Development of Compact Flywheel Energy Storage System (ComFESS)

Transcription

1 TECHNICAL PAPER Development of Compact Flywheel Energy Storage System (ComFESS) A. KUBO H. KAMENO R. TAKAHATA The objective of this research program is to develop a flywheel energy storage system that can replace the lead battery, which has inherent problems in regard to maintenance and environmental protection. The basic development target has been a "1 kwh class system (ComFESS) capable of providing a backup power supply of 3 watts for approximately 3 hours," for which challenges were reduction of windage loss and rotation loss. Therefore, the flywheel is housed in a vacuum container, and an active magnetic bearing has been adopted to support the flywheel without contact. This paper concerns the watt loss as related to the specifications of the active magnetic bearing (AMB) and motor/generator. Specifically, discussion will be focused on the approaches employed to cut down on overall watt loss of the flywheel system to approximately a quarter of that involved in the conventional system. This improvement stems mainly from the reduction of AMB loss achieved by means of the zero-power nonlinear control. This zero-power nonlinear control, being able to dispense with the bias current, enables both AMB loss and power consumption for AMB control to be minimized. Key Words: flywheel energy storage system, ComFESS, active magnetic bearings zero-power nonlinear control, hysteresis motor 1. Introduction Along with the advance of the IT revolution, the impact of disrupted communications due to blackouts and the like on social and personal life has become extremely serious. That is why many lead batteries are currently used as the backup power sources, and rapid expansion of their usage is expected. However, the lead batteries have service life of only several years (3~4 years) and require maintenance. To make matters worse, as they contain chemical substances and heavy metals that are hazardous to the human body and plants, the batteries cannot help facing the environmental problems. Under such circumstances, clean, high-performance energy storage systems that can replace the lead batteries having maintenance and environmental problems are demanded. The flywheel energy storage system is one of the candidates, regarding which development has been progressing worldwide 1). This research program has been aimed at development of elemental technology that would comprise a compact flywheel energy storage system (hereinafter referred to as "ComFESS"), which would be a 1 kwh class system capable of supplying 3 W power for approximately 3 hours, replacing the lead batteries. In order to ensure 3 hours backup power supply, it is necessary to minimize the windage loss and bearing loss of the ComFESS. Therefore, the flywheel is located in a vacuum chamber and the flywheel rotor is supported by active magnetic bearings (hereinafter referred to as "AMB") that provide non-contact support with minimum loss. 2. Target of Loss Reduction With a view to constructing a system of 3 W 3 hours, a conceptual system design was first worked out 2). The structure of the ComFESS is shown in Fig. 1. Flywheel Vacuum container Hub Axial AMB Lower side RaAMB unit Main shaft upper side RaAMB unit Motor / Generator Fig. 1 3D drawing of ComFESS To make the system compact, we incorporated a flywheel 44mm in diameter made of CFRP (carbon fiber reinforced plastic). Inside the bore of the flywheel is mounted the rotor unit comprised of the hub and the shaft. The rotor unit is accelerated up to 24 min 1, and the electric power is taken out while the unit is subsequently decelerated. Here, however, a part of the energy stored in the system is consumed as the driving power for the AMB controller and the inverter. Taking this into account and assuming that the power of 3 W 3 hours is taken out, calculation is made to approximate the rotational decay of the rotor as shown in Fig. 2. In this figure, the "+" marked curve represents the rotational decay pattern Koyo Engineering Journal English Edition No.167E (25) 29

