Method for retrofitting a wind turbine with an energy generating unit

Abstract

A method for retrofitting a wind turbine having a tower and a first energy generation unit with a second energy generation unit. The method includes: analyzing a first natural frequency of the tower relative to a first rated operating frequency of the tower having the second energy generation unit; when the first natural frequency is within the first rated operating frequency, modifying one or both of the tower and the second energy generation unit such that the modified one or both of the tower and the second energy generation unit have non-overlapping second natural frequencies and second rated operating frequencies; and replacing the first energy generation unit with the second energy generation unit.

Description

Technical Field

[0001] The present invention generally relates to wind turbines and, more particularly, to a method of retrofitting a wind turbine having a tower and a first energy generation unit with a second improved energy generation unit and operating the retrofitted wind turbine in a manner that extends the expected life of the tower. Background Art

[0002] Wind turbines are used to generate electrical energy using renewable resources and without burning fossil fuels. Generally, a wind turbine converts kinetic energy from the wind into electricity. A horizontal axis wind turbine includes a tower and an energy generation unit positioned at the top of the tower. The energy generation unit typically includes a nacelle for housing mechanical and electrical components (such as a generator) and a rotor operatively coupled to components in the nacelle via a main shaft extending from the nacelle. The rotor in turn includes a central hub and a plurality of blades that radially extend from the central hub and are configured to interact with the wind to cause rotation of the rotor. The rotor is supported on the main shaft that is directly or indirectly operatively coupled to a generator housed within the nacelle. Thus, when the wind forces the blades to rotate, the generator generates electrical energy.

[0003] Generally, over the life of a wind turbine, which can span several decades, new technologies become available. As an example, a new generation of power generation turbines with higher power production at a lower price (Levelized Cost of Energy - LCOE) becomes available. At some point, it may become economical to replace an old generation of power generation turbines with a new generation of power generation turbines before the end of the life of some components of the wind turbine. This is typically done by replacing the entire wind turbine, including the base. However, this is quite expensive, and thus the wind turbine industry is looking for solutions to improve the business case for retrofitting with a new generation of power generation turbines. Summary of the Invention

[0004] For these and other purposes, a method of retrofitting a wind turbine is provided. The wind turbine has a tower and a first energy generation unit. During the retrofit process, the first energy generation unit is replaced with a second energy generation unit. The method includes: analyzing a first natural frequency of the tower relative to a first rated operating frequency of the tower with the second energy generation unit; when the first natural frequency is within the first rated operating frequency, modifying one or both of the tower and the second energy generation unit such that the modified one or both of the tower and the second energy generation unit have non - overlapping second natural frequencies and second rated operating frequencies; and replacing the first energy generation unit with the second energy generation unit.

[0005] In one embodiment, the step of modifying one or both of the tower and the second energy generating unit includes increasing the height of the tower from a first tower height to a second tower height, the height difference between the first tower height and the second tower height being sufficient to reduce a first natural frequency to a second natural frequency, and the second natural frequency not being within a second rated operating frequency. As an example, the step of modifying the tower may include adding an adapter to the tower to increase the first tower height to the second tower height.

[0006] In one embodiment, the step of modifying one or both of the tower and the second energy generating unit includes decreasing the height of the tower from a first tower height to a second tower height, the height difference between the first tower height and the second tower height being sufficient to reduce a first natural frequency to a second natural frequency, and the second natural frequency not being within a second rated operating frequency.

[0007] In one embodiment, the tower has at least a first section and a second section, and the step of adding an adapter to the tower includes positioning the adapter between the first section and the second section. In this embodiment, the adapter may be generally cylindrical. In an alternative embodiment, the step of adding an adapter to the tower includes positioning the adapter at the top of the tower, and the step of replacing the first energy generating unit with the second energy generating unit includes coupling the second energy generating unit to the adapter. In this embodiment, the adapter may be generally conical.

[0008] In addition, in one embodiment, the tower has at least a first section and a second section, and the step of adding an adapter to the tower includes removing the first energy generating unit and the first section, coupling the adapter to one of the first section and the second section, and coupling the other of the first section and the second section to the adapter, with the adapter positioned between the first section and the second section.

[0009] In yet another embodiment, the tower is coupled to a base and the step of adding an adapter to the tower includes positioning the adapter between the tower and the base and the step of replacing the first energy generating unit with the second energy generating unit includes coupling the second energy generating unit to the tower. In this embodiment, the adapter may be generally cylindrical or conical.

[0010] In one embodiment, the step of modifying one or both of the tower and the second energy generating unit includes restricting the operating parameters of the second energy generating unit to change the limit of the second rated operating frequency to be lower than or higher than the second natural frequency. More specifically, the step of modifying one or both of the tower and the second energy generating unit includes: restricting the operating parameters of the second energy generating unit to increase the lower limit of the second rated operating frequency to a frequency higher than the second natural frequency. As an example, the step of changing the operation of the wind turbine to change the rated operating frequency may include pitching one or more blades on the second energy generating unit. Optionally, the step of changing the operation of the wind turbine to change the rated operating frequency of the tower may include restricting the speed of the rotor on the second energy generating unit.

