Self-aligned rotatable wind turbine support system

Abstract

A self-aligning rotatable wind turbine comprising a tower, a rotor nacelle assembly, and a rotatable base. The tower has a cross section in the shape of a teardrop truncated by a Kamm back; and is rig

Description

Background 24 08 23

[0001] Most of the current wind generation turbine uses horizontal axis turbine. The three basic elements of the prevailing support system for horizontal axis wind turbine including fixed base, cylindrical tower, and io active yaw control at the top of the tower have not been changed since its inception. This invention has presented a new and cheaper support system.

[0002] Energy generated by wind has seen rapid growth in the recent years from both onshore and offshore wind for its sustainability. However the 15 levelized cost of electricity (LCOE) is relatively high especially for offshore wind where foundation needs to be constructed using specialist equipment / operation for marine operations. The LCOE can be even higher for recent offshore wind development projects in deeper water and more hostile site than those in the previous years. 20

[0003] To reduce LCOE, larger turbines are created. While theoretically the increase in turbine size will lead to the reduction of the overall per megawatt balance of plant cost as the number of foundations is reduced, the dependence can be more complex especially when the choice for larger 25 turbine leads to more expensive form of foundation such as jacket instead of monopile for smaller turbine. There are several practical limitations in the current foundation technology in copying with the large load brought by large turbine. The cheapest form of foundation type, the monopile, is growing in size with ever larger turbine so that it is reaching the limits of 30 crane in installation and fabrication capacity. Therefore more expensive form of foundation such as jacket or more complex installation procedure must be used. With ever more challenging site conditions and deeper water depth the increased cost will impact on the feasibility of the development of 24 08 23 offshore wind and therefore hinder the development of sustainable energy generation.

[0004] The use of current prevailing support system to resist much larger 5 load demand can become a bottleneck due to both the availability of fabrication / installation technology and the high cost for offshore wind development. It is therefore not only beneficial to have a cost effective support system but urgently needed to overcome these bottlenecks. io Summary of The Invention

[0005] The present invention presents a cheaper support solution for horizontal axis wind turbine. This system utilises the wind itself to align 15 blades to face with wind by a combination of an inventive tower section design and rotatable base, therefore it can reduce loads onto the support structures substantially.

[0006] The invention design comprises of the following main elements: 20 1) Tower cross section, aerodynamically shaped to reduce wind load but simplified for easy fabrication and installation, and in the same time easy for structural optimization and weight reduction, 2) Rotatable base, which rotate from wind forces on the tower asymmetric section to align with wind direction with built in passive yaw control 25 3) The tower section has the ability of wind driven self propelled alignment of turbine blade to face wind due to its exterior shape and the non axial symmetric section and the rotatable base 4) New concept of yaw control. The active Yaw control system in conventional design is removed from the rotor nacelle assembly (RNA) 30 at the hub on the top of the tower and is replaced by a passive or semiactive yaw control system at the base of the tower

[0007] Notably, the new design adopted an unconventional design philosophy: 24 08 23 1) Almost 90% load reduction in the longitudinal direction of the tower cross section, 2) the wind force on the transverse direction is used for the benefit of restoring alignment between wind direction and the tower longitudinal 5 direction 3) Instead of a tower system fixed with the base, the tower is to turn with wind with a soft base; 4) Instead of strengthening the support structure for increasing load from large turbine, the key feature of the innovation is to reduce the load input io from all possible sources including a rotatable base, 5) Instead of designing the tower and support structure separately, the turbine, tower and support are designed as a whole.

[0008] This system can reduce the cost of foundation construction due to 15 load reduction and reduce operation cost due to passive yaw system at base, while increasing the power production efficiency for the generation of wind energy. It can be used for both future large turbine and current range of smaller turbine, and for both offshore and onshore, floating or fixed. It can also be used for repurposing existing offshore wind turbine 20 installations. Related Technology

[0009] The innovation in this patent uses a rotatable wind vaning system to 25 support horizontal axis wind turbine. Until now, almost 100% of the horizontal axis turbine has the rotor nacelle assembly with at the top of tower which is often referred as hub. There are variations of this arrangement but at least the following components are required: 1) Blades (mostly 3 blades) 30 2) Rotor 3) Generator 4) Controller 5) Pitch 6) Nacelle 24 08 23 7) Yaw drive / motor 8) Gearbox (optional)

