Benefits of Battery-U Itracapacitor Hybrid Energy Storage Systems by Ian C. Smith B.S., Electrical Engineering Northeastern University, 2009 Submitted to the Department of Electrical Engineering and Computer Science on May 17, 2012, in partial fulfillment of the requirements for the degree of ARCHIVES Master of Science in Electrical Engineering MASSACHUSETTS INSTITUTE OF TE CNLOG7Y at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY JUL R LOR1I7 ,AiS June 2012 © Massachusetts Institute of Technology, 2012. All rights reserved. Author Department of Electrical Engineering and Computer Science May 17, 2012 ,,-~7 / *--7 j ii Certified by Thesis Supervisor: John G. Kassakian Professor of Electrical Engineering and Computer Science Accepted by Leslie A. Kolodziejski Chair, Department Committee on Graduate Students Benefits of Battery-Ultracapacitor Hybrid Energy Storage Systems by Ian C. Smith Submitted to the Department of Electrical Engineering and Computer Science on May 17, 2012, in partial fulfillment of the requirements for the degree of Master of Science in Electrical Engineering Abstract This thesis explores the benefits of battery and battery-ultracapacitor hybrid energy storage systems (ESSs) in pulsed-load applications. It investigates and quantifies the benefits of the hybrid ESS over its battery-only counterparts. The metric for quantifying the benefits is charge efficiency - the amount of energy delivered to the load per unit charge supplied by the battery. The efficiency gain is defined as the difference in charge efficiency between the hybrid and the battery-only ESS. A custom experimental apparatus is designed and built to supply the current control for charging and discharging the batteries, as well as the data acquisition for measuring energy and current output. Experiments are performed on both ESSs under four different pulsed load profiles: 1. 436 ms pulse period, 10% duty cycle, 8 A pulse amplitude 2. 436 ms pulse period, 25% duty cycle, 8 A pulse amplitude 3. 436 ms pulse period, 10% duty cycle, 16 A pulse amplitude 4. 436 ms pulse period, 25% duty cycle, 16 A pulse amplitude Circuit models are created to accurately represent the battery and ultracapacitors. These models are used in simulations of the same test cases from the physical experiments, and efficiency gains are compared. The circuit models differed from the experimentation by less than 1%. Both experimental and simulated data demonstrate significantly increased charge efficiencies of hybrid ESSs over battery-only ESSs, 3 with demonstrated gains between 10% and 36%. These benefits were greatest for the 16 A, 10% duty cycle test case because it combined the highest pulse amplitude and the shortest duty cycle. It is concluded that high-amplitude, low duty cycle, and low period pulsed- load profiles yield the highest efficiency gains. Thesis Supervisor: John G. Kassakian Professor of Electrical Engineering and Computer Science 4 Acknowledgements I have many people to thank for making my time at MIT an amazingly rich and rewarding experience. I would like to first thank my thesis advisor - Professor John Kassakian - for his immense amount of knowledge, wisdom, and patience. Having access to such an outstanding and distinguished MIT professor has been an amazingly educational experience, and has undoubtedly bolstered my growth as an engineer and as a person. I must also extend thanks to Professor Joel Schindall for mentoring me throughout the project. His cool-headed voice of reason and industry experience has pulled me back onto the right path too many times to count. I would also like to thank the other members of the LEES community. It takes a lab to raise an engineer and I feel incredibly fortunate to have had 2 years in LEES. A special thanks, in particular, is due to the following people for always being there to help me in a time of need and to keep the mood light when it was getting too dark: Sam, Justin, Richard, Andy, Minjie, Jackie, Wardah, and Juan. Thank you, Dave Otten, for being a levelheaded voice of reason during all of the chaos. And thank you to everybody else in 10-061 and 10-050 for some memorable moments and good times. To my parents, I cannot thank you enough for always supporting me. Your advice and love have always kept me on the right course and I am proud to say that I am your son. May this thesis serve as a dedication to the values that I learned from both of you. Thank you Gordon and Alan for being wonderful brothers and always, always having a joke ready to keep the laughter flowing. And finally, I would like to thank Dan, Brian, Jose, Matt, Tony, and everybody else at Nest for giving me the opportunity to return for my degree and for creating a wonderful place to call home after its completion. I could not be more excited for the future and what it brings - the sky is the limit! 5 6 Contents 1 - INT RODUCTION............................................................................ 11 1.1 GENERAL CHARACTERISTICS OF ULTRACAPACITORS......................... 12 1.2 A BRIEF HISTORY OF ULTRACAPACITORS......................................... 16 1.3 PRIOR RESEARCH AND APPLICATIONS .............................................. 17 1.4 THESIS OVERVIEW ...................................................................... 20 2 - EXPERIM ENTATION ............................................................... 23 2.1 HARDWARE DESIGN .................................................................. 26 2.2 CALIBRATION AND DATA PROCESSING.......................................... 35 2.3 ANALYSIS AND RESULTS ............................................................. 44 2.4 CONCLUSION .......................................................................... 49 3 - SIMU LA TION.......................................................................... 51 3.1 ULTRACAPACITOR CIRCUIT MODEL ............................................. 52 3.2 BATTERY CIRCUIT MODEL ........................................................... 58 3.3 HYBRID ESS CIRCUIT MODEL ...................................................... 65 3.4 SIMULATION PROCEDURE ........................................................... 67 3.5 SIMULATION RESULTS................................................................. 69 3.5 CONCLUSION .......................................................................... 78 4 - CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK..... 79 4.1 CONCLUSIONS ......................................................................... 