[0011] In another embodiment, a method of retrofitting a wind turbine having a first energy generating unit with a second energy generating unit, wherein the wind turbine includes a modular tower having at least a first section and a second section, the method includes: inserting an adapter between the first section and the second section to increase the height of the tower; and coupling the second energy generating unit to one of the first section and the second section.

[0012] In yet another embodiment, a wind turbine includes a modular tower having at least two conical sections configured to be coupled to a cylindrical section, wherein the cylindrical section is configured to be located between the at least two conical sections. The wind turbine may further include a second cylindrical section configured to be coupled between one of the conical sections and the base. The energy generating unit is configured to be coupled to the other conical section. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The drawings incorporated in and forming a part of this specification illustrate one or more embodiments of the invention and, together with the general description of the invention given above and the detailed description given below, serve to explain the invention.

[0014] Figures 1A to 1B is a perspective view of a wind turbine having a tower and a first energy generating unit;

[0015] Figure 2 is a wind turbine having Figures 1A to 1B a tower and retrofitted with a second energy generating unit; Figure 3A is Figures 1A to 1B a schematic diagram of the natural frequency and rated operating frequency of the wind turbine shown in

[0016] Figure 3BSchematic diagram of the natural frequency of the existing tower and the rated operating frequency after modification according to an embodiment of the present invention;

[0017] Figure 3C Schematic diagram of the natural frequency of the existing tower and the natural frequency of the tower after modification according to an embodiment of the present invention;

[0018] Figure 4 Shows an exemplary system for controlling an energy generation unit;

[0019] Figure 5A 、 Figure 5B and Figure 5C Shows a modification process according to an embodiment of the present invention;

[0020] Figure 6A 、 Figure 6B 、 Figure 6C and Figure 6D Shows a modification process according to an embodiment of the present invention; and

[0021] Figure 7 Shows a base adapter according to an embodiment of the present invention. Detailed Description

[0022] Refer to Figures 1A to 1B, the wind turbine 10 includes a modular tower 12 and an energy generating unit 14 disposed at the top of the tower 12. As is conventional, the modular tower 12 may be coupled to a base 16 at its lower end. Exemplary modular tower 12 includes three segments 12a, 12b, and 12c, which together define a generally vertical tower axis 18 about which the energy generating unit 14 may be rotated by a yaw mechanism (not shown). The base 16 may be a relatively large mass, e.g., concrete, steel, etc., embedded in the ground and the forces on the wind turbine 10 may ultimately be transmitted through the base. Although not shown, in alternative embodiments, the base 16 may include an offshore platform or the like for offshore wind turbine applications. The energy generating unit 14 includes the portion of the wind turbine that converts the energy of the wind into electrical energy. To this end, the energy generating unit 14 generally includes a housing or nacelle 20, a rotor 22 having a central hub 24 and one or more blades 26 (e.g., three blades) mounted to and radially extending from the central hub 24, and a generator (not shown) for converting mechanical energy into electrical energy. In one embodiment, the energy generating unit 14 may further include a drive train (not shown) that includes gearing interconnecting the rotor 22 and the generator. Most of the generator and the drive train may be located inside the nacelle 20 of the wind turbine 10. In addition to the generator, the nacelle 20 generally houses the various components required to convert wind energy into electrical energy as well as the various components required to operate, control, and optimize the performance of the wind turbine 10. The wind turbine blades 26 are configured to interact with the free stream air flow (wind 62) to generate a lift force that causes the rotor 22 to rotate or revolve generally within the plane defined by the wind turbine blades 26. Thus, the energy generating unit 14 is capable of generating power from the air flow passing through the swept area of the rotor 22. The energy generating unit 14 is attached to the tower 12 at a top flange 28. The tower 12 supports the loads provided by the energy generating unit 14 and operates to raise the energy generating unit 14 (particularly the rotor 22) to a height above the ground plane or sea level where there is generally a faster moving air flow with less turbulence.

[0023] At some point in the life of a wind turbine, an existing wind turbine may be replaced. During a retrofit process, the existing energy generating unit may be replaced with another improved energy generating unit. This is shown as an example in Figure 2 In the modified wind turbine 40 should be designed to work in a manner that extends the service life of the tower 12 at least to and preferably beyond the expected life design value of the tower 12.

[0024] To this end, the inventors recognized the problem that must be analyzed before installing the second energy generation unit 42 on the tower 12. This problem relates to considering the natural frequency of the wind turbine with respect to the range of the rated operating frequencies of the wind turbine 40 having the new energy generation unit 42 after the following retrofit process. The rated operating frequency is the vibration frequency generated during the optimal power generation of the wind turbine. In the case where the new energy generation unit has been installed on the existing tower, during the normal operation of the retrofitted wind turbine, the new energy generation unit must not generate a rated operating frequency that overlaps with the natural vibration frequency of the new energy generation unit on the wind turbine tower. The natural frequency of the wind turbine is the frequency at which the wind turbine tower oscillates without any driving or damping forces.

