Patent Description:
A direct drive generator driven by the blades of the wind turbine is efficient and has minimal losses due to transmission of torque from the turbine blades to the DC generator. Direct drive conventional generators on wind turbine towers generally have a power rating of <NUM> megawatts (MW) or less.

Conventional direct drive generators typically have low torque density and become too heavy for a wind turbine tower at power ratings above about <NUM> MW. Gearboxes may be unreliable and not suitable for long life service in a wind turbine.

In addition, alternating current (AC) wind generators must undergo total power conversion to convert the generated power to <NUM>-<NUM> cycle AC at a particular voltage desired by the grid connection. This conversion is typically accomplished by a power electronics (PE) converter consisting of a rectifier in the first stage to convert the AC to direct current (DC) and then an inverter stage to produce the desired AC.

There is a long felt need for direct drive generators for wind turbines capable of generating higher electrical power, e.g., <NUM> MW or more. In addition, there is a need for a DC generator which allows the up-tower weight to be reduced, thereby providing reductions in cost, size and weight, which allow for economical shipping and installation on a wind turbine tower and concomitant reliability increase.

Documents <CIT>, <CIT>, <CIT> and <CIT> relate to generators having a superconducting coil.

In one aspect, a superconducting generator in accordance with claim <NUM> is provided. The superconducting generator includes an annular armature connectable to rotate with a rotating component of a wind turbine. A stationary annular field winding is coaxial to the armature and separated by a gap from the armature. The field winding includes superconducting coils, and there is a non-rotating support for the field winding. The non-rotating support is a torque tube. The torque tube is a member formed of a composite material, or a member formed of a plurality of segmented sections, a space frame or strut torque carrying assembly. The torque tube is connected to a thermal shield casing or a field winding housing.

In another aspect, a wind turbine in accordance with claim <NUM> is provided, which includes a tower, a nacelle mounted on top of the tower, a hub connected to the nacelle and supported by the tower, and a plurality of blades connected to the hub. A superconducting generator is housed within the nacelle. The superconducting generator includes an annular armature connectable to rotate with a rotating component of a wind turbine. A stationary annular field winding is coaxial to the armature and separated by a gap from the armature. The field winding includes superconducting coils, and there is a non-rotating support for the field winding. The non-rotating support is a torque tube. The torque tube is a member formed of a composite material, or a member formed of a plurality of segmented sections, a space frame or strut torque carrying assembly. The torque tube is connected to a thermal shield casing or a field winding housing.

These and other features and improvements of the present disclosure will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the several drawings and the appended claims.

The above and other aspects, features, and advantages of the present disclosure will become more apparent in light of the subsequent detailed description when taken in conjunction with the accompanying drawings.

A superconducting direct current (DC) generator has been developed with a stationary, or non-rotating, field winding and a rotating armature driven directly by a turbine, such as a wind turbine. The superconducting DC generator may be mounted in the upper region of wind turbine tower and coupled directly to the rotating component of the wind turbine, e.g., the blades. The direct drive generator is sufficiently lightweight to be mounted on top of a wind turbine tower and coupled to rotating wind turbine blades via the hub.

The superconducting DC generator provides high torque density, which allows the DC generator to be lightweight and transportable, despite the added components needed to cool and insulate the superconducting coils in the field winding. The stationary field winding includes a series of racetrack shaped superconducting coils cooled to cryogenic temperatures. The rotating armature and iron yoke (optional) are connected directly to and turned by the wind turbine. A commutator assembly transfers the current generated by the rotating armature to conductors that may extend down through the frame of the wind turbine.

<FIG> is a front view of a wind turbine <NUM> that includes a tower <NUM> anchored to the ground. A superconducting DC generator <NUM> is housed within a nacelle <NUM> mounted on top of the tower. The nacelle may rotate about an axis of the tower to align the turbine blades <NUM> with respect to the wind direction. The blades extend radially outward from a hub <NUM>. The blades <NUM> typically face into the wind and are turned by the energy of the wind. The DC generator <NUM> is housed within the nacelle <NUM> and is driven directly by the hub <NUM> and blades <NUM>. The rotation of the blades and hub directly drives the armature of the DC generator.