[0010] A typical schematic representation with gear box can be shown in 5 figure 1. RNA can have the variation without gear box but all of them would have the active yaw control built in. The active yaw control system has the yaw drive rotating the nacelle on upwind turbines to keep them facing the wind when wind direction changes with wind direction sensors. The yaw motors power the yaw drive to make this happen. io

[0011] The various components of the wind turbine yaw systems normally include a means of rotatable connection between nacelle and tower (yaw bearing), a means of active variation of the rotor orientation (i.e. yaw drive), a means of restricting the rotation of the nacelle (yaw brake) and a control 15 system which processes the signals from wind direction sensors (e.g. wind vanes) and gives the proper commands to the actuating mechanisms.

[0012] Almost universally, the tower system to support the turbine RNA are made of cylindrical columns fixed to the support system for both onshore 20 and offshore wind farms. The thickness and diameters of the tower section can vary along the length based on load resistance / stiffness requirements.

[0013] Some methods have been proposed to change this arrangement including the following: 25 1) UK patent 10,194 (1909) wherein the tower is constructed to turn freely to the wind on two rollers or ball bearings, one being at the base of the girder framework and the other immediately under the wind turbine; with the annular ring securely anchored to the ground by means of steel guy ropes or rods; 30 2) UK patent 780,381 (1955) wherein a rotating tower with protruding fins is stabilized by 3 legs and the turbine; 3) US 20130156596, wherein a tower comprises a single airfoil symmetric about its major chord, or a pair of spaced apart asymmetric airfoils; 4) WO 2009 / 068521(2009) wherein the rotating turbine is supported at the middle point of the tower under an active yaw system by three or more legs, The rotating truss tower was not aerodynamically shaped; 24 08 23 The Invention 24 08 23 Tower Cross Section

[0014] The cylindrical shaped towers attract wind forces which then is 5 transferred to the foundation below. The wind force is predominantly drag force proportional to the so called drag coefficient. Tubular circles have drag coefficient between 0.6 and 1.2 depending the criticality regime defined by the Reynolds’ number. By changing the shape of the circular tube to a tera drop shape, this drag coefficient can be substantially reduced by as io much as 90%, as shown in Figure 2.

[0015] The drag for an airfoil section is from the combination of skin friction effect and wake effects. To reduce drag, airfoils are designed in the way that the air going over them will not separate so that the wake effects are 15 small. As seen in Figure 3, when the length over width ratio is 2.5, the drag coeffect Cd over a range of Reynolds number (from 106 to 107) is the smallest. Or the length over width = 3.9 for an airfoil appears to give the smallest Cd (0.06) when Re=105, for a teardrop section. 20

[0016] The Reynolds number for the diameter d=3m and wind speed from 5m / s to 50m / s, typically for offshore wind turbine, is in the range be 1.125x106 and 1.126x107, i.e., about the same range of from 106 to 107. Hence the approximate optimal ratio of the length over width is between 2.5 and 3.9 for a shape like elliptical or tear drop. 25

[0017] To simplify fabrication, Kamm back is introduced in the design as shown in Figure 4. The change of drag by Kamm back is small and can be represented by the formula related to the ratio of cut to overall length. (S F Hoerner). When the cut is about 30% of the length, the drag coefficient is 30 changed from 0.06 to 0.25, still significantly below the drag for a rounded body between 0.6 and 1.2. The cut is also necessary to avoid stress concentration and its associated fatigue issues.

[0018] The proposed shape of the cross section is in the figure 6 as part of the system in Figure 5. The shape can be composed several easily fabricatable elements: • one circular segments in the front, 5 • two plates or bent plates on the sides, • and one u shape like bent plates as Kamm back. 24 08 23 Wind Driven Self Alignment

[0019] The non-axial-sym metric cross section and the set out of the 5 rotational centre enables self-alignment when wind is not blowing to the front of the tear drop section. Figure 7 shows 7 Flow on the tower section when wind is blowing to the head. Figure 8 shows the air flow passing through the tower cross section and the wake when wind is blowing 45 degree from the head of the. The total combined wind force produces a io restoring moment to realign with wind direction. There can be alternative section with the same principle to reduce wind direct wind load. An alternative section is shown in figure 10. 24 08 23 15                            Passive Yaw control

[0020] The drawbacks of current yaw control system at the RNA include 1) adding mass and cost to the rotor nacelle assembly, 2) introducing gyroscopic forces and torsion to the support structure when applying turning 20 forces and 3) requiring high maintenance 4) The alignment with wind will also depend the system response of sensor and yaw control mechanics.