[0025] Generally and with reference to Figures 1A to 1B , as the rotational speed of the rotor 22 increases, the vibration frequency on the wind turbine tower 12 increases. The vibration on the wind turbine tower 12 may be caused by the periodic movement of each blade 26 as it sweeps past the tower 12. At startup, the rotational frequency of the rotor 22 increases from the stationary position. The rotational speed of the rotor 22 increases until it reaches the optimal rotational speed at which the power output is optimized. This occurs at a predetermined wind speed determined by the rated speed of the rotor 22. The output power of the wind turbine 10 is maintained relatively constant under variable wind conditions by controlling the pitch of the blades 26. Thus, as the wind speed changes, the blades 26 can pitch to maintain the optimal rotational speed. Despite the presence of controls (e.g., blade pitch controls) designed to optimize power generation, there are still some variations in the rotational frequency of the rotor 22 during the optimal power generation process. That is, during the rated operation, there are some variations in the rated operating frequency on the wind turbine tower 12. This variation results in a range of rated operating frequencies. As mentioned herein, during startup or shutdown, the rated operating frequency does not include the vibration frequency on the wind turbine tower 12. Each of these can result in a vibration frequency that coincides with the natural frequency of the wind turbine tower. However, the coincidence time is short because the rotational speed of the rotor 22 increases towards the optimal rotational speed at which the rated operating frequency is generated, or the rotational speed decreases from the optimal rotational speed towards the stationary position.

[0026] As an example and with reference to Figures 1A to 1B and Figure 3A the exemplary wind turbine 10, the wind 62 moves the nacelle 20. That is, the wind 62 causes the tower 12 to flex (in the direction of arrow 62), thereby displacing the nacelle 20 from the axis 18. In addition to the periodic displacement motion, the wind causes the rotor 22 to rotate. The rotation of the rotor generates periodic vibrations that are transmitted throughout the wind turbine 10 (including the tower 12). Considering the wind turbine 10, the tower 12 has a natural frequency 64 ( Figure 3A)。If the tower 12 is subjected to an external periodic force (e.g., wind 62 and / or rotor rotation) near its natural frequency 64, the tower 12 will resonate. For this reason, as Figure 3A shown, the wind turbine 10 is designed such that the natural frequency 64 is outside the rated operating frequency 66 of the wind turbine 10. These rated operating frequencies 66 are generated by the rotation of the rotor 22 during optimal power generation, as described above. As shown, there is no overlap between the range of the natural frequency 64 and the rated operating frequency 66. The tower 12 is designed to resonate at a frequency that does not typically occur during optimal power generation of the wind turbine 10. This design avoids excessive vibrations that could cause the tower 12 to degrade due to resonance during operation. Thereby, the lifespan of the wind turbine tower 12 can be extended.

[0027] Although Figure 3A the rated operating frequencies 66 shown in span a frequency range above 0 Hz and above the natural frequency 64, it should be understood that these frequencies are not the only frequencies generated by the wind turbine 10 during operation. As described above, for example, before startup, the rotor 22 is stationary. During startup, the rotational speed of the rotor 22 increases, and as the rotor 22 rotates towards the rotational speed for optimal power generation, the vibration frequencies experienced by the wind turbine 10 can overlap with the natural frequency 64. This typically occurs temporarily because the rotor rotational speed continuously increases before reaching the optimal rotational speed at which the wind turbine tower 12 experiences the rated operating frequency 66. Similarly, during shutdown, as the rotational speed of the rotor 22 slows down, the rotational speed of the rotor 22 will generate vibration frequencies that temporarily overlap with the natural frequency 64. In this regard, the rated operating frequency 66 represents the rated operating range of the vibration frequencies experienced by the wind turbine 10 during optimal power generation. The rated operating frequency excludes those frequencies observed during startup and shutdown of the wind turbine 10.

[0028] When retrofitting the wind turbine 10 with the second energy generation unit 42, as described below, the goal is to maximize power generation until the expected lifespan of the second energy generation unit 42 and / or the tower 12 is reached (as described above, preferably they reach the end of their service life together). In this regard, the goal is to extend the lifespan of the tower 12 as long as possible. For this reason, the natural frequency 64 is considered in relation to the prediction of the rated operating frequency of the tower with the second energy generation unit 42. Based on the results of this analysis, various scenarios can address the overlap between the rated operating frequency and the natural frequency of the tower with the second energy generation unit 42.

