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Fuel cell electric vehicles (FCEV) are a promising alternative to conventional as well as battery electric vehicles (BEV). Hydrogen-based powertrains are especially competitive in heavy-duty transport applications, such as trucks, buses, coaches or trains, since they allow high mileage and short refueling times. Polymer electrolyte fuel cells (PEFC) are the most suitable fuel cell technology for automotive applications, since they show high power densities and system efficiencies, a fast dynamic response and decent freeze-start capabilities. However, to achieve a significant market penetration, fuel cell costs have to be further reduced. The cooling system of polymer electrolyte fuel cells contributes significantly to system volume, mass and cost. On the one hand, conventional liquid cooling requires a complex, multi-layered design of bipolar plates and on the other hand, an increased operating temperature is necessary to reject the entire waste heat to the environment. To ensure a suitable ionic conductivity of the polymer electrolyte membrane at elevated temperatures, an external humidifier is considered in many system architectures. This, however, leads to additionally system volume, mass and costs. Evaporative cooling is a combined cooling and humidification concept, which allows to overcome these hurdles. It reduces the complexity of the bipolar plates and eliminates the need for external humidification. However, the interactions between operating conditions and the fuel cell performance, cooling power and humidification are complex and have not been investigated in detail yet. Therefore, a combined numerical and experimental analysis of evaporative cooling for PEFC is conducted in this work and the concept is evaluated on different scales from cell to stack and system level. Isothermal in situ and operando measurements are performed under technical cell boundary conditions to analyze the evaporation behavior, cooling power, internal humidification, electrochemical performance and operational stability of an evaporatively cooled fuel cell. Additionally, a zero dimensional fuel cell system model is developed to analyze optimal stack operating conditions and to investigate the interactions between the fuel cell stack and balance of plant (BoP). Results show the viability of evaporative cooling over a broad range of operating conditions. The entire waste heat is effectively removed and a sufficient membrane humidification is achieved. The optimum system power and highest system efficiency are achieved at high temperatures (80 to 90 °C), low gas pressures (125 to 150 kPa) and a corresponding cathode stoichiometry between 1.5 and 3. Under these conditions, sufficient water can be retrieved from the exhaust gas to ensure a closed water loop. Therefore, filling and storage of liquid water is not required. However, the performance as well as the pressure drop of the exhaust gas condenser is key to an efficient operation. The experimentally observed electrochemical performance is comparable to conventional cooling with humidified gases at the inlet, center and outlet of the cell. An insignificant reduction in electrochemical performance is offset by the volume and weight saving potential of up to 30% and thus considerably reduced cost. In conclusion, this work provides a principal proof of concept for the evaporative cooling approach. |
