Study On Time-Delay Characteristic Of Fuel Cell And Fuel Cell-Battery Hybrid Power Supply System

With more and more attention from nations in the world focusing on green and clean energy, proton exchange membrane fuel cell (PEMFC), which is a high-efficient energy conversion device by using the renewable energy source hydrogen as its fuel, has been attracting more and more attentions and interest of the world, owing to its irreplaceable and distinguished advantages such as zero pollution byproducts, high efficiency, low operating noise and temperature, and so on. However, the disadvantages of PEMFC such as large voltage variation range, soft output characteristic, and obvious time-delay impact on its response to load sudden change are the main barriers to its commercialization. In real applications, in order to ensure that PEMFC is able to supply loads chronically, stably and economically, DC-DC converters and extra auxiliary energy sources are needed to insert, while proper control methods are necessary to decrease its output fluctuation and frequency.Through analyzing the flow and distribution situation of the fuel gas in the internal pipe of the PEMFC stack, the two key factors, the value of hydrogen pressure and the distance length between each cell and gas inlet port, were found to answer for the cell dynamic feature. Based on the intensive study on the cold-start process of the PEMFC system, the time-delay characteristic on electrical output of the PEMFC system was radically explained by the reason that the terms ENerns, of all the cells in the stack are not able to build up synchronously due to the mechanical structure and the surrounding variety of the stack. A qualitative function was presented to describe the relationship between the delay-time phenomenon of single cell and the specific parameters of the PEMFC. Additionally, a new viewpoint for explaining the "inverse polarity" observation in the experiments was proposed, introducing the thought of equivalent internal resistance. Therefore, the necessarity that constanting the output current of the PEMFC to enhancing the whole performance of the power supply system was clarified.A double-input buck converter (DIBC) topology which has higher efficiency, less component count and lower cost, and its corresponding power management strategy and closed feedback control method were proposed and designed, with the purpose of regulating both the DIBC output voltage and output current of the PEMFC constant simultaneously. Thanks to the battery used to compensate the shortage or absorb the surplus of the energy from the PEMFC, the current ripple of the PEMFC was controlled to the minimum, by which way the operating life span of the PEMFC can be prolonged, the manufacturing cost of the system can be cut down, and most of all, the threat of the time-delay electrical characteristic of the PEMFC to the entire system can be contained.Referring to the proposed theoretical analysis and system design, a prototype of the PEMFC-battery hybrid power supply system and a suit of control programs were developed. Based on TI digital signal processor TMS320F2808, the programs were designed to take charge of the operation, debug, monitor and protect of the whole system. The experiments were carried out to verify correctness and implementability of the presented DIBC topology, control method, and power management strategy. The results showed the ideal output characteristics and control effect, which means the power supply system with the proposed circuit topology and operational control programs has practical research value and application prospect.