79 4.2 SUGGESTIONS FOR FUTURE W ORK .............................................. 81 BIBLIOGRAPHY............................................................................. 85 APPENDIX A ................................................................................... 89 APPENDIX B................................................................................ 123 APPENDIX C................................................................................. 163 APPENDIX D............................................................................... 171 7 List of Figures Figure 1. Ultracapacitor structure [6].......................................................... 14 Figure 2. The ultracapacitor and NiMH battery cell used in the ESS ....24 Figure 3. Test rig block diagram ................................................................... 27 Figure 4. High-current circuitry and control. Discharge circuitry is highlighted in blue, charge in orange, and the energy storage system is highlighted in green................................................................. 29 Figure 5. ADC m easurement nodes............................................................. 31 Figure 6. Op amp scaling circuits and their scaling constants...........31 Figure 7. Battery voltage and temperature during a constant-current charge. The vertical line denotes the time at which charging c e as e d ..................................................................................................................... 33 Figure 8. The test rig PCB layout. The red traces are on the top layer, the large blue planes are the inner ground layer, and the small blue traces are on the bottom layer. The inner power layer is h id d e n ..................................................................................................................... 34 Figure 9. Example calibration of pulse-high and pulse-low scenarios for an 8 A, 25% duty cycle pulse. The blue data points represent the test rig sampling points. The green line is data from the o scillo sco pe .................................................................................................... . . 38 Figure 10. Data output stream structure ................................................... 40 Figure 11. Plot of the sampling times, overlaid on top of a current p u ls e ........................................................................................................................ 42 Figure 12. A single ultracapacitor circuit model......................................53 Figure 13. Comparing the original ultracapacitor model to experiment during a long-term charge test................................................................ 55 Figure 14. Comparing the original ultracapacitor model with calculated values to the experimental discharge pulses ..................................... 56 Figure 15. Comparing the four-branch ultracapacitor model simulation to experimental data in a 10A pulsed discharge test......................58 Figure 16. Initial Panasonic Nickel-metal Hydride Battery circuit model ................................................................................................................................... 59 AV Figure 17. Calculating battery ESR. rESR ~~ ......................................... 60 AI Figure 18. Battery setting voltage to find the transient branch re sista nce s ....................................................................................................... . 60 Figure 19. Comparing the two-branch battery model to experimental d a ta .......................................................................................................................... 62 Figure 20. Final battery model, consisting of three RC branches.........63 8 Figure 21. Comparing the three-branch battery model to experimental d a ta ............................................................................................................. . . . 64 Figure 22. Hybrid ESS model with 1 battery pack in parallel with 3 ultracapacitors in series......................................................65 Figure 23. Comparing the hybrid ESS model to experimental data. Load voltage is on top, battery current on bottom. ........................ 66 Figure 24. Numerical Simulation Flowchart for a given time, t.........68 Figure 25. Optimizing the pulse variables for efficiency gain. Figure 25a) Pulse duty cycle; Figure 25b) amplitude; Figure 25c) period. 74 ................................................................................... 7 Figure 26. Efficiency gains for low and high pulse periods.............76 Figure 27. Hybrid circuit model that was analyzed for the simulation s c ript s .................................................................................................................... 164 Figure 28. Hybrid circuit model during a discrete time step. Capacitors are represented as voltage sources....................................165 9 List of Tables Table 1. Ultracapacitor and battery component specs [31]. [32].........24 Table 2. Example unaltered readings from a 16 A 25% duty cycle current pulse, with the data comma-separated in Microsoft Excel.41 Table 3. 16 A 25% duty cycle pulse variable multipliers. The 6 rows represent a different sample during a 25% duty cycle pulse...........42 Table 4. Same stream of data from Table 2 after multiplying by the multipliers from Table 3. The values are displayed in volts.....43 Table 5. Experimental data results for the short tests.......................... 45 Table 6. 16 A, 10% duty cycle tests, comparing 'short' test results to the depletion test results (note the difference in units of energy an d cha rg e )................................................................................................... . . 47 Table 7. Original three-branch ultracapacitor model component values, based upon the procedure in [34].........................................54 Table 8. The final component values used for the four-branch ultracapacitor m odel in Figure 12........................................................... 57 Table 9. Component values for the two-branch battery model........62 Table 10. Battery model component values..............................................64 Table 11. Simulation results for the same test cases that were run in the experim ental chapter.......................................................................... 70 Table 12. Comparing simulated to experimental data outputs ........ 72 10