2 for the case of conventional AMB technology and the motor/generator. Specifically, the AMB relies on the linear control method consuming bias current 3), whereas the motor/generator is such that the induction motor is driven by a conventional electric converter. Thus, the figure indicates that the conventional system can take out the power only for 1.5 hours. Table 1 summarizes various losses estimated for the rotational speed condition of 24 min 1. The values in the "conventional" column are for the conventional technology while those in the "Target" column are the values at which the new system is aimed. Rotational speed, min : Target : Conventional Time, hours Fig. 2 Estimated rotational decay of the rotor Table 1 Classification of ComFESS min 1 Conventional [W] Target [W] Loss of AMB 5 1 Windage loss (at 1Pa) 5 5 Loss of motor / generator 2 95 Total Design of ComFESS In an attempt to achieve the above target, we worked on design of ComFESS. The results are as follows Motor/Generator & Converter As motor/generator candidates for this system, a hysteresis motor and permanent magnet motor (hereinafter referred to as "PM") were comparatively appraised. As a result, the hysterisis motor was selected because of its having the following features: Simple structure with excellent cost performance Smaller loss (PM motor has 4~5 times greater loss) No need to develop converter (IGBT type converter available in the market was selected) 3. 2 Magnetic Bearing (AMB) 1) PM Biased axial magnetic bearing (AxAMB) In this project, a PM biased axial magnet (PM biased AxStator) that allows PM attracting force was adopted, which can substitute for the magnetic force required to sustain the dead weight of the rotor unit. 2) Radial active magnetic bearing (RaAMB) A pair of radial active magnetic bearings provided on each of the upper and the lower parts of the spindle respectively are also required to save energy consumption. In this project, therefore, the "zero-power non-linear control method" was adopted to minimize the bias current of RaAMB (total current supplied to RaAMB, I = bias current, I + control current, I c ) Rotor Unit The rotor unit of ComFESS is comprised of the shaft, the hub and the flywheel. Its natural frequency was calculated as shown in Fig. 3. The main specifications of the ComFESS are summarized in Table 2 6). The target value for loss of AMB is set around 1 W, which is expected to be attained by adopting the non-linear control (zero-power control) 4) and other improvements. The windage loss of 5 W is based on the pressure of 2 Pa, which cannot be further improved. On the other hand, the loss of the motor/generator is targeted for around 95 W by adopting the motor with smaller loss than the induction motor. The rotational decay curve expected if the targets in Table 1 are achieved is plotted in Fig. 2 with " " marks. This result indicates that the taking power of 3 W 3 hours would be attainable 5). Rotor unit radius, mm f2b : 1 185Hz (1st Bending mode of shaft) # # f1b : 145Hz (1st Bending mode of hub) 1 # : Bearing : Sensor Rotor unit length, mm 5 Fig. 3 Result of mode shape simulation 3 Koyo Engineering Journal English Edition No.167E (25)

3 Table 2 Main specifications of ComFESS Rotor Total mass of rotor: 75kg Main shaft: Outer dia. 75mm Length 258mm Flywheel: Outer dia. 44mm Inner dia. 3mm Length 26mm Energy storage capacity 9Wh (effective) Kinetic energy of the rotor 1.6kWh (24 min 1 ).1kWh (5 min 1 ) Power capacity AxAMB AMB (1 DOF) PM biased AxAMB Control method: Nonlinear control RaAMB AMB (4 DOF) Control method: Linear control Electromagnets: Hetero-polar Rotor lamination: Silicon steel plate Touch down bearings Emergency support for Radial Upper side: direction Lower side: Radial direction and axial direction AMB controller Power supply for magnets: 8~15DCV Max. current for AMB: 4A (total) 8A (max.) for each electromagnet Amplifier: PWM 4. Basic Element Tests and Results Basic element tests were carried out to confirm the performance of each element with basic design. Therefore, three kinds of test machines were designed and manufactured. The first one was ComFESS BB. The flywheel was comprised of CFRP and the main shaft including the flywheel and hub was supported only by ball bearings. This test machine was used for testing such elements as the motor, flywheel and vacuum chamber. The second test machine was a dummy ComFESS using a bakelite flywheel and active magnetic bearing. It was used for element tests of AMB and the natural frequencies of the rotor. The third test machine was a flywheel AMB system (AMX95) designed to evaluate the zero power control. After all element tests, ComFESS with AMB and CFRP flywheel was assembled by using some parts of Dummy ComFESS and ComFESS BB as shown in Fig. 3. The results of the basic element tests are as follows Measurement of Loss Each category of loss measured on the ComFESS BB at 24 min 1 is shown in Table 3. The motor/generator controller had a loss of 2 W, which clearly was far smaller than the target of 7 W. Thanks to this, the total measured loss on ComFESS BB was 3 W, which met the target of 345 W. Table 3 Measurement of ComFESS BB loss ComFESS BB Target Measurement Motor / Generator (W) 25 3 Motor / Generator controller (W) 7 2 Motor / Generator total loss (W) A 95 5 Ball bearing (W) 2 2 Windage loss (W) 5 5 Rotational loss (W) B Total loss (W) A+B Test on PM Biased Axial Magnetic Bearing The rotor unit of the ComFESS was successfully levitated stably. At that moment, the waveform of electric current was measured with the results as shown in Fig. 4. Total current, A Total current, A Time, sec (a) PM biased AxAMB 2.5A Mean :.22A N = [rpm] Time, sec (b) Conventional AxAMB Fig. 4 Measured results of PM biased min 1 The total current supplied to the PM biased AxAMB was significantly influenced by the noise. Nevertheless, the average current was held around.22 A which was far lower than approx. 2.5 A required with the conventional AxAMB, confirming the energy saving effect of the PM biased AxAMB. Koyo Engineering Journal English Edition No.167E (25) 31

