Patent Description:
Wind turbines are used to produce electrical energy using a renewable resource and without combusting a fossil fuel. Generally, a wind turbine converts kinetic energy from the wind into electrical power. A horizontal-axis wind turbine includes a tower and an energy generating unit positioned atop of the tower. The energy generating unit typically includes a nacelle to house mechanical and electrical components, such as a generator, and a rotor operatively coupled to the components in the nacelle through a main shaft extending from the nacelle. The rotor, in turn, includes a central hub and a plurality of blades extending radially therefrom and configured to interact with the wind to cause rotation of the rotor. The rotor is supported on the main shaft, which is either directly or indirectly operatively coupled with the generator which is housed inside the nacelle. Consequently, as wind forces the blades to rotate, electrical energy is produced by the generator.

Oftentimes during a lifetime of a wind turbine, which can span multiple decades, new technology becomes available. As an example, new generation turbines that have a higher electricity production at a lower price (Levelized Cost of Energy - LCOE) become available. At some point, before the end of the lifetime of some components of a wind turbine, it may become economical to replace an old generation turbine with a new generation turbine. This is often done by replacing the entire wind turbine including the foundation. However, this is quite costly, so the wind turbine industry is searching for solutions to improve a business case for retrofitting with a new generation turbine. <CIT> is an example of retrofitting a wind turbine on an existing tower.

To these and other ends, a method of retrofitting a wind turbine is provided. The wind turbine has a tower and a first energy generating unit. During retrofitting, the first energy generating unit is replaced with a second energy generation unit. The method includes analyzing a first natural frequency of the tower relative to first rated operation frequencies of the tower having the second energy generating unit; when the first natural frequency lies within the first rated operation frequencies, modifying one or both the tower and the second energy generating unit so that the modified one or both the tower and the second energy generating unit have a second natural frequency and second rated operation frequencies that do not overlap; and replacing the first energy generating unit with the second energy generating unit.

In one embodiment, modifying one or both the tower and the second energy generating unit includes increasing a height of the tower from a first tower height to a second tower height, the difference in height 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 is not within the second rated operation frequencies. By way of example, modifying the tower may include adding an adapter to the tower to increase the first tower height to the second tower height.

In one embodiment, modifying one or both the tower and the second energy generating unit includes decreasing a height of the tower from a first tower height to a second tower height, the difference in height 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 is not within the second rated operation frequencies.

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

Moreover, in one embodiment, the tower has at least a first section and a second section and adding the 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, the adapter being between the first section and the second section.

In yet a further embodiment, the tower is coupled to a foundation and adding the adapter to the tower includes positioning the adapter between the tower and the foundation and replacing the first energy generating unit with the second energy generating unit includes coupling the second energy generating unit to the tower. The adapter may be generally cylindrical or conical in this embodiment.

In one embodiment, modifying one or both the tower and the second energy generating unit includes limiting an operating parameter of the second energy generating unit to change a limit of the second rated operation frequencies to be below or above the second natural frequency. More specifically, modifying one or both the tower and the second energy generating unit includes limiting an operating parameter of the second energy generating unit to increase a lower limit of the second rated operation frequencies to a frequency above the second natural frequency. By way of example, altering the operation of the wind turbine so as to change the rated operation frequency may include pitching one or more blades on the second energy generating unit. Alternatively, altering the operation of the wind turbine so as to change the rated operation frequency on the tower may include limiting the speed of the rotor on the second energy generating unit.

In a further example not part of the invention, 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 with at least a first section and a second section, 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.

In yet another example not part of the invention, 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 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 a foundation. An energy generating unit is configured to be coupled to the other conical section.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the invention.

