May 13, 2022, ©. Leeham News: Last week, we looked at advanced developments for hydrogen-burning gas turbines.
Now we look at the alternative hydrogen-based propulsion system, which uses a Fuel Cell to convert the energy in hydrogen to electric power that drives motors to spin propellers or fans, Figure 1.
As a recap (Figure 2), the gaseous hydrogen enters on the Anode side, where its positive protons diffuse through the Proton Exchange Membrane (PEM), and the electrons take the external path to the Cathode side (thus forming the fuel cell current), where the reduction of H2 and O to H2O takes place.
The PEM fuel cell type can be developed to have acceptable volumetric and mass characteristics so that it can be used in aeronautical applications.
There are two types of PEMs fuel cells for our purposes, the classical or Low-Temperature fuel cells (called LT-PEM fuel cells or just fuel cells from now on) and the High-Temperature PEM fuel cells (called HT-PEM fuel cells).
The cells have in common that they produce more heat than electric energy from the process H2+O = H2O. For air vehicle applications, the heat management of the fuel cell propulsion system is its major problem. The magnitude is dependent on what type of PEM fuel cell is used.
In the classical low-temperature PEM fuel cell, the Proton Exchange Membrane is dependent on stringent water management inside the cell for a proper function; not too little (conductivity suffers), not too much (access of O2 to the process suffers). The water management limits the cell’s operating temperature to around 80°C and makes it sensitive to freezing.
The large heat flow with a low temperature from the cell makes heat management difficult for LT-PEM cells in aircraft applications. It creates a mass, volume, and drag problem (large air-cooled heat exchanger surfaces are necessary). The heat is manageable for stationary and rolling vehicle applications as these have fewer constraints on mass and volume for the heat management system.
The LT-PEM cell is a mature product with several manufacturers and application areas (ground transport, portable power generation). The key problem area for ground transportation (trucks, buses, cars..) is the cost, partly because of the platinum used as a catalyst in the reaction process. The research focus for LT-PEMs outside aerospace is, therefore, cost down, not mass or volume down (as aerospace would like it).
The HT-PEM cell has a typical heat exhaust temperature of 180°C, and its PEM is not dependent on the correct humidity. The PEM uses absorbed Phosphoric acid as the electrolyte. The type has been developed over the last 30 years for stationary applications, primarily as it’s less dependent on the purity of the hydrogen. It can use derivative fuels like methanol that is reformed into a hydrogen-rich gas.
The higher operating temperature eases the cell’s cooling as the heat exchanger surface area depends on the temperature difference, all other parameters being equal. Therefore, the HT-PEM is interesting for aeronautical applications. It’s a less mature variant, therefore it needs further development before the first aircraft applications.
A PEM fuel cell needs several external systems to operate correctly. The management of these systems and the fuel cell is called Balance Of Plant, BOP.
Figure 3 shows the systems needed for an LT-PEM system. The graph is from the same NASA report as Figure 2. I included the figure text as it explains the different circuits of air and heat management necessary for the function of the stack.
All these loops must be controlled to give the fuel cell stack optimal working conditions at all phases of flight.
An HT-PEM system can skip the humidifier in the air supply loop, and the water out from the stack is an open-loop waste process. Its heat management system can also be made lighter and smaller.
In the following Corners, we look deeper into PEM fuel-based propulsion systems and their integration into an aircraft.