May 27, 2022, ©. Leeham News: Last week, we looked at the power levels we need in a fuel cell and electric motor system. We listed the required powers and durations for takeoff, climb, and maximum continuous power levels for a 70-seater turboprop.
Now we go deeper into the fuel cell system design, looking at system powers and thermals.
We have previously looked at the components we need in a fuel cell system, Figure 1. To discuss the sizing of such a system over the different phases of flight and how all the various components behave is complex. It’s a system with a lot of moving parts.
Figure 1 is taken from a NASA report (you find it here) that discusses an aeronautical fuel cell system that uses gaseous hydrogen. Consequently, it doesn’t use the cooling capacity inherent in an LH2 fueled system. We have previously shown that a gaseous hydrogen tank system has unacceptable mass and volume characteristics for an airliner. For our liquid hydrogen-fueled 70 seater turboprop, we want to discuss a cryocooled fuel cell system.
For this purpose, we use a recent report from a team from the Norwegian University of Science and Technology (NTNU) where a liquid hydrogen fuel cell system is described and simulated. You find the NTNU report here. It uses the LH2 to cool the power cables, inverters, and electric motors to superconducting status. As a result, the power losses and masses for these parts are reduced to a fraction of a non-cryogenic system.
The NTNU report observes that the components in such a system are in their early stage of development (low TRL level). It simulates a system ready for prime time by 2035. To cater to system uncertainty it simulates a base case, a conservative variant, and an optimistic scenario. We use the base case.
Figure 2 shows the system which uses the LH2 cool sink to turn the power cabling, inverter, and motor to superconduction. Note the blue flow path of the LH2, which goes from LH2 at 20K, vaporizes at 22.164K (by it absorbing 0.44MJ/kg of heat), and then flows backward in the system, cooling each major component until it has the required 358K (85°C) for the fuel cell entry. LH2 absorbs about 12kJ/kg and K after vaporization.
The red cooling path is a classical liquid (water/glycol) additional cooling system to top up where the LH2 cooling is exhausted. In practice, the blue cooling is done with a Nitrogen system as hydrogen is difficult to handle inside the system units. The liquid hydrogen transfers its cooling capacity to this system in a heat exchanger before entering the fuel cell as 85° H2.
The system’s power flow and heat losses are shown in Figure 3.
The black notations are for power/heat flows during takeoff, where the report assumes 2.05MW shaft power per side, and grey values are for the cruise at FL200, where 2*1.25MW is assumed.
Observe the enormous losses. For takeoff, we need 4.1MW shaft power for the propellers. The system draws 11.1MW LH2 from the tank. 4.8% of the 11.1MW leaves as unused H2. The remaining 6.47MW is absorbed by the blue and red cooling systems.
As discussed last week, the system can be designed to absorb takeoff and the initial climb heat losses but must be sized for the losses from continued climb and cruise.
At cruise at FL200, the system draws 6.7MW LH2 and delivers 2.5MW shaft power. 4.8% leaves as unused H2 as before, and we need to cool away 3.85MW. The 3.85MW and any additional heat generated during the climb to cruise size the heat management of the system.
Note the minimal heat the hydrogen cooled DC/DC converter, HTS cabling, DC/AC inverter, and SCM motor generate. The simulation does not analyze the emergency cases where the superconduction is lost for these units.
The overall system efficiency is defined as shaft power out versus power drawn from the tank. It’s on average 38.6%. The fuel cell enthusiast will say the fuel cell is closer to 50%. Correct, about 45% at a cruise load, lower at takeoff, and higher when not stressed (depends on the fuel cell sizing). But we are not looking at component efficiencies. What is to compare with an alternative propulsion system is the shaft power generated to the propeller/fan to the power drawn from the energy store.
In the report, NTNU discusses the possibility of cutting the top power of the system (and thus heat) by using a battery as an energy source, Figure 4.
The NTNU team probably make the same mistake we discussed in the hybrid articles. If the battery-assisted power levels are needed for emergencies (for example 100% power for an early go-around), the batteries cannot be used for other purposes during the flight; thus, the fuel cell must be sized for the full load.
Next week we use the NTNU and the NASA reports to look at what happens should we not have LH2 at -253°C to aid our system’s cooling.