Fuel cell test facilities for low-pressure operation

Kurzfassung

Caused by the increasing request of electrical power in modern as well as in future aircrafts fuel cells for on-board generation of the electrical power becomes interesting systems. Possible advantages of fuel cells are a higher electrical efficiency compared to the turbines and the diversification of the electrical power generation (increased reliability). However, the analysis of the feasibility of using fuel cells in these applications demands accurate predictions of fuel cell behaviour at the conditions encountered in the flight scenarios. In particular, a passive fuel cell operating on an aircraft at 12000 meters will need to be able to produce power using air that has a pressure only one-quarter of that at sea level, and withstand temperatures of –55 °C or lower. In order to develop and optimize fuel cells for aircraft applications the components must be tested under in-fly conditions. Therefore, test of systems and stacks, as well as single cells should be performed in the pressure range of interest. The adaptation of a fuel cell system to the conditions at high elevations requires a tremendous effort. In order to design the various subsystems and to select appropriate components it is necessary to first identify the requirements and the operation characteristic of fuel cell systems and stacks. For this reason a test facility for small fuel cells systems under low-pressure conditions was design and realized. A complete home-built PEFC fuel cell system of 300 W was tested down to a pressure of 0.2 bar. The goalof these measurements was to ascertain and validate theoretical evaluations and simulations with real experiments. The fuel cell system was integrated into a vacuum chamber with an internal volume of 2.5 m3 and a minimal possible pressure of 0.1 mbar. The necessary adaptations included a hydrogen safety system for low-pressures and installation of a lock systems with valves to ascertain an adequate air supply. Also modifications of the fuel cell system were necessary for reliable operation like an improved cooling fort he compressor and stack and an improvement of the humifidfier. The controls for the system had to be installed outside of the chamber. With this tes facility the German Aerospace Center (DLR) tested for the first time in Europe a fuel cell system under low-pressure conditions of 0.2 bar (abs.), which corresponds to an altitude of 12000 m. However, in order to consider all relevant aspects for this application also stack and cell design have to be improved for this specific application. In the stack design the pressure drop must be reduced compared to stacks for operation under normal conditions, because the pressure drop significant increases with decreasing operating pressure (Δp∼1/p). The second fundamental problem is the water management in low pressure operation. In order to ensure a good humidification of the cell the water partial pressure in the feed gas should be the saturated water vapor pressure of the cell temperature. However, under flying conditions the saturated water vapor pressure of water can be higher than the total operating pressure; e.g. at 80°C the saturated water vapor pressure is approx. 45000 Pa (450 mbar). Caused by the high water partial pressure the concentration of the oxygen in the fuel cell will be additionally decreased and the performance will be reduced.. Therefore DLR has built-up an additional test facility for cell characterization for low pressures. The test facility can operate fuel cells down to an operating pressure of 20000 Pa (0.2 bar). In the test facility short stacks and single cells can be tested and investigated with the DLR tool for current density measurements as well as for electrochemical impedance spectroscopy can be integrated. The test facility is fully controlled by a programmable logic controller (PLC) which is also used for the data acquisition.