Abstract:
A solid steel flywheel rotor having improved material properties offers improved energy storage at reduced cost. A process for manufacturing the rotor is also provided.

Description:
GOVERNMENT RIGHTS LEGEND 
       [0001]    This invention was made with government support under contract OE-0000232 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
     
    
     BACKGROUND 
       [0002]    1. Field of Art 
         [0003]    This description generally relates to energy storage, and particularly to energy storage using flywheels. 
         [0004]    2. Description of the Related Art 
         [0005]    Many energy sources, particularly clean energy sources such as wind turbines and solar panels, generate energy that does not temporally match the load experienced. In much of the developed world, energy generation follows experienced load, such that energy is provided as needed. Under circumstances of high load, techniques such as the use of peaker generators and spinning and non-spinning reserves on thermal generators allow for generation that matches high and variable load. However, despite the availability of such techniques, there are often instances where energy storage is important for meeting energy load. 
         [0006]    Currently existing energy storage systems all have drawbacks of one form or another. Size, price, storage efficiency, efficacy, and safety are all concerns when designing an energy storage system. Generally, smaller size, lower price, reduced loss in both inputting energy for storage and extracting it for distribution, reduced losses for continuous operation, and safe disposal are all preferred characteristics of energy storage systems. 
         [0007]    A flywheel is one type of energy storage system that stores energy as rotational kinetic energy. A flywheel rotor is a weighted, rotationally symmetric mass that spins while physically coupled, directly or indirectly, to a motor/alternator that itself is electrically coupled to a converter, such as a back-to-back inverter system, constituting an AC-AC conversion subsystem. When power is received for storage, the rotor is driven, increasing the rotational speed of the flywheel rotor. When power is to be extracted, the flywheel rotor drives the motor/alternator. The faster a flywheel rotor can spin, the more energy it can store, but the more stress is induced on the rotor. Generally, the amount of stress a rotor is able to sustain while operating is a function of the design, materials, and processes used to make the rotor. Specifically, the amount of stress that can be sustained depends on a combination of the rotor material&#39;s yield strength, fracture toughness, maximal intrinsic defect size, cyclic fatigue characteristics, and the rotor&#39;s shape, among other factors. Generally, a flywheel&#39;s bearing and suspension subsystem is designed to minimize energy losses due to friction, and other loss sources. 
         [0008]    Cost relative to the amount of energy that can be stored is of particular importance for a flywheel system. The cost of a flywheel system can be roughly divided into two portions, the cost of manufacturing the flywheel rotor, and the balance of system costs for supporting elements such as bearings, mountings, enclosure, etc. In the past, flywheel rotors have been very expensive to manufacture. As a result, flywheel systems have primarily been used in applications involving only seconds to minutes of energy storage, as it was simply too costly to either manufacture a single rotor that can store tens to hundreds of kWh of energy, or to use many individual rotors that are cost inefficient with respect to the balance of systems costs for the supporting elements used in conjunction with the rotors. 
         [0009]    Some existing flywheel rotors are made of common, low alloy steels such as American Iron and Steel Institute (AISI) 4340 and AISI 4140. These steels have low costs and other desirable properties, however such rotors are limited to thin sections due to limitations in through-hardenability, which is required to achieve a useful yield strength and therefore that can handle a significant amount of stress. For example, although these rotor materials can achieve ultimate tensile strengths (UTSs) of 2 gigapascal (GPa), and fracture toughness of 40 megapascal square root meter (MPa·m 0.5 ), such rotors are limited to maximum cross-sectional thicknesses of 3-6 inches. 
         [0010]    Other steel flywheel rotors are made with high-alloy steels such as maraging steels, Aermet steels, and some stainless steels. These flywheel rotors are able to sustain higher stresses throughout cross-sectional thicknesses greater than 6 inches. These rotors achieve these stresses without the need for multiple separate sections, but are cost prohibitive due to the high content of expensive alloying elements such as nickel and cobalt. Other modern flywheel rotors are made of carbon fiber and therefore allow for significantly higher working stresses, however the high cost of carbon fiber and the ancillary components needed to achieve the corresponding higher rotational speeds makes carbon fiber rotors prohibitively expensive, despite their high working strength-to-weight ratios. 
