Cellsius: Pioneering Sustainable Aviation

Blog by Cellsius Project H2

Cellsius is an organization of students in mechanical and electrical engineering at ETH Zurich who are passionate about aviation and are working to make flying more environmentally friendly. After E-Sling, our first electrically powered aircraft, performed its maiden flight in 2022 and continues to prove its abilities in many test flights, we wanted to look for ways to improve its very limited flight time in a sustainable way. This was the beginning of project H2.

Before we can integrate our systems into an aircraft, we need to validate our powertrain on the ground. Two specific components that we laid our special focus on, are the traction battery, as well as our traction inverter. As there are not many commercial components available specifically designed for a hydrogen aircraft, we need to develop most components ourselves. Our goal this year is to be able to draw maximum power on ground tests. That is 100kW of continuous power generated by our fuel cell stack. If we can demonstrate this capability, the next year's team will be able to build the system into the actual plane itself and prepare for take-off.

Traction battery

Our battery acts as a buffer to the fuel cell, delivering extra power during take-off, being charged during cruise flight and acting as a fail-safe in case of an emergency. Our battery is rather small, as it does no tact as our primary energy source. It has a nominal voltage of 645 V and a capacity of 5.8 kWh. We intend to discharge it up to 6C, hence we also have two cooling plates on each side of the battery. We have a configuration of 180s2p Li-Ion cylindrical cells, split into 15 modules. Each module has its own BMS-Slave, reading voltage and temperature of every parallel cell pair. All BMS slaves are connected via an ISO-SPI bus, ending in a BMS master. Our BMS master was developed on the FoxBMS platform from the Frauenhofer institute. This BMS has custom flashed SoCs in an BGA package, which would be impossible for us to solder ourselves reliably. Thus, we took advantage of the PCB assembly feature of Eurocircuits. We sent them our custom parts and got the manufactured and assembled boards back. We also recently finished our assembly of the battery as well and can now move on to testing.

Traction Inverter

We also build our own inverter to complement our custom air-cooled motor. We have a 3-phase synchronous motor with an encoder. To be able to power the motor with the correct phase current and voltages, we need to measure multiple sensors like our three current sensors at each phase, our dc link bus voltage, the encoder, as well as some MOSFET parameters. To combine all these data and write our motor control on top of them, we developed our own inverter controller. We were able to validate this controller and do motor tests up 20kW of electrical power and are currently only limited by the wall power we can draw. The next step is to get the fuel cell system up and running and push the inverter up to its intended limit of 100kW.