With reference to <FIG>, a wind turbine <NUM> includes a modular tower <NUM> and an energy generating unit <NUM> disposed at the apex of the tower <NUM>. As is conventional, the modular tower <NUM> may be coupled to a foundation <NUM> at a lower end thereof. The exemplary modular tower <NUM> includes three sections 12a, 12b, and 12c that collectively define a generally vertical tower axis <NUM> about which the energy generating unit <NUM> may rotate via a yaw mechanism (not shown). The foundation <NUM> may be a relatively large mass, e.g., concrete, steel, etc., embedded in the ground and through which forces on the wind turbine <NUM> may be ultimately transferred. Although not shown, in an alternative embodiment, the foundation <NUM> may include an offshore platform or the like used in offshore wind turbine applications. The energy generating unit <NUM> includes the part of the wind turbine which transforms the energy of the wind into electrical energy. In this regard, the energy generating unit <NUM> typically includes a housing or nacelle <NUM>, a rotor <NUM> having a central hub <NUM> and one or more blades <NUM> (e.g., three blades) mounted to the central hub <NUM> and extending radially therefrom, and a generator (not shown) for converting mechanical energy into electrical energy. In one embodiment, the energy generating unit <NUM> may further include a drive train (not shown), including a gear arrangement, interconnecting the rotor <NUM> and the generator. The generator and a substantial portion of the drive train may be positioned inside of the nacelle <NUM> of the wind turbine <NUM>. In addition to the generator, the nacelle <NUM> typically houses miscellaneous components required for converting wind energy into electrical energy and various components needed to operate, control, and optimize the performance of the wind turbine <NUM>. The wind turbine blades <NUM> are configured to interact with a free stream air flow (the wind <NUM>) to produce lift that causes the rotor <NUM> to spin or rotate generally within a plane defined by the wind turbine blades <NUM>. Thus, the energy generating unit <NUM> is able to generate power from the airflow that passes through the swept area of the rotor <NUM>. The energy generating unit <NUM> is attached to the tower <NUM> at a top flange <NUM>. The tower <NUM> supports the load presented by the energy generating unit <NUM> and operates to elevate the energy generating unit <NUM>, and especially the rotor <NUM>, to a height above ground level or sea level at which faster moving air currents of lower turbulence are typically found.

At some point in the lifetime of the wind turbine, an existing wind turbine may be replaced. During a retrofit process, an existing energy generating unit may be replaced with another, improved energy generating unit. This is shown, by way of example, in <FIG>. A retrofit approach calls for the same tower <NUM> to be used for a second energy generating unit <NUM>. A retrofitted wind turbine <NUM> should be operated in a manner that extends the service life of the tower <NUM> at least to and preferably beyond the life expectancy design value of the tower <NUM>.

To this end, the inventors identified a problem that must be analyzed prior to installation of the second energy generating unit <NUM> on the tower <NUM>. That issue involves consideration of a natural frequency of a wind turbine relative to a range of rated operation frequencies of the wind turbine <NUM> with a new energy generating unit <NUM> following a retrofit process, described below. Rated operation frequencies are vibrational frequencies that are produced during optimal power generation of the wind turbine. The new energy generating unit, when installed on an existing tower, must not produce rated operation frequencies that overlap the natural vibration frequency of the new energy generating unit on the wind turbine tower during normal operation of the retrofitted wind turbine. A natural frequency of the wind turbine is the frequency at which the wind turbine tower will oscillate in the absence of any driving or damping force.

In general and with reference to <FIG>, as the rotation of the rotor <NUM> increases, the vibration frequency on the wind turbine tower <NUM> increases. Vibration on the wind turbine tower <NUM> may be due to the periodic motion of each blade <NUM> as it sweeps past the tower <NUM>. At startup, the rotation frequency of the rotor <NUM> increases from a stationary position. The rotation of the rotor <NUM> increases until it reaches an optimal rotational rate at which power output is optimized. This occurs at a predetermined wind speed, which is determined by a rated speed of the rotor <NUM>. Power output from the wind turbine <NUM> is held relatively constant in variable wind conditions by controlling a pitch of the blades <NUM>. Thus, with variation in wind speed, the blades <NUM> may be pitched to maintain optimal rotation rate. Although there are controls designed to optimize power production, for example, blade pitch control, during optimal power generation there is still some variation in rotational frequency of the rotor <NUM>. That is, during nominal operation there is some variation in the rated operation frequency on the wind turbine tower <NUM>. This variation produces a range in the rated operation frequencies. As is referenced herein, a rated operation frequency does not include vibrational frequencies on the wind turbine tower <NUM> during startup or shutdown. Each of these can cause vibrational frequencies that can coincide with the natural frequency of the wind turbine tower. However, the time of coincidence is short because the rotor <NUM> rotation rate is increasing toward optimal rotation rate at which the rated operation frequencies are produced or the rotation rate is decreasing toward a stationary position from the optimal rotation rate.