[0029] For this reason, as Figure 3BAs shown, when the tower 12 is retrofitted using the second energy generating unit 42, the new wind turbine 40 may have a new rated operating frequency 68. The new rated operating frequency 68 can pose problems regarding the natural frequency 64 of the tower 12. The natural frequency 64 of the tower 12 mainly depends on the stiffness and mass of the tower 12. Although not limited by theory, it is believed that replacing the energy generating unit 14 with the second energy generating unit 42 has a negligible effect on the natural frequency 64 of the tower 12 (i.e., the natural frequency does not change significantly), but due to the improved efficiency provided by the second energy generating unit 42, the second energy generating unit 42 can cause a significant change in the rated operating frequency 68 relative to the rated operating frequency 66. However, it should be noted that if the new energy generating unit has a significantly different mass from the existing energy generating unit, a change in the natural frequency can occur.

[0030] As an example and continuing to refer to Figure 3B , without modifying the tower 12, the new rated operating frequency 68 of the wind turbine 40 with the new second energy generating unit 42 may include the natural frequency 64 of the tower 12. That is, an analysis of the operation of the wind turbine 40 via modeling or other mathematical means can reveal that at certain wind speeds, one or more rated operating frequencies will be close to or even coincide with the natural frequency 64 of the tower 12. If the life of the tower 12 is to be maintained or extended, an overlap between the rated operating frequency 68 and the natural frequency 64 needs to be avoided.

[0031] To this end, the present inventors have developed a solution to address the overlap of the rated operating frequency 68 and the natural frequency 64 of the tower 12 during the retrofit process. Generally, the solution involves changing the natural frequency 64 of the tower 12 by modifying the tower 12 and / or involves changing the rated operating frequency 68 of the retrofitted wind turbine 40. By changing either the natural frequency or the rated operating frequency, the overlap between the two is avoided.

[0032] One way to change the rated operating frequency 68 is to control the operation of the retrofitted wind turbine 40. As described above, the tower life rate depends on the vibration frequency applied to the tower 12 during the operation of the retrofitted wind turbine 40. The vibration frequency applied to the tower 12 can be controlled to some extent by the operation of the wind turbine 40. Referring to Figure 3B, once the rated operating frequencies 68 are known or determined, they can be compared with the natural frequencies 64 of the tower 12 (such as in the memory of the controller; see below). Using this information, any overlap between the rated operating frequencies 68 and the natural frequencies 64 of the tower 12 can be determined. The operation of the wind turbine 40 can be controlled to change the rated operating frequencies 68 to a frequency different from (e.g., lower or higher than) the natural frequencies 64 of the tower 12. As an example, the operating frequency can be changed by changing the speed of the rotor 22.

[0033] In an exemplary embodiment, and continuing to refer to Figure 3B and Figure 4 , the exemplary system 50 can monitor the vibration frequencies acting on the tower 12, including the rated operating frequencies of the wind turbine 40. In this regard, the system 50 includes a central controller 52 and one or more sensors 54 that are operatively coupled to the wind turbine 40 and are configured to directly or indirectly indicate the vibration of the tower 12. As an example, in one embodiment, the sensors 54 can be directly coupled to the tower 12. However, in alternative embodiments, the sensors 54 can be coupled to another part of the wind turbine 40 but are configured to measure parameters related to the vibration acting on the tower 12 (e.g., an accelerometer located in the nacelle 20). The central controller 52 can be the main controller for the wind turbine 40 or can be a separate controller that is operatively coupled to the main controller of the wind turbine 40.

[0034] As shown, the controller 52 can be operatively coupled to a pitch mechanism 56 that is capable of pitching one or more blades 26 on the second energy generation unit 42. By pitching the blades 26 in an appropriate manner, wind energy can be harvested (i.e., captured by the wind turbine) to maintain the rotational speed of the rotor 22. This can establish a lower limit 70 of the rated operating frequency ( Figure 3B ), to narrow the rated operating frequency 68 to a new rated operating frequency 72. In this case, the lower limit of the rated operating frequency 68 increases in the direction of arrow 74 such that the lower limit 70 of the rated operating frequency 72 is greater than the natural frequency 64 of the tower 12. In this way, an overlap between these rated operating frequencies 72 and the natural frequency 64 of the tower 12 is avoided. As shown, the controller 52 can be operatively coupled to a drag mechanism 58 (e.g., a generator) that provides drag to the rotation of the rotor 22 and thus controls the speed at which the rotor 22 rotates.

[0035] either alone or in combination with Figure 4Another solution used in conjunction with the system 50 shown is to change the mass or stiffness of the tower 12 during the retrofit process. That is, during the retrofit to install the second energy generation unit 42, one or both of the mass or stiffness of the tower 12 are changed. This structural change to the tower 12 causes the natural frequency to shift away from the rated operating frequency of the wind turbine 40.

[0036] In Figure 3C , the natural frequency 64 shifts away from the rated operating frequency 68. Specifically, the natural frequency 64 is directly proportional to the stiffness of the tower 12 and inversely proportional to the mass of the tower 12. According to one embodiment of the present invention, the natural frequency 64 of the tower 12 is shifted outside the rated operating frequency 68 of the retrofitted wind turbine 40 by changing one or both of the stiffness and mass. Stiffness is the resistance to deformation. By way of example only, changing the stiffness of the tower 12 can be achieved by changing the height of the tower 12. Increasing the height reduces the stiffness and thus reduces the natural frequency of the tower 12. Also, decreasing the height of the tower 12 increases the stiffness and increases the natural frequency.

