Felix Smyrek from Recklinghausen investigated how the components of a dual fuel cell drive in a truck need to be coordinated so that they work as economically as possible while still delivering high performance in his bachelor's thesis at Fachhochschule Dortmund. His findings underline the usefulness of fuel cell drives in trucks.
Several truck manufacturers use drives consisting of two fuel cell systems (FCS) and a battery. This offers advantages, such as a longer service life, if the FCSs are used alternately when power requirements are low, for example in city traffic.
With higher power requirements, the idea is to distribute the power evenly between the two FCSs so that both are used as little as possible.
In cooperation with the Center for Fuel Cell Technology (ZBT) in Duisburg, Felix Smyrek investigated whether this is actually the case in two stages using a computer-simulated model of a truck from an Asian manufacturer. His work was recognized as the faculty's best Bachelor's thesis at the Academic Year Celebration at the end of 2023.
Static operation
In the first step, the man from Recklinghausen looked at a scenario with a constant power requirement, for example when driving at a constant speed on a flat road. Does it make a difference if the FCSs deliver different levels of power? "The efficiency curve of a fuel cell system is quite easy to illustrate," says Felix Smyrek. It first rises steeply, reaches its maximum ("peak") and then falls back down much flatter.
Smyrek's series of experiments showed that even if a single FCS can easily deliver the required power, efficiency is highest when each FCS is operated as close to the peak as possible.
This leads to the conclusion that it is more efficient to operate only one cell if the power requirements are below the peak. If the demand is higher, as in the example, both FCSs should be operated in equal parts.
Dynamic operation
In dynamic operation, which simulates a real car journey with starting, accelerating, braking and so on, the battery also comes into play. It serves as a buffer in certain situations, i.e. to help with power peaks. Felix Smyrek has now investigated exactly when its use is most efficient.
The vehicle development student found out: Whenever there are rapid power changes, it is advisable to buffer as much as possible with the battery. For two reasons. Firstly, the losses of a FCS increase during load changes, such as those that occur during rapid power peaks, and energy is lost as a result.
Secondly, a little more complicated: when braking, the braking energy is recuperated, i.e. fed back into the battery. However, this is only possible if the battery is not already full at that moment. That's why it makes sense to keep using the battery while driving to prevent it from being fully charged.
Valuable data
Because the fuel cell drive systems of truck manufacturers differ in many respects and the manufacturers do not provide details on things such as efficiency and the exact coordination of the FCS and battery, Felix Smyrek's findings cannot be easily transferred to other trucks. Nevertheless, it provides basic knowledge about how the interaction between two fuel cells and a battery can be controlled as efficiently as possible, and is therefore a further step on the way to optimizing the use of the drive system.
The ZBT also benefits from the amount of data generated. Felix Smyrek says that the cooperation with the Duisburg institute made his work possible in the first place. "I am therefore all the more pleased that I was able to give the team at the ZBT a small benefit and that they can use the findings for further projects."
Prof. Dr. Markus Thoben and Prof. Dr. Sönke Gößling supervised the bachelor thesis.
Why fuel cells at all? Aren't purely battery-powered vehicles more efficient anyway?
Battery electric vehicles have a higher overall efficiency than fuel cell vehicles. This means that they convert a higher proportion of the energy supplied to them - their charge - into drive energy, namely up to 80 percent, in exceptional cases even more. By way of comparison, a fuel cell drive system uses up to 60 percent, while combustion engines use a maximum of 45 percent.
The disadvantage of a battery is its weight. The greater the range of a vehicle, the larger and heavier the battery must be in order to provide sufficient energy. For trucks that cover many hundreds of kilometers at a time, the battery would be so large and heavy that it would be unprofitable to operate. In addition, charging stations with a charging capacity of one megawatt would be required - seven times as much as an electric car needs.
This is where the fuel cell drive comes into play. This is because its fuel - hydrogen - is not only more space-saving and easier to transport, but can also be refueled much faster. This means that for long ranges, the fuel cell drive is ultimately more efficient despite its lower efficiency.