As one example only, the tower <NUM> may be between <NUM> and <NUM> meters in height, one to two (<NUM> to <NUM>) meters in diameter at the top and four (<NUM>) meters in diameter at the ground base. The tower may be constructed of tapered tubular steel, but may also be made from a lattice structure or from concrete sections. The turbine blades <NUM> are equally spaced around the hub <NUM>, and the resulting rotor diameter may be about <NUM> meters to about <NUM> meters or more. While the blades may be made of any suitable material, they are typically formed of a carbon or glass fiber reinforced plastic or epoxy. The blades may have a fixed pitch or a variable pitch, depending on whether a variable pitch gearbox is included in the hub. The dimensions of the tower and blades and their compositions may be chosen as desired in the specific application.

<FIG> is a schematic diagram showing in cross-section a direct drive generator <NUM> having an annular rotating armature <NUM> and a stationary superconducting field winding assembly <NUM> surrounded by the armature. The rotating armature <NUM> is an outer annular ring around the field winding assembly <NUM>. The armature <NUM> may comprise conductive windings <NUM>, e.g., coils or bars, arranged longitudinally (axially) along the length of the armature and on an inside cylindrical surface of the armature. By way of example, the longitudinal sections of the armature windings may be <NUM> to <NUM> inches in length, have a thickness of <NUM> to <NUM> inches and an inside diameter of between <NUM> to <NUM> inches. The coils or bars may be connected at their opposite ends to one another by conductive end turns <NUM>. The end turn connections between the longitudinal coils or bars are dependent on their number and arrangement, and the phases of electricity to be generated in the armature windings. The inside cylindrical surface of the armature windings is separated by a narrow air gap, e.g., about <NUM>-<NUM> inches, from the outer surface of the stationary superconducting field winding assembly <NUM>.

The annular rotating armature <NUM> includes a cylindrical yoke <NUM> that supports the coils and bars <NUM>. The outer surface of the yoke <NUM> is fixed to a cylindrical housing <NUM> that rotates with the armature. The diameter of the housing <NUM> may be, for example, between <NUM> to <NUM> feet and have an axial length of <NUM> feet. The housing is fitted to a circular disc <NUM> that supports the housing and armature <NUM>. The disc has a circular aperture at its center that is mounted to an annular bracket <NUM> to which is attached the annular base <NUM> of the hub <NUM> of the wind turbine. The bracket <NUM> and base <NUM> may be secured together by bolts arranged in a circular array around the bracket and base. The disc <NUM> may have optional openings or holes <NUM> for weight reduction. The bracket <NUM> is mounted on an end of a rotating cylindrical support tube <NUM> that is radially inward of the armature winding. A reinforcing ring <NUM> may be fixed to the inner corner between the bracket <NUM> and support tube <NUM>. The support tube <NUM> may be, for example, between <NUM> to <NUM> feet in diameter.

To convert the generated mechanical energy into DC electrical energy, as the rotating armature <NUM> turns, the current in the armature coils <NUM> is commutated to produce a direct current output by a commutator assembly <NUM>. In the illustrated example, the commutator assembly <NUM> is configured as a plurality of circumferential rings proximate an outside surface of the housing <NUM> or circular disc <NUM>. The commutator assembly <NUM> may be disposed at any convenient location exterior the rotating housing <NUM>. Positioning of the commutator assembly <NUM> closer to the axis of rotation will allow shorter circumferences. The commutator assembly <NUM> is generally comprised of a rotating first portion <NUM> that rotates with the rotating armature <NUM> as it turns and a stationary second portion <NUM> that remains stationary. The commutator assembly <NUM>, and more particularly the rotating first portion <NUM> is comprised of a plurality of conductive segments <NUM>, formed of a material such as copper. In this example, the commutator assembly <NUM> is comprised of at least two rotating commutator segments <NUM>. The commutator segments <NUM> are configured to rotate with, and are electrically connected to, the rotating armature coils <NUM> via a plurality of electrical connections <NUM>. The number of segments and electrical connections are variable and determined by the number of electrical poles selected by the generator designer. The stationary second portion <NUM> of the commutator assembly <NUM> is completed by a fixture holding a plurality of commutator brushes <NUM>, and in this particular embodiment, at least two commutator brushes <NUM>. The commutator brushes <NUM> are typically comprised of carbon, remain stationary, and are held by a stationary platform <NUM> configured to take an output current from the commutator brushes <NUM> to a power conversion system <NUM>. The commutator brushes <NUM> are configured to ride or brush on the rotating commutator segments <NUM> as they rotate. The commutator brushes <NUM> may be held in position by way of spring tension, and may include grounded and ungrounded brushes. As electrical energy is generated, the energy is conducted through the commutator brushes <NUM> and the rotating commutator segments <NUM> to the stationary platform <NUM>, and ultimately to the power conversion system <NUM> that is coupled to a power utility grid, factory or other electrical power load so that the electricity can be used.

