A Sensorless Methanol Concentration Controller Based on the Amplitude of Output Voltage for Direct Methanol Fuel Cells
We have found that there is a close relationship between the methanol concentration and the amplitude of voltage fluctuation of DMFC. The output voltage of DMFC experiences fluctuations during operation even under a constant electric load because of uneven reaction rates at the electrodes that are caused by mainly formation of CO2 slugs at the anode and water flooding at the cathode. The CO2 slugs and water flooding can clog the pores of the electrodes and retard the access of reactants to the catalytic sites, leading to a fluctuation of output voltage. Water flooding at the cathode is estimated to be more influential than the CO2 slugs at the anode. In addition, the methanol concentration in the feed affects the methanol crossover rate from the anode to the cathode and thus influences water flooding at the cathode. Thereby the degree of voltage fluctuation is proportional to the methanol concentration.
This work presents a novel sensor-less control strategy for adjusting the methanol concentration in re-circulating direct methanol fuel cell (DMFC) systems by utilizing the amplitude of voltage fluctuation of DMFC as a feedback parameter. The effects of various operating conditions on the fluctuation amplitude are analyzed, and a new sensor-less algorithm is proposed based on the relationship between the methanol concentration in the feed and the amplitude of voltage fluctuation. The feasibility of the newly proposed algorithm is evaluated and compared with a control method using a methanol sensor in a continuous run of a 200 W-class DMFC system. It is confirmed that the algorithm works very well and is able to control the methanol feed concentration within a small error bound. Moreover, this method enables steady operations of DMFC systems by minimizing the flooding phenomenon at the cathode. This sensor-less algorithm could be applied to any type of DMFC systems because it has an auto-tuning function.