State of health of proton exchange membrane fuel cell in aeronautic applications
Introduction
The projected exponential increase of commercial flights by 2050 encourages minimizing carbon footprint from the transport sector to sustain the environment. Hence, the European commission set out goals for aircrafts that include emission-free taxiing, 65% noise reduction and 90% reduction in emission of NOx [1]. Additionally, proton exchange membrane fuel cell (PEMFC) not only generates electrical energy but has useful by-products in aeronautic environment (oxygen-depleted air, heat and water) and has attractive operating properties that assist in sensitive issues (noise reduction in airports due to silent operation and zero greenhouse gas emission in situ) [2,3]. The several studies that propelled the “more-electric airplanes” and incorporating fuel cell-powered systems in aircrafts successfully demonstrated the feasibility of deploying fuel cell technology in aeronautic applications. The review by Dyantyi et al. eloquently outlined the successful projects, progress made and identified future prospects [4].
One of the prospects identified was the lack of understanding of the fundamental behaviour of PEMFC in aeronautic conditions. The fundamental behaviour refers to operating conditions of the fuel cell that result in optimal performance and minimal degradation of its components. The operating conditions are operating temperature, relative humidity/dew point temperature (% RH), stoichiometry and pressure. Consequently, there was a follow-up paper that sought to understand the effect of the start-up/shutdown (SUSD) cycling [5]. The study examined the nature and extent of degradation of the fuel cell components exposed to the SUSD cycling incorporated with heating and cooling to mimic real world aeronautic operating conditions. The operating conditions used were considered nominal for aeronautic conditions with 2.5 cathode stoichiometry compensating the low ambient pressure and low oxygen content at high altitudes, where 60 °C is the nominal operating temperature for PEMFC, and 100% RH. Oxygen content is constant but believed to decrease with altitude due to decreasing partial pressure when estimated using the ideal gas law equation for pressure and altitude. Uno et al., employed back-pressure regulator to increase partial pressure while Horde et al., observed improved PEMFC performance at 2.5 air stoichiometry [[6], [7], [8]]. Effects of relative humidity during the SUSD studies are only pronounced at 100% RH [9,10]. 100% RH on cathode has little or positive impact on fuel performance since reducing anode humidification facilitates water movement through back diffusion. On the other hand, 100% RH on both anode and cathode may require special flow field design of GDL that can facilitate adequate removal of excess water even at low flow rates of gases. Flooding not only induce starvation but also promote carbon corrosion at high voltages [11,12]. Therefore, 100% RH was chosen as an extra parameter of the AST conditions for possible flooding caused by rapid change of load demand and consequent water generated at higher current densities. The key findings were that the high voltages and rapidly changing load demand were too aggressive on the catalyst layer and subsequently caused severe performance loss of 0.196 mV h−1 average degradation rate [5]. The significant degradation of the fuel cell components during the SUSD cycling was in-line with the findings from Pei et al., which showed that SUSD and load changing are major contributors to fuel cell degradation [[13], [14], [15], [16], [17], [18]]. The observed fuel cell degradation propelled further study for the rest of the aircrafts’ load profile to explore the effects of the flight stages on its life and components.
The aim of this study is to examine and compare the behaviour of PEMFC components exposed to load profile of an aircraft (namely idling, take-off, cruise (constant and variable load demand), and landing). The estimated operational times for the stages are: idling and taxiing (8 min), take-off and climbing (20 min), cruise (depends on route but averaged to 4 h), descent and landing (25 min) and ground taxiing (7 min) [19]. The flight stages assimilated known accelerated stress tests (AST) such as idling/open circuit voltage (ID), cruise/current hold (CR), take-off and landing/SUSD (TL) and variable load demand/potential cycling (VL). The major difference between aeronautic and terrestrial (vehicular applications) are low ambient pressure at high altitudes and low oxygen content. AST studies conducted on vehicular applications tend to be difficult to compare or establish a benchmark due to numerous modifications of the US DOE protocols. For instance, the review by Rodgers et al., tabled different operating conditions that made it challenging to define trends for scaling performances and durability [20]. The novelty of this paper is that it not only modified US DOE AST methods but also incorporated tested operating conditions for PEMFC in aeronautics and from the Development of Reference Test Procedures project as a contribution towards standardization of PEMFC testing protocol. The conditions are cathode stoichiometry of 2.5, anode stoichiometry of 1.8 and nominal temperature of 60 °C [[21], [22], [23], [24]]. PEMFCs operated in aeronautic environment is likely to be exposed to temperatures up to 90 °C [21]. Hence, the choice of examining all the flight stages at nominal 60 °C and extreme 90 °C to study the effect of temperature. The small size of available database on PEMFC behaviour in aeronautic conditions further encouraged the study. The database is essential in predictive modelling for optimizing PEMFC performance and abating degradation in order to improve its reliability and lifetime.
Section snippets
Setup
The tested PEMFC single cell (see Fig. 1) in this study comprised of a 25 cm2 commercial membrane electrode assembly (MEA) fitted in a commercial single cell fixture made of graphite bipolar plates with a serpentine/parallel flow pattern design and gold-plated current collectors. The cathode and anode catalyst loadings were 1.0 mg cm−2 Pt. Each flight stage used fresh MEA. The tests were carried out on a Greenlight G20 Fuel Cell Test Station equipped with Emerald™ control and automation
Overall performance
This section explores the ability of PEMFC under extreme rapid changing operating conditions that an aircraft is likely to encounter. Fig. 2 graphically demonstrated the performance of the PEMFC at the various flight stages which was evaluated by monitoring the power output against current density through measuring IV curves.
The peak power density output for a fresh MEA was 0.20 W cm−2. The 0.05 W cm−2 generated power difference after all the stages confirmed that the PEMFC is capable of
Conclusion
The effect of load profile of aircraft on PEMFC performance and degradation was investigated by testing the fuel cells under simulated aeronautic conditions for each flight stage. The flight stages (idling, take-off, variable load demand and landing) at the extreme temperatures returned similar overall performance loss, except for the cruise mode. One of the causes of the performance loss was dehydration since it is difficult to humidify at 90 °C due to faster water absorbing capacity of air on
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is supported by Hydrogen and Fuel Cell Technologies RDI Programme (HySA), funded by the Department of Science and Technology in South Africa (Project KP3-S03).
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