In an alternative example, as best illustrated in a partial schematic cross-section view in <FIG>, the rotating first portion <NUM> of the commutator assembly <NUM> that rotates with the rotating armature <NUM> may be comprised of the plurality of commutator brushes <NUM> and the stationary second portion <NUM> of the commutator assembly <NUM> that remains stationary, may be comprised of the plurality of commutator segments <NUM>. In this alternative embodiment, the plurality of rotating commutator brushes <NUM> are configured to ride or brush on the stationary commutator segments <NUM> as they rotate. Similar to the reverse configuration previously disclosed for the first and second portions <NUM>, <NUM> of the commutator assembly <NUM>, as electrical energy is generated, the energy is conducted through the commutator segments and the rotating commutator brushes to the stationary platform <NUM>, and ultimately to the power conversion system <NUM> that is coupled to a power utility grid, factory or other electrical power load so that the electricity can be used.

Referring back to <FIG>, a pair of annular bearings <NUM> arranged on opposite ends of the support tube <NUM> rotatably support the support tube <NUM> on a stationary base tube <NUM> is attached to a mount <NUM> that is supported by the floor of the nacelle. A ring bracket <NUM> may attach mount <NUM> to a bracket <NUM> for the base tube. Bolts secure the brackets <NUM>, <NUM> together. The pair of bearings <NUM> may be of the same type. Alternatively, the annular bearing <NUM> near the hub <NUM> may have a longer length, e.g., <NUM> to <NUM> inches, than the annular bearing <NUM> near the tower, which may have a length of <NUM> to <NUM> inches. The bearing <NUM> near the hub is longer because it more directly receives the downward force of the hub and blades and wind, which may be <NUM>,<NUM> pounds of force, and receives a bending moment from the hub, blades and wind, which moment may be <NUM>×<NUM><NUM> inch-pounds at the base <NUM> and bracket <NUM>.

The support tube <NUM> may have constant thickness along its length. Alternatively, the base tube <NUM> may be thick, e.g., two inches, near the tower and thin, e.g., one inch, near the hub. The base tube may reduce in thickness in a step or a taper. The reduction in the thickness reduces the weight of the tube. Similar weight reducing features may include cutouts or holes in the disc <NUM>, light weight materials, e.g., composites, in the housing <NUM>. A disc brake <NUM> grasps an annular lip <NUM> on an end of the housing <NUM>. The brake can slow or stop the rotation of the blades, if the wind becomes excessive and the blades rotate too fast. Thin and lightweight gussets <NUM> extend from circular disc <NUM> to the support tube <NUM>. The gussets structurally reinforce the disc <NUM>.

