GB2461753A - Bracing Arrangement for Large Horizontal-Axis Wind-Turbine - Google Patents

Abstract

Tension bracing members 4 e.g. cables or solid ties or rods are attached to blades 2 via collars 3 which may be fitted to the outer blade surface or to a central blade spine or spar. The collar may comprise a bearing to preserve the blade pitch adjustment or may be rigidly attached to a fixed spar about which an aerofoil surface may rotate for pitch adjustment. There may be minor blades 5 between main blades to adjust the brace element angle in plane or the braces may pass directly between main blades and there may be an up-wind pylon (7,fig.5) to provide an axial bracing component. The turbine may have a conventional generator shaft output or be an integral conversion wind turbine, ICWT, with blade mounted masses that move as the rotor turns for internal energy conversion. Mitigates the oscillatory component of in-plane blade loads due to gravity and the steady out of plane loads tending to bend the blades downwind.

Description

BRACING ARRANGEMENT FOR LARGE

HORIZONTAL-AXIS WIND TURBINE.

Field of the Invention:

This invention relates mainly to the provision of bracing onto the blades of large horizontal-axis wind-turbines such that the tip deflections at the ends of these blades and the bending moments at their roots are reduced. This bracing has the highest value in the context of wind-turbines which convert the energy extracted from the wind through allowing mass to move freely within the main rotor of the turbine. However the bracing described here also has substantial value for large horizontal-axis turbines of a more conventional nature which pass the power through the main shaft as the product of torque and rotational speed.

Background.

Most -if not all present-day large horizontal-axis wind-turbines have a small number of blades supported only at a hub. The norm is for there to be three blades. Economic considerations seem to dictate that as the turbine tip diameter, D, increases, the cost per unit power produced over a lifetime reduces. In short, it has been the case that larger is better. However, the scaling laws of mechanical engineering indicate that this cannot continue to be the case for conventional wind-turbine designs as the diameter increases. Two specific considerations apply: (1) The power from a wind-turbine increases in proportion to D2 whereas the rotational speed decreases in proportion to D. This means that if the power is converted by extracting mechanical power through the main shaft of the wind-turbine as the product (T x Q) -where T represents torque and where �= represents the rotational speed of the main rotor the torque, T is proportional to L). Now, if the wind-turbine drives a generator directly (rather than through a gearbox), the cost of that generator rises in direct proportion to the torque. It seems also to be the case that for wind-turbines which utilise a gearbox to increase the rotational speed of the generator compared with the rotational speed of the main rotor, the cost of the gearbox is in direct proportion to the magnitude of the input torque. Ultimately, therefore, at sufficiently large diameters, the costs of the generator or gearbox will dwarf all other costs. * ***

:::: (2) The importance of gravity as a load increases further and further with increasing D. If an entire turbine is simply "scaled-up" from some original value, then the aerodynamic forces will increase in proportion to D2, aerodynamic moments will increase in proportion to D3 and * the (bending) stresses associated with aerodynamic forces remain constant. By contrast, * *. gravitational forces increase in proportion to D3, gravitational moments increase in proportion * : ": to D4 and the (bending) stresses associated with gravitational forces rise in proportion to D. * :. These two facts support the argument that a logical structure for the rotors of large wind turbines is one in which (at least some of) the power extracted from the wind is converted directly within the rotor of * * the wind-turbine itself by allowing masses running within passages fixed to the main rotor to be driven by gravity. This scheme has been outlined in recently-filed patent applications (GB 0613249.2 and subsequently PCT1GB2007f002477), where the movement of masses within the turbine blades relative to the blades themselves is used as a mechanism by which to harvest the power directly from the wind.

The present patent application relates primarily to such machines -where some or all of the power extracted from the wind by aerofoils present on blades is converted by the gravity-driven movement of masses contained within the rotor of the wind-turbine relative to the structure of the wind-turbine rotor itself. The term Integral Conversion Wind Turbines (ICWTs) will be used to describe these machines henceforth. In the following section, we will show that for IC Wi's, the average moment at the root of any one blade is close to zero in the plane normal to the rotation axis. Analysis has shown, however, that the bending moments at blade roots (in the plane normal to the rotation axis) varies significantly during one rotation cycle of the wind turbine rotor [11. Structural features which can couple the whole set of blades can be very valuable in reducing the oscillatory bending moments at the roots of blades.

