Patent Application
20190346013
Existing flywheel energy storage systems intended for high power systems (100+ Kw) and short duration (seconds to minutes) delivery of energy primarily utilize complex systems of advanced composite materials and active magnetic bearings to take advantage of high-speed capability (20 krpm and above) and to minimize physical size and the quantity of materials require for their construction. These systems have not yet become low enough in cost to result in wide acceptance and multiple product providers. Where low alloy steels have been utilized for these applications, the steel rotors are large cylinders with thick cross-sections, rather than thin disks, resulting in forgings that require expensive processing and more difficult heat treatment due to the longer time constants involved in heating and quenching thick cross-sections. Non-destructive product inspection is also more difficult, less reliable and more expensive than when the cross-sections are thinner. As a result, these steel alloy wheels have not resulted in lower pricing than the composite counterparts.
The opposite end of the flywheel spectrum, which comprises lower power to energy ratios and where charge and discharge durations are multiple hours, is dominated by low alloy steel rotors that are more economical due to the economies of scale. While rotors in high energy systems have weights typically expressed in tons, the lower energy/high power systems are typically a few hundred pounds. As such, installations of these lower power to energy ratio systems can consist of hundreds of machines, rather than single units or a few dozen units at most as is the case with high power to energy ratio applications. This compounds the economies of scale even farther, considering the large volume of steel required.
Most flywheel designs previously produced utilize permanent magnets and/or electro-magnets to either fully levitate the rotating flywheel mass, where magnetic bearings are used (U.S. Pat. Nos. 5,998,899, 5,864,303), or to “off-load” most of the mass when ball bearings are used, to maximize bearing life and lengthen the time between service intervals (U.S. Pat. No. 9,136,741B2). The axial position measurements of a levitated rotor are critical to systems including levitated rotating assemblies. In cases where magnetics are only utilized to off-load ball bearings (or contact bearings of any sort), force sensors have been used to control off-load using magnetic lift to achieve a preferred bearing load, as measured by the force sensor, such as the load cell shown in U.S. Pat. No. 9,136,741B2. In such a system, small movements can result in large load fluctuations, making it difficult to achieve stable operation at preferred low loads as small deviations in the axial location can result in significant change in the magnitude of the bearing load and as a result preferred load levels are difficult to maintain.
As set forth herein, a preferred embodiment of a flywheel system for providing high power over a short time duration takes advantage of the benefits of low alloy steel, namely high quality and ease of inspection that results from a small cross-section, by employing a thin disk cross-section. A significant portion of the market for high power to energy ratios can be addressed using flywheel rotors constructed of a low alloy steel disc less than 3-inches thick and less than 3 feet in diameter. However, the design improvements described herein are also suitable for use in thicker cross-sections. While the shape of the flywheels used herein do not constitute new matter, as they have been utilized for a considerable period of time, the embodiments addressed herein incorporate new and unobvious peripheral components added to the general flywheel configurations employed in high power, short duration flywheel energy storage systems.
More specifically, the improvements to flywheel systems include, but are not limited to,
While the feedback control loop is described for use in a flywheel energy storage and motor-generator, preferably in a vertical arrangement, its application is not limited to such systems and is useful in a wide variety of machines that include rotating shaft assemblies in support bearings with the shafts mounted vertically, horizontally or any angular orientation therebetween.
Mounted to a lower (internal) surface of the upper housing is a magnetic off-loader 36. The conductor wire comprising the magnetic coil 38 in the magnetic off-loader 36 is shown as having a round cross-section but can have various different cross-sections such as square or rectangular or be composed of multiple layers of a sheet material. For clarity only four coil turns are shown but numerous turns are generally used. As best shown in
Positioned in the upper roller bearing housing 42 above the roller bearing 40 is a spring assembly 44. The spring assembly 44 can be composed of various different spring mechanism, such as a stack (for example 4) of Bellville or conical washers, curved disc washers, split disc washers (such as lock washers), coil springs, etc., the purpose of which is explained below. Also located within the upper roller bearing housing 42 and above the spring assembly 44 a defined distance and spaced from a top end 46 of the upper shaft 20, defining a space 41 or gap, is sensor 48 for measuring or sensing the distance of the top end of the shaft 46 from a lower surface of the sensor 48. The sensor 48 is preferably a capacitive sensor, but may also be an inductive sensor, an optical sensor or other suitable sensor for measuring or sensing changes in the lateral distance in the space or gap 41 between the sensor 48 and the shaft top end.