As an example and with reference to the exemplary wind turbine <NUM> of <FIG> and <FIG>, the wind <NUM> moves the nacelle <NUM>. That is, the wind <NUM> causes the tower <NUM> to flex (in the direction of the arrow <NUM>) so that the nacelle <NUM> is displaced from the axis <NUM>. In addition to that periodic displacement motion, the wind rotates the rotor <NUM>. Rotor rotation produces periodic vibrations that are transmitted throughout the wind turbine <NUM>, including the tower <NUM>. Considering the wind turbine <NUM>, the tower <NUM> has a natural frequency <NUM> (<FIG>). If the tower <NUM> is subject to an external periodic force (e.g., the wind <NUM> and/or rotor rotation) at close to its natural frequency <NUM>, the tower <NUM> will resonate. For that reason, as shown in <FIG>, the wind turbine <NUM> is designed with the natural frequency <NUM> outside of rated operation frequencies <NUM> of the wind turbine <NUM>. The rated operation frequencies <NUM> result from the rotation of the rotor <NUM>, described above, during optimal power generation. As shown, there is no overlap between the natural frequency <NUM> and a range of the rated operation frequencies <NUM>. The tower <NUM> is designed to resonate at a frequency that does not typically occur during optimal power generation of the wind turbine <NUM>. This design avoids excessive vibrations, ones that may result in degradation of the tower <NUM> from resonance during operation. In this way, the life of the wind turbine tower <NUM> may be prolonged.

While the rated operation frequencies <NUM> illustrated in <FIG> span a range of frequencies above <NUM> and above the natural frequency <NUM>, it will be appreciated that these are not the only frequencies that the wind turbine <NUM> produces during operation. As is described above, for example, prior to startup, the rotor <NUM> is stationary. During startup, the rotational rate of the rotor <NUM> increases, and as the rotor <NUM> spins up toward a rotation rate for optimal power generation, the vibrational frequencies experienced by the wind turbine <NUM> may overlap the natural frequency <NUM>. This is typically a momentary occurrence because the rotor rotation rate continues to increase until the optimal rotation rate is reached at which time the wind turbine tower <NUM> experiences rated operation frequencies <NUM>. Likewise, during shutdown, the rotation rate of the rotor <NUM> will produce a vibrational frequency that momentarily overlaps with the natural frequency <NUM> as the rotation rate of the rotor <NUM> slows down. In this regard, the rated operation frequencies <NUM> represent a nominal operational range of vibrational frequencies experienced by the wind turbine <NUM> during optimal power generation. The rated operation frequencies exclude those frequencies observed during startup and shutdown of the wind turbine <NUM>.

Upon retrofitting the wind turbine <NUM> with the second energy generating unit <NUM>, as is described below, the goal is to maximize power production until the life expectancy of the second energy generating unit <NUM> and/or the tower <NUM> are reached (as noted above, preferably they reach the end of their service life together). In that regard, the goal is to extend the life of the tower <NUM> as long as possible. To that end, the natural frequency <NUM> is considered relative to a prediction of rated operation frequencies of the tower with the second energy generating unit <NUM>. Depending upon the result of that analysis, a number of scenarios are possible to address an overlap between rated operation frequencies and the natural frequency of the tower with the second energy generating unit <NUM>.

To that end, as shown in <FIG>, when the tower <NUM> is retrofitted with the second energy generating unit <NUM>, the new wind turbine <NUM> will likely have new rated operation frequencies <NUM>. The new rated operation frequencies <NUM> could present a problem with regard to the natural frequency <NUM> of tower <NUM>. The natural frequency <NUM> of the tower <NUM> depends primarily on a stiffness and a mass of the tower <NUM>. While not being bound by theory, it is believed that replacing the energy generating unit <NUM> with the second energy generating unit <NUM> has a negligible effect on the natural frequency <NUM> of the tower <NUM> (i.e., the natural frequency does not change appreciably), but the second energy generating unit <NUM> can cause a significant shift in the rated operation frequencies <NUM> relative to the rated operation frequencies <NUM> because of the improved efficiencies provided with the second energy generating unit <NUM>. However, it is noted that if a new energy generating unit has a mass that is significantly different from the existing energy generating unit, a shift in the natural frequency may occur.

By way of example and with continued reference to <FIG>, the new rated operation frequencies <NUM> of the wind turbine <NUM> with the new second energy generating unit <NUM> in the absence of modifications to the tower <NUM> can encompass the natural frequency <NUM> of the tower <NUM>. That is, an analysis of the operation of the wind turbine <NUM> via modeling or other mathematical means may reveal that at certain wind velocities one or more of the rated operation frequencies will approach or even coincide with the natural frequency <NUM> of the tower <NUM>. Overlap of the rated operation frequency <NUM> with the natural frequency <NUM> is to be avoided if the life of the tower <NUM> is to be maintained or prolonged.