The base tube <NUM> supports a field winding support disc <NUM> on which is mounted the stationary field winding assembly <NUM>. The field windings <NUM> are a series of conductive loops (or coils) through which current circulates, and once a current (or voltage) is ramped up to a desired level the cold superconducting temperature of the windings permits the current to circulate with zero resistance. This circulating current acts as a magnet to impose a magnetic field in the armature windings. The assembly of the base tube <NUM> and support disc <NUM> is an exemplary non-rotating support for the field winding assembly <NUM>. The disc may have cutouts or holes <NUM> to reduce weight. The disc <NUM> is attached to an end of a cryostat housing <NUM> containing the superconducting coils of the field winding <NUM>. The housing <NUM> and its cooling components form a cryostat that cools the superconducting coils of the field winding. The housing for the cryostat <NUM> may be annular, rectangular in cross section, have an outside diameter of between <NUM> to <NUM> feet, and a length of <NUM> to <NUM> feet. The dimensions of the housing <NUM> and other components of the DC generator and wind turbine are a matter of design choice and may vary depending on the design of the wind turbine.

The cryostat <NUM> insulates the superconducting coils so that they may be cooled to near absolute zero, e.g., to <NUM> Kelvin (K) and preferably to <NUM>. To cool the windings, the housing <NUM> includes insulated conduits <NUM> to receive liquid helium (He) or other similar cryogenic liquid (referred to as cryogen). A two-stage re-condenser <NUM> mounted in an upper region of the nacelle, on top of the nacelle or on top of the tower, and above the field windings provides cryogen, e.g., liquid He, using a gravity feed. The cryogen flows around the superconducting coil magnets of the field windings and cools the coil magnets to achieve a superconducting condition. The coils are cooled, e.g., to <NUM> degree K, as the He at least partially vaporizes. The He vapor flows through one of the conduits <NUM> to the re-condenser <NUM>, where the He is cooled, liquefied and returned via conduit <NUM> to the coils magnets. The power conductors for the superconducting coils also pass through the housing <NUM> with the insulated conduits <NUM> for the helium.

Torque is applied by the hub <NUM> to turn the rotating armature <NUM> around the stationary super-conducting field winding assembly <NUM>. The rotating support disc <NUM> transmits the torque from the hub to the rotating armature <NUM>. Torque is applied by the rotating armature <NUM> to the stationary super-conducting field winding assembly <NUM> due to electromagnetic force (EMF) coupling. The torque applied to the stationary super-conducting field winding assembly <NUM> is transmitted by the field winding housing <NUM> to the stationary support disc <NUM> and to the mount <NUM> of the tower <NUM>.

Referring to <FIG>, the interior of the housing <NUM> is evacuated and that forms an insulating vacuum around the thermal shield <NUM>. A first torque tube <NUM> suspends the thermal shield <NUM> in the evacuated interior of the cryostat housing <NUM>. The torque tube <NUM> is mounted to an annular flange <NUM> inside the housing. The flange elevates the tube from the inside wall of the housing <NUM>. Another annular flange <NUM>, at the opposite end of the torque tube <NUM>, elevates the thermal shield <NUM> from the tube and centers the thermal shield inside the housing <NUM>. The torque tube <NUM> also transmits torque from the thermal shield <NUM> to the housing <NUM>, and provides thermal insulation to the low temperature thermal shield from the ambient temperature housing <NUM>. The thermal shield <NUM> is preferably formed of a lightweight material. Suspended in the thermal shield <NUM> is an annular casing <NUM>.

A second torque tube <NUM> is supported on one end by a flange <NUM> on an inner wall of the thermal shield <NUM>. The flange <NUM> may extend into the interior of the chamber defined by the thermal shield <NUM> or may comprise two flanges (one inside the thermal shield and the other outside the thermal shield). The flanges may be formed of an insulating material. The second torque tube <NUM> thermally insulates and suspends the annular casing <NUM> from the thermal shield. The second torque tube <NUM> transmits torque from the coils to the first torque tube <NUM>. Both the first and second torque tubes may be formed of lightweight materials. The torque tube is an essential structural component of a superconducting generator. The torque tube reacts to electro-magnetic torque, which is generated by interaction with the rotating armature coils by transferring torque into the external structure and eventually to the wind turbine tower. The torque tube also carries the static weight of the cooled field structure and thermal shield, and it minimizes conduction heat loads to the superconducting coils and thermal shield.

