February 5, 2021, ©. Leeham News: Last week, we started the discussion around fuel cells as a source of electric energy in airliners. We went through the principle and asked some vital questions.
Now we look at different types of fuel cells and for what applications these are suited.
Last week, we learned fuel cells use an electrochemical process to release the energy stored in hydrogen. The knowledge around fuel cells has been built over the last 90 years after Francis Bacon developed a stationary alkaline fuel cell in the 1930s. This type is still in use today, predominately as backup power in industrial applications (first column in Figure 2).
The fuel cell type for aircraft applications is the second type, Proton Exchange Membrane, or PEM fuel cells. These are in intense development for mobility applications such as forklift trucks (operating indoors), trucks, buses, cars, and now aircraft applications.
The PEM cell is the lightest type, and it has good volumetric density. Present developments aim for a system installed weight (fuel cells need control and cooling systems) of 2kW/kg and a volumetric density of 5kW/l.
The fuel cell emits electricity, heat (the 2H2 + O2 → 2H2O reaction is highly exothermic), and water from an input of hydrogen and oxygen. The heat from the cells is a cooling problem as long as it can’t be used somewhere on the aircraft. In static building applications, the heat is used for heating, and then the system efficiency reaches 80% or more.
The PME fuel cell efficiency of 50%-60% is on the cell level. If we include system losses due to the fuel cell’s cooling and control, the system-level efficiency drops around 10%-15%.
The PEM fuel cell has an anode, the PEM, and a cathode. Each cell delivers around 0.7V, and the current dependens on the cell surface. To get practical power levels, several cells are combined into stacks.
Figure 3 shows a stack from the PEM fuel cell company Ballard. This stack is designed for cars in cooperation with Audi and has a 140kW rating.
What we don’t see in Figure 3 are the control electronics and the cooling system. Fuel cells have no moving parts, which makes them reliable. But they are sensitive to the environment where they operate, see the table in Figure 2. There is, therefore, a lot of further development required before a fuel cell system can operate in the non-pressurized part of an aircraft.
Airbus has started this work with a German fuel cell supplier, ElringKlinger, and Universal Hydrogen, that targets the turboprop market with their ready-filled hydrogen gas tube stacks, cooperates with fuel cell company Plug Power.
We now have two ways to provide energy for propulsion and aircraft systems, hydrogen-burning gas turbines and fuel cells. Which type is suited for what purpose? It will be the subject of next week’s Corner.