       SUMMARY 
       [0011]    A high strength metal alloy flywheel rotor is described that offers improved kinetic energy storage at reduced cost. The flywheel rotor&#39;s performance is based in part on its material characteristics. In one embodiment it has a yield strength of at least 900 MPa, a fracture toughness of at least 70 MPa·m 0.5 , and a maximal intrinsic defect size of at most 2 millimeters (mm). These characteristics are consistent throughout its entirety. 
         [0012]    In the same or a different embodiment, the rotor may be made of 300M vacuum-arc-remelting (VAR) steel that is through-hardened and tempered. 
         [0013]    In various implementations, the rotor may also have additional characteristics beyond any of those described above. For example, the rotor may be formed with a diameter greater than its length (or thickness), or vice versa. The rotor may be rotationally symmetric. The rotor may be a monolithic shape without any bored holes. The rotor may include one or more journals protruding from the rotor allowing an external shaft to be physically attached to each journal. In one embodiment, the shaft may be attached using a shrink fit. 
         [0014]    An exemplary method for manufacturing the rotor is also described. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]      FIG. 1  is a block diagram of a flywheel energy storage system according to one embodiment. 
           [0016]      FIG. 2  is a cross sectional view of a flywheel rotor according to one embodiment. 
           [0017]      FIG. 3  is a cross sectional view of a journal of a flywheel rotor according to one embodiment. 
           [0018]      FIG. 4  is an exemplary process for manufacturing the flywheel rotor according to one embodiment. 
       
    
    
       [0019]    The figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
       DETAILED DESCRIPTION 
     I. Flywheel Energy Storage System 
       [0020]      FIG. 1  is a block diagram of a flywheel energy storage system  100  according to one embodiment. The energy storage system includes a flywheel rotor  130 , a motor/alternator  140 , a first inverter  150 , a capacitor  160 , a second inverter  170 , and an AC line  180 . Energy is drawn from, or delivered to, an AC line  180 , such as a conventional three-phase 60 Hz line. The first  150  and second  170  inverters as well as capacitor  160  illustrate an exemplary back-to-back converter system for converting the input alternating current into an alternating current acceptable to the motor/alternator  140 . The motor/alternator  140  converts between electrical and mechanical energy, so that energy can be stored in or drawn from the flywheel rotor  130 . The motor/alternator  140  is physically coupled to the flywheel rotor  130  either directly or indirectly using a shaft. The motor/alternator  140  is coupled to the remainder of the system  100  via wires or other electrical couplings. Generally, although only one of each component is shown, in practice a flywheel energy storage system  100  may include multiples of each individual component.  FIG. 1  is one exemplary type of ac-to-ac conversion system. In general, the inventions described herein pertain to a broad range of ac-to-ac conversion topologies, as well as systems that interface directly to a direct current (dc) line. The latter are of especial relevance for dc microgrid and solar photovoltaic applications. 
       II. Flywheel Rotor Shape 
       [0021]      FIG. 2  is a cross sectional view of a flywheel rotor  130  (or simply rotor) according to one embodiment. The rotor  130  is formed of a single mass of material. However, two different portions of the rotor  130  generally can be said to perform different functions. A primary rotational mass  230  makes up most of the mass of the rotor and stores the majority of the kinetic energy stored by the rotor. Two journals  212  extend perpendicularly from either side of the primary rotational mass and assist in coupling the rotor to separate shafts (not shown). Each of these portions is further described below. In some instances, the rotor may also include elements on its outer surface, for example discrete masses to provide centrifugal loading. 
         [0022]    The rotor is generally rotationally symmetric, and thus the rotor can be described using a cylindrical coordinate system where the origin is through the center rotational axis of the rotor. In implementations including other elements on the outer surface, such as discrete masses, the rotor and the discrete mass elements are both uniformly distributed about the origin 
         [0023]    To provide an example of scale, in one embodiment, the rotor  130  is between 36-72 inches in diameter, and weighs between 2-5 tons. 