With reference to
It should be noted that the embodiment also shows a sensor 48 in the lower bearing housing 35. Various different position sensing technologies, known to those skilled in the art can be utilized. However, capacitive position sensing is preferred due to the stability of operation and the generation of clean electrical signals, even in the presence of large fluctuating magnetic fields range from about 4,000 rpm to about 13,000 rpm.
Capacitive sensors are non-contacting devices capable of high-resolution measurements of changes, in the nanometer range, in the position of a conductive target spaced small distances (referred to as the gap) from the lower surface of the sensor. These devices generate an electrical signal which changes in a defined manner as the distance across the gap varies. For the feedback loop 100, the value of capacitance measured across the gap is correlated to the distance of the sensor face from the rotor surface. The field strength of the lift magnet (the magnetic off-loader 36) in combination with the spring compression is then modulated to change (raise or lower) the location of the rotor adjusting the lateral positions of the rotor to a preferred location to optimize the bearing life.
For high power/short duration operation a single flywheel assembly is typically sized to provide from 100 to 1000 kW for 15 to 30 seconds. While not limiting the scope and operation of the disclosed embodiment, a preferred assembly utilizes a flywheel rotor consisting of a single steel plate, usually up to about 4 inches thick and up to about 48 inches in diameter. In a more preferred embodiment for electric rail transportation applications the rotor is about 2 inches thick and about 32 inches in diameter to provide 100 kW/0.6 kWh (100 kW for 20 second duration) and would have an operating speed range between 4,000 rpm and 13,000 rpm.
By utilizing the combination of relatively low compression, appropriately sized soft springs in the spring assembly 44 located above the upper roller bearing 42, sensing the location, small fluctuations in the location and dynamic travel of the top end 46 of the upper shaft 20, use of a position feed-back control system 100 stabilizes the magnetic force provided by the magnetic off-loader 36 which adjusts the axial position of the rotor 12 in the air gap 41 from about 0.5 mm to 3 mm to provide sufficiently low bearing loads to provide bearing life that can exceed 10 years. In other words, using the feedback control loop 100, the electrical output of the position sensor 48 is fed to a controller 102 which then sends a signal that controls the electrical current through the electro-magnets in the magnetic off-loader 36, adjusting the vertical location of the flywheel rotor 12 up or down, keeping the lateral position of the rotor 12 in a preferred target location to minimize bearing loads. Any one of several position sensing technologies can be utilized.
Using position sensor feedback to control axial shaft position, particularly a capacitive position sensor because they do not provide false data in presence of a magnetic field, the current output from the sensor allows electromagnet control of the magnetic lift force, which in turn determines the shaft axial position, the shaft position determining the magnitude of the compression force provided by the spring assembly in the bearing housing, thus maximizing bearing service life.
For applications where the duty cycle is high, such as when the motor/generator is not coasting and in its energy delivery mode, it is advantageous to utilize the permanent magnet rotor to take advantage of the compact size (less material for cost savings) and high efficiency capability. Such is the case in electric rail energy recycling, which is a primary application of the devices described herein. However, permanent magnet machines have greater coasting losses because the permanent magnetic field causes eddy currents in the surrounding stationary metallic parts, even during coasting. The permanent magnet coasting loss can be avoided using alternative motor types such as Homo Polar, Synchronous Reluctance and Induction Motors.
As an alternative embodiment of the permanent magnet rotor of
The homo-polar motor-generator 52 can be utilized to boost or buck the field of the permanent magnet motor generator rotor 26. In the boost case, as mentioned above, the homo-polar machine 52 adds to the power output so that the permanent magnet motor generator rotor 26 does not have to be sized for the lowest operating speeds. Another advantage of the presence of the homo-polar motor generator 52 is that it can be used to effectively cancel the field of the rotating permanent magnet when the machine is coasting. This is the case whenever the machine is idling and not motoring or generating. As indicated above, a big disadvantage of permanent magnetic machines in flywheel applications is that the rotating magnetic field produces eddy currents in surrounding stationary electrically conductive material, resulting in drag on the rotating body and lost energy. This lost energy also generates heat, adding to the system heat load and necessitating the use of cooling systems. The controls for the hybrid motor-generator 50 can effectively be utilized to cancel out the field caused by the rotating permanent magnet 26.
