GB2463957A

GB2463957A - Multiple rotor vertical axis wind turbine

Info

Publication number: GB2463957A
Application number: GB0909155A
Authority: GB
Inventors: James O'donnell
Original assignee: MICROGEN TECH Ltd
Current assignee: MICROGEN TECH Ltd
Priority date: 2008-10-06
Filing date: 2009-05-28
Publication date: 2010-04-07
Anticipated expiration: 2029-05-28
Also published as: GB0818216D0; GB0909155D0; GB2463957A8; GB2464132B; GB2464132B8; GB2464132A8; GB2463957B8; GB2464132A; GB2463957B

Abstract

A vertical axis wind turbine comprises a cylindrical vertical structure 1 supporting a plurality of vertical axis wind turbine rotors 2, 5, 12, 13, and houses a plurality of electric generators 8. Main drive gears of each rotor and generator may mesh through openings in the vertical structure wall. Each rotor may incorporate an annular ring gear, within the rotor assembly, which drives a gearing system inside the structure, this being connected to a respective generator. Rotor bearings located on the vertical structure may allow the turbine rotors to rotate freely around the structure which acts as its static shaft. An annular rotor hub, of lightweight caged construction, may include blade hubs and a blade auto-pitching mechanism. A pitch limitation mechanism may be provided in the hub to allow the pitch to be altered relative to the wind direction to apply alternate lift and drag forces to diametrically opposed rotor blades. A variable speed and frequency generator drive system may be mounted eccentrically within the vertical structure.

Description

I

Multiple Rotor Vertical Axis Wind Turbine This invention relates to a method of overcoming the design limitations in efficient power extraction from wind energy currently employed by current designs of horizontal axis wind turbines (HAWT) and vertical axis wind turbines (VAWT).

In this invention a number of vertical axis wind turbines are mounted within the same structure housing the generators, turbine rotors and stator sections to optimise incoming airflow. The use of a multiple turbine approach on a single structure overcomes the difficulties associated with conventional wind farms employing horizontal axis wind turbine (HAWT) technology in that more turbines can be accommodated within a given acreage. Current designs of horizontal axis wind turbines (HAWT) which have become the de facto standard for high mega watt power output wind farms have a number of disadvantages in their application. As an example generator access is particularly poor, particularly in exposed offshore locations and maintenance access may not be feasible due to sea conditions or inability to safely operate helipad to allow access by air. A vertical axis wind turbine can afford easy access from sea and from above by air thereby offering a higher operating availability if the limitations of vertical axis wind turbine designs can be overcome. Although horizontal axis wind turbines are becoming more efficient the spacing required between them in a wind farm result in the wind farm itself being relatively inefficient when viewed as an overall power generating site due to the array losses between wind turbines within the wind farm.

The ability to make an efficient wind turbine in the vertical axis is extremely difficult primarily due to the out of balance forces generated in the shaft when the turbine is rotating horizontally with the shaft in the vertical plane. This is principally due to the shaft having significantly less inertia than the rotating element and therefore induced vibration, due to uneven torque, can be amplified which in turn propagates harmonic motion ultimately leading to mechanical failure. Additionally current designs of vertical axis wind turbines impart very low torque to the driven element and hence have a low power output. To overcome this vertical axis wind turbines are generally of light construction and cannot compete with the horizontal axis HAWT designs in terms of power output. However the horizontal axis wind turbine HAWT design suffers limitations in the amount of power extraction in that the propeller must be large and as a consequence wind farms require significant spacing between the individual wind turbines to allow them to operate efficiently. Another significant disadvantage of the horizontal axis (HAWT) machine is that it requires to be directed to optimal wind direction. This requires a complex series of mechanical systems commonly referred to as yaw control. A vertical axis wind turbine (VAWT) system does not require yaw control and can thus operate in any wind direction relative to it.

It is the ability to operate in any wind direction that offers significant advantages to a vertical axis (VAWT) system allowing a relatively simple design to be constructed if the problem of induced vibration and bearing loadings in the shaft can be overcome.