         [0024]    II.A Primary Rotational Mass 
         [0025]    Beyond being rotationally symmetric, the primary rotational mass may be formed in a variety of different shapes, each designed to achieve specific performance goals. In one implementation, the primary rotational mass  230  of the rotor has a “fishtail” shape, when viewed in cross-section. 
         [0026]    The fishtail shape helps ensure a nearly uniform distribution of stress throughout the primary rotational mass  230  due to rotational forces exerted on the rotor. The fishtail shape is an exemplary shape for optimizing rotor mass and material volume utilization, i.e. for optimizing the shape factor. Generally, the fishtail shape includes a center section and an adjoining peripheral mass. In the center section, the rotor is thicker closer to a first radius  202  near the center axis  226 , and continuously decreases in thickness out to a second radius  204 , away from the origin. In one embodiment, this central section is governed by a profile of the form: 
         [0000]        t=he   −βr     2     (1)
 
         [0000]    where t is the longitudinal thickness of the rotor, h is the central thickness, r is the distance away from the origin along the polar axis, and β is a constant. 
         [0027]    Regarding the peripheral mass&#39;s shape, between the second radius  204  and a third radius  206  near the outer diameter  210  of the rotor, the primary rotational mass  230  continuously increases in thickness in the longitudinal axis. Between the third radius  206  and a fourth radius  208 , the rotor maintains a consistent thickness in the longitudinal axis for a short distance along the polar axis. The fourth radius  208  is located at or near the outer surface  210  of the rotor along the polar axis. Near the fourth radius  208 , the rotor&#39;s edges may be rounded or squared. 
         [0028]    Regarding the relative proportions of the center section relative to the peripheral mass, the majority of the mass of the fishtail portion is located in the center section. 
         [0029]    In the example illustrated in  FIG. 2 , the outer surface  210  of the rotor  130  has a diameter that is greater than the widest thickness  228  of the fishtail portion of the rotor  130 . As will be further described below, any shape of rotor that allows the entirety of the rotor to be through-hardened is capable of achieving relatively high levels of working stress. Generally, rotors with diameters greater than their thickness rotate at slower speeds than their counterparts with thicknesses greater than their diameter. Slower rotational speeds reduce the operational requirements of the bearing assembly that allows the rotor to rotate, thereby reducing the overall cost of the flywheel system. 
         [0030]    In another embodiment, rather than having the fishtail shape, the rotor instead has a cylindrical shape. 
         [0031]    II.B Journals 
         [0032]    Along the longitudinal axis (or center rotational axis) of the rotor, the rotor includes two journals  212  for attaching and detaching a shaft for transferring energy between the rotor and the bidirectional motor/alternator  140 . The journals  212  remove the need for a bore to couple the rotor to the shaft. A bore results in a doubling of hoop stress at the inner diameter of the bore. Such bores are often drilled into rotors after manufacturing of the rotor, or the rotors are deliberately designed and manufactured with such a hole in mind. In contrast, replacing a bore with the journals  212  allows stress to be more evenly distributed throughout the primary rotational mass, thereby avoiding a stress riser where the bore would otherwise be placed. 
         [0033]      FIG. 3  is a cross sectional view of a journal  212  of the rotor according to one embodiment. Each journal  212  extends outward  218  from a mound  214  of increased thickness that itself extends outward from the origin of the center section of the primary rotational mass  230 . The mound  214  has a tapered shape that has a thickness greatest adjacent to the journal  212 , and which tapers gradually in thickness as radius increases. The gradually tapering shape of the mound  214  isolates the journal from experiencing a stress riser or peak stress at the point where the journal  212  adjoins the primary rotational mass  230 . 