[0037] As Figure 3C shown in the exemplary embodiment of, by increasing the height of the tower 12, the natural frequency 64 decreases relative to the rated operating frequency 68 of the wind turbine 10. The modified tower 82 is shown in Figures 5A to 6D and described below. That is, increasing the height reduces the stiffness and the natural frequency decreases. If the height is increased enough, the new natural frequency 76 of the modified tower is less than the rated operating frequency 68 after retrofit with the second energy generation unit 42. This is schematically shown by the arrow 78 in Figure 3C . Advantageously, a taller tower may be beneficial for other reasons.

[0038] By positioning the second energy generation unit 42 at a higher location in an atmosphere with faster air flow and less turbulence, it is believed that the annual energy production (AEP) of the wind turbine will increase due to the increased height. The increased height of the tower 12 increases the bending moment acting on the modified tower 82 (e.g., considering a cantilever beam with a large load at the end) and thus reduces the life of the tower 12. Nevertheless, depending on the specific application, it may be possible to increase the height of the tower 12 and operate the second energy generation unit 42 at a rated power curve such that the modified wind turbine tower and the second energy generation unit 42 reach the end of their service life simultaneously.

[0039] Figures 5A to 5C shows an example of combining the height modification of the tower 12 with the retrofit including the second energy generation unit 42. In the figure, the height H1 of the tower 12 ( Figure 5A ) is increased to the new height H2 (Figure 5C ), as a result, the second energy generating unit 42 is at a new height H2. Once retrofitted, the modified wind turbine tower 82 ( Figure 5C ) corresponds to Figure 3C the natural frequency 76 and the rated operating frequency 68 shown in Figures 1A to 1B . That is, the tower 12 having a natural frequency of 64 shown in Figure 3C is modified during the retrofit process. As a result of the modification, the modified wind turbine tower 82 has a height H2 and has a natural frequency of 76. As shown in

[0040] , this natural frequency is lower than the rated operating frequency 68 of the modified tower 82 having the second energy generating unit 42. Figure 5A For this purpose, in an exemplary retrofit, the tower 12 shown in Figure 5A is constructed in sections. In the example shown, the tower 12 includes three sections 12a, 12b, and 12c that jointly define a height H1. Embodiments of the present invention are not limited to three sections, as towers having two or more sections can be retrofitted in the manner described herein. Referring to

[0041] In Figure 5B and Figure 5C , the height H1 of the tower 12 is increased by adding a tower transition adapter 86 at the top 84 of the tower section 12c. That is, the height H2 of the modified tower 82 (shown in Figure 5C ) is equal to the height H1 plus the length of the tower transition adapter 86. In addition to increasing the height relative to the tower 12, the tower transition adapter 86 can also provide a geometric shape match between the top 96 of the modified tower 82 and the second energy generating unit 42. As an example, the second energy generating unit 42 may be from a manufacturer different from that of the tower 12, such that there is a geometric shape mismatch between the tower 12 and the second energy generating unit 42. The tower transition adapter 86 can increase the height of the tower 12 while also compensating for changes in the geometry between the first energy generating unit 14 and the second energy generating unit 42.

[0042] In this regard and referring to Figure 5B, the tower transition adapter 86 includes a first end 92 having a first interface 94 sized to engage an interface 90 on the top end 84 of the tower segment 12c. The tower transition adapter 86 further includes a second end 96 having a second interface 98 sized to engage an interface 100 on the second energy generating unit 42. The tower transition adapter 86 is coupled to the top end 84 of the tower segment 12c. More specifically, the first interface 94 at the first end 92 of the tower transition adapter 86 can be coupled to the interface 90 at the top end 84 of the tower segment 12c, for example, by welding or a flange connection. In an exemplary embodiment, the interfaces 94, 98 of the tower transition adapter 86 can include flanges (e.g., annular flanges). The interfaces 90, 100 on the tower segment 12c and the second energy generating unit 42 can also include flanges, respectively. Fasteners (such as nuts / bolts) can be used to couple the corresponding flanges together, as is commonly known in the art. In an alternative embodiment, these flanges can be omitted, and the tower transition adapter 86 can be coupled to the tower segment 12c, for example, by welding.

[0043] In an exemplary embodiment, to address the potential mismatch between the modified tower 82 and the second energy generating unit 42, the sizes of the interfaces 94 and 98 can be different from each other. More specifically, the diameters of the interfaces 94, 98 can be different. In one embodiment, for example, the diameter of the first interface 94 can be about 3 meters, and the diameter of the second interface 98 can be about 4 meters, and vice versa. In this regard, the tower transition adapter 86 can have a conical configuration to address the dimensional differences. However, depending on the specific application, other sizes and shapes are possible. Additionally, the length of the tower transition adapter 86 can be varied to position the second energy generating unit 42 at a desired height H2. As an example, the length of the tower transition adapter 86 can be between about 2 meters and about 30 meters. Additionally, depending on the specific application and the desired natural frequency 76 of the modified tower 82 relative to the natural frequency 64 of the tower 12 (shown in Figure 3C ), other lengths are possible.