To overcome these problems a multiple rotor approach is used in this vertical axis wind turbine VAWT design albeit at a lower power output for the individual turbine compared to a similar sized single horizontal axis turbine. Overall for a given footprint, i.e. using a single vertical structure, a higher power output from a multiple turbine and generator unit compared to a horizontal axis wind turbine (HAWT) occupying the same footprint in the wind farm can be achieved. More vertical axis wind turbines (VAWTs) in a multiple turbine single vertical structure configuration can be incorporated within a given acreage of windfarm thereby boosting its overall power output. Given that wind direction no longer a constraint these units can provide greater operating availability and are less sensitive to location relative to prevailing winds. The vertical axis wind turbine using a long horizontally extended rotor can also self regulate performance. Therefore a complex pitch control mechanism to avoid over speed events is not required. The rotor blade and hub can be designed in detail to balance forces while maximising torque applied to the power generator The invention will now be described by the attached drawings: Figure 1 Elevation of a large multiple vertical axis wind turbine VAWT unit in an offshore environment with helicopter access Figure 2 Elevation of a large onshore windfarm multiple vertical axis wind turbine unit Figure 3 Section of an illustrative rotating element and interaction with power generator using a planetary gear drive from rotor to generator and gear box.

Figure 4 Section of an illustrative rotating element and interaction with a variable speed generator eccentrically mounted within the support structure.

Figure 5 Exploded view of an example of main components in rotating element.

Figure 6 Section through blade illustrating a simple auto-pitch control Figure 7 Illustrative rotor blade hub design for multiple vertical axis wind turbine (VAWT) Figure 8 Illustrative example of wind farm interaction in an array of multiple rotor vertical axis wind turbines.

Figure 9 Overview of principle of operation of multiple rotor vertical axis wind turbine Figure 10 illustrative section of a multiple generator and rotor vertical axis wind turbine employing eccentrically mounted variable speed generators housed within the support structure.

With reference to figure 1 a vertical tower structure I incorporates a number of vertical axis wind turbine (VAWT) rotor sections 13, 5, 12, blade 2 and stator 3 stages. The vertical axis wind turbine (VAVVT) stages are equal in number and each counter rotates to the stage above or below as illustrated in figure 9 to allow optimal operation. This counter rotation of the stages assists in balancing the forces across the structure and reduceemploying counter rotational rotors also allows for the turbines to be matched and paired in a windfarm array so that wind energy extraction is optimised for a given area in the wind farm and wind turbine interaction is not counter-productive to the neighbouring turbines in the array. Torsional loading on the main structure housing the generators and turbines is minimised by an equal number of wind turbine rotors counter rotating to each other around the same structure. In figure 9 a four blade rotor design is shown as an example however a number of blade designs and any number of blades can be employed, four blades are however an optimal number to ensure a smooth power output and to avoid stalling due to suboptimal position relative to incoming airflow. In figure 1, 2 and 10 four stages are shown with four blades at each stage, this is only an example and more stages and different numbers of blades can be accommodated subject to the limitation of the structure. The vertical spacing between rotors is determined by the air flow and interaction between rotor stages in the vertical attitude and the overall height of structure that can be accommodated in the windfann or individual turbine unit location. A single turbine stage can be used also in this design, however multiple turbines is the preferred configuration to maximise power generation efficiency. The optional stator 3 mounted from the wall of the structure acts as a wind deflector and reduces the vertical interaction of air turbulence between adjacent stages thereby allowing the vertical distance between the rotors to be minimised. The stator/deflector 3 is a fixed horizontal thin blade element projecting out from the main structure 1. This is illustrated in figure 1, 2 and 9 however is omitted from the configuration shown in figure 10. The vertical axis wind turbine (yAWl) stage.rotors rotate on the structure 1 via a bearing arrangement mounted within the structure in a recess in the walls 10 and 11 referencing figures 3, 4 and 5 any combination of bearing configuration can be used in mounting on the structure 1 outer wall an supporting the rotor. The bearings are of conventional design for a number of rotating equipment applications for larger rotating machinery. The bearings absorb rotational forces from the rotor and allow it to rotate freely around the structure with minimal vibration loadings. In effect the structure 1 acts as the rotor shaft, thereby removing the problem of induced vibration due to the high rotational inertia of the rotor in relation to the inertia of the shaft when a vertical axis wind turbine (VAWT) directly drives the shaft of a generator or coupling to a gear box. Utilising the structure as the shaft also allows the multiple vertical axis wind turbines to be placed on top of each other with the structure I acting as the non-rotating shaft for the turbine rotors. The rotor for the purpose of illustration is made up of three sections, 12 incorporating the main body to which the rotor blades via the hub 18 are attached and 5 the annular gear where the inner face of the gear meshes with the drive mechanism for generator and gear box 8 or in figure 4 a variable speed generator with no gear box 24. The annular ring gear 5 on the rotor meshes with a planetary gearing mechanism 6 in figure 3 or a single ring gear 26 in figure via an opening 7 in the main structure 1. The gears generator mesh with the ring gear 5 on the rotor and the rotor rotated by the wind drives the generator housed inside the main structure.