4 4. 3 Element Test on Zero-Power Control of Radial Magnetic Bearing 7) 1) Flywheel AMB system (AMX95) Figure 5 shows the modeling simulation and figure of the flywheel AMB system used for the experiment. This flywheel AMB system consisted of a CFRP flywheel AMB, a control unit and a high-frequency inverter. The parameters for this flywheel AMB system are shown in Table 4. L2 L1 Lu L1 U3 U7 Sensor1 i3 y hy i7 Magnet3 x x f3 Magnet7 Sensor2 f7 G z x f1 Magnet1 Magnet5 x f5 (a) Model of system Fig. 5 Flywheel-AMB system (AMX95) Table 4 Parameters of rotor-amb system Symbol Value Unit M kg I r.173 kgm 2 I a.186 kgm 2 L u.499 m L l.1676 m L m K u Nm 2 /A 2 K l Nm 2 /A 2 X, Y m 2) Test results For this test, a feed back control system was constructed by means of a digital signal processor (DSP). This control system receives input of 4-direction displacement information from 4- position sensors and gives output control current to 8- electromagnets, i.e. a 4-input-8-output system. The control output is supplied to the electromagnet via a D/A converter and power amplifier. The control performance was evaluated by the orbit waveforms of the rotor, of which examples recorded at 6 6 min 1 for upper and lower magnetic bearings were shown in Fig. 6. Figure 7 shows the waveforms of the control current on two electromagnets facing each other at the lower position of the spindle. These waveforms demonstrate that at each moment only one of the two electromagnets is energized in accordance with the displacement of the rotor 4), or that the basic control action is properly performed. i1 i5 x U1 U5 (b) Picture of system yu, m i5, A x=11hz xu, m (a) Upper magnetic bearing x=1hz yl, m x=11hz xl, m (b) Lower magnetic bearing Fig. 6 Orbits of rotor (6 6min 1 ) Time, s i7, A x=1hz Time, s Fig. 7 Control current (6 6min 1 ) However, the measured current value was still high due to unbalance of the rotor. It is possible to reduce this current by additionally applying the unbalance force rejection control (UFRC) which rotates the rotor around the center of inertia without controlling rotation synchronous component. An experimental attempt to perform the zero-power nonlinear control utilizing the conventional magnetic bearing mechanism for the linear control was successful. As shown in Table 5, the energy consumption was reduced to 6% of that required for conventional magnetic bearing control. This technology will make it possible to realize a highly efficient flywheel energy storage system that has not yet been achieved. It is also confirmed that the new technology can be applied to such other systems that involve high gyroscopic action or an elastic rotor. As further improvement is expected at higher rotating speed, it is conceivable to realize perfect zero-power control. Table 5 Rotational speed vs. power consumption Rotational speed (min 1 ) Zero-bias method (W) Bias, method (W) Energy reduction (%) Koyo Engineering Journal English Edition No.167E (25)

