Control Of Fuel Cell System For Vehicle Based On Oxygen Excess Ratio

Energy shortage and environmental pollution produced by automobile with traditional energy utilization patterns have been prominent and have hindered the sustainable development of economic. Hence, utilization of clean and renewable energy sources has become an inevitable trend throughout the world. Fuel cells which have low noise, zero emissions, high efficiency and scalable power have attracted much attentions. Especially, a proton exchange membrane fuel cell (PEMFC), which is suitable as power source for vehicles, has become a new field in electric vehicle.Firstly, this paper discusses four kinds of dynamic equivalent circuit models of PEMFC. In these models, the simplified equivalent circuit visually reflects voltage output of fuel cell and the effect of double layer capacitor, and its component parameters can be calculated by current interrupt method. An improved simplified equivalent circuit models based on the characteristic of current step is proposed for automotive fuel cell. The dynamic resistance and inductance in parallel could be used to simulation concentration change of gases in transportation, which is series with the open circuit potential source. In the improved circuit, the values of open circuit potential and of ohmic resistance are calculated in theory. The capacitance is obtained through empirical formula, and other parameters are evaluated with current interrupt experimental data. The improved circuit and parameter deciding method are tested by experiments and good match results are obtained.The results show linear fitting method in computing of activation voltage and activation resistance works better.Secondly, an oriented-control model including each part of PEMFC system is proposed and the following research are conducted:(1) Analysis of flow, pressure and power consumption of compressor. When compressor produces a certain air flow and outlet pressure, the higher ambient temperature and the altitude are, the greater the power consumed by compressor is. Under the terms of its outlet pressure and speed remain unchanged, the outlet flow increases with atmospheric pressure. (2) Analysis of air and oxygen pressures on cathode flow field. Under set environmental and stack temperature, the pressures are determined by the rotational speed of the compressor, increasing along with the speed. Whereas the oxygen pressure decreases correspondingly when the stack temperature increases. (3) Analysis of stack voltage, the net power and efficiency. If the stack temperature increase, the stack voltage and the net power increase, but the efficiency decreases. With the increase of oxygen excess ratio (OER), power consumption increases and efficiency decreases monotonically, while there is an optimal value for the net power. (4) Analysis of the flow and pressure at inlet of stack anode. They are affected by voltage of solenoid valve, rotational speed of recirculation pump and open degree of purge valve. The rotational speed adjusts the flow most strongly in these three parameters, same as the voltage of valve for the pressure. (5) Analysis of the dynamics of the pressure drop in return manifold. It is affected by the open degree of purge valve, purge duration and pressure at return manifold. The pressure drops are small for small open degree, short purge duration and low pressure, and vice versa. (6) Analysis of the temperature in the stack and the temperature difference across the stack. The temperature can be adjusted effectively by the voltage of the blower in the radiator, and be changed in a certain range by the open degree of bypass valve. The temperature difference could be regulated by the voltage of the recirculation pump. The higher the voltage of blower is, the faster the system responses. The lower voltages of the blower and the pump are benefit for system efficiency.Finally, in this paper the control strategies of PEMFC system for vehicle are investigated and their rationality and accuracy are tested by simulations. For air supply system, a control based on optimal OER is proposed. The optimal OER is evaluated by stack current, then the feed-forward control is used to calculation reference of air flow through compressor, lastly fuzzy controller adjusts the compressor. The two methods of searching the optimal OER are compared. Besides, OER region is divided into three areas:oxygen starvation, oxygen moderation, oxygen saturation. Formula that estimates the minimum OER value in oxygen moderation region is proposed and then current regulators are presented against phenomenon of oxygen starvation and its saturation. For hydrogen supply system, the dynamic matrix control (DMC) adjusts effective cross-sectional area of solenoid valve in order to change the pressure at inlet of stack anode. Combined with air supply system, the hydrogen supply control based on the optimal OER is proposed. For thermal management, a second state equation is built by means of linearizing steady state operation point. Two PI controllers, quadratic optimal controller and model-based predictive controller based on state equations are adopted and compared for the stack temperature and its temperature difference. Similar to hydrogen supply, the thermal control strategy based on the optimal OER is proposed. The tests on the system combining these three subsystem show each controller can coordinate the work of the various subsystems and achieve satisfactory accuracy.