In the conventional present-day wind-turbines, the average bending moment at the root of any one blade is certainly not close to zero when the turbine is working at rated conditions. The benefits of structural features coupling the various wind-turbine blades are less pronounced for conventional wind-turbines than they are for ICWTs -but there are some benefits nevertheless.

This patent application deals specifically with the concept of employing bracing (ties which may be sections of cable) to reduce the deformation of blades relative to a frame of reference attached to the hub of the machine. Circumferential bracing is proposed to curtail the deformation of the turbine blades in the plane normal to the axis. Axial-radial bracing is proposed to curtail the deformation of the turbine blades in directions parallel to the axis of the machine. In both cases, the bending moments at the roots of the blade are reduced.

Blade Root Moments: In-Plane.

Consider a wind-turbine having N blades attached to the hub. In most modem machines, N=3. The action of the wind on these blades at any one instant is to create a circumferential force distribution on the blade tending to move each point on the blade in the same (positive) circumferential direction. In any one operating condition, the distribution of circumferential force on the blades can be considered to be equivalent to a single circumferential force F on the blades at radius R. Then the total aerodynamic torque developed by the wind-turbine in that particular condition is TA (F x R x A/). The product of the aerodynamic torque and the rotational speed of the rotor, , is the instantaneous power being extracted from the wind, P (TA x) (F x R x N x The maximum power available in a given swept-area of wind-turbine is a fixed quantity for a specified : (rated) wind-speed. For a given turbine tip diameter, the turbine rotational speed is limited to a given range by efficiency concerns. If rotational speed is too low, the torque must be high to produce the power and then the air behind the turbine is left with a high degree of kinetic energy in the form of swirl. If the rotational speed is too high, then drag losses on the blades are substantial compared with ,* * the total power extracted. The optimal rotational speed is expressed in terms of the tip-speed ratio (blade tip circumferential speed divided by the incident wind speed) which is typically in the range 3-8.

: Higher tip-speed ratios are preferable for smaller numbers of blades and lower ratios for higher numbers of blades. * * S

Since the range of effective rotational speeds is fixed for a given turbine diameter and for a given rated velocity of incident wind, the total airgap torque TA is also fixed within a range. As reasoned earlier, this torque varies in direct proportion to D3.

Now consider how this aerodynamic torque is transmitted through the hub and into the main shaft. The outside radius of the hub, r, is usually very small compared with this mean radius of action of the blade lift, R. The mean aerodynamic blade root moment is M= (F x (R -r)) = (T4 (R -r) / (RN)). This moment occurs in the plane normal to the axis. Because (r/R) is usually small, M TA IN.

For conventional wind-turbines, the mean blade root moment has an average value of (approximately) (TA/N). The actual bending moment oscillates around this mean value influenced by gravity. As one blade rises, the bending moment on it is reduced in magnitude (aerodynamic forces act in opposition to gravity forces). As the blade falls again, the bending moment is increased in magnitude (aerodynamic forces act in sympathy with gravity forces). The ratio between the magnitude of the oscillation and the mean value increases with increasing turbine diameter, D. For ICWTs, the mean blade root moment has an average value close to zero. The only torque transmitted into the main shaft is that which is needed to support the bearing loss torque. This may clearly be very small. Analysis shows, however, that the oscillatory components of blade root moment can be very large if no bracing is present [1]. The movement of mass causes substantial Coriolis forces to exist and also accounts for variation of the circumferential components of gravitational force over and above the natural sinusoidal variation that gravitational force undergoes as a blade turns around a horizontal axis.