To that end, the inventors have developed solutions that address the overlap of the rated operation frequencies <NUM> and the natural frequency <NUM> of the tower <NUM> during a retrofit process. Generally, the solution involves shifting the natural frequency <NUM> of the tower <NUM> by modifying the tower <NUM> and/or shifting the rated operation frequencies <NUM> of the retrofitted wind turbine <NUM>. By modifying either of the natural frequency or the rated operation frequencies, an overlap between the two is avoided.

One method of shifting the rated operation frequencies <NUM> is via control of the operation of the retrofitted wind turbine <NUM>. As noted above, the tower life rate depends on the vibrational frequencies being imposed on the tower <NUM> during operation of the retrofitted wind turbine <NUM>. The imposed vibrational frequencies on the tower <NUM> may be controlled to a certain extent through operation of the wind turbine <NUM>. With reference to <FIG>, once the rated operation frequencies <NUM> are known or determined, they may be compared to the natural frequency <NUM> of the tower <NUM> (such as in the memory of a controller; see below). With that information, any overlap between the rated operation frequencies <NUM> and the natural frequency <NUM> of the tower <NUM> can be determined. Operation of the wind turbine <NUM> may be controlled to change the rated operation frequencies <NUM> to frequencies different from (e.g., lower or higher than) the natural frequency <NUM> of the tower <NUM>. As an example, the operation frequencies may be changed by changing the speed of the rotor <NUM>.

In an exemplary embodiment and with continued reference to <FIG> and <FIG>, an exemplary system <NUM> may monitor the vibrational frequencies acting on the tower <NUM>, including the rated operation frequency of the wind turbine <NUM>. In this regard, the system <NUM> includes a central controller <NUM> and one or more sensors <NUM> operatively coupled to the wind turbine <NUM> and configured to indicate, directly or indirectly, the vibrations of the tower <NUM>. By way of example, the sensors <NUM> may be directly coupled to the tower <NUM> in one embodiment. In an alternative embodiment, however, the sensors <NUM> may be coupled to another part of the wind turbine <NUM> but configured to measure a parameter that correlates to the vibrations acting on the tower <NUM> (e.g., accelerometers located in the nacelle <NUM>). The central controller <NUM> may be the primary controller for the wind turbine <NUM> or may be a separate controller which is operatively coupled to the primary controller of the wind turbine <NUM>.

As shown, the controller <NUM> may be operatively coupled to a pitch mechanism <NUM> capable of pitching one or more of the blades <NUM> on the second energy generating unit <NUM>. By pitching the blades <NUM> in an appropriate manner, wind energy may be harnessed (i.e., captured by the wind turbine) to maintain the rotation rate of the rotor <NUM>. This may establish a lower limit <NUM> (<FIG>) on the rated operation frequencies to narrow the rated operation frequencies <NUM> to a new rated operation frequencies <NUM>. In this case, a lower limit of the rated operation frequencies <NUM> is increased according to arrow <NUM> so that the lower limit <NUM> of the rated operation frequencies <NUM> is greater than the natural frequency <NUM> of the tower <NUM>. In this way, overlap of the rated operation frequencies <NUM> and the natural frequency <NUM> of the tower <NUM> is avoided. As shown, the controller <NUM> is operatively coupled to a resistance mechanism <NUM> (e.g., a generator) that provides resistance to the rotation of the rotor <NUM> and thus controls the speed at which the rotor <NUM> rotates.

Another solution that may be used separately or in conjunction with the system <NUM> shown in <FIG> is to change a mass or a stiffness of the tower <NUM> during the retrofit process. That is, during retrofit to install the second energy generating unit <NUM>, one or both of the mass or stiffness of the tower <NUM> is modified. This structural modification to the tower <NUM> shifts the natural frequency away from the rated operation frequencies of the wind turbine <NUM>.

In <FIG>, the natural frequency <NUM> is shifted away from the rated operation frequencies <NUM>. Specifically, the natural frequency <NUM> is directly proportional to the stiffness and inversely proportional to the mass of the tower <NUM>. According to an embodiment of the invention, changing one or both of the stiffness and the mass shifts the natural frequency <NUM> of the tower <NUM> outside of the rated operation frequencies <NUM> of the retrofitted wind turbine <NUM>. Stiffness is resistance to deformation. By way of example only, changing the stiffness of the tower <NUM> may be achieved by changing the height of the tower <NUM>. Increasing the height decreases the stiffness and thus decreases the natural frequency of the tower <NUM>. And, decreasing the height of the tower <NUM> increases the stiffness and increases the natural frequency.