In figure 3 inside the structure 1 the planetary gears 6 protrude through an opening 7 in the structure I wall to mesh with the annular ring gear 5 on the rotor 12, 13. The rotor bottom section can accommodate an additional ring 13 which interfaces with the bottom annular bearing housing on the tower 11. The annular gear 5 drives a planetary gear 6 directly connected to the sun gear 9. The sun gear 9 meshes with the planetary gear 6 which has been rotated by the annular gear via an aperture 7 opening in the structure allowing the rotational force from the rotor 2 to be transferred to the generator and gear box 8 via the planetary gearing system. The gear sizes in relation to each other can be determined by the required gear ratio for the particular application. Conventional electrical generator technology for wind power plants can be employed with the generator mounted in the vertical as opposed to horizontal plane. For the purposes of describing this invention the generator referred to is an electrical generator of various suitable designs including its gear box and associated electrical conversion equipment and controls 8. In figure 4 the same concept is illustrated using a variable frequency/speed generator 24 this dispenses with the need for a gear box and planetary gearing system to drive the generator. To allow this type of generator to be employed a final drive gear 26 mounted on the generator assembly meshes with the annular ring gear 5 on the rotor through an opening 7 on the structure 1 wall. To allow the gears 26, 5 to mesh through the opening 7 the variable speed generator 24 has to be mounted eccentrically to the vertical axis of the structure as illustrated in figure 4.

All stages of the vertical axis wind turbine (VAVVT) on the tower operate identically with the exception of half of the wind turbine units rotating in a counter clockwise direction and the other half rotating clockwise. Therefore for each vertical axis wind turbine unit mounted on the structure I a generator 8, 24 is connected, access for maintenance to each stage of the generators can be achieved between the generator units as illustrated in figure 10. Additionally for installation the turbine units incorporating generator and rotor sections can be fitted in modular form on top of each other and constructed with appropriate methods for the structural load rating of * the structure. It can be possible to operate the multistage vertical axis wind turbine with all stages rotating in the same direction but this would require a structure designed to absorb harmonic motion and a large imbalance of force due to the torque application across the structure in a single direction.

A significant disadvantage of most vertical axis wind turbine designs is the low torque generated. However the blades 2 utilised on this design can be sufficiently long to generate significant torque output for operation of the generator 8, 24 via the drive system mounted within the structure 1. It is essential that the rotor incorporating the hub must be of lightweight construction. On the rotor main body section 12 this achieved by the rotor body being constructed to form a cage structure to reduce weight compared to a solid body rotor section. To improve the aerodynamic efficiency of the rotor body 12, it is clad in a lightweight cover 14, thereby the overall rotor assembly, 12, 5, 14, 13 is much lighter than had the main rotor body been solid. The individual wind turbine units may require assisted starting in low winds and a starter motor 16 can be located at each stage. The starter motor will drive one of the planetary gears 6 to initiate the rotational motion in the stage and bring the unit on line. Conventional starter motor design and engagement technology can be applied for this application as used in many large industrial applications. The starter motor 16 will disengage from the planetary gear 6 on a signal from a speed sensor monitoring the motion of the main rotor to indicate that it is rotating at a sufficient speed to maintain rotation independently. If the rotor is auto-rotating prior to a start demand signal the speed sensor will not permit engagement of the starter motor via the motor control logic. Wind speed input to the start sequence for all individual turbine starts will be via an anemometer device 25 mounted at the top of the structure. A wind direction vane is not required as no yaw control is utilised in a vertical axis wind turbine. For the variable speed/frequency electrical generator 24, starting can either be via a starter motor meshed to the drive gear 26 (not illustrated) or via reversed current directed from the grid or distribution network to initiate rotational motion and power generation.

Numerous braking system designs can be used from disc to electro braking methods dependent on suitability for an individual unit detail design. A braking mechanism is therefore not required to be illustrated here.

To impart maximum torque to the drive mechanism for a given wind speed the rotor blade 2 must be able to alter its pitch relative to the wind direction. The ability to self alter pitch within a limited travel allows the maximising of lift forces relative to drag forces across the blade. Pitch control on the wind facing side can be achieved by the trailing edge 17 rotating the blade along its axis in an upward direction by simply rotating on the blade root 21 acting as a pivot within the blade hub 18. To avoid an over speed event the upward movement of the trailing edge is limited by a stop 20 concentrically fitted around the blade root 21. This ensures that in high winds the torque and speed applied by the acting side of the turbine are limited by the braking motion of the blade with its leading edge facing the wind. The bearings located on the tower structure 11, 4, 10 have to absorb the varying thrust as the wind turbine rotates, however the action of the planetary gear system, 5, 6, 9 can also aid in damping the motion resultant from the rotor blade pitch variation and position relative to the prevailing wind during the rotational cycle causing imbalance in torque. The blade hub 18 is attached to bearing housings at either end that take up the load of the blade 2 and the blade root 21, while also allowing the blade to auto pitch within the limits in the mechanical stop 20. The mechanical stop 20 mates against the machined profile within the blade hub 18 thereby limit the pitch of the blade 2. The blade hub has to withstand a high degree of force generated by the pitch cycling and also has to be of lightweight construction therefore the blade hub 18 casting has to have void spaces 29 to reduce the weight that would otherwise penalise its performance.