         [0034]    A fillet is present where the journal  212  and mound  214  are joined. The fillet avoids stress risers around the journal  212 . The journal&#39;s outer surface is substantially planar along the polar axis. The outer surface of journal  212  is narrower  220  than a connecting end  222  of the shaft  226  configured to attach to the journal  212 . The shaft then narrows  224  considerably for the majority of its length. Generally, the diameter  220  of the interference fit between the upper surface of the journal  212  and the shaft is greater than the diameter  224  of the shaft. Having a comparatively large diameter  220  for an interference fit is beneficial for further reducing stress risers inside the rotor  130  near the journal  212 , since only a relatively light interference fit is needed with such a large diameter. Further, the interference stresses induced in the journal region are generally compressive, and thus work to mitigate centrifugally induced stresses in the journal  212 . Thus, the combination of the tapering shape and wide diameter  220  of the journal  212  result in a mechanism for coupling with the shaft that minimizes the stress impact of the coupling on the rotor  130  as a whole. 
         [0035]    In one embodiment, the shaft  226  is coupled to the journal  212  via a shrink fit. For example, the shaft can be heated prior to attachment to the journal  212 , causing the shaft to thermally expand. After heating, the journal  212  and shaft can be attached. The shaft is then allowed to cool, thereby thermally contracting to create an interference fit with the journal  212 . In another embodiment, an internal press fit may be used, with cooling of the shaft used to create the interference fit between the journal  212  and the shaft  226 . The shaft  226  may also be coupled to the journal  212  via a press fit, or with a central axially oriented retaining bolt if a hollow cylindrical shaft is used. These are examples of numerous alternatives for coupling the shaft  226  to the journal  212 . 
         [0036]    As an example, in one embodiment the journal  212  has an outer diameter of approximately 4-6 inches, and protrudes outward from the mound approximately one inch. The shaft has a connecting end  222  outer diameter of 6-8 inches. Away from the connecting end, the shaft  226  has an outer diameter of 1-3 inches, which is narrower than the 4-6 inch outer diameter of the journal  212 . 
         [0037]    The rotor, including the primary rotational mass  230  and journals  212 , is manufactured as a single piece of material, for example using the example materials and example process described below. Thus, the rotor has a single body construction where there are no welds, joints, seams, holes, or differences in construction between the primary rotational mass and journals  212 . However, also as further described below, different portions of the single body/single piece rotor may be subjected to different treatments and/or manufacturing processes to vary the properties of the rotor at different points. For example, the surface of the rotor may receive different treatments than the interior of the rotor. 
       III. Rotor Material Properties and Manufacturing 
       [0038]    The performance of the rotor is based on several parameters of the materials that make up the rotor, as well as the manufacturing processes performed to convert the raw materials into the final state as they appear in the rotor. These parameters include the yield strength of the rotor, the fracture toughness of the rotor, the maximal intrinsic defect size (or maximum initial crack size) in the rotor, and the cyclic fatigue (or cyclic crack growth rate). The rotor may also be described in terms of other properties that are either known equivalents of these properties or that can be converted into/derived from these properties. 
         [0039]    In one embodiment, the rotor has parameter values such that the yield strength σ yield  of the rotor is greater than a first threshold, the fracture toughness σ fracture  of the rotor is greater than a second threshold, and the maximal intrinsic defect size a intr  is less than a threshold size. Defined in this way the rotor achieves significant performance in the working stress σ working  it can endure over its operational lifetime. During the operation the rotor will always meet the following condition: 
         [0000]      σ working &lt;ασ yield   (1)
 
         [0000]    where α is a parameter for derating between 0 and 1. Further, the rotor material is designed such that during the operational lifetime of the rotor, the cyclic crack growth, or growth of an initial crack present in the rotor during manufacturing as it grows towards the critical crack size, grows slowly enough to permit tens of thousands of complete stress cycles. 