It is instructive to put some numbers around these moments. For a 3-bladed machine of conventional design with tip-diameter 200m and designed for a rated wind speed of 15 mIs, rated rotational speed might be 0.54 radls (5.16 rev/mm) and rated power would be around 18 MW. The mean (in-plane) 1ade rt mon'Pnt cm each blade of this machine would be 1 1.1MNm. Gravity accounts for an additional �26.2 MNm. For a 4-bladed ICWT machine (c.t. ij), iiii iucm (i-pr) bendi.g moment in each blade is 0 but the oscillatory component reaches peaks of nearly 70 MNm.

The bracing described in this document is extremely effective in reducing the oscillatory components of in-plane blade-root bending moments. Obviously, this presents the greatest advantage in the context of the ICWTs but it has clear advantage also in the context of even conventional wind turbines. The bracing does not contribute much to reducing the mean in-plane bending moments.

Blade Root Moments: Out-Of-Plane Moments.

* When wind is blowing directly from the West with a velocity which is not excessive for the turbine : operation, the axis of the turbine is oriented East-West. Then, in addition to the intended effect of wind * on the wind-turbine (the circumferential forces acting on blades) the wind has the effect of bending all of the blades of the turbine in a direction parallel to the axis -i.e. East. The associated bending moments at the blade roots are out-of-plane. These bending moments are equally important in the cases ***. of the ICWTs and conventional wind-turbines. The axial-radial bracing proposed here allows for the minimisation of these. Note that although a wind-turbine control system can decide to produce zero net * : aerodynamic torque in wind-speeds above some threshold value, it cannot achieve a zero drag in those winds because it cannot reduce the projected frontal area of the blades to zero. These blade-root : : moments can be very large in high winds. * **. *

Summary of the Invention.

A horizontal-axis wind-turbine comprises a rotating subsystem and a stationary subsystem. The rotating subsystem, henceforth simply called the rotor, comprises a main shaft to which a hub is attached carrying blades. The blades extend (mainly) radially from the hub.

Each blade has one or more collars located along its length where bracing may be applied. In the case of a conventional wind turbine where the entire blade body is pivoted about a radial axis extending from the centre of the hub, these collars would be fitted to the blade through bearings (probably rolling-element bearings) such that cables or ties could be attached to these collars and empowered to transmit axial and circumferential forces into the blades. In the case of an ICWT, each blade comprises a spine fixed into the hub around which several independent aerofoils may be articulated, the collars can be fixed rigidly to the spines.

Circumferential bracing would extend between all blades in the form of tension elements attached to the collars of the blades. Sometimes, in addition to the full-length blades which define the turbine tip diameter (major blades), it is appropriate to have minor blades also at intermediate angular positions.

This is especially appropriate for turbines having fewer than four major blades. The minor blades serve the important function of increasing the angle between the circumferential braces and the major blades.

In effect, the minor blades caused the circumferential bracing to follow a more circular path. There would be at least one complete ring of circumferential bracing. In some cases, two or more complete rings of circumferential bracing are provided.

The most advantageous radius at whicn to aiiact cfcrtia! bran is approximately at two-thirds of the distance between hub and tip. If two rings of circumferential bracing are present, these would be located at approximately at 40% and 80% of the radial extents of the blades.

The same collars which provide for attachment of circumferential bracing also provide for the attachment of axial-radial bracing to help react forces exerted by the wind tending to deflect the rotor blades in the same direction as the axis of rotation. For the axial-radial bracing, a pyion extends forward (into the wind) from the centre of the hub culminating in an attachment point for bracing. The axial-radial bracing then comprises a number of slender tie-elements between the tip of the hub pylon and the collars on the blades. The pylon must extend forward sufficiently for there to be a reasonable angle between the bracing elements and a plane normal to the axis of rotation. An angle of 20° would SS. . . . . . . . : be typical. The consideration here is that if the angle is too shallow, then when the bracing is reacting substantial axial forcing on the blades there will be a very high tension in the axial-radial bracing and the radial component of this bracing must be reacted by the sum of compressive stress in the blades between the hub and the collar and centrifugal loads. In this case, the blades could potentially buckle. *..S * * ***.

