The role of supercapacitors in designing fuel cell powered portable applications

In this paper, the role of a supercapacitor in the design of fuel cell powered systems is discussed. First, the electric equivalent circuit model of the fuel cell and the supercapacitor is obtained using frequency analysis, which is essential in establishing a design procedure to size the required supercapacitor. It is evident from the nonlinear model of the supercapacitor that the capacitance increases with voltage. Since the fuel cell voltage is inversely proportional to the output power supplied, it is shown that combining fuel cell with supercapacitor is particularly advantageous to guard against load transients as higher energy is stored while supplying lighter loads. The other advantages of connecting the supercapacitor in parallel with the fuel cell are shown to be: (a) increased steady state stability when powering constant power loads, (b) voltage and system performance stability during fuel cell dynamics (purging), and (c) increased fuel efficiency (i.e. reduced hydrogen consumption). For transient stability analysis, the effect of fuel cell internal impedance (extra element), along with the impedance of the nonlinear supercapacitor, on the transfer function of the DC/DC converter is analyzed. Finally, experimental evaluation and comparison of fuel consumption in the conventional and hybrid systems is performed, showing that the hybrid source has improved fuel utilization. From these results, it is shown that the proposed approach permits optimization of energy management and improvement of dynamic performance of the power conditioner. The experimental results obtained on 20 W and 30 W PEM fuel cell/boost converter systems demonstrate the validity of the proposed approach.