         [0040]    As a specific example, in one embodiment the rotor has a yield strength σ yield  of at least 900 MPa, a fracture toughness σ fracture  of at least 70 megapascal per square root meter (MPa·m 0.5 ), and a maximal intrinsic defect size that is 2 millimeters (mm) or smaller. In another embodiment, the rotor has a yield strength σ yield  between 900 MPa and 2 GPa, inclusive, a fracture toughness σ fracture  between 40 and 200 MPa·m 0.5 , inclusive, and a maximal intrinsic defect size between 0.05 mm and 2 mm, inclusive. In other embodiments, the rotor may have properties within any sub-range within the above described ranges. For example, in one embodiment, the rotor has a yield strength a σ yield  of between 900-1000 MPa, 1000-1100 MPa, 1100-1200 MPa, 1200-1300 MPa, 1300-1400 MPa, 1400-1500 MPa, 1500-1600 MPa, 1600-1700 MPa, 1700-1800 MPa, 1800-1900 MPa, 1900-2000 MPa, or any combination of sub-ranges thereof. In the same or a different embodiment, the rotor has a fracture toughness σ fracture  of between 40-50 MPa·m 0.5 , 50-60 MPa·m 0.5 , 60-70 MPa·m 0.5 , 70-80 MPa·m 0.5 , 80-90 MPa·m 0.5 , 90-100 MPa·m 0.5 , 100-110 MPa·m 0.5 , 110-120 MPa·m 0.5 , 120-130 MPa·m 0.5 , 130-140 MPa·m 0.5 , 140-150 MPa·m 0.5 , 150-160 MPa·m 0.5 , 160-170 MPa·m 0.5 , 170-180 MPa·m 0.5 , 180-190 MPa·m 0.5 , 190-200 MPa·m 0.5 , or any combination of sub-ranges thereof. In the same or a different embodiment, the rotor has a maximal intrinsic defect size of between 0.5-0.6 mm, 0.6-0.7 mm, 0.7-0.8 mm, 0.8-0.9 mm, 0.9-1.0 mm, 1.0-1.1 mm. 1.1-1.2 mm, 1.2-1.3 mm, 1.3-1.4 mm, 1.4-1.5 mm, 1.5-1.6 mm, 1.6-1.7 mm, 1.7-1.8 mm, 1.8-1.9 mm, 1.9-2.0 mm, or any combination of sub-ranges thereof. 
         [0041]    A rotor that meets the above exemplary thresholds can be made of 300M steel. 300M steel is described by Aerospace Material Standard (AMS) Society of Automotive Engineers (SAE) 6257 (referred to simply as SAE-6257). 300M steel has a proportional chemical composition of 1.6% Silicon (Si), 0.82% Chromium (Cr), 1.8% Nickel (Ni), 0.40% Molybdenum (Mo), 0.08% Vanadium (V), and a range of 0.40-0.44% Carbon (C), with remainder being Iron (Fe). 300M steel has a relatively low cost, and thus is advantageous for reducing the cost of a flywheel energy storage system including a rotor made of this material. The V and Si are alloying elements that offer improved hardenability and allow thick-section rotors to be made that are up to 14″ thick and entirely through-hardened, for example in the fishtail shape as described above. 
         [0042]    However, mere specification of 300M steel alone is insufficient to ensure the parameters specified above. Additional manufacturing steps are used to improve the performance of the rotor. These steps include refining, multi-step forging, heat treatments, surface treatments, and machining. 
         [0043]    The 300M steel is refined using a refinement process such as vacuum-arc-remelting (“VAR”), electro-slag-remelting (“ESR”), or vacuum induction melting (VIM). These processes help remove defects larger than the desired maximal intrinsic defect size. In contrast, if the 300M steel were instead melted in open air, it would tend to have defects larger than this desired maximal intrinsic defect size such as inclusions or other impurities. VAR refinement helps ensure that the maximal intrinsic defect size is 2 mm or smaller. 
         [0044]    Multi-step forging introduces directional grains into the rotor. Generally, grain orientation is determined based on the forging process used. A single step forging process may be insufficient to ensure the presence of consistent directional grains throughout the entirety of the rotor. Performing multiple forging steps helps ensures consistent grain orientation throughout the entirety of the rotor. Controlling grain orientation also has the added benefit of shaping and orienting any inclusions present in the rotor. 