[0021] The self-alignment ability of the tower design in this invention somehow overcomes these drawbacks by removing the active yaw control 25 system. The yaw control system can be passive as the tower rotate with wind and adjust the orientation of the wind turbine blade with the wind direction.

[0022] The base is rotatable and supported by a bearing system to have 30 minimum resistance to circular movement while taking gravity / horizonal loads and bending moment. A plane view in is Figure 9. A reference bearing system can be found in the typical turret for floating production storage system for offshore oil and gas production. The rotatable base also contains semi-active yaw control, which is only needed for 29 10 24

[0022] The base is rotatable and is supported by a bearing system to have minimum resistance to circular movement while taking gravity / horizonal loads and bending moment. A reference bearing system can be found in 5 the typical turret for floating production storage system for offshore oil and gas production. The rotatable base also contains semi-active yaw control, which is only needed for: a) Stopping excessive rotation out of wind alignment b) Disentangling cables io c) Orienting turbine away from wind direction if required. Differentiations to Prior Art

[0023] Some alternative tower designs have been proposed such as: 15 1) UK patent 10,194 (1909) wherein the tower is constructed to turn freely to the wind on two rollers or ball bearings, one being at the base of the girder framework and the other immediately under the wind turbine; with the annular ring securely anchored to the ground by means of steel guy ropes or rods. 20 2) UK patent 780,381 (1955) wherein a rotating tower with protruding fins is stabilized by 3 legs and the turbine. 3) WO 2009 / 068521(2009) wherein the rotating turbine is supported at the middle point of the tower under an active yaw system by three or more legs. The rotating truss tower was not aerodynamically shaped. 25

[0024] This patent differs from prior art in several fundamental aspects: 1) In geometry, the tower section is shaped in a hybrid manner combining simplified tear drop at the front with one of the options being circular, and Kamm back. This is not only significantly different from the prior 30 inventions but also has the combined embodiments of wind load reduction, simple fabrication and easy installation, and facilitation for structural weight optimisation. 2) Benefiting from the hybrid cross section shape and the offset of the centre of rotation to centre of action, the tower has the ability to self-align List of Figures: 24 08 23 Figure 1: Current typical RNA with active yaw control (Credit: U.S. Department of Energy). 5 Figure 2: Comparison of drag coefficients between circular section and tear drop section (reference 1) Figure 3: Comparison of drag coefficients for different Re values io (reference 2) Figure 4: Increase of drag caused by Kamm Back (Ref 1, Hoemer) Figure 5: Elevation of Self-aligned Rotatable Wind Turbine Support 15 System Figure 6: Cross section of tower with Kamm tail (plus) characteristics Figure 7: Flow on the tower section when wind is blowing to the head 20 Figure 8: Restoring wind force with flow on the tower section when wind is blowing at 45 Degree from the head Figure 9: Plane view of the bottom of the tower 25 Figure 10: Alternative Tower Cross Section

Claims

I claim:29 10 241. A self-aligned rotatable wind turbine support system comprising of:1) A tower with cross section composed of an airfoil in tear drop like shape at the front and Kamm back at the back;2) A Rotor Nacelle Assembly system without yaw control, rigidly connected with the tower;3) A rotatable base with bearing supports with a passive yaw control system.

2. A system according to claim 1, The tower cross section airfoil can be simplified by four easily fabricated segments, one circular segment in the front, two plates or bent plates on the sides, and one u shape like bent plate as Kamm back.

3. A system according to claim 1, the rotational centre is purposefully set to be away from the combined wind force centre to enable the wind load on the side plate of the cross section exerting a restoring rotational moment to align the tower with the wind direction.

4. A system according to claim 2, for simplicity, the rotational centre can be set at the centre of the front circular section.

5. A system according to claim 2, for simplicity, the width of the Kamm back is set to be the radius of the front circular segment.

6. A system according to claim 1, the Kamm back flat piece can be rolled with bent ends to enable longitudinal welding away from high stress hot spots.

7. A system according to claim 2, provisionally the angle a=18.5 deg.

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