[0044] Referring to Figure 5C , in the case where the tower transition adapter 86 is attached to the tower segment 12c, the height of the modified tower 82 is H2. The height H2 of the modified tower 82 is greater than the height H1 of the tower 12. This causes the natural frequency of the modified tower 82 to be reduced relative to the tower 12. Next, the second energy generating unit 42 can be coupled to the tower transition adapter 86. More specifically, the interface 100 of the second energy generating unit 42 can be coupled to the second interface 98 at the second end 96 of the tower transition adapter 86, for example, by welding or a flange connection, to complete the retrofit process.

[0045] It should be appreciated that in an alternative embodiment, the second energy generating unit 42 may be coupled to the second end 96 of the tower transition adapter 86, which assembly is then subsequently coupled to the top 84 of the tower section 12c. In any event, after the retrofit process, the retrofitted wind turbine 40 may be returned to service and operation. The retrofitted wind turbine 40 should preferably be operational until the service life of the modified tower 82 and the second energy generating unit 42 has expired. By extending the service life of the wind turbine during the retrofit process, additional time can be provided to the wind turbine operator within which to recoup their investment in the wind turbine.

[0046] In many areas in which wind turbines are located, there may be local or regional laws, regulations, ordinances, etc. that limit the height to which a structure (such as a wind turbine) can extend above the ground. These may exist, for example, as a safety precaution to avoid aviation accidents or for other safety considerations. In any event, when increasing the overall height of the retrofitted wind turbine 40, the wind turbine operator may want to verify the tip height of one of the blades 26 of the second energy generating unit 42 when the blade is in the twelve o'clock position (i.e., at the maximum height of the wind turbine 40) to ensure that the retrofitted wind turbine 40 meets applicable standards. If the tip height of the blade 26 exceeds a predetermined threshold determined by the laws, regulations, ordinances, etc. of the locale in which the retrofitted wind turbine 40 is located, then it may be necessary to reduce the height of the tower transition adapter 86 (or possibly even the height of the tower 12, see below) in order to comply with applicable laws, regulations, ordinances, etc.

[0047] As described above, the tower transition adapter 86 is added to the top 84 of the tower 12 such that substantially the entire tower 12 is "reused" in the retrofitted wind turbine 40. This represents an efficient use of existing structure during the retrofit process. However, the present invention is not limited to this embodiment. In this regard and in alternative embodiments, a portion of the original tower 12 may be removed to create a new tower interface at which the tower transition adapter 86 may be coupled to vary the overall height of the modified tower 82.

[0048] As Figures 6A to 6D shown, in one embodiment, the height H1 of the tower 12 may be increased by inserting an adapter at a location other than the top 84 of the tower 12. In this regard, any two of the tower sections 12a, 12b, and 12c of the tower 12 are separable, and the tower transition adapter 110 may be coupled between these tower sections. As an example and as Figure 6A and Figure 6BAs shown, the tower transition adapter 110 is inserted between tower segments 12b and 12c. Although not shown, the tower transition adapter can be inserted between segments 12a and 12b or between segment 12a and the base 16, as described in connection with Figure 7 as described.

[0049] To this end, and with reference to Figure 6A and Figure 6B , the energy generation unit 14 and tower segment 12c can be removed. This leaves the remaining tower segments 12a and 12b in place. As shown, the top end 112 of tower segment 12b forms an interface 114 to receive the tower transition adapter 110.

[0050] Next, and with reference to Figure 6B , the tower transition adapter 110 has a first end 116 that defines a first interface 120 for coupling to tower segment 12b and has a second end 122 that defines a second interface 124. The first interface 120 at the first end 116 of the tower transition adapter 110 couples to the interface 114 of tower segment 12b. In the exemplary embodiment shown, the tower transition adapter 110 is cylindrical. As an example, each of the interfaces 120 and 124 defines a circle with the same diameter. Advantageously, the construction of the tower transition adapter 110 is simplified because the dimensions of each end 116 and 122 do not vary with the length of the tower transition adapter 110. The tower transition adapter 110 can be coupled to the interface 114 via a flange connection. Alternatively, the tower transition adapter 110 can be welded to the interface 114.