As is shown in the exemplary embodiment of <FIG>, the natural frequency <NUM> is decreased relative to the rated operation frequencies <NUM> of the wind turbine <NUM> by increasing the height of the tower <NUM>. A modified tower <NUM> is shown in <FIG> and described below. That is, increasing the height decreases the stiffness and the natural frequency decreases. With a sufficient height increase, the new natural frequency <NUM> of the modified tower is less than the rated operation frequencies <NUM> following retrofit with the second energy generating unit <NUM>. This is schematically shown by arrow <NUM> in <FIG>. Advantageously, a taller tower may be beneficial for other reasons.

By placing the second energy generating unit <NUM> higher within the atmosphere where faster air currents and less turbulence exist, it is believed that the annual energy production (AEP) of the wind turbine will be increased as a result of the increased height. The increased height of the tower <NUM> increases the bending moment acting on the modified tower <NUM> (e.g., think of a cantilevered beam with a large load on its end) and thus reduces the life of the tower <NUM>.

Nevertheless, depending on the particular application, it may be possible to increase the height of the tower <NUM> and operate the second energy generating unit <NUM> at a rated power curve such that the modified wind turbine tower and the second energy generating unit <NUM> reach the end of their service life at the same time.

An example of a height modification of the tower <NUM> in combination with a retrofit to include the second energy generating unit <NUM> is illustrated in <FIG>. In the figures, a height H1 (<FIG>) of the tower <NUM> is increased to a new height, H2 (<FIG>) with the second energy generating unit <NUM> at the new height H2. Once retrofitted, a modified wind tower <NUM> (<FIG>) corresponds to the natural frequency <NUM> and rated operation frequencies <NUM> shown in <FIG>. That is, the tower <NUM> shown in <FIG> having a natural frequency <NUM> is modified during a retrofit process. As a result of the modification, the modified wind turbine tower <NUM> has height H2 having the natural frequency <NUM>. This natural frequency is below the rated operation frequencies <NUM> of the modified tower <NUM> having the second energy generating unit <NUM> as is shown for example in <FIG>.

To that end, in the exemplary retrofit, the tower <NUM> shown in <FIG> is built in sections. In the example shown, the tower <NUM> includes three sections 12a, 12b, and 12c that collectively define the height H1. Embodiments of the invention are not limited to three sections as a tower with two or more sections may be retrofit as is described herein. With reference to <FIG>, in one embodiment, the first energy generating unit <NUM> is removed from a top end <NUM> of the tower section 12c. This may be achieved by a crane (not shown) or other lifting device.

In <FIG> and <FIG>, the height H1 of the tower <NUM> is increased by adding a tower transition adapter <NUM> at the top end <NUM> tower section 12c. That is, the height H2 of the modified tower <NUM> (shown in <FIG>) is equal to the height H1 plus the length of the tower transition adapter <NUM>. In addition to increasing the height relative to the tower <NUM>, the tower transition adapter <NUM> may also provide geometry matching between the top end <NUM> of the modified tower <NUM> and the second energy generating unit <NUM>. By way of example, the second energy generating unit <NUM> may be from a manufacturer different than the manufacturer of the tower <NUM> such that a geometry mismatch between the tower <NUM> and second energy generating unit <NUM> exists. The tower transition adapter <NUM> may increase the height of the tower <NUM> while also remedying a variation in geometry between the first energy generating unit <NUM> and the second energy generating unit <NUM>.

In this regard and with reference to <FIG>, the tower transition adapter <NUM> includes a first end <NUM> with a first interface <NUM> sized for engaging with the interface <NUM> on the top end <NUM> of the tower section 12c. The tower transition adapter <NUM> further includes a second end <NUM> with a second interface <NUM> sized for engaging with an interface <NUM> on the second energy generating unit <NUM>. The tower transition adapter <NUM> is coupled to the top end <NUM> of the tower section 12c. More particularly, the first interface <NUM> at the first end <NUM> of the tower transition adapter <NUM> may be coupled to the interface <NUM> at the top end <NUM> of the tower section 12c, such as by welding or a flanged connection. In an exemplary embodiment, the interfaces <NUM>, <NUM> of the tower transition adapter <NUM> may include a flange (e.g., an annular flange). The interfaces <NUM>, <NUM> on the tower section 12c and the second energy generating unit <NUM>, respectively, may also include a flange. A fastener, such as a nut/bolt may be used to couple the respective flanges together, as is generally known in the art. In an alternative embodiment, the flanges may be omitted and the tower transition adapter <NUM> may be coupled to the tower section 12c through welding, for example.