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Specific Embodiment #1: No minor blades.1 Circumferential bracing only.

A horizontal-axis wind-turbine comprises a rotating subsystem and a stationary subsystem. Figure 1 illustrates the most important parts of the rotating subsystem. The rotating subsystem, henceforth simply called the rotor, comprises a main shaft to which a hub (1) is attached carrying blades (2). The blades extend (mainly) radially from the hub and they are invariably equipped with pitch control mechanisms for their aerofoils. Each blade carries at least one collar (3) at a position more than half-way between the hub and the tip diameter. The collars are fixed in such a way that they do not impede the normal pitch-control actions of the blade aerofoils. Attached to these collars are circumferential bracing elements (4). These bracing elements would usually be cables or plain solid ties. Because these bracing elements will cause unproductive drag on the turbine and will account for at least some loss of energy, the frontal projected area of the bracing is kept as small as possible. The inefficiency caused by the presence of the bracing is offset by the fact that the bracing enables larger turbines to be built than Figure 1 shows a turbine having four blades. The system is workable also with three blades -though the stiffness added in this case is greatly reduced because of the shallow angles between the bracing elements (4) and the blade centrelines. The system of Figure 1 is not applicable to two-blade turbines -since the bracing elements in this case would offer negligible additional in-plane stiffness to the blades.

Specific Embodiment #2: Minor blades present to enhance circumferential bracing A horizontal-axis wind-turbine comprises a rotating subsysu.eiii (Iviur) and a s kiuaiy uiyiii.

Figure 2 illustrates. The rotor comprises a main shaft to which a hub (1) is attached carrying major blades (2) as well as minor blades (5). The major blades serve to convert the bulk of the power from the air and the major blades would invariably be equipped with pitch control mechanisms for their aerofoils. The minor blades would usually also be equipped with aerofoils. These might or might not be equipped with a capability for pitch adjustment.

All of the blades extend (mainly) radially from the hub. Each major blade (2) carries at least one collar (3) at a position more than half-way between the hub and the tip diameter and arranged such that it does not impede the normal function of pitch control for the major blade aerofoils. Each minor blade (5) also carries at least one collar (6). If pitch control is provided on the minor blades, these collars are arranged so as not to impede that function. * ..*

:::.: Circumferential bracing elements (4) are attached between the collars to describe one or more polygons. The vertices of these polygons would ordinarily be at similar distances from the main rotation axis. Two conflicting considerations are borne in mind in the determination of these radial * :: distances; firstly that the net centrifugal loads between hub and collar (or between successive pairs of * collars) is different for the major and minor blades and secondly that the oscillatory in-plane bending : moments acting on the major blades are higher than those acting on the minor blades.

* :. The circumferential bracing elements would usually be cables or plain solid ties. To minimise unproductive drag on the turbine and consequent inefficiency, the frontal projected area of the bracing * : * is kept as small as possible. The small degree of inefficiency caused by the bracing is offset by the fact that this bracing enables larger turbines to be built than would otherwise be possible.

Figure 2 shows a four-blade turbine having major and minor blades with one minor blade between each pair of successive major blades. The concept of circumferential bracing involving minor blades is applicable to turbines having two or more major blades. Note that without the minor blades, circumferential bracing could not be applied usefully to two-bladed turbines and its use with three-bladed machines would be limited. If circumferential bracing is applied to turbines designed to have two major blades, then there would typically be four or more minor blades.

Figures 3a and 3b illustrate the use of minor blades and circumferential bracing in the cases of wind turbines having two and three major blades respectively. In Figure 3a, there are six minor blades and only two major blades. The role of the minor blades and the circumferential bracing is especially obvious here. When the centreline of the two major blades lies in a horizontal plane, the circumferential tension in the bracing above this centreline carries the bulk of the gravitational load on the blades so that the blade roots do not have to carry this as a very large moment. It is also especially clear in this case that the circumferential bracing does not help to reduce the average bending moment at the roots of the blades.

Specific Embodiment #3: Axial-radial bracing only.