The blade design will be such that the lift and downward force characteristic are such that when the leading edge 23 is facing wind the blade will be at or close to stall to maximise lift forces and result in reduced loads on the bearings. For extreme wind conditions the blade design can accommodate a safety factor in its flexibility or a number of safety devices and features can be accommodated in the blade design that exist in current horizontal axis wind turbine (HAWT) technology.

Other features relevant to this invention include the ability to ensure that the wind turbine system is weather proof examples being the weather protection covers on the rotor 27 and the ability to completely avoid exposure of any sensitive elements of the generator 8, 24 to weather by housing it completely within the main structure 1.

To optimise operation in a multiple structure I and turbine wind farm the units 30, 31 can be constructed to place the turbine rotors at different heights to each other as illustrated in figure 8. In addition to mounting the turbine rotors 12713,5 at different and complimentary heights on the multiple vertical axis wind turbine units 30, 31 they can be located so that the counter rotational effect of the rotors on the multiple vertical axis wind turbines 30,31 counter rotate to each other. The counter rotational effect and the aerodynamic design of the support structure 1 allows a square pitch spacing to be employed in the wind farm thereby maximising the harnessing of wind energy from the multiple vertical axis wind turbine units 30, 31 installed on the site.

Claims

Claims 1. A vertical axis wind turbine comprising a single cylindrical vertical structure supporting a plurality of vertical axis wind turbine rotors while housing a plurality of electrical power generators.
2. A vertical structure acting as the housing for multiple generators driven by wind turbine rotors with main drive gears of wind turbine rotor section and generator meshing through openings in the vertical structure wall.
3. A plurality of vertical axis wind turbine rotors incorporating annular ring gears within the rotor assembly driving gearing systems inside the structure connected to dedicated electrical power generators also housed within the support structure.
4. Rotor bearings and bearing housing located on the vertical structure allows for the vertical axis wind turbine (yAWl) rotor to rotate freely around the structure utilised as its static shaft.
5. An annular rotor hub of lightweight caged construction incorporating, drive gear, blade hubs housing and blade auto-pitching mechanism within blade hubs.
6. A pitch limitation mechanism within the blade hub that allows the blade to alter pitch relative to wind direction to apply alternate drag and lift forces to blades diametrically opposite to each other on the rotor hub.
7. A variable speed and frequency generator drive system with the generator mounted eccentrically within the housing structure to allow generator drive gear to mesh with wind turbine rotor gear via a single opening in the structure.Amendments to the Claims have been filed as follows Claims 1. A vertical axis wind turbine comprising a single walled cylindrical vertical structure acting as the static shaft for a plurality of vertical axis wind turbine rotors driving a plurality of electrical power generators housed inside the vertical structure and mounted eccentrically to the vertical centre line of the structure.2. A vertical axis wind turbine as defined in claim I wherein main drive gears of each wind turbine rotor section and generator mesh through a respective opening in the vertical structure wall.3. A vertical axis wind turbine as defined in claim I wherein each wind turbine rotor incorporates an annular ring gear which drives a single generator gear inside the structure, the generator gear being connected to the electrical power generator which is of the variable frequency type.4. A vertical axis wind turbine as defined in claim I wherein each wind turbine rotor incorporates an annular rotor hub of caged construction incorporating a drive gear and a blade hub which houses a blade auto-pitching mechanism.5. A vertical axis wind turbine as defined in claim 4 wherein each wind turbine rotor incorporates a pitch limitation mechanism within the blade hub that allows the blade to alter pitch relative to wind direction to apply : rs alternately predominant drag and lift forces to blades diametrically opposite to each other on the rotor hub.6. A vertical axis wind turbine as defined in claim I wherein a plurality of said * wind turbines can be sited in an array to allow paired counter rotation of rotors on adjacent turbines.7. A vertical axis wind turbine as defined in claim 6 where wind turbine rotors .* . can be mounted at different heights to each wind turbine rotor on an adjacent vertical axis wind turbine.