one rotor (32), the path traced by the lower radial end of said rotor during one rotation of the rotor extends radially beyond, in a projection onto a horizontal plane, the triangle (Ti) ofthe edge vertices (11) ofthe polygon ofthe stage level (NE) which is located just belowthe rotor.

14. The tower according to any one of claims 11 to 13, wherein each wind turbine rotor (32) comprises three blades (33).

15. The tower according to any one of claims 11 to 14, further comprising antennas (40), in particular cellular téléphoné antennas, fixed to the structure above the stage level (NE) that cornes after the highest wind turbine rotor (32) in the upward direction of the tower.