Challenges of Using Fuel Cells to Deliver Aircraft Propulsion Power

Abstract

Beside sustainable aviation fuels (SAFs), which offer an easy but expensive drop-in solution the use of hydrogen is discussed as an important way to decarbonise air traffic. According to a study performed by McKenzie on behalf of the “Fuel Cell and Hydrogen 2” and “Clean Sky 2” Joint undertakings hydrogen is the economical more viable option for small and medium range aircrafts1. For the smaller aircrafts electric propulsion using fuel cells to deliver the required electric power are the preferred choice from the economic perspective.Operating fuels cell on board of a flying aircraft is however not necessarily easy. An evident issue is the reduced air pressure at high altitudes. So, the barometric pressure at an altitude of 11,000 m is only 0.29 bar. Oxygen partial pressure is equally reduced to 61 mbar at constant oxygen concentration of 21 vol.%. In order to supply the stack either much higher flow rates are required at reduced pressures or a much higher compression. The former will create challenges regarding water management as a high gas flow in combination with reduced pressure accelerates the drying out of the membrane. Using HT-PEMFC instead would avoid the impact of water evaporation. However, the conditions are also likely to foster evaporation of phosphoric acid from the membrane electrode assembly. Here it will be investigate from published data if new HT-PEMFC membranes e.g. from Los Alamos National Laboratory2 would allow such a low pressure operation. Operating at high pressure will definitively challenge the system efficiency. Also, the characteristics of typical compressors render it difficult to achieve the required air compression rate of about 4.5 to increase pressure from 0.29 bar ambient to 1.3 bar stack pressure. This is, because during typical cruise operation power consumption and thus air need is strongly reduced.The variation of load and subsequently the likely variation of stack pressure can also cause challenges, in particular if after a period of low power requirements, a sudden increase in power demand needs to be answered. Such a situation will regularly be encountered for high load take off acceleration after taxying and potential waiting. Here hybridisation with a battery can be helpful to allow the fuel cell to power-up in time before take-off charging the battery used for electric taxying. More difficult is the irregular but for safety reason important situation of a go-around. During descend engines are typically operated at close to idle condition for a longer period of time. If no measures are taken to maintain stack temperature the temperature will reduce so that the immediate supply of power required for a go-around might not be available. Keeping the temperature at low load may however again cause dry-out or acid evaporation depending on whether the stack is LT- or HT-PEMFC based.In this contribution are more detailed analysis will be presented also striving to assign issues and their potential mitigation to component, stack or system level. Hydrogen-powered aviation: A fact-based study of hydrogen technology, economics, and climate impact by 205 0, Luxembourg, Publications Office of the European Union.K.-S. Lee, S. Maurya, Y. S. Kim, C. R. Kreller, M. S. Wilson, D. Larsen, S. E. Elangovan and R. Mukundan, Energy Environ. Sci., 11(4), 979–987 (2018).