[0051] Once the tower transition adapter 110 is fixed to tower segment 12b, tower segment 12c is fixed to the tower transition adapter 110. In this regard and with reference to Figure 6C and Figure 6D , segment 12c includes a first end 126 that defines a first interface 130 and includes a second end 132 that defines a second interface 134. The second interface 134 can be a top flange 28 (in Figures 1A to 1B ). The first interface 130 at the first end 126 of tower segment 12c couples to the interface 124 at the second end 122 of the tower transition adapter 110. As an example, the tower transition adapter 110 can be coupled to the first end 126 via a flange connection. Alternatively, the tower transition adapter 110 can be welded to the first end 126. The length of the tower transition adapter 110 can be varied to position the second energy generation unit 42 at a desired height H3, where H3 is greater than H1 by the length of the tower transition adapter 110. As an example, the length of the tower transition adapter 110 can be between about 2 meters and about 30 meters. Additionally, depending on the particular application and the desired natural frequency 76 of the modified tower 82 relative to the natural frequency 64 of the tower 12 (inFigure 3C As shown (not shown), other lengths are possible. Although not shown, the tower transition adapter 110 may include an internal platform and ladder that transition between the internal ladders of adjacent segments 12b and 12c.

[0052] Referring Figure 6D , the second energy generation unit 42 may then be coupled to the tower segment 12c. More specifically, the interface 100 of the second energy generation unit 42 may be coupled to the second interface 134 at the second end 132 of the tower segment 12c. It should be appreciated that in an alternative embodiment, the second energy generation unit 42 may be coupled to the second end 132 of the tower segment 12c, and then the assembly may subsequently be coupled to the tower transition adapter 110. Advantageously, in the case where a wind turbine operator retrofits the first energy generation unit 14 with a second energy generation unit 42 manufactured by the same manufacturer, it may not be necessary to address a mismatch between the modified tower 82 (i.e., tower segment 12c) and the second energy generation unit 42. The interfaces 100 and 134 may be the same size. More specifically, the diameters of the interfaces 100 and 134 may be the same. Inserting the tower transition adapter 110 between any two tower segments 12a, 12b, and 12c eliminates the need for a transition between the modified tower 82 and the second energy generation unit 42.

[0053] In one embodiment, for example, the diameter of the interface 100 may be approximately 3 meters and the diameter of the second interface 134 may be approximately 3 meters. However, other sizes are possible depending on the specific application. It is possible that the addition of the tower transition adapter 110 between tower segments 12a and 12b or 12b and 12c avoids issues associated with the design and construction of an adapter that can receive the second energy generation unit 42 at the interface of the adapter. Constructing a cylindrical adapter such as the tower transition adapter 110 is relatively easier and less expensive. As an example, the tower transition adapter 110 may be constructed with upper and lower L-shaped flanges at the interfaces 130, 134 that are coupled to each of the tower segments 12b and 12c (or coupled between segments 12a and 12b), while an adapter to be directly coupled to the second energy generation unit 42 is more difficult to design and more costly to construct.

[0054] In this embodiment and similar to the above, it should be appreciated that the new height of the modified tower 82 of the retrofitted wind turbine 40 may be greater than or less than the original height of the tower 12, depending on the desired variation of the natural frequency relative to the expected rated operating frequency of the wind turbine 40. The tip height of the blade 26 may also be checked to confirm that the height of the retrofitted wind turbine 40 is within applicable standards.

[0055] Figure 6D and Figure 7Another embodiment of the present invention is shown. As a supplement or alternative to the tower transition adapters 86, 110, an adapter may be inserted between the tower section 12 and the base 16. As Figure 6D shown, the increased height H3 relative to H1 may be due in whole or in part to the base adapter 156. In addition to modifying the natural frequency of the tower 12 as shown, for example, in Figure 3C , the base adapter 156 may solve other problems in the industry.

[0056] As an example and with reference to Figure 7 , one problem in the industry is that there are an excessive number of unused wind turbine bases 150. The base 150 is composed of reinforced concrete 152. A plurality of anchor bolts 154 are embedded in the concrete 152. These anchor bolts are positioned in the concrete before the concrete sets and are therefore non-removable without damaging the base 150. Although not shown, the anchor bolts 154 may be arranged in a ring of a predetermined size and are adapted to receive a specific tower design. Generally, the size and dimensions of the layout of the anchor bolts 154 are manufacturer-specific and may be customized for the site. This presents a particular problem since, due to design variations between wind turbine towers, one turbine manufacturer may not typically use the base of another turbine manufacturer. Thus, once the base 150 has been installed, if it is not used to secure a wind turbine dedicated to that base, the base will be unavailable.