5 5. Feasibility of 3 W 3 hours System (ComFESS) Discussion on the current technology level and issues to be addressed to realize the targeted system are shown as follows: Applying the findings of the basic element tests to the ComFESS, rotational decay of the rotor was calculated with the results as shown in Fig. 8. Supposing the effect of the hysteresis motor to be added on the rotational decay curve for the conventional technology ("+" marked), a new curve marked with "&" can be drawn. On top of that, if the effect of PM biased AMB and zero biased control at the elemental development level so far are added, "(" marked and "S" marked curves are obtained. Obviously, however, they are still far larger than the target (" " marked). And yet, there is potential improvement by application of zero-power control including unbalanced force rejection control ("'" marked curve), that would make it possible to achieve the target. In the future, therefore, UFRC development will be pursued to complete the ComFESS incorporating the insights obtained in this study. Rotational speed, min : Conventional : Hysteresis motor : PM bias AMB (bias control AMB) : Zero bias AMB : Target : Zero bias AMB (Unbalance control) 2 Time, hours Fig. 8 Rotational decay of rotor Conclusion As a result of this research work, the following element technology has been established. 1) Reduction of loss by optimum driving of the hysterisis motor 2) Reduction of power consumption of the axial magnetic bearing by utilizing the attractive force of the permanent magnet for supporting the dead weight of the rotor unit 3) Reduction of power consumption of the radial magnetic bearing through application of zero-power non-linear control On the other hand, issues needing to be addressed for commercialization have been clarified. 4) For the zero-power-non-linear control to be applied to the ComFESS, the unbalance force rejection control needs to be added, that would enable further reduction of energy consumption. Based on the findings of this study, the development program will be shifted to the commercialization phase, wherein development of commercial products will be pursued. At the same time, there has arisen a new commercial needs or development of a power backup system that can provide several hundred kilowatts for around a minute. To meet this demand for high-output-short-time backup systems, further advance of these elemental technologies will be pursued. This research program has been performed under the sponsorship of the NEDO (New Energy and Industrial Technology Development Organization in Japan) as one of the NEDO International Joint Research Projects (NEDO-Grant Project) for these three years. Acknowledgement Taking this opportunity of publication, we wish to extend our deepest gratitude to our research partners, Professor Nonami of Chiba University, Dr. Thoolen of Centrum voor Constructie en Mechatronica (CCN: the Netherlands), Emeritus Professor Schloesser of Eindhoven Institute of Technology (the Netherlands), and President Nishimoto of NTRK Co., Ltd. for their assistance and support. Koyo Engineering Journal English Edition No.167E (25) 33

6 References 1) R. Takahata: Tribologist, 49, 5 (24) ) H. Kameno, A. Kubo, S. Gachter and R. Takahata: Koyo Engineering Journal, 163E (23) 44. 3) Y. Miyagawa et al, A.5 kwh Flywheel Energy Storage System using A High-Tc Superconducting Magnetic Bearing, IEEE Transactions on Applied Superconductivity, 9, 2 (1999) ) S. Sivrioglu, K. Nonami, R. Takahata and A. Kubo: "Adaptive Output Backstepping Control of a Flywheel Zero-Power AMB System with Parameter Uncertainty," Proceeding of 42nd IEEE Conference on Decision and Control (CDC), Hawaii-USA (23) ) H. Kameno et al.: Basic Design of 1 kwh Class Flywheel Energy Storage System, Proceedings of the Eighth International Symposium on Magnetic Bearings (ISMB-8), (22) ) A. Kubo et al.: Dynamic Analysis and Levitation Test in 1 kwh Class Flywheel Energy Storage System, Proceeding of 7th International Symposium on Magnetic Technology (ISMST-7), (23) ) R. Takahata, A. Kubo, F. Thoolen and K. Nonami: Compact Flywheel Energy Storage System, Proceedings of FY21 International Joint Research Program (NEDO Grant) Conference. A. KUBO * H. KAMENO * R. TAKAHATA ** * Mechatronic Systems Research & Development Department, Research & Development Center ** Mechatronic Systems Research & Development Department, Research & Development Center, PhD 34 Koyo Engineering Journal English Edition No.167E (25)