The hybrid permanent magnet homo-polar motor generator 50 combination thus increases the overall efficiency of the system that allows reduction of the size and costs of the system and/or provides additional variable load capability. The hybrid combination also avoids the necessity of sizing the permanent magnet alone to achieve power requirements at the lower end of the operating speed range, allowing for a smaller and therefore less expensive permanent magnet component. Minimizing the size of the permanent magnet also minimizes associated coasting losses.
Control of rotating field strength contribution of the homo-polar section 52 can be used to boost and/or add field strength to the field established by the permanent magnet, achieving rated power from a magnet that could not be accomplish otherwise. Controlling the rotating field contribution of the homo-polar section used to buck or cancel some or all of the field strength of the permanent magnet minimizes eddy current coasting losses otherwise caused by the rotating magnetic field of the permanent magnet.
The presence of the permanent magnet section also minimizes the total size required of the homo-polar magnet added to the machine, thus reducing material cost of the homo-polar magnet, which, if used alone is otherwise larger and of lower power density than most other motor-generator types.
Typical flywheels that are utilized in electric rail systems have a tall cylindrical configuration, and thus are not suitable for stacking. Because they have higher operating speeds, their footprint provides a higher power density for a single unit. Their length to diameter ratio requires those units to use large cross-section steel forgings or carbon fiber composites resulting in a higher cost than the systems described herein. By stacking the flywheel units 60 (flywheel assembly 10), which have thinner rotating disks (flywheel rotors 12), the power density requirements can be met while maintaining a much lower cost per unit of power ($/MW).
To enable a higher power density footprint, as shown in
While it is not novel to stack power units the rack configuration itself provides for stable multi-flywheel operation and ease of loading/maintenance. Flywheel stacking has been suggested in the past; however, implementation has not been successfully applied. The present system, because of its lighter weight and operating stability of the units 60, enables the implementation of stable stacked configurations. Vibration stability can also be maintained by using isolation mounts (not shown) between each of the units 60 or each unit 60 and the posts 64 forming the rack structure, and isolation mounts comprising tension mounts 68 and compression mounts 70, which may also include dampening springs attached to or resting on a supporting floor. The result is dynamic stability, ease of use in loading the units 60 into the frame 62 and mobility of the rack and unit assembly.
Rather than adding expense and complication, while also reducing efficiency by using DC-DC conversion to interface mismatched voltages, for example when a 750 Vdc flywheel system is to be utilized on 1500 Vdc electric rail system, the system shown and described herein allows the flywheel to be configured in various alternative voltage configurations.
Most flywheels manufactured for world-wide electric metro-train applications are 750 Vdc. However, there is also a growing number of 1500 Vdc systems. With reference to the stator 28, a dual voltage capability is accomplished with stator coil groupings that have separate parallel connections 80 or a serial connection 82 configuration around the iron support portion of the stator, to provide the two separate voltage configurations, for example 750V and 1500V (
Throughout this disclosure, the preferred embodiments herein and examples illustrated are provided as exemplars, rather than as limitations on the scope of the present disclosure. As used herein, the terms “invention,” “method,” “system,” “present method,” “present system” or “present invention” refers to any one of the embodiments incorporating features of the invention described herein, and any equivalents. Furthermore, reference to various feature(s) of the “invention,” “method,” “system,” “present method,” “present system,” or “present invention” throughout this document does not mean that all claimed embodiments or methods must include the referenced feature(s).
It is also understood that when an element or feature is referred to as being “on” or “adjacent” another element or feature, it can be directly on or adjacent the other element or feature or intervening elements or features that may also be present. Furthermore, relative terms such as “outer”, “above”, “upper”, “lower”, “below”, and similar terms, may be used herein to describe a relationship of one feature to another. It is understood that these terms are intended to encompass different orientations in addition to the orientation depicted in the figures and do not limit the structures shown to any vertical or horizontal orientation.
Although the terms first, second, etc. may be used herein to describe various elements or components, these elements or components should not be limited by these terms. These terms are only used to distinguish one element or component from another element or component. Thus, a first element or component addressed herein could be termed a second element or component without departing from the teachings of the present invention. As used herein, the term “and/or” includes any and all combinations of one or more of the associated list items.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. For example, when the present specification refers to “a” component or “a” material it is understood that this language, in the first instance, encompasses a single component or a plurality or array of components and, in the second instance, a single or multiple material. It will be further understood that the terms “comprises,” “comprising,” “includes” and/or “including” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
This application claims benefit of U.S. Provisional Application Ser. No. 62/669,520 filed May 10, 2018.
| Number | Date | Country | |
|---|---|---|---|
| 62669520 | May 2018 | US |