In an exemplary embodiment, to resolve the potential mismatch between the modified tower <NUM> and the second energy generating unit <NUM>, the size of the interfaces <NUM> and <NUM> may be different from each other. More particularly, the diameter of the interfaces <NUM>, <NUM> may be different. In one embodiment, for example, the diameter of the first interface <NUM> may be about <NUM> meters and the diameter of the second interface <NUM> may be about <NUM> meters and vice versa. In that regard, the tower transition adapter <NUM> may have a conical configuration to account for the difference in dimensions. Other sizes and shapes, however, are possible depending on the particular application. Moreover, the length of the tower transition adapter <NUM> may vary to locate the second energy generating unit <NUM> at the desired height H2. By way of example, the tower transition adapter <NUM> may be between about <NUM> meters and about <NUM> meters in length. Again, other lengths may be possible depending on the particular application and the desired natural frequency <NUM> of the modified tower <NUM> relative to the natural frequency <NUM> of the tower <NUM> (shown in <FIG>).

Referring to <FIG>, with the tower transition adapter <NUM> attached to the tower section 12c, the height of the modified tower <NUM> is H2. The height H2 of the modified tower <NUM> is greater than the height H1 of the tower <NUM>. This produces a decrease in the natural frequency of the modified tower <NUM> relative to the tower <NUM>. Next, the second energy generating unit <NUM> may be coupled to the tower transition adapter <NUM>. More particularly, the interface <NUM> of the second energy generating unit <NUM> may be coupled to the second interface <NUM> at the second end <NUM> of the tower transition adapter <NUM>, such as by welding or a flanged connection, to complete the retrofit process.

It should be recognized that in an alternative embodiment, the second energy generating unit <NUM> may be coupled to the second end <NUM> of the tower transition adapter <NUM> and then that assembly is subsequently coupled to the top end <NUM> of the tower section 12c. In any event, subsequent to the retrofit process, the retrofitted wind turbine <NUM> may be returned to service and operated. The retrofitted wind turbine <NUM> should preferably be operational until the service life of the modified tower <NUM> and the second energy generating unit <NUM> have expired. By extending the service life of the wind turbine through the retrofit process, the wind turbine operator may be provided additional time in which to obtain a return on their investment in the wind turbine.

In many regions where wind turbines are located, there may exist local or regional laws, regulations, ordinances, etc. which limit the height at which structures, such as wind turbines, may 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 <NUM>, the wind turbine operator may want to verify the blade tip height when one of the blades <NUM> of the second energy generating unit <NUM> is at the twelve o'clock position (i.e., at the maximum height of the wind turbine <NUM>) to ensure the retrofitted wind turbine <NUM> meets the applicable standards. If the tip height of the blades <NUM> exceed a predetermined threshold established by the laws, regulations, ordinances, etc. in which the retrofitted wind turbine <NUM> is located, then the height of the tower transition adapter <NUM> (or possibly even the height of the tower <NUM>, see below) may have to be reduced in order to comply with the applicable laws, regulations, ordinances, etc..

As described above, the tower transition adapter <NUM> was added to the top end <NUM> of the tower <NUM>, such that substantially the entirety of the tower <NUM> is "reused" in the retrofitted wind turbine <NUM>. This represents an efficient use of the existing structure in the retrofit process. The invention, however, is not limited to that embodiment. In this regard and in an alternative embodiment, a portion of the original tower <NUM> may be removed to establish a new tower interface at which the tower transition adapter <NUM> may be coupled to change the overall height of the modified tower <NUM>.

As illustrated in <FIG>, in one embodiment, the height H1 of the tower <NUM> may be increased by insertion of an adapter at locations other than at the top end <NUM> of the tower <NUM>. In that regard, any two sections 12a, 12b, and 12c of the tower <NUM> may be separated and a tower transition adapter <NUM> may be coupled between the tower sections. By way of example and as illustrated in <FIG> and <FIG>, the tower transition adapter <NUM> is inserted between tower section 12b and 12c. Although not shown, a tower transition adapter may be inserted between sections 12a and 12b or between the section 12a and the foundation <NUM> as is described in conjunction with <FIG>.

To that end, and with reference to <FIG> and <FIG>, the energy generating unit <NUM> and the tower section 12c may be removed. This leaves the remaining tower sections 12a and 12b in position. As shown, a top end <NUM> of the tower section 12b forms an interface <NUM> to receive the tower transition adapter <NUM>.

