Wind turbines larger than 6 MW are becoming common, particularly for offshore installations, where consolidating several smaller wind turbines into one large version reduces manufacturing, installation, and sustainment costs in accordance with economies of scale.
The overall reliability of the wind turbine is mostly determined by the mechanical gearbox, which converts the practical low speed of the wind-turbine rotor propeller hub (e.g., 5 to 20 RPM) to the variable high speed of a conventional electric generator system (e.g., two pole-pair, 60 Hz, 450 to 1800 RPM). In turn, large wind turbines are eliminating the gearbox (and its compounded inefficiency, complexity, size, and cost) with low-speed, directly driven electric generator systems. However, as physics dictates, low-speed electric generator systems are necessarily large in diameter (and heavy).
For example, the radial-flux, direct-drive, rare-earth permanent-magnet (RE-PM) electric generator of the 12-MW Haliade-X wind turbine from GE is approximately 11 meters (or 36 ft) in diameter and consumes a significant portion of the 20.6-meter (67.5 ft) long nacelle weight, which is over 600 metric tons (or 660 tons). All of that must be transported to the wind-turbine installation site over specially prepare routes and then lifted more than 130 meters to the tower hub. Accordingly, the manufacture, installation, maintenance, and transportation of large wind-turbine generator systems requires uncommonly large, specialized, and expensive handling equipment with complicated transportation logistics.
Under an ARPA-E program, the U.S. Department of Energy is asking for a âlightweightâ (or small) high-power, low-speed (e.g., 5 to 20 RPM), direct-drive electric generator system for the next generation of large wind turbines. But a âlightweightâ direct-drive electric generator seemingly contradicts standard electric motor or generator (i.e., electric machine) design principles and trade space as follows:
In accordance with the previous standard electric-machine design principles and trade space, a low-speed, direct-drive electric generator âsystemâ will necessarily be large in diameter, regardless of the flux-density potential of superconductors or the deep air-gap depth conveniently supported by RE-PMs. Instead, a practical lightweight, low-speed, high-power, direct-drive electric generator for wind turbines is only provided with:
1. A true multiphase wound-rotor (synchronous) doubly-fed electric machine circuit and control architecture that conveniently accommodates the axial-flux form and effectively halves the frame size and weight per unit of power.
2. An electric-machine circuit and control architecture that conveniently accommodates ultra-low-frequency excitation control to reduce pole-pair count and resulting diameter.
3. A 3D printer that conveniently accommodates the additive manufacture of large axial-flux electric machines with the highest performance materials to reduce size.
4. An electric-machine circuit and control architecture that conveniently accommodates safe separation into multiple components of transportable size, diameter, and weight. In turn, these components should be able to be conveniently lifted, reassembled, and power stacked lengthwise inside the nacelle at the installation site to incrementally meet the wind-turbine power rating.
Itâs possible to stack two fully assembled and functional SYNCHRO-SYMs (1) and (2). Each has its own bearing and frame assembly, axle assembly, and integrated electronic control. As a result, the power stack shows the accumulated power, which is independent from the wind-turbine rotor hub and bearing assembly. The stator (3) and rotor (4) are duplicate axial-flux disk assemblies. The axle assembly (5), which is attached to the rotor assembly (4), and the stator assembly (3) have bayonet plugs (6) that align and mate (8) with the bayonet sockets (7) to form a rigid but separate integrated stack of stators (3) and rotors (4) of multiple assemblies.
Looking at the figure, another method for mating âstatorsâ would comprise a set of sliding frames (or rails) that span at least the full length of the stack of SYNCHRO-SYMs. The rails are a portion of the âstatorâ bayonet plugs (6) with a similar portion of sockets (7) configured as channel blocks for inserting the sliding frames. With each lightweight SYNCHRO-SYM component lifted to the nacelle and positioned onto the sliding frames by an internal nacelle crane, such as the rotor (4) and axle (5) assembly, the stator (3) and bearing assembly, or each complete SYNCHRO-SYM in the stack, the component would slide along the rails for alignment and attachment to another component inside the nacelle. As a result, all stators of the SYNCHRO-SYM stack are joined as one by the stator rails (or sliding frames) and stator bayonet methods (6,7). And separately, all rotors of the SYNCHRO-SYM stack are joined as one by the rotor bayonet method (6,7).
The same field installation and assembly process (using the internal nacelle crane as just discussed) is also utilized to replace or maintain any rotor, stator, or fully functional component within the entire stack. Or more importantly, itâs used to retrofit legacy wind-turbine systems, which have peaked their useful operational life.
Likely, the âlegacy systemsâ have an obsolete mechanical gearbox, generator, and electronic drive, all of which can be hastily disassembled and removed, and then replaced with a small-diameter, low-speed, âdirect-drive stackâ of SYNCHRO-SYM components. Since the legacy electronic drive, gearbox, and generator upgrade is only 18% of total wind-turbine cost, the 80/20% tax credit rule is satisfied, while upgrading to a higher-performing, more efficient, more reliable, integrated, variable-speed wind-turbine system.
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Using a computer-aided-design (CAD) tool specifically developed for the axial-flux SYNCHRO-SYM manufactured with MOTORPRINTER, called BEM-CAD, resulted in the table shown below. The table presents specification comparisons between the 12-MW Haliade-X wind turbine from GE and a resilient power stack of 12, small, lightweight, self-contained, 1-MW SYNCHRO-SYMs. Each SYNCHRO-SYM can be separated again into smaller rotor and stator components for easiest handling and transportation.
This table compares a GE 12-MW Haliade-X wind turbine with a power stack of 12, small, lightweight, self-contained, 1-MW SYNCHRO-SYMs.
BRTEC brings superconductor electric-machine systems closer to reality by exciting the conventional sinusoidally distributed active winding set with pure sinusoidal excitation waveforms, which reduces cryogenic refrigeration by avoiding harmonic heating of the superconductor electromagnet, and by brushlessly relocating the superconductor electromagnet to the stationary body (stator) for convenient logistical support. NOTE: When alternating-current (AC) superconductors become a reality, only the fully electromagnetic SYNCHRO-SYM will be the superconductor electric machine of choice.
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