Load following fuel cell systems depend on control of reactant flow and regulation of DC bus voltage during load (current) drawn from them. To this end, we model and analyse the dynamics of a fuel cell system equipped with a compressor and a DC-DC converter. We then employ model-based control techniques to tune two separate controllers for the compressor and the converter. We demonstrate that the lack of communication and co-ordination between the two controllers entails a severe tradeoff in

more »

... ieving the stack and power output objectives. A co-ordinated controller is finally designed that manages the air and the electron flow control in an optimal way. We demonstrate our results during specific and critical load changes around a nominal operating point. Although our analysis does not cover wide operating region, it provides insight on the level of controller co-ordination necessary in non-hybridized fuel cell power supply. The shut-down and start-up procedures will be investigated in future work. cell system is typically combined with a battery or capacitor into a hybrid power generation system. A complete PEM fuel cell power system includes several components apart from the fuel cell stack and battery, such as an air delivery system which supplies oxygen using a compressor or a blower, a hydrogen delivery system using pressurized gas storage or reformer, a thermal and water management system that handles temperature and humidity, DC-DC converters to condition the output voltage and/or current of the stack and finally electric loads (Rajashekara, 2000; Yang et al., 1998) . Figure 1 shows the configuration of a typical fuel cell power system which is constructed with fuel cell, DC-DC converter and battery. The DC-DC converter transforms unregulated DC power of the FC to regulated DC bus power. Research on the DC-DC converters for fuel cells is focused on soft voltage sources which accounts for the cell voltage variation due to the electrochemical characteristic at different operation conditions (U.S. Department of Energy, 2004). Sometimes the converter is used to filter the current from the fuel cells to avoid imposing transients that can lead to FC failure or degradation. In both cases, the coupled dynamics of current and voltage in fuel cells and the converter affects the system performance. Specifically, limiting the current drawn from the fuel cell enhances fuel cell performance but degrades the voltage regulation performance in DC-DC converter. This direct conflict can be addressed easily with hybridization. Hybridization in the fuel cell power system may also achieve higher fuel cell efficiency by levelling peak power demand to the battery, allowing the fuel cell to operate on its optimum range. Cunningham et al. (2003) showed that battery-hybrid fuel cell vehicle associated with regenerative braking improves efficiency up to 15%. The efficiency gain in a fuel cell hybrid vehicle depends on the degree of hybridization (Ishikawa et al., 2004 ). The hybrid system efficiency can be even worse than the stand-alone fuel cell in some driving cycles (Friedman, 1999; Ramaswamy et al., 2004) . Also, efficiency of a hybridized auxiliary power unit (APU) or distributed power generation, which has no energy recovery apparatus like regenerative braking, is not yet addressed. These unexplored issues highlight the importance of defining the achievable performance and limitation of a fuel cell power system before hybridization. The purpose of this paper is to define the dynamic limitation of a FC power system which is augmented with a DC-DC converter but without a battery. To investigate the coupled dynamics with currents and voltages in the fuel cell power system, it is necessary to establish an analytic model for the fuel cell with DC-DC converter and design the overall system.