Next, and with reference to <FIG>, the tower transition adapter <NUM> has a first end <NUM> defining a first interface <NUM> for coupling to the tower section 12b and has a second end <NUM> defining a second interface <NUM>. The first interface <NUM> at the first end <NUM> of the tower transition adapter <NUM> is coupled to the interface <NUM> of the tower section 12b. In the exemplary embodiment shown, the tower transition adapter <NUM> is cylindrical. As an example, each of the interfaces <NUM> and <NUM> define a circle having the same diameter. Advantageously, construction of the tower transition adapter <NUM> is simplified as the dimensions of each end <NUM> and <NUM> do not vary with the length of the tower transition adapter <NUM>. The tower transition adapter <NUM> may be coupled to the interface <NUM> via a flanged connection. Alternatively, the tower transition adapter <NUM> may be welded to the interface <NUM>.

Once the tower transition adapter <NUM> is secured to the tower section 12b, the tower section 12c is secured to the tower transition adapter <NUM>. In that regard and with reference to <FIG> and <FIG>, the section 12c includes a first end <NUM> that defines a first interface <NUM> and includes a second end <NUM> that defines a second interface <NUM>. Second interface <NUM> may be the top flange <NUM> (in <FIG>). The first interface <NUM> at the first end <NUM> of the tower section 12c is coupled to the interface <NUM> at the second end <NUM> of the tower transition adapter <NUM>. By way of example, the tower transition adapter <NUM> may be coupled to the end <NUM> via a flanged connection. Alternatively, the tower transition adapter <NUM> may be welded to the end <NUM>. The length of the tower transition adapter <NUM> may vary to locate the second energy generating unit <NUM> at the desired height H3, with H3 being greater than H1 by the length of the tower transition adapter <NUM>. By way of example, the tower transition adapter <NUM> may be between about <NUM> meters and about <NUM> meters in length. Again, other lengths may be possible depending on the particular application and the desired natural frequency <NUM> of the modified tower <NUM> relative to the natural frequency <NUM> of the tower <NUM> (shown in <FIG>). Although not shown, the tower transition adapter <NUM> may include an internal platform and ladder that transitions between internal ladders of adjacent sections 12b and 12c.

Referring to <FIG>, the second energy generating unit <NUM> may then be coupled to the tower section 12c. More particularly, the interface <NUM> of the second energy generating unit <NUM> may be coupled to the second interface <NUM> at the second end <NUM> of the tower section 12c. It should be recognized that in an alternative embodiment, the second energy generating unit <NUM> may be coupled to the second end <NUM> of the tower section 12c and then that assembly is subsequently coupled to the tower transition adapter <NUM>. Advantageously, where the wind turbine operator is retrofitting the first energy generating unit <NUM> with the second energy generating unit <NUM> made by the same manufacturer there may be no need to resolve a mismatch between the modified tower <NUM>, that is, the tower section 12c, and the second energy generating unit <NUM>. The size of the interfaces <NUM> and <NUM> may be the same. More particularly, the diameter of the interfaces <NUM> and <NUM> may be the same. Inserting the tower transition adapter <NUM> between any two tower sections 12a, 12b, and 12c eliminates a requirement for a transition between the modified tower <NUM> and the second energy generating unit <NUM>.

In one embodiment, for example, the diameter of the interface <NUM> may be about <NUM> meters and the diameter of the second interface <NUM> may be about <NUM> meters. Other sizes, however, are possible depending on the particular application. It may be that the addition of the tower transition adapter <NUM> between the tower sections 12a and 12b or 12b and 12c avoids problems associated with design and construction of an adapter capable of receiving the second energy generating unit <NUM> at an interface of the adapter. It is comparatively more straight forward and less expensive to construct a cylindrical adapter, such as the tower transition adapter <NUM>. By way of example, the tower transition adapter <NUM> may be constructed with upper and lower L flanges at interfaces <NUM>, <NUM> that couple to each of the tower sections 12b and 12c (or between sections 12a and 12b) whereas an adapter that is to be coupled directly to the second energy generating unit <NUM> is more difficult to design and more costly to construct.

In this embodiment and similar to the above, it should be recognized that the new height of the modified tower <NUM> of the retrofitted wind turbine <NUM> may be greater than or less than the original height of the tower <NUM> depending on the desired shift in the natural frequency relative to the anticipated rated operation frequencies of the wind turbine <NUM>. The tip height of the blades <NUM> may also be checked to confirm that the height of the retrofitted wind turbine <NUM> is within the applicable standards.

