Type of Document Dissertation Author Wang, Lei URN etd-09272010-121647 Title Power Electronics Enabled System design and Power Management Control in Future Fuel Cell Vehicle Degree Doctor of Philosophy Department Electrical and Computer Engineering, Department of Advisory Committee
Advisor Name Title Hui Li Committee Chair Chiang Shih Committee Member Jim P. Zheng Committee Member Simon Y. Foo Committee Member Emmanuel G. Collins University Representative Keywords
- Bidirectional Dc-dc Converter
- Power Management
- Fuel Cell
- Energy Storage System
Date of Defense 2010-09-10 Availability unrestricted Abstract
Advanced vehicle technology including hybrid electric vehicles, plug-in hybrid electric vehicles, and fuel cell hybrid electric vehicles are attracting attention recently, as climate change and energy shortage become a more and more severe problem. Among all the options, fuel cell hybrid electric vehicles (FCHV) are considered to be next generation dominant technology because of its zero emission characteristic and other advantages.
While active research is continuously being pursued in the use of energy storage elements and power management strategies in fuel cell hybrid vehicle applications, the research reported so far in literature has limited guidance in real application. They consider energy storage sizing, power management strategy, and performance target as independent blocks, thus the design does not follow any systematic approach. Furthermore, the design methods to choose the configuration of FCHV, energy storage and appropriate power management will depend on many factors, which include vehicle characteristics such as mass, fuel economy and performance requirement, therefore it is challenging to find an optimized design solution.
The proposed research first focuses on developing a systematic design strategy for a fuel cell vehicle. An optimization function was derived in order to achieve maximum fuel economy, with variables such as power Degree-of-Hybridization (DH), total weight of fuel cell with Energy Storage System (ESS), and ESS to total weight ratio. The results show that optimum variables are firmly depending on the characteristics of fuel cells, batteries and ultracapacitors. The vehicle powers in both urban driving cycles and highway driving cycles are analyzed. Finally, optimum energy sources total weight and ESS weight ratio are found based on combined driving cycle, which is from the Environmental Protection Agency (EPA) range estimations. Unlike the previous two parameters, DH is not a fixed value but always varying in real driving based on the driving profile. The optimized DH is close to its possible maximum value for both urban and highway driving from analysis. Furthermore, the analysis also found Li-ion battery with an ultracapactor is the best choice for the ESS. The optimum size of ultracapacitor within the total ESS is also derived based on best fuel economy.
To achieve maximum fuel economy in real driving, DH needs to be adjusted in real time. When DH is always close to its maximum value, it is called full hybrid strategy. In this research, a novel energy management strategy for a full hybrid FCHV is proposed. This strategy is divided into three main parts: (1) FC power reference generating to maintain a proper DH; (2) Battery power and ultracapacitor power allocation to fully utilize ultracapacitor high power capability; and (3) Ultracapacitor and Li-ion battery SOC control to maintain the appropriate SOC window.
Compared to the full hybrid strategy, mild hybrid strategy is defined when degree-of-hybridization (DH) is 20%-30% of its maximum value. The fuel economy will be less than a full hybrid, but the smaller size of the ESS makes it more economic in industry mass production. This research also proposed a novel mild hybrid strategy of two-level power control, which simplifies fuel cell hydrogen/air control; therefore the fuel cell dynamic response is improved according to traditional load-following mode.
Since this system includes both battery and ultracapacitor, a 3-port dc-dc converter is utilized. To maintain soft-switching condition in a wide input voltage range of battery and ultracapacior, a new asymmetrical duty cycle control method is proposed for this converter, it can achieve wider ZVS range under varied input voltage comparing to other methods. Furthermore, the laws regarding the minimum circulating losses among the three ports are also studied, the power flow direction of battery and ultracapacitor ports are synchronized to minimize circulation loss. Theoretical analysis and experiments show that proposed method has higher efficiency.
The proposed optimal design has been described in detail in chapter 3. The corresponding energy management and control is provided in chapter 4, a new control method for 3-port dc-dc converter is proposed in chapter 5, and simulation and experimental results are provided in chapter 6. The last chapter is conclusions and future work.
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