June 17, 2022, ©. Leeham News: Last week, we looked at the installation effects and efficiencies of the fuel cell systems we discussed in earlier parts of the series.
We could see the variants were significantly heavier than the propulsion system they would replace for an ATR72 size aircraft. The discussion assumed classical PEM fuel cells, also called Low Temperature PEM Fuel Cells. Now we look at if High Temperature PEM Fuel Cells can improve the installation situation.
High Temperature PEM Fuel Cells
High Temperature PEM Fuel Cells (called HTFC from now on) were developed over the last 30 years to overcome some problems with Low Temperature PEM Fuel Cells (LTFC from now on).
LTFCs have water as the electrolyte that conducts the protons in the fuel cell PEM and, therefore, cannot run at temperatures above 80°C as the water then evaporates from the PEM. The low temperature process also makes the LTFC sensitive to H2 purity, which must be over 99.9%. The higher operating temperature of typically 160°C and a phosphor acid PEM electrolyte makes the HTFC less sensitive to H2 impurities; it works with 97% pure H2.
The other advantage is the higher process temperature makes the cooling easier. The cooling of an LTFC is a real problem as the system must function in, say, the Middle East or African 40+°C climate, which means a ram-air heat exchanger has a 40°C delta for cooling. Surface cooling solutions must also handle a departing aircraft’s high-intensity sun radiation. With the HTFC at 160°C, the delta increases to 120°C, a 3X improvement.
The negative with HTFC is that these need heating to get to the working temperature of 160°C before the process runs. Therefore, we can expect to find an LTFC as an APU to supply the power to heat up and start the propulsion systems in an HTFC-based aircraft.
While the HTFC allows a simpler and lighter heat management system, it had a higher stack mass as its fuel cell process was less developed than the LTFC’s.
A California startup, Hypoint, has attacked the mass issue by developing lighter aluminum-based bi-polar stack plates and has decided to use air cooling for the balance of plant. Its design uses HTFC modules stacked in parallel and serial to get the desired power and voltage levels to feed the systems DC/DC converter, Figure 2.
The initial 20kW module will be ready this summer to combine into a system with the lead customer, Piasecki Aircraft, for an eVTOL application.
An example of how these modules can be combined into a 160kW system is shown in Figure 3.
The system has a circular form to fit existing aircraft nacelles, but the modules can be stacked in any convenient format. They must be pressurized and cooled by a compressor, however (a radial compressor is housed in the center of the unit), so pressure air ducting to the modules is required.
If the oxygen-depleted exit and cooling air energy shall be recovered, the air must be ducted back to a combined turbine+compressor unit, where part of the heat energy from the modules can be recovered in the turbine to drive the system compressor.
The modules and their applications are in development. First-round modules are at 20kW, whereas next-generation modules shall triple the power level.
As the balance of plant requires fewer components (no air humidifier, no liquid-based thermal management system), Hypoint envisages a system power density of 2kW/kg, including the balance of plant components.
The Hypoint approach is new and therefore less mature than converting existing LTFC systems to airborne use. Still, it is more adapted to airborne applications with its improved cooling, a simpler balance of plant, and lower mass.
Once higher power modules are available, and there is more experience in configuring these into systems with the power levels of Megawatts, it’s an attractive alternative to standard PEM Fuel Cells.