Abstract
Conventional cooling systems of polymer electrolyte fuel cells are responsible for a significant share of stack and system volume, mass and cost. Evaporative cooling shows the potential to overcome these hurdles by simplifying the design of bipolar plates and eliminating the need for an external humidifier. Thus, evaporative cooling can significantly contribute towards reaching the DOE fuel cell system power density target of 850 W/L. This paper investigates the potentials and limits of evaporative cooling at stack level. For this, a zero-dimensional model has been developed, incorporating mass and energy balances as well as electrochemistry and evaporation. Main findings show that evaporative cooling is feasible over a wide range of operating conditions. The cooling performance is a function of temperature, gas pressures and stoichiometric ratios, where the temperature shows the largest leverage. A feasible operating window is proposed, which is slightly shifted towards higher temperatures (80–95 °C), lower pressures (100–200 kPa) and higher cathode stoichiometric ratios (>1.5) compared to conventional fuel cells. A slight decrease in electrochemical performance (ca. 3% at 1.5 A/cm 2 ) is easily compensated by the volume and weight saving potential of up to 30% and thus substantially reduced cost. • Potentials and limits of evaporative cooling for PEFC are analyzed. • A zero-dimensional fuel cell stack model is developed. • Optimal operating conditions for evaporative cooling are quantified. • An operating window for evaporative cooling is proposed. • The high mass, volume and cost saving potential of evaporative cooling is proven.
Highlights
Fuel cell electric vehicles (FCEV) [1,2,3,4] are a promising alternative to conventional internal combustion engine vehicles (ICEV) and battery electric vehicles (BEV), for heavy-duty transport applica tions, since they allow an efficient and emission-free conversion of hydrogen, provide high mileage [5] and short refueling times [6]
Conventional cooling systems of polymer electrolyte fuel cells are responsible for a significant share of stack and system volume, mass and cost
It has been shown that evaporative cooling works effectively over a broad range of operating conditions and a suitable operating windows has been quantified
Summary
Fuel cell electric vehicles (FCEV) [1,2,3,4] are a promising alternative to conventional internal combustion engine vehicles (ICEV) and battery electric vehicles (BEV), for heavy-duty transport applica tions, since they allow an efficient and emission-free conversion of hydrogen, provide high mileage [5] and short refueling times [6]. Polymer electrolyte fuel cells (PEFC) are a favorable propulsion system for automotive applications, since they reach peak system efficiencies above 60% [3], system power densities of more than 640 W/L [7], respectively 659 W/kg [8] and allow start-up times below 10 s at 20 ◦C and 20 s at − 20 ◦C (to 50% of rated power) [2] as well as freeze-start capabilities at temperatures below − 30 ◦C [9]. The state-of-the-art cooling solution for transport applications aims at transferring the waste heat to a liquid coolant that flows through separate cooling channels in the bipolar plates. The shape of these cooling channels determines the cooling performance as well as the pressure drop significantly [13]. To ensure a proper humidification and ionic con ductivity of the proton exchange membrane [16] at elevated temperatures, an external humidifier is often required which adds to the system volume
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