The insulated power cables <NUM>, <NUM> for the superconducting coils <NUM> pass through sealed apertures in the housing <NUM>, thermal shield <NUM> and, for the first conduit <NUM> to the casing <NUM> for the superconducting coils. The housing, thermal shield, and casing provide an insulated and cooled environment within which the superconducting coils can be cooled to cryogenic temperatures, e.g., <NUM> degree Kelvin. The torque tubes arranged in opposite directions thermally and mechanically isolate the windings and their casings from ambient conditions. The casing may be annular and rectangular in cross section. The curvature of the casing conforms to the curvature of the annular chamber <NUM>. The casing <NUM> may include an annular array of hollow recesses <NUM> that each receive a race-track shaped coil <NUM> and a supply of liquid helium. A support bracket <NUM> is seated in the recess and above each coil magnet.

<FIG> illustrates a simplified schematic diagram showing in cross-section a portion of the cryostat for the superconducting field winding, according to an aspect of the present disclosure. The cryostat comprises the annular chamber <NUM> and a cylindrical shell <NUM> is disposed around a casing <NUM> and field windings <NUM>. A thermal resistant support <NUM> is connected to a torque tube <NUM>. A second thermal resistant support or thermal anchor point <NUM> is connected to a second torque tube <NUM>, and this second torque tube <NUM> is anchored to the annular chamber via a third thermal resistant support <NUM>. The first and second torque tubes <NUM>, <NUM> are comprised of frustoconical shaped members, which are generally a truncated cone shape. The torque tubes <NUM>, <NUM> may have stiffening ribs <NUM>, <NUM> incorporated therein to increase the circumferential and axial rigidity of the torque tubes. The stiffening ribs may extend circumferentially and/or axially along the torque tubes. In <FIG>, the circumferential direction would be into or out of the page along an arc, and the axial direction is generally horizontal to the left or right.

<FIG> illustrates a simplified schematic diagram showing in cross-section a portion of the cryostat for the superconducting field winding, according to an aspect of the present disclosure. The cryostat comprises the annular chamber <NUM> and a cylindrical shell <NUM> is disposed around a casing <NUM> and field windings <NUM>. A thermal resistant support <NUM> is connected to a torque tube <NUM>. A second thermal resistant support <NUM> is connected to a second torque tube <NUM>, and this second torque tube <NUM> is anchored to the annular chamber via a third thermal resistant support <NUM>. The first and second torque tubes <NUM>, <NUM> are comprised of cylindrical or generally cylindrical shaped members. The torque tubes <NUM>, <NUM> may have stiffening ribs <NUM>, <NUM> incorporated therein to increase the circumferential and axial rigidity of the torque tubes.

The torque tubes may be comprised or formed of a composite material, such as, a fiber reinforced plastic, an epoxy fiberglass laminate, a phenolic fiberglass laminate, a phenolic fiberglass with wound filaments, a polyester fiberglass laminate, a polyester fiberglass laminate with wound filaments, a polymide fiberglass laminate, a carbon epoxy, or a fiberglass epoxy laminate. In some known superconducting generators, metal (e.g., titanium) torque tubes were employed or investigated for use. However, titanium has substantially better thermal transfer properties than fiberglass epoxies, and that quality is not desired in superconducting generators. In fact, the opposite is desired, and a thermally insulative material would be more desired to maintain the cold temperatures in the annular chamber. The composite materials identified above can provide sufficient rigidity to resist torsional buckling, and also exhibit satisfactory fatigue resistance at cold temperatures, while also having low thermal transfer properties.

<FIG> illustrates a partial exploded view of a torque tube <NUM>, according to an aspect of the present disclosure. The torque tube <NUM> is constructed of a composite material having a plurality of layers (only four of which are shown for clarity). It is to be understood the number of layers may be more than one and up to any desired number as desired in the specific application. The layers <NUM>-<NUM> are comprised of hoop oriented fibers oriented at + or - <NUM> degrees. The degrees are measured from a radial plane at a single axial distance, with respect to the generator. For example, layers <NUM> and <NUM> have fibers oriented at + <NUM> degrees, and layers <NUM> and <NUM> have fibers oriented at - <NUM> degrees. The fiber angles provide a good balance of providing structural rigidity and also providing a longer thermal conduction path to opposing ends of the torque tube. The fiber angles may also range between about + or - <NUM> degrees to about + or - <NUM> degrees.