A horizontal-axis wind-turbine comprises a rotating subsystem and a stationary subsystem. Figure 4 presents a side-elevation of the most important parts of the rotating subsystem. The rotating subsystem, (the rotor) comprises a main shaft (0) to which a hub (I) is attached carrying blades (2). Blades extend (mainly) radially from the hub and they are invariably equipped with pitch control mechanisms for their aerofoils. Each blade carries at 1e.st ne rnlliir () t ruitirrn niore thi hif-'.'y eer the hub and the tip diameter. The collars are fixed in such a way that they do not impede the normal pitch-control actions of the blade aerofoils.

Fixed onto the front of the hub (1) is a pylon (7) which is capable of carrying substantial axial compression load. At the fore-most point on the pylon is a collar (8) to which bracing cables or tie-bars may be attached. Bracing (9) then extends from this collar to at least one collar on each blade. The effect of this axial-radial bracing (9) is that when strong winds act to bend the blades backwards out of the plane, this bracing helps to react the loads so that they do not have to be carried as out-of-plane bending moments at the roots of the blades.

Figure 5 shows an extension of Figure 4 where minor blades are present in addition to the major blades * :. and axial-radial bracing is applied also to the minor-blades. In most cases, if minor blades were present and if axial-radial bracing was applied to the major blades, it would be usual to apply it also to the minor blades but because of the contrast in length, the ties to the minor blades would be much lighter.

Other Embodiments.

* ****. * *

The bracing described here is applicable to "conventional" horizontal-axis wind-turbines where all of the power extracted from the wind is transmitted along the main shaft as the product (TX C�=) where T represents torque and where) represents rotational speed. It is also applicable to Integral Conversion * : Wind Turbines (ICWTs) where the power extracted from the wind is converted by allowing the movement of mass within the main rotor of the wind-turbine under the influence of gravity. The collars (3) have a slightly different implementation in the two cases. For ICWTs, each of the blades has an integral spine (10) about which aerofoils are articulated. Figures 6 and 7 illustrate how this articulation (11) can be achieved. The collars are then fixed directly onto the blade spines. For wind-turbines in which the entire blade body is rotated at the root for the purposes of pitch control, the collars must be attached to the blade body via a bearing.

It is obvious that the axial-radial bracing of Figure 4 solves a problem which is different from the problem solved by the circumferential bracing. The primary purpose of the circumferential bracing is to reduce the oscillatory components of the in-plane blade root moments and to reduce the in-plane deflections of the wind-turbine blades. The primary purpose of the axial-radial bracing is to reduce the down-wind out-of-plane blade deflections and the associated out-of-plane moments. What is common to the two of them is that both require the provision of attachment collars on the major blades at significant radial distances from the hub. Once these fixing collars are present, both types of bracing can be fitted The benefits of incorporating minor blades into the wind-turbine rotor are shown in Specific Embodiment #2. When circumferential bracing is being considered for a large wind-turbine, the incorporation of minor blades between the major blades should be evaluated. One or more minor blades may be installed between each pair of successive major blades. The minor blades would ordinarily be fitted with aerofoils which might or might not have adjustable pitch. In the case of ICWTs, the minor blades would be likely to have provisions for mass to move internally within them also.

The axial-radial bracing described in Specific Embodiment #3 presupposes that the main wind-turbine rotor is upwind of the tower. Axial-radial bracing is also possible where the main rotor is downwind of the tower but less attractive in this case as the bracing members would be on the "compressive stress" side oI the biaties atid so wouki ic iii iu th iicivc3. Tc pylcn (7) f Figrc 5 then be replaced by a simple tension element.

From the above paragraphs, therefore, it is evident that embodiments are possible where: (a) The wind-turbine is a "conventional" (T x Q) machine or an ICWT.

(b) Circumferential bracing and/or axial-radial bracing is present (c) Minor blades are present between the major blades serving to make the circumferential bracing more normal to the major blades.

(d) The wind-turbine main rotor may be either upwind or downwind of the tower. *..* * * * *. * I.. * * S... * . * S S... * II I. S *S.

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