Another embodiment of the invention is shown in <FIG> and <FIG>. In addition or as an alternative to the tower transition adapter <NUM>, <NUM>, an adapter may be inserted between the tower <NUM> and the foundation <NUM>. As is shown in <FIG>, the increased height H3 relative to H1 may be wholly or in part because of a foundation adapter <NUM>. In addition to modifying the natural frequency of the tower <NUM>, for example shown in <FIG>, the foundation adapter <NUM> may solve other problems in the industry.

By way of example and with reference to <FIG>, one problem in the industry is that there is an excess of unused wind turbine foundations <NUM>. The foundations <NUM> are constructed of steel reinforced concrete <NUM>. Embedded in the concrete <NUM> are a plurality of anchors <NUM>. These are positioned in the concrete prior to the concrete setting and are therefore not removable without destroying the foundation <NUM>. While not shown, the anchors <NUM> may be arranged in a ring of predetermined size and adapted to receive a specific tower design. Generally, the size and dimensions of a layout of the anchors <NUM> are manufacturer specific and may be customized to the site. This presents the specific problem in that one turbine manufacturer may generally not utilize another turbine manufacturer's foundation because of design variations between wind towers. Thus, once the foundation <NUM> is installed, if not used to secure a wind turbine specific to that foundation, the foundation is not usable.

In one embodiment of the invention, a foundation adapter <NUM> is coupled between the tower section 12a and the foundation <NUM> where the size and bolt arrangement between the tower section 12a and the anchors <NUM> do not cooperate. In that regard, the foundation adapter <NUM> includes a shell <NUM> that defines a sidewall <NUM>. The sidewall <NUM> may be in the form of a circular cylinder matching the dimensions of the tower section 12a. At one end <NUM> of the sidewall <NUM>, an L-flange <NUM> extends inwardly with a plurality of through-bores <NUM> that are spaced apart to each receive an anchor <NUM>. As shown, a nut is threaded onto the anchor <NUM> and secures the adapter <NUM> to the foundation <NUM>. Opposite the end <NUM>, at an end <NUM>, a T-flange <NUM> defines a plurality of interior through bores <NUM> and a plurality of exterior through bores <NUM>. The bores <NUM> and <NUM> align with corresponding bores <NUM> in the tower section 12a. Aligned bores <NUM> and <NUM> and <NUM> and <NUM> receive bolts or other fasteners <NUM> so that the tower section 12a can be secured to the adapter <NUM>. The bores <NUM> in the T-flange <NUM> do not align with the bores <NUM> in the L-flange <NUM>. In this way, the foundation adapter <NUM> permits one manufacturer to utilize another manufacturer's unused foundation and advantageously permits a wind turbine to be installed on an otherwise unusable foundation. The adapter <NUM> also elevates the height of the second energy generating unit <NUM> and modifies the natural frequency of the modified tower <NUM> in accordance with the embodiments shown in <FIG> and <FIG>. While <FIG> illustrates an adapter having a lower L-flange and an upper T-flange, embodiments of the invention are not limited to the configuration shown. By way of example, a foundation adapter may include a lower L-flange and an upper L-flange, a lower T-flange and an upper L-flange, or a lower T-flange and an upper T-flange. Furthermore, while a cylindrical adapter is described, the foundation adapter <NUM> is not limited to a cylinder (e.g., circular). In that regard, the foundation adapter <NUM> may have a reverse cone configuration that transition between different overall dimensional differences between the foundation <NUM> and the tower section 12a.

Claim 1:
A method of retrofitting a wind turbine, the wind turbine having a tower (<NUM>) and a first energy generating unit (<NUM>), wherein the first energy generating unit (<NUM>) is replaced with a second energy generating unit (<NUM>), the method comprising:
analyzing a first natural frequency (<NUM>) of the tower relative to first rated operation frequencies (<NUM>) of the tower having the second energy generating unit (<NUM>);
when the first natural frequency (<NUM>) lies within the first rated operation frequencies (<NUM>), modifying one or both the tower (<NUM>) and the second energy generating unit (<NUM>) so that the modified one or both the tower (<NUM>) and the second energy generating unit (<NUM>) have a second natural frequency (<NUM>) and second rated operation frequencies that do not overlap; and
replacing the first energy generating unit (<NUM>) with the second energy generating unit (<NUM>).