         [0045]    Generally, heat treatments are used to increase yield strength and hardness of steel. In a heat treatment, the steel is heated (or austenetized) into austenite. The time and temperature of the heating in part defines the grain size of the rotor. The austenite is then rapidly cooled (or quenched). The quenching converts the austenite into one of several other material phases of steel, such as pearlite and martensite. Due to the physics of heat transfer, not all depths within the steel will cool at the same rate, meaning that shallower depths of the steel will often quench into a significant proportion of martensite (e.g., greater than 50% martensite), whereas deeper depths of the steel may quench into a significant proportion of pearlite or other material phases, with only a minority of the steel quenching into martensite (e.g., less than 50% martensite). The proportions of various material phases a steel quenches into is governed by the material&#39;s transition curve (referred to as a TTT curve). Martensite, specifically, is desirable for use in a rotor because it has very high yield strength and also very high hardness. A piece of steel is said to be through-hardened when at every thickness the steel contains at least 50% martensite. 
         [0046]    In one embodiment, VAR 300M steel is used in the rotor because it is possible to through-harden the 300M steel to depths of 8-14 inches, making it very useful for forming a rotor of sufficient size to store a significant amount of kinetic energy. Particularly, the Si and V alloying elements in 300M delay the formation of pearlite during quenching in favor of the transition to martensite, resulting in increased through-hardness at significant depths within the steel. In a VAR 300M rotor, through-hardening allows the rotor to achieve a yield strength σ yield  of up to 2 GPa. 
         [0047]    Quenched steels have a drawback of having a low fracture toughness. Consequently, the quenching step can be followed by a tempering step. Tempering maintains the steel at a temperature lower than austenetizing temperature (e.g., 600-1200 Fahrenheit (F)) for a period of time (e.g., several hours) before cooling slowly back to room temperature. At the expense of some yield strength σ yield , tempering significantly improves fracture toughness, and eliminates residual internal stresses. In a VAR 300M rotor, tempering allows the rotor to achieve a fracture toughness σ fracture  of at least 70 MPa·m 0.5  while also maintaining a σ yield  of over 900 MPa. 
         [0048]    Surface treatments protect the surface of the rotor. Several different surface treatments may be used. A first is shot peening, where compressive stress is imparted to the surface of the rotor to harden it. A second is nitrogen and/or carbon treatments that similarly increase hardness as well as the yield strength of the rotor&#39;s surface. Other surface treatments may also be used. 
         [0049]    One advantage of a rotor constructed as described above is that a significantly larger rotor, one capable of storing tens to hundreds of kWh of energy, can be manufactured at low cost relative to other potentially conceivable processes. Further, the total cost of a flywheel system incorporating such a rotor is also lowered relative to existing flywheel systems that use many smaller rotors. This is due to the fact that using a large rotor reduces the need for multiple rotors and their associated supporting elements. For example, it is much less expensive for a flywheel system to use a larger bearing to support a larger rotor versus using many smaller rotors each using their own smaller bearings. Further, a single monolithic rotor is also generally more economical than a rotor assembled from a stack of separate rotor components. 
       IV. Method of Manufacture 
       [0050]      FIG. 4  is an exemplary process for manufacturing a rotor according to one embodiment. In the example of  FIG. 4 , elements are alloyed  401  to manufacture steel with the desired material composition. For example, if 300M is to be used, Si, Cr, Ni, Mo, V, C, and Fe are alloyed together. The alloyed elements are then refined  403  to remove large defects. Continuing with the example above, the VAR process may be used to refine the 300M alloy. The refined alloy is then forged  405  to near net shape using a multi-step process to orient grain size and direction. A heat treatment is applied 407 to through-harden the forged rotor material to improve yield strength. Tempering  409  is then performed to improve fracture toughness. The rotor material may then be machined  411  to form the rotor into the desired shape. 
         [0051]    Upon reading this disclosure, those of skill in the art will appreciate still additional alternative structural and functional designs through the disclosed principles herein. Thus, while particular embodiments and applications have been illustrated and described, it is to be understood that the disclosed embodiments are not limited to the precise construction and components disclosed herein. Various modifications, changes and variations, which will be apparent to those skilled in the art, may be made in the arrangement, operation and details of the method and apparatus disclosed herein without departing from the spirit and scope defined in the appended claims.