<FIG> illustrates a cross-sectional view, in a radial plane, of a torque tube <NUM> formed from a plurality of segmented sections. The torque tube <NUM> is a generally cylindrical or frustoconical member, which is formed of a plurality of segmented sections <NUM>-<NUM> joined together. Each of the sections <NUM>-<NUM> is formed of an arcuate plate, and when they are joined together form the cylindrical torque tube shown in <FIG>. Preferably, the material of each section is a composite laminate, however, metallic materials/alloys may be used if desired and determined suitable for the desired application.

<FIG> illustrates a cross-sectional view, in a radial plane, of a torque tube <NUM> formed from a plurality of segmented sections. The torque tube <NUM> is a generally cylindrical, polygonal or frustoconical member, formed of a plurality of segmented sections <NUM>-<NUM> joined together. Each of the sections <NUM>-<NUM> is formed of a flat plate, and when they are joined together form the generally cylindrical or polygonal torque tube shown in <FIG>.

<FIG> illustrates a plan view of a torque tube section, according to an aspect of the present disclosure. Section <NUM> may be any one (or all) of segmented sections <NUM>-<NUM> or <NUM>-<NUM> of <FIG>. Section <NUM> may be a profiled plate or a hollow tube or an arcuate plate and includes one or more apertures <NUM>-<NUM> formed in the main body of the section. The apertures <NUM>-<NUM> serve multiple functions. First, they provide access windows to the windings <NUM> housed in casing <NUM>. Secondly, the apertures <NUM>-<NUM> reduce the overall weight of the torque tube, as material has been removed to create the apertures. Thirdly, the apertures <NUM>-<NUM> decrease heat transfer along the axial section thereof, due to the reduced cross-sectional area created by the apertures.

<FIG> illustrates a cross-sectional view in a radial plane of a multi-strap torque carrying assembly, according to an aspect of the present disclosure. The field coil casing <NUM> is connected to a plurality of straps <NUM>, which are in turn connected to the torque tube <NUM>. The plurality of straps <NUM> function to support axial and transverse or circumferential loads. An additional set of straps <NUM> (not shown) may be used to connect the first torque tube <NUM> to the second torque tube <NUM> (not shown).

Claim 1:
A superconducting generator comprising:
an annular armature (<NUM>) connectable to rotate with a rotating component of a wind turbine;
a stationary annular field winding (<NUM>) coaxial to the armature (<NUM>) and separated by a gap from the armature (<NUM>), wherein the field winding (<NUM>) includes superconducting coils;
a non-rotating support for the field winding (<NUM>),
the non-rotating support comprising an external cryostat housing (<NUM>) disposed around a thermal shield (<NUM>) disposed around the field winding (<NUM>), and a first torque tube (<NUM>, <NUM>, <NUM>, <NUM>) and a second torque tube (<NUM>, <NUM>, <NUM>, <NUM>) connected to the first torque tube (<NUM>, <NUM>, <NUM>, <NUM>), and an annular casing (<NUM>) for housing the field winding (<NUM>), the annular casing (<NUM>) being suspended in the thermal shield (<NUM>),
wherein the first and second torque tubes are configured to transmit torque from the field winding (<NUM>) to the external cryostat housing (<NUM>) and to provide thermal insulation,
wherein the first torque tube suspends the thermal shield (<NUM>) in an interior of the external cryostat housing (<NUM>) and is configured to transmit torque from the thermal shield (<NUM>) to the external cryostat housing (<NUM>) and is configured to provide thermal insulation between the thermal shield (<NUM>) and the external cryostat housing (<NUM>), and
wherein the second torque tube suspends the annular casing (<NUM>) and thermally insulates the annular casing (<NUM>) from the thermal shield (<NUM>),
characterized in that
at least one of the first and second torque tubes (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) is cylindrical or frustoconical and formed of a member comprising a composite material and/or formed of a plurality of segmented sections (<NUM>-<NUM>).