[0057] In one embodiment of the present invention, the base adapter 156 is coupled between the tower section 12a and the base 150, where the dimensions and bolt arrangement between the tower section 12a and the anchor bolts 154 do not match. In this regard, the base adapter 156 includes a housing 160 defining a sidewall 162. The sidewall 162 may be in the form of a cylinder that matches the dimensions of the tower section 12a. At one end 164 of the sidewall 162, an L-shaped flange 166 extends inwardly, with a plurality of through-holes 170 that are spaced apart to each receive an anchor bolt 154. As shown, nuts are screwed onto the anchor bolts 154 and secure the adapter 156 to the base 150. Opposite the end 164, at the end 172, a T-shaped flange 178 defines a plurality of internal through-holes 174 and a plurality of external through-holes 176. The holes 174 and 176 are aligned with corresponding holes 180 in the tower section 12a. The aligned holes 174 and 180 and 176 and 180 receive bolts or other fasteners 184 such that the tower section 12a can be secured to the adapter 156. The holes 174 in the T-shaped flange 178 are not aligned with the holes 170 in the L-shaped flange 166. In this way, the base adapter 156 allows one manufacturer to utilize the unused base of another manufacturer and advantageously allows a wind turbine to be installed on a base that would otherwise be unavailable. The adapter 156 also raises the height of the second energy generating unit 42 and according toFigures 5A to 5C and Figures 6A to 6D the embodiment shown in modifies the natural frequency of the modified tower 82. Although Figure 7 an adapter having a lower L-shaped flange and an upper T-shaped flange is shown, embodiments of the present invention are not limited to the construction shown. By way of example, the base adapter may include a lower L-shaped flange and an upper L-shaped flange, a lower T-shaped flange and an upper L-shaped flange, or a lower T-shaped flange and an upper T-shaped flange. Additionally, although a cylindrical adapter has been described, the base adapter 156 is not limited to being cylindrical (e.g., circular). In this regard, the base adapter 156 may have a reverse conical configuration that transitions between different overall dimensional differences between the base 150 and the tower segment 12a.

[0058] Although the present invention has been shown by way of illustration of various preferred embodiments and although these embodiments have been described in some detail, it is not the intention of the applicant to limit or in any way restrict the scope of the appended claims to such details. Further advantages and modifications will be apparent to those skilled in the art. Accordingly, the various features of the present invention may be used singly or in any combination in accordance with the needs and preferences of the user.

Claims

1. A method for retrofitting a wind turbine having a tower and a first energy generation unit, wherein, the first energy generation unit is replaced by a second energy generation unit, and the method includes: analyzing a first natural frequency of the tower relative to a first rated operating frequency of the tower having the second energy generation unit; when the first natural frequency is within the first rated operating frequency, modifying one or both of the tower and the second energy generation unit such that the modified one or both of the tower and the second energy generation unit have non-overlapping second natural frequencies and second rated operating frequencies; and replacing the first energy generation unit with the second energy generation unit.

2. The method according to claim 1, wherein, the step of modifying one or both of the tower and the second energy generation unit includes: reducing the height of the tower from a first tower height to a second tower height, the height difference between the first tower height and the second tower height being sufficient to reduce the first natural frequency to the second natural frequency and the second natural frequency not being within the second rated operating frequency.

3. The method according to claim 1, wherein, the step of modifying one or both of the tower and the second energy generation unit includes: increasing the height of the tower from a first tower height to a second tower height, the height difference between the first tower height and the second tower height being sufficient to change the first natural frequency to the second natural frequency and the second natural frequency not being within the second rated operating frequency.

4. The method according to claim 3, wherein, the step of modifying the tower includes: adding an adapter to the tower to increase the first tower height to the second tower height.

5. The method according to claim 4, wherein, the tower has at least a first section and a second section, and the adapter is cylindrical, and the step of adding the adapter includes: positioning the adapter between the first section and the second section.

6. The method according to claim 4, wherein, the adapter is conical, and the step of adding the adapter includes: positioning the adapter at the top of the tower, and the step of replacing the first energy generation unit with the second energy generation unit includes: coupling the second energy generation unit to the adapter.

7. The method according to claim 4, wherein, the tower is coupled to a base and the adapter is cylindrical, and wherein the step of adding the adapter includes: positioning the adapter on the base.

8. The method according to claim 4, wherein, the tower has at least a first section and a second section, and the step of adding the adapter includes: removing the first energy generation unit and the first section; coupling the adapter to one of the first section and the second section; and Couple the other of the first section and the second section to the adapter, the adapter being located between the first section and the second section.

9. The method according to any one of claims 1 to 8, wherein, the step of modifying one or both of the tower and the second energy generating unit includes: restricting the operating parameters of the second energy generating unit to change the limit of the second rated operating frequency to be lower than or higher than the second natural frequency.

10. The method according to any one of claims 1 to 8, wherein, the step of modifying one or both of the tower and the second energy generating unit includes: restricting the operating parameters of the second energy generating unit to reduce the upper limit of the second rated operating frequency to a frequency lower than the second natural frequency.

11. The method according to any one of claims 1 to 8, the method further comprises: providing a controller and one or more sensors operatively coupled to the wind turbine; receiving, at the controller, information from the one or more sensors for indicating the second rated operating frequency acting on the tower; and using the controller to change the operation of the second energy generating unit to change the limit of the second rated operating frequency such that the second natural frequency is lower than or higher than the limit and not within the modified second rated operating frequency range.

12. The method according to claim 11, wherein, the step of changing the operation of the wind turbine to change the limit of the second rated operating frequency of the tower includes: pitching one or more blades on the second energy generating unit.

13. The method according to claim 11, wherein, the step of changing the operation of the wind turbine to change the limit of the second rated operating frequency of the tower includes: restricting the speed of the rotor on the second energy generating unit.

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