Abstract
Based on fundamental works and longstanding experiences with the design and setup of fuel cell system components as well as similar components from internal combustion engines, a complete air supply system for a fuel cell demonstrator vehicle was set up, tested and optimised at FEV. The work especially included the compressor, an electric motor as compressor drive, an intermittend electromagnetic injection valve for de-ionized water, the development of an innovative system concept with air pre-heating and last but not least the corresponding control strategies. The excellent operation of the air supply and humidification system was successfully demonstrated under extreme conditions by crossing the Simplon pass in January 2002.
The boosting strategy is one of the most essential parameters for design and operation of a fuel-cell-system. High pressure ratios enable high power densities, low size and weight. Simultaneously, the demands in humidification and water recovery for todays systems are reduced. But power consumption and design effort of the system strongly increases with the pressure level.
Therefore, the main focus must be on the system efficiencies at part load. In addition, certain boundary conditions like the inlet temperature of the fuel-cell stack must be maintained. With high pressure levels the humidification of the intake air upstream, into or downstream the compressor might not be sufficient to dissipate enough heat. Vaporization during the compression process shows efficiency advantages while the needs in heat dissipation decreases.
The optimisation of the integrated air supply and humidification concept is based on fundamental analysis as well as experiences from similar applications in a first step. Water injection into the compressor shows benefits in efficiency of up to 5% and improves the controllability. Injection maps have been generated in order to meet desired inlet conditions of the fuel cell concerning gas temperature and dewpoint. For further development, testing and optimization of the airsupply system, a hardware-in-the-loop system based on the modeling and simulation environment MATLAB/SIMULINK was used. It is possible to control the compressor and the fuel cell test bench from the beginning as a part of the software model of the complete passenger vehicle. In this way, the feedback of the other system components could be taken into consideration. The control strategies were developed by detailed simulations, essentially based on measurement data from the compressor and the fuel cell test bench. The control strategy for part load operation was verified under transient operating conditions.
With the simulation tool MATLAB/SIMULINK, supplemented by detailed self-developed design tools for compressor and expander units and hardware-interfaces for hardware-in-the-loop simulations, fuel-cell-systems can be modeled, simulated and verified accurately. The access to a wide database from compressor and fuel cell test benches